Rapid Recombination by Cadmium Vacancies in CdTe.

Seán R Kavanagh,David O Scanlon,Aron Walsh

doi:10.1021/acsenergylett.1c00380

Seán R Kavanagh, David O Scanlon + Show 1 more

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https://doi.org/10.1021/acsenergylett.1c00380

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Abstract

CdTe is currently the largest 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 (VCd) 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 VCd, while reproducing the VCd1– 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.

Highlights

Cadmium telluride (CdTe) is currently the largest thin-film photovoltaic technology
Undoped CdTe grown from the melt is typically found to exhibit native p-type behavior,[14] which has often been attributed to the presence of vacancies in the Cd sub-lattice.[18]
For CdTe, using a screened hybrid Density Functional Theory (DFT) functional with spin−orbit coupling (HSE+SOC), we find that the room-temperature experimental bandgap of 1.5 eV is reproduced at a Hartree−Fock exchange fraction αexx = 34.5%, a value which reproduces the experimental lattice constant to within 1%

Summary

To quantify the effect of this recombination channel on

CdTe solar cell performance, we calculate the trap-limited conversion efficiency (TLC),[66] which incorporates the effects of defect-mediated non-radiative recombination via the Shockley−Read−Hall model.[68]. As depicted in the current−voltage curve, we find that cadmium vacancies can significantly reduce the open-circuit voltage (VOC,TLC = 1.04 V), minority carrier lifetime (τe = 29 ns), and the maximum achievable photovoltaic efficiency from the ideal 32.1% to 26.7% Computational methods; supporting notes S1 and S2; Figures S1−S3 and Tables S1−S4, showing bandgapcorrected hybrid DFT functional; bulk electronic structure; vacancy bonding, structural, and electronic analysis, including; discrepancies in theoretical studies; carrier capture model, results, and analysis, experimental identification of tellurium dimerization; defect electronic densities of states; and chemical potentials (PDF).

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