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Flexible photovoltaic power systems: integration opportunities, challenges and advances

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2017 Flex. Print. Electron. 2 013001

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Flex. Print. Electron. 2 (2017) 013001 https://doi.org/10.1088/2058-8585/aa5750

TOPICAL REVIEW

Flexible photovoltaic power systems: integration opportunities,challenges and advances

Aminy EOstfeld1 andAnaClaudiaAriasDepartment of Electrical Engineering andComputer Sciences, University of California, Berkeley, CA 94720,United States of America1 Author towhomany correspondence should be addressed.

E-mail: aostfeld@berkeley.edu and acarias@eecs.berkeley.edu

Keywords: photovoltaics, flexible electronics, printed electronics, energy harvesting, energy storage, power electronics, photo-rechargeabledevices

AbstractPhotovoltaic power systems, consisting of solarmodules, energy storage, and powermanagementelectronics, are of great importance for applications ranging fromoff-grid and portable power toambient light harvesting for sensor nodes. Co-design and integration of the components usingprinting and coatingmethods onflexible substrates enable the production of effective andcustomizable systems for these diverse applications. In this article, we review photovoltaicmodule andenergy storage technologies suitable for integration intoflexible power systems.We discuss the designof electrical characteristics for these systems that enable them to power desired loads efficiently, as wellas strategies for physically combining the components. Functions and design considerations of powermanagement electronics are presented alongwith recent progress toward printed andflexible powerelectronics.We analyze both hybrid and fully flexible photovoltaic systems and the critical role of theapplication in the choices ofmaterials and architectures for the system components.

1. Introduction

Development of large-scale, reliable and cost-effectivephotovoltaic (PV) power systems is critical for achiev-ing a sustainable energy future, as the Sun is the largestsource of clean energy available to the planet [1].Photovoltaics are also an ideal power source forremote locations without electric grid access [2], andare of interest for numerous smaller scale applicationsincluding consumer electronics and wearable devices,as well as energy harvesting from indoor light sourcesfor low-power applications such as sensor nodes [3].For indoor and outdoor applications alike, creating astandalone PV system that can reliably meet thecurrent and voltage demands of electronic loadsrequires the addition of energy storage and powermanagement electronics. Energy storage devices suchas batteries are necessary to manage the temporalvariations in PV module output, for example due tovariations in solar irradiance, as well as variations inload. Powermanagement electronics play a number ofvital roles in PV systems, including preventing over-charging and over-discharging of the battery, ensuringmaximum power is being extracted from the PV

module, and converting the PV module’s directcurrent (DC) output power to alternating current(AC) or to a different DC voltage.

Typically, power management circuits consist ofdiscrete components acquired from various manu-facturers and soldered to a rigid printed circuit board(PCB). Similarly, most commercial PV modules andbatteries are rigid and bulky, and the conventional PVsystem implementation involves connecting these dis-parate components with wires as shown in figure 1(a).Over the past several years, however, flexible electro-nic materials and solution-based fabrication techni-ques have emerged as a potentially transformative setof technologies for photovoltaics, energy storage, andother electronics industries. The use of flexiblemateri-als can reduce weight, improve portability, and sim-plify PV system installation [4–6], in addition toenabling entirely new applications such as wearablesensors and smart labels [7–10]. Additive printing andcoating techniques allow materials to be depositedover large areas at high speeds and low temperatures,enabling customizable electronic systems on plasticsubstrates with low cost and low embodied energy[10–12]. The successes of these technologies have

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18October 2016

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6 January 2017

PUBLISHED

6March 2017

© 2017 IOPPublishing Ltd

inspired visions of fully printed, flexible, integratedsystems such as that illustrated in figure 1(b), in whichflexible PV module and battery layers are integratedwith printed powermanagement electronics and prin-ted load devices.

The power levels of PV systems span several ordersof magnitude, from indoor energy harvesting for low-power sensors to large-scale outdoor solar installa-tions. These diverse applications impose varyingrequirements in terms of performance, cost, mechan-ical properties and degree of integration. As a result,reported photovoltaic system designs cover a widespectrum from the conventional approach illustratedin figure 1(a), to hybrid approaches consisting of bothprinted and conventional components, to fully flexiblesystems resembling figure 1(b), depending on theuse case.

This review will evaluate recent progress towardthe vision of integrated, printed, flexible photovoltaicsystems. Advances in printed and flexible photovoltaicmodules, energy storage devices, and power electroniccomponents will be reviewed. Both electrical and phy-sical aspects of system design will be analyzed in termsof performance, manufacturing viability, and suit-ability for applications. Focus will be given to how theapplication drives design choices such as PV moduleand energy storage materials and structures, powermanagement circuit topology, and the use of fullyprinted versus hybrid electronics. Examples of effec-tive photovoltaic systems with varying scales and pur-poses will be reviewed, and major remainingroadblocks will be identified.

2. Photovoltaicmodules

Numerous material and manufacturing processoptions exist for photovoltaic modules. Ideally, aphotovoltaic module would be composed of abun-dant, inexpensive materials that provide good long-term stability and power conversion efficiency underthe expected operating conditions (indoors, outdoors,or both). Manufacturing processes should be scalable

to large areas and multi-cell modules at high speed,and should have minimal energy consumption andmaterial waste. A photovoltaic module technologywith these characteristics will have low cost per watt,large electricity production potential, and short energypayback time, and will therefore be most appropriatefor large-scale production [13, 14]. Although applica-tions such as integration into rigid electronic productsdo not require the PV module to be mechanicallyflexible, flexibility is beneficial in many other situa-tions. For example, the use of flexible materials canreduce solar module weight by eliminating the needfor bulky protective packaging and allow the modulesto be rolled or folded for transportation. As a result,flexible solar cells are ideal for applications such asportable lighting systems in off-grid rural regions [4]and portable power for the military [15]. Flexible solarmodules are advantageous for larger-scale installationsand building-integrated photovoltaics because theycan be installed very quickly (by simply unrolling) andcan be laminated onto surfaces such as roofs and walls[5, 6, 16]. They are also of interest for powering flexibleelectronics such as health monitoring devices [8, 9]and smart packaging [17] because they can be locatednear the devices, enabling wireless operation, withoutcompromising the system’s flexibility or low profile.However, the solar industry is currently dominated bycrystalline silicon, which is brittle and energy intensiveto produce, and to a lesser extent the thin-filmmaterials cadmium telluride and copper indiumgallium selenide, which rely on rare elements [18].Amorphous silicon (a-Si), organics, and perovskitesare promising alternatives based on abundant andflexible materials. This section will summarize themain advantages, challenges, and recent advances ineach of these technologies.

2.1. Amorphous silicona-Si was the first thin-film PV technology to reachcommercial production [19]. a-Si solar cells aretypically deposited by plasma-enhanced chemicalvapor deposition in a p–i–n structure: light is primarilyabsorbed in the central intrinsic region, and the

Figure 1. Illustrations of (a) conventional and (b) fully printed, flexible, integrated photovoltaic systems.

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built-in electric field aids in charge carrier collection.Although they have higher recombination rates andtherefore lower efficiency than crystalline silicon,amorphous silicon solar cells can nevertheless have ashorter energy payback time due to the reduction inenergy used to produce the cells [20]. Furthermore,the ability to manufacture a-Si solar cells at relativelylow temperatures enables the use of a number ofsubstrates, from common inexpensive plastics such aspolyethylene terephthalate (PET) and polyethylenenaphthalate (PEN) [21–23], to the cardboard used infood and beverage packaging [17]. A major challengefor a-Si solar cells is the Staebler–Wronski effect [24],in which light creates defects which act as recombina-tion centers, causing the efficiency to decrease overtime until it reaches a stabilized value. This effect canbeminimized by optimizing the fabrication process orby reducing the active layer thickness [19]. Sincereducing thickness improves charge carrier collectionefficiency but reduces absorption, it has becomecommon practice to create multijunction a-Si devicesby stacking thin individual cells. Higher efficienciescan be achieved for multijunction cells incorporatingmaterials with different bandgaps, such as amorphoussilicon–germanium alloys or microcrystalline Si, inaddition to the standard a-Si, because a greater portionof the solar spectrum can be absorbed [25]. Due totheir low cost and the variety of form factors available,amorphous silicon solar cells are the most commontype of PV device to be integrated into consumerproducts [3]. Specifically, since small flexible amor-phous silicon PV modules have been commerciallyavailable for years, they have been the module ofchoice in a number of integrated flexible energyharvesting systems, including many that will bediscussed throughout this review [26–34].

2.2.OrganicsOrganic photovoltaics (OPV) utilize semiconductingpolymers and small molecules to form the active layer.Typically, an electron donor (usually a polymer) andan electron acceptor (usually a fullerene) are mixed insolution and cast together to form a bulk heterojunc-tion, an interpenetrating network of donor andacceptor domains. Light absorbed in the donorgenerates excitons, which must diffuse to an interfacewith the acceptor where, assisted by the difference inelectron affinity of the two materials, they split intofree holes in the donor and free electrons in theacceptor. The bulk heterojunction structure enablesthe active layer to simultaneously have sufficientthickness for light absorption (∼100 nm) and domainsizes small enough (∼10 nm) for efficient excitoncollection [35]. Due to the very small thicknessrequired for light absorption, OPVs can be fabricatedwith total thickness (including the substrate) less than2 μm [36, 37], which is appealing for portable andwearable applications where weight must be mini-mized. Additionally, the chemical structure of OPVactivematerials can be customized to achieve a desiredcolor, and the film thickness can be customized toproduce semitransparent solar cells, making OPVsideal for aesthetic and building-integrated applications[38–44]. OPVs have demonstrated similar maximumefficiencies to amorphous silicon cells in sunlight, inthe range of 10%–11% [45], and can offer higherperformance under indoor lighting [46, 47]. Further-more, due to the low embodied energy of solution-processed carbon-based materials, OPV have thepotential for energy payback times on the order of afewmonths, among the best of PV technologies [14].

Improvement of OPV performance and manu-facturing scalability is ongoing. For example, there hasbeen a great effort to replace the incumbent

Figure 2. Flexible organic photovoltaicmodules. (a)Photograph and (b) current density–voltage characteristic of amodulewith 14series-connected cells. Reprinted from [70], copyright 2014, with permission fromElsevier. (c)Current–voltage characteristic and (d)photograph of a large-area high-voltageOPV installation [6]JohnWiley& Sons. © 2013WILEY-VCHVerlag GmbH&Co.KGaA,Weinheim.

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transparent electrode material used in OPVs, indiumtin oxide, with low-cost, high-conductivity, flexibleand solution-processable alternatives such as metalgrid/conducting polymer composites [48], carbonnanotube networks [49, 50], or silver nanowire net-works [51]. Vacuum deposited low work functionmetals, typically used as the electron collecting elec-trode, are also being replaced with electrodes based onsolution-processed organic materials [52] or metaloxides [53], enabling fully solution processed devices[39, 42, 54–56]. Efficiency and stability, which havehistorically been poorer for OPVs than for most otherPV technologies, are being improved through thedesign of new active layer and contact materials; aswell as new architectures such as tandem cells andmodules with high geometric fill factors [57–64]. Dueto these improvements, many monolithically inte-grated flexible OPVmodules have been achieved usingroll-to-roll or roll-to-roll-compatible coating pro-cesses [6, 65–71]. The state of the art includes smallmodules with efficiency surpassing 3% [69, 70], asshown in figures 2(a), (b), and high-voltage moduleswith areas in the tens of square meters [6, 67], asshown infigures 2(c), (d).

2.3. PerovskitesDespite being the newest of the PV technologiesreviewed here, perovskite solar cells have yielded thehighest efficiencies, with several recent examples of18% efficiency or higher [72–74]. Perovskite solar cellsevolved from dye-sensitized solar cells, which com-bine a light-absorbingmaterial withmesoporous TiO2

for electron collection [75]. Unlike conventional dye-sensitized solar cells, perovskite solar cells are solid-state devices, and typically utilize organometal leadtrihalide perovskites as the absorbing layer along withan organic hole transporting layer.

As with organic solar cells, a great deal of researchis taking place in order to improve the viability of per-ovskite solar cells for commercialization and integra-tion. One thrust of perovskite solar cell research is toimprove the stability and eliminate the hysteresis inthe current–voltage characteristics that has plagued

many early perovskite solar cells [74, 76–78]. Anotheris to fabricate cells at low temperatures [79–83]enabling the use of inexpensive, flexible plastic sub-strates [84–92]. Several of the lessons learned fromOPVdevelopment have also been useful for perovskitesolar cells, such as the use of scalable processes forcoating the active layers from solution [77, 93, 94].Many active layer and contact materials developed forOPV have also been employed as electron or hole col-lecting layers in perovskite solar cells [74, 84–86,94–97]. Efforts to improve the health and environ-mental aspects of perovskite solar cells include produ-cing the lead perovskite material from recycled lead-acid car batteries [98] and replacing it altogether withlead-free, tin-basedmaterials [99–101]. In the past twoyears, a number of small perovskite solar moduleshave been demonstrated [78, 95, 96, 102–104].Recently, a flexible perovskite solar module wasdemonstrated by Di Giacomo et al [105]. Althoughpower conversion efficiencies of ∼10% have beenachieved for small individual flexible perovskite solarcells, further optimization is needed to reach the sameperformance levels with larger-area flexible modules,which are currently on par with OPVmodules, as illu-strated infigure 3.

