Record efficiencies for dry processed cadmium free CIGS solar cells with indium sulfide buffer layers prepared by atomic layer deposition (ALD)

CIGS based solar cells are presently excellent candidates for a new generation of large scale and low-cost photovoltaic thin film modules. To allow exclusively vacuum processing, a dry alternative for the buffer layer deposition is desirable. For environmental reasons, it is also desirable to get Cd-free devices. Atomic layer chemical vapor deposition (ALCVD or ALD) is a technique ensuring conformal growth of thin films from the vapor phase in a scalable manner. This paper presents optimization studies on the formation of In2S3 buffer layers by ALD for high efficiency CIGS thin film solar cells. The main parameters investigated are the deposition temperature (between 150 and 260degreesC) and the effect of the thickness of the buffer (15 nm and 30 nm), on growth rates, properties of the films and solar cells parameters. The highest efficiency of 16.4 % has been obtained at 220degreesC for a layer thickness of 30 nm. Diffusion of copper and sodium has been evidenced by XPS measurements which may have a positive role in the junction formation processes. The importance of interface chemistry for dry buffer is underlined.