Solution-Processed Copper Iodide as an Inexpensive and EffectiveAnode Buffer Layer for Polymer Solar CellsWeihai Sun,† Haitao Peng,‡ Yunlong Li,† Weibo Yan,† Zhiwei Liu,† Zuqiang Bian,*,†

and Chunhui Huang†

†Beijng National Laboratory for Molecular Science, State Key Laboratory of Rare Earth Materials and Applications, College ofChemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China‡Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic ofChina

*S Supporting Information

ABSTRACT: In this work, a CuI anode buffer layer prepared from a facilesolution-processed method was introduced in polymer solar cells (PSCs). TheCuI films obtained under different spin-coating speeds and anneal treatments,and the performances of corresponding device fabricated with these CuI filmswere systematically investigated. The results showed that the devices based onCuI anode buffer layer displayed superior performance than conventionaldevices using poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) sulfonicacid (PEDOT/PSS). The highest power conversion efficiency of the PSCs basedon CuI layer reached 4.15% under the illumination of AM 1.5G, 100 mW/cm2.Furthermore, the pristine CuI layer could be applied in PSCs immediately,which shortened the production time of PSCs.

■ INTRODUCTION

Polymer solar cells (PSCs) as a potential candidate for solarenergy conversion technologies have attracted considerableattention in recent years due to their advantages, such asmechanical flexibility, low-cost, and easy fabrication.1−5 Despitethe potential of PSCs, the highest power conversion efficiencies(PCE) of 9−10% obtained in recent research6 are still notsatisfactory for commercial applications. To improve theperformance of PSCs, a variety of strategies have beenproposed, including the design of new light-harvestingmaterials,7,8 the new process of controlling film morphology,9,10

and the optimization of device structures.11 In addition, theinterface between polymer materials and electrodes is crucialfor device performance. One of the promising strategies is toinsert an interfacial buffer layer12−14 between polymer materialsand electrodes, which can improve PSCs by tuning the workfunction of electrodes,15,16 preventing undesired quenching ofexcitons in the surface of electrodes17 or diminishing therecombination of photogenerated carriers.18

In conventional PSCs, the most frequently used anodeinterfacial buffer layer is poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) due to its high con-ductivity, high transparency, and high work function.19,20

Nevertheless, the acidic and hygroscopic nature of thePEDOT/PSS could cause device degradation.21,22 Thesedeficiencies have led to extensive exploration of new materialsto replace PEDOT/PSS. Inorganic materials such as NiO,17

MoO3,23,24 V2O5,

25 and WO326 have been extensively used as

anode buffer layers for their ambient stability and suitable

optical and electrical properties. Meanwhile, comparable oreven higher PCE has been achieved by incorporating inorganicmaterials in PSCs comparing with the devices based onPEDOT/PSS. Due to the insolubility in most solvents,inorganic materials are usually deposited by vacuumtechniques,17,27 which are substantially more expensive andmore complicated than solution-based approaches. To formthese inorganic buffer layers, several solution processingmethods were reported by using different inorganic precur-sors,28 nanoparticles,29 or colloidal particles.30 And yet, hightemperature treatment or long annealing time is necessary forpost processing of the solution-processed inorganic buffer layerin order to remove solvent and achieve the crystalline phase.These preparation processes of inorganic buffer layer arecomplicated and incompatible with high throughput and scale-up production. There are only a few studies on the use ofpristine or low temperature solution-processed inorganicmaterial films for anode buffer layers31,32 in PSCs. Lately,Zilberberg et al.25 demonstrated an isopropyl alcohol solutionprocessed vanadium oxide layer as anode buffer layer withoutany postdeposition heat treatment. The devices based onvanadium oxide layer had comparable efficiency and substan-tially higher stability, comparing with those devices using