3. Energy storage

Energy storage is a vital component of a PV system.Solar irradiance is variable and often unpredictableduring the day, and entirely unavailable at night.Manyloads, on the other hand, require a constant amount ofpower, or must operate at night, as in the case oflighting. Other loads, such as sensor nodes, oftenrequire relatively large pulses of current for sensoroperation or data transmission, with periods of muchlower power consumption in between. A PV modulesized tomeet the sensor node’s peak demandwould beunderutilized at non-peak times. Electrochemicalenergy storage devices such as batteries and super-capacitors address these issues by storing excess energyduring periods of high irradiance or light load andreleasing that energy as needed at a later time. Both

Figure 3. Flexible perovskite photovoltaics. (a)Current density–voltage characteristic and photograph of small flexible cells.Reproduced from [88]with permission of The Royal Society of Chemistry. (b)Photograph and (c) current density–voltagecharacteristics of aflexiblemodule with 4 series-connected cells [105] JohnWiley& Sons. © 2015WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim.

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batteries and supercapacitors can be produced inflexible form factors using scalable coating techniques,and are therefore of great interest for flexible powersystems [12, 106–111]. This section will briefly com-pare flexible battery and supercapacitor technologiesand provide some guidelines for the selection ofappropriate energy storage devices for flexible PVsystems.

An ideal energy storage device for applications inflexible PV systems would have a high specific energy(Wh l–1 or Wh kg–1) so that sufficient energy storagecapacity can be achieved in a thin, flexible form factor.The device would retain its capacity over a large num-ber of charge–discharge cycles, so that it can functionover the long term to offset daily variation of PV out-put as well as second- or minute-scale changes in load.It would also have high energy efficiency and low cost.Charge/discharge rate capability, usually defined bythe highest charge or discharge rate the device canexperience without a substantial drop in capacity orefficiency, is often an important figure of merit as well.Energy storage device architectures with high rate cap-ability allow the peak PV current to be accepted andpeak load current to be provided efficiently withoutnecessarily requiring a large capacity.

Lithium-ion batteries are the industry leader forportable electronics due to their high specific energy,high energy efficiency, and long lifetime [112]. They

are also beginning to replace lead-acid batteries in off-grid PV energy storage and nickel metal hydride(NiMH) batteries in electric and hybrid vehicles,despite their higher up-front cost, due to these char-acteristics and the potential for cost reduction as pro-duction scales up [113]. Lithium-ion batteries havealso received a great deal of attention for integrationinto flexible electronic systems, and numerous lithiumbatteries have been demonstrated recently in planarflexible [27, 33, 114–124] and stretchable [125–128]forms as well as in the form of flexible wires or fibers[129–133] for such applications. Thin active layerstend to provide higher rate capability and greatermechanical flexibility, but at the cost of a lower capa-city per unit area. Efforts are therefore under way todesign batteries with simultaneously high capacity,rate capability and flexibility, for example through theuse of nanostructured active materials [123] andcurrent collectors [122]. While many state-of-the-artbattery architectures are demonstrated on small(few cm2) areas, Kim et al developed larger-area(25 cm2) flexible lithium-ion batteries based on textileelectrodes and explored the integration of thesebatteries into higher-voltage multi-cell modules [134].Figures 4(a)–(d) show a schematic of the module andphotographs of the module attached onto flexible sur-faces. This work also investigated the battery behaviorat 45 °C, an expected operating temperature if the

Figure 4. Flexible batteries designed for integration into systems. (a)–(d) Flexible lithium-ion batterymodule: (a) illustrations offlexible batterymodule integratedwith a rollable display, (b) layout of 16-cell batterymodule, photographs ofmodule integrated into(c) a tent and (d) a rollingwindow shade. Reproduced from [134]with permission of The Royal Society of Chemistry. (e), (f)Photographs of printed flexible zinc–manganese batteries (e) as fabricated and (f) integrated into a sensingmodulewith otherelectronics [135]©2015 IEEE. Reprinted, with permission, fromProceedings of the IEEE 103, 535–553 (2015).

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battery is exposed to sunlight, and reported a declinein capacity of 23% after 10 cycles. Further invest-igation is therefore necessary to optimize battery per-formance for integration with PVmodules in outdoorapplications.

Alkaline chemistries such as silver–zinc and zinc–manganese are non-toxic and less reactive thanlithium chemistries [136]. As a result, they can be pro-cessed in air and require less robust encapsulation, andare preferable for applications such as wearables wherethey will come into close contact with the human body[137, 138]. Silver–zinc, zinc–manganese, and lithium-ion batteries all tend to have similar practical values ofspecific energy, in the range of 135–150 Wh kg–1 [12].However, the voltage of alkaline batteries (1–2 V) islow compared to the 3.6–4.2 V range of standardlithium-ion battery chemistry, requiring multiple bat-teries in series [137, 139–141] or power electronics tomeet the voltage needs of many loads. Rechargeablealkaline batteries have also tended to suffer from poorcycle life compared to lithium batteries [137, 142], dueto irreversible processes such as migration of silverions and zinc dendrite formation. However, theseissues can be mitigated through the optimization ofthe electrolyte and separator materials [138, 143].Imprint Energy has recently developed printed flexiblezinc–manganese batteries based on solid polymerelectrolytes that proved to be more mechanicallyrobust than commercial flexible lithium-ion batteries[135]. These batteries were integrated with a temper-ature sensor and circuitry into a flexible sensing mod-ule, as shown in figures 4(e), (f).

While state-of-the-art flexible batteries can chargeand discharge as quickly as a few minutes [123], fur-ther increasing the charge or discharge rate can lead toreduced efficiency or longevity of the battery. Super-capacitors, on the other hand, can operate morerapidly, charging and discharging completely in a fewseconds [111]. They also tend to have greater cyclingstability than batteries: numerous flexible super-capacitors have been reported with lifetimes of manythousands of cycles [109]. Thus, it may be practical touse a supercapacitor as the sole energy storage comp-onent for applications in which a load, such as a sensornode with a bluetooth wireless communication mod-ule, requires large pulses of current relatively fre-quently. However, capacitors also self-discharge morerapidly than batteries, making them unsuitable forlong-term energy storage on the scale of days [135].Combined energy storage schemes using both bat-teries and capacitors have therefore been proposed formany systems, so that both peak current andlong-term energy storage demands can be met[2, 135, 144, 145]. Supercapacitor structures are oftensimpler than batteries, often consisting of carbon-based electrode materials on either side of a separatorand electrolyte. This simplicity is appealing from theperspectives of manufacturing and integration,although more complex structures are also being

developed that incorporate conducting polymers ormetal oxides, which undergo redox reactions andgreatly increase the energy density [108, 109].

Unlike batteries, the voltage of a supercapacitor iszerowhen the capacitor is fully discharged.As a result, ifa supercapacitor connected to a PV module is allowedto discharge completely, the operating point of the PVmodule will be far away from the maximum powerpoint, and the efficiency of chargingwill be lowuntil thevoltage has increased again. The efficiency can beimproved by not allowing the capacitor to dischargecompletely, thus maintaining the PV module closer toits maximum power point; however, this reduces theusable energy storage capacity of the supercapacitor.This is the case when a battery and supercapacitor areconnected in parallel: the supercapacitor is maintainedat the same voltage as the battery. Alternatively, powerconversion electronics can be added between the PVmodule and supercapacitor to maximize the efficiency,aswill be discussed inmore detail in section 4.3.

4. Integrated photovoltaic power systems

The previous two sections summarized recentadvances in photovoltaic module and electrochemicalenergy storage technologies that have enabled highperforming, scalable, and flexible devices. To achievean effective power system for a particular application,these components must be designed not only for highindividual device performance but also for highperformance of the complete system under theexpected range of operating conditions. Power systemdesign should also take into account any physicalconstraints imposed by the application, such as weightlimits and flexibility or stretchability requirements.Selecting power electronics with form and functionappropriate for the particular system is essential aswell. Figure 5 illustrates an example design processflow for an energy harvesting and storage system. First,an energy storage device is selected based on thevoltage, capacity, and mechanical requirements of theapplication. Then, a PV module is designed to chargethe device efficiently with minimum required powerelectronics. The power management electronics thatare still necessary, such as a blocking diode and batteryprotection circuit, are incorporated. Finally, solarcharging performance is evaluated. This section willreview in more detail the electrical design considera-tions and physical integration strategies of PVmodulesand energy storage, followed by a discussion of devicesand circuits for power management and progresstoward printed and flexible circuits. Examples ofeffective systemdesignwill be highlighted throughout.

4.1. Electrical considerationsWhile power electronics are beneficial in many cases,designing the system to perform well without sub-stantial power electronics is ideal, to minimize

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manufacturing complexity, costs and potential pointsof failure. Therefore, the voltage of the power systemshould be designed to match the requirements of theload devices whenever possible, so that power electro-nics are not required to convert to the load voltage.The PVmodule open-circuit voltage should be higherthan the maximum voltage of the battery or super-capacitor, so that a full charge is possible. Since theopen-circuit voltage of a PV module is roughlylogarithmically dependent on the irradiance [146],attention should be paid to the range of possible open-circuit voltages when designing a system that willexperience a range of illumination conditions.Furthermore, the system should be designed so thatthe operating voltage of the PV module is as close aspossible to the maximum power point, as often aspossible, so that maximum power is collected. Forexample, if the system uses a lithium-ion battery,which has a voltage between 3.6 and 4.2 V during thecharging period, the maximum power point of the PVmodule should be in that range. Achieving the desiredPV module voltage involves selecting the appropriatenumber of series-connected cells; finer tuning of the

voltage is also possible through selection of the activelayer and contactmaterials.

The power flows and energy storage capacity of thesystem are also crucial parameters that can be designedindependently of the voltage. For reliable long-termoperation without additional power sources, the aver-age power generated by the PVmodulemust be at leastequal to the average load power consumption. Theenergy storage capacity should also be sufficient toovercome the variability in power generation and load.For example, in an outdoor system powered by theSun, the battery must have enough capacity to powerthe load during the night, and the solar module mustbe able to produce at least that much excess energyduring the day. PV module current and battery capa-city depend on the active area as well as the materialsand thicknesses used for the active layers. Improve-ments in efficiency of thin-film PV modules and arealcapacity of thin-film batteries allow these require-ments to be met with smaller device footprints. Insome applications, the duty cycle of the load can bevaried to adjust the average power consumptiondepending on the amount of power available.

Figure 5.Aprocess flow for the design and evaluation of aflexible PV energy harvesting and storage system. The general steps areillustratedwith an example systemusing a thin-film lithium-ion battery [33], a printedOPVmodule, and basic batterymanagement.The battery voltage during chargingwith theOPVmodule, limited to 4.1V by the batterymanagement circuit, is shown in step 4.

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Several recent reports on PV systems have eval-uated the impact of conditions such as the light sourceand intensity and the PVmodule orientation on poweroutput and charging rate [29–33, 145, 147–149]. Ofthese, several were specifically designed to powerwearable health monitoring devices, and were there-fore characterized under conditions they would likelyexperience as their wearers move around [30–33, 145, 147]. In some cases the power systems weredesigned concurrently with the loads, allowing morethorough optimization of power flows. Somewearablesystems combining power sources and medical sen-sors are shown infigure 6.

One example of such a wearable system is the Soli-Band developed by Dieffenderfer et al, a healthcareplatform that continuously monitors the wearer’sphotoplethysmogram (PPG) signal to determine heartrate and blood oxygenation [8, 145]. The power sys-tem for the SoliBand consisted of monocrystalline sili-con solar cells, a lithium battery, a supercapacitor, andcharge management electronics, integrated with theelectronics for PPGmeasurement, data processing andcommunication into a wrist-worn device as shown infigure 6(a). Although all the individual componentswere rigid, the use of flexible connections betweencomponents resulted in a system with adequate flex-ibility for the application of interest. The battery capa-city, 20 mAh, was selected in order to provide thesystem’s power demand of 13.7 mW for 4 h withoutsunlight, and the solar module was able to charge the

battery in 30 min under direct sunlight or just over anhour in indirect sunlight. When minimal light wasavailable and the battery charge was low, the systemwas designed to enter a power-saving mode of lowduty cycle, pulse-like operation powered by the super-capacitor, with average power consumption of only0.57 mW. Similarly, Toh et al designed a wearable sen-sor node to sense body temperature and transmit thedata wirelessly [30]. A supercapacitor was selected asthe sole energy storage device, since the load profile ofthe sensor node consisted of 1 s pulses of relativelyhigh current (40 μA) during data transmission sepa-rated by 60 s periods of much lower current (1.3 μA).A flexible amorphous silicon solar module was used tocharge the supercapacitor during the sleep mode, withsufficient area to recharge the supercapacitor com-pletely between pulses under indoor lighting condi-tions. We have also developed a wearable system topower health monitoring devices, based on a flexibleamorphous silicon PVmodule layered on top of a flex-ible lithium-ion battery [33]. Figures 6(b), (c) show aphotograph of the device and a schematic of a pro-posed wearable healthcare system integrating thepower source and the components of a pulse oximeter.The solar module voltage was selected in order tocharge the battery under either sunlight or indoorlight, and the footprint of the devices was selected toprovide a capacity of 48mAh for powering a pulse oxi-meter. The duty cycle of the pulse oximeter, whichconsumed 20 mAwhile measuring the PPG signal and

Figure 6.Power source design for wearable healthcare devices. (a)Photographs and schematic of the SoliBand, integrating a rigid PVmodule, battery, supercapacitor, and components of a pulse oximeter into awristband [8]©2015 IEEE. Reprinted, with permission,fromProceedings of the IEEE 103, 665–681 (2015). (b)Photograph offlexible energy harvesting and storage system and (c) integrationconcept for energy system integratedwith components of a pulse oximeter. (d)Voltage and current delivered from the energy systemto a loadwith the optimal duty cycle tomatch average PVmodule and load currents. The dotted black line indicates that the batteryvoltage and state of charge are the same after each load cycle. Reproduced from [33]. CCBY 4.0 International license.

