Fate of Low-Lying Charge-Transfer Excited States in a Donor:Acceptor Blend with a Large Energy Offset

In an effort to gain a comprehensive picture of the interfacial states in bulk heterojunction solar cells, we provide a combined experimental-theoretical analysis of the energetics and dynamics of low-lying electronic charge-transfer (CT) states in donor:acceptor blends with a large frontier orbital energy offset. By varying the blend composition and temperature, we unravel the static and dynamic contributions to the disordered density of states (DOS) of the CT-state manifold and assess their recombination to the ground state. Namely, we find that static disorder (conformational and electrostatic) shapes the CT DOS and that fast nonradiative recombination crops the low-energy tail of the distribution probed by external quantum efficiency (EQE) measurements (thereby largely contributing to voltage losses). Our results then question the standard practice of extracting microscopic parameters such as exciton energy and energetic disorder from EQE.