Density of electronic states model for organic solar cells

The analysis of the transport and recombination in organic bulk heterojunction (BHJ) solar cells requires knowledge of the electronic density of states. The electronic structure of the interface between the polymer donor and the fullerene acceptor is of particular importance because of its role in carrier generation and recombination. Data from electronic transport, photoconductivity spectral response and theoretical calculations are described, and we show that these provide consistent and quantitative information about the electronic structure. In particular, the three techniques each give independent evidence for an approximately exponential band tail of localized states. The photoconductivity spectral response shows an exponential Urbach tail at low energy characteristics of band tail optical transitions, and also provides a measure of the interface band gap and the band offset between the polymer and fullerene. Time-of-flight photoconductivity shows dispersive transport characteristic of multiple trapping in a band tail and also measured the recombination lifetime. The theoretical calculations demonstrate that an exponential band tail arises from disorder in the pi-pi stacking of the polymer chains, and also gives the density of mobile states. Together the results provide the information to construct a quantitative density of states model. We discuss how the model is used to understand carrier transport and recombination properties of BHJ solar cells.