We explored relationships between photophysical processes and solar cell characteristics in solution-processable bulk heterojunctions (BHJs), in particular: (1) polymer donor:fullerene acceptor:small-molecule (SM) nonfullerene acceptor, (2) polymer donor:SM donor:SM nonfullerene acceptor, and (3) SM donor:SM nonfullerene or fullerene acceptor. Addition of a nonfullerene SM acceptor to “efficient” polymer:fullerene BHJs led to a reduction in power conversion efficiency (PCE), mostly due to decreased charge photogeneration efficiency and increased disorder. By contrast, addition of an SM donor to “inefficient” polymer:SM nonfullerene acceptor BHJs led to a factor of two to three improvement in the PCE, due to improved charge photogeneration efficiency and transport. In most blends, exciplex formation was observed and correlated with a reduced short-circuit current (Jsc) without negatively impacting the open-circuit voltage (Voc). A factor of ∼5 higher PCE was observed in SM donor:fullerene acceptor BHJs as compared to SMBHJs with the same SM donor but nonfullerene acceptor, due to enhanced charge carrier photogeneration in the blend with fullerene. Our study revealed that the HOMO and LUMO energies of molecules comprising a blend are not reliable parameters for predicting Voc of the blend, and an understanding of the photophysics is necessary for interpreting solar cell characteristics and improving the molecular design of BHJs.

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Published in Journal of Photonics for Energy, v. 5, no. 1, article 057208, p. 057208-1 through 057208-15.

Copyright 2015 Society of Photo Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.

Kallarakkal Ramakrishnan Rajesh, Keshab Paudel, Brian Johnson, Rawad Hallani, John Anthony, Oksana Ostroverkhova, "Design of Organic Ternary Blends and Small-Molecule Bulk Heterojunctions: Photophysical Considerations," Journal of Photonics for Energy, Volume 5, Number 1, Article 057208, 2015. DOI: http://dx.doi.org/10.1117/1.JPE.5.057208

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This work was supported by University Grants Commission, India via Indo-US Raman Postdoctoral Fellowship- F.No. 5-50/2013(IC) and the National Science Foundation grant DMR-1207309.

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