Special Issue: Michael Gratzel Festschrift

Received: December 30, 2013Revised: April 17, 2014

Article

pubs.acs.org/JPCC

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PEDOT/PSS. However, the V2O5 layers were fabricated atambient air for 1 h to undergo hydrolysis, which could not beapplied in PSCs immediately after spin-coating. Therefore,there is an urgent need to find new inorganic interfacialmaterials amenable to solution-processing of PSCs.Cuprous iodide (CuI) with a wide band gap of 3.1 eV has

three crystalline phases of α, β and γ. Among the differentcrystalline phases, γ-phase CuI behaves as a p-type semi-conductor. It acts as an I-VII semiconductor with zinc blendestructure. Currently, γ-CuI has been presently applied in thefabrication of light emitting diodes,33,34 organic catalysts,35,36

and vacuum fluorescent displays.37 Meanwhile, to solve theproblems of electrolyte leakage due to the incomplete sealingand electrode corrosion caused by the use of liquid electrolyte,CuI also has been employed as the hole conductor in solid-state, dye-sensitized38−40 and perovskite solar cells.41 Recently,Xie et al.42 first introduced CuI by thermal evaporation asanode buffer layer to fabricate P3HT/PCBM blend solar cells.They found that the CuI buffer layer not only provides anOhmic contact with the active layer, but also induces the self-organization of P3HT chains into a well-ordered structure.Although the PSCs with solution-processed CuI interfacial layerwas obtained by spin-coating CuI nanoparticles dispersed inethanol, the PCE of 2.6% is poorer compared with the 3.1%achieved by the device with the vacuum deposited CuI layer.In this paper, we introduced a solution-processed CuI film as

the anode buffer layer of PSCs based on poly(3-hexylth-iophene) (P3HT) as donor and [6,6]-phenyl-C60-butyric acidmethyl ester (PCBM) as acceptor. The fabrication of anodebuffer layer was a simple and time-saving procedure withoutany post-treatment. The influence factors of the morphologiesof CuI layers and the corresponding devices have beenoptimized and investigated carefully.

■ EXPERIMENTAL SECTION

Materials. Indium tin oxide (ITO) coated glass substrateswith a sheet resistance of 24 Ω/sq and an ITO thickness of 64nm were purchased from CSG Holding Co., Ltd. CuI (purity>99%), PEDOT/PSS (Clevious P VP AI 4083), P3HT(Regioregular), and PCBM (>99%) were purchased fromSigma-Aldrich, H. C. Stark Company, Rieke Metals, and Sigma-Aldrich, respectively. All these commercially available materialswere used as received without further purification.Device Fabrication. The ITO-coated glass substrates were

cleaned ultrasonically in detergent, deionized water, acetone,and isopropyl alcohol sequentially and ultraviolet-ozone treatedfor 10 min. The PEDOT/PSS aqueous solution was filteredthrough a 0.45 μm filter and spin-coated at 4000 rpm for 60 son the cleaned ITO substrates. Subsequently, the PEDOT/PSSfilm (about 40 nm) was baked at 140 °C for 10 min in theoven. The CuI films were prepared by spin-coating (1000,3000, 5000 rpm) with copper iodide acetonitrile solution (10mg/mL) on the ITO electrode (i.e., CuI-1000, CuI-3000, CuI-5000, respectively). Annealed CuI layer (a-CuI) was fabricatedthrough baking at 100 °C on a hot plate for 10 min comparedwith pristine CuI layer. The bulk-heterojunction layer was spin-cast onto the PEDOT/PSS or CuI layer at a coating speed of1000 rpm for 60 s from a chlorobenzene solution containing 10mg/mL of P3HT and 8 mg/mL PCBM (or 20 mg/mL ofP3HT and 16 mg/mL PCBM). The substrates were annealedat 150 °C for 10 min. Finally, they were transferred to a vacuumchamber and 0.5 nm of LiF and 70 nm of Al were thermally

deposited on the photoactive layer under a vacuum of 5 × 10−5

Pa. The active area of the device was 5 mm2.Characterization of the Thin Films and the Perform-

