Efficient PEDOT:PSS-Free Polymer Solar Cells with an EasilyAccessible Polyacrylonitrile Polymer Material as a Novel Solution-Processable Anode Interfacial LayerYong-Jin Noh,† Sae-Mi Park,† Jun-Seok Yeo,‡ Dong-Yu Kim,‡ Seok-Soon Kim,*,§ and Seok-In Na*,†

†Professional Graduate School of Flexible and Printable Electronics, Polymer Materials Fusion Research Center, Chonbuk NationalUniversity, 664-14 Deokjin-dong, Deokjin-gu, Jeonju-si, Jeollabuk-do 561-756, Republic of Korea‡Heeger Center for Advanced Materials (HCAM), School of Material Science and Engineering, Gwangju Insititute of Science andTechnology, Gwangju 500-712, Republic of Korea§Department of Nano and Chemical Engineering, Kunsan National University, Kunsan, Jeollabuk-do 753-701, Republic of Korea

*S Supporting Information

ABSTRACT: We demonstrate that an easily accessiblepolyacrylonitrile (PAN) polymer can efficiently function as anovel solution-processable anode interfacial layer (AIL) toboost the device performances of polymer:fullerene-based solarcells (PSCs). The PAN thin film was simply prepared withspin-coating of a cost-efficient PAN solution dissolved indimethylformamide on indium tin oxide (ITO), and the thinpolymeric interlayer on PSC parameters and stability weresystemically investigated. As a result, the cell efficiency of thePSC with PAN was remarkably enhanced compared to thedevice using bare ITO. Furthermore, with PAN, we finally achieved an excellent power conversion efficiency (PCE) of 6.7% anda very high PSC stability in PTB7:PC71BM systems, which constitute a highly comparable PCE and superior device lifetimerelative to those of conventional PSCs with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Theseresults demonstrate that the inexpensive solution-processed PAN polymer can be an attractive PEDOT:PSS alternative and ismore powerful for achieving better cell performances and lower cost PSC production.

KEYWORDS: polymer solar cells, bulk heterojunction, polymeric interlayer, polyacrylonitrile, anode interfacial layer,work-function modification

1. INTRODUCTION

Polymer-based bulk-heterojunction (BHJ) solar cells (PSCs)have been regarded as a next-generation solar energy sourcebecause of their flexibility, solution processability and mass-production ability.1−8 However, for real commercialization, thepower conversion efficiency (PCE) and cell stability of PSCswill need to be further improved.6−8 To this end, developinginterfacial layers between electrodes and BHJ is crucial becausethe PSC performances such as open circuit voltage (Voc), shortcircuit current density (Jsc), fill factor (FF), and cell lifetime arehighly dependent on the interlayers.9−12 For these reasons,significant and challenging studies on electrode modificationhave been conducted by introducing an interfacial layer, and inparticular, the metal oxides and carbon-based materials havebeen actively studied as a promising interfacial layer in PSCsbecause of their solution processability, low-temperatureprocesses, and excellent selective carrier extraction.12−22

Recently, besides metal oxides and carbon-based materials,solution-processable polymeric materials could also beemployed as an efficient electrode interfacial modifier to realizelow-cost printable solar cells via a simple and solution-processed nonvacuum process.23−31 In particular, it has been

well-demonstrated that conjugated-polyelectrolyte-based thinpolymeric interlayers with a thickness of a few nanometers canform an interfacial dipole and induce a vacuum level shift ofadjacent metal electrodes,32,33 resulting in an increase of thebuilt-in potential, Voc, and PCE.24−26 This excellent work-function tunability of the conjugated polyelectrolytes couldrender them widely applicable as interfacial materials. However,the conjugated polyelectrolytes still have some disadvantages.For example, to synthesize such ionic conjugated polymericmaterials, relatively time-consuming multistep synthesis pro-cesses are typically required.26 In addition, most of the solution-processed polymer materials have been used as a cathodeinterfacial layer in PSC as mentioned in the recent paper.28

Studies on the uses of the polymeric materials as anodeinterfacial materials are very limited, and the subject has rarelybeen addressed, despite the urgent need to replace the widelyused poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) with a high hydroscopic material having an

Received: January 22, 2015Accepted: October 21, 2015Published: October 21, 2015

Research Article

www.acsami.org

© 2015 American Chemical Society 25032 DOI: 10.1021/acsami.5b07841ACS Appl. Mater. Interfaces 2015, 7, 25032−25038

acidic nature. Therefore, studies on cost-efficient polymericlayers prepared using a simple synthesis process as well asstudies on their use as anode interfacial layers (AILs) could behighly valuable for low-cost and high-performance PSCs.In this study, we demonstrate that an easily accessible

