Journal of Materials Chemistry AMaterials for energy and sustainabilitywww.rsc.org/MaterialsA

ISSN 2050-7488

Volume 2 Number 19 21 May 2014 Pages 6671–7118

PAPERXianyu Deng et al.Environmentally friendly biomaterials as an interfacial layer for highly effi cient and air-stable inverted organic solar cells

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Research Centre for Advanced Function

Engineering Lab of Flexible Transparent Co

of Advanced Materials, School of Materi

Graduate School, Harbin Institute of Tech

E-mail: xydeng@hitsz.edu.cn; Fax: +86-755-

† Electronic supplementary informa10.1039/c4ta00152d

Cite this: J. Mater. Chem. A, 2014, 2,6734

Received 10th January 2014Accepted 29th January 2014

DOI: 10.1039/c4ta00152d

www.rsc.org/MaterialsA

6734 | J. Mater. Chem. A, 2014, 2, 673

Environmentally friendly biomaterials as aninterfacial layer for highly efficient and air-stableinverted organic solar cells†

Riming Nie, Aiyuan Li and Xianyu Deng*

Appropriate interfacial modification plays an important role in the high performance of organic solar cells.

We report that a transparent cathode of indium tin oxide (ITO) modified with an ultrathin layer of peptide, an

environmentally friendly biomaterial, shows an obvious reduction in the work function. Investigation of the

device exhibits that the power conversion efficiency (PCE) was significantly increased from 2.12% to 8.13%

with the use of the peptide modification. The inverted device with the peptide-modified ITO as the cathode

showed significantly longer time efficiency delay in air than conventional forward devices with an active

metal as the cathode. Because peptides are biological materials that exist naturally in living things,

the results provide an environmentally safe method to fabricate highly efficient and air-stable organic

solar cells.

Introduction

Organic based solar cells (OSCs) have been attracting growinginterest due to their unique advantages, such as low fabricationcost, light weight, and high mechanical exibility, all of whichmake them suitable for potential applications in low cost andrenewable energy sources.1–4 With the continuous improvementof organic solar cells, the future of fabrication will require moreenvironmentally friendly techniques and better integration ofgreen technology for sustainable organic devices. Biomaterialsand natural compounds have the particular advantages thatthey are environmentally friendly and naturally occurring, sothey can contribute sustainably to organic electronics. However,despite these advantages, few biomaterials have been reportedas functional elements in the fabrication of organic based solarcells. Here, we report the use of peptide, the biomaterial that isfound widely in biology, as an interfacial modier in thefabrication of organic solar cells.

The basic structure of organic solar cells includes a bulkheterojunction layer, where conjugated polymers act as electrondonors and fullerene derivates act as electron acceptors, whichis sandwiched between a bottom electrode of transparentindium tin oxide (ITO) and a metal counter electrode.Compared with conventional forward organic solar cells, where

al Materials and Devices, Shenzhen

nductive Films, Shenzhen Key Laboratory

als Science and Engineering, Shenzhen

nology, Shenzhen 518055, P. R. China.

26033504; Tel: +86-755-26033211

tion (ESI) available. See DOI:

4–6739

the ITO is a hole collection electrode, inverted organic solarcells with the ITO being an electron collection electrode aremore stable due to the lack of acidic conducting polymer andactive metal.5–8 Inverted solar cells also take advantage ofvertical phase separation, which results in richer acceptormaterials near the bottom electrode and richer donor materialsnear the top electrode.9 However, ITO electrodes generally donot have good electron selectivity because their work function(about 4.7 eV) is seriously mismatched with the lowest unoc-cupied molecular orbital (LUMO) energy level of organicacceptors. Therefore, lowering the work function of the ITO iskey to realizing highly efficient inverted organic solar cells.Some alternative methods have been employed to overcomethe problem of work function mismatch. One method was theinsertion of an n-type metal oxide between the ITO and theorganic active layer, including TiOx, ZnOx, or an interfacial layerof alkali metal salt such as Cs2CO3.10–13 Another method wassolution-depositing a 5–10 nm layer of organic material on topof the ITO, including polymer electrolytes, neutral insulatingpolymers and zwitterions.14–20 These additional layers can adjustthe energy barrier and form an electron selective contactbetween the ITO and the organic active layer, resulting in largelyimproved electron collection efficiency and an increased opencircuit voltage (Voc). However, the inserted layers have someintrinsic drawbacks, in that they unavoidably affect propertiessuch as the conductivity, transparency, and surface roughnessof the ITO. To overcome this shortcoming, modication ofthe ITO surface with a self-assembled molecular monolayer(SAM) has been employed, since SAM modication requiresonly a small amount of material and it can tune thesurface work function with little change to the bulk electricalproperties. Several SAMs have been reported to lower the ITO

