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Reduced Nonradiative Voltage Loss in Terpolymer Solar Cells

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journal contribution
posted on 30.04.2020, 19:13 by Qingzhen Bian, Birhan A. Abdulahi, Zewdneh Genene, Ergang Wang, Wendimagegn Mammo, Olle Inganäs
The dissociation of hybrid local exciton and charge transfer excitons (LE–CT) in efficient bulk-heterojunction nonfullerene solar cells contributes to reduced nonradiative photovoltage loss, a mechanism that still remains unclear. Herein we studied the energetic and entropic contribution in the hybrid LE–CT exciton dissociation in devices based on a conjugated terpolymer. Compared with reference devices based on ternary blends, the terpolymer devices demonstrated a significant reduction in the nonradiative photovoltage loss, regardless of the acceptor molecule, be it fullerene or nonfullerene. Fourier transform photocurrent spectroscopy revealed a significant LE–CT character in the terpolymer-based solar cells. Temperature-dependent hole mobility and photovoltage confirm that entropic and energetic effects contribute to the efficient LE–CT dissociation. The energetic disorder value measured in the fullerene- or nonfullerene-based terpolymer devices suggested that this entropic contribution came from the terpolymer, a signature of higher disorder in copolymers with multiple aromatic groups. This gives new insight into the fundamental physics of efficient LE–CT exciton dissociation with smaller nonradiative recombination loss.