polyacrylonitrile (PAN) polymeric film can effectively act as anovel AIL to enhance overall PSC performances. The PANsimply synthesized via free radical polymerization of acryloni-trile has been well-known as one of the least expensiveprecursors for the fabrication of various carbon materials suchas carbon fiber and carbon-based transparent electrodes, andbasically, PAN also has desirable properties for use as interfaciallayers such as good solubility to common solvents, goodformation of thin film, and good chemical stability.34−38 Inaddition, Summan demonstrated that the electrical conductivityof PAN materials can be improved with increasing treatmenttemperature owing to enhanced conjugation of carbon−nitrogen in PAN materials.39 Recently, Jiao et al. reportedthat the work functions of PAN materials were controlled from4.66 to 4.48 eV with increasing temperature, indicating that thework functions of PAN can be suitable for carrier injection andextraction in organic devices.40 From these observations, inparticular considering that PAN work function and conductivitycan be easily tunable and also that PAN already possesses thebasic requirements for use as anode interfacial materials, it wasbelieved that PAN could be a promising AIL candidate in PSCs.Nevertheless, the study of PAN as interfacial material and itsreal application in electronic devices have seldom beenaddressed.37,39,41 In addition, no studies have been conductedon PAN material as an anode interfacial layer in polymer solarcells. Herein, we actively investigated PAN materials as AILsand systemically studied theirs effects on PSC parameters andstability. As a result, the cell efficiencies of the PSCs with PANAILs were remarkably enhanced compared with those of thePSCs using bare ITO. Furthermore, with PAN, we finallyachieved an excellent power conversion efficiency of 6.7% and avery high PSC stability based on PTB7:PC71BM, whichconstitute highly comparable cell efficiency and superior PSC

lifetime relative to those of to reference PSCs with theconventional PEDOT:PSS. These observations demonstratethat the inexpensive solution-processed PAN polymer can be aviable PEDOT:PSS alternative material and more beneficial forachieving better cell performance and lower-cost PSCproduction.

2. EXPERIMENTAL DETAILSFor the fabrication of the polymer:fullerene-based PSCs, typically withthe ITO/AIL/BHJ/Ca (20 nm)/Al (100 nm) multilayer stackingshown in Figure 1a, the patterned ITO (∼10 ohm/sq.) substrates werefirst cleaned with acetone, deionized water, and isopropyl alcohol. Thesubstrates were then dried at 100 °C for 30 min and UV−ozonetreatments were applied for 30 min. The PAN powder (150 000 Mw,Sigma-Aldrich) was dissolved in dimethylformamide (DMF) tofabricate 0.01, 0.1, 0.5, and 1 wt % PAN solutions. The PAN solutionswere spin-coated at 5000 rpm for 40 s onto ITO substrates and driedat 350 °C for 30 min under an air atmosphere for a typical stabilizationprocess.37,42 The thickness values of PAN films prepared with 0.01,0.1, 0.5, and 1 wt % PAN solution were ∼0.5, ∼1, ∼2, and ∼5 nm,respectively. PEDOT:PSS (Clevios P VP AI 4083, Heraeus) as thereference AIL was also prepared by spin coating at 5000 rpm for 40 s,followed by drying at 120 °C for 10 min. The photoactive layers(∼230 nm) based on poly(3-hexylthiophene) (P3HT, 4002-EE,50 000−70 000 Mw, ∼96% regioregularity, Rieke Metals) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM, 99.5% purity, Nano-C)were then fabricated by spin-coating of the blend solution mixed withP3HT (25 mg) and PCBM (25 mg) in 1,2-dichlorobenzene (DCB, 1mL) at 700 rpm for 60 s. This was followed by solvent annealing for120 min and thermal annealing at 110 °C for 10 min in N2. For theformation of another active layer, a BHJ mixed solution composed of10 mg of thieno[3,4-b]thiophene/benzodithiophene (PTB7, ∼97 000Mw, 1-Material), 15 mg of [6,6]-phenyl C71-butric acid methyl ester(PC71BM, > 99% purity, Nano-C), and 1,8-diiodictance (5 vol %,Tokyo Chemical Industry Co. Ltd.) in 1 mL of chlorobenzene (CB)was also prepared, and the resulting blend solution was spin-coated at2000 rpm for 60 s for the fabrication of a PTB7:PC71BM layer (∼100nm). Finally, the thermal evaporation of a metal cathode having Ca(20 nm)/ Al (100 nm) was performed under vacuum at 10−6 Torr.The photocurrent density (J)−voltage (V) power curves of thefabricated solar cells were measured using Keithley2400 equipment

Figure 1. (a) Device structure of the tested polymer solar cells and (b) representative current density−voltage (J−V) curves of P3HT-based PSCswith various PAN films and PEDOT:PSS. The inset shows the Rs data of various PAN films in P3HT-based PSCs. Influence of PAN thickness on (c)PCE and FF and (d) Jsc and Voc of P3HT-based PSCs.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.5b07841ACS Appl. Mater. Interfaces 2015, 7, 25032−25038

25033

and a solar simulator (Class AAA) under the standard AM 1.5G and100 mW/cm2 measurement condition. The PSC stability was alsorecorded without encapsulation according to the ISOS-D-1 protocol,measured as a function of exposure time in air under ambienthumidity/temperature and AM 1.5G solar simulator.43 The surfaceimages, work function, and UV−vis transmission data of each filmwere investigated using atomic force microscopy (AFM, VeecoDimension 3100), a Kelvin probe (KP 6500, McAllister TechnicalServices. Co. Ltd.), X-ray photoelectron spectroscopy (XPS, ThermoScientific K-Alpha), and a Scinco S-3100 spectrophotometer,respectively. More detailed PSC fabrication and measurement weredescribed in our previous reports.17,44

