Analysis of device stability through measurements of the electronic structure of organic solar cells

Abstract

We describe experimental measurements which determine the band tail density of states distribution in bulk heterojunction solar cells, with measurements made on P3HT:PCBM and PCDTBT:PCBM. Measurement of the density of states enables analysis of the effects of annealing and environmental stability. Transient photoconductivity experiments allow measurement of the electron and hole drift mobility and the dispersive nature of the electronic transport is directly related to the band tail density of states. We have developed techniques to measure the band tail over a range of energies. In each type of cell we observe an approximately exponential band tail, but also find an additional distribution of deep states. Measurements of the photocurrent spectral response to low energy provide a 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. These two experiments allow the changes in the density of states and the corresponding changes in transport and recombination to be measured as a function of cell processing and exposure. The paper will discuss the effects of annealing and chemical exposure.