Band Gap Profiling and High Speed Deposition of Cu(In,Ga)Se2 for Thin Film Solar Cells

The Cu(In,Ga)Se2-based thin film solar cell is a promising candidate for becoming one of the more important solar cell technologies in the near future. In order to realize such a development a significant reduced production cost of the Cu(In,Ga)Se2 (CIGS) layer is needed. This work shows a possible way towards such a reduction by increasing the deposition rate and decreasing the CIGS thickness with almost maintained device efficiency. Obtaining an improved device performance in CIGS-based solar cells by using an in-depth variation of the band gap has earlier been investigated without any clear conclusions. In this work an extensive experimental study of the beneficial effect of band gap profiling has been performed and firmly based conclusions have been made. For standard CIGS devices the band gap profiling can result in an improved efficiency of around 0.4 % units. This gain is related to improved field-assisted carrier collection. For reduced CIGS thicknesses the importance of a band gap profiling is enhanced, and at a CIGS thickness of 0.5 μm an efficiency gain of 2.5 % units is obtained, resulting in a 13.4 % efficient device. The main reason for the gain is passivation of the back contact, which becomes increasingly detrimental for the device performance as the CIGS thickness is reduced. With an optimized band gap profile the CIGS thickness can be reduced 3-4 times, with almost solely absorption related losses. The potential for increasing the deposition rate of co-evaporated CIGS layers is shown to be large. An increase of up to 10 times compared to commonly used deposition rates is possible with only minor losses in efficiency. By using band gap profiled thin CIGS layers deposited at high rates, the production from a single evaporation system can be increased up 30 times. Such an increase will lead to the needed reduction of the production cost of the complete solar cell module