Small Molecule with Extended Alkyl Side Substituents for Organic Solar Cells
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Small Molecule with Extended Alkyl Side Substituents for Organic Solar Cells Chenyu Zheng1,2,3, Ishita Jalan2, Jeremy A. Cody2, and Christopher J. Collison1,2,3 1
Microsystems Engineering, 2School of Chemistry and Materials Science (SCMS), and 3NanoPower Research Laboratory (NPRL), Rochester Institute of Technology, Rochester, NY 14623, U.S.A.
ABSTRACT In this work, we have investigated two aniline based squaraine molecules, DBSQ(OH)2 and DHSQ(OH)2, for their potential application in organic photovoltaics. These two squaraine molecules are only different in side chain length (i.e. butyl vs. hexyl). Yet, their solar cell properties are drastically different (PCE = 3.6% vs. 1.9%). We have further investigated the reason behind the superior performance of DBSQ(OH)2 in absorbance spectra, hole mobility characterization and transmission electron microscopy. The results show that DBSQ(OH)2 has a higher hole mobility (5.1×10-4 cm2/V•s vs. 1.4×10-4 cm2/V•s) and is able to mix well with the fullerene acceptor compared to DHSQ(OH)2. Our work shows clearly that the long solubilizing alkyl side chain might be detrimental for OPV performance and that shorter side chains with enough solubility have great value when designing small molecules. INTRODUCTION Solution processed organic photovoltaic (OPV) technology is attractive because it promises low cost, flexible and aesthetically applicable solar energy harvesting devices. With the dominant usage of fullerene derivatives, such as PC61BM or PC71BM, for electron accepting materials, many research efforts have been made to develop highly absorptive donors, either polymers or small molecules. Squaraine (SQ) dyes are among the most investigated small molecule donor materials in solution processed OPV application due to their ease of synthesis and purification, and their high extinction coefficients in the near-infrared region1–5. With continuous efforts in synthesis of new squaraines,6,7 the solar cell efficiency employing these small molecular donor has been improved to 6-7%, which is comparable to many other small molecule systems8,9. Yet, more systematic studies on squaraine molecules are needed to provide inside guidance that lead to smarter synthesis. Here we investigate two aniline based squaraine molecules, with linear di-butyl (i.e. DBSQ(OH)2) and di-hexyl (i.e. DHSQ(OH)2) side chains. Our study clearly shows that both self-aggregation, bulk heterojunction morphology, and electronic properties of squaraines are strongly influenced by side chain length. DBSQ(OH)2 outperforms the DHSQ(OH)2 in solar cell devices, reaching a power conversion efficiency of 3.6 % with a simple device structure. Our result is surprisingly different from the studies of the influence of polymer side chain length for polymer based solar cells.10 We consider the large effect of side chain modification to be related to the unique properties of small molecules, such as strong aggregation tendency,11 strict packing geometry in the solid state5 and shorter intermolecular distance12; and thus the structure-
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