Solution-processed star-shaped oligomers in normal and inverted organic solar cells

•PCE of normal and inverted solar cells for 3 star-shaped oligomers is evaluated.•The normal structure provides better photovoltaic performance than the inverted one.•Higher PCE of the normal devices is partly due to higher shunt resistance. The performance of three different star-shaped oligomers (SSOs) as electron donor materials for organic solar cells is investigated. These promising donor components are blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) fullerene acceptor and solution-processed normal and inverted organic solar cells are fabricated. These SSOs are based on a triphenylamine core and differ in the solubilizing groups and the oligothiophene arm length. We have found that the power conversion efficiency (PCE) is by 10–60% higher in the normal structure, mainly due to an enhanced open-circuit voltage and fill factor. The observed difference in device performance can be assigned partly to the lower leakage currents. By using contact angle measurements and atomic-force microscopy studies, we estimate the degree of vertical phase separation in bulk heterojunctions. The latter has a good correlation to the corresponding photocurrent differences obtained in the normal and inverted structure devices.