Solar Energy Materials & Solar Cells

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1 Solar Energy Materials & Solar Cells 105 (2012) Contents lists available at SciVerse ScienceDirect Solar Energy Materials & Solar Cells journal homepage: High-performance polymer solar cells with moderately reduced graphene oxide as an efficient hole transporting layer Ye-Jin Jeon a, Jin-Mun Yun b, Dong-Yu Kim b, Seok-In Na a,n, Seok-Soon Kim c,nn a Professional Graduate School of Flexible and Printable Electronics, Department of Flexible and Printable Electronics, Chonbuk National University, , Deokjin-dong, Deokjin-gu, Jeonju-si, Jeollabuk-do , Republic of Korea b School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju , Republic of Korea c Department of Nano and Chemical Engineering, Kunsan National University, Kunsan, Jeollabuk-do , Republic of Korea article info Article history: Received 20 January 2012 Received in revised form 18 May 2012 Accepted 19 May 2012 Available online 23 June 2012 Keywords: Polymer solar cells Graphene oxide Thermal treatment Hole transporting layer Stability abstract As an alternative to the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer (HTL) in polymer solar cells (PSCs), moderately reduced graphene oxide (GO) films fabricated by simple and fast thermal treatment of solution processed GO were investigated. PSC with thermally treated GO at 250 1C exhibited best performance with a power conversion efficiency (PCE) of 3.98%, compared to the PSC containing conventional PEDOT:PSS HTL with a PCE of 3.85%. Furthermore, the PSC with thermally treated GO showed superior stability compared to the PSC with conventional PEDOT:PSS HTL under the atmosphere condition without any encapsulation process. Our demonstration suggests that moderately reduced GO by simple thermal treatment could be promising HTL replacing PEDOT:PSS in PSCs as well as other organic electronics. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Solution-processable polymer bulk-heterojunction (BHJ) solar cells have attracted constant attention as a cost-efficient power source [1 12]. In conventional BHJ solar cells, a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM) blend layer is sandwiched between a transparent anode and a low workfunction metal cathode such as Ca/Al or LiF/Al. In this case, direct electrical contacts between the interfaces of active layer and electrodes lead to recombination of carriers and current leakage. Therefore, various hole transport layers (HTL) are being used to circumvent this issue. Water-soluble poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been widely used as an appropriate HTL for more efficient hole collection via alignment of work functions of P3HT and transparent ITO anode as well as improvement of contact between active layer and transparent anode by minimizing the detrimental effects of ITO roughness [13,14]. Although this layer helps PSCs achieve an improvement in efficiency, many of research group have tried to replace PEDOT:PSS due to the several problems including highly acidic and hygroscopic properties, leading to poor long-term stability [15]. To solve these problems, wide band gap p-type like inorganic materials such as V 2 O 5,MoO 3,andNiOhave n Corresponding author. nn Corresponding author. addresses: nsi12@jbnu.ac.kr (S.-I. Na), sskim@kunsan.ac.kr (S.-S. Kim). been reported [16 20]. However, most of the inorganic HTLs are deposited using vacuum system, which is incompatible with solution-processable and printable PSCs. For this reason, very recently interest in a thin film of graphene oxide (GO) and reduced GO as an efficient HTL for high-performance PSCs has been emerged [21 24]. GO is a graphene sheet functionalized with oxygen groups in the form of epoxy and hydroxyl groups on the basal plane and various other types at the edges [25,26]. In GO, because most of carbon atoms bonded with oxygen is sp 3 hybridized, it disrupts the sp 2 conjugation of the hexagonal graphene lattice. Hence, the substantial sp 3 fraction in GO makes it an insulating material. Therefore, when the insulating GO was used as HTL, the device performance will be highly dependent on the thickness of GO due to its insulating property [24]. Jung et al. reported that incremental removal of oxygen in GO induces transition of GO from electrical insulator to semiconductor and ultimately to a graphene-like semimetal [27]. Although the reduction of GO can be effectively performed by chemical process using a hydrazine reagent, it is inadequate for mass production due to the toxicity of chemical reducing agent and multiple-steps [24]. In addition, the dispersion concentration of the reduced GO produced using hydrazine is low, which could also be disadvantageous for practical applications to the devices. Recently, it is reported that GO can be reduced with thermal methods, which are believed to be green methods without using any hazardous reductants. There have been two kinds of thermal methods reported. One is solvothermal reduction method, which usually needs harsh solvents such as N,Ndimethylformamide or /$ - see front matter & 2012 Elsevier B.V. All rights reserved.

