Current Applied Physics 12 (2012) 908e910

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Annealing-free Poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester-based organic solar cells Dae Sung You a, b, Chang Su Kim a, *, Yong Jin Kang a, Kyounga Lim a, Sunghoon Jung a, Do-Geun Kim a, Jong-Kuk Kim a, Sungjin Jo c, Joo Hyun Kim b, **, Jae-Wook Kang a, * a b c

Korea Institute of Materials Science, Changwon, Gyeongsangnamdo 642-831, Republic of Korea Organic Opto-electronic Material Laboratory, Bukyong National University, Busan 608-739, Republic of Korea School of Energy Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 September 2011 Received in revised form 6 December 2011 Accepted 6 December 2011 Available online 13 December 2011

We report on the fabrication of efficient annealing-free organic solar cells using co-solvent solution considered as a promising method for low-cost and time-saving manufacturing. Higher device efficiency could be obtained compared to the pure solvent casted device, resulting from the improved crystallinity, optical absorption and transport properties. The power conversion efficiency of 2.8% was obtained, demonstrating the feasibility of achieving low-cost and high-efficiency organic solar cells without any additional treatment and processing additives. Ó 2011 Elsevier B.V. All rights reserved.

Keywords: Organic solar cell Annealing-free P3HT crystallinity

1. Introduction Organic solar cells are of extensive research interest mainly because they can be fabricated at low costs by using solutionprocess [1e4]. In comparison to small molecular or oligomer materials, polymer active layers have superior solution processability. However, solution-processed polymer active layers always comprise amorphous regions along with crystalline domains, which is the essentially reason for their relatively poor carrier transport properties [5,6]. Thermal annealing at around 150
C or higher is a commonly used practice to improve the crystallinity of the polymer active layers to obtain higher transport properties. Although high efficiency can be obtained by applying thermal annealing, the device performance is sensitive to the annealing conditions, and these additional processes prolong device fabrication time. This poses a challenge in the cost effective production of printed organic solar cells in a high-throughput, roll-to-roll manufacturing method. Another potential issue associated with high-temperature thermal annealing is that common plastic substrates such as polyethersulfone (PES), polyethylene terephthalate (PET) films become dimensionally unstable at

* Corresponding authors. ** Corresponding author. E-mail addresses: [email protected] (C.S. Kim), [email protected] (J.H. Kim), [email protected] (J.-W. Kang). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.12.007

a temperature of 150
C or higher [7,8]. To eliminate the thermal annealing step, we report on the fabrication and characterization of efficient solar cells using co-solvent solution considered as a promising method for low-cost and time-saving manufacturing. 2. Experiment The conventional organic solar cells fabricated on the ITO-coated glass substrate were first cleaned in an ultrasonic bath containing acetone, and then in boiling isopropyl alcohol. The substrates were then dried in an oven and treated with UVeozone for 5 min. A buffer layer of poly(3,4-ethylenedioxylenethiophene) (PEDOTePSS, Baytron P):isopropyl alcohol (IPA) (PEDOTePSS:IPA ¼ 1:2) was prepared on ITO substrate using a spin coater after passing through a 0.45 mm filter with a thickness of approximately 40 nm. The coated PEDOTePSS film was dried at 150
C for 1 min on a hot plate in a glove box. The Poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend solution was prepared in a mass ratio 1:1 in pure chlorobenzene (CB) and co-solvent (1:1, v/v) of CB and 1, 2, 4-trichlorobenzene (TCB). The photoactive layer was deposited by spin-coating with a thickness of 240 nm in a glove box. Finally, a 120 nm-thick Al electrode was deposited on the photoactive layer by thermal evaporation at 3  106 Torr. The active area of the device was 0.38 cm2 with an island-type of electrode geometry. The current densityevoltage (JeV) characteristics of the organic solar cell devices were measured under AM1.5 simulated illumination with an intensity

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of 100 mW/cm2. The JeV curves were recorded automatically with a Keithley SMU 2410 source meter by illuminating the devices. P3HT:PCBM blend films, characterized by UVevis absorption, X-ray diffraction (XRD), atomic force microscopy (AFM), were prepared on ITO-glass substrates for an accurate comparison. 3. Results and discussion

Fig. 1. (a) UVevis absorption spectra of P3HT:PCBM blend films cast from pure CB and CB:TCB co-solvent solutions. (b) Out-of-plane grazing incident XRD intensities of P3HT:PCBM blend films cast from pure CB and CB:TCB co-solvent solutions.

