Harnessing Structure-Property Relationships for Poly(alkyl thiophene)-Fullerene Derivative Thin Films to Optimize Performance in Photovoltaic Devices

Nanoscale bulk heterojunction (BHJ) systems, consisting of fullerenes dispersed in conjugated polymers have been actively studied in order to produce high performance organic photovoltaics. How the BHJ morphology affects device efficiency, is currently ill-understood. Neutron reflection together with grazing incidence X-ray and neutron scattering and X-ray photoelectron spectroscopy are utilized to gain understanding of the BHJ morphology in functional devices. For nine model systems, based on mixtures of three poly(3-alkyl thiophenes, P3AT) (A = butyl, hexyl, octyl) blended with three different fullerene derivatives, the BHJ morphology through the film thickness is determined. It is shown that fullerene enrichment occurs at both the electrode interfaces after annealing. The degree of fullerene enrichment is found to strongly correlate with the short circuit current (J(SC)) and to a lesser degree with the fill factor. Based on these findings, it is demonstrated that by deliberately adding a fullerene layer at the electron transport layer interface, J(SC) can be increased by up to 20%, resulting in an overall increase in power conversion efficiency of 5%.