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The formation of MoSe2 has been studied on polycrystalline Mo layers and on Mo single crystals in dependence of the Mo orientation, the Na concentration, and also as a function of the Se source and the substrate temperatures. The Mo substrates were selenized by evaporation of Se. The samples were analyzed by means of X-ray diffraction, Rutherford backscattering spectrometry, elastic recoil detection analysis, and by conventional and high-resolution transmission electron microscopy. It was found that the crystal structure and orientation of the MoSe2 layer change with increasing substrate temperature. However, the texture of MoSe2 does not depend on the orientation of the Mo substrate. It was also found that the MoSe2 growth is significantly influenced by the Na concentration at substrate temperatures of 450°C and 580°C.

To characterize the influence of sodium diffusion on the Cu(In,Ga)Se2 (CIGS) layer, samples with and without an A12O3 diffusion barrier were investigated by transmission electron microscopy. The soda lime glass acts as a sodium source for samples without diffusion barrier while a post deposition treatment with NaF adds sodium to the CIGS layer for samples with diffusion barrier. For low deposition temperatures in the three-stage growth process for CIGS slightly columnar CIGS grains near the Mo back contact are found and larger grains near the top surface. Samples subject to high-temperature post deposition treatments show slightly larger grain sizes in the CIGS layer than samples prepared at the same temperature but with sodium from the soda lime glass. The surface smoothness of the CIGS layer and therefore the continuity of the following layers is improved for samples with higher deposition temperature. Occasional defects are found on the surface of the ZnO layer which can locally alter reflectivity and absorption properties of these solar cells. Despite the high CIGS surface roughness, the CdS layer with an average thickness of only 50 nm appears continuous and even fills trenches between CIGS grains near the interface.

Cu(In, Ga)Se2 (CIGS) layers were grown with different vacuum evaporation recipes and the effects of sodium on the grain growth were investigated by scanning electron microscopy. A reduced grain size mainly in the lower part of films grown with the 3-stage process was observed when Na was available during growth. The growth kinetics were found to be influenced during the second stage of the process. In contrast, no clear change in grain size could be observed for CIGS layers grown with constant evaporation rates, with a bi-layer and with an ’extreme’ bi-layer process. A mechanism for the action of Na is proposed, which is based on the assumption that Na passivates CIGS grain boundaries and that this results in a surfactant effect. The mechanism explains the different influences of Na on CIGS growth for different evaporation recipes, the occurrence of reduced grain size in the 3-stage process and the surface smoothing effect observed mainly for CIGS grown with the bi-layer process.In-diffusion of Na after CIGS growth is shown to be an alternative Na incorporation method that does not reduce the CIGS grain size. A conversion efficiency improvement from 10.4% to 14.2% was achieved due to the post-deposition treatment on CIGS absorbers grown without Na at a maximum substrate temperature of 450°C.

Low substrate temperatures have to be used for polymer substrates. Therefore, using soda- lime glass (SLG) substrates with and without an alkali barrier (Al2O3), a three-step CIGS coevaporation process for a substrate temperature of 450 °C has been developed and compared to film deposition with constant evaporation rates. The three-step process was found to enhance grain nucleation. An efficiency of 14.0 % has been achieved with this process for solar cells on SLG. Since polymers in general do not contain Na, a way of Na addition to the absorber is needed. It is shown that NaF coevaporation can be used to control the Na content in CIGS. Also incorporation of Na in CIGS by diffusion from a NaCl layer through a polyimide is demonstrated. With such SLG/NaCl/polyimide structures flexible solar cells can be obtained using a lift-off process. A cell efficiency of 11.6 % (0.99 cm2 area) has been achieved.