Critical electron transfer rates for exciton dissociation governed by extent of crystallinity in small molecule organic photovoltaics

Solution-processed bulk heterojunction organic solar cells fabricated with 1,3-bis[4-(N,N-diisopentylamino)-2,6-dihydroxyphenyl]squaraine and phenyl-C61-butyric acid methyl ester were found to exhibit unexpectedly low external quantum efficiency in the squaraine regions upon annealing. X-ray diffraction (XRD), spectral response, and time-resolved microwave absorption were all used to characterize the materials used and the devices prepared from them. An explanation for the drop in efficiency is proposed using Marcus-Hush theory to tie together the changes in coherent crystal domain size found by XRD and the external quantum efficiency results. Exciton dissociation at the interface was determined to be the rate-limiting step in efficient current generation for these devices.