Observing electron transport and percolation in selected bulk heterojunctions bearing fullerene derivatives, non-fullerene small molecules, and polymeric acceptors

Hang Yin, Jie Yan, Johnny Ka Wai Ho, Delong Liu, Pengqing Bi, Carr Hoi Yi Ho, Xiaotao Hao, Jianhui Hou, Gang Li, Shu Kong So

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17 Citations (Scopus)


We systematically study how electrons achieve percolation pathways in a bulk-heterojunction (BHJ) blend as the weight fraction of the electron acceptor is altered. Three different classes of BHJs are explored: BHJs that contain (1) fullerene-based acceptors, (2) non-fullerene small molecules, and (3) polymer acceptors. The nano-topologies of acceptors in (1) to (3) can be approximately associated with spheres, oblates (plates) and prolates (long rods), respectively. We discover that the weight fraction of the acceptor for electron percolation to complete is successively reduced in the order of fullerenes (0.32), non-fullerene (NF) small molecules (0.25), and polymers (0.11). A new parameter, namely, the effective volume fraction, χ, is introduced to quantify the fraction of the acceptor moieties that can contribute to electron transport in the BHJs. The electron percolation regions can be fitted with a model that considers the shapes of the acceptors and their χ in the BHJs. An all-polymer BHJ containing PTB7-Th:N2200 exhibits the largest χ (0.6–0.8), indicating N2200 acceptors are the most effective in transportation of electrons. All-polymer BHJs can better preserve their electron transport capability arising from composition variations, and they are immuned from the introduction of inert materials or the exposure of moisture, in stark contrast to fullerene-based BHJs. Our results correlate the electron transport behaviors to the nano-topologies of acceptors for BHJ cells, and offer guidance to tune the compositions and understand the nano-morphology of BHJ cells using different classes of acceptors.

Original languageEnglish
Article number103950
JournalNano Energy
Publication statusPublished - Oct 2019


  • Acceptor topology
  • All-polymer solar cells
  • Effective volume fraction
  • Electron percolation
  • Moisture stability
  • Organic solar cells

ASJC Scopus subject areas

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

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