ance of the Solar Cells. The current−voltage curves of solarcells were measured by Keithley 4200 SemiconductorCharacterization System and illuminated by a standard siliconsolar cell calibrated Oriel 300 W solar simulator (Thermo Oriel91160−1000) with AM 1.5G filter at an intensity of 100 mW/cm2. All the photovoltaic performance measurements wereperformed in ambient air. The thickness of the thin films wascharacterized by using a KLA-Tencor α-Step Surface Profiler.SEM images and AFM images for the buffer layers wereobtained using Hitachi S-4800 and SPA400 SPM (SeikoInstrument Inc.). The optical transmittance and the workfunction of PEDOT/PSS and CuI thin film were measured on aUV-3100 spectrophotometer (Shimadzu) and by ultravioletphotoelectron yield spectroscopy (Riken Keiki). The X-raydiffraction (XRD) patterns of the deposited CuI films wererecorded on a D/MAX-2000 X-ray diffractometer withmonochromated Cu Kα irradiation (λ = 1.5418 Å). Unlessotherwise specified, the CuI films for characterization anddevice fabrication were prepared under the optimal spin-coatingspeed of 3000 rpm and without heat treatment.

■ RESULTS AND DISCUSSIONFigure 1 shows the X-ray diffraction patterns of CuI-3000 thinfilms (before annealing) and a-CuI-3000 films (after annealing).

Both of the CuI films exhibited an intense peak at the (111)reflection, which was assigned to γ-phase with a zinc blendeface centered cubic structure.43 Comparing the later to theformer, a significant increase in the intensity of the CuI (111)reflection was observed and another weak peak at the (222)plane was found in a-CuI-3000 films, which indicated that thecrystallinity of CuI film was improved after the annealingprocess at 100 °C. The average crystallite size at the CuI (111)plane could be obtained by applying Scherrer’s formula. Forannealing-free film, the average diameter of CuI crystallite canbe calculated to about 31 nm. The particle size of CuI increasedto 43 nm after the thermal treatment. And the average diameterof CuI crystallite was basically not influenced by the spin-coating speed.The transmission spectra of PEDOT/PSS and various CuI

films on quartz substrates are shown in Figure 2. All of thesefilms were highly transparent in the visible range between 400and 800 nm. Except for CuI-1000 film, other CuI filmsexhibited high transparency of more than 95%, which was

Figure 1. XRD patterns of CuI-3000 thin film (a) and a-CuI-3000 film(b).

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better than the transmittance of 90% obtained by the PEDOT/PSS film. The transmittance of CuI films reduced slightly afterheat treatment, due to the enhancement of scattering caused bythe increase of the crystallization and grain size of CuI films,which is in good agreement with the XRD results. An obvioushump at about 408 nm was observed in a-CuI-3000 film, whichmay be caused by the excitation of electrons from the sub-bands to the conduction bands of CuI.44 With the increase ofthe spin-coating speed, CuI films exhibited better transmittancein visible region due to the decrease of CuI thickness.The open-circuit voltage (Voc) of PSCs is related to the

difference between the highest occupied molecular orbital(HOMO) of the donor and the lowest unoccupied molecularorbital (LUMO) of the acceptor, only when Ohmic contactsare formed in interface. Introducing the interfacial buffer layerscan be an effective way to adjust the work functions ofelectrodes to achieve Ohmic contact. The ultraviolet photo-electron yield spectroscopy was used to assess for the workfunction of different anode interfacial layers (see Figure S1 inthe Supporting Information). The HOMO energy level of CuI-3000 film was found to be 5.1 eV from the photoemissioncutoff, which was slightly smaller than the previously reportedvalue.42 Although the measured work function of CuI (5.1 eV)was a bit lower than the work function of PEDOT/PSS (5.3eV), both of them were higher than the work function of ITO(4.6 eV) and the HOMO of P3HT (5.0∼5.2 eV),7 whichimproved the alignment of the levels of energies in the interfacebetween the anode and the donor materials and facilitated thehole extraction. Besides, the LUMO level of CuI (2.0 eV) washigher than most of the donor materials. Consequently, it canact as an electron blocking layer compared with the inadequatecapability of PEDOT/PSS.45,46