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Fig. 2 (a) Survey XPS profiles and (b) high-resolution In 3d XPS profilesof the bare ITO, the ITO with Gly–Gly and the ITO with Gly–L-Leu.

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work function. These SAM materials include N(C4H9)4OH,tetrakis(dimethylamino)ethylene (TDAE), and 3-amino-propyltriethoxysilane (APTES).21–23 However, most of theprevious SAM materials are chemically toxic and are processedin organic solvents, which are toxic and harmful to theenvironment.

In this contribution, we report the results of themodicationof ITO with a SAM of the non-toxic and water soluble bioma-terial peptide. It signicantly lowered the work function of theITO and provided a good match of energy levels of the ITO andorganic active materials to achieve highly efficient and air-stableinverted OSCs. Fig. 1 shows the device structure, in whichthe active layer is a blend of poly[4,8-bis(2-ethylhexyloxy)-benzo-[1,2-b:4,5-b0]dithiophene-2,6-diyl-alt-(4-octanoyl-5-uoro-thieno-[3,4-b]thiophene-2-carboxylate)-2,6-diyl] (PBDTTT-CF) andphenyl-C71-butyric acid methyl ester (PC71BM), and the chem-ical structure of two peptides used in this study, glycylglycine(Gly–Gly) and glycine–L-leucine (Gly–L-Leu). Because thepeptides contain carboxylic acid groups (–COOH), the peptidescan be chemically adsorbed on the surface of a metal oxide bythe reaction of –COOH with the metal oxide or –OH on themetal oxide surface. The adsorption of peptides on the ITOsurface produces molecular dipole moments arranged on thesurface. Consequently, the work function of the ITO can belargely reduced to achieve high efficiency inverted organic solarcells. X-ray and ultraviolet photoelectron spectroscopy (XPS andUPS) were used to investigate the work function of the ITOsubstrates and the effect of the peptide SAMs on the surfacecomposition.

Results and discussion

Fig. 2(a) shows the full spectrum of XPS acquired from the bareITO, Gly–Gly treated ITO, and Gly–L-Leu treated ITO aer awater rinse. The appearance of the N 1s peak related to –N– ataround 400 eV, and the disappearance or weakening of the

Fig. 1 Molecular structures of the two peptides and a schematic of theinverted organic solar cells.

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peaks due to indium atoms on the Gly–Gly deposited ITO clearlyindicate the adsorption of Gly–Gly on the surface of ITO. Fromthe high-resolution XPS picture shown in Fig. 2(b), an obviousshi occurred in the In 3d peak of the Gly–Gly treated ITOcompared to that of the bare ITO, which proves the presence ofa chemically adsorbed monolayer of Gly–Gly on top of ITO. Asimilar result was also observed for the other peptide Gly–L-Leu,as shown in Fig. 2.

