Optimization & Developmentof new Perovskite Precursors

for Device Fabrication

Michael StringerRegistration Number, 140148240

PhD Supervisor, Prof. David Lidzey.

The University of Sheffield,Department of Physics & Astronomy.

EPSRC Centre for Doctoral Training,‘New and Sustainable Photovoltaics’

Abstract

In recent years perovskite based solar cells have become a promising photovoltaic technology, most notablyfor their high power conversion efficiencies and potential for cheap solution-processable module production.

In this report perovskite devices are fabricated using the common perovskite precursor MAI:PbCl2:PbI2(4:1:1 stoichiometry) and using alternative new precursors: MAI:PbAc2 (3:1) and FAI:PbI2:MABr:PbBr2(5.67:5.67:1:1) in an attempt to optimize device efficiency. The FAI:PbI2:MABr:PbBr2 precursor holdsthe record for highest device efficiency in literature. This report includes data on the implementationof these precursor to produce MAPbI3−xClx, MAPbAc3, (FAPbI3)0.85(MAPbBr3)0.15 perovskite filmsin p-i-n device architectures, with PEDOT:PSS as a electron transport medium (HTM), PCBM as ahole transport medium (ETM) in a ETM/Perovskite/HTM photovoltaic device, a structure that is lesscommon in literature.

Improved fabrication routine procedure and device optimization experiments lead to a championefficiency of 12.1% incorporating a MAPbI3−xClx active layer and 8.1% using a MAPbAc3 active layer.

1st Year Report

October 2015

Contents

1 Introduction 11.1 Perovskite Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Research Aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Theory 12.1 Perovskite Crystal Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 A Perovskite Active Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.3 PSSC Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3 Experimental Methods 43.1 Fabrication Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.2 Analysis Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 Experimental Results 84.1 Reference Devices - I201

MAI:PbCl2:PbI2 (4:1:1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.1.1 Precursor Additives

HI & HPA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.1.2 Swabbing the ITO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.1.3 Extending the Cathode

& Changing the Cathode (0 & 5 nm Ca) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.1.4 Changing PC70BM thickness

& Changing the Cathode (0 & 5 nm Ca) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.2 Alternative Perovskite Recipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.2.1 MAI:PbAc2 (3:1) Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2.2 FAI:PbI2:MABr:PbBr2 (5.67:5.67:1:1)

I401 Devices & Solvent Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Conclusion 13

6 Short-Term Outlook 14

7 Doctoral Development Program(DDP) Requirements 14

1 Introduction

1.1 Perovskite Background

The photovoltaic community has made many significant de-velopments in the last few years, the most outstanding re-search topic being the fabrication and development of thefirst perovskite sensitized solar cells (PSSCs). In 2006 adye sensitized solar cell (DSSC) adapted with perovskitewithin its nanoporous titanium oxide (titania or TiO2) layerachieved a power conversion efficiency (PCE) of 2.2%.[1,2].In 2006 the perovskite used was an organometal halideCH3NH3PbBr3, a crystal structure containing methylam-monium (CH3NH3), lead (Pb) and bromide (Br). The mostprominent perovskite active areas in literature use iodine(I) as the halide. There have been many reiterations andattempts to change the perovskite formula with the goal toimprove efficiency, lengthen stability and search for a metalless toxic than Pb, with no carcinogenic manufacturing bi-product.[3,4]

The reason why the scientific community is so excited bythe advent of the PSSC is best explained by observing therapid increase in reported PCEs. By the end of 2014 a de-vice with an active area made from a formamidinium (FA)and methylammonium (MA) perovskite blend (FAPbI3)0.85(MAPbBr3)0.15 achieved a 20.3% PCE.

Perovskites have had the advantage of immediately inte-grating into the 20 years of engineering already completedon their predecessor DSSC. PSSCs utilize the same archi-tecture as DSSCs and lean on the same chemical and elec-tronic understanding of the supporting electron and holetransport mediums (ETMs & HTMs), the electrode con-tacts, the transparent conducting oxides (TCOs) and all ofthe interface physics involved with planar and mesoporousstructures between each layer.[4]

1.2 Research Aims

This report focusses on the optimization of recipes used tofabricate thin film perovskite planar solar cells. The maingoal of optimization of these devices is to increase the PCEof planar reverse structure p-i-n devices, which are madefrom a series of thin films deposited on patterned IndiumTin Oxide (ITO) substrates, the films (with their thick-ness’s) in order of deposition are: poly(3,4-ethylenedioxyth-iophene) (PEDOT:PSS, 30-40nm), perovskite(300-400nm),[6,6]-Phenyl-C71-butyric acid methyl ester(PC70BM, 40-200nm), Calcium (Ca, 0-5nm) and Aluminium (Al, 100nm)cathodes. Optimization is achieved by investigating the finedetails of device fabrication, probing the parameter space ofspin cast annealed thin films, and by fundamentally chang-ing the materials used to produce the precursor.

Figure 1: The crystal structure of a perovskite in a cubic orpseudocubic -phase (α). The positions and common substancesfor the organic cations A, metal cations B and halide anions X inthe ABX3 formula are indicated. Whilst in the cubic phase, theperovskites unit cell may be taken as a repetition of A organiccations (as shown) or as a unit cell of B metal cations. Thehalides form octahedra which may tilt depending on the size ofthe surrounding and enclosed cations.

Current density-voltage (J-V) data provides results thatshow the removal of Ca from the cathode and a simplifica-tion in architecture can be done without seriously impact-ing the device PCE and that the thinning of the PC70BMcan enhance the PCEs of perosvkite devices. Several pre-cursors are optimized to achieve 12.1%, 8.1% and 4.3% PCEdevices with MAPbI3−xClx, MAPbAc3 and(FAPbI3)0.85(MAPbBr3)0.15 active layers respectively.