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1 mA between measurements, was chosen so that theaverage load current and average PV module outputcurrent would be equal, maintaining the battery at aconstant state of charge as shown infigure 6(d).

4.2. Physical integrationThe initial electrical design and characterization of asolar energy harvesting and storage system can oftenbe carried out without much attention to the physicalintegration and form factor of the system. Forexample, in such demonstrations as solar charging of anovel flexible battery with coplanar architecture [27]and charging a supercapacitor with a high-perfor-mance perovskite solar cell [150], the solar cells andenergy storage devices were simply located side by side.However, to create a viable product for a particularmarket or application, it is necessary to select anappropriate structure and manufacturing process forthe system. There are many strategies for physicallycombining the components of a PV system, frommanufacturing the components in individual packagesand installing them in separate locations, to assemblingindividually manufactured flexible solar cells andstorage devices into a single stack, to single multi-functional devices that perform both energy harvestingand storage simultaneously. The ideal degree ofintegration andflexibility depends on the requirementsof the application; common objectives include max-imizing power per unit area, maximizing longevity ofthe system,minimizing weight, or enabling customiza-tion or replacement of components.

It is often advantageous to locate the energy sto-rage and circuitry beneath the solar module, to mini-mize the footprint required to produce a given amount

of power. If the energy storage, solar module, and sub-strate for the circuitry are all flexible, the entire systemcan be flexible, enabling attachment to flexible orcurved surfaces or integration with flexible load devi-ces. Many flexible PV power systems have thereforebeen produced by fabricating the solarmodule, energystorage device, and circuitry using separate manu-facturing lines, then laminating the layers together[29, 33, 119, 152, 153]. For example, Krebs and co-workers developed multiple designs for integratedsolar-powered lamps based on in-house printed OPVmodules and commercial lithium-ion batteries[4, 151, 154]. In the earliest of these designs [4], thecircuitry was printed directly onto the back of the solarmodule, but due to frequent misalignments and mal-functions of the circuitry, yield was low and manyfunctional solarmodules were discarded. Later designsimproved yield and reduced cost by utilizing a separatesubstrate for the circuitry [151, 154]. Since the lampswere distributed to the public, they needed to be dur-able. Therefore, additional layers were added toimprove mechanical robustness, such as a top overlayand a spacer with similar thickness to the batteryintended to reduce strain on the battery. Figure 7shows the complete structure and photographs of oneof these solar lamps [151]. These lamination techni-ques are also of interest for adhering flexible PV sys-tems onto other devices. For example, Gambier et alattached a flexible PV module and battery onto a flex-ible piezoelectric cantilever to produce a multi-functional energy harvesting and storage system [28].

The choice of substrates and adhesives in these sys-tems is very important, as their mechanical propertiesdictate those of the entire system. For example,

Figure 7. (a)Photographs and (b) structure of a solar lamp integrating anOPVmodule, lithium-ion battery, and light-emitting diode(LED). Reproduced from [151]with permission of The Royal Society of Chemistry.

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polydimethylsiloxane (PDMS) is a popular adhesivematerial due to its high elasticity, low cost, and resist-ance to heat and aging [29]. Despite the use of flexiblematerials for each layer, multilayer structures some-times suffer from low flexibility due to their relativelylarge thickness [152]. In some cases, such as the one infigure 7, rigid layers are intentionally added, eventhough the active components are flexible, to ensurethe system is durable [151]. In other cases, such aswearable applications, higher flexibility is desired,necessitating alternative designs such as textile sub-strates with designated folding points [119]. Somewearable applications even require a stretchable powersystem with properties similar to skin. Lee et al pre-sented a technique to achieve such stretchable systems,in which many small, rigid devices are connected bystretchable serpentine interconnects within an ultra-low-modulus elastomer and supported by a higher-modulus elastomer substrate [155]. Figure 8 shows theconstruction of the system, including arrays of chip-scale batteries and solar cells as well as a chip for powermanagement. While the solar cell and battery arrayswere initially placed side by side, the excellent stretch-ability of the system also allowed it to be folded in halfto reduce the footprint.

Currently, the benefits of integrating a flexible bat-tery or supercapacitor with the PV module apply pri-marily to consumer products, portable systems andindoor energy harvesting applications. This is becausebatteries tend to be sensitive to extreme temperaturesand have lifespans shorter than the often-cited 20 yearPV module lifespan [3, 18, 112, 113]. Thus, for large-scale outdoor solar installations, which have high cost,are exposed to strong sunlight for long periods of time,

and are expected to perform well for many years, it isgenerally preferable to locate the battery in a separatecontainer where it can be kept away from the Sun andcan be replaced independently of the solar modules.There has been some investigation of flexible batteryperformance at elevated temperatures, but moreresearch and optimization is still needed before bat-teries are ready for reliable long-term integration intooutdoor PV systems [134]. As an example of a semi-integrated design, Garcia-Valverde et al developed aportable 15 W PV system for outdoor applications asshown in figure 9 [156]. This system consisted of aflexible OPV module that could be rolled around aplastic cylinder for transport; inside the cylinder was arigid battery pack and power management electronicson a rigid circuit board. The battery’s location under-neath the circuit board kept it shaded from direct sun-light, but the product design was nevertheless user-friendly and free of external wiring.

These heterogeneous integration approachesallow a great deal of freedom in the fabrication pro-cesses, as the processing conditions of one device donot need to be compatible with the other devices, andallow each component to be optimized independentlyfor highest performance. On the other hand, there isalso great interest in leveraging printing and coatingtechniques to build the energy harvesting and storagedevices onto the same substrate, or deposit one devicedirectly onto the other [111, 157–159]. These approa-ches could potentially improve the flexibility andweight of the completed power system, since the com-ponents are not encapsulated separately, as well as thesimplicity of manufacturing and ease of use. Forexample, Singh et al developed a process to spray-coat

Figure 8. Stretchable PV systembased on small rigid devices connected by stretchable serpentine interconnects and encapsulated in astretchable elastomer. (a) Schematic illustration and (b) photograph of the system, showing arrays of solar cells and batteries, powermanagement electronics, ultralow-modulus elastomer core and higher-modulus elastomer shell. (c) Illustration of the foldingprocess, withfinite element analysis showing the low strain experienced by all parts of the system. Reproduced from [155].

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a lithium-ion battery onto a number of substrates,including metal foils, ceramic tiles, and plastic sheets[160]. A solar power system was demonstrated by glu-ing a solar module onto the spray-coated battery on aceramic tile substrate, but in theory the battery couldbe sprayed directly onto the solar module instead. InYe et al, sputtering was used to deposit the layers of asolid-state lithium battery onto the back of an amor-phous silicon solar module [161]. Wee et al built asupercapacitor on top of an organic solar cell by drop-casting a carbon nanotube network electrode onto thecompleted solar cell, then stacking a polymer electro-lyte film and a free-standing carbon nanotube elec-trode on top [162]. There have also been numerousreports of integrated energy harvesting and storagedevices (often called photo-rechargeable devices) inwhich a dye- or quantum dot-sensitized solar cell andenergy storage device are built on opposite sides of ashared electrode [163–173]. The shared-electrodeconfiguration has been shown to reduce the seriesresistance, compared to externally connected devices[162]. The non-shared electrodes of the solar cell andenergy storage device are shorted together, thus creat-ing a parallel connection. In addition to these planarintegrated structures, several fiber- or wire-shapedenergy harvesting and storage systems have beendemonstrated, in which the same conductive fiber orwire substrate is coated with photovoltaic and energystorage materials in a side-by-side [174–178] or core-sheath [179] arrangement. These systems are of part-icular interest for wearable applications as they canpotentially be woven into electronic textiles. Addi-tional integrated energy harvesting and storage solu-tions include two-electrode photoelectrochemicalcells that generate and store charge in light-inducedredox reactions at both electrodes [180] and integra-tion of ferroelectric materials into photovoltaic cellsfor energy storage [181, 182].

Although these novel photo-rechargeable devicesare promising in many respects, several challengesremain before they become viable for use on a largescale. First, the use of shared electrodes greatly con-strains the design of the system, as the shared layermust perform well as both a solar cell electrode and abattery or supercapacitor electrode. The architecture

andmaterial choice for the other layers of the solar celland energy storage device must also be compatiblewith the shared layer. For example, most of the inte-grated photo-rechargeable devices reported to datehave utilized a dye-sensitized solar cell and a super-capacitor because both rely on similar nanostructuredelectrodes. Second, nearly all of the photo-recharge-able devices consist of a single solar cell and a singleenergy storage device, and as a result the voltage is lim-ited to the open-circuit voltage of the solar cell. Thevoltage can be increased by connecting several photo-rechargeable devices in series [172]. However, fabri-cating the solar module and battery separately allowsthe use of battery chemistries with higher voltages thanthat of a single solar cell, enabling higher energy den-sity than the fully integrated photo-rechargeable devi-ces. The solar module can then be produced with theideal number of series-connected cells to charge thatbattery. A few higher-voltage photo-rechargeabledevices have been demonstrated, through the use oftandem solar cells and series-connected solar cells.Wee et al connected two organic solar cells in series,one of which was used as the substrate for the fabrica-tion of a supercapacitor, allowing the voltage to bedoubled [162]. The structure and charge–dischargecharacteristics of this 1 V device are shown infigures 10(a)–(c). Guo et al developed a 3 V photo-rechargeable device as shown in figures 10(d)–(f), inwhich two tandem solar cells were connected in seriesto charge a lithium-ion battery [167]. One of the solarcells shared the common electrode with the battery,while the other was separated from the common elec-trode using an insulating polymer layer. However, theneed for the additional insulating layers and differingorientations of the solar cells resulted in a highly com-plex structure. Finally, the direct parallel connectionbetween solar cell and energy storage used in nearly allthe photo-rechargeable devices is often presented asan advantage due to its simplicity. For most applica-tions, though, it is preferable to include some amountof electronics between the PVmodule and energy sto-rage to maintain each in its optimal voltage range andmaximize the stored energy.

Figure 9.Photographs of a rollable solar charger, inwhich aflexibleOPVmodule can bewrapped around a cylinder containing a rigidbattery and powermanagement electronics. Reprinted from [156], copyright (2016), with permission fromElsevier.

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4.3. Electronics for powermanagementInmany lab-scale demonstrations of PVmodules usedwith energy storage devices, the two components areconnected directly to each other without any addi-tional electronics, in order to prove the viability of anovel component architecture or the physical integra-tion scheme for the components. However, powermanagement electronics are necessary to transformthese proofs of concept into robust systems thatoperate safely and efficiently under time-varyingconditions. Functions of power management electro-nics include protecting batteries from over-chargingor over-discharging, ensuring maximum power isextracted from the PVmodule even as illumination orload conditions change, and converting from theoutput power characteristics of the PV system to therequirements of the load. Characteristics such asamount of power, type of energy storage device, andvariability of illumination conditions determinewhichtypes of power management electronics are needed.Figure 11 illustrates the common types of powermanagement electronics in their respective locationsin the system.

A blocking diode, the most basic addition to a PVsystem, is a diode connected in series between the PVmodule and the energy storage device. It allows cur-rent to flow out of the PV module but prevents dis-charge of the energy storage device into the PVmodulein the absence of light. Blocking diodes are thereforenecessary in any system that may be in the dark attimes. As a result, even systems without any otherpower management electronics usually include ablocking diode [4, 26, 27, 33, 119, 151, 161, 183].In these systems, the PV module should be designedwith a slightly higher voltage to account for the

forward-bias voltage drop of the blocking diode,which is usually on the order of several tenths of a volt.In low-voltage systems, the power loss resulting fromthis voltage drop can be significant, but in higher-volt-age systems it is negligible. Some systems have used anLED as the blocking diode, so the user can visuallymonitor whether the battery is charging [27, 119].Alternatively, an additional rectifying barrier layerincorporated directly into the PV module may per-form the same function without requiring an externaldiode [184].

When multiple PV cells or modules are connectedin series, and one or more of the cells is shaded, thecurrent through the entire array is limited by the cur-rent of the shaded cell. As a result, diodes can also beconnected in parallel with each cell, allowing currentto bypass that cell if it becomes shaded. Bypass diodesare most common in large-area systems, where it islikely that the irradiance will vary over the area of thePV array. Switched-capacitor voltage balancing is anactive approach that offers the potential for higherenergy collection efficiency than bypass diodes [185].This technique takes advantage of the fact that whilethe current at the maximum power point dependsstrongly on irradiance, the voltage at the maximumpower point varies only slightly. A string of capacitorsand transistors is used alongside the string of PV cellsor modules; the transistors act as switches alternatelyconnecting each capacitor in parallel with one PV cellor its neighbor. The switched capacitors provide analternate path for any mismatch in current betweenthe cells, and also maintain all of the cells at the samevoltage, allowing near-maximum power to be extrac-ted from each cell. Since the switched-capacitorapproach is more complex than bypass diodes, it is of

Figure 10. Integrated photo-rechargeable devices. (a) Structure of a photo-rechargeable device consisting of two series-connectedorganic solar cells, with a supercapacitor printed onto the bottom cell. (b)Voltage and (c) current during two charging and dischargingcycles of the device. Reproduced from [162]with permission of The Royal Society of Chemistry. (d) Structure of a photo-rechargeabledevice consisting of two series-connected tandemdye-sensitized solar cells, one of which shares an electrodewith a lithium-ionbattery (LIB). (e), (f)Current–voltage characteristics of the two tandem cells: (e) SC I and (f) SC II. Reprintedwith permission from[167]. Copyright 2012AmericanChemical Society.

