Abstract
We report on a study of post-processing treatments in hybrid solar cells based on poly(3-hexylthiophene) (P3HT) being in contact with nanocrystalline TiO2 films. The power conversion efficiency of the hybrid polymer/TiO2 solar cells is increased over three-fold through the simultaneous application of external bias voltage and UV illumination. These treatments enhance short-circuit current density, J(SC), from 1 mA/cm(2) to over 3 mA/cm(2) under simulated air mass (AM) 1.5 conditions (100 mW cm(-2)) and lead to a peak external quantum efficiency of over 16%. The AM 1.5 open circuit voltage reaches 0.47 V and the fill factor reaches 0.53, resulting in an overall power conversion efficiency of 0.74%. The improved performance following UV exposure is correlated to a slowing of over one order of magnitude in the dynamics of charge recombination as monitored by transient optical spectroscopy. We further demonstrate that while the UV radiation present in a simulated AM 1.5 solar spectrum is sufficient to cause the increase in the J(SC) and efficiency after two hours of exposure, no change results from exposure to the same spectrum when a UV blocking filter is used. We propose that UV exposure modifies the nature or density of surface trapping species in the nanocrystalline TiO2, resulting in reduced recombination rates and a higher efficiency of collection of photogenerated charges.
