# 20%-efficient polycrystalline Cd(Se,Te) thin-film solar cells with compositional gradient near the front junction

## TL;DR

Researchers improved Cd(Se,Te) solar cells by creating a bandgap gradient, achieving a 20.03% efficiency and higher voltage.

## Contribution

They introduced a Cd(O,S,Se,Te) region to enable a bandgap gradient in Cd(Se,Te) solar cells for the first time.

## Key findings

- A bandgap gradient reduced hole density and suppressed nonradiative recombination in the front junction.
- The champion device achieved 20.03% efficiency with an open-circuit voltage of 0.863 V.
- The method used commercial SnO2 buffers, making it practical for industry.

## Abstract

Bandgap gradient is a proven approach for improving the open-circuit voltages (VOCs) in Cu(In,Ga)Se2 and Cu(Zn,Sn)Se2 thin-film solar cells, but has not been realized in Cd(Se,Te) thin-film solar cells, a leading thin-film solar cell technology in the photovoltaic market. Here, we demonstrate the realization of a bandgap gradient in Cd(Se,Te) thin-film solar cells by introducing a Cd(O,S,Se,Te) region with the same crystal structure of the absorber near the front junction. The formation of such a region is enabled by incorporating oxygenated CdS and CdSe layers. We show that the introduction of the bandgap gradient reduces the hole density in the front junction region and introduces a small spike in the band alignment between this and the absorber regions, effectively suppressing the nonradiative recombination therein and leading to improved VOCs in Cd(Se,Te) solar cells using commercial SnO2 buffers. A champion device achieves an efficiency of 20.03% with a VOC of 0.863 V.

Bandgap gradient is a promising approach to improve the open-circuit voltage in thin film solar cells. Here, authors incorporate a Cd(O,S,Se,Te) region to realize the bandgap gradient at front interface and demonstrate Cd(Se,Te) solar cells with reduced recombination and a champion efficiency of 20.03%.