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most interest for large-scale systemswhere the avoidedpower losses are greatest.

In systems that include batteries, electronicsknown as battery management or charge controllersare important tomaximize battery performance, long-evity and safety. Batteries generally have a maximumvoltage corresponding to a full charge, and a mini-mum voltage corresponding to the fully dischargedstate; beyond these values irreversible damage canoccur. The simplest form of battery management is azener diode in parallel with the battery [154]. Thezener diode turn-on voltage should be equal to themaximum allowable battery voltage, thus ensuringthat the battery voltage never surpasses that value.Additional functions of battery management includedisconnecting the load if the battery voltage reaches aset minimum value (indicating that the battery hasbecome fully discharged), and charging the batteryusing a specified charging profile. In constant current-constant voltage charging, for example, the battery ischarged with constant current until it reaches its max-imum voltage, then maintained at that voltage untilthe current decays to zero. Constant current-constantvoltage charging allows the battery to be charged com-pletely, at a higher rate than simple constant-currentcharging, without exceeding the voltage limits [12]. Ifthe illumination conditions and load profile are verypredictable, the sizes of the solar module and batterycan potentially be designed so that the battery willnever become fully charged or fully discharged, mak-ing batterymanagement un-necessary. However, if theload or illumination are variable, or if fast charging isdesired, battery management is an important additionto a PV system.

DC–DC converters, circuits that convert from oneDC voltage to another, have several potential applica-tions in PV systems. First, the voltage of a system

containing batteries or supercapacitors varies depend-ing on the current and state of charge. If a load requiresa constant voltage, or one that is not a multiple of theindividual battery voltage, a DC–DC converter is nee-ded. For example, organic circuits and light-emittingdevices often require a voltage somewhat higher thanthat of a single lithium battery [186–190]. It can also beadvantageous to print solar modules with very highvoltages of hundreds to thousands of volts, to simplifyassembly and minimize resistive losses, then convertto a lower voltage for use by loads [6, 67, 156]. A DC–DC converter can also be used to ensure that a PVmodule is operating at its maximum power pointregardless of the battery or supercapacitor voltage orload impedance; this type of circuit is called a max-imum power point tracker (MPPT). In general, DC–DC converter circuits consist of switches (imple-mented with transistors or diodes), passive compo-nents (inductors and/or capacitors) that performshort-term energy storage, and a control system.Figure 12 shows three commonDC–DC converter cir-cuits: the boost converter (a), which increases voltage;the buck converter (b), which decreases voltage; and aswitched-capacitor voltage doubler, also known as acharge pump (c). In the buck and boost converters, acontrol system determines the duty cycle of theswitches, which sets the conversion ratio, while inswitched-capacitor converters the conversion ratiodepends on the circuit topology. Depending on thecontrol system, the same basic circuit design can beused to achieve different objectives, such as regulatingthe output voltage to a constant value, regulating theinput voltage to a constant value, or maximizing theoutput power.

The efficiency of amaximumpower point trackingcircuit depends on what control algorithm is used, aswell as the losses in the circuit components. Indirect

Figure 11. Schematic of a PV system, highlighting powermanagement options in their respective locations.

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MPPT approaches such as constant voltage and frac-tional open circuit voltage are desirable for small andportable systems, because fewer and smaller compo-nents can be used, potentially reducing cost and facil-itating integration [191]. However, direct MPPTschemes such as the perturb-and-observe and incre-mental conductance methods tend to have higher effi-ciency and versatility [192]. Thus, when consideringMPPT strategies for a given PV system, it is importantto consider the expected efficiencies with MPPT cir-cuits of varying complexity as well as with noMPPT atall. Since battery voltages tend to vary over a relativelysmall range (typically 10%–15%) during charging[12, 107, 110, 138], the PV module can be maintainedclose to its maximum power point simply by choosinga battery with appropriate voltage. For low-cost, low-power systems, the best option may be to connect thePVmodule and battery directly with noMPPT, allow-ing cost and complexity to be minimized [193]. Forlarge systems, on the other hand, a MPPT circuit ismore desirable because the potential power loss due tothe varying battery voltage is more significant, the los-ses in the power electronic components are less sig-nificant by comparison, and the cost of high-performance components is more easily justified.MPPT circuits can be even more advantageous in sys-tems that use supercapacitors, because supercapacitorvoltage ranges tend to be greater than those of bat-teries. When a supercapacitor is discharged, its voltageis zero, meaning a PV cell connected directly will bevery far from its maximum power point. For this rea-son, the energy conversion and storage efficiency ofsolar supercapacitor charging has typically been abouthalf of the power conversion efficiency of the solar cell[170, 172, 176]. Using a MPPT circuit to convert fromthe solar cell maximum power point voltage to thesupercapacitor voltage could greatly increase the effi-ciency of this process.

Finally, PV systems may also include inverters,which convert from DC to AC power. An inverter isnecessary if a system is connected to the grid, or usedto power loads that are designed to plug into wall out-lets. AC power is also of interest for wireless sensorsbecause it can be transmitted wirelessly through cou-pled inductors, capacitors, or antennas, avoiding theneed for physical connection between energy harvest-ing and load devices [34]. Like DC–DC converters,inverters often use a combination of transistors and

passive components: the transistors are switched togenerate an AC waveform, and the passive compo-nents filter the waveform to create a sinusoid withminimal distortion.

4.4. Printed andflexible power electronicsIntegration of power management electronics intoflexible energy harvesting and storage systems can beaccomplished through the use of conventional rigidcomponents on flexible substrates, flexible compo-nents, or some combination of the two. Flexibleprinted circuit boards (flex-PCB) are an establishedtechnology in which conventional surface-mounttechnology (SMT) components are attached usingsolder to photolithographically patterned copper-coated flexible plastic substrates. To fabricate a com-plex circuit such as MPPT, many components aresoldered to the flex-PCB; as long as the componentsare relatively small and sparsely populated, the systemremains flexible [29, 30]. The highly conductivecopper interconnects and compatibility with SMTcomponents such as silicon integrated circuits (ICs)allow power electronics with high efficiency at highpower levels to be achieved in flex-PCB technology.For example, Acanski et al demonstrated a MPPTcircuit on flex-PCB with 87% efficiency at a power of100 W, to be integrated into flexible PV panels [194].Small and low-profile SMT packages (2.2 mm thick orless) were selected for all of the components tominimize bulk and maximize flexibility. The mostchallenging component to implement in such a lowprofile SMT package is usually the inductor, a criticalpart of many DC–DC converters. As a result, severalplanar spiral inductors for power electronics have beendeveloped, using the PCB copper layers for the wind-ings [194–198]. Several of these designs have incorpo-rated magnetic cores, either by screen printing[195, 197] or laminating [194, 196] layers of magneticmaterials onto the spirals.

As an alternative to flex-PCB, conductive materi-als can be printed directly onto plastic substrates toform the pads and interconnects. A number of techni-ques exist for attaching SMT components to printedpads at low temperature, including isotropically andanisotropically conductive tapes and metal-filledepoxies [199]. These techniques combined with thelow curing temperature of many printable conductorsenable the use of less expensive substrates with lower

Figure 12. Schematics of commonDC–DCconverters: (a) boost converter, (b) buck converter, (c) charge pump.

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melting points, such as PET and PEN, in place of thepolyimide that is typically used in flex-PCBs. Printingcan also be employed to fabricate circuits directly onthe back of a solarmodule [4]. Since printing processesare additive, they can potentially reduce process com-plexity and material waste compared to subtractiveprocesses such as the etching of flex-PCBs [200]. Fur-thermore, since many types of electronic materials canbe printed, including conductors, dielectrics, semi-conductors, and magnetic materials, there is interestin using printing to deposit the power electronic com-ponents themselves in addition to the interconnects[191]. Since many devices require the same materials,such as metal contacts, multiple components of a cir-cuit can be printed simultaneously, allowing the entirecircuit to be produced with a minimum number ofsteps by a single manufacturer. Replacing SMT com-ponents with printed thin-film components could alsoreduce weight and volume and improve the mechan-icalflexibility of the circuit.

Passive components—inductors, capacitors, andresistors—play important roles in DC–DC converters,

as seen in figure 12. Inductors and capacitors act asshort-term energy storage elements in buck and boostconverters and switched-capacitor converters, respec-tively, enabling the voltage conversion to take place.Capacitors additionally filter out voltage ripples inboth types of converters. Resistor networks are used tomeasure voltages and currents, providing feedback tothe control system in order to regulate the conversionratio. To date, printed and flexible passive compo-nents have most often been designed for applicationsin radio frequency (RF) data transmission or energyharvesting [7, 201–210]. However, in a few recentworks, printed and flexible passive components havebeen designed specifically for power electronics[211, 212]. For example, we have developed screen-printed flexible inductors, capacitors, and resistors, asshown infigure 13(a), focusing primarily on the designof the inductor fabrication process and geometry tominimize resistive losses [211]. An inductor and tworesistors were integrated into a boost converter with anIC to light organic LEDs at a constant voltage of 5 Vusing a flexible lithium-ion battery as the power

Figure 13.Printed andflexible passive components for power electronics. (a)Photograph of printed flexible inductor, capacitor, andresistor. (b)Photograph of hybrid boost converter powering organic LEDs from a lithium-ion battery. (c)Performance of boostconverter using printed inductor and resistors compared to all-SMT circuit. Reproduced from [211]. CCBY 4.0 International license.(d)Hybrid switched-capacitorDC–DCconverter consisting of twoflexible capacitors and an IC. (e)Performance of the converterwith flexible capacitors compared to SMT capacitors. © 2016 IEEE. Reprinted, with permission, from IEEETransactions on PowerElectronics 31, 2695–2708 (2016) [212].

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source, as shown in figure 13(b). The converter withthe printed components gave up to 90% of the effi-ciency of a converter with all SMT components(figure 13(c)). Van Tassell et al developed flexiblecapacitors based on a spray-coated high-k compositedielectric of barium titanate nanocrystals in an organicmatrix [212]. These capacitors outperformed off-the-shelf capacitors in terms of both power density andcost, and were integrated into a 15 W switched-capa-citor DC–DC converter with up to 90% efficiency. Aphotograph of the converter, consisting of two capaci-tors and an IC on a flexible substrate, is shown infigures 13(d), and (e) compares the performance of thecircuit with the flexible capacitors versus off-the-shelfceramic capacitors.

Diodes are used in PV systems as blocking diodes,bypass diodes, and components of DC–DC con-verters. As for the passive components, RF energy har-vesting applications have spurred the developmentof a number of flexible thin-film diodes [204–207, 213, 214]. Since the output voltage of a simple RFrectifying antenna is lower than that required by someapplications, voltagemultiplying charge pump circuitshave been integrated into some of the energy harvest-ers. For example, voltage tripling circuits havebeen developed based on printed polyaniline/ZnOSchottky diodes [204] and printed ZnO/Al Schottkydiodes [205]. 4x voltage multiplying charge pumpshave also been demonstrated, using diodes based onprinted poly(3-hexylthiophene) (P3HT) [214], a com-mon active layer material for OPVs, and poly(triar-ylamine) [206]. Although the focus of these works hasbeen on AC–DC converters, the same basic printeddiode structures are also of interest for DC–DC chargepumps [206]. Additionally, since solar cells themselvesbehave as diodes in the dark, Steim et al explored theuse of organic bypass diodes integrated into organicPV modules [215]. The diodes utilized exactly thesame materials and structure as the solar cells, butwere intentionally shaded. By characterizing the solarmodule power output with one cell shaded, theauthors found the organic bypass diodes to be just aseffective as conventional Si or GaAs diodes, but with agreatly simplifiedmanufacturing process.

While printed passive components [211, 212] andorganic bypass diodes [215] have been demonstratedwith similar performance to their conventional SMTcounterparts in low-power systems, the same has notbeen true for thin-film transistors (TFTs). Funda-mental limitations, such as the limited mobility of dis-ordered materials, as well as processing challenges,such as the difficulty of achieving short channellengths and thin gate dielectrics with high yield byprinting, limit the performance of TFTs. As a result,TFTs are generally characterized by high on-resistanceand limited operating frequency, and remain the lar-gest hurdle toward high-performing fully printed orthin-film power electronics. Nevertheless, severalworks have begun to explore the use of TFTs in power

electronics. For example, Ng et al used printed organicTFTs in a voltage multiplier to boost the voltage ofpulses to switch organic memory cells [216]. Pastorelliet al designed high-current printed P3HT TFTs forpowering an electrochromic display, using an organicsolar module as the power source [217]. The TFTscould support a current as high as 45 mA, but onlywith gate and source-drain voltages of 30 V each; whenthe source-drain voltage was reduced to 2 V, the cur-rent decreased correspondingly. Marien et al devel-oped several circuits, including a DC–DC converter,using organic TFTs based on pentacene on plastic foil[218, 219]. A switched-capacitor DC–DC convertertopology was chosen because inductor-based con-verters would require lower on-resistance for theTFTs, as well as a large amount of space for the induc-tor. The first version of the converter operated at a lowfrequency of 100 Hz and had a conversion efficiency ofonly 5% [218]. In the second design, the dimensions ofthe transistors and capacitors were modified, and thevoltage drop under load was reduced somewhat [219].Although the current carrying capability was still verylow (∼10 nA), this result highlights one of the benefitsof using TFTs for integrated power electronics: thetransistor structure can be re-designed during produc-tion to optimize performance or meet changingrequirements. Hong et al took advantage of the higherfield-effect mobility of indium gallium zinc oxide(IGZO) TFTs to develop a higher performing DC–DCconverter with efficiency up to 66.6% [220]. This con-verter was designed to be integrated into displays formobile devices as a replacement for the conventionalamorphous silicon TFTs, and was therefore fabricatedon glass rather than aflexible substrate.

