Donor-acceptor organic solar cells often show high quantum yields for charge collection, but relatively low open-circuit voltages (VOC) limit power conversion efficiencies to around 12%. We report here the behavior of a system, PIPCP:PC61BM, that exhibits very low electronic disorder (Urbach energy less than 27 meV), very high carrier mobilities in the blend (field-effect mobility for holes >10-2 cm2 V-1 s-1), and a very low driving energy for initial charge separation (50 meV). These characteristics should give excellent performance, and indeed, the VOC is high relative to the donor energy gap. However, we find the overall performance is limited by recombination, with formation of lower-lying triplet excitons on the donor accounting for 90% of the recombination. We find this is a bimolecular process that happens on time scales as short as 100 ps. Thus, although the absence of disorder and the associated high carrier mobility speeds up charge diffusion and extraction at the electrodes, which we measure as early as 1 ns, this also speeds up the recombination channel, giving overall a modest quantum yield of around 60%. We discuss strategies to remove the triplet exciton recombination channel.
KAUST Repository Item: Exported on 2020-10-01
Acknowledgements: S.M.M., R.H.F., M.K.R., S.A.-A., and J.-L.B. acknowledge support from the KAUST Competitive Research Grant Program. M.K.R., S.A.-A., and J.-L.B. also acknowledge generous support of their work by KAUST and the Office of Naval Research Global (Award N629091512003); they thank the KAUST IT Research Computing Team and Supercomputing Laboratory for providing computational and storage resources. N.A.R., M.W., T.-Q.N., and G.C.B. acknowledge support from the Department of the Navy, Office of Naval Research (Award Nos. N00014-14-1-0580 and N00014-16-1-25200. A.S. would like to acknowledge the funding and support from the India–UK APEX project. H.L.S. acknowledges support from the Winton Programme for the Physics of Sustainability. M.N. and H.S. gratefully acknowledge financial support from the Engineering and Physical Sciences Research Council though a Programme Grant (EP/M005141/1).