Rapid Recombination by Cadmium Vacancies in CdTe

TL;DR

This study reveals that cadmium vacancies in CdTe significantly accelerate charge recombination, limiting solar cell efficiency, through mechanisms involving tellurium dimerization and metastable structures, as shown by ab initio calculations.

Contribution

It provides a detailed ab initio analysis of cadmium vacancy behavior in CdTe, clarifying its electronic structure and role in recombination processes, which was previously debated.

Findings

Cadmium vacancies facilitate rapid charge-carrier recombination in CdTe.

Recombination reduces maximum efficiency by over 5%.

Tellurium dimerization and metastable structures are key mechanisms.

Abstract

CdTe is a key thin-film photovoltaic technology. Non-radiative electron-hole recombination reduces the solar conversion efficiency from an ideal value of 32% to a current champion performance of 22%. The cadmium vacancy (V_Cd) is a prominent acceptor species in p-type CdTe; however, debate continues regarding its structural and electronic behavior. Using ab initio defect techniques, we calculate a negative-U double-acceptor level for V_Cd, while reproducing the V_Cd^-1 hole-polaron, reconciling theoretical predictions with experimental observations. We find the cadmium vacancy facilitates rapid charge-carrier recombination, reducing maximum power-conversion efficiency by over 5% for untreated CdTe -- a consequence of tellurium dimerization, metastable structural arrangements, and anharmonic potential energy surfaces for carrier capture.