Carrier generation and collection in CdS/CdSe-sensitized SnO_2 Solar cells exhibiting unprecedented photocurrent densities

摘要

CdS/CdSe-sensitized nanostructured SnO_2 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 TiO_2. An overall power conversion efficiency of 3.68% has been achieved for the SnO_2-based SSCs, which compares very favorably to efficiencies obtained by the TiO_2-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 SnO_2 photoanodes is apparently too large to be measured by IS, yet for otherwise identical TiO_2 electrodes, clear electron transport features could be observed in impedance spectra, tacitly implying slower charge transport in TiO_2. Despite this, electron diffusion length measurements suggest that charge collection losses are negligible in both kinds of cell. SnO_2-based SSCs exhibit higher IPCEs compared with TiO_2-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 SnO_2-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 TiO_2-based SSCs, despite the conduction band minimum of SnO 2 being hundreds of millielectronvolts lower than that of TiO _2.