Both IGZO and pentacene are deposited undervacuum, by evaporation or sputtering, and thereremains a lack of high performing printed TFTs sui-table for power electronics in PV systems. An addi-tional challenge for TFT-based power electronics is thedifficulty of achieving complementary circuits in com-patible processes: IGZO is inherently n-type, forexample, and p-type organic semiconductors tend tohave substantially higher mobility than n-type organicsemiconductors. Most circuits are thus designed usingonly p-type or only n-type transistors. Identifyinghigh-performance n-type organic TFT materials is anactive area of research, however, and Fuketa et alrecently developed a flexible voltage regulating circuitbased on complementary organic TFTs [31]. The cir-cuit was a shunt regulator designed to protect the sen-sitive organic sensing circuitry in a PV-poweredwearable healthcare device. By limiting the voltage ofthe PV module under high illuminance conditions,the addition of the voltage regulator increased theoperational illuminance range by a factor of 7.3.Although a battery was not used in this demonstra-tion, a similar organic shunt regulator could alsopotentially be used to protect a battery from over-charging.

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Finally, two recent reports have shown integratedflexible PV systems where a PV module, battery, andpower management electronics are all implementedusing thin-film technology [34, 221]. Meister et aldesigned a system consisting of a thin-film OPVmod-ule, a thin-film NiMH battery, and IGZO TFT-basedpower management electronics, in a layered structureas shown in figures 14(a), (b). Both a 6 V/14.4 mAhsystem and a 24 V/5.5 mAh system were demon-strated; the dimensions of eachwere selected to allow afull charge in 4 h under full sun. A diode-connectedTFT (gate and drain connected together)was used as ablocking diode. Since the PV open-circuit voltagedropped below the battery maximum voltage underlow light conditions, charge pump circuits weredesigned to boost the voltage and enable low-lightcharging. The switching signals for the charge pumpswere provided by IGZO TFT ring oscillators. The lowefficiency of 4% for the charge pump was justified bythe argument that even a low-efficiency charge pumpexpands the range of lighting conditions under whichthe system can operate: without it the voltage underlow light would be too low to charge the battery at all.

In the second integrated flexible PV system,designed specifically for indoor energy harvesting,Rieutort-Louis et al used amorphous silicon for boththe PV module and the power management electro-nics, and a lithium-ion battery pack [34]. The systemwas designed both to store energy locally for poweringintegrated ‘on-sheet’ loads and to transfer power wire-lessly to ‘off-sheet’ loads, as shown in figures 14(c), (d).Therefore, the circuitry included battery managementas well as an inverter and inductive interface for wire-less power transfer. For the blocking diode, a diode-connected TFT was compared against amorphous andnano-crystalline silicon diodes, and the amorphoussilicon diode was found to provide the best balance ofhigh on-current and low reverse leakage. The batterymanagement circuit was designed, by optimizing thecircuit topology as well as the geometries of the powertransistors, to disconnect the load when the batterypack voltage dropped to its minimum allowable valueof 10 V. A power of nearly 5 mW could be delivered toon-sheet loads, at an efficiency of 60%, and up to8 mW could be transferred wirelessly at an efficiencyof 21% for powering off-sheet loads.

Figure 14.Complete flexible PV energy harvesting and storage systems including thin-film powermanagement electronics.(a) Schematic diagram and (b) photograph of system based onOPVmodule, NiMHbattery, and IGZO electronics. ©2015 IEEE.Reprinted, with permission, fromEuropeanConference onCircuit Theory andDesign (ECCTD) (IEEE, Trondheim,Norway, 2015),1–4 [221]. (c) Schematic diagram and (d) photograph of systemusing amorphous silicon solarmodule and electronics with lithium-ion battery, including inductive link forwireless power transfer. ©2014 IEEE.Reprinted, with permission, from IEEE Journal ofPhotovoltaics 4, 432–439 (2014) [34].

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In light of the many challenges and opportunitiesof using thin-film components for power electronics,an important question facing the designer of a flexiblePV system is which components to implement in thin-film technology. Fabricating all of the components inthe same printing or coating process has the advan-tages of minimal process complexity and maximumdesign freedom for every component. Furthermore,avoiding SMT components entirely can potentiallyminimize bulk and maximize flexibility of the system.However, thin-film components generally have largerfootprints and lower performance (particularly in thecase of TFTs) relative to their SMT counterparts, lead-ing to serious tradeoffs. For example, if the operatingfrequency of a DC–DC converter is reduced in orderto increase TFT performance, then greater inductanceand/or capacitance is needed, requiring larger foot-prints and potentially increasing losses in the passives.Hybrid systems, consisting of some SMT and somethin-film components, are considered ideal for manyother flexible electronics applications. In medical sen-sing, for example, thin-film sensors are preferable tointerface with the flexible surfaces of the body, but sili-con is needed for data processing [9]. Similarly, thehybrid approachmay offer a good compromise for PVsystems, in which the large-area components (PVmodule and battery) are printed and flexible, whilehigh-performing silicon electronics are used for thepowermanagement.

The ideal power electronic circuit design dependson the application. For example, systems with lowercurrents and higher voltages can more easily toleratethe high resistances that characterize many thin-filmcomponents. On the other hand, if a solar module andbattery can be designed with good voltage matching,the overall energy collection efficiency may be higherwith no MPPT circuit than with a low-performingthin-film MPPT circuit. Once the decision has beenmade that an ICwill be used, it is advantageous to inte-grate as many components as possible into the IC, sothat few additional SMT components or printing stepsare required. Since inductors are often the most bulkycomponents in SMT form and most difficult to inte-grate into an IC, the ideal approach for a DC–DC con-verter may consist of SMT components plus a thin-film inductor, or a single-chip switched-capacitorsolution.

5. Conclusion

In summary, we have reviewed promising technolo-gies for printed and flexible photovoltaic modules,energy storage, and power management electronics,and assessed their readiness for incorporation intointegrated flexible photovoltaic energy systems. Wehave discussed approaches for physical integration ofthe components and the electrical considerationsrequired to design a high-performing system, and

conclude that the ideal system architecture is highlydependent on the illumination conditions and loadprofile of the particular application. Higher power PVsystems tend to have more stringent requirements interms of power efficiency, reliability and longevity,particularly in permanent outdoor installations. Therealization of fully integrated printed and flexiblesystems on a large scale will therefore require con-tinued research in several areas, including thedevelopment of battery technologies with good per-formance and stability at high temperatures andsubstantial improvement in performance of printedpower electronic components. Hybrid approachesconsisting of printed and flexible PV modules inte-grated with conventional power electronic compo-nents currently offer an appealing compromise. Onthe other hand, there have already been multipledemonstrations of integrated flexible PV energyharvesting and storage systems that can meet the lowpower demands of many portable and wearableelectronic devices and sensor nodes. Innovative inte-gration strategies such as shared electrodes for the PVand energy storage layers and incorporation of printedand flexible power electronic components can help toimprove the flexibility and simplicity of these powersystems and potentially reduce cost. The availability ofhigh-speed and customizable printing processes,numerous printable electronic materials, and techni-ques for integration of printed and conventionalcomponents can also enable application-specific sys-tem design including performance optimization andintegration of PV system components with loaddevices.

Acknowledgments

This work was supported in part by the NationalScience Foundation under Grant No. ECCS-1610899.AEO was supported by the NSF Graduate ResearchFellowship Program under Grant No. 1106400. Theauthors acknowledgeDrAbhinavGaikwad for provid-ing theflexible lithium-ion battery used in the examplesystem process flow and Dr Balthazar Lechêne forhelpful discussions.

References

[1] LewisN S 2007 Powering the planetMRSBull. 32 808–20[2] InganäsO andAdmassie S 2014 25th anniversary article:

organic photovoltaicmodules and biopolymersupercapacitors for supply of renewable electricity: aperspective fromAfricaAdv.Mater. 26 830–48

[3] ApostolouG andReinders AHME2014Overview of designissues in product-integrated photovoltaics Energy Technol. 2229–42

[4] Krebs FC,Nielsen TD, Fyenbo J,WadstrømMandPedersenMS 2010Manufacture, integration anddemonstration of polymer solar cells in a lamp for the‘Lighting Africa’ initiativeEnergy Environ. Sci. 3 512–25

[5] Darling S B andYou F 2013The case for organicphotovoltaicsRSCAdv. 3 17633–48

18

Flex. Print. Electron. 2 (2017) 013001 AEOstfeld andACArias

[6] Krebs FC, EspinosaN,HöselM, Søndergaard RR andJørgensenM2014 25th anniversary article: rise to power—OPV-based solar parksAdv.Mater. 26 29–39

[7] NilssonH-E,Unander T, Siden J, AnderssonH,Manuilskiy A,HummelgardMandGullikssonM2012System integration of electronic functions in smart packagingapplications IEEETrans. Compon. Packag.Manuf. Technol. 21723–34

[8] Misra V et al 2015 Flexible technologies for self-poweredwearable health and environmental sensing Proc. IEEE 103665–81

[9] KhanY,Ostfeld A E, LochnerCM, Pierre A andArias AC2016Monitoring of vital signswith flexible andwearablemedical devicesAdv.Mater. 28 4373–95

[10] Street RA,NgTN, SchwartzDE,WhitingG L, Lu J P,Bringans RD andVeres J 2015 Fromprinted transistors toprinted smart systemsProc. IEEE 103 607–18

[11] Krebs FC 2009 Fabrication and processing of polymer solarcells: a review of printing and coating techniques Sol. EnergyMater. Sol. Cells 93 394–412

[12] GaikwadAM,Arias AC and Steingart DA2015Recentprogress on printed flexible batteries:mechanical challenges,printing technologies, and future prospects Energy Technol. 3305–28

[13] WadiaC, Alivisatos A P andKammenDM2009Materialsavailability expands the opportunity for large-scalephotovoltaics deployment Environ. Sci. Technol. 43 2072–7

[14] Lizin S, Van Passel S, De Schepper E,MaesW, Lutsen L,Manca J andVanderzandeD 2013 Life cycle analyses oforganic photovoltaics: a review Energy Environ. Sci. 6 3136–49

[15] Trautz KM et al 2013Mobile solar power IEEE J. Photovolt. 3535–41

[16] Jelle B P, Breivik C andRøkenesHD2012 Building integratedphotovoltaic products: a state-of-the-art review and futureresearch opportunities Sol. EnergyMater. Sol. Cells 100 69–96

[17] Vicente A, ÁguasH,Mateus T, AraújoA, LyubchykA,Siitonen S, Fortunato E andMartins R 2015 Solar cells forself-sustainable intelligent packaging J.Mater. Chem.A 313226–36

[18] Fraunhofer 2015Photovoltaics Report Technical ReportFraunhofer Institute for Solar Energy Systems, ISE, Freiburg,Germany

[19] ShahA, Torres P, Tscharner R,WyrschN andKeppnerH1999 Photovoltaic technology: the case for thin-film solarcells Science 285 692–8

[20] Bhandari KP, Collier JM, EllingsonR J andApulD S 2015Energy payback time (EPBT) and energy return on energyinvested (EROI) of solar photovoltaic systems: a systematicreview andmeta-analysisRenew. Sustain. Energy Rev. 47133–41

[21] SöderströmT,Haug F-J, Terrazzoni-Daudrix V andBallif C2008Optimization of amorphous silicon thinfilm solar cellsfor flexible photovoltaics J. Appl. Phys. 103 114509

[22] Fernández S, Santos J,MunueraC,García-HernándezMandNaranjo F 2015 Effect of argon plasma-treated polyethyleneterepthalate onZnO:Al properties forflexible thinfilm siliconsolar cells applications Sol. EnergyMater. Sol. Cells 133 170–9

[23] WilkenK, PaetzoldUW,MeierM, PragerN, FahlandM,Finger F and SmirnovV 2015Nanoimprint texturing oftransparent flexible substrates for improved lightmanagement in thin-film solar cells Physica Status Solidi 9215–9

[24] StaeblerD L andWronski CR 1977Reversible conductivitychanges in discharge-produced amorphous SiAppl. Phys.Lett. 31 292

[25] ShahAV, SchadeH,VanecekM,Meier J, Vallat-Sauvain E,WyrschN,Kroll U, DrozC andBailat J 2004Thin-film siliconsolar cell technology Prog. Photovolt., Res. Appl. 12 113–42

[26] KimHS, Kang J S, Park J S,HahnHT, JungHC andJoung JW2009 Inkjet printed electronics formultifunctionalcomposite structureCompos. Sci. Technol. 69 1256–64

[27] Kim J-S, KoD, YooD-J, JungDS, YavuzCT, KimN-I,Choi I-S, Song J Y andChoi JW2015Ahalfmillimeter thick

coplanar flexible batterywithwireless recharging capabilityNano Lett. 15 2350–7

[28] Gambier P, Anton SR, KongN, Erturk A and InmanD J 2012Piezoelectric, solar and thermal energy harvesting for hybridlow-power generator systemswith thin-film batteriesMeas.Sci. Technol. 23 015101

[29] Carmo J P, Gomes JM,Gonçalves LMandCorreia JH 2013Aflexible thin-film for powering stand alone electronicdevicesMeasurement 46 4145–51

[30] TohWY, TanYK, KohWS and Siek L 2014Autonomouswearable sensor nodeswithflexible energy harvesting IEEESens. J. 14 2299–306

