Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading

TL;DR

This paper demonstrates that optimal bandgap grading and periodic backreflector corrugation can significantly improve the efficiency of ultrathin CIGS solar cells, enabling high performance with reduced material use.

Contribution

It introduces a coupled optoelectronic model to optimize bandgap grading in ultrathin CIGS cells, achieving higher efficiencies than homogeneous bandgap designs.

Findings

Sinusoidal bandgap grading predicts 27.7% efficiency for 2200-nm CIGS layer.

Grading increases efficiency to 22.89% with only 600-nm thickness.

Periodic corrugation has a smaller but positive effect on efficiency.

Abstract

The power conversion efficiency of an ultrathin CIGS solar cell was maximized using a coupled optoelectronic model to determine the optimal bandgap grading of the nonhomogeneous CIGS layer in the thickness direction. The bandgap of the CIGS layer was either sinusoidally or linearly graded, and the solar cell was modeled to have a metallic backreflector corrugated periodically along a fixed direction in the plane. The model predicts that specially tailored bandgap grading can significantly improve the efficiency, with much smaller improvements due to the periodic corrugations. An efficiency of 27.7% with the conventional 2200-nm-thick CIGS layer is predicted with sinusoidal bandgap grading, in comparison to 22% efficiency obtained experimentally with homogeneous bandgap. Furthermore, the inclusion of sinusoidal grading increases the predicted efficiency to 22.89% with just a 600-nm-thick CIGS layer. These high efficiencies arise due to a large electron-hole-pair generation rate in the narrow-bandgap regions and the elevation of the open-circuit voltage due to a wider bandgap in the region toward the front surface of the CIGS layer. Thus, bandgap nonhomogeneity, in conjunction with periodic corrugation of the backreflector, can be effective in realizing ultrathin CIGS solar cells that can help overcome the scarcity of indium.