3. RESULTS AND DISCUSSION

To investigate the ability of PAN to work as AILs and thethickness effects of PAN AIL on solar cell parameters, PSCswith and without PAN interlayers were fabricated andcompared. As can be seen in Figure 1 and Tables 1 and 2,the FF and Jsc values in PAN-based PSCs tended to decreasewith increasing thickness of interfacial materials, with theexception of the FF of 0.5 nm PAN, whereas Voc in PAN-basedPSCs showed similar values, irrespective of the AIL thickness.This demonstrates that the overall device parameters can bestrongly affected by the AIL film thickness; thus, PAN-thicknessfor the use of AILs should be optimized. As shown in Figure 1c,the 0.5 nm PAN-based PSC showed poor cell efficiency of∼2.5%, probably because of the fully uncovered PAN film onthe ITO surface, which can result in a nonuniform contact atBHJ/electrode interfaces, thus producing higher seriesresistance (Rs), lower shunt resistance (Rsh), and lowerFF,44,45 which can be confirmed in the J−V curve and filmmorphology of PAN in Figures 1b and S1. With increasingPAN thickness, PAN-based PSCs showed more enhanced cell

efficiencies of up to an average PCE value of ∼3.6%, probablybecause of a more uniform and fully covered PAN film.15,45

However, the further increase of AIL thickness beyond ∼1 nmreduced the cell efficiencies, mainly because of the lower FFand Jsc, which could be induced by the reduced transmittancewith an increase in AIL film thickness and the increased surfaceroughness as shown in Figures S1 and S2;15,45 beyond 1 nmPAN, the transmittance and rms roughness were changed from84.59% and ∼0.72 nm to 82.27% and ∼2.94 nm (2 nm PAN)or 75.18% and ∼13.9 nm (5 nm PAN), respectively. Inparticular, as shown in the inset of Figure 1b, as PAN thicknessincreased up to ∼5 nm, the Rs value was further increased,probably because of the charge blocking effect shown in aninsulating material thicker than the monolayer,46 which resultedin the relatively low FF shown in the 5 nm PAN-based device.Consequently, the best cell efficiency was obtained in the PSCwith 1 nm PAN, and more importantly when considering thatPSCs with PAN AILs showed dramatically enhanced devicecharacteristics compared with the PSCs with no PAN, it can beconfirmed that the easily accessible and cost-efficient PANmaterial can work effectively as the anode/BHJ interfacial layerto provide a high-PCE device.For a better feasibility test, we further investigated PAN-

based PSCs and directly compared them with the conventionalreference PSCs using PEDOT:PSS AIL materials. Variances inthe PSC parameters (PCE, Voc, FF, and Jsc) resulting fromchanging the AIL are summarized in Figure 2. With the use ofAIL materials, all of the PSC performances were improved,which resulted from the significant enhancement in the Voc andFF values and the relatively small enhancement in Jsc values, asdepicted in Figure 2; the solar cell with bare ITO had anaverage PCE of 1.148%, an average Voc of 0.423 V, an average

Table 1. Representative Cell Performances of P3HT-Based PSCs with Various PAN Films

Voc (V) Jsc (mA/cm2) FF (%) PCE (%) Rs (Ω cm2)

ITO 0.449 8.751 29.03 1.140 13.940.01 wt % PAN 0.571 10.38 42.51 2.52 4.1090.1 wt % PAN 0.606 10.09 58.48 3.575 3.8570.5 wt % PAN 0.582 10.07 53.38 3.130 4.8051 wt % PAN 0.577 9.543 42.34 2.331 8.674PEDOT:PSS 0.608 9.901 61.13 3.681 3.204

Table 2. Average and Standard Deviation Values of Photovoltaic Parameters of P3HT-Based PSCs with PAN Interfacial Layersas a Function of Concentration

Voc (V) Jsc (mA/cm2) FF (%) PCE (%)

0.01 wt % PAN 0.563 (±0.011) 10.3725 (±0.013) 42.905 (±0.559) 2.504 (±0.023)0.1 wt % PAN 0.606 (±0.004) 10.04 (±0.051) 59.49 (±1.035) 3.621 (±0.053)0.5 wt % PAN 0.583 (±0.002) 10.040 (±0.082) 52.185 (±1.044) 3.055 (±0.068)1.0 wt % PAN 0.566 (±0.009) 9.414 (±0.129) 37.313 (±4.359) 1.993 (±0.293)

Figure 2. Influence of different interfacial layers on the (a) PCE and FF and (b) Jsc and Voc of P3HT-based PSCs.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.5b07841ACS Appl. Mater. Interfaces 2015, 7, 25032−25038

