Seminar: Triplet formation in non-fullerene acceptor organic solar cells, with Dr Alexander Gillett, University of Cambridge
Location: Lecture Theatre PP2, People's Palace, Mile End Campus
The development of electron accepting materials that are not based on fullerene derivatives has been key to the recent resurgence of organic solar cells (OSCs), with power conversion efficiencies of 17% reported.(1) However, despite the rapid progress made, the voltage losses of OSCs associated with non-radiative (NR) recombination events is still significantly higher than rival inorganic technologies, such as perovskites.(2) Therefore, the future of OSCs depends on our ability to highlight and minimise all major NR channels. One key NR loss pathway is the formation of low-energy triplet exciton states on either the electron donor (D) or acceptor (A) that cannot be re-dissociated into free charge carriers.(3) In this work, we systematically study triplet formation in a wide range of high-performance fullerene and non-fullerene acceptor (NFA) OSCs. Surprisingly, we observe the signatures of triplets in all NFA systems studied, except one. A detailed mechanistic study reveals that triplet exciton formation in efficient NFA OSCs occurs almost exclusively after the non-geminate recombination of free charge carriers into spin-triplet charge transfer states. Finally, through a thorough computational study of the "triplet-free" blend, we suggest design rules to ensure that triplet formation can be suppressed in future NFA OSCs.
(1) Y. Lin, B. Adilbekova, Y. Firdaus, E. Yengel, H. Faber, M. Sajjad, X. Zheng, E. Yarali, A. Seitkhan, O. M. Bakr, A. El?Labban, U. Schwingenschlögl, V. Tung, I. McCulloch, F. Laquai and T. D. Anthopoulos, Adv. Mater., 2019, 31, 1902965.
(2) S. M. Menke, N. A. Ran, G. C. Bazan and R. H. Friend, Joule, 2018, 2, 25–35.
(3) A. Rao, P. C. Y. Chow, S. Gélinas, C. W. Schlenker, C.-Z. Li, H.-L. Yip, A. K.-Y. Jen, D. S. Ginger and R. H. Friend, Nature, 2013, 500, 435–439.