jz0c00756_si_001.pdf (1.86 MB)
Buildup of Triplet-State Population in Operating TQ1:PC71BM Devices Does Not Limit Their Performance
journal contribution
posted on 2020-03-27, 17:04 authored by Safakath Karuthedath, Julien Gorenflot, Armantas Melianas, Zhipeng Kan, Martijn Kemerink, Frédéric LaquaiTriplet
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.