Light-soaking issue in polymer solar cells: Photoinduced energy level alignment at the sol-gel processed metal oxide and indium tin oxide interface

We report the origin of the strong UV-irradiation dependence, generally known as a "light-soaking" process, in inverted polymer solar cells (I-PSCs) using the interface of an sol-gel processed titanium sub-oxide (TiO_x) and indium tin oxide (ITO) cathode. When I-PSCs incorporating TiO_x as an electron-selecting layer were fabricated, the as-prepared devices exhibited an anomalous J-V curve with a kink shape, resulting in an extremely low efficiency. However, the kink shape disappeared after white light irradiation for considerable duration, after which the device parameters recovered the normal values expected for this class of devices. By using electrochemical impedance spectroscopy and by measuring the contact potential difference and transient photoconductivity of the TiO_x layer, we found that the light-soaking process in I-PSCs originates from the photoinduced "rearrangement of the Fermi levels" at the sol-gel processed TiO_x and ITO cathode interface together with trap sites existing in the TiO_x layer. Based on our data, we optimized I-PSC devices with a high fill factor (FF) of ∼70%.