Fig. 1(a) shows UVevis absorption spectrum of the P3HT:PCBM blend films cast from pure CB and CB:TCB co-solvent solutions. The UVevis spectrum indicates the degree of P3HT crystalline ordering in the blend films. The P3HT:PCBM blend film cast from pure CB solvent contained only one peak approximately 460 nm corresponding to the dissolved P3HT chains in solution in the absence of molecular ordering [9,10]. While P3HT:PCBM blend film cast from CB:TCB co-solvent shows that the absorption maximum of the P3HT peak shifts to higher wavelengths, due to an increase in conjugation length and therefore rearrangement of the P3HT chains. In addition, clear vibronic shoulders appeared at long wavelengths (between 550 and 620 nm) as the enhanced interchain pep* stacking, and increased absorption intensity [11,12]. The crystallinity of P3HT in the P3HT:PCBM blend films was also evaluated by XRD measurements. Fig. 1(b) shows the out-of-plane X-ray diffraction patterns of blend films cast from pure CB and CB:TCB co-solvent solutions. The P3HT:PCBM blend film cast from pure CB solvent is in the amorphous phase which infer that the film fabricated from pure CB solvent was not sufficiently crystallized. In contrast, P3HT:PCBM blend film cast from CB:TCB co-solvent shows a crystalline peak around a 2q value of 5.4
, representing a P3HT crystallinity with a-axis orientations (main chain parallel and side chains perpendicular to the substrate) [13,14]. No crystallinity with b- and c-axis orientation was detected. The blend film cast from CB:TCB co-solvent showed higher crystallinity than the film cast from pure CB solvent, which agreed with the results from the UV absorption measurements. The improved P3HT crystallinity can enhance not only the carrier transport but also the light harvesting capabilities, improving the cell performance [15]. Fig. 2 shows the AFM (a) height and 3D images of P3HT:PCBM blend film cast from CB pure solvent and (b) height and 3D images

Fig. 2. AFM height and 3D images of P3HT:PCBM blend films cast from (a) pure CB and (b) CB:TCB co-solvent solutions.

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efficiencies is mainly attributed to the increase of short circuit current and fill factor. Since the short circuit current and fill factor are strongly dependent on the transport properties of the networks in the bulk-heterojunction film, we hypothesize that the improved device performance results from improved carrier transport as a result of improved P3HT crystallinity [18,19]. No further improvements in device efficiency were observed when the blend film cast from CB:TCB co-solvent were annealed at 150
C for 10 min. Consequently, we can conclude that the blend film cast from CB:TCB co-solvent was already crystallized regardless of posttreatment. It can be agreed with the results from the UV absorption, XRD and AFM measurement. 4. Conclusion In summary, we have demonstrated the fabrication of efficient solar cells using co-solvent solution without any annealing process. The P3HT:PCBM blend film cast from CB:TCB co-solvent exhibited a better crystallinity, optical absorption and morphology than blend film cast from pure CB solvent. The power conversion efficiency of 2.8% was obtained, demonstrating the feasibility of achieving low-cost and high-efficiency organic solar cells without any additional treatment. By carefully selecting solvent mixtures, the performance of the diodes could be enhanced, which provides an approach to efficient organic solar cells. Acknowledgments

Fig. 3. JeV characteristics (a) in the dark and (b) under illumination (100 mW/cm2) of P3HT:PCBM organic solar cells fabricated using pure CB and CB:TCB co-solvent solutions. The inset shows a schematic structure of conventional organic solar cells.

of P3HT:PCBM blend film cast from CB:TCB co-solvent. The blend film cast from CB pure solvent has a very smooth surface with rms roughness of 0.6 nm in Fig. 2(a). The rms roughness of blend film cast from CB:TCB co-solvent increases to 5.2 nm with rough hillock on surface in Fig. 2(b). This increase in surface roughness can be directly correlated with improved P3HT crystallinity [16,17]. Fig. 3(a) shows the JeV characteristics measured in the dark for devices cast from pure CB and CB:TCB co-solvent solutions. The JeV characteristics in the dark were performed to check whether the device is working (diode characteristic) and to determine the rectification ratio. The rectification ratio is defined as the absolute value of forward current divided by the absolute value of the backward current. They present the important information how good the diode is. The device cast from pure CB solvent showed poor diode behavior with a rectification ratio of 7  102 at 1 V. The device cast from CB:TCB co-solvent shows an improved rectification ratio compared with device cast from pure CB solvent (rectification ratio is 3  105 at 1 V). This improvement is mainly due to a decrease in the reverse bias current. Fig. 3(b) shows the JeV characteristics under illumination (100 mW/cm2) for devices cast from pure CB and CB:TCB co-solvent solutions. The device cast from pure CB solvent exhibits a power conversion efficiency of 0.51%, with a short circuit current, Jsc, of 1.82 mA/cm2, a fill factor, FF, of 0.42, and an open circuit voltage, Voc, of 0.66. The device cast from CB:TCB co-solvent exhibit substantially improved performance with a Jsc of 8.15 mA/cm2, a FF of 0.62, and a Voc of 0.55. Accordingly, the device efficiency increases to 2.80%. The enhancement of

