Rubidium fluoride post deposition treatment of CIGS for perovskite-CIGS tandem applications
J.C. Cortes Chitiva (TU Delft - Electrical Engineering, Mathematics and Computer Science)
Olindo Isabella – Mentor (TU Delft - Photovoltaic Materials and Devices)
JG Buijnsters – Graduation committee member (TU Delft - Micro and Nano Engineering)
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Abstract
During the last years, large improvements in the efficiency on the CIGS technology have been achieved, making them excellent candidates as bottom cells for double junction tandem devices. This improvement is a result of the implementation of post deposition treatments (PDT) using alkali-metal fluorides. These alkalies prove to enhance the performance of the solar cells by passivating defects in the bulk of the CIGS, and also allow to use of thinner buffer layers. Therefore, understanding the role of the different variables of the PDT, and finding the optimal combination of them is essential to improve the performance of the device.
In this work, the identification of the optimum combination of the most relevant variables of RbF PDT on 3-stage co-evaporation low band-gap (1.00-1.01 eV) CIGS absorbers is performed by carefully examining the impact of the preselected variables such as substrate temperature, material flux, and stage duration on the performance of the solar cell. Similarly, an optimization of the buffer layer is conducted to enable proper deposition and enhanced absorption in the NIR. To interpret the impact of the PDT variables, the different sets of samples are evaluated electronically and compositionally with multiple characterization techniques.
It was determined that the concentration of Cu present in the surface of the absorber prior starting the PDT, together with the substrate temperature plays the most important role in the effectiveness of the PDT. This is mainly due to facilitating optimally Rb absorption and diffusion. Next to this, appropriate rinsing of the excess of alkali present on the surface of the absorber must be performed before the deposition of the buffer layer, in order to prevent the formation of a detrimental photoactive barrier at the CdS/CIGS interface.