Light-modulated TiOx Interlayer Dipole and Contact Activation in Organic Solar Cell Cathodes

Abstract

Understanding working mechanisms of selective interfacial layers and the underlying energetics of the organic semiconductor/electrode interface is an issue of primary concern for improving organic solar cell technologies. TiO x interlayers are used here to tune the selectivity of the cathode contact to electrons by controlled UV-light activation. The S-shaped kink observed for deactivated titania interlayers completely disappears after 2 min of UV-light exposure yielding high fill factor (≈60%) and adequate efficiencies. UV-light activation of complete cells alters the work function of the oxide that decreases about 650 mV as observed by Kelvin probe measurements. Capacitive techniques reveals a light-intensity dependent shift in flat-band voltage of up to 1.2 V under 1 sun illumination (without UV) in the case of deactivated TiO x interlayers. An increase in the magnitude of the light-modulated dipole present at the oxide layer accounts for that voltage shift. Although the sign of the interface dipole would favor the extraction of electrons, the concomitant modification of the band bending in the organic semiconductor hinders an efficient extraction of carriers at positive voltages and originates the S-shaped characteristics. After contact activation, the dipole strength does not change with the light intensity