Opportunity to improve thin film solar cell performance identified

Although thin film solar cell efficiencies have increased significantly in recent years, there is still plenty of room for improvement. Focusing on cadmium-telluride film technology, Moseley et al. studied theoretically how inhomogeneity in the distribution of donors and acceptors at the nanometer scale, so-called “fluctuations”, can lead to significant performance losses. The results shine light on how fluctuations hinder expected performance gains.

“Doping activation and interface recombination issues need to be solved for cadmium-telluride technology to achieve its performance potential,” said author John Moseley.

For decades, cadmium-telluride cells have been doped with copper, which limited carrier densities to the 10¹⁴ cm⁻³ range. A switch to arsenic doping has improved carrier densities by two orders of magnitude, but the overall performance is still short of the theoretical potential.

The authors sought to identify the source of this limitation. Through lab-based characterization, they found arsenic doping activation levels are limited to 1 to 5 percent in polycrystalline thin films, compared to the 10 to 50 percent demonstrated in single-crystal materials. These results suggest a high degree of carrier compensation compared to the single-crystal materials. Further analysis indicated that the high degrees of carrier compensation can lead to fluctuating potentials, which directly impact the solar cell’s open circuit voltage and thus decrease cell efficiency.

Looking forward, the authors hope their findings will draw attention to the importance of doping activation to further increase cell efficiency. They also note that since doping issues occur across a range of compound thin film solar cell materials, their results are applicable beyond cadmium-telluride.

Source: “Impact of dopant-induced optoelectronic tails on open-circuit voltage in arsenic-doped Cd(Se)Te solar cells,” by John Moseley, Sachit Grover, Dingyuan Lu, Gang Xiong, Harvey L. Guthrey, Mowafak M. Al-Jassim, and Wyatt K. Metzger, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0018955 .