25034

FF of 30.66%, and an average Jsc of 9.067 mA/cm2, whereasPAN raised the average PCE, Voc, FF, and Jsc up to 3.621%,0.606 V, 59.49%, and 10.04 mA/cm2, respectively. Morenotably, the device performances in PAN-based PSCs closelyapproached those of the reference PEDOT:PSS-AIL-basedPSCs.To further study why PAN raised the PSC performances,

various investigations on the work function (WF), XPS,transmittance, surface AFM morphology, Rs, and Rsh wererecorded as shown in Figure 3. As shown in Figure 3a, aconsiderable change was observed in the WFs. However, asshown in Figures 3c,d, the corresponding transmittance andsurface morphology, which can also have a high impact on PSCperformances,44 did not show a significant difference; thetransmittance at 500 nm and the rms roughness of the bareITO, PAN, and PEDOT:PSS PSCs were 87.66% and 0.69 nm,84.59% and 0.72 nm, and 85.53% and 0.90 nm, respectively.The ITO WF was 4.66 eV, but PAN presented an increasedWF value of 5.10 eV, very similar to that of the PEDOT:PSSPSC of 5.09 eV. As shown in Figure 3b, the C 1s and N 1s XPSdata of the pristine PAN material showed a strong CN triple-bond peak. In contrast, the stabilized PAN exhibited variouspeaks with CN, CO, and NO bonds: the C 1s peak of stabilizedPAN exhibited C−C (285 eV), C−N (285.9 eV), C−O (286.9eV), CO (288.2 eV), and C−(O)−O (289.77 eV), and theN 1s peak of stabilized PAN showed C−N (398.5 eV), CN(401 eV), and N−O (403 eV).39,47−49 These changes shown inFigure 3b are in agreement with the previously reportedstabilization and oxidation reaction of PAN.37,39,47,48 Moreimportantly, considering that the CN, CO, and NO bonds withdifferent electronegativity can induce a high internal dipolemoment,17,26,50,51 it was believed that the larger WF of PANcould be due to the presence of the CN, CO, and NO bonds.

Furthermore, when considering that the anode−electrode WFand the HOMO level of the donor-conjugated polymer shouldbe better matched to achieve a maximum built-in potential asshown in the inset of Figure 3a52,53 thus improving the Voc, FF,and Jsc values,

9−13 it was believed that the larger WF of PANmight be responsible for the largely increased PSC perform-ances shown in PAN-based PSCs. In addition, theseobservations concurred with the Rs and Rsh results shown inFigure 3e; the introduction of PAN as an anode-interfacemodifier also had an impact on the Rs and Rsh values in solarcells. As shown in Figure 3e, the average Rs and Rsh values ofPSCs without AILs were 14.37 and 110.3 Ω cm2, respectively,whereas when PAN AIL was employed, the Rs and Rsh werelargely improved up to 4.637 and 1838 Ω cm2, respectively.Such values for Rs and Rsh closely approach those of thereference PSCs with PEDOT:PSS having an average Rs of 4.117Ω cm2 and average Rsh of 2021 Ω cm2, indicating that PAN filmcan also provide a better ohmic contact and effectively preventcharge-carrier recombination at the BHJ/ITO interface.25,45

Furthermore, from the overall results shown in Figure 3, thelargely improved PSC characteristics obtained in PAN-basedcells are believed to be mostly due to the increased built-inpotential between the anode and cathode electrodes throughthe use of PAN AILs to provide a better ohmic contact andcharge collection.9−13,25 More notably, the quantitativelysimilar device parameters of PAN-based PSCs relative tothose of the PEDOT:PSS-based cells effectively demonstratethat PAN can be as effective as the representative PEDOT:PSSAIL typically used for providing high efficiency in PSCs.To expand the value of PAN as a PEDOT:PSS alternative,

the PSC stability of PAN-based solar cells was investigatedaccording to air-exposure time and the ISOS-D-1 protocol,43

which was also compared with the reference PEDOT:PSS cell.

Figure 3. (a) Work functions of bare ITO, PAN, and PEDOT:PSS. The inset shows dark J−V characteristics of PSCs with and without interfaciallayer. (b) C 1s and N 1s XPS spectra of the pristine and stabilized PAN. (c) Optical transmission spectra of PAN and PEDOT:PSS on ITO. (d)AFM topographic images of ITO, PAN, and PEDOT:PSS. (e) Rs and Rsh data of various AIL materials in P3HT-based PSCs.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.5b07841ACS Appl. Mater. Interfaces 2015, 7, 25032−25038