2 Theory

2.1 Perovskite Crystal Structure

The perovskite chemical structure is of the form ABX3,[1,3–5]

consisting of A and B Cations with X Anions forming acrystal structure demonstrated in figure 1. In the organic-inorganic organometal halide hybrid perovskites implementedin typical perovskite solar cells: A is typically the large or-ganic cations MA (CH3NH3) or FA (HC(NH2)2), B hasremained as Pb for all high PCE devices but a few researchgroups are looking at the replacement tin (Sn), and thehalides iodine (I), chloride (Cl) and bromide (Br) are allimplemented individually or as halide blend perovskites.(APbX3 (A= MA, FA)(X = I, Br, Cl)). Successful per-ovskite blends include MAPbI3−xClx,[6,7] MAPb(I1−xBrx)3,[8]

FAPbIyBr3−y,[9] FAPbI3−xClx[10] and a recent blend with

record breaking PCE, (FAPbI3)0.85(MAPbBr3)0.15.[11]

1

Figure 2: A p-n junction band diagram, showing how incidentphotons cause the charge generation of electron and holes andhow the built-in field separates charges.

2.2 A Perovskite Active Area

The functionality of a typical silicon solar cell can be rep-resented as a p-n junction, where-by silicon doped with ahigher hole concentration (p-type) is brought into contactwith silicon doped with a higher electron concentration (n-type). The resulting contact causes a band bending be-tween the two materials as their relative Fermi levels align.Figure 2 demonstrates the band banding and shows the di-rection of the induced built-in field caused by charges build-ing up on both sides of the junction, under equilibrium thediffusion of charges is balanced by the current caused bythe built-in field causing a region of charge depletion. Thep-n junction can act as a photovoltaic device as incidentphotons generate electron-hole pairs which are swept outof the p-n junction before they can recombine.

A typical perovskite solar cell device can be described as be-ing a heterostructure with a central active area which actsas an intrinsic semiconductor. This active layer is capa-ble of producing electron-hole pairs under illumination butdoes not preferentially spit the charge and without help thecharges would recombine. Unlike silicon the perovskite isnot doped in order to achieve a built-in voltage, instead theactive area needs heterojunctions with surrounding electronand hole transport layers which have band structures thatcause selective extraction of charges when in contact withthe band structure of the perovskite active area. The wholeheterostructure is built up of a series of materials with bandgaps which create an energy landscape to selectively splitselectrons and holes towards a cathode and anode, figure 5demonstrates a typical perovskite p-i-n energy landscape.

To summarise, photons above the energy of the band gap(which is dependant on the choice of chemical compositionand stoichiometry of the perovskite) cause the generation ofan electron and hole pair which are separated by the electricaffinity generated by the appropriate choice of charge trans-port layers. Alternatively the diffusion lengths for electronand holes within the perovskite are so large it may be bet-ter to consider the transport layers as blocking layers whichsimply do not allow the diffusion of one type of carrier.

2.3 PSSC Architecture

The success of the perovskite as a semi-conducting activearea, capable of achieving high PCE, is due to the remark-able properties of the perovskite. The perovskites largeabsorption coefficients allow many photons to be collectedwith the perovskite layer being only a few 100nm thick.A high charge mobility of 8cm2(Vs)−1 for MAPbI3 and11.6cm2(Vs)−1 for MAPbI3−xClx and a low bi-molecularcharge recombination encourage a high fill factor (FF).[12]

Short Circuit Current Density (JSC) is high as charge gen-eration occurs on the order of picoseconds whilst recom-bination is on the order of microseconds.[13] A low non-radiative recombination rate results in a small difference of∼450meV between Open Circuit Voltage (VOC) and Eg/q,better than many PV semiconductors.[13,14]

The perovskite was originally implemented in a photovoltaicactive area as a hybrid of a standard DSSC in 2006;[2]

achieving 2.2% PCE. Figure 3 is the schematic for a typi-cal DSSC. The photoactive dye generates free charges fromincoming photons, the electrons transfer into the opticallytransparent TiO2 mesoporous scaffold and the holes passthrough the liquid electrolyte to the electrode and counterelectrode respectively. The perovskite directly replaced thedye to produce the first ever PSSC. In 2009 the same groupreplaced the bromide with iodide in a similar architectureand reached 3.8% PCE[15] and by 2011 another group hadreached 6.2% PCE by sensitizing the TiO2 with perovskitequantum dots (QDs).[16] It was at this time that perovskiteswere observed to be easily dissolved in the liquid electrolyte,causing damaging losses to the PCE.[5]

In 2012 the move away from PSSCs using the DSSC liq-uid redox electrolyte occurred with two major pieces ofpublished work, completed by H.Snaith’s and M.Gratzel’sgroups. They produced device with PCEs 10.9% and 9.7%respectively.[18,19] The liquid electrolyte was replaced by theHTM spiro-MeOTAD, doped with bis(trifluoromethane) sul-fonimide lithium (Li-TFSI) and 4-tert-butylpyridine (TBP).The device structure implemented is similar to schematic(a) in figure 4.

2

Figure 3: From Nature,[17] a Dye-Sensitized Solar Cell(DDSC). The well-established DDSC configuration, which wasadapted by replacing the dye with halide perovskite to make thefirst ever perovskite sensitized solar cell (PSSC).

Figure 4: From Nature Materials,[3] the evolution of the per-ovskite solar cell away from DSSCs. (a) The liquid electrolytereplaced by a hole transport medium (HTM), the perovskitecoats the titanium dioxide (Ti02) scaffold or an alternative alu-minium Oxide (Al2O3 scaffold). This all remains upon a trans-parent conductive oxide layer (TCO) and Compact TiO2 layer.(b) The perovskite is a layer above the scaffold material and theHTM is a separate layer above this, due to the highly conduc-tive nature of the perovskite this structure has been proven tofunction effectively, even without the HTM. (c) Thin film planarstructure with HTM above and no mesoporous scaffold.

