Measurements of the localized state electronic structure of organic solar cells

Experimental measurements of the band tail density of states distribution in bulk heterojunction solar cells, are described, with measurements made on P3HT:PCBM and PCDTBT:PCBM. The transient photoconductivity experiment measures the electron and hole drift mobility in the solar cells. The dispersive nature of the electronic transport is directly related to the band tail density of states distribution. The technique is extended to measure the band tail DOS over a range of energies. In each type of cell we observe an approximately exponential band tail, but also find an additional broad distribution of deep states. Measurements of the photocurrent spectral response to low energy provide an alternative measure of the optical absorption spectrum for excitations from the polymer HOMO to the PCBM LUMO. These data also reveal an exponential distribution of band tail states with a slope that is the same as the electron transport measurements.