25035

As depicted in Figure 4, although the PEDOT:PSS provided anexcellent device efficiency similar to that of PAN, theconventional PEDOT:PSS-based device exhibited a rapidefficiency degradation and was not working after 3600 min,whereas PAN-based PSC showed an excellent device lifetimewith the PCE maintaining ∼60% of its initial PCE value, evenafter 13 440 min. Considering that PEDOT:PSS, well-known tohave high acidity and hydrophilicity, experimentally showed apH of ∼2 and a contact angle of 11.03° and that suchPEDOT:PSS properties can result in a poor PSC stability, therelatively neutral (pH ∼8) and hydrophobic (contact angle =50.63°) properties of PAN material could be responsible for themore enhanced PSC lifetime compared with that ofPEDOT:PSS.17,54 To further expand the application ability ofPAN materials as AILs in solar cells, we also studied PAN AILand applied it to different BHJ-based PSCs withPTB7:PC71BM. As described in Figure 5, the PTB7:PC71BM-

based PSC with PAN AIL exhibited excellent PSC parameters,with a PCE of 6.70%, a Voc of 0.713 V, a Jsc of 15.19 mA/cm2,and a FF of 61.83%, and their values were very similar to thoseof the reference PTB7:PC71BM-based PSC with the widelyemployed PEDOT:PSS AIL, even in the low-band-gappolymer-based PSC, which showed a PCE of 6.905%, a Voc

of 0.790 V, a Jsc of 13.89 mA/cm2, and a FF of 62.87%. Similardevice-stability results and trends were also detected in PTB7-based PSCs, as shown in the bottom inset of Figure 5. Theseobservations indicate that PAN is sufficient to be used as AILsin various PSCs and more beneficial than PEDOT:PSS forgreater efficiency and stability of polymer photovoltaic cells.

4. CONCLUSIONSWe employed a cost-effective, solution-processed, and easilyaccessible polymer material, polyacrylonitrile (PAN), as aninterfacial layer between the polymer:fullerene active layer andITO anodes for boosting the device performance of polymersolar cells. The thin polymeric interlayer was systemicallyinvestigated to determine its feasibility as a new PEDOT:PSSalternative AIL, and as a result, the cell efficiency of the PSCwith PAN was remarkably enhanced compared with that of thedevice using bare ITO because of the enhanced built-inpotential and effective shielding effect from the direct contactbetween ITO and BHJ through the use of PAN AIL to providea better ohmic contact and charge collection. Furthermore, withPAN, we finally achieved a high power conversion efficiency of6.7% and an excellent PSC stability in PTB7:PC71BM systems,which constitute a highly comparable PCE and superior PSClifetime relative to those of the conventional referencePEDOT:PSS-based PSCs. These observations suggest thateasily accessible and solution-processed PAN is sufficient foruse as AILs for polymeric solar cells and more beneficial thanPEDOT:PSS for achieving better PSC performances and lower-cost PSC production.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.5b07841.

AFM topographic images and optical transmissionspectra of PAN with different concentrations (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: sskim@kunsan.ac.kr (S.-S.K.).*E-mail: nsi12@jbnu.ac.kr (S.-I.N.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis paper was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF)funded by the Ministry of Science, ICT & Future Planning(MSIP) (2013R1A1A1011880).

■ REFERENCES(1) Brabec, C., Scherf, U., Dyakonov, V., Eds. Organic Photovoltaics:Materials, Device Physics, and Manufacturing Technologies; Wiley:Weinheim, Germany, 2011.

Figure 4. Changes in the photovoltaic parameters of P3HT-based PSCs with PAN and PEDOT:PSS films under an ambient atmosphere. The insetshows the initial J−V curve of P3HT-based PSCs with PEDOT:PSS and PAN.

Figure 5. Representative J−V curves for PTB7-based PSCs with PANand PEDOT:PSS films. The inset shows the device stability of PTB7-based PSCs with PAN and PEDOT:PSS.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.5b07841ACS Appl. Mater. Interfaces 2015, 7, 25032−25038