There has been a lot of attention focused on the use ofa scaffold of TiO2 as the ETM. In 2012 a Al2O3 scaffoldwas implemented to replace TiO2 and it was believed thatopen circuit voltage (VOC) was boosted by this change, pro-ducing a 10.9% device.[5,14,18–20] In addition, several articlessince have outed TiO2 as having a bad interface with typicalperovskite materials.[21,22] A study focusing on the origin ofJ-V sweep hysteresis (a common problem with perovskitedevice testing) compared several combinations of perovskiteinterlayers and came to the conclusion the interface betweenthe scaffold TiO2 and the perovskite could be a signifi-cant contributor to hysteresis.[22] Several studies on stabil-ity have also led to conclusion that the TiO2/perovskite in-terface is ultraviolet (UV) unstable as it is a photo-catalystfor oxidizing materials. Iodine may be extracted by the pro-cess described in equation 1. The solutions offered by oneof these studies recommends the replacement of Al2O3 orthe introduction of an antimony sulfide (Sb2S3) interlayerbetween the perovskite and the TiO2 layer.[13]

2I− ↔ I2 + 2e−

3CH3N+3 ↔ 3CH3NH2 + 3H+

I− + I2 + 3H+2e− ↔ 3HI [13] (1)

The next step for perovskite hybrids was the analysis ofMAPbI3−xClx which was found to have a electron and holediffusion length Ld,e of well over one micron, over 10 timesthat of MAPbI3 with Ld/e∼100nm. This allows charge tobe extracted over thicker perovskite films. In addition ex-perimental evidence revealed the perovskite itself could pro-duce working devices as a thicker perovskite active area, notjust as a sensitizing substance[18,19,23] (See Figure 4 (b)).It was also found that the perovskite later didn’t have tobe fully intercalated with an ETM scaffold, in fact, devicescould function without an ETM scaffold at all[24](See Figure4 (c)). In 2013 a planar compact titania c-TiO2/MAPbI3−x

Clx/Spiro-OMeTAD device reached a PCE above 15%. Withlonger diffusion lengths these perovskite films no longerneeded the scaffold to sufficiently extract charges into theHTM.[13] The variation in charge diffusion lengths that oc-curs with different compositions of perovskite allows flexi-bility in the device architecture, and opened several avenuesof research for the ever growing perovskite community.[14]

Device architectures for perovskites from 2013 onwards typ-ically include the mesoporous structure already shown infigure 4 (b) or the planar structure in figure 4 (c). Devicesfocused on this report do not have the HTM above the per-ovskite (n-i-p devices), and have been fabricated with al-ternative HTMs below the perovskite. Choices of materialfor ETMs and HTMs are limited by use of appropriate elec-tron affinity to create an ideal energy landscape for strongcharge extraction and also by solvent matching in the fab-rication process.[25]

3

Figure 5: From Scientific Reports,[30] planar device schematicand energy level diagram for a reverse structure devicewith HTM PEDOT:PSS (poly(3,4-ethylenedioxythiophene)polystyrene sulfonate) below the MAPbI3 perovskite and ETMPC60/70BM (Phenyl-C61/C71-butyric acid methyl ester).

Figure 5 shows a device with ETM Phenyl-C61(or C71)-butyric acid methyl ester (PCBM) layer above the per-ovskite, alongside the energy levels associated with eachlayer. In this case PCBM could not have been fabricatedbelow the perovskite as the typical solvent used for deposit-ing the perovskite is dimethylformamide (DMF), which alsodissolves PCBM.

The band gap, energy landscape and morphology of everylayer involved varies between different choices of: device ar-chitectures, perovskite formulas, transport layers and inter-layers. Optimization by fabrication or computational mod-els help refine ideal formulas, such as the optimum thick-ness’s of each layer for charge generation and extraction.Every interface within the perovskite is important, chargeextraction processes and degradation effects are often foundto depend heavily on the perovskite/transport layer inter-faces. Each material and fabrication technique used for alayer will effect the electronic structure and morphologyof that layer, which in turn also effects the structure andmorphology of the layer deposited above it.[26–29]

3 Experimental Methods

3.1 Fabrication Techniques

The first step in any perovskite development or optimiza-tion experiment is to make the perovskite precursor. Eachchemical may be significantly different depending on thesupplier, the batch or the purity of that product and socare should taken in the first step of actually selecting thesupplier and product used to produce the perovskite. Thereis not much published literature on this, although there hasbeen a slight shift towards more recent publications givingthe CAS number and supplier of the chemicals they haveused. A decent example is the solubility of PbI2 which isradically different depending on its purity, for example athicker perovskite film can be obtained from a 99% purePbI2 ink derivative as the solvents can hold high concen-tration of PbI2.

Given that many of the chemicals that make up the precur-sor are hygroscopic, it is common practise to avoid atmo-spheric conditions and use only anhydrous solvents whenstoring and weighing out chemicals for perovskite precur-sors. Whilst the degradation pathways of the perovskiteinclude moisture and oxygen there is very little publishedinformation on the effect of water present in the precursor.Theoretically storing chemicals or pre-made precursors inthe dark in a nitrogen atmosphere should preserve theirquality, however the best possible way to reduce variabil-ity in device data during the precursor making stage is toalways make the perovskite ink fresh and to heat it for asshort as time as possible. Adding a magnetic stir bar orsonicating the ink may help agitate the chemicals and helpthem go into solution quicker with less heat.

The materials refereed to and used to collect data for thisreport include: [6,6]-Phenyl-C71-butyric acid methyl esteror C70 PCBM or PC70BM a electron transport medium(ETM) shown in figure 6, poly(3,4-ethylenedioxythiop -hene) or PEDOT:PSS a hole transport medium (HTM)shown in figure 7, Calcium or Ca an injection cathode, Alu-minium or Al a cathode, methylammonium iodide or MAI,formamidinium iodide or FAI, methylammonium bromideor MABr, lead iodide (99% purity) or PbI2, lead bromide orPbBr2, lead chloride or PbBr2 and lead acetate tri-hydrateor PbAc2. Glass ITO patterned substrates are cleaned us-ing hellmanex in boiling deionized (DI) water, isopropanol(IPA) and a ultra-violet (UV) plasma cleaner. Other chem-icals include hydriodic acid (HI) 57% wt in water with 0.3%hypophosphorous acid (HPA) and HPA 50 wt % in water.

