Polymer Triplet Energy Levels Need Not Limit Photocurrent Collection in Organic Solar Cells

We study charge recombination via triplet excited states in donor/acceptor organic solar cells and find that, contrary to intuition, high internal quantum efficiency (IQE) can be obtained in polymer/fullerene blend devices even when the polymer triplet state is significantly lower in energy than the intermolecular charge transfer (CT) state. Our model donor system comprises the copolymer PIDT-PhanQ poly(indacenodithiophene-co-phenanthro[9,10-b]-quinoxaline), which when blended with phenyl-C-71-butyric acid methyl ester (PC71BM) is capable of achieving power conversion efficiencies of 6.0% and IQE approximate to 90%, despite the fact that the polymer triplet state lies 300 meV below the interfacial CT state. However, as we push the open circuit voltage (V-OC) higher by tailoring the fullerene reduction potential, we observe signatures of a new recombination loss process near V-OC = 1.0 V that we do not observe for PCBM-based devices. Using photoinduced absorption and photoluminescence spectroscopy, we show that a new recombination path opens via the fullerene triplet manifold as the energy of the lowest CT state approaches the energy of the fullerene triplet. This pathway appears active even in cases where direct recombination via the polymer triplet remains thermodynamically accessible. These results suggest that kinetics, as opposed to thermodynamics, can dominate recombination via triplet excitons in these blends and that optimization of charge separation and kinetic suppression of charge recombination may be fruitful paths for the next generation of panchromatic organic solar cell materials with high V-OC and J(SC).