Ultrafast Dynamics Group
Frédéric Laquai's Group

Page 03/24/2020 23:00:24

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

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


S. Karuthedath, J. Gorenflot, A. Melianas, Z. Kan, M. Kemerink, F. Laquai. J. Phys. Chem. Lett. 2020, DOI: 10.1021/acs.jpclett.0c0075​

S. Karuthedath, J. Gorenflot, A. Melianas, Z. Kan, M. Kemerink, F. Laquai
TQ1:PC71BM, Organic solar cells, Triplet state dynamics
2020
​Triplet generation in organic solar cells has been considered a major loss mechanism. 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 co-polymer 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 non-geminate 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 build-up, TQ1:PC71BM devices exhibit only moderate geminate recombination and significantly reduced non-geminate charge recombination, with reduction factors between 10-4 to 10-3 compared to Langevin recombination.