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Co-evaporant Induced Crystalline Donor: Acceptor Blends in Organic Solar Cells Toshihiko Kaji,* Minlu Zhang, Satoru Nakao, Kai Iketaki, Kazuya Yokoyama, Ching W. Tang, and Masahiro Hiramoto Organic solar cells (OSCs) are actively being developed as a low-cost technology for solar-to-electric power conversion.[1–5] Currently, major research efforts are focused on improving the cell efficiency through optimization of the bulk heterojunction (BHJ) architecture,[6] a blend film consisting of a mixture of donor (p-type) component and acceptor (n-type) component of various organic materials. High-efficiency OSCs are mostly based on BHJs with a solution-cast blend film of a conjugated polymer as the donor and a small molecule as the acceptor,[7–9] such as P3HT:PCBM. For these polymeric BHJs, the ability to control the film morphology and crystallinity is essential, which is usually done through selection of proper solvents[10,11] and fine-tuning the post-deposition conditions in a solution-cast process. For vacuum-deposited small molecules, which in principle offer many significant advantages and are well proven to be useful in the related organic light emitting diode (OLED) technology, however, the implementation of morphology and crystallinity control of BHJs is much more limited as the effect of solvent is absent. In this work, we have succeeded in producing high-quality and morphologically-oriented crystalline blend films based on small molecules by using a liquid as a non-sticking co-evaporant source during vacuum deposition of the blend film and observed striking improvements in OSC performance, particularly in the photocurrent generation with the use of a relatively thick (∼400 nm) blend film for greater light absorption. The origin of BHJs can be traced to earlier work on vacuumdeposited OSCs, which are based on small-molecule donors and acceptors, first reported in 1986 on a p-n bi-layer architecture,[12] and later in 1991 on a p-i-n tri-layer architecture,[13] where the i-interlayer, an equivalent of BHJ, is a vacuum-deposited blend film of a phthalocyanine and a perylene derivative. Currently, vacuum-deposited OSCs with improved BHJ formulations,
Prof. T. Kaji, Dr. S. Nakao, Dr. K. Iketaki, K. Yokoyama, Prof. M. Hiramoto Institute for Molecular Science 5-1 Higashiyama, Okazaki, 444-8787, Japan E-mail:
[email protected] Prof. T. Kaji, Prof. M. Hiramoto The Graduate University for Advanced Studies [Sokendai] 5-1 Higashiyama, Okazaki, 444-8787, Japan Prof. T. Kaji, M. Zhang, Prof. C. W. Tang Department of Chemical Engineering University of Rochester 206 Gavett Hall, Rochester, New York 14627, USA
DOI: 10.1002/adma.201101305
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notably with fullerene (C60) as acceptor, have demonstrated efficiencies as high as 4–5%[14-16] for single cells. Similar to polymeric OSCs, these improvements were achieved mainly through fine-tuning of the morphology of the blend films and thereby their electrical properties. For the vacuum-deposited OSCs, morphology tuning is generally limited to controlling the deposition rates, the substrate temperature during deposition,[16,17] and post-deposition annealing[18] without the benefits of solvent induced effects inherent in polymeric OSCs. This deficiency is apparently a major factor in limiting the performance of vacuum-deposited OSCs based on small molecules. Compared to polymeric blend films, the electrical conductivity of most vacuum-deposited blend films are relatively low, thus only very thin (