2 Y.-J. Jeon et al. / Solar Energy Materials & Solar Cells 105 (2012) N-methyl-2-pyrrolidinone, high pressure, and/or long reacting time [28 30]. The other is solid heating reduction method necessarily requiring ultra-high vacuum under Ar and H 2, and/ or rapid heating (4200 1C min 1 ) up to C under Ar gas or up to 800 1C under H 2 gas [31 32]. These complicated and high temperature processes over 500 1C are not adequate to the simple and cost-efficient production of PSCs on various substrates. In this study, we demonstrate a facile, low-cost and fast route to fabricate highly efficient PSCs containing moderately reduced GO by thermal treatment of GO films in air. GO films were prepared via solution process on ITO anode and then annealed under the various conditions of temperature, from 100 to 350 1C in air. Effect of heat treatment of GO HTL films on the cellperformances was characterized, and stability of PSCs with moderately reduced GO was compared to conventional systems including PEDOT:PSS HTL. 2. Experiments For the use of GO as HTL, GO was prepared according to the previous report [33]. Graphene oxide was prepared by stirring 1 g of powdered graphite (Alfar Aesar, 325 mesh) and 3 g of potassium permanganate (Sigma-Aldrich) into 23 ml of H 2 SO 4 (Sigma-Aldrich, 98%) at room temperature for 30 min. And then, the temperature of the reaction mixture was increased to 40 1C, where it was maintained for 6 h. At the end of 6 h, 50 ml of deionized (DI) water was slowly added into the pasty mixture. The suspension was poured over 500 g of ice containing 10 ml of H 2 O 2 (35 wt%, Sigma-Aldrich), and then the resulting bright yellow suspension was filtered through the PTFE membrane (polytetrafluoroethylene). After this, the remaining gel-like GO was thoroughly washed with diluted 1 M HCl followed by acetone and DI water. Finally, GO powder was freeze dried at 60 1C for 1 day. For device fabrication, this prepared GO powder was dispersed into DI water, and then the aqueous dispersion of GO was further diluted with N,N-dimethylformamide (DMF) at a concentration of 1.5 mg/ml for the use as a coating solution. ITO (Samsung Corning Co, Ltd.)-coated glass substrates were cleaned with a special detergent followed by ultrasonication in acetone and isopropyl alcohol and then kept in an 100 1C oven for 30 min. Before the preparation of HTL, all substrates were treated with UV/O 3 for 20 min to increase wettability of ITO surface. First, for the conventional device structure, PEDOT:PSS (Baytron P AI 4083) HTL with a thickness of 30 nm was spin-coated onto the UV/O 3 -treated ITO/glass substrates followed by annealing at 120 1C for 10 min. To characterize the effects of thermal treatment of GO on the device performance, GO HTL was spin-coated using the prepared dispersion of GO. Spin-coated GO layer was thermally treated at 150, 250, and 350 1C. P3HT:PCBM active layers were spin-coated onto the HTL coated substrate at 700 rpm for 60 s using o-dichlorobenzene (o-dcb) solution containing a 25 mg/ml of P3HT (Rieke Metals) and a 25 mg/ml of PCBM (Nano-C). Then, to obtain highly ordered active layer, the active layer coated substrates were kept in a glass jar at room temperature to evaporate o-dcb solvent slowly for 2 h in an N 2 -filled glove box, followed by annealing at 110 1C for 7 min inside the glove box. Finally, top electrodes composed of LiF (0.7 nm)/al (80 nm) with an area of 4.14 mm 2 were deposited using a thermal evaporator in vacuum with a pressure of 10 6 Torr. Cell performance was measured using a Keithley 2400 instrument under 1 sun (100 mw/cm 2 ) using a xenon light source and AM 1.5 global filter. A reference Si solar cell certified by the International System of Units (SI) (SRC-1000-TC-KG5-N, VLSI Standards, Inc) was used for calibration for accurate measurement. To analysis the effect of HTL on the stability of devices, change of cell performance was recorded as a function of exposed time in air using the same instrumental setup without any encapsulation process. The optical properties of various GO and PEDOT:PSS HTLs were investigated via. UV vis spectrophotometer with a Varian, AU/ DMS-100S. The successful formation of various GO HTLs on ITO and surface morphologies of GO and PEDOT:PSS were measured by AFM using