Significant Performance Improvement of Porphyrin-Sensitized TiO2 Solar Cells under White Light Illumination

A significant improvement in the photovoltaic performance of porphyrin-sensitized TiO2 solar cells under white light illumination is reported. The most significant improvement occurs within the first hour of light exposure and is irreversible within at least the studied 3-month period. Heat treatment in the dark produced only moderate performance improvement, whereas light treatment using a UV long-pass filter (>475 nm) led to an improvement similar to that obtained with the full simulated AM 1.5 spectrum, suggesting that the effect is linked to the photoexcitation of the porphyrin dye molecules. Light exposure resulted in simultaneous improvements in the short-circuit current (J(sc)), the open-circuit voltage (V-oc), and the fill factor (FF). The J(sc) improvement is attributed to better charge injection demonstrated by thin-film APCE measurements. Photovoltage decay measurements showed a factor of 2-3 increase in the electron lifetime after light exposure, accompanied by a comparable decrease in the electron diffusion coefficient. The improved electron lifetime combined with the increased J(sc) resulted in increased electron densities under open-circuit conditions, leading to improved V-oc. Electrochemical impedance measurements showed a reduced charge-transfer resistance for I-3(-) reduction at the Pt counter electrode, which is thought to be responsible for the increased filling factor. Furthermore, ATR-FTIR and X-ray reflectometry measurements indicated no significant change in the dye layer morphology after light exposure. An alternative mechanism involving the photogeneration of electronic states within the band gap of TiO2 leading to improved injection, slower electron transport, and improved electron lifetime is discussed.