Intermediate Band Solar Cells Based on Cr:ZnS: Device Characterization and Simulation
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- Institutt for fysikk 
Intermediate band solar cells (IBSCs) are emerging, potentially high efficiency, solar cells. In this work, IBSCs based on zinc-sulfide with a high chromium doping density (Cr:ZnS), are fabricated, characterized and gradually improved. The first devices showed an efficiency of (0.008 ± 0.001) %. Improvements in the cell design led to a record efficiency of (2.05 ± 0.12) %.Thin undoped and Cr-doped ZnS films, on a silicon substrate, were processed into complete devices and characterized with current-voltage and open circuit voltage versus light intensity measurements. Solar cell simulations were made with a numerical simulation program (SCAPS), to get a better understanding of our solar cells. Different solar cell parameters, absorption coefficients, and ZnS film thicknesses were varied and investigated in the simulations. Addition of the chromium doping in the zinc-sulfide films resulted in large improvements in the cell efficiencies, compared to the undoped cells, due to below band gap absorption and a higher conductivity in this layer. This was seen in both the experimental and simulated cells. Aging effects were observed for some of the solar cells, when the zinc-sulfide layer is exposed to air. However, when a thin top layer of aluminum doped zinc-oxide was used, the aging effect was strongly reduced or removed. This work is a contribution to the realization of high performance IBSCs, based on abundant, non-toxic, and inexpensive materials. The total efficiencies achieved are still low, in comparison to established solar cell technologies, but the trend is going the right direction. Hopefully, these solar cells will approach their theoretical limits in near future and contribute to a revolution in the worldwide electricity production.