Mixed Domains Enhance Charge Generation and Extraction in Bulk-Heterojunction Solar Cells with Small-Molecule Donors

Obaid Alqahtani, Maxime Babics, Julien Gorenflot, Victoria Savikhin, Thomas Ferron, Ahmed H. Balawi, Andreas Paulke, Zhipeng Kan, Michael Pope, Andrew J. Clulow, Jannic Wolf, Paul L. Burn, Ian R. Gentle, Dieter Neher, Michael F. Toney, Frédéric Laquai*, Pierre M. Beaujuge, Brian A. Collins

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

44 Scopus citations


The interplay between nanomorphology and efficiency of polymer-fullerene bulk-heterojunction (BHJ) solar cells has been the subject of intense research, but the generality of these concepts for small-molecule (SM) BHJs remains unclear. Here, the relation between performance; charge generation, recombination, and extraction dynamics; and nanomorphology achievable with two SM donors benzo[1,2-b:4,5-b]dithiophene-pyrido[3,4-b]-pyrazine BDT(PPTh2)2, namely SM1 and SM2, differing by their side-chains, are examined as a function of solution additive composition. The results show that the additive 1,8-diiodooctane acts as a plasticizer in the blends, increases domain size, and promotes ordering/crystallinity. Surprisingly, the system with high domain purity (SM1) exhibits both poor exciton harvesting and severe charge trapping, alleviated only slightly with increased crystallinity. In contrast, the system consisting of mixed domains and lower crystallinity (SM2) shows both excellent exciton harvesting and low charge recombination losses. Importantly, the onset of large, pure crystallites in the latter (SM2) system reduces efficiency, pointing to possible differences in the ideal morphologies for SM-based BHJ solar cells compared with polymer-fullerene devices. In polymer-based systems, tie chains between pure polymer crystals establish a continuous charge transport network, whereas SM-based active layers may in some cases require mixed domains that enable both aggregation and charge percolation to the electrodes.

Original languageEnglish (US)
Article number1702941
JournalAdvanced Energy Materials
Issue number19
StatePublished - Jul 5 2018


  • charge transport
  • domain purity
  • microscopy
  • mixed domains
  • organic solar cells
  • photovoltaic devices
  • resonant X-ray scattering
  • small molecules
  • transient spectroscopy

ASJC Scopus subject areas

  • Renewable Energy, Sustainability and the Environment
  • General Materials Science


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