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Journal of Photonics for Energy

Design of organic ternary blends and small-molecule bulk heterojunctions: photophysical considerations
Author(s): Kallarakkal Ramakrishnan Rajesh; Keshab Paudel; Brian Johnson; Rawad Hallani; John Anthony; Oksana Ostroverkhova
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Paper Abstract

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.

Paper Details

Date Published: 9 January 2015
PDF: 15 pages
J. Photon. Energy 5(1) 057208 doi: 10.1117/1.JPE.5.057208
Published in: Journal of Photonics for Energy Volume 5, Issue 1
Show Author Affiliations
Kallarakkal Ramakrishnan Rajesh, Oregon State Univ. (United States)
Keshab Paudel, Oregon State Univ. (United States)
Brian Johnson, Oregon State Univ. (United States)
Rawad Hallani, Univ. of Kentucky (United States)
John Anthony, Univ. of Kentucky (United States)
Oksana Ostroverkhova, Oregon State Univ. (United States)

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