Mechanistic investigation into the light soaking effect observed in inverted polymer solar cells containing chemical bath deposited titanium oxide

In the glass-indium tin oxide (ITO)/titanium oxide (TiOx)/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS)/Au cell (TiOx cell), which contains amorphous titanium oxide prepared by chemical bath deposition and dried at 150 °C, a light soaking effect has been observed upon irradiation with white light. In contrast, in ITO/titanium oxide (TiO2)/P3HT:PCBM/PEDOT:PSS/Au cell (TiO2 cell), which contains anatase titanium oxide prepared by heat treatment at 450 °C, the maximum power conversion efficiency was obtained just after irradiation with white light. The number of P3HT+• cation radicals in the quartz-ITO/TiOx and TiO2/P3HT:PCBM substrates was estimated by ESR measurements at room temperature upon irradiation with white light. It increased gradually with an increase in irradiation time for the TiOx substrate but increased only slightly just after light irradiation for the TiO2 substrate. Upon irradiation with UV-cut light, the performance of the TiOx cell was inferior to that of the TiO2 cell. This could be related to the resistances of the P3HT:PCBM layers which were estimated by alternating current impedance spectroscopy. The resistance of the P3HT:PCBM layer in the TiOx cell was much larger than that in the TiO2 cell, though the difference between the two cells was merely heat treatment temperature of titanium oxide films using as electron collection layers. That is, the concentration of photocarriers in the P3HT:PCBM of the TiOx cell was significantly less than that in the P3HT:PCBM of the TiO2 cell. From these experimental results, the light soaking effect could be reasonably explained by assuming the existence of charge recombination centers in the TiOx near the TiOx/P3HT:PCBM interface.