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Moritz Futscher1 Lucie McGovern1 Jumin Lee1 Bruno Ehrler1

1, AMOLF, Amsterdam, , Netherlands

Conventional solar cells are limited to a theoretical maximum power conversion efficiency of 34%, commonly known as the Shockley-Queisser limit. Singlet exciton fission, a process by which one high-energy singlet exciton is converted into two triplet excitons of half the energy, is a promising way of overcoming this limit. This process has been observed in a range of organic semiconductors where pentacene is the most studied for this prospect. Triplet excitons cannot decay to radiatively to the ground state, so that trap states are presumably the main decay channel. To understand behavior of tripled excitons we investigate the charge-carrier-trap distribution in pentacene-based devices using deep-level transient spectroscopy and admittance spectroscopy. We find that Schottky diodes made from pristine Pentacene have a trap state with a density of about 1017 cm-3 and an activation energy of 0.5 eV. By systematically adding impurities into the pristine pentacene layer we show that the variety and the density of charge-carrier traps can be altered, affecting the triplet lifetime. We then investigate the performance of singlet fission solar cells with various trap state densities.

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