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)benzo[1,2-b:4,5-b']dithiophene-co- octylthieno[3,4-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. The performance of poly(di(2-ethylhexyloxy) benzo[1,2-b:4,5-b']dithiophene-co-octylthieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) bulk heterojunction solar cells strongly depends on the polymer molecular weight, and processing these bulk heterojunctions with a solvent additive preferentially improves the performance of low molecular weight devices. It is demonstrated that polymer aggregation in solution significantly impacts the thin-film bulk heterojunction morphology and is vital for high device performance. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledged KAUST grant number(s): KUS-C1-015-21
Acknowledgements: The authors acknowledge Eric Hoke and Sean Sweetnam for helpful discussion and thank the Advanced Imaging and Characterization Laboratories at King Abdullah University of Science and Technology (KAUST) for technical support. This publication was based on work supported by the Center for Advanced Molecular Photovoltaics (CAMP) (Award No KUS-C1-015-21), made possible by KAUST. J.A.B. acknowledges government support by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program, and P.M.B. and A.E.L. acknowledge financial support under Baseline Research Funding from KAUST. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. The beamline 4-2 is part of the SSRL Structural Molecular Biology Program which is supported by the DOE Office of Biological and Environmental Research, and by the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393) and the National Center for Research Resources (P41RR001209). The contents of this publication are solely the responsibility of the authors and do not necessarily represent the official views of NIGMS, NCRR or NIH. ChemMatCARS Sector 15 is principally supported by the National Science Foundation/Department of Energy under grant number NSF/CHE-0822838. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
ASJC Scopus subject areas
- Renewable Energy, Sustainability and the Environment
- General Materials Science