Light illumination intensity dependence of current-voltage characteristics in polymer solar cells with solution-processed titanium chelate as electron extraction layer

Solution-processed titanium (IV) oxide bis(2,4-pentanedionate) (TOPD) as an electron extraction layer (EEL) and the blend of thieno[3,4-b]thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as a photoactive layer with a power conversion efficiency (PCE) of 9.15% are demonstrated. Analysis of the current-voltage (J-V) characteristics under dark conditions provides information on the weaker leakage current of TOPD-based polymer solar cells (PSCs) compared to ZnO-based PSCs. By investigating the photovoltaic performance under different light illumination intensities, we achieved an improvement of photovoltaic performance under weak light illumination intensity for TOPD-based PSCs. The enhanced photovoltaic performance under weak light illumination intensity for TOPD-based PSCs can be experimentally explained by the higher open-circuit voltage (VOC) value under lower light illumination intensity compared to ZnO-based PSCs. This indicates that the TOPD layer in the PSCs reduced the probability of charge carrier recombination at trap sites and improved the charge carrier collection and transport efficiency under weaker light illumination intensity. Simultaneously, the similar fill factor and short circuit current density of two PSCs can be experimentally explained by the weak bimolecular recombination between the photoactive layer and indium-tin-oxide electrode with the different EEL materials under different light illumination intensity.