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

Abstract

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
Volume8
Issue number19
DOIs
StatePublished - Jul 5 2018

Bibliographical note

Funding Information:
O.A. was supported by Prince Sattam bin Abdulaziz University in Saudi Arabia and the Saudi Arabian Cultural Mission in the United States. T.F. was supported by the Washington State University Seed Grant Program. A.P. and D.N. acknowledge financial support from the German Ministry of Science and Education (project UNVEIL). V.S. acknowledges financial support from the NDSEG fellowship. This research used resources described above of the Advanced Light Source, which is a DOE Office of Science User Facility under Contract No. DE-AC02-05CH11231 and Stanford Synchrotron Radiation Lightsource. P.M.B. acknowledges support by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No. CRG_R2_13_BEAU_KAUST_1 and under KAUST Baseline Research Funding. The authors also wish to acknowledge the Australian Centre for Neutron Scattering (formerly the Bragg Institute at the time of the measurements) and the Australian Nuclear Science and Technology Organisation (ANSTO) for providing the neutron research facilities for the NR experiments. The NR measurements were supported by an Australian Research Council Discovery Program (DP120101372). The authors would like to thank Dr. Andrew Nelson, Dr. Ravi Chandra Raju Nagiri, and Mr. Jake McEwan for their assistance in performing the NR measurements.

Publisher Copyright:
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Keywords

  • 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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