Optical absorption by defect states in organic solar cells

Organic bulk heterojunction solar cells enjoy a well established reputation, reaching conversion efficiencies up to 4% at present. Information on electronic states in the gap, whether or not connected with defects, that can influence the power conversion efficiency, are still lacking. A photoelectrical study of the optical absorption processes in encapsulated cells is presented. Fourier-transform photocurrent spectroscopy is used to measure the transitions connected with free charge carrier generation. The dynamic range obtained with this method reaches over at least eight orders of magnitude. Measurements on solar cells with an active layer of MDMO-PPV:PCBM (1:4 weight ratio) enabled a study of the intrinsic defect generation due to UV-illumination. Temperature annealing effects in P3HT:PCBM (1:2 weight ratio) cells and the induced morphological changes are related with the changes in absorption spectrum. These morphological changes were separately probed with optical and transmission electron microscopy.