Rapid Recombination by Cadmium Vacancies in CdTe

Slides for my talk on "Rapid Recombination by Cadmium Vacancies in CdTe" at MRS Spring 2021.

YouTube talk here:
https://youtu.be/bzyek49JU0c

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

Transcript

1. Reconciliation of Theory with Experiment for Defects in CdTe – Cadmium Vacancies Seán R. Kavanagh, Aron Walsh, David O. Scanlon sean.kavanagh.19@ucl.ac.uk EN07.02.02
2. CdTe: Cadmium Vacancies (VCd ) • Key solar photovoltaic technology (first thin-film absorber to be commercialised). • Carrier recombination reduces efficiencies from the detailed-balance limit of 32%, to a current record of 22%.1 • VCd known to be an important acceptor species, affecting both conductivity and electron-hole recombination. • Despite >70 years of research, atomistic mechanisms have remained unclear. 1 Best Research-Cell Efficiency Chart — Photovoltaic Research — NREL. www.nrel.gov/pv/cell-efficiency
3. Vacant Cd site (tetrahedral Initially) CdTe: Cadmium Vacancies (VCd )
4. CdTe: VCd -1 1. Meyer, B. K. and Hofmann