The Influence of Compound Target Preparation, Sputtering Power and Substrate Temperature on the Achievement of Cu(In,Ga)Se2 Precursors Suitable to Get High Efficiency Solar Cells

In the past, we reported Cu(In,Ga)Se2 solar cells exhibiting an efficiency of about 16% obtained by sputtering and selenization, starting from targets of InSe, GaSe and Cu. Here, we report on the requirements, which have to be respected in order to get good precursors suitable to give high efficiency by simple selenization in a Se-vapour. First of all, the targets of InSe and GaSe should exhibit a density higher than 97% to avoid their damage during the sputtering deposition. Secondly, especially for InSe, the sputtering power has to be regulated. If it is too low then films with excess of Se are obtained and this can cause the peeling of the film during selenization. If the sputtering power is too high, segregation of In on the surface of the target is observed. We found out that the right sputtering power is about 3.8 W/cm2 corresponding to a deposition rate of ~ 20 Å/sec. Another important parameter is the substrate temperature. During the InSe deposition, if the temperature substrate is higher than 670 K then the film will contain In-excess. If the temperature is lower than 470 K the film can contain Se-excess. The substrate temperature is less critical when GaSe or Cu are deposited. In this case a higher substrate temperature, in the range 670 ÷ 720 K, is used in order to get a good mixing of InSe, GaSe and Cu. During the selenization, an important parameter to be considered is the deposition rate of Se. It has been found out that the grain size of the finished Cu(In,Ga)Se2 film is larger when the evaporation rate of Se is not too high and it is around 1 μm/sec. By controlling the above parameters, high quality Cu(In,Ga)Se2 films with a grain size larger than 1 μm are reproducibly obtained. These films, used to prepare solar cells, gave us the possibility to obtain an efficiency larger than 16%.