Mechanisms behind CdSeTe solar cell high efficiency explained

Alloying cadmium tellurium (CdTe) solar cells with selenium (Se) has led to a surge of efficiency from 16.7% to 22.1%. Zheng et al. investigate the interplay of bandgrading, lifetime, band alignment and carrier concentration in the performance of CdSeTe solar cells.

“By varying the ratio of Se and Te throughout the solar cell, electro-optical properties are altered,” said author Xin Zheng. “However, physical modeling to understand these multiple effects on solar cell performance has been largely absent.”

By comparing different combinations of the four mechanisms, the researchers identified the prominent causes for the efficiency increase and provided some guidance to further enhance performance.

According to the authors, although the efficiency can be increased by lowering the bandgap, it is not the only contributor to the performance gained by CdSeTe alloying. Likewise, the adjustments of CdSeTe compositions and bandgrading depth are also not enough to account for the observed improvements by themselves.

“Performance gains from grading the Se composition and adjusting the absorption coefficient in the solar cell are relatively small compared to the effects of Se composition on carrier lifetime,” said Zheng.

Instead, the researchers identified the lifetime increase by Se alloying as a major contributing factor toward the surge in efficiency. They believe that for the efficiencies to surpass 25% without the help of back-surface reflectors or fields, the hole density of the solar cells must be increased.

In the future, the authors intend to further investigate the impact of Se composition and carrier concentration profiles on performance.

Source: “Roles of bandgrading, lifetime, band alignment, and carrier concentration in high-efficiency CdSeTe solar cells,” by X. Zheng, E. Colegrove, J. N. Duenow, J. Moseley, and W. K. Metzger Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0013726 .