Buildup of Triplet-State Population in Operating TQ1:PC71BM Devices Does Not Limit Their Performance

Safakath Karuthedath; Julien Gorenflot; Armantas Melianas; Zhipeng Kan; Martijn Kemerink; Frédéric Laquai

doi:10.1021/acs.jpclett.0c00756

Buildup of Triplet-State Population in Operating TQ1:PC₇₁BM Devices Does Not Limit Their Performance

J Phys Chem Lett. 2020 Apr 16;11(8):2838-2845. doi: 10.1021/acs.jpclett.0c00756. Epub 2020 Mar 27.

Authors

Safakath Karuthedath¹, Julien Gorenflot¹, Armantas Melianas², Zhipeng Kan¹, Martijn Kemerink³, Frédéric Laquai¹

Affiliations

¹ Materials Science and Engineering Program (MSE), Physical Sciences and Engineering Division (PSE), KAUST Solar Center (KSC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia.
² Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics, Center of Organic Electronics (COE), Linköping University, 58183 Linköping, Sweden.
³ Complex Materials and Devices, Department of Physics, Chemistry and Biology, Linköping University, 58183 Linköping, Sweden.

PMID: 32202789
DOI: 10.1021/acs.jpclett.0c00756

Abstract

Triplet generation in organic solar cells has been considered a major loss channel. Determining the density of the triplet-state population in an operating device is challenging. Here, we employ transient absorption (TA) spectroscopy on the quinoxaline-thiophene copolymer TQ1 blended with PC₇₁BM, quantify the transient charge and triplet-state densities, and parametrize their generation and recombination dynamics. The charge recombination parameters reproduce the experimentally measured current-voltage characteristics in charge carrier drift-diffusion simulations, and they yield the steady-state charge densities. We demonstrate that triplets are formed by both geminate and nongeminate recombination of charge carriers and decay primarily by triplet-triplet annihilation. Using the charge densities in the rate equations describing triplet-state dynamics, we find that triplet-state densities in devices are in the range of charge carrier densities. Despite this substantial triplet-state buildup, TQ1:PC₇₁BM devices exhibit only moderate geminate recombination and significantly reduced nongeminate charge recombination, with reduction factors between 10^-4 and 10^-3 compared to Langevin recombination.