The impact of hot charge carrier mobility on photocurrent losses in polymer-based solar cells
Bronson Philippa, Martin Stolterfoht, Paul L. Burn, Gytis Juska, Paul Meredith, Ronald D. White, and Almantas Pivrikas. Scientific Reports, doi:10.1038/srep05695 (2014).
A typical signature of charge extraction in disordered organic systems is dispersive transport, which implies a distribution of charge carrier mobilities that negatively impact on device performance. Dispersive transport has been commonly understood to originate from a time-dependent mobility of hot charge carriers that reduces as excess energy is lost during relaxation in the density of states. In contrast, we show via photon energy, electric field and film thickness independence of carrier mobilities that the dispersive photocurrent in organic solar cells originates not from the loss of excess energy during hot carrier thermalization, but rather from the loss of carrier density to trap states during transport. Our results emphasize that further efforts should be directed to minimizing the density of trap states, rather than controlling energetic relaxation of hot carriers within the density of states.