[31] FuketaH,HamamatsuM,Yokota T, YukitaW, Someya TT,Sekitani T, TakamiyaM, Someya TT and Sakurai T 2015Energy-autonomous fever alarm armband integrating fullyflexible solar cells, piezoelectric speaker, temperaturedetector, and 12 Vorganic complementary FET circuits 2015IEEE International Solid-State Circuits Conf.—(ISSCC)Digestof Technical Papers (CA: San Francisco: IEEE) pp 1–3

[32] Dionisi A,Marioli D, Sardini E and SerpelloniM2016Autonomouswearable system for vital signsmeasurementwith energy-harvestingmodule IEEE Trans. Instrum.Meas.65 1423–34

[33] Ostfeld AE,GaikwadAM,KhanY andArias AC 2016High-performance flexible energy storage and harvesting systemfor wearable electronics Sci. Rep. 6 26122

[34] Rieutort-LouisW,Huang L,HuY, Sanz-Robinson J,Wagner S, Sturm JC andVermaN2014A complete fullythin-film PVharvesting and power-management systemonplastic with on-sheet batterymanagement andwireless powerdelivery to off-sheet loads IEEE J. Photovolt. 4 432–9

[35] Kippelen B andBrédas J-L 2009Organic photovoltaics EnergyEnviron. Sci. 2 251–61

[36] KaltenbrunnerM,WhiteMS,Głowacki ED, Sekitani T,Someya T, Sariciftci N S andBauer S 2012Ultrathin andlightweight organic solar cells with high flexibilityNat.Commun. 3 770

[37] Jean J,WangA andBulovićV2016 In situ vapor-depositedparylene substrates for ultra-thin, lightweight organic solarcellsOrg. Electron. 31 120–6

[38] Krebs FC, BiancardoM,Winther-Jensen B, SpanggardH andAlstrup J 2006 Strategies for incorporation of polymerphotovoltaics into garments and textiles Sol. EnergyMater.Sol. Cells 90 1058–67

[39] Guo F et al 2013 ITO-free and fully solution-processedsemitransparent organic solar cells with high fill factorsAdv.EnergyMater. 3 1062–7

[40] Galagan Y, ShanmugamS, Teunissen J, Eggenhuisen T,Biezemans A, VanGijseghemT,GroenWandAndriessen R2014 Solution processing of back electrodes for organic solarcells with inverted architecture Sol. EnergyMater. Sol. Cells130 163–9

[41] van derWiel B, EgelhaafH-J, IssaH, RoosMandHenzeN2014Market readiness of organic photovoltaics for buildingintegrationMRSProc. 1639mrsf13-1639-y10-03

[42] YimJH, JoeS-Y,PangC, LeeKM, JeongH,Park J-Y,AhnYH,deMello JC andLeeS 2014Fully solution-processedsemitransparent organic solar cellswith a silver nanowirecathode and a conducting polymer anodeACSNano 8 2857–63

[43] Berny S et al 2015 Solar trees: first large-scale demonstrationof fully solution coated, semitransparent, flexible organicphotovoltaicmodulesAdv. Sci. 3 1500342

[44] ForberichK,Guo F, Bronnbauer C andBrabec C J 2015Efficiency limits and color of semitransparent organic solarcells for application in building-integrated photovoltaicsEnergy Technol. 3 1051–8

[45] GreenMA, EmeryK,HishikawaY,WartaWandDunlop ED2016 Solar cell efficiency tables (version 47)Prog. Photovolt.,Res. Appl. 24 3–11

[46] LungenschmiedC,DennlerG,NeugebauerH, Sariciftci SN,GlatthaarM,Meyer T andMeyer A 2007 Flexible, long-lived,large-area, organic solar cells Sol. EnergyMater. Sol. Cells 91379–84

19

Flex. Print. Electron. 2 (2017) 013001 AEOstfeld andACArias

[47] Minnaert B andVeelaert P 2010The appropriateness oforganic solar cells for indoor lighting conditions SPIEPhotonics Europe ed P LHeremans et al (Brussels, Belgium:International Society forOptics and Photonics) pp 77221P(http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=749480)

[48] Aernouts T, Vanlaeke P, GeensW, Poortmans J,Heremans P,Borghs S,Mertens R, Andriessen R and Leenders L 2004Printable anodes for flexible organic solar cellmodulesThinSolid Films 451-452 22–5

[49] DuPasquier A, UnalanHE, Kanwal A,Miller S andChhowallaM2005Conducting and transparent single-wallcarbon nanotube electrodes for polymer-fullerene solar cellsAppl. Phys. Lett. 87 203511

[50] Ostfeld AE,Catheline A, LigsayK, KimK-C, ChenZ,Facchetti A, Fogden S andArias AC 2014 Single-walledcarbon nanotube transparent conductive films fabricated byreductive dissolution and spray coating for organicphotovoltaicsAppl. Phys. Lett. 105 253301

[51] De S,Higgins TM, Lyons P E,Doherty EM,Nirmalraj PN,BlauW J, Boland J J andColeman JN2009 Silver nanowirenetworks asflexible, transparent, conducting films:Extremely highDC to optical conductivity ratiosACSNano 31767–74

[52] ZhouY et al 2012Auniversalmethod to produce low-workfunction electrodes for organic electronics Science 336 327–32

[53] ZhangH, StubhanT, Li N, TurbiezM,Matt G J, Ameri T andBrabec C J 2014A solution-processed bariumhydroxidemodified aluminumdoped zinc oxide layer for highlyefficient inverted organic solar cells J.Mater. Chem.A 218917–23

[54] GaynorW, Lee J-Y and Peumans P 2010 Fully solution-processed inverted polymer solar cells with laminatednanowire electrodesACSNano 4 30–4

[55] AngmoD et al 2014All-solution-processed, ambientmethodfor ITO-free, roll-coated tandempolymer solar cells usingsolution-processedmetalfilms Energy Technol. 2 651–9

[56] Eggenhuisen TM et al 2015High efficiency, fully inkjetprinted organic solar cells with freedomof design J.Mater.Chem.A 3 7255–62

[57] HeZ, ZhongC, Su S, XuM,WuHandCaoY 2012 Enhancedpower-conversion efficiency in polymer solar cells using aninverted device structureNat. Photon. 6 593–7

[58] You J et al 2013Apolymer tandem solar cell with 10.6%power conversion efficiencyNat. Commun. 4 1446

[59] Adams J et al 2014Air-processed organic tandem solar cellson glass: toward competitive operating lifetimes EnergyEnviron. Sci. 8 169–76

[60] Lechêne B, Perrier G, Emmanouil K, Kennou S,Bouthinon B and de Bettignies R 2014Design of intermediatelayers for solution-processed tandemorganic solar cells:guidelines from a case study onTiOx andZnO Sol. EnergyMater. Sol. Cells 120 709–15

[61] LiN, BaranD, SpyropoulosGD, ZhangH, Berny S,TurbiezM, Ameri T, Krebs FC andBrabec C J 2014Environmentally printing efficient organic tandem solar cellswith high fill factors: a guideline towards 20%powerconversion efficiencyAdv. EnergyMater. 4 1400084

[62] Spyropoulos GD,Kubis P, LiN, BaranD, Lucera L,SalvadorM, Ameri T, VoigtMM,Krebs FC andBrabec C J2014 Flexible organic tandem solarmodules with 6%efficiency: combining roll-to-roll compatible processingwithhigh geometric fill factors Energy Environ. Sci. 10 3284–90

[63] LiN andBrabecC J 2015Air-processed polymer tandemsolar cells with power conversion efficiency exceeding 10%Energy Environ. Sci. 8 2902–9

[64] Hong S et al 2016A series connection architecture for large-area organic photovoltaicmodules with a 7.5%moduleefficiencyNat. Commun. 7 10279

[65] ManceauM,AngmoD, JørgensenMandKrebs FC2011ITO-free flexible polymer solar cells: from smallmodeldevices to roll-to-roll processed largemodulesOrg. Electron.12 566–74

[66] AngmoD,Gevorgyan SA, Larsen-Olsen TT,Søndergaard RR,HöselM, JørgensenM,Gupta R,Kulkarni GU andKrebs FC 2013 Scalability and stability ofvery thin, roll-to-roll processed, large area, indium-tin-oxidefree polymer solar cellmodulesOrg. Electron. 14 984–94

[67] Sommer-Larsen P, JørgensenM, Søndergaard RR,HöselM andKrebs FC 2013 It is all in the pattern- high-efficiency power extraction frompolymer solar cells throughhigh-voltage serial connection Energy Technol. 1 15–9

[68] Andersen TR et al 2014 Scalable, ambient atmosphere roll-to-rollmanufacture of encapsulated large area,flexibleorganic tandem solar cellmodulesEnergy Environ. Sci. 72925–33

[69] Carlé J E,HelgesenM,MadsenMV, Bundgaard E andKrebs FC 2014Upscaling from single cells tomodules—fabrication of vacuum- and ITO-free polymer solar cells onflexible substrates with long lifetime J.Mater. Chem.C 21290–7

[70] Kubis P et al 2014High precision processing offlexibleP3HT/PCBMmodules with geometric fill factor over 95%Org. Electron. 15 2256–63

[71] Kapnopoulos C,Mekeridis ED, Tzounis L, Polyzoidis C,Zachariadis A, Tsimikli S, Gravalidis C, Laskarakis A,VouroutzisN and Logothetidis S 2016 Fully gravure printedorganic photovoltaicmodules: a straightforward process witha high potential for large scale production Sol. EnergyMater.Sol. Cells 144 724–31

[72] ZhouH,ChenQ, LiG, Luo S, Song T-B,DuanH-S,HongZ,You J, Liu Y andYang Y 2014 Interface engineering of highlyefficient perovskite solar cells Science 345 542–6

[73] JeonN J,Noh JH, YangWS, KimYC, Ryu S, Seo J andSeok S I 2015Compositional engineering of perovskitematerials for high-performance solar cellsNature 517476–80

[74] NieW et al 2015High-efficiency solution-processedperovskite solar cells withmillimeter-scale grains Science 347522–5

[75] UpadhyayaHM, Senthilarasu S,HsuM-HandKumarDK2013Recent progress and the status of dye-sensitised solarcell (DSSC) technologywith state-of-the-art conversionefficiencies Sol. EnergyMater. Sol. Cells 119 291–5

[76] JeonN J,Noh JH,KimYC, YangWS, Ryu S and Seok S I2014 Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cellsNat.Mater. 13 897–903

[77] MeiA et al 2014Ahole-conductor-free, fully printablemesoscopic perovskite solar cell with high stability Science345 295–8

[78] Fakharuddin A et al 2015Vertical TiO2 nanorods as amedium for stable and high-efficiency perovskite solarmodulesACSNano 9 8420–9

[79] Ball JM, LeeMM,HeyA and SnaithH J 2013 Low-temperature processedmeso-superstructured to thin-filmperovskite solar cells Energy Environ. Sci. 6 1739–43

[80] CarnieM J, CharbonneauC,DaviesML, Troughton J,Watson TM,Wojciechowski K, SnaithH andWorsleyDA2013A one-step low temperature processing route fororganolead halide perovskite solar cellsChem. Commun. 497893–5

[81] Wang J TW et al 2014 Low-temperature processed electroncollection layers of graphene/TiO2 nanocomposites in thinfilm perovskite solar cellsNano Lett. 14 724–30

[82] Wojciechowski K, SalibaM, Leijtens T, Abate A andSnaithH J 2014 Sub-150Cprocessedmeso-superstructuredperovskite solar cells with enhanced efficiency EnergyEnviron. Sci. 7 1142–7

[83] AbdollahiNB,Gharibzadeh S, Ahmadi V and ShahverdiHR2016New scalable cold-roll pressing for post-treatment ofperovskitemicrostructure in perovskite solar cells J. Phys.Chem.C 120 2520–8

[84] Docampo P, Ball JM,DarwichM, EperonGE and SnaithH J2013 Efficient organometal trihalide perovskite planar-heterojunction solar cells onflexible polymer substratesNat.Commun. 4 2761

20

Flex. Print. Electron. 2 (2017) 013001 AEOstfeld andACArias

[85] Roldán-CarmonaC,MalinkiewiczO, SorianoA,Mínguez EG,Garcia A, Reinecke P, Kroyer T,DarM I,NazeeruddinMKandBolinkH J 2014 Flexible highefficiency perovskite solar cells Energy Environ. Sci. 7 994–7

[86] You J et al 2014 Low-temperature solution-processedperovskite solar cells with high efficiency andflexibilityACSNano 8 1674–80

[87] LiuD andKelly T L 2013 Perovskite solar cells with a planarheterojunction structure prepared using room-temperaturesolution processing techniquesNat. Photon. 8 133–8

[88] Jung JW,Williams ST and JenAK-Y 2014 Low-temperatureprocessed high-performance flexible perovskite solar cells viarationally optimized solvent washing treatmentsRSCAdv. 462971–7

[89] KaltenbrunnerM et al 2015 Flexible high power-per-weightperovskite solar cells with chromiumoxide-metal contactsfor improved stability in airNat.Mater. 14 1032–9

[90] SunK, Li P, Xia Y, Chang J andOuyang J 2015Transparentconductive oxide-free perovskite solar cells with PEDOT:PSSas transparent electrodeACSAppl.Mater. Interfaces 715314–20

[91] KimB J et al 2015Highly efficient and bending durableperovskite solar cells: toward awearable power source EnergyEnviron. Sci. 8 916–21

[92] XuX et al 2015Workingmechanism forflexible perovskitesolar cells with simplified architectureNano Lett. 15 6514–20

[93] DengY, Peng E, ShaoY, Xiao Z,DongQ andHuang J 2015Scalable fabrication of efficient organolead trihalideperovskite solar cells with doctor-bladed active layersEnergyEnviron. Sci. 8 1544–50