a Veeco Dimension 3100 instrument operated in tapping mode with a silicon cantilever. Degree of reduction of GO by thermal treatment under various temperature was compared through the XPS measurements using an AXIS-NOVA (Kratos) system with a monochromatized Al Ka under a pressure of Torr. 3. Results and discussion As shown in the schematics of device structure in Fig. 1, to confirm the potential of solution processed GO as HTLs and characterize the effects of simple thermal treatments of solidified GO films on the electronic properties of GO and performance of PSCs containing GO based HTLs, GO HTLs with a thickness of 3 nm are prepared by spin-coating followed by thermal annealing at 150, 250, and 350 1C for 10 min in air. As we mentioned, because usual thermal reduction of GO requires ultra-high vacuum under Ar and H 2 and high temperature of 800 1C C under Ar gas or under H 2 gas, these conditions are not adequate to the simple production of PSCs on transparent electrode/glass substrate, even on flexible polymer films. Hence, we tried to characterize the possibility of moderately reduced GO Fig. 1. Schematic of PSCs fabricated with PEDOT:PSS and GO.

3 98 Y.-J. Jeon et al. / Solar Energy Materials & Solar Cells 105 (2012) as a HTL for highly efficient PSCs by simple thermal treatment of GO films in air. Firstly, to determine successful formation of GO films and to confirm morphological features of GO films that are thermally treated at various temperatures, GO films were investigated using AFM. Because the thickness of a single layer of GO nanosheet (1 nm) is less than the roughness and granular structure of ITO, it is difficult to obtain accurate information on the morphology of GO films. Therefore, we prepared GO layers on flat glass substrates via spin coating followed by thermal treatment at 150, Fig. 2. AFM topography image of a GO without thermal treatment (a), thermally treated GO at 150 1C (b), 250 1C (c), and 350 1C (d), and PEDOT:PSS (e).

4 Y.-J. Jeon et al. / Solar Energy Materials & Solar Cells 105 (2012) , and 350 1C. The PEDOT:PSS film, commonly used HTL in normal architecture PSCs, was also prepared on glass substrate to characterize surface morphology. Fig. 2 shows the AFM images of PED- OT:PSS and GO prepared by spin coating without and with thermal treatment at 150, 250, and 350 1C. As shown in Fig. 2(a) (c), the GO Fig. 3. Transmittance spectra of bare ITO and ITO containing PEDT:PSS and GO without and with thermal treatment at various temperature. samples without and with thermal treatment at 150 and 250 1C showed a well formed 2-dimensional nanosheet. On the other hands, in the case of GO films thermally treated at 350 1C, some aggregations of nanosheets and relatively non-uniform morphology were observed, compared with other samples. The thicknesses of nanosheets of GO without and with thermal treatment at 150, 250, and 350 1C were estimated to 1.18, 1.07, 0.87, and 0.85 nm, respectively, and these values are well consistent with previous reports [34,35]. Decrease in the thickness of single sheet, resulting from the removal of oxygen group, indicates that solution processed GO films are reduced by simple thermal treatment at 250 and 350 1C. The root mean square (RMS) roughness of GO films prepared without and with thermal treatment at 150 and 250 1C is similar with each other and less than 1 nm. GO films thermally treated at 350 1C showed higher roughness of 2 nm, and PEDOT:PSS films showed roughness of 1.48 nm. In PSCs, since the incident light is illuminated to the active layer through the HTL coated transparent anodes, optical properties of HTL and transparent electrode can affect cell performance. The optical transmission spectra of bare ITO and ITO with PEDOT:PSS and GO layers are shown in Fig. 3. Although the optical transmittance of GO is slightly different with PEDOT:PSS HTL, all of the HTLs are highly transparent in the range of nm with a transmittance values up to 85%. The transmittance was slightly decreased after incorporation of HTLs, but these HTLs do not significantly alter the transparency of ITO electrode. Fig. 4. XPS spectra of GO without thermal treatment (a) and thermally treated GO at 150 1C (b), 250 1C (c), and 350 1C (d): (1) C C, (2) C O, (3) CQO, and (4) CQO O H.

