Increased open circuit voltage in Cu In,Ga S2 based solar cells prepared by rapid thermal processing of metal precursors

In this work, we present the progress made in Cu In,Ga S2 based solar cells prepared from sputtered metals subsequently sulfurized using rapid thermal processing in sulfur vapor. In a previous paper, we showed that Cu In,Ga S2 solar cells with efficiencies of 12.2 could be achieved. The current collection of the obtained cells as confirmed by the external quantum efficiency was excellent. However the used process resulted in absorbers with very expressed CuInS2 CuGaS2 top bottom segregation [1]. The electronic properties, including fill factor and open circuit voltage, were thus inferior to those obtained in our earlier work with multi source evaporated absorbers, where we could show that the best performance is obtained when there is a gallium content of about five percent in the upper part of the absorber layer [2]. In the current contribution, we show that the required degree of interdiffusion can be achieved by choosing appropriate sulfurization temperature profiles. Incomplete sulfurization is avoided and the process stability and reproducibility of cell properties is greatly enhanced. The corresponding material and cell properties are similar to multi source evaporation and a new record cell is obtained. The process leads to an effective band gap between those of the gallium free reference and the most efficient multi source evaporated cells. It was possible to increase the effective band gap resulting in open circuit voltages up to 842 mV. The structure and electronic properties of absorbers grown by different approaches and cells prepared from them has been analyzed. We will discuss the correlation between structure, gallium depth profile and cell performance. Voltage and quantum efficiency in relation to the effective band gap are compared to those of the multi source evaporated cells. Process control of the buffer layer preparation suggests different absorber surface properties and the need to re adjust heterojunction preparation in order to fully exploit the efficiency potential of our sequentially prepared Cu In,Ga S2 absorber layer