Growth mechanism of thermally processed Cu(In,Ga)S2 precursors for printed Cu(In,Ga)(S,Se)2 solar cells

We investigate a process used for the selenisation of particle-based precursors to prepare low-cost Cu(In,Ga)(S,Se)2 (CIGS) solar cells. It is suitable for high throughput with a short optimum selenisation duration of 35 min and employs a rapid thermal annealing system with elemental selenium vapour. Homogeneous crack-free Cu(In,Ga)S2 precursor films of up to 1 mu m are obtained via doctor blading. The high selenium vapour pressure in the selenisation reaction chamber results in the formation of a compact Cu(In,Ga)(S,Se)2 layer on top of a carbon-rich underlayer. In order to investigate the phase development in the film, the selenisation process was interrupted at different stages and the samples were monitored via XRD and surface-sensitive Raman measurements. We find the formation of a polycrystalline Cu(In,Ga)Se2 phase already after 1 s at the target temperature of 550 degrees C. Furthermore, the effect of initial precursor thickness on solar cell parameters is discussed. Complete solar cells are prepared by conventional methods, leading to conversion efficiencies well above 8%. ((c) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)