Since the adhesion between the anode buffer layer and activelayer is critical for good contact and stability, the water contactangle was measured on the disparate materials to explore thesurface properties (see Figure S2 in the SupportingInformation). It was found that the water contact angle variedfrom 14.3° for PEDOT/PSS film to 52.7° for CuI-3000 film,which indicated that the surface of CuI-3000 film was morehydrophobic than the surfaces of PEDOT/PSS film.42 Themore hydrophobic CuI could provide a closer contact to thefollowed organic materials than that of PEDOT/PSS layer.To achieve a better understanding of the effect of interfacial

layers on device performance, the morphologies of the differentbuffer layers were observed by AFM and SEM. The surfacetopographies of ITO, ITO/PEDOT/PSS, ITO/CuI-3000, andITO/a-CuI-3000 are shown in Figure 3. Moreover, AFM

images in Figure S3 illustrated the three-dimensional structureof anode interfacial layers. The root-mean-square (RMS)roughness of bare ITO was 1.2 nm, while the RMS roughnessslightly increased to 1.5 nm after being covered with thePEDOT/PSS layer. The flat surface was in contrast with therough films spin-cast from CuI acetonitrile solution, in whichRMS roughness increased sharply to 12.3 nm. Obviously, theCuI particles grew larger and the RMS roughness (19.9 nm)increased after the annealing process at 100 °C for 10 min,which is consistent with the result of XRD. As shown in FigureS3, some of CuI particles on the anode were as high as 100 nm,which means they can poke through the subsequentlydeposited organic active layer (ca. 100 nm) to reach thecathode. Figure 4 shows the SEM images of the PEDOT/PSS,CuI-1000, CuI-3000, CuI-5000, and a-CuI-3000 deposited onthe ITO substrates. As shown, the PEDOT/PSS film exhibiteda highly smooth surface and covered the ITO completely.Unlike the PEDOT/PSS film, the morphology of CuI films wasdramatically influenced by spin-coating speed. The CuImolecules could form a more continuous film at low speedthan that formed at high speed. As shown in Figure S4, CuIparticles showed a tendency to form discontinuous CuI islandswith the increase of the spin-coating speed. Moreover, therewere many cracks in the CuI film after the heat treatment,which could reduce the anode interfacial layer coverage for ITOsubstrates.J−V characteristics of photovoltaic devices with a structure of

ITO/CuI-1000 (noted as Device A), CuI-3000 (noted asDevice B), CuI-5000 (noted as Device C), PEDOT/PSS(noted as Device D), and a-CuI-3000 (noted as Device E)/P3HT/PCBM/LiF (0.5 nm)/Al (70 nm) were measured underdark and simulated AM 1.5 irradiation and are shown in Figure

Figure 2. Optical transmission spectra of PEDOT/PSS films andvarious CuI films on quartz substrates.

Figure 3. AFM topography images of (a) ITO, (b) ITO/PEDOT/PSS, (c) ITO/CuI-3000, and (d) ITO/a-CuI-3000.

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5, and the photovoltaic parameters of the correspondingdevices are summarized in Table 1. As shown, the Device Dfabricated with PEDOT/PSS layer exhibited an open circuitvoltage (Voc) of 0.627 V, a short circuit current (Jsc) of 8.88mA/cm2, a fill factor (FF) of 50.7%, and a power conversionefficiency (PCE) of 2.82%. In contrast, other devicesincorporated with CuI layer exhibited the enhanced PCEthan that of devices based on PEDOT/PSS layer. Theimprovement of device performance was basically attributedto the increase in FF. As a matter of fact, a shunt resistance, Rsh,and a series resistance, Rs, play a significant part in FF, which isone of the most important factors affecting performance ofdevices. An increase in Rsh, a decrease in Rs, or both is essential