To investigate the effect of the peptides on the morphologyof the ITO surface, we carried out AFM measurements on theITO substrates with and without peptide SAMs. The AFMimages shown in Fig. 3(a) illustrate that the roughness valuesof the bare ITO, the rinsed Gly–Gly treated ITO, and the rinsedGly–L-Leu treated ITO are 3.46 nm, 3.28 nm, and 3.36 nm,respectively, indicating that the peptide SAMs modicationdid little to affect the morphology of the ITO surface.According to previous reports,24,25 the morphology and orien-tation of organic layers covering the modied ITO may beaffected because of the change in surface affinity. We alsocompared the morphologies of PBDTTT-CF:PC71BM lms thatwere spin-coated on ITO substrates with and without Gly–Glyor Gly–L-Leu. As shown in Fig. 3(b), the roughness values of thePBDTTT-CF:PC71BM lms on the original ITO, the Gly–Glymodied ITO, and the Gly–L-Leu modied ITO are 1.12 nm,1.36 nm, and 1.41 nm, respectively, indicating that the

Fig. 3 (a) AFM images of bare ITO, Gly–Gly treated ITO, and Gly–L-Leu treated ITO, after washing. (b) AFM images of PBDTTT-CF:PC71BMfilms deposited on bare ITO and ITO modified with Gly–Gly or Gly–L-Leu.

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modication of ITO had little effect on the morphology of theactive organic layer.

Fig. 4(a) shows the UPS spectra of ITO with and withoutpeptide modication. For all of the curves in the kinetic energyspectra, the edge of the Fermi-level was close to the energy of theHeI source (21.22 eV), which is at near zero in the bindingenergy spectra. Thus, the value of the work function wasdetermined by the cut-off edge of kinetic energy according tothe method discussed in the literature.20,26 The work function ofthe bare ITO was 4.69 eV, which was similar to the value given inprevious reports.27 With the modication of the ITO using Gly–Gly and Gly–L-Leu, the work function was reduced to 3.33 eV and3.63 eV, respectively. The decrease in the work function of theITO was attributed to the surface dipole moment of self-assembled molecules, which is shown in Fig. 1. The change inthe work function of substrate, Df, can be given by the simpleequation:28

Df(n) ¼ eNm(n)cos(q)/303SAM (1)

where N is the surface density of the molecules, m is themolecular dipole moment, q is the average dipole tilt angle fromthe normal to the surface, and 3SAM is the dielectric constant ofthe SAM. The positive amine group and the negative carboxylicgroup in the peptide molecule produced a large intrinsic dipolemoment on the surface of the ITO. On the other hand, it waseasy for the peptides to form hydrogen bonds between thecarbonyl group (C]O) and the amide group (NH).29,30 This isdifferent from other modifying molecules containing aminogroups in neutral molecules or polymers and conjugated ornon-conjugated polyelectrolytes.15,16,18,23 The hydrogen bondsare stronger than van der Waals forces, which help produce themolecule dipoles along the direction perpendicular to the ITOsurface. This helps achieve a large reduction of the workfunction.

Fig. 4 (a) Normalized UPS spectra of bare ITO and ITO modified withGly–Gly or Gly–L-Leu. (b) Normalized UPS spectra of a PBDTTT-CF:P71BM blend film spin-coated on bare ITO and ITO modified withGly–Gly or Gly–L-Leu. (c) Schematic energy level alignment based onthe UPS measurements.

6736 | J. Mater. Chem. A, 2014, 2, 6734–6739

To further investigate the effect of the peptides on the elec-tronic structure of the interface between the ITO and theorganic active layer, UPS measurements of an ultra-thinPBDTTT-CF:PC71BM were made. Fig. 4(b) shows the valenceband spectra of the PBDTTT-CF:PC71BM on top of the bare ITOand ITO with peptide modication. The highest occupiedmolecular orbital (HOMO) level of the PBDTTT-CF:PC71BM onthe original ITO was 4.9 eV, which was close to the HOMO ofPBDTTT-CF reported in the literature.31 This phenomenonsuggests that the top of the lm surface was rich in PBDTTT-CFaer a vertical phase separation of PC71BM and PBDTTT-CF,which has also been demonstrated in another commonly usedblend system, P3HT:PCBM.32–34 With the addition of peptide,there was a shi of the HOMO toward higher binding energy(BE) by 0.5 eV for Gly–Gly, and by 0.7 eV for Gly–L-Leu. Theenergy level alignments are shown in Fig. 4(c). They indicatethat the modication of the ITO substrate with Gly–Gly and Gly–L-Leu results in a 0.8 eV and a 0.5 eV decrease of the barrierheight between the Fermi level and the lowest unoccupiedmolecular orbital (LUMO), respectively.