The solvents used to collect data for this report include:dimethylformamide or (CH3)2NCH or DMF, dimethyl sul-foxide or (CH3)2SO or DMSO, gamma-Butyro- lactone orC4H6O2 or GBL & chlorobenzene or C6H5Cl or CB.

The MA/FA halides are mixed with Pb halide materialsat different stiochiometrys in various solvents to producethe perovskite precursor. The precursors dealt with inthis report include: MAI:PbCl2:PbI2 (4:1:1 stoichiometry)in DMF 40% wt, MAI:PbAc2 (3:1) in DMF 40% wt andFAI:PbI2:MABr: PbBr2 (5.67:5.67:1:1) in GBL:DMSO (7:3volume ratio). These are be refereed to as i201, PbAc2 andi401 precursors respectively.

PEDOT:PSS is typically 1.3 to 1.7 wt% in water. PC70BMis dissolved in CB, the concentration of which is a param-eter studied in this report, for reference i201 it is typicallymade to be 50mg/ml.

The primary fabrication technique for thin film depositionused for data in this first year report is dynamic or staticsingle step spin-coating. Equation 2 is the relationship be-tween spin speed and film thickness.

4

Figure 6: From OSSILA,[31] Molecular structure of [6,6]-Phenyl-C71-butyric acid methyl ester or C70 PCBM orPC70BM.

Figure 7: From OSSILA,[31] Molecular structure of poly(3,4-ethylenedioxythiop -hene) or PEDOT:PSS.

Figure 8: From OSSILA,[31] Steps 1-4 demonstrate how a so-lute (black) in a solvent (red) may form a film through spincoating the solution as the solvent evaporates.

Figure 8 demonstrates the spin coating process as it de-posits a thin film of solute.

t ∝ 1√w

[31] (2)

For each precursor there is an optimum choice of parame-ters for spin coating, the parameter space in includes: theviscosity and concentration of the precursor, static or dy-namic application of the precursor, spin speed, accelerationand spinning time. Optimizing the spin coating parametersshould improve film uniformity, decrease film roughness andremove spin defects and is a simple way to improve thequality of films and hence boost device performance. Thecontact layers Ca and Al are thermally evaporated underhigh vacuum and a UV activated epoxy are used as en-capsulation. The necessity for good encapsulation is alsodemonstrated within the data obtained for this report.

It is worth noting that a visibly ’good perovskite film’ is notnecessarily indicative of good device performance and othertechniques are needed to qualitatively or quantitatively de-termine the quality of the device. Figure 9 is a completebreak down of the potential fabrication of perovskite de-vices. There are several deposition routes, each with itsown large parameter space, many of which have been cov-ered in detail in the literature report section of this report.The typical structure of all p-i-n perovskite devices fabri-cated for this report is shown in figure 10. One key featureof this device architecture is the ITO patterning which al-lows an easy connection for device testing but requires theswabbing of the films to remove them from the ITO areasof the film, the film should be swabbed up to end of theITO fingers so to reduce the chance of charges shorting be-tween the cathode and anode without going through all ofthe other layers. Alternatively the devices can be scribedbetween the ITO fingers and perovskite before the contactsare thermally evaporated. Some of the experimental datain this report looks the replacement of normal short con-tacts (which shall be refereed to as contact type A) as seenin figure 10 and longer contacts which go all the way to theedge of substrate (contact type B). These different contactsare be made with different size thermal evaporations masksA and B.

Notably, most research groups work on standard n-i-p archi-tecture devices not the p-i-n architecture which the EPMMgroup at Sheffield currently focus on. The other distinct dif-ferences in this structure from those common in literatureare the 5nm calcium cathode injection layer and the UV ac-tivated encapsulation, which were both originally optimizedfor organic devices. This report does contain experimentaldata on removing the calcium from the cathode howeverthe effect of the epoxy and UV curing on the perovskitehas not been thoroughly investigated.

5

Figure 9: Flow diagram showing potential fabrication options and parameter space for a perovskite device run.

6

Figure 10: Typical p-i-n reverse perovskite device architectureas made by the EPMM group in Sheffield.

3.2 Analysis Techniques

Thin film characteristics can be used to understand andoptimize the perovskite without having to produce a fulldevice. Typically the thickness, roughness and quality ofthe film are studied using surface profiling equipment suchas a Dektak or Atomic Force Microscope (AFM), an opticalmicroscope, steady state PL and absorption measurements.

The thickness, roughness and surface coverage of the per-ovskite layer are important as the PCE of a device im-proves with greater surface coverage, and there is an op-timum thickness and roughness of the perovskite for im-proved charge extraction into thinner electon and hole block-ing layers. When investigating new perovskite blends, theabsorption spectrum and PL allow the determination ofthat blends band gap and Urbach energetic disorder froma Tauc plot of (αE)0.5 vs E, where α is the absorption co-efficient calculated from absorption spectroscopy and E ishν, the energy of photons illuminating the perovskite film.

This report will not include many absorption and PL mea-surements, but will focus on the J-V data of perovskitedevices. To build a more complete picture of the chemicaland morphological structure of each new perovskite blend,Scanning Electron Microscopy (SEM), Time-Resolved Pho-toluminescence (TRPL) and Raman spectroscopy could beused in addition to PL and absorption data.

Figure 11: From The Physics of Solar Cells by Jenny Nel-son,[32] An ideal photo-voltaic J-V curve, that identifies the keypoints of interest, the maximum power point’s current Jm andvoltage Vm, Jsc and Voc. From these values a FF and PCE ofthe device can be determined.

Another alternative route to device optimization would bethe use of a transfer matrix, to model the perovskite as acavity and use published n-k data to predict the optimumthickness of the perovskite and surrounding charge trans-port layers.