25036

(2) You, J.; Chen, C.-C.; Hong, Z.; Yoshimura, K.; Ohya, K.; Xu, R.;Ye, S.; Gao, J.; Li, G.; Yang, Y. 10.2% Power Conversion EfficiencyPolymer Tandem Solar Cells Consisting of Two Identical Sub-Cells.Adv. Mater. 2013, 25, 3973−3978.(3) Na, S.-I.; Kim, S.-S.; Jo, J.; Kim, D.-Y. Efficient and Flexible ITO-Free Organic Solar Cells Using Highly Conductive Polymer Anodes.Adv. Mater. 2008, 20, 4061−4067.(4) Norrman, K.; Madsen, M. V.; Gevorgyan, S. A.; Krebs, F. C.Degradation Patterns in Water and Oxygen of an Inverted PolymerSolar Cell. J. Am. Chem. Soc. 2010, 132, 16883−16892.(5) Krebs, F. C.; Nielsen, T. D.; Fyenbo, J.; Wadstrøm, M.; Pedersen,M. S. Manufacture, Integration and Demonstration of Polymer SolarCells in a Lamp for the “Lighting Africa” Initiative. Energy Environ. Sci.2010, 3, 512−525.(6) Dou, L.; You, J.; Yang, J.; Chen, C.-C.; He, Y.; Murase, S.;Moriarty, T.; Emery, K.; Li, G.; Yang, Y. Tandem Polymer Solar CellsFeaturing a Spectrally Matched Low-Bandgap Polymer. Nat. Photonics2012, 6, 180−185.(7) Søndergaard, R.; Helgesen, M.; Jørgensen, M.; Krebs, F. C.Fabrication of Polymer Solar Cells Using Aqueous Processing for AllLayers Including the Metal Back Electrode. Adv. Energy Mater. 2011, 1,68−71.(8) Go, Y.-J.; Yun, J.-M.; Noh, Y.-J.; Yeo, J.-S.; Kim, S.-S.; Jung, C.-H.;Oh, S.-H.; Yang, S.-Y.; Kim, D.-Y.; Na, S.-I. Efficient Polymer SolarCells with a Solution-Processed Gold Chloride as an Anode InterfacialModifier. Appl. Phys. Lett. 2013, 102, 163302.(9) Chen, L.-M.; Hong, Z.; Li, G.; Yang, Y. Recent Progress inPolymer Solar Cells: Manipulation of Polymer:Fullerene Morphologyand the Formation of Efficient Inverted Polymer Solar Cells. Adv.Mater. 2009, 21, 1434−1449.(10) Yeo, J.-S.; Yun, J.-M.; Jung, Y.-S.; Kim, D.-Y.; Noh, Y.-J.; Kim, S.-S.; Na, S.-I. Sulfonic Acid-Functionalized, Reduced Graphene Oxide asan Advanced Inrerfacial Materials Leading to Donor Polymer-Independent High-Performance Polymer Solar Cells. J. Mater. Chem.A 2014, 2, 292−298.(11) Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K.-Y. InterfaceEngineering for Organic Electronics. Adv. Funct. Mater. 2010, 20,1371−1388.(12) Steim, R.; Kogler, F. R.; Brabec, C. J. Interface Materials forOrganic Solar Cells. J. Mater. Chem. 2010, 20, 2499−2512.(13) He, C.; Zhong, C.; Wu, H.; Yang, R.; Yang, W.; Huang, F.;Bazan, G. C.; Cao, Y. Origin of the Enhanced Open-Circuit Voltage inPolymer Solar Cells via Interfacial Modification Using ConjugatedPolyelectrolytes. J. Mater. Chem. 2010, 20, 2617−2622.(14) Liu, J.; Xue, Y.; Gao, Y.; Yu, D.; Durstock, M.; Dai, L. Hole andElectron Extraction Layers Based on Graphene Oxide Derivatives forHigh-Performance Bulk Heterojunction Solar Cells. Adv. Mater. 2012,24, 2228−2233.(15) Yun, J.-M.; Yeo, J.-S.; Kim, J.; Jeong, H.-G.; Kim, D.-Y.; Noh, Y.-J.; Kim, S.-S.; Ku, B.-C.; Na, S.-I. Solution-Processable ReducedGraphene Oxide as a Novel Alternative to PEDOT: PSS HoleTransport Layers for Highly Efficient and Stable Polymer Solar Cells.Adv. Mater. 2011, 23, 4923−4928.(16) Gao, Y.; Yip, H.-L.; Chen, K.-S.; O’Malley, K. M.; Acton, O.;Sun, Y.; Ting, G.; Chen, H.; Jen, A. K.-Y. Surface Doping ofConjugated Polymers by Graphene Oxide and Its Application forOrganic Electronic Devices. Adv. Mater. 2011, 23, 1903−1908.(17) Kim, S.-H.; Lee, C.-H.; Yun, J.-M.; Noh, Y.-J.; Kim, S.-S.; Lee, S.;Jo, S. M.; Joh, H.-I.; Na, S.-I. Fluorine-Functionalized andSimultaneously Reduced Graphene Oxide as a Novel Hole Trans-porting Layer for Highly Efficient and Stable Organic PhotovoltaicCells. Nanoscale 2014, 6, 7183−7187.(18) Sekine, N.; Chou, C.-H.; Kwan, W. L.; Yang, Y. ZnO Nano-Ridge Structure and Its Application in Inverted Polymer Solar Cell.Org. Electron. 2009, 10, 1473−1477.(19) Noh, Y.-J.; Na, S.-I.; Kim, S.-S. Inverted Polymer Solar CellsIncluding ZnO Electron Transport Layer Fabricated by Facile SprayPyrolysis. Sol. Energy Mater. Sol. Cells 2013, 117, 139−144.