This research was supported by grants from the fundamental R&D program for core technology of materials and the new and renewable energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (Grant No. 20103020010050) funded by the Ministry of Knowledge Economy, Republic of Korea. J. H. Kim would like thank the support from the Converging Research Center Program through the Ministry of Education, Science and Technology (2010K001079). References [1] S.Y. Park, Y.J. Kang, S. Lee, D.G. Kim, J.K. Kim, J.H. Kim, J.W. Kang, Sol. Energy Mater. Sol. Cells 95 (2011) 852. [2] S.Y. Park, W.I. Jeong, D.G. Kim, J.K. Kim, D.C. Lim, J.H. Kim, J.J. Kim, J.W. Kang, Appl. Phys. Lett. 96 (2010) 173301. [3] F. Zhang, X. Xu, W. Tang, J. Zhang, Z. Zhuo, J. Wang, J. Wang, Z. Xu, Y. Wang, Sol. Energy Mater. Sol. Cells 95 (2011) 1785. [4] C.F. Shih, K.T. Hung, J.W. Wu, K.T. Huang, S.H. Wu, Jpn. J. Appl. Phys. 49 (2010) 040204. [5] E. Verploegen, R. Mondal, C.J. Bettinger, S. Sok, M.F. Toney, Z. Bao, Adv. Funct. Mater. 20 (2010) 3519. [6] H. Yan, S. Swaraj, C. Wang, I. Hwang, N.C. Greenham, C. Groves, H. Ade, C.R. McNeill, Adv. Funct. Mater. 20 (2010) 4329. [7] W.A. MacDonald, J. Mater. Chem. 14 (2004) 4. [8] V. Zardetto, T.M. Brown, A. Reale, A.D. Carlo, J. Polym. Sci. Part B Polym. Phys. 49 (2011) 638. [9] Y.D. Park, H.S. Lee, Y.J. Choi, D. Kwak, J.H. Cho, S. Lee, K. Cho, Adv. Funct. Mater. 19 (2009) 1200. [10] T. Yamamoto, D. Komarudin, M. Arai, B.L. Lee, H. Suganuma, N. Asakawa, Y. Inoue, K. Kubota, S. Sasaki, T. Fukuda, H. Matsuda, J. Am. Chem. Soc. 120 (1998) 2047. [11] Y. Kim, S. Cook, S.M. Tuladhar, S.A. Choulis, J. Nelson, J.R. Durrant, D.D.C. Bradley, M. Giles, I. McCulloch, C.S. Ha, M. Ree, Nat. Mater. 5 (2006) 197. [12] L. Li, G. Lu, X. Yang, J. Mater. Chem. 18 (2008) 1984. [13] J. Jo, S.S. Kim, S.I. Na, B.K. Yu, D.Y. Kim, Adv. Funct. Mater. 19 (2009) 866. [14] G. Li, Y. Yao, H. Yang, V. Shrotriya, G. Yang, Y. Yang, Adv. Funct. Mater. 17 (2007) 1636. [15] V.D. Mihailetchi, H. Xie, B. de Boer, L.J.A. Koster, P.W.M. Blom, Adv. Funct. Mater. 16 (2006) 699. [16] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 4 (2005) 864. [17] H. Kim, W.W. So, S.J. Moon, Sol. Energy Mater. Sol. Cells 91 (2007) 581. [18] C.S. Kim, L.L. Tinker, B.F. Disalle, E.D. Gomez, S. Lee, S. Bernhard, Y.L. Loo, Adv. Funct. Mater. 21 (2009) 3110. [19] C.S. Kim, S. Lee, L.L. Tinker, S. Bernhard, Y.L. Loo, Chem. Mater. 21 (2009) 4583.