5 100 Y.-J. Jeon et al. / Solar Energy Materials & Solar Cells 105 (2012) As mentioned earlier, when the insulating GO was used as HTL, the cell-performance is highly sensitive to the thickness of GO and incremental removal of oxygen in GO can induce transition of GO from electrical insulator to semimetal. That is the reason why we try to make a moderately reduced GO for efficient PSCs through facile thermal treatment in the air. XPS was employed to analyze removal of oxygen groups of GO after thermal treatment. As shown in Fig. 4(a), the C 1 s spectrum of GO before thermal treatment clearly indicates a considerable degree of oxidation with four components that correspond to carbon atoms in different functional groups: the non-oxygenated ring C (284.9 ev), the C in C-O bonds (286.4 ev), the carbonyl C (CQO, ev), and the carboxyl group (CQO O H, ev). In the case of GO after thermal treatment at 150 1C, although slightly decreased peak intensity corresponding to C O bonds was shown, considerable degree of oxidation was observed in the XPS data similar to GO without thermal treatment. In the data of GO after thermal treatment at 250 and 350 1C, dramatic decreased intensity of the peaks corresponding to C O, CQO, and CQO O H was observed. It obviously demonstrates that oxygen functional groups, resulting in the insulating properties of solution processed GO films, can be efficiently removed through facile thermal treatment at 250 and 350 1C [24]. For further confirmation on the reduction of GO, the average conductivity values of GO films without and with thermal treatments were measured through the 4-point probe measurement. The conductivity values of GO films without thermal treatment and thermally treated GO at 150 1C were and , respectively, whereas the similar average conductivity values of 1.8 and 2.6 S/cm were obtained in the case of thermally treated GO films at 250 and 350 1C, respectively. It indicates that facile thermal treatment of solution processed GO films can efficiently induce a moderate reduction of GO to facilitate the carrier transport more efficiently. Fig. 5. Representative J V characteristics of devices prepared using thermally reduced GO as an efficient HTL (a). Effects of the temperatures of solution process GO on the PCE (b), FF (c), J sc (d), and V oc (e).

6 Y.-J. Jeon et al. / Solar Energy Materials & Solar Cells 105 (2012) Table 1 Representative cell performance of solar cells with various HTL. PCE (%) FF (%) J sc (ma/cm 2 ) V oc (V) No HTL PEDOT:PSS No Annealing C C C To evaluate the possibility of GO and moderately reduced GO as HTL in PSCs, P3HT:PCBM based PSCs were fabricated and compared with conventional PSC with PEDOT:PSS HTL. Fig. 5(a) shows representative J V data of the PSCs and the performance characteristics were summarized in Table 1. As shown in Fig. 5(a), all solar cell parameters were increased by introducing HTL between ITO electrode and active layer. PSC including GO HTL without thermal treatment shows a low efficiency of 1.75%. However, cell performance was improved by applying thermal treatment after the fabrication of GO HTL, specially solar cell parameters of devices with thermally treated GO at 250 and 3501C were dramatically increased and showed comparable or higher efficiency compared to PEDOT:PSS based conventional device. In particular, the PSC with GO thermally treated at 250 1C exhibited the highest performance with opencircuit voltage (V OC ) of 0.59 V, short-circuit current density (J SC )of ma/cm 2, fill factor (F.F) of 65.92%, and overall power conversion efficiency (PCE) of 3.94%. This improvement might be attributed to the use of moderately reduced GO HTL having better conducting property than the insulating GO HTL. Although the GO thermally treated at 350 1C showed similar electrical properties with GO thermally treated at 250 1C, PSC with GO thermally treated at 350 1C showed relatively low efficiency of 3.6%. This slightly lower efficiency is believed to arise from the aggregations of nanosheets and non-homogeneous film morphology with higher RMS