Towards an understanding of light activation processes in titanium oxide based inverted organic solar cells

The light activation phenomenon in inverted P3HT:PCBM bulk heterojunction organic solar cells based on titanium oxide sublayer (TiO_x) is characterized by fast acquisition of current-voltage (J-V) curves under light bias as function of time. TiO_x layers were thermally treated under inert atmosphere at different temperatures prior active layer deposition and for every device an activation time was extracted. It is shown that the higher the TiO_x annealing temperature, the faster the activation. The improvement of the overall device performances is also observed for devices with TiO_x layers baked above 100 °C. The evolution of the characteristic of the organic semiconductors (OSC) device, from dielectric to diode, is attributed to the increase of TiO_x conductivity by three orders of magnitude upon white light illumination. Additionally, devices based on baked TiO_x present higher conductivity than those based on unbaked TiO_x which would explain the gain in performances and the short activation time of the OSC. In order to understand the origin of the phenomenon, deactivation experiments were also performed under different conditions on OSC. The deactivation process was shown to be thermally dependent and fully reversible under inert atmosphere, which suggest that deep traps are responsible for the activation phenomenon. An optimal annealing temperature was found at 120 °C and gives a reasonable short activation time of approximately 1 min and photo conversion efficiency up to 4%.