Electronic Structure and Transition Energies in Polymer-Fullerene Bulk Heterojunctions

Photocurrent spectroscopy is used to measure both the charge transfer and exciton optical absorption spectra of various bulk heterojunction organic solar cells. The energy difference between the polymer HOMO energy and the fullerene LUMO energy is obtained from the spectra, along with the disorder energy. Combining information from cells with several different polymers and fullerenes allows measurements of the energy differences between HOMO or LUMO energies for about 10 different polymers and fullerenes, with an estimated uncertainty of SO meV. Heterojunction band offsets are obtained for the various cells, distinguishing between the excitonic and the single-carrier band offsets. The cell open-circuit voltage is shown to be closely correlated with the interface band gap. The exciton disorder energy is directly correlated to the band-tail disorder and we also consider the effects of exciton thermalization on the charge generation mechanism. The data indicate that an energy offset between the polymer exciton and the charge transfer ground state below about 0.25 eV adversely affects the cell performance, while a HOMO band offset below about 0.2-0.3 eV also degrades cell performance but by a different mechanism.