Substantial improvement of the photovoltaic characteristics of TiO2/CuInS2 interfaces by the use of recombination barrier coatings

The recent success of ultrathin insulator oxide coatings on mesoporous TiO2 films (such as Al2O3 and MgO) with regard to the efficiency of solid state dye-sensitized solar cells (Jap. J. Appl. Phys. 40 (2001) L732) has received substantial attention (J. Am. Chem. Soc. 125 (2003) 475; Chem. Mater. 14 (2002) 2930). While the physical origin for the beneficial effect of these coatings (referred to as recombination barrier coatings in this text) is still under discussion, it is certainly accompanied by a decrease of the interface recombination rate (J. Am. Chem. Soc. 125 (2003) 475). These findings inspired us to investigate the effect of recombination barrier coatings at nanostructured TiO2/CuInS2 interfaces. Due to the high internal interface area, interface recombination can be expected to be the dominant recombination pathway in this type of solar cells and the investigation of concepts for its passivation is therefore crucial. Apart from the oxide coatings we also included In(OH)xSy and In2S3 surface layers into our studies. In this contribution we first provide experimental evidence for the sensitivity of the TiO2/CuInS2 interface with respect to recombination and then report on the substantial improvement of the photovoltaic characteristics, in particular the photocurrent density of flat and nanostructured TiO2 /CuInS2 interfaces using Al2O3 barrier and/or In2S3 buffer coatings. These findings lead for the first time to the practical realization of a nanostructured cell of this type (TiO2/Al2O3/In2S3/CuInS2) with a current density well above 10 mA/cm2 at 100 mW/cm2 illumination intensity and an overall efficiency of almost 3%.