Fig. 5 shows the current density versus voltage (J–V) charac-teristics and external quantum efficiency (EQE) of the PBDTTT-CF:PC71BM based OSCs with and without SAMs of peptides onthe surface of the ITO. The device performance was optimizedby altering various parameters during the process of the devicefabrication (see Fig. S1 to S5†). As expected, the device using thebare ITO exhibited the lowest power conversion efficiency(PCE), 2.12%, while the device with ITO/Gly–Gly and the devicewith ITO/Gly–L-Leu had PCEs of 8.13% and 7.92%, respectively.The enhanced PCE was due to a comprehensive enhancementin open circuit voltage (Voc), short circuit current density (Jsc)and ll factor (FF). The Voc, the Jsc and the FF were increased,respectively, from 0.37 V, 15.20 mA cm�2 and 38% to 0.73 V,17.69 mA cm�2 and 63% for the device with Gly–Gly, and to 0.72V, 17.22 mA cm�2 and 64% for the device with Gly–L-Leu. Thelarge increase in the Jsc and FF was attributed to the decrease ofthe series resistance (Rs). As shown in Fig. 5, it was observed thatthe Rs of the devices with SAMs of peptides signicantlydecreased from 12.02 to 7.03 (Gly–Gly) and 6.73 (Gly–L-Leu) Ucm2, respectively. The dramatic enhancement of the deviceperformance can be rstly attributed to the lowered workfunction of the ITO resulting from the modication withpeptides, which led to a reduction of the barrier height of the

Fig. 5 J–V characteristics (a) and EQE (b) of the inverted PBDTTT-CF:PC71BM based solar cells with a transparent cathode of bare ITO,Gly–Gly treated ITO, or Gly–L-Leu treated ITO.

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electron collection interface, thus improving the ohmic contactof the ITO with the photoactive layer. The enhanced ohmiccontact facilitates electron transport to the ITO, which can notonly produce high efficiency electron collection to increase Jscand FF, but can also help obtain the maximum achievable Voc,which is dependent on the difference between the quasi-Fermilevels of the holes and the electrons.35,36 The peptide modiedITO has a work function that is obviously lower than that of thebare ITO, which indicates that the device with peptide modiedITO has a much larger quasi-Fermi level difference than that ofthe device without the modication. Thus, the OSCs with thepeptide modied ITO have much higher Voc compared with theOSCs using the bare ITO.

The stability of the inverted OSCs was measured underconditions where the devices were exposed to air without anyencapsulation. Fig. 6 shows the changes in Voc, Jsc, FF and PCEwith time for the two inverted OSCs with peptide modication,and the traditional forward OSC with Ca. The forward devicewith Ca shows rapid degradation in performance. Aer 5 hours,the efficiency of the device with Ca reduces by nearly 80% of itsinitial value, accompanied by obvious decrease of Voc, Jsc, andFF. However, the efficiency of the device containing ITO/peptideremains at approximately 90% of its intial value aer 24 hours.The results indicate that the inverted OSCs with peptide as atransparent cathode modier exhibit signicantly improved airstability compared with the conventional device with Ca. Thedecreased performance of the forward device is primarilycaused by the rapid reaction of metallic Ca with H2O and O2

from the air, which enter into the device through pin holes onthe evaporated metal electrode. The acidic conducting polymerPEDOT:PPSS could also possibly affect the device stability,because it releases acidic or ionic matter that may damage theITO electrode and the active layer. The inverted devices do not

Fig. 6 Changes in the Voc, Jsc, FF and PCE of the forward OSCs withCa and the inverted OSCs with Gly–Gly/ITO or Gly–L-Leu/ITO withtime.

This journal is © The Royal Society of Chemistry 2014

contain any active metal elements or acidic polymers. Themodifying peptides are molecularly stable and chemicallybonded on top of the ITO, which makes the interface of thecathode with ITO more stable. Thus, the OSCs with peptidemodied ITO show better air stability than the conventionaldevice with Ca.