[94] HwangK, Jung Y-S,HeoY-J, Scholes FH,Watkins S E,Subbiah J, JonesD J, KimD-Y andVakD 2015Toward largescale roll-to-roll production of fully printed perovskite solarcellsAdv.Mater. 27 1241–7

[95] Matteocci F, Razza S, DiGiacomo F, Casaluci S,Mincuzzi G,BrownTM,D’EpifanioA, Licoccia S andDi Carlo A 2014Solid-state solarmodules based onmesoscopic organometalhalide perovskite: a route towards the up-scaling processPhys. Chem. Chem. Phys. 16 3918–23

[96] Seo J, Park S, KimYC, JeonN J,Noh JH, Yoon SC andSeok S I 2014 Benefits of very thin PCBMand LiF layers forsolution-processed p–i–n perovskite solar cells EnergyEnviron. Sci. 7 2642–2626

[97] Li R, XiangX, TongX, Zou J and LiQ 2015Wearable double-twisted fibrous perovskite solar cellAdv.Mater. 27 3831–5

[98] ChenP-Y,Qi J, KlugMT,DangX,HammondPT andBelcher A 2014 Environmentally-responsible fabrication ofefficient perovskite solar cells from recycled car batteriesEnergy Environ. Sci. 7 3659–65

[99] Hao F, StoumposCC,CaoDH,Chang RPHandKanatzidisMG2014 Lead-free solid-state organic-inorganichalide perovskite solar cellsNat. Photon. 8 489–94

[100] KumarMH et al 2014 Lead-free halide perovskite solar cellswith high photocurrents realized through vacancymodulationAdv.Mater. 26 7122–7

[101] NoelNK et al 2014 Lead-free organic–inorganic tin halideperovskites for photovoltaic applications Energy Environ. Sci.7 3061–8

[102] Fakharuddin A, PalmaAL,DiGiacomo F, Casaluci S,Matteocci F,Wali Q, RaufM,DiCarlo A, BrownTMandJose R 2015 Solid state perovskite solarmodules by vacuum-vapor assisted sequential deposition onNd:YVO4 laserpatterned rutile TiO2nanorodsNanotechnology 26 494002

[103] Moon S-J, Yum J-H, Lofgren L,Walter A, Sansonnens L,BenkhairaM,Nicolay S, Bailat J and Ballif C 2015 Laser-scribing patterning for the production of organometallichalide perovskite solarmodules IEEE J. Photovolt. 5 1087–92

[104] QiuW et al 2016 Pinhole-free perovskite films for efficientsolarmodulesEnergy Environ. Sci. 9 484–9

[105] DiGiacomo F et al 2015 Flexible perovskite photovoltaicmodules and solar cells based on atomic layer depositedcompact layers andUV-irradiated TiO2 scaffolds on plasticsubstratesAdv. EnergyMater. 5 1401808

[106] GwonH,Hong J, KimH, SeoD-H, Jeon S andKangK2014Recent progress on flexible lithium rechargeable batteriesEnergy Environ. Sci. 7 538–51

[107] HuY and SunX 2014 Flexible rechargeable lithium ionbatteries: advances and challenges inmaterials and processtechnologies J.Mater. Chem.A 2 10712–38

[108] Li L,WuZ, Yuan S andZhangX-B 2014Advances andchallenges for flexible energy storage and conversion devicesand systemsEnergy Environ. Sci. 7 2101–22

[109] WangX, LuX, Liu B, ChenD, Tong Y and ShenG 2014Flexible energy-storage devices: design consideration andrecent progressAdv.Mater. 26 4763–82

[110] ZhouG, Li F andChengH-M2014 Progress inflexiblelithiumbatteries and future prospects Energy Environ. Sci. 71307–38

[111] VladA, SinghN,GalandeC andAjayan PM2015Designconsiderations for unconventional electrochemical energystorage architecturesAdv. EnergyMater. 5 1402115

[112] Scrosati B andGarche J 2010 Lithiumbatteries: status,prospects and future J. Power Sources 195 2419–30

[113] Diouf B and Pode R 2015 Potential of lithium-ion batteries inrenewable energyRenew. Energy 76 375–80

[114] HuL, Choi JW, Yang Y, Jeong S, LaMantia F, Cui L-F andCui Y 2009Highly conductive paper for energy-storagedevices Proc. Natl Acad. Sci. USA 106 21490–4

[115] HuL,WuH, LaMantia F, Yang Y andCui Y 2010Thin,flexible secondary Li-ion paper batteriesACSNano 4 5843–8

[116] KooM, ParkK-I, Lee SH, SuhM, JeonDY, Choi JW,KangK and LeeK J 2012 Bendable inorganic thin-film batteryfor fullyflexible electronic systemsNano Lett. 12 4810–6

[117] LiN, ChenZ, RenW, Li F andChengH-M2012 Flexiblegraphene-based lithium ion batteries with ultrafast chargeand discharge rates Proc. Natl Acad. Sci. USA 109 17360–5

[118] Liu B, Zhang J,WangX, ChenG, ChenD, ZhouC and ShenG2012Hierarchical three-dimensional ZnCo2O4 nanowirearrays/carbon cloth anodes for a novel class of high-performance flexible lithium-ion batteriesNano Lett. 123005–11

[119] Lee YH et al 2013Wearable textile battery rechargeable bysolar energyNano Lett. 13 5753–61

[120] Liu B,WangX, ChenH,WangZ, ChenD,Cheng Y-B,ZhouC and ShenG2013Hierarchical silicon nanowires-carbon textilesmatrix as a binder-free anode for high-performance advanced lithium-ion batteries Sci. Rep. 3 1622

[121] Choi K, Cho S, Chun S, Yoo J and LeeC 2014Heterolayered,one-dimensional nanobuilding blockmat batteriesNanoLett. 14 5677–86

[122] GaikwadAM,KhauBV,DaviesG,Hertzberg B,Steingart DA andArias AC2014Ahigh areal capacity flexiblelithium-ion battery with a strain-compliant designAdv.EnergyMater. 5 1401389

[123] WangX, Liu B,HouX,WangQ, LiW,ChenD and ShenG2014Ultralong-life and high-rate web-like Li4Ti5O12 anodefor high-performance flexible lithium-ion batteriesNanoRes.7 1073–82

[124] KimS-H,Choi K-H, Cho S-J, Choi S, Park S and Lee S Y 2015Printable solid-state lithium-ion batteries: a new routetoward shape-conformable power sources with aestheticversatility for flexible electronicsNano Lett. 15 5168–77

[125] Xu S et al 2013 Stretchable batteries with self-similarserpentine interconnects and integratedwireless rechargingsystemsNat. Commun. 4 1543

[126] SongZ et al 2015Kirigami-based stretchable lithium-ionbatteries Sci. Rep. 5 10988

[127] WengW, SunQ, Zhang Y,He S,WuQ,Deng J, FangX,GuanG, Ren J and PengH2015A gum-like lithium-ionbattery based on a novel arched structureAdv.Mater. 271363–9

[128] LiuW,ChenZ, ZhouG, SunY, LeeHR, LiuC, YaoH,BaoZ andCui Y 2016 3Dporous sponge-inspired electrodefor stretchable lithium-ion batteriesAdv.Mater. 28 3578–83

[129] KwonYH et al 2012Cable-typeflexible lithium ion batterybased on hollowmulti-helix electrodesAdv.Mater. 24 5192–7

21

Flex. Print. Electron. 2 (2017) 013001 AEOstfeld andACArias

[130] LinH,WengW,Ren J, Qiu L, Zhang Z, Chen P,ChenX,Deng J,WangY and PengH2014Twisted aligned carbonnanotube/silicon composite fiber anode forflexible wire-shaped lithium-ion batteryAdv.Mater. 26 1217–22

[131] Ren J, Zhang Y, BaiW,ChenX, Zhang Z, FangX,WengW,Wang Y andPengH 2014 Elastic andwearable wire-shapedlithium-ion battery with high electrochemical performanceAngew. Chem. 126 7998–8003

[132] WengW, SunQ, Zhang Y, LinH, Ren J, LuX,WangMandPengH2014Winding aligned carbon nanotube compositeyarns into coaxialfiber full batteries with high performancesNano Lett. 14 3432–8

[133] ZhangY,BaiW,ChengX,Ren J,WengW,ChenP, FangX,ZhangZ andPengH2014Flexible and stretchable lithium-ionbatteries and supercapacitors basedonelectrically conductingcarbonnanotubefiber springsAngew.Chem.12614792–6

[134] Kim J S, Lee Y-H, Lee I, KimT-S, RyouM-HandChoi JW2014 Large areamulti-stacked lithium-ion batteries forflexible and rollable applications J.Mater. Chem.A 2 10862–8

[135] MacKenzie J D andHoC2015 Perspectives on energy storagefor flexible electronic systemsProc. IEEE 103 535–53

[136] IngaleND,Gallaway JW,NyceM,Couzis A andBanerjee S2015Rechargeability and economic aspects of alkaline zinc–manganese dioxide cells for electrical storage and loadleveling J. Power Sources 276 7–18

[137] Berchmans S, Bandodkar A J, JiaW, Ramírez J,Meng Y S andWang J 2014An epidermal alkaline rechargeable Ag–Znprintable tattoo battery forwearable electronics J.Mater.Chem.A 2 15788–95

[138] Zamarayeva AM,GaikwadAM,Deckman I,WangM,KhauB, Steingart DA andArias AC 2016 Fabrication of ahigh-performance flexible silver–zincwire batteryAdv.Electron.Mater. 2 1500296

[139] GaikwadAM, Zamarayeva AM, Rousseau J, ChuH,Derin I and Steingart DA 2012Highly stretchable alkalinebatteries based on an embedded conductive fabricAdv.Mater. 24 5071–6

[140] GaikwadAM, Steingart DA,NgTN, SchwartzDE andWhitingG L 2013Aflexible high potential printed battery forpowering printed electronicsAppl. Phys. Lett. 102 233302

[141] WangZ andMitra S 2014Development offlexible secondaryalkaline battery with carbon nanotube enhanced electrodesJ. Power Sources 266 296–303

[142] KettlgruberG, KaltenbrunnerM, Siket CM,Moser R,Graz IM, Schwödiauer R andBauer S 2013 Intrinsicallystretchable and rechargeable batteries for self-poweredstretchable electronics J.Mater. Chem.A 1 5505–8

[143] HoCC, Evans JW andWright PK 2010Direct writedispenser printing of a zincmicrobattery with an ionic liquidgel electrolyte J.Micromech.Microeng. 20 104009

[144] Kan SY, VerwaalM andBroekhuizenH 2006The use ofbattery-capacitor combinations in photovoltaic poweredproducts J. Power Sources 162 971–4

[145] Dieffenderfer J P, Beppler E,Novak T,Whitmire E,Jayakumar R, Randall C,QuW,RajagopalanR andBozkurt A2014 Solar poweredwrist worn acquisition system forcontinuous photoplethysmogrammonitoring 2014 36thAnnual Int. Conf. IEEE Engineering inMedicine and BiologySociety (EMBC) (Chicago, IL: IEEE) pp 3142–5 (http://ieeexplore.ieee.org/articleDetails.jsp?arnumber=694428)

[146] Nelson J 2003The Physics of Solar Cells (London: ImperialCollege Press)

[147] Liu Z andWangXC2013Manufacture and performanceevaluation of solar garment J. Cleaner Prod. 42 96–102

[148] Tuukkanen S, VälimäkiM, Lehtimäki S, VuorinenT andLupoD2016 Behaviour of one-step spray-coated carbonnanotube supercapacitor in ambient light harvester circuitwith printed organic solar cell and electrochromic display Sci.Rep. 6 22967

[149] Lechêne BP, CowellM, Pierre A, Evans J,Wright PK andArias AC 2016Organic solar cells and fully printedsupercapacitors for indoor light energy harvestingNanoEnergy 26 631–40

[150] XuX, Li S, ZhangH, Shen Y, Zakeeruddin SM,GraetzelM,Cheng YB andWangM2015Apower pack based onorganometallic perovskite solar cell and supercapacitorACSNano 9 1782–7

[151] Krebs FC, Fyenbo J and JørgensenM2010 Productintegration of compact roll-to-roll processed polymer solarcellmodules:methods andmanufacture using flexographicprinting, slot-die coating and rotary screen printing J.Mater.Chem. 20 8994–9001

[152] Dennler G et al 2007A self-rechargeable and flexible polymersolar battery Sol. Energy 81 947–57

[153] GaoZ, Bumgardner C, SongN, Zhang Y, Li J and Li X 2016Cotton-textile-enabled flexible self-sustaining power packsvia roll-to-roll fabricationNat. Commun. 7 11586

[154] Krebs FC, Fyenbo J, TanenbaumDM,Gevorgyan SA,Andriessen R, vanRemoortere B, Galagan Y and JørgensenM2011TheOE-AOPVdemonstrator anno domini 2011EnergyEnviron. Sci. 4 4116–23

[155] Lee JW et al 2016 Soft, thin skin-mounted powermanagement systems and their use inwireless thermographyProc. Natl Acad. Sci. USA 113 6131–6

[156] Garcia-Valverde R, Villarejo J A,HöselM,MadsenMV,Søndergaard RR, JørgensenMandKrebs FC 2016 Scalablesingle point power extraction for compactmobile and stand-alone solar harvesting power sources based on fully printedorganic photovoltaicmodules and efficient high voltageDC/DCconversion Sol. EnergyMater. Sol. Cells 144 48–54

[157] ChenT, Yang Z and PengH 2013 Integrated devices to realizeenergy conversion and storage simultaneouslyChemPhysChem 14 1777–82