J-V curves are a powerful tool for determining the per-formance of a completed device and can be used to identifythe limitations that might be reducing the performance ofa device. Figure 11 shows the behavior of a solar cell underillumination when under various applied bias and identifiessome key features of the J-V curve. The maximum powerpoint’s current Jm and voltage Vm along side with Jsc andVoc can be used in equation 3 to determine the fill factor(FF) and hence the efficiency of the device.

FF =JmVmJscVoc

, η =JscVocFF

Ps

[32] (3)

The equivalent circuit in figure 12 indicates the true be-havior of a real device which may contain large series re-sistances Rs and shorts between layers of the device thatreduce the shunt resistance Rsh. Even a well fabricated de-vice may have thick transport layers which cannot transportcharge efficiently and incomplete surface coverage whichwould lead to shorts, this would lead to a high series re-sistance RS and low shunt resistance respectively. A J-Vcurve shows if either of these parasitic effects are occurringas indicated in figure 12 which gives an example of a devicewith high Rs and one with low Rsh.A complete picture of a photo-voltaic device can be ex-pressed as equation 4 which takes into account a non-idealityfactor m, the area of the device A, and temperature T.

J = JSC − JO(eq(V+JARs)/mkbT − 1)− V + JARs

Rsh

[32] (4)

7

Figure 12: From The Physics of Solar Cells by Jenny Nel-son,[32] An equivalent circuit of a photo-voltaic with graphsshowing the effect of Rs and Rsh on J-V curves.

In addition to identifying the parasitic resistances, the J-Vsweep Voc is an indication of how well your choice of ex-traction layers and contacts is generating a energy affinitylandscape and the Jsc is a good indicator of the quality ofthe perovskite film.

If Voc is low then the device structure may need alternativetransport layers with different electron and hole affinities ormore inter-layers, if Jsc is low then the perovskite may needre-engineering or the transport layers may be too thick.

4 Experimental Results

4.1 Reference Devices - I201MAI:PbCl2:PbI2 (4:1:1)

In this section of results the reference i201 recipe that pro-duces a MAPbI3−xClx is optimized via several experiments:Additives hydriodic acid (HI) and hypophosphorous acid(HPA) are added to the precursor which fails to boost PCE,the difference in swabbing the ITO for all layers at oncewith one solvent or every layer separately is found to notreduce PCE, the contact size is changed to produce a de-vice with no swabbing stage without PCE losses, and thethickness of the PC70BM layer is changes to increase devicePCEs.

For the majority of this research the reference devices im-plement the p-i-n structure as demonstrated in figure 10with the perovskite active layer made from the Ossila pre-cursor i201, this ink contains MAI:PbI2:PbCl2 at 40%wt inDMF with a stoichiometry of 4:1:1.

Figure 13: Absorbance of a i201 MAPbI3−xClx perovskite filmafter fabrication and after being aged in ambient conditions for51 hours.

These perovskite layers are processed in a nitrogen atmo-sphere is the glovebox and are typically annealed at 80 to90oC and spun at conditions that obtain a thickness of 300-400nm. The typical i201 recipe as published on the Ossilawebsite can produce perovskite films with a roughness of<10nm and efficiencies up to 12%.

Some basic spectroscopic information obtained from an i201film can be seen in figure 13, which presents data from anaging study completed for a proposal for a diamond beamline run in spring 2015. The band edge of the MAPbI3−xClxperovskite film is ∼ 1.55 eV which matches DFT calculatedand experimental values reported in literature.[33,34] After51 hours the films absorption profile has changed signif-icantly, it closely matches that of pure PbI2, this resultdemonstrates the instability of i201 without encapsulation.The steady state PL of i201 stored in ambient conditions isshown in figure 14, the PL spectra do not demonstrate thetrend of perovskite instability with the normalised peak PLintensity increasing and then decreasing over time.

The standard i201 recipe is: PEDOT:PSS spun at 6000rpmon 120oC cleaned substrates, i201 spun at 4000rpm coldwith 120 minute 85oC anneal and PC70BM (50mg/ml inCB) at 1000rpm with a 5nm Ca and 100nm Al thermallyevaporated cathode.

4.1.1 Precursor AdditivesHI & HPA

It has been reported that HI into the precursor could im-prove device performance by enhancing the solubility of theprecursor materials and changing the surface morphologyof the film to produce a smoother perovskite.[9]

8

Figure 14: Multiple PL scans of i201 MAPbI3−xClx aged inambient conditions. Under 450nm illumination.

Figure 15: i201 device PCEs with and without(ref i201) 5%v/V HI in the precursor.

I201 devices were fabricated using the same conditions withand without 5%v/V HI in the precursor. It had previouslybeen established that too much HI in the precursor causeda red-phase perovskite which required a higher temperatureto convert, ultimately resulting in a rougher film.

Figure 15 clearly shows how the HI did not improve PCE inthe i201 devices. HI may only improve PCE device perfor-mance of specific precursors, particularly those that requirehigher annealing temperatures or those which contain ma-terials which do not dissolve easily in DMF/DMSO/GBL.HPA stabilizing acid has also been linked to increased de-vice PCE. [2015 Super Solar Conference in Bath]

HPA contains HI and water and supposedly increases thePCE of devices for the same reasons HI might, with the ad-ditional effect of it being a redox agent capable of reducingthe amount of oxidized perovskite in an attempt to increaseperovskite stability. HPA was added to i201 at a 0.3% v/Vratio, which made devices consistently worse. Figure 16demonstrates batches of the best i201 devices, containing achampion pixel of 12.1% PCE compared to i201 with HPA.

Following the i201 recipe with these replacements and ad-ditives has no clear positive results, a more thorough opti-mization is needed when changing a chemical component oradding something else to the precursor, which may involvechanging other parameters in the fabrication.

