Effects of solvent vapour annealing on the performances of benzo[1,2-b:4,5-b′]dithiophene and 4,7-di(4-hexyl-thiophen-2-yl)-5,6-difluorine-2,1,3-benzothiadiazole-based alternating polymer solar cells with different configurations

Solvent vapour annealing (SVA) treatments have been proven to be an efficient approach to optimize the morphology of the active layer in bulk heterojunction polymer solar cells. For an alternating polymer based on dioctylthiophene substituted benzo[1,2-b:4,5-b′]dithiophene and 4,7-di(4-hexyl-thiophen-2-yl)-5,6-difluorine-2,1,3-benzothiadiazole (PBDTT-DFDTBT), enhanced power conversion efficiency (PCE) from 7.65% to 8.06% was achieved in regular device configuration as ITO/PEDOT:PSS/PBDTT-DFDTBT:phenyl-C71-butyric acid methyl ester (PC₇₁BM)/poly(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluoren)(PFN)/Al after tetrahydrofuran (THF) SVA treatment. As for the inverted device configuration of ITO/ZnO/PFN/PBDTT-DFDTBT:PC₇₁BM/MoO₃/Al with THF SVA post-treatment, simultaneously decreased open-circuit voltage (V_OC) and short-circuit current density (J_SC) were obtained, giving rise to a reduced PCE of 6.60%. We observed increased absorption intensity, improved hole/electron mobility and phase purity for the THF treated blend films, implying the improvement of the photovoltaic performances. However, the THF SVA-induced upper migration of the PC₇₁BM in the active layer and the resulting trap-induced recombination are ascribed to the suppressed photovoltaic parameters of inverted device relative to the regular one.