Balanced Electric Field Dependent Mobilities: A Key to Access High Fill Factors in Organic Bulk Heterojunction Solar Cells

Hang Yin, Pengqing Bi, Sin Hang CHEUNG, Wai Leong Cheng, Ka Lok Chiu, Carr Hoi Yi Ho, Ho Wa Li, Sai Wing Tsang, Xiaotao Hao, Shu Kong SO*

*Corresponding author for this work

Research output: Contribution to journalJournal articlepeer-review

48 Citations (Scopus)


The compositions of most lab-based bulk-heterojunction (BHJ) solar cells are optimized by their power conversion efficiencies (PCEs). In this report, we suggest that the compositions should be optimized by their fill-factors (FFs) instead. With the optimized-FF approach, BHJ cells tend to have higher acceptor contents and possess better thermal and operational stabilities. Three model systems, namely, PTB7:PC71BM, PTB7-Th:ITIC, and PBDB-T:ITIC BHJs, are chosen as case studies. Charge carrier transport measurements are used to reveal the origin of the enhanced FFs of these BHJ solar cells. We demonstrate that these acceptor-rich BHJs possess better balanced field-dependent electron-to-hole mobility ratios due to improved electron mobilities near open-circuit conditions. We introduce a new parameter, known as charge imbalance factor (Δ), to quantify the impact of field dependent mobilities on the FF of the OPV cells. The improved mobility ratio (reduced Δ) suppresses carrier recombinations (especially at the open-circuit conditions). Despite having slightly reduced PCEs, the FF-optimized cells enjoy a much better stability. Our results suggest that FF-optimized BHJ cells with higher acceptor contents should be considered for practical applications, due to better thermal and operational stability.

Original languageEnglish
Article number1700239
JournalSolar RRL
Issue number4
Publication statusPublished - 1 Apr 2018

Scopus Subject Areas

  • Atomic and Molecular Physics, and Optics
  • Energy Engineering and Power Technology
  • Electronic, Optical and Magnetic Materials
  • Electrical and Electronic Engineering

User-Defined Keywords

  • balanced field dependent mobilities
  • bulk heterojunction cells
  • device stability
  • imbalance factors


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