(20) Girotto, C.; Voroshazi, E.; Cheyns, D.; Heremans, P.; Rand, B.P. Solution-Processed MoO3 Thin Films As a Hole-Injection Layer forOrganic Solar Cells. ACS Appl. Mater. Interfaces 2011, 3, 3244−3247.(21) Zilberberg, K.; Gharbi, H.; Behrendt, A.; Trost, S.; Riedl, T.Low-Temperature, Solution-Processed MoOx for Efficient and StableOrganic Solar Cells. ACS Appl. Mater. Interfaces 2012, 4, 1164−1168.(22) Chen, S.; Manders, J. R.; Tsang, S.-W.; So, F. Metal Oxide forInterface Engineering in Polymer Solar Cells. J. Mater. Chem. 2012, 22,24202−24212.(23) He, Z.; Wu, H.; Cao, Y. Recent Advances in Polymer SolarCells: Realization of High Device Performance by IncorporatingWater/Alcohol-Soluble Conjugated Polymers as Electrode BufferLayer. Adv. Mater. 2014, 26, 1006−1024.(24) Zhou, Y.; Li, F.; Barrau, S.; Tian, W.; Inganas, O.; Zhang, F.Inverted and Transparent Polymer Solar Cells Prepared with Vacuum-Free Processing. Sol. Energy Mater. Sol. Cells 2009, 93, 497−500.(25) Na, S.-I.; Kim, T.-S.; Oh, S.-H.; Kim, J.; Kim, S.-S.; Kim, D.-Y.Enhanced Performance of Inverted Polymer Solar Cells with CathodeInterfacial Tuning via Water-Soluble Polyfluorenes. Appl. Phys. Lett.2010, 97, 223305.(26) Lim, K.-G.; Choi, M.-R.; Kim, H.-B.; Park, J. H.; Lee, T.-W.High-Efficiency Polymer Photovoltaic Cells Using a Solution-Processable Insulating Interfacial Nanolayer: The Role of theInsulating Nanolayer. J. Mater. Chem. 2012, 22, 25148−25153.(27) Li, C.-Y.; Wen, T.-C.; Guo, T.-F. Sulfonated Poly-(diphenylamine) as a Novel Hole-Collecting Layer in PolymerPhotovoltaic Cells. J. Mater. Chem. 2008, 18, 4478−4482.(28) Zhou, H.; Zhang, Y.; Mai, C.-K.; Collins, S. D.; Nguyen, T.-Q.;Bazan, G. C.; Heeger, A. J. Conductive Conjugated Polyelectrolyte asHole-Transporting Layer for Organic Bulk Heterojunction Solar Cells.Adv. Mater. 2014, 26, 780−785.(29) Kang, R.; Oh, S.-H.; Kim, D.-Y. Influence of the IonicFunctionalities of Polyfluorene Derivatives as a Cathode InterfacialLayer on Inverted Polymer Solar Cells. ACS Appl. Mater. Interfaces2014, 6, 6227−6236.(30) Sun, K.; Zhao, B.; Kumar, A.; Zeng, K.; Ouyang, J. HighlyEfficient, Inverted Polymer Solar Cells with Indium Tin OxideModified with Solution-Processed Zwitterions as the TransparentCathode. ACS Appl. Mater. Interfaces 2012, 4, 2009−2017.(31) Lee, B. H.; Jung, I. H.; Woo, H. Y.; Shim, H.-K.; Kim, G.; Lee,K. Multi-Charged Conjugated Polyelectrolytes as a Versatile WorkFunction Modifier for Organic Electronic Devices. Adv. Funct. Mater.2014, 24, 1100−1108.(32) Seo, J. H.; Nguyen, T.-Q. Electronic Properties of ConjugatedPolyelectrolyte Thin Film. J. Am. Chem. Soc. 2008, 130, 10042−10043.(33) Seo, J. H.; Yang, R.; Brzezinski, J. Z.; Walker, B.; Bazan, G. C.;Nguyen, T.-Q. Electronic Properties at Gold/Conjugated-Polyelec-trolyte Interfaces. Adv. Mater. 2009, 21, 1006−1011.(34) Karacan, I.; Erdogan, G. The Influence of Thermal StabilizationStage on the Molecular Structure of Polyacrylonitrile Fibers Prior tothe Carbonization Stage. Fibers Polym. 2012, 13, 295−302.(35) Saha, B.; Schatz, G. C. Cabonization in Polyacrylonitrile (PAN)Based Carbon Fibers Studied by ReaxFF Molecular DynamicsSimulations. J. Phys. Chem. B 2012, 116, 4684−4692.(36) Zhang, F.; Di, C.-a.; Berdunov, N.; Hu, Y.; Hu, Y.; Gao, X.;Meng, Q.; Sirringhaus, H.; Zhu, D. Ultrathin Film Organic Transistors:Precise Control of Semiconductor Thickness via Spin-Coating. Adv.Mater. 2013, 25, 1401−1407.(37) Na, S.-I.; Noh, Y.-J.; Son, S.-Y.; Kim, T.-W.; Kim, S.-S; Lee, S.;Joh, H.-I. Efficient Organic Solar Cells with Solution-ProcessedCarbon Nanosheets as Transparent Electrodes. Appl. Phys. Lett. 2013,102, 043304.(38) Jung, C.-H.; Kim, W.-J.; Jung, C.-H.; Hwang, I.-T.; Khim, D.;Kim, D.-Y.; Lee, J.-S.; Ku, B.-C.; Choi, J.-H. A Simple PAN-BasedFabrication Method for Microstructured Carbon Electrodes forOrganic Field-Effect Transistors. Carbon 2015, 87, 257−268.(39) Summan, A. M. Electrical Conductivity of Heat TreatedPolyacrylonitrile and Its Copper Halide Complexes. J. Polym. Sci., PartA: Polym. Chem. 1999, 37, 3057−3062.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.5b07841ACS Appl. Mater. Interfaces 2015, 7, 25032−25038