roughness, as observed in the AFM image of GO films thermally treated at 350 1C. For further investigation on the effect of thermal conditions and reliability of device performance, we fabricated several devices with GO HTL thermally treated at the temperature from 100 1C to 350 1C, at the intervals of 50 1C. As shown in Fig. 5(b) (e), the performance was improved with increasing the temperature. In particular, the devices with GO treated at 250 and 300 1C exhibited excellent performance characteristics and those values exceeded the values of the conventional PEDOT:PSS based PSCs. These results clearly demonstrated that our moderately reduced GO, exhibiting an improved carrier transport property and increased conductivity, by simple and fast thermal treatment can be a promising and cost-effective candidate for HTLs in PSCs, even in various organic semiconductor based electronics. Apart from the high efficiency, improvement of the stability of PSC and development of novel process are equally important issues for the realization of PSC as a next generation power source [36]. Herein, we further investigated the device stability that was recorded as a function of exposure time in air without any encapsulation. As demonstrated by Fig. 6, the PCE of the conventional device with PEDOT:PSS showed a rapid degradation and dropped to 0% after exposure in air for 10 h. The poor stability is attributed to the highly acidic and hygroscopic properties of PEDOT:PSS, resulting in the corrosion of ITO, indium migration into the photoactive layers, and easy penetration of water molecules into the PEDOT:PSS, thus inevitably inducing the degradation of device performance. On the contrary, the efficiencies of PSCs with moderately reduced GO HTL thermally treated at 250 and 350 1C were retained above 70% after 48 h. It suggests Normalized PCE (%) that the moderately reduced GO, which is produced by facile and fast thermal treatment of solution processed GO film, is a more promising HTL than the conventional PEDOT:PSS for the fabrication of highly efficient PSCs with outstanding stability. 4. Conclusion In conclusion, we demonstrated the application of moderately reduced GO as an efficient HTL for highly efficient and stable PSCs. Instead of complicate thermal reduction process of GO, requiring ultra-high vacuum under Ar and H 2 and high temperature of 800 to C, moderately reduced GO HTLs were prepared by the thermal treatment of solution processed GO film at 250 1C in air. Reduction of GO by simple thermal treatment was confirmed through the decreased thickness of nanosheet and decreased intensities of C O, CQO, and C (O) O in the XPS spectra, resulting from the removal of oxygen group in GO during thermal treatment. The device with thermally treated GO over 250 1C showed comparable or slightly higher efficiency in comparison with PSC containing PEDOT:PSS HTL. Moreover, the PCE of PEDOT:PSS based device rapidly dropped to 0% after exposure in air for 10 h, whereas the efficiency of PSCs with moderately reduced GO HTL was retained above 70% after 48 h. These results indicates that moderately reduced GO fabricated by facile and fast thermal treatment, which is very suitable to the high speed mass production based on the roll-to-roll system, can be an excellent candidates as HTL for highly efficient and stable PSCs. Acknowledgments This paper was supported by the R&D program funded by Jeollabuk-do, research funds of Chonbuk National University in 2012, and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No ). References 0 PEDOT: PSS GO_250 C GO_350 C Time (h) Fig. 6. Stability of PSCs with PEDOT:PSS and moderately reduced GO HTL during exposure to air. [1] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wedl, Photoinduced electron transfer from a conducting polymer to buckminsterfullerene, Science 258 (1992) [2] G. Yu, J. Gao, J.C. Hemmelen, F. Wudl, A.J. Heeger, Polymer photovoltaic cells enhanced efficiencies via a network of internal donor-acceptor heterojucntions, Science 270 (1995)

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