Data related to manuscript [‘Short Excited State Lifetimes Mediate Charge Recombination Losses in Organic Solar Cell Blends with Low Charge Transfer Driving Force', Advanced Materials (2021), https://doi.org/10.1002/adma.202101784]

Abstract

We investigate a blend of a low optical-gap diketopyrrolopyrrole polymer and a fullerene derivative, with near-zero driving force of 50 meV for interfacial electron transfer. Using femtosecond transient absorption and electro-absorption spectroscopy, we quantify the charge transfer (CT) and recombination dynamics as well as the transport at early timescales. Electron transfer is found to be ultrafast, which is consistent with a semiclassical Marcus-Levich-Jortner description at low driving force and low reorganization energy. However, we observe significant geminate recombination and unusually short S1 and CT state lifetimes in the investigated system (13-14 ps). At low S1-CT offset, a short excited state lifetime mediates charge recombination because i) back-transfer from the CT to the S1 state followed by S1 recombination can occur and ii) additional S1-CT hybridization can decrease the CT lifetime. Both effects are confirmed by density functional theory calculations. In addition, we observe relatively slow (tens of picoseconds) dissociation of charges from the interfacial CT state, in contrast to polymer:fullerene blends with high CT driving force. We identify low local charge carrier mobility as a primary reason for the slow rise of free charge population. Simulations using a four-state kinetic model entailing the effects of energetic disorder reveal that the free charge yield could be increased from the observed 12% to 60% by increasing the S1 and CT lifetimes to 150 ps. Alternatively, decreasing interfacial CT state disorder while increasing bulk disorder of free charges enhances the yield to 65% in spite of the short lifetimes.