Using Ultralow Dosages of Electron Acceptor to Reveal the Early Stage Donor–Acceptor Electronic Interactions in Bulk Heterojunction Blends

Carr Hoi Yi Ho, Sin Hang CHEUNG, Ho Wa Li, Ka Lok Chiu, Yuanhang Cheng, Hang Yin, Mau Hing CHAN, Franky So, Sai Wing Tsang, Shu Kong SO*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

43 Citations (Scopus)

Abstract

Tuning the donor–acceptor (D–A) weight ratio is an essential step to optimize the performance of a bulk heterojunction (BHJ) solar cell. The unoptimized regime with a low acceptor concentration is generally unexplored despite it may reveal the early stage electronic D–A interactions. In this study, PTB7:PC71BM is used to examine factors that limit the device performance in unoptimized regime. The key limiting factor is the creation of traps and localized states originated from fullerene molecules. Photothermal deflection spectroscopy is used to quantify the trap density. Starting with pristine PTB7, addition of small concentration of fullerene increases the electron trap density and lowers the electron mobility. When the D–A weight ratio reaches 1:0.1, fullerene percolation occurs. There is an abrupt drop in trap density and simultaneously a six orders of magnitude increase in the electron mobility. Furthermore, the fill factors of the corresponding photovoltaic devices are found to anticorrelate with the trap density. This study reveals that electron trapping is the key limiting factor for unoptimized BHJ solar cells in low fullerene regime.

Original languageEnglish
JournalAdvanced Energy Materials
Volume7
Issue number12
DOIs
Publication statusPublished - 21 Jun 2017

Scopus Subject Areas

  • Renewable Energy, Sustainability and the Environment
  • Materials Science(all)

User-Defined Keywords

  • bulk heterojunction
  • carrier transport
  • photothermal deflection spectroscopy
  • photovoltaics
  • traps

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