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.