25037

(40) Jiao, F.; Zhang, F.; Zang, Y.; Zou, Y.; Di, C.; Xu, W.; Zhu, D. AnEasily Accessible Carbon Material Derived from Carbonization ofPolyacrylonitrile Ultrathin Films: Ambipolar Transport Properties andApplication in a CMOS-Like Inverter. Chem. Commun. 2014, 50,2374−2376.(41) Joh, H.-I.; Lee, S.; Kim, T.-W.; Hwang, S. Y.; Hahn, J. R.Synthesis and Properties of an Atomically Thin Carbon NanosheetSimilar to Graphene and Its Promising Use as an Organic Thin FilmTransistor. Carbon 2013, 55, 299−304.(42) Na, S.-I.; Lee, J.-S.; Noh, Y.-J.; Kim, T.-W.; Kim, S.-S.; Joh, H.-I.;Lee, S. Efficient ITO-Free Polymer Solar Cells with Pitch-ConvertedCarbon Nanosheets as Novel Solution-Processable TransparentElectrodes. Sol. Energy Mater. Sol. Cells 2013, 115, 1−6.(43) Reese, M. O.; Gevorgyan, S. A.; Jørgensen, M.; Bundgaard, E.;Kurtz, S. R.; Ginley, D. S.; Olson, D. C.; Lloyd, M. T.; Morvillo, P.;Katz, E. A.; Elschner, A.; Haillant, O.; Currier, T. R.; Shrotriya, V.;Hermenau, M.; Riede, M.; Kirov, K. R.; Trimmel, G.; Rath, T.;Inganas, O.; Zhang, F.; Andersson, M.; Tvingstedt, K.; Lira-Cantu, M.;Laird, D.; McGuiness, C.; Gowrisanker, S.; Pannone, M.; Xiao, M.;Hauch, J.; Steim, R.; DeLongchamp, D. M.; Rosch, R.; Hoppe, H.;Espinosa, N.; Urbina, A.; Yaman-Uzunoglu, G.; Bonekamp, J.-B.; vanBreemen, A. J. J. M.; Girotto, C.; Voroshazi, E.; Krebs, F. C.Consensus Stability Testing Protocols for Organic PhotovoltaicMaterials and Devices. Sol. Energy Mater. Sol. Cells 2011, 95, 1253−1267.(44) Noh, Y.-J.; Park, S.-C.; Hwang, I.-T.; Choi, J.-H.; Kim, S.-S.;Jung, C.-H.; Na, S.-I. High-Performance Polymer Solar Cells withRadiation-Induced and Reduction-Controllable Reduced GrapheneOxide as an Advanced Hole Transporting Materials. Carbon 2014, 79,321−329.(45) Gao, Y.; Yip, H.-L.; Hau, S. K.; O’Malley, K. M.; Cho, N. C.;Chen, H.; Jen, A. K.-Y. Anode Modification of Inverted Polymer SolarCells Using Graphene Oxide. Appl. Phys. Lett. 2010, 97, 203306.(46) Lim, K.-G.; Choi, M.-R.; Kim, J.-H.; Kim, D. H.; Jung, G. H.;Park, Y.; Lee, J.-L.; Lee, T.-W. Rolle of Ultrathin Metal Fluoride Layerin Organic Photovoltaic Cells: Mechanism of Efficiency and LifetimeEnhancement. ChemSusChem 2014, 7, 1125−1132.(47) Wu, M.; Wang, Q.; Li, K.; Wu, Y.; Liu, H. Optimization ofStabilization Conditions for Electrospun Polyacrylonitrile Nanofibers.Polym. Degrad. Stab. 2012, 97, 1511−1519.(48) Yue, Z.; Benak, K. R.; Wang, J.; Mangun, C. L.; Economy, J.Economy, J. Elucidating the Porous and Chemical Structures of ZnCl2-Activated Polyacrylonitrile on a Fiberglass Substrate. J. Mater. Chem.2005, 15, 3142−3148.(49) Yang, D.-Q.; Sacher, E. A Spectroscopic Study of CNxFormation by the keV N2

+ Irradiation of Highly Oriented PyrolyticGraphite Surfaces. Surf. Sci. 2003, 531, 185−198.(50) Brown, R. D.; Burden, F. R.; Garland, W. Microwave Spectrumand Dipole Moment of Pyridine-N-Oxide. Chem. Phys. Lett. 1970, 7,461−462.(51) Akada, K.; Terasawa, T.; Imamura, G.; Obata, S.; Saiki, K.Control of Work Function of Graphene by Plasma Assisted NitrogenDoping. Appl. Phys. Lett. 2014, 104, 131602.(52) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.;Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors inBulk-Heterojuction Solar Cells-Toward 10% Energy-ConversionEfficiency. Adv. Mater. 2006, 18, 789−794.(53) Yao, S.; Li, P.; Bian, J.; Dong, Q.; Im, C.; Tian, W. Influence of aPolyelectrolyte Based-Fluorene Interfacial Layer on the Performanceof a Polymer Solar Cell. J. Mater. Chem. A 2013, 1, 11443−11450.(54) Liu, Z.; Li, J.; Yan, F. Package-Free Flexible Organic Solar Cellswith Graphene Top Electrodes. Adv. Mater. 2013, 25, 4296−4301.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.5b07841ACS Appl. Mater. Interfaces 2015, 7, 25032−25038

25038