Effect of the solvent used to prepare the photoactive layer on the performance of inverted bulk heterojunction polymer solar cells

The initial performance and subsequent degradation of inverted polymer solar cells [indium-tin oxide/titanium oxide (TiOx)/[6,6]-phenyl C61 butyric acid methyl ester (PCBM): regioregular poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxylenethiophene):poly(4- styrene sulfonic acid)/Au, TiOx cell] are studied by photocurrent-voltage measurements as well as ac impedance spectroscopy (IS) and carrier mobility measurements. The TiOx cells containing a P3HT:PCBM layer prepared from a solution of chlorobenzene (CB) showed a maximum power conversion efficiency (PCE) of 2.23%. In contrast, the TiOx cells containing a P3HT:PCBM layer prepared from a solution of 1,2,3,4- tetrahydronaphthalene (tetralin) containing 2 vol- 1,8-octanedithiol (ODT) exhibited a maximum PCE of 2.92%. However, after exposure to light irradiation for 100 h, the maximum PCE of the tetralin:ODT cell decreased to 68% of its initial value. On the other hand, over 96% of the maximum PCE was maintained in the CB cell after 100 h of irradiation. The IS measurement results suggest that the degradation of the Tetralin: ODT cell was caused by a morphological change of the P3HT:PCBM layer that made efficient photoinduced charge separation difficult.