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Four different types of solar cells prepared in different laboratories have been characterized by impedance spectroscopy (IS): thin-film CdS/CdTe devices, an extremely thin absorber (eta) solar cell made with microporous TiO2/In(OH)xSy/PbS/PEDOT, an eta-solar cell of nanowire ZnO/CdSe/CuSCN, and a solid-state dye-sensitized solar cell (DSSC) with Spiro-OMeTAD as the transparent hole conductor. A negative capacitance behavior has been observed in all of them at high forward bias, independent of material type (organic and inorganic), configuration, and geometry of the cells studied. The experiments suggest a universality of the underlying phenomenon giving rise to this effect in a broad range of solar cell devices. An equivalent circuit model is suggested to explain the impedance and capacitance spectra, with an inductive recombination pathway that is activated at forward bias. The deleterious effect of negative capacitance on the device performance is discussed, by comparison of the results obtained for a conventional monocrystalline Si solar cell showing the positive chemical capacitance expected in the ideal IS model of a solar cell.

Annealing effects on the physical properties of ZnO/CdSe core−shell nanowires have been analyzed in the range of 150−400 °C. Annealing at temperatures higher than 350 °C induce a structural transition in nanocrystalline CdSe nanowire shell, from cubic zinc blende to hexagonal wurtzite structure. This transition takes place at temperatures higher than those in CdSe bulk crystals (95 °C), underlying that the CdSe structure is determined not only by the temperature but also by the crystal size. The role of free surface energy in low-dimensional CdSe systems is emphasized. Our hypothesis explains the behavior observed in CdSe nanowire shell, as well as the results from other authors in CdSe nanocrystals. Annealing at temperatures ≥350 °C also result in the increase of particle size constituting the CdSe nanowire shell from 3 nm for the as-deposited to ≥9 nm. This considerably enhances the electronic properties of the ZnO/CdSe nanowires. The improvement may result from an easier charge carrier transport in CdSe nanowire shell promoted by the loss of quantum confinement, which is revealed by optical spectroscopy. High external quantum efficiencies (>70%) in ferro-/ferricyanide solutions have been obtained for ZnO/CdSe core−shell nanowire arrays annealed at 400 °C, demonstrating their potential in nanostructured solar cells.

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