Quantifying phase separation and interfacial area in organic photovoltaic bulk heterojunction processed with solvent additives

Abstract

“Solvent additives” process is a simple and efficient way to achieve high power conversion efficiency in bulk heterojunction organic solar cells. The morphology measurements at the nanoscale confirm that the addition of solvent additives may promote the 3′H-Cyclopropa[8,25] [5,6] fullerene-C70-D5h(6)-3′ -butanoicacid, 3′-phenyl-, methyl ester (PCBM) aggregation in poly(3-hexylthiophene-2,5-diyl) (P3HT: PCBM) blend films. Larger phase separation between P3HT and PCBM caused by solvent additives facilitates the formation of bicontinuous network and therefore charge carriers can transport efficiently to the electrodes. Small angle X-ray scattering was applied to quantify the cluster size of PCBM and the interfacial area between P3HT and PCBM. The results demonstrate that the introduction of solvent additives may enlarge the size of PCBM clusters without decreasing interfacial area. Solvent additives could balance the large interfacial area for exciton dissociation and bicontinuous pathways for carrier transportation. The effect of thermal annealing on the nanomorphology of blend film is also investigated for comparison.