Mixed Domains Enhance Charge Generation and Extraction in Bulk‐Heterojunction Solar Cells with Small‐Molecule Donors |
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| Authors: | 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 |
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| Institution: | 1. Department of Physics and Astronomy, Washington State University, Pullman, WA, USA;2. King Abdullah University of Science and Technology (KAUST), KAUST Solar Center, Physical Sciences and Engineering Division, Material Science and Engineering Program, Thuwal, Saudia Arabia;3. Electrical Engineering Department, Stanford University, Stanford, CA, USA;4. Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA, USA;5. Institut fur Physik und Astronomie, Physik weicher Materie, University of Potsdam, Potsdam‐Golm, Germany;6. Centre for Organic Photonics & Electronics, The University of Queensland, Queensland, Australia;7. Chemical Sciences Program, Physical Sciences and Engineering Division, (PSE), KAUST Solar Center (KSC), King Abdullah University of Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia |
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| 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 benzo1,2‐b:4,5‐b]dithiophene‐pyrido3,4‐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. |
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| Keywords: | charge transport domain purity microscopy mixed domains organic solar cells photovoltaic devices resonant X‐ray scattering small molecules transient spectroscopy |
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