Three-Dimensional Bulk Heterojunction Morphology for Achieving High Internal Quantum Efficiency in Polymer Solar Cells By Jang Jo, Seok-In Na, Seok-Soon Kim, Tae-Woo Lee, Youngsu Chung, Seok-Ju Kang, Doojin Vak, and Dong-Yu Kim* 1. Introduction Recently, various technologies designed to generate power by harnessing solar energy have been gaining interest because of dwindling supplies of natural resources and the environmental problems associated with fossil fuels. [1] Solar-energy generation, especially that of photovoltaics, has great potential as a renewable energy source because of its limitless and non-polluting properties. However, the high cost of man- ufacturing conventional silicon solar cells makes the development of more economic materials a critical issue in the research on solar-energy conversion. Fortunately, bulk heterojunction (BHJ) organic material pairs, particularly semiconducting polymers and fullerene (C 60 ) derivatives, have provided an affordable solution for the low-cost manu- facturing of large-area photovoltaics as con- jugated polymer:C 60 interpenetrating net- works exhibit ultrafast photoinduced charge transfers. [2,3] Recently, several research groups have reported that BHJ solar cells based on a composite film using poly(3- hexylthiophene) (P3HT) as an electron donor and [6,6]-phenyl- C 61 -butyric acid methyl ester (PCBM) as an electron acceptor show a power-conversion efficiency near 5%, which is the best reported performance for solution-processed polymer solar cells. [4–7] In general, the performance of BHJ solar cells can be maximized by controlling the morphology of the active layer, because efficient photoinduced charge generation, transport, and collection at each electrode crucially depend on the nanometer-scale morphology of the composite films. [8,9] First, the crystallization of P3HT improves sunlight harvesting by extending the conjugation length and increases hole mobility, which is a limiting factor for the transport balance of charges dissociated from the bound excitons. [10] In addition, the interpenetrating network of BHJ materials must generally have periods of optimum length, which depends on the exciton diffusion length of the polymer used as the light-harvesting and hole-transporting material. In the case of a P3HT:PCBM composite, the optimum donor/acceptor period has been reported as approximately 20 nm. [11] An ordered BHJ morphology can be achieved by the inter-diffusion of each molecule in the composite films. FULL PAPER www.afm-journal.de [*] Prof. D.-Y. Kim, J. Jo, S.-I. Na, S.-J. Kang Heeger Center for Advanced Materials Department of Materials Science and Engineering Gwangju Institute of Science and Technology 1 Oryong-Dong, Buk-Gu, Gwangju 500-712 (Republic of Korea) E-mail: kimdy@gist.ac.kr Prof. S.-S. Kim School of Materials Science and Chemical Engineering Kunsan National University Kunsan, Chonbuk 573-701 (Republic of Korea) Prof. T.-W. Lee Department of Materials Science and Engineering Pohang University of Science and Technology San 31, Hyoja-dong, Nam-gu, Pohang 790-784 (Republic of Korea) Dr. Y. Chung Samsung Advanced Institute of Technology Mt. 14–1, Nongseo-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446– 712 (Republic of Korea) Dr. D. Vak Bio21 Institute, Holmes Laboratory, University of Melbourne Building 102 (Level 4), 30 Flemington Road, Parkville, Victoria 3010 (Australia) DOI: 10.1002/adfm.200900183 Here, an investigation of three-dimensional (3D) morphologies for bulk heterojunction (BHJ) films based on regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) is reported. Based on the results, it is demonstrated that optimized post-treatment, such as solvent annealing, forces the PCBM molecules to migrate or diffuse toward the top surface of the BHJ composite films, which induces a new vertical component distribution favorable for enhancing the internal quantum efficiency (h IQE ) of the devices. To investigate the 3D BHJ morphology, novel time-of-flight secondary-ion mass spectroscopy studies are employed along with conventional methods, such as UV-vis absorption, X-ray diffraction, and high-resolution transmission electron microscopy studies. The h IQE of the devices are also compared after solvent annealing for different times, which clearly shows the effect of the vertical component distribution on the performance of BHJ polymer solar cells. In addition, the fabrication of high- performance P3HT:PCBM solar cells using the optimized solvent-annealing method is reported, and these cells show a mean power-conversion efficiency of 4.12% under AM 1.5G illumination conditions at an intensity of 100 mW cm 2 . 2398 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2009, 19, 2398–2406