[158] SongT and SunB 2013Towards photo-rechargeable textilesintegrating power conversion and energy storage functions:Canwe kill two birds with one stone?ChemSusChem 6408–10

[159] SchmidtD,HagerMDand SchubertU S 2016 Photo-rechargeable electric energy storage systemsAdv. EnergyMater. 6 1500369

[160] SinghN,GalandeC,Miranda A,Mathkar A, GaoW,Reddy ALM,VladA andAjayan PM2012 Paintable batterySci. Rep. 2 481

[161] YeRB, YoshidaK,Ohta K andBabaM2013 Integrated thin-film rechargeable battery onα-Si thin-film solar cellAdv.Mater. Res. 788 685–8

[162] WeeG, SalimT, LamYM,Mhaisalkar SG and SrinivasanM2011 Printable photo-supercapacitor using single-walledcarbon nanotubes Energy Environ. Sci. 4 413–6

[163] NagaiH and SegawaH2004 Energy-storable dye-sensitizedsolar cell with a polypyrrole electrodeChem. Commun. 974–5

[164] Miyasaka T, IkedaN,Murakami TNandTeshimaK 2007Light energy conversion and storagewith soft carbonaceousmaterials that solidifymesoscopic electrochemical interfacesChemi. Lett. 36 480–7

[165] ChenH-W,HsuC-Y, Chen J-G, LeeK-M,WangC-C,HuangK-C andHoK-C 2010 Plastic dye-sensitized photo-supercapacitor using electrophoretic deposition andcompressionmethods J. Power Sources 195 6225–31

[166] Saito Y, Uchida S, KuboT and SegawaH2010 Surface-oxidized tungsten for energy-storable dye-sensitized solarcellsThin Solid Films 518 3033–6

[167] GuoW,XueX,WangS, LinCandWangZL2012An integratedpowerpackof dye-sensitized solar cell andLi battery basedondouble-sidedTiO2nanotube arraysNanoLett.12 2520–3

[168] Liu P, YangH, Ai X, Li G andGaoX 2012A solar rechargeablebattery based on polymeric charge storage electrodesElectrochem. Commun. 16 69–72

[169] YanNF, Li GR, PanGL andGaoXP 2012TiNnanotubearrays as electrocatalytic electrode for solar storablerechargeable battery J. Electrochem. Soc. 159A1770–4

[170] Skunik-NuckowskaM,Grzejszczyk K,Kulesza P J, Yang L,VlachopoulosN,Haggman L, Johansson E andHagfeldt A2013 Integration of solid-state dye-sensitized solar cell withmetal oxide charge storagematerial intophotoelectrochemical capacitor J. Power Sources 234 91–9

22

Flex. Print. Electron. 2 (2017) 013001 AEOstfeld andACArias

[171] YangZ, Li L, LuoY,HeR,Qiu L, LinH and PengH2013Anintegrated device for both photoelectric conversion andenergy storage based on free-standing and aligned carbonnanotube film J.Mater. Chem.A 1 954–8

[172] Xu J,WuH, Lu L, Leung S-F, ChenD,ChenX, Fan Z,ShenG andLiD 2014 Integrated photo-supercapacitor basedon bi-polar TiO2 nanotube arrayswith selective one-sideplasma-assisted hydrogenationAdv. Funct.Mater. 24 1840–6

[173] Shi C,DongH, ZhuR, LiH, SunY, XuD, ZhaoQ andYuD2015An ‘all-in-one’mesh-typed integrated energy unit forboth photoelectric conversion and energy storage in uniformelectrochemical systemNano Energy 13 670–8

[174] Bae J, Park Y J, LeeM,Cha SN,Choi Y J, Lee C S, Kim JMandWang Z L 2011 Single-fiber-based hybridization of energyconverters and storage units using graphene as electrodesAdv.Mater. 23 3446–9

[175] ChenT,Qiu L, Yang Z, Cai Z, Ren J, LiH, LinH, SunX andPengH2012An Integrated ‘energy wire’ for bothphotoelectric conversion and energy storageAngew. Chem.,Int. Ed. 51 11977–80

[176] FuY,WuH, Ye S, Cai X, YuX,Hou S, KafafyH andZouD2013 Integrated power fiber for energy conversion andstorageEnergy Environ. Sci. 6 805–12

[177] ChenX, SunH, Yang Z,GuanG, Zhang Z,Qiu L and PengH2014Anovel ‘energyfiber’ by coaxially integrating dye-sensitized solar cell and electrochemical capacitor J.Mater.Chem.A 2 1897–902

[178] Zhang Z, ChenX, Chen P,GuanG,Qiu L, LinH, Yang Z,BaiW, LuoY and PengH2014 Integrated polymer solar celland electrochemical supercapacitor in aflexible and stablefiber formatAdv.Mater. 26 466–70

[179] SunH, Jiang Y, Xie S, Zhang Y, Ren J, Ali A, Doo SG, Son IH,HuangX and PengH 2016 Integrating photovoltaicconversion and lithium ion storage into a flexible fiberJ.Mater. Chem.A 4 7601–5

[180] WangQ,ChenH,McFarland E andWang L 2015 Solarrechargeable batteries based on lead-organohalide electrolyteAdv. EnergyMater. 5 1501418

[181] LoCW, Li C and JiangH2011Direct solar energy conversionand storage through coupling between photoelectrochemicaland ferroelectric effectsAIPAdv. 1 042104

[182] ZhangX,HuangX, Li C and JiangH2013Dye-sensitizedsolar cell with energy storage function through PVDF/ZnOnanocomposite counter electrodeAdv.Mater. 25 4093–6

[183] Jensen J, DamHF, Reynolds J R, Dyer A L andKrebs FC2012Manufacture and demonstration of organic photovoltaic-powered electrochromic displays using roll coatingmethodsand printable electrolytes J. Polym. Sci.B 50 536–45

[184] Agbo SN,Merdzhanova T, Yu S, TempelH, KunglH,Eichel RA, RauU andAstakhovO2016Photoelectrochemical application of thin-film silicon triple-junction solar cell in batteries Physica Status Solidi a 2131926–31

[185] Stauth J T, SeemanMDandKesarwani K 2012A resonantswitched-capacitor IC and embedded system for sub-modulephotovoltaic powermanagement IEEE J. Solid-State Circuits47 3043–54

[186] Kopola P, TuomikoskiM, SuhonenR andMaaninenA 2009Gravure printed organic light emitting diodes for lightingapplicationsThin Solid Films 517 5757–62

[187] Irimia-VladuM et al 2010 Biocompatible and biodegradablematerials for organic field-effect transistorsAdv. Funct.Mater. 20 4069–76

[188] SmaalW et al 2012Complementary integrated circuits onplastic foil using inkjet printed n- and p-type organicsemiconductors: fabrication, characterization, and circuitanalysisOrg. Electron. 13 1686–92

[189] LochnerCM,KhanY, PierreA andAriasAC2014All-organicoptoelectronic sensor for pulse oximetryNat.Commun.5 5745

[190] UnoM et al 2015 Short-channel solution-processed organicsemiconductor transistors and their application in high-speed organic complementary circuits and organic rectifiersAdv. Electron.Mater. 1 1500178

[191] Ababei C, Yuvarajan S and SchulzD L 2010Towardintegrated PVpanels and power electronics using printingtechnologies Sol. Energy 84 1111–23

[192] Salas V, Olías E, Barrado A and Lázaro A 2006 Review of themaximumpower point tracking algorithms for stand-alonephotovoltaic systems Sol. EnergyMater. Sol. Cells 901555–78

[193] Nasiri A, Zabalawi S A andMandic G 2009 Indoor powerharvesting using photovoltaic cells for low power applicationsIEEETrans. Ind. Electron. 56 4502–9

[194] AcanskiM, Popovic-Gerber J and Ferreira B 2012Design of aflexible very low profile high step-up PVmodule integratedconverter 2012 IEEE Energy Conversion Congress andExposition (ECCE) (Raleigh, NC: IEEE) pp 2942–8

[195] Brandon E,Wesseling E, ChangV andKuhnW2003 Printedmicroinductors onflexible substrates for power applicationsIEEETrans. Compon. Packag. Technol. 26 517–23

[196] Waffenschmidt E, AckermannB andWilleM2005 Integratedultra thin flexible inductors for low power converters IEEE36th Conf. on Power Electronics Specialists, 2005 (Recife, Brazil:IEEE) pp 1528–34

[197] BangD and Park J 2009Ni–Zn ferrite screen printed powerinductors for compactDC–DCpower converter applicationsIEEETrans.Magn. 45 2762–5

[198] Kundrata J and Baric A 2012Design of a planar inductor forDC–DCconverter onflexible foil applications 2012 Proc. 35thInt. ConventionMIPRO (Opatija, Croatia: IEEE) pp 54–9

[199] AnderssonHA,Manuilskiy A,Haller S,HummelgårdM,Sidén J, HummelgårdC,OlinH andNilssonHE 2014Assembling surfacemounted components on ink-jetprinted double sided paper circuit boardNanotechnology 25094002

[200] SubramanianV et al 2008 Printed electronics for low-costelectronic systems: technology status and applicationdevelopment 38th European Solid-State Device Research Conf.ESSDERC 2008 (Edinburgh: IEEE) pp 17–24

[201] RedingerD,Molesa S, Yin S, Farschi R and SubramanianV2004An ink-jet-deposited passive component process forRFID IEEETrans. ElectronDevices 51 1978–83

[202] Leung S YY andLamDCC2007 Performance of printedpolymer-based RFID antenna on curvilinear surface IEEETrans. Electron. Packag.Manuf. 30 200–5

[203] LimN,Kim J, Lee S, KimNandChoG2009 Screen printedresonant tags for electronic article surveillance tags IEEETrans. Adv. Packag. 32 72–6

[204] JungM et al 2010All-printed and roll-to-roll-printable13.56 MHz-operated 1 bit RF tag on plastic foils IEEETrans.ElectronDevices 57 571–80

[205] ParkH,KangH, Lee Y, Park Y,Noh J andChoG2012 Fullyroll-to-roll gravure printed rectenna on plastic foils forwireless power transmission at 13.56MHzNanotechnology 23344006

[206] Heljo P, Lilja KE,MajumdarH S and LupoD2014Highrectifier output voltages with printed organic charge pumpcircuitOrg. Electron. 15 306–10

[207] Jung Y et al 2015 Fully printed flexible and disposable wirelesscyclic voltammetry tag Sci. Rep. 5 8105

[208] Hester J G et al 2015Additivelymanufacturednanotechnology and origami-enabled flexiblemicrowaveelectronics Proc. IEEE 103 583–606

[209] McKerricher G, Perez J G and ShamimA2015 Fully inkjetprinted RF inductors and capacitors using polymer dielectricand silver conductive inkwith through vias IEEE Trans.ElectronDevices 62 1002–9

[210] Voutilainen J-V,Happonen T,Hakkinen J and Fabritius T2015All silk-screen printed polymer-based remotely readabletemperature sensor IEEE Sens. J. 15 723–33

[211] Ostfeld AE,Deckman I, GaikwadAM, Lochner CMandArias AC 2015 Screen printed passive components forflexible power electronics Sci. Rep. 5 15959

[212] VanTassell B et al 2016Metacapacitors: printed thin film,flexible capacitors for power conversion applications IEEETrans. Power Electron. 31 2695–708

23

Flex. Print. Electron. 2 (2017) 013001 AEOstfeld andACArias

[213] MynyK, Steudel S, Vicca P, Genoe J andHeremans P 2008Anintegrated double half-wave organic Schottky diode rectifieron foil operating at 13.56MHzAppl. Phys. Lett. 93 093305

[214] Mutlu S,Haydaroglu I and SevimAO2011Realization ofpolymer charge pump circuits using polymer semiconductorsOrg. Electron. 12 312–21

[215] SteimR, Schilinsky P, Choulis S A andBrabec C J 2009Flexible polymer photovoltaicmodules with incorporatedorganic bypass diodes to addressmodule shading effects Sol.EnergyMater. Sol. Cells 93 1963–7

[216] NgTN, SchwartzDE,Mei P, Kor S, Veres J, Bröms P andKarlssonC 2016Pulsed voltagemultiplier based on printedorganic devices Flexible Print. Electron. 1 015002

[217] Pastorelli F, SchmidtTM,HöselM, SøndergaardRR,JørgensenMandKrebs FC2015Theorganic power transistor:roll-to-rollmanufacture, thermal behavior, andpowerhandlingwhendrivingprinted electronicsAdv. Eng.Mater.18 51–5

[218] MarienH, SteyaertM, vanVeenendaal E andHeremans P2011Organic dual DC–DCupconverter on foil for improvedcircuit reliability Electron. Lett. 47 278

[219] MarienH, SteyaertMS J, vanVeenendaal E andHeremans P2012Analog building blocks for organic smart sensor systemsin organic thin-film transistor technology onflexible plasticfoil IEEE J. Solid-State Circuits 47 1712–20

[220] Hong S-H, Yang I-S, Kang J-S, HwangT-H, KwonO-K,ByunC-W,CheongW-S,HwangC-S andChoK-I 2010DC–DCconverters using indium gallium zinc oxide thin filmtransistors formobile display applications Japan. J. Appl.Phys. 49 03CB05

[221] Meister T et al 2015 Bendable energy-harvestingmodulewith organic photovoltaic, rechargeable battery, and a-IGZOTFT charging electronics 2015 EuropeanConf. onCircuitTheory andDesign (ECCTD) (Trondheim,Norway: IEEE)pp 1–4

24

Flex. Print. Electron. 2 (2017) 013001 AEOstfeld andACArias