Carrier generation and collection in CdS/CdSe-sensitized SnO2 Solar cells exhibiting unprecedented photocurrent densities

Abstract

CdS/CdSe-sensitized nanostructured SnO2 solar cells exhibiting record short-circuit photocurrent densities have been fabricated. Under simulated AM 1.5, 100 mW cm-2 illumination, photocurrents of up to 17.40 mA cm-2 are obtained, some 32% higher than that achieved by otherwise identical semiconductor-sensitized solar cells (SSCs) employing nanostructured TiO2. An overall power conversion efficiency of 3.68% has been achieved for the SnO2-based SSCs, which compares very favorably to efficiencies obtained by the TiO2-based SSCs. The characteristics of these SSCs were studied in more detail by optical measurements, spectral incident photon-to-current efficiency (IPCE) measurements, and impedance spectroscopy (IS). The apparent conductivity of sensitized SnO2 photoanodes is apparently too large to be measured by IS, yet for otherwise identical TiO2 electrodes, clear electron transport features could be observed in impedance spectra, tacitly implying slower charge transport in TiO2. Despite this, electron diffusion length measurements suggest that charge collection losses are negligible in both kinds of cell. SnO2-based SSCs exhibit higher IPCEs compared with TiO2-based SSCs which, considering the similar light harvesting efficiencies and the long electron diffusion lengths implied by IS, is likely to be due to a superior charge separation yield. The resistance to charge recombination is also larger in SnO2-based SSCs at any given photovoltage, and open-circuit photovoltages under simulated AM 1.5, 100 mW cm-2 illumination are only 26-56 mV lower than those obtained for TiO2-based SSCs, despite the conduction band minimum of SnO 2 being hundreds of millielectronvolts lower than that of TiO 2. © 2011 American Chemical Society.