to achieving a high FF. According to Table 1, the devices withCuI layer displayed a lower Rs compared with device based onPEDOT/PSS layer, which resulted in higher FF and PCE. Inaddition, the J−V curves in the dark showed a typical diodebehavior in all of the three devices in Figure 5. We observedthat the dark current density in CuI devices was obviouslyhigher than that of in PEDOT/PSS device with the additionalvoltage increases. It can be explained by the work function ofPEDOT/PSS measured as 5.3 eV, which was much higher thanthe work function of ITO (4.6 eV) and created a high energybarrier between the ITO electrode and PEDOT/PSS. Thework function of 5.1 eV for the CuI layer was more beneficialfor improving the energy level alignment at the interfacebetween the ITO electrode and active layer, where it can forman Ohmic contact and facilitate the hole transportation andcollection. The device structure of the PSCs and the schematicenergy diagram of the materials in devices are illustrated inFigure 6. The lowest unoccupied molecular orbital (LUMO) ofCuI layer was estimated to be 2.0 eV from the work function(5.1 eV) and band gap (3.1 eV) of CuI, which was lower thanthat of PCBM and thus easy to block electrons away from ITOelectrode. On the contrary, the electron blocking capability ofPEDOT/PSS is uncertain since PEDOT/PSS could beconsidered as an electron collecting electrode.45,46 In additionto the above parameters, the hydrophobic nature of CuI canprovide better compatibility with the subsequent hydrophobicorganic material like P3HT, which could promote an orderedgrowth of P3HT layer and reduce the contact resistancebetween the anode interfacial buffer layer and active layer.47

The devices showed high FF and PCE, but poor Voc, when theanode buffer layer of PEDOT/PSS was replaced by CuI. At thesame time, we noted the CuI layer induced a decrease of Rshdue to some leakage paths, which may be one reason48,49 forthe decrease of Voc. As shown by the SEM study, the ITOelectrode could be entirely covered with PEDOT/PSS, but notby CuI, which formed some leakage paths from bare ITO to theactive layer. Furthermore, the leakage paths can also beconfirmed by the AFM study. It showed that the small islands,which formed a high density of peaks, were observed in CuIlayer, and they could dramatically affect the active layer andthus cause large leakage current and small Rsh. After the CuIfilm was annealed for 10 min, the Rsh and Voc in Device Ebecame smaller owing to the rougher surface, as shown inFigure 3. Besides, the performance of CuI devices performancewas influenced by the morphology of CuI layers obtained at

Figure 4. SEM images of (a) the PEDOT/PSS, (b) CuI-1000, (c)CuI-3000, (d) CuI-5000, and (e) a-CuI-3000 deposited on ITOsubstrates.

Figure 5. J−V curves of devices with a structure of ITO/(differentanode buffer layer: (A) CuI-1000; (B) CuI-3000; (C) CuI-5000; (D)PEDOT/PSS; (E) a-CuI-3000) /P3HT/PCBM/LiF (0.5 nm)/Al (70nm), under dark (hollow) and simulated AM 1.5 irradiation (solid).

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different spin-coating speeds. With the increase of the speed ofCuI from 1000 to 3000 rpm, PCE of the devices with CuIlayers increased from 2.80% for Device A to 3.41% for DeviceB, and the Rs decreased from 14.3 to 7.86 Ω·cm2. This fact maybe caused by the reduced thickness of CuI with the increase ofthe spin-coating speed. On the other hand, with the furtherincrease of the spin-coating speed to 5000 rpm, PCE and FF ofDevice C decreased dramatically to 2.90 and 49.2%,respectively. Moreover, the Rsh dropped markedly from 453to 242 Ω·cm2, whereas the Rs increased from 7.86 to 9.12 Ω·cm2. It was found that the uncovered area of ITO distinctlyincreased with the increasing spin-coating speed, which led to alower Voc and smaller Rsh.The PCE data of Device B and Device D fabricated with CuI

(at a spin-coating speed of 3000 rpm and without thermaltreatment) and PEDOT/PSS, respectively, was subjected topairwise t test. The results of pairwise t test are shown inSupporting Information, Table S1. As can been seen from thetable, the P-value is 0, which is less than 0.05 and means there isa significant difference in PCE between CuI-based devices andPEDOT/PSS-based devices. As the mean of PCE of CuI-baseddevice is higher than that of PEDOT/PSS-based device, CuI isa better anode interfacial layer than PEDOT/PSS.