A comparison of the performance of inverted OSCs withdifferent interlayers used to modify ITO is shown in Table 1. Foreach interlayer type, we used different deposition conditions tooptimize the performance. The values shown in Table 1 are theaverages obtained from ten devices. The devices containingsolution-processed TiO2 showed the lowest repeatability, thereason for which is the difficulty of forming TiO2 crystals withuniform crystallinity and size by a sol–gel process. For thedevices with the commercial polymer poly(ethylene oxide) (PEO)as the interfacial material, although a high Voc of 0.75 V and Jscof 17.23 mA cm�2 were achieved, the PCE was only 6.34% due tothe low FF of 49%. The reason for this is that the insertion ofPEO increased the series resistance due to the strongly insu-lating nature of PEO. We note that the use of PEO as an inter-facial modier produced signicantly enhanced performance inan inverted OSC based on P3HT:PCBM as an active layer,according to a previous report.19 The different effects of PEO inP3HT and PBDTTT-CF based devices could be attributed to theirdifferent fabrication processes. For a high performanceP3HT:PCBM solar cell, the active layer generally needs toundergo a slow drying process and an annealing treatment ataround 110 �C, which could change the formation of the PEO onthe surface of ITO. PEO has also been widely used as an inter-facial material to achieve high performance in organic lightemitting diodes (OLEDs) and OSCs with a traditional forwardstructure.37,38 However, the original mechanics of the PEO in theinverted structure may be different to that of PEO in a conven-tional forward structure, where the reaction occurring betweenthe –O– of PEO and the thermally evaporated metal atom canproduce n-type doping at the interface between the metalcathode and the organic layer.37,39 The high electron concen-tration from the doping could achieve high conductivity,which can compensate for the disadvantage of the highlyinsulating character PEO in a forward structure. In thiswork, peptide molecules are chemically adsorbed on the ITOsurface so that they are insusceptible to the deposition of theorganic layer on top of it. An obvious enhancement ofperformance induced by the peptide modication was alsoobserved for P3HT:PCBM based inverted solar cells (seeFig. S6†). Chemisorption also covers the ITO with a self-assembled monolayer, which effectively improves the surfaceproperties but has little affect on the characteristics of thebulk. On the other hand, peptides are good alternative inter-layer materials because they are environmentally friendly andare deposited via a non-toxic process. Moreover, there aremany structures and types of peptides found in nature, andthe ITO electrode is also a window electrode widely used inother optoelectronic devices, such as inorganic–organichybrid solar cells. Therefore, it is promising to use peptides tofurther enhance the efficiency of organic solar cells and otheroptoelectronic devices.

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Table 1 Performances of the inverted PBDTTT-CF:PC71BM based solar cells with TiO2 covered ITO, PEO covered ITO, Gly–Gly treated ITO andGly–L-Leu treated ITO

Modifying material Voc (V) Jsc (mA cm�2) Calculated Jsc (mA cm�2) FF (%)

PCE (%)

Average Best

TiO2 0.74 17.39 17.20 57 5.18 7.29PEO 0.75 17.23 17.09 49 6.26 6.34Gly–Gly 0.73 17.69 17.40 63 8.02 8.13Gly–L-Leu 0.72 17.22 17.00 64 7.84 7.92

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Conclusions

In conclusion, we have demonstrated that highly efficientinverted organic solar cells can be produced by modifying thesurface of indium tin oxide (ITO) cathodes with self-assembledpeptide molecules. With Gly–Gly as the modier, the invertedorganic solar cells based on PBDTTT-CF:PC71BM exhibited thehighest PCE of 8.13% accompanied by a Jsc of 17.69 mA cm�2, aVoc of 0.73 V and a FF of 63%. The role of the peptide SAMs wasto effectively decrease the work function of the ITO surface toimprove the ohmic contact between the ITO electrode and theorganic active layer. As a biomaterial found widely in livingthings, and produced by a solution-processing method in water,the peptides provide a low cost, non-toxic and environmentallyfriendly route for the fabrication of organic solar cells.