4.1.2 Swabbing the ITO

A side experiment in figure 16 demonstrated that swab-bing the device for the cathode and anode ITO connectioncould be done all at once after the PC70BM (e.g i201 swab)instead of layer by layer (e.g i201 usual) without limitingdevice PCEs, although the data indicates that swabbingall layers at once might increase the variability of devices.Either way, the process of swabbing perovskite to completethe device architecture, which is done after annealing inthe i201 recipe, may effect device performance and preventthe perovskite from crystallizing to an optimum form. Fi-nally 16 also concludes that ink made on site on the dayof the experiment (labelled ‘my’) was equivalent to batchespurchased from a supplier.

4.1.3 Extending the Cathode& Changing the Cathode (0 & 5 nm Ca)

To further understand the actual effect of swabbing theperovskite after annealing a batch of devices was made us-ing evaporation mask B which extended the cathode to theedge of the substrates, the devices could then have the per-ovskite scribed instead of swabbed. The effect of swabbingappears very minimal, figure 17 includes data sets on refer-ence swabbed devices with the mask A, and devices whichhave been swabbed or scribed with mask B. The variationbetween batches is small which indicates that the advan-tages of avoiding swabbing the perovskite are limited.

A clear difference between our devices and those commonlyfound in literature is the inclusion of a 5nm calcium injec-tion layer, the removal of the calcium layer is possible butthe devices are consistently a little bit worse than referencedevices. This reduction in PCE without Ca can be shownwhen comparing figure 18 to figure 17 where the effect ofswabbing and scribing with different masks is also com-pared in both cases. Extended cathode B appears to haveno advantage over cathode A, although it is also interestingto observe that the new mask performs just as well whenthe perovskite is neither swabbed or scribed.

An apparent problem with the extended masks is that partof the contact is un-encapsulated, these contacts becomevisibly degraded, and in some cases cause the pixel to stopworking over the timespan of a few hours. This happensfor both Al only cathodes and Ca/Al cathodes, which in-dicates it is not Ca alone that is causing instability in ourdevices, but more likely a reaction involving the perovskite,PC70BM and any choice of contact.

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Figure 16: i201 device PCEs with and without HPA (0.3%v/V), with FAI instead of MAI and step by step swabbing(usual) vs complete swabbing (swab). ’my’ refers to precursorswhich were made on site and not purchased from a supplier.

For a short time after fabrication these edge un-encapsulatedcontacts don’t seem to vary in PCE from normal referencedevices, but will eventually fail. In comparison a completelyun-encapsulated device, where the active area and its cap-ping PC70BM and contacts are exposed to air will sufferdamaging loses to the PCE. It can also be observed thatfor data presented in figure 18 to figure 17 the general PCEof all devices is lower than previous reference devices (Dataset labelled best refs), this was later identified and as a con-tamination problem in the chemical storage and weighingout procedure and has since been rectified.

4.1.4 Changing PC70BM thickness& Changing the Cathode (0 & 5 nm Ca)

Another distinct difference between high PCE devices in lit-erature and i201 device architecture is the thickness of thePC(60/70)BM layer, a lot of published p-i-n recipes have aPC60BM layer of 30-50nm whilst the current i201 recipeuses a 150-200nm thick PC70BM layer. The advantage ofsuch a thick layer is that you are guaranteed to planarizethe device transport layer film, making sure that the per-ovskite does not come into contact with the cathode ma-terials, the disadvantage being that the PC(60/70)BM hassignificantly reduced charge transport properties in com-parison to the perovskite; its conductivity and electron andhole diffusion lengths are orders of magnitude lower thanthe perovskite. Given that the i201 recipe regularly pro-duces perovskite films with ∼ 10nm roughness, a thinnerPC70BM layer should theoretically still planarize the de-vice.

Figure 19 and 20 show the effect of thinning PC70BM withand without the 5nm Ca interlayer respectively. In thecase of this device run it seems that something went wrongwith reference i201 devices, and the devices without Ca hadhigher PCE’s in general. It is promising to see that in bothcases 200nm what not the optimum PC(60/70)BM thickness,

Figure 17: i201 device PCEs with reference 5nm Ca / 100nmAl cathode, swabbed or scribed using thermal evaporation maskA or B to achieve a short contact type A or long contact B thatextends to the edge of substrates.

Figure 18: i201 device PCEs with 0nm Ca / 100nm Al cathode,swabbed or scribed using thermal evaporation mask A or B toachieve a short contact type A or long contact B that extendsto the edge of substrates.

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Figure 19: i201 device PCEs with reference 5nm Ca / 100nmAl cathode, varying PC70BM thickness.

Figure 20: i201 device PCEs with 0nm Ca / 100nm Al cathode,varying PC70BM thickness.

it appears that 140nm for 5nm Ca / 100nm Al devices and70nm for 0nm Ca / 100nm Al devices are the optimumthicknesses. 140nm and 70nm PC70BM layers were madefrom spinning a 35mg/ml PC70BM in CB at 1000rpm and4000rpm respectively.

The champion reference device for the i201 recipe producedthe non light soaked J-V sweep shown in figure 21 and setsthe bench mark for alternative perovskite recipes.

4.2 Alternative Perovskite Recipes

4.2.1 MAI:PbAc2 (3:1) Devices

In literature PbAc2 has been implemented into spray coatedand spin coated devices successfully as an alternative leadsource, the advantage of PbAc2 being that the MAAc unre-acted unwanted component sublimes off the film at 97.4oC,which is a much lower temperature than MACl (226.7.4oC)and MAI(245oC).[35] The annealing stage of the PbAc2 isreported to be fast and can apparently produce a ’ultra’smooth film, with ∼ 14% PCE devices requiring only a 5minute anneal at 100oC.[35] There is little information onthe use of PbAc2 on PEDOT:PSS in literature, and thedetails of the exact chemical to use and molar ratio are

Figure 21: J-V curve of non light soaked champion i201 refer-ence device, the reverse bias scan was taken at 0.05V/s.

Figure 22: MAI:PbAc2 device PCEs using 3:1 Tri-hydrate mo-lar ratio, for different drying times after spin coating before an-nealing, and also for precursor spun hot at 70oC. 2000 rpm with5 minute 100oC anneal.

unclear. The product most likely used is a Sigma Aldrichtri-hydrate which is material used in this study. A 40%wt 3:1 molar ratio MAI:PbAc2 in DMF is the base pre-cursor, however it is unclear whether it is better to use thetri-hydrate molecular weight or the anhydrous molecularweight to balance the stoichiometric and get the maximumamount of material reacted in the deposition stage. Themolecular weight of PbAc(Tri) is 379.33 g/mol whilst themolecular weight of PbAc(anh) is 325.29 g/mol.

During the device fabrication runs of various PbAc2 recipesit became apparent that the precursor ink could not beheated to aid the dissolution of the chemicals in the solvent,this is most likely because the decomposition temperatureof PbAc2 is ∼ 80oC. Inks heated above 60oC for longer than10 minutes began to no longer form the perovskite crystalstructure when deposited. Formamidinium based chemi-cals also have lower decomposition temperatures and henceshould also be treated with similar care.

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Figure 23: MAI:PbAc2 device PCEs using 3:1 Tri-hydrate mo-lar ratio, for inks left at room temperature and 45oC overnight,and for precursors with added HPA. 2000 rpm, 0/1 minute drywith 5 minute 100oC anneal.

Figure 24: Table of MAI:PbAc2 device PCEs using 3:1 Anhy-drous molar ratio, matrix of spin speeds and drying times.

Figure 25: Table of MAI:PbAc2 device PCEs using 3:1 Tri-hydrate molar ratio, matrix of spin speeds and drying times.

Figure 26: J-V curve of non light soaked champion MAI:PbAc2device PCEs using 3:1 Anhydrous molar ratio, the reverse biasscan was taken at 0.05V/s.

PbAc2 based perovskite devices may require a drying stageafter deposition before annealing to achieve the best surfacemorphology. Figure 22 shows an initial investigation intodrying times and some insight on if the ink should be de-posited hot or cold, and figure 40 is part of a data set usedto ensure the precursor was stable below 50oC, including adevice batch looking at adding HPA into the MAI:PbAc2precursor. The number of failed devices in these two exper-iments was high, and HPA gave no obvious PCE boost, buta few of the good devices encouraged the investigation of amatrix of spin speeds and drying times for cold and hot inksusing the anhydrous and tri-hydrate molar ratios. Figure24 and figure 25 show that the most promising MAI:PbAc2devices come from cold precursor spun at 2000rpm with a 0or 1 minute dry, using either the anhydrous or tri-hydratemolar ratio.

The champion device pixel is shown in the J-V sweep figure26. The 8.1% PCE device had sufficient FF of 75.2% butthere is equal room for improvement in the 0.89 V VOC

and the 12.22 mA/cm2 JSC . There is a lot more parame-ter space to probe, and it seems that the MAI:PbAc2 per-ovskite can fundamentally function on-top of PEDOT:PSS,which is a relatively unpublished device recipe.

4.2.2 FAI:PbI2:MABr:PbBr2 (5.67:5.67:1:1)I401 Devices & Solvent Engineering

The highest recorded PCEs for perovskite devices mostlyimplement the (FAPbI3)0.85(MAPbBr3)0.15 active layer ina standard n-i-p structure.[11] The advantages of this choiceof perovskite are well published. FAI should intrinsicallytake in a wider part of the spectrum and has better chargetransport properties than MAI, the inclusion of PbBr2 isassociated with larger crystals and longer stability of theperovskite.

12

The inclusion of MABr also phase locks the perovskite struc-ture so it does not enter the non-semiconducting hexagonalphase ensuring the perovskite is phase and temperature sta-ble within the standard operating temperatures of a solarcell.[36]

Such perovskites require a DMSO/GBL blend of solventsat a 3:7 volume ratio and a toluene quench.[37] The com-position of the film after deposition is concentrated by theevaporation of GBL and DMSO, however when the non-solvent miscible toluene is added to the system all of theconstituents are frozen with the DMSO rapidly forming intoa complex of MAI-PbI2-DMSO. Any unreacted solvent iscleared away by the toluene, the remaining DMSO complexretards the immediate formation of the perovskite allowingthe film to spread evenly and uniformly so that when theDMSO is eventually driven out during the annealing stagethe remaining perovskite film is uniform and dense.[37]

There is no evidence in literature of this kind of perovskiteworking in the reverse p-i-n architecture, it is unclear ifit can be successfully deposited on PEDOT:PSS. It is alsocurrently unclear if such solvent engineering could improvethe more common perovskite blends such as Ossila i101 andi201.

The ink used to test (FAPbI3)0.85(MAPbBr3)0.15 is cur-rently referred to as i401 and is a 40% wt stoichometric of5.67:5.67:1:1 FAI:PbI2:MABr:PbBr2, care must be taken touse lower purity PbI2 in order to produce a thick enoughperovskite film. The toluene quench is an incredibly sen-sitive process, a difference in 10s between perovskite depo-sition and toluene can entirely change the perovskite mor-phology. By probing the parameter space, an ultra reflec-tive 300nm perovskite film with ∼ 5nm roughness can bedeposited, however the base of the perovskite layer appearsto be randomly covered in cracks. The cracks do not ap-pear to be caused by: an unstable spin coater, comets orother spin defects and may well be due to an extreme stressbetween the PEDOT:PSS and perovskite during the rapidformation of the MAI-PbI2-DMSO complex.

Currently two spinning recipes have reached some success:figure 27 shows ∼ 4% PCEs from a i401 spun at 1500 rpmfor 50s then 4500 rpm for 20s with a toluene quench att=50s and, a i401 spun at 1500 rpm for 40s then 4500 rpmfor 25s with a toluene quench at t=50s. The 100oC an-nealed devices have not converted the perovskite, and thedevices without a toluene quench appear extremely roughwith low film coverage and low PCE. i401 also has plenty ofparameter space left to probe, but it should become quicklyapparent if these cracks will only subside if the perovskiteis deposited on a rougher mesoporous TiO2 structure withless surface stress between the perovskite and HTM.

Figure 27: PCEs of i401 recipes annealed at 100oC and 150oCfor 10 minutes using recipes: T1, i401 spun at 1500 rpm for 50sthen 4500 rpm for 20s with a toluene quench at t=50s and T2,spun at 1500 rpm for 40s then 4500 rpm for 25s with a toluenequench at t=50s.

Figure 28: J-V curve of non light soaked champion i401 devicePCEs, the reverse bias scan was taken at 0.05V/s.

A J-V curve of the current i401 devices in figure 28 hasa FF and JSC that are particularly low, indicating that ahigh quality perovskite film has not yet been achieved. Itis plausible that the cracks are not detrimental to the de-vice peformance and that a higher annealing temperatureand/or time will drive the DMSO out of the MAI-PbI2-DMSO and produce a higher PCE device.

5 Conclusion

The 12.1%, 8.1% and 4.3% champion PCEs achieved fordevices with MAPbI3−xClx, MAPbAc3 and (FAPbI3)0.85(MAPbBr3)0.15 have been achieved through a combinationof parameter space optimization experiments and using in-formation from published literature. Whilst there havebeen no huge breakthroughs in PCE, the complexity of ref-erence device recipe has been reduced by developing theoption to remove Ca and swab all layers at once.

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The 12.1% device currently remains the highest PCE ob-tained using the i201 ink precursor but there should be clearimprovements to PCE with reduced PC70BM thickness if asmoother perovskite film can be engineered. With limitedoptimization so far, the champion PCEs of the other pre-cursors remains relatively low but there is a large amountof parameter space left to probe.

6 Short-Term Outlook

After completion of this report the focus of my lab work willbe to continue to optimize the alternative perovskite pre-cursor i401 as a device active layer, with supporting spec-troscopic studies of the material itself. This optimizationwill involve a complete change in device structure as I be-gin to work on standard n-i-p architecture with TiO2 andspiro-MeOTAD.

I will also investigate the use of original Ossila inks i101and i201 in standard structure in an attempt to reach PCEsas quoted in literature. I may continue to use optimiza-tions that appear in literature to boost my reference de-vice performance (i201 reverse device structure) by addingadditives into the perovskite or inter-layers in the devicestructure. I have completed some preliminary work thatsuggests inclusion of 4-8% DCB into i201 does not improvedevice performance and would like to confirm this.

Finally I propose that a FAI based device that uses a PbAc2lead source and other additives could produce a dense smoothperovskite film without the need for a toluene quench, Iwould like to implement this in a device with thinner trans-port layers and no Ca cathode, using either a reverse struc-ture device with a PTAA interlayer above the PEDOT:PSSand a 70nm PCBM layer or a standard structure device.

7 Doctoral Development Program(DDP) Requirements

This year has been a combination of CDT modules com-pleting the requirements for my EPSRC funded PhD andwork done in the University of Sheffield to complete thestandard DDP requirements for PhD students.

A outline of topics and techniques I have gained experi-ence with during my CDT modules includes: An overviewof the fundamentals and photovoltaics including semi con-ductor physics and experimental experience with ExternalQuantum Efficiency (EQE), J-V and Hall Effect measure-ments. An overview of alternative comparable Renewableenergy sources, the governmental road-map of renewableenergy, the physics of Exergy and a week long course inEntrepreneurship provided by Cambridge Judge Business

school. Research Skills and PV in action, including ex-perience with thin film photovoltaic fabrication, Raman,Optical, Steady Steady Photo luminescence (PL) and ab-sorption spectroscopy. Mathematical Methods and Nan-otechnology including the physics of semiconductors de-rived from first principles, the use of nanotechnology in pho-tovoltaic devices, Time-Resolved Photoluminescence (TRPL)and reflectance spectroscopy. Designs for high performancephotovoltaics and further business schools including the useof impedance spectroscopy and further thin film device fab-rication. A complete picture of advances sustainable mate-rials including a full device fabrication and analysis includ-ing steady state and TRPL PL, EQE, J-V and Kelvin Probespectroscopy to create a mock publication. And finally theuse of photovolatic materials in real world PV modules andsystems, including real lamination, large area device test-ing and solar farm scale modeling and site analysis. Themajority of this work as not formed any experimental workthat has been written in the report but has provided witha wide breath of knowledge and some initial experience tohelp me with my own research.

The experimental work I present in this report is my ownresearch outside of my work completed for the CDT. Thisresearch has been mostly focused on the fabrication of smallarea perovskite devices. I have familiarized myself withclean room and glove box fabrication recipes that includethe used of chemicals and solvents to create an ink precur-sor, the use of spin coaters and hotplates to produce op-timized perovskite films with appropriate transport layers,the use of a thermal evaporator to produce metal contactsand the proper short term encapsulation of thin film de-vices using UV activated epoxy.

I have attempted to develop and optimize several differentprecursors in an attempt push efficiencies here in the Elec-tronic and Photonic Molecular Materials Group (EPPM)to reach those commonly found in literature. In order toanalyze my results I have mainly relied on surface mor-phology images obtained from a Dektak profiler or opticalmicroscope images, J-V device performance data, or PLand absorption spectroscopy images. I hope to continueto use these techniques and build my experience in othertechniques such as spray coating as I continue to investi-gate alternative perovskite device recipes, I may focus onstandard architecture devices which are currently not beingmade by the EPPM group here in Sheffield.

In addition to these two main bodies of work I have demon-strated in 2nd year computing and 2nd year Physics lab-oratories for the past two semesters and will continue todemonstrate and mark lab books for the 1st year laboratoryfor the next university semester. I have also maintainedgood attendance to departmental colloquia including a se-ries of soft matter seminars.

14

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