The images of SEM and AFM showed that the surface of CuIfilm was rough, and the height of particles could be as high as100 nm. These large particles could poke through the organicactive layer to reach the cathode, which could lead to increasethe chance of direct shorts and local high fields. Consequently,the carrier recombination and leakage current could occur indevices. In order to reduce the chances of the direct contactbetween the CuI layer and the Al cathode, the thickness oforganic active layer was increased by spin-coating with achlorobenzene solution containing 20 mg/mL of P3HT and 16mg/mL PCBM. In these devices, the CuI layers were preparedat a spin-coating speed of 3000 rpm and without thermaltreatment. The maximum PCE of 4.15%, along with a Voc of0.61 V, a Jsc of 11.21 mA/cm

2, and a FF of 60.5%, was achievedand the J−V curve under white light illuminations is shown inFigure 7, and also, the photovoltaic parameters of PEDOT/PSS

and CuI devices with the doubled P3HT/PCBM concentrationis displayed in Supporting Information, Table S2. Comparedwith the data in Table 1, both of the Jsc of PEDOT/PSS andCuI devices were increased due to the more light absorbedfrom the increased thickness of organic layer. The degree ofincrease of Jsc was greater in CuI devices owing to the greaterinterface between the CuI layer and organic layer, where itmade a contribution to the separation of charge carrier.Meanwhile, the FF of the PEDOT/PSS device was decreasedowing to a larger series resistance of the thicker organic layer.However, the FF of CuI device was remained unchanged sincethe CuI layer could improve P3HT chain packing andcrystallization which may facilitate hole extraction and leadsto a decreased interfacial resistance.42 And there will be plentyof room for improving the PCE of devices through controllingthe surface morphology of CuI layer, which are actively pursuedby us.

Table 1. Photovoltaic Parameters of PSCs with Various Anode Buffer Layers (Device A, CuI-1000; Device B, CuI-3000; DeviceC, CuI-5000; Device D, PEDOT/PSS; Device E, a-CuI-3000)

device Voc (V) Jsc (mA/cm2) FF (%) PCE (%) best PCE (%) Rs (Ω cm2)a Rsh (Ω cm2)a

A 0.622 ± 0.007 8.67 ± 0.39 52.1 ± 1.4 2.80 ± 0.12 2.99 14.3 427B 0.603 ± 0.010 9.77 ± 0.14 58.1 ± 2.6 3.41 ± 0.13 3.60 7.86 453C 0.605 ± 0.006 9.75 ± 0.29 49.2 ± 2.2 2.90 ± 0.10 3.15 9.12 242D 0.627 ± 0.006 8.88 ± 0.31 50.7 ± 2.5 2.82 ± 0.11 2.97 20.8 2488E 0.580 ± 0.012 9.92 ± 0.21 56.7 ± 1.6 3.27 ± 0.12 3.50 8.13 386

aThe series resistance, Rs, stands for the slope of the J−V curve at J = 0 for the best device and the shunt resistance, Rsh, stands for the slope of the J−V curve at J = Jsc for the best device.

Figure 6. (a) Device structure of polymer solar cells. (b) Schematicenergy diagram of various materials in devices.

Figure 7. J−V curve of PSC with the ITO/CuI-3000/P3HT/PCBM(20:16 mg/mL)/LiF (0.5 nm)/Al (70 nm).

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In conclusion, we demonstrated a high-efficiency P3HT/PCBM solar cell incorporating CuI as an anode buffer layer bya simple solution process. The evaluations of CuI layers andPSC performance indicated that the CuI layers could act as anefficient anode interfacial layer, which showed higher perform-ance than the devices based on PEDOT/PSS layers. Theenhanced device performance was due to the improvement ofthe Jsc and FF. Furthermore, higher efficiency could be realizedwithout any heat treatment for CuI layer, which indicated thepreparation of CuI layer could be a convenient and time-savingprocess.

■ ASSOCIATED CONTENT*S Supporting InformationThe UPS of bare ITO, PEDOT/PSS, and CuI layers, watercontact angle of PEDOT/PSS and CuI layers, AFM images ofthe three-dimensional structure of ITO and different anodebuffer layers, and SEM images of CuI films obtained at differentrates are discussed. This material is available free of charge viathe Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: bianzq@pku.edu.cn. Tel.: (+86)-010-62753544.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Basic ResearchProgram (2011CB933303) and the National Natural ScienceFoudation (NSFC) of China (90922004).

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