Experimental sectionMaterials

Indium tin oxide (ITO) coated glass substrates with a sheetresistance of 7–10 U sq�1 were purchased from CSG HoldingCo., Ltd. Peptides were purchased from Aladdin®. The electrondonor material PBDTTT-CF and electron acceptor PC71BM forsolar cells were purchased from Solarmer Energy., Inc. andSigma-Aldrich®, respectively, and they were used in the state inwhich they were received.

Modication of ITO

ITO substrates with patterns were rst ultrasonically cleanedwith detergent, deionised water, acetone, and isopropyl alcohol,for 10 min each. The cleaned substrates were dried under anitrogen stream and subsequently treated with UV–ozone for 15min. Aer this, the substrates were soaked in a 10 mM aqueouspeptide solution at 90 �C for 2 h in order to form self-assembledSAMs of peptides on the surface of the ITO. Then, the substrateswere rinsed with pure water to remove the residual physicallyadsorbed peptide molecules, and then transferred into anitrogen glove box with H2O and O2 contents less than 1 ppm todry for 1 h.

Device fabrication

Inverted solar cells were fabricated on top of the peptidedeposited ITO and the original cleaned ITO as a comparison. Asolution of 10 mg ml�1 PBDTTT-CF and 25 mg ml�1 PC71BMwas blended in dichlorobenzene solvent with the additive 1,8-

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diiodooctane (DIO) (3% v/v) and spin-coated on top of thepeptide modied ITO sheets with a speed of 1000 rpm for 60 s.Then, they were dried under nitrogen gas at room temperaturefor 10 min. Finally, a 5 nm thick MoO3 layer and a 120 nm thickAl layer were successively deposited by thermal evaporationonto the top of the active layer in a vacuum chamber with apressure of less than 10�4 Pa. The typical active area of thedevices in this study was set to about 0.12 cm2 by using ashadow mask during the anode evaporation.

Device and lm characterization

The current density (J)–voltage (V) characteristics of the deviceswere recorded with a computer-controlled Keithey 2400 sourcemeter. Simulated AM 1.5 sunlight with an intensity of 100 mWcm�2 was provided by a solar simulator whose light intensitywas calibrated by using a Newport Oriel 91150V PV cell as astandard single-crystal Si cell. The measurement of incidentphoton–current conversion efficiency (IPCE) was carried outwith a quantum efficiency (QE)/IPCE measurement system(SR830, Stanford Research Systems). A standard Si photodiodecalibrated from Hamamatsu was tested as a reference beforeeach sample measurement. The standard Si photodiode wasregularly calibrated with another standard Si photodiode cali-brated from Newport, to conrm whether it was being degradedor not. Work function was measured by ultraviolet photoemis-sion spectroscopy (UPS) under ultra-high vacuum (UHV). UPSmeasurements were performed with an unltered HeI (21.22eV) gas discharge lamp and a total instrumental energy reso-lution of 100 meV. All the UPS measurements of the onset ofphotoemission for determining the work function were doneusing standard procedures, with a �10 V bias applied to thesample to enhance the signals with kinetic energy (KE) nearzero. XPS measurements were carried out using a mono-chromatic Al Ka source (1486.6 eV). Both the UPS and the XPSmeasurements were performed at room temperature. Themorphologies of the samples were characterized by atomic forcemicroscopy (AFM) from Veeco NanoScope IV Multi-Mode intapping mode.

Acknowledgements

We are grateful to Mr S. Z. Zheng, Prof. K. Y. Wong (the ChineseUniversity of Hong Kong) and Mr Nick K. C. Ho (the Hong KongUniversity of Science and Technology) for the XPS and UPSmeasurements. We acknowledge nancial support from the

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Natural Scientic Research Innovation Foundation at the Har-bin Institute of Technology (HIT.NSRIF, 2009142), the Shenz-hen Research Foundation Project (JC201005260119A,C201105160573A, JC201104220174A) and NSFC (no. 60706017).

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