Controlling Solution‐Phase Polymer Aggregation with Molecular Weight and Solvent Additives to Optimize Polymer‐Fullerene Bulk Heterojunction Solar Cells |
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Authors: | Jonathan A Bartelt Jessica D Douglas William R Mateker Abdulrahman El Labban Christopher J Tassone Michael F Toney Jean M J Fréchet Pierre M Beaujuge Michael D McGehee |
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Institution: | 1. Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA;2. Department of Chemistry, University of California, Berkeley, CA, USA;3. Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia;4. Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA, USA |
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Abstract: | The bulk heterojunction (BHJ) solar cell performance of many polymers depends on the polymer molecular weight (M n) and the solvent additive(s) used for solution processing. However, the mechanism that causes these dependencies is not well understood. This work determines how M n and solvent additives affect the performance of BHJ solar cells made with the polymer poly(di(2‐ethylhexyloxy)benzo1,2‐b:4,5‐b′]dithiophene‐co‐octylthieno3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD). Low M n PBDTTPD devices have exceedingly large fullerene‐rich domains, which cause extensive charge‐carrier recombination. Increasing the M n of PBDTTPD decreases the size of these domains and significantly improves device performance. PBDTTPD aggregation in solution affects the size of the fullerene‐rich domains and this effect is linked to the dependency of PBDTTPD solubility on M n. Due to its poor solubility high M n PBDTTPD quickly forms a fibrillar polymer network during spin‐casting and this network acts as a template that prevents large‐scale phase separation. Furthermore, processing low M n PBDTTPD devices with a solvent additive improves device performance by inducing polymer aggregation in solution and preventing large fullerene‐rich domains from forming. These findings highlight that polymer aggregation in solution plays a significant role in determining the morphology and performance of BHJ solar cells. |
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Keywords: | organic electronics photovoltaics solar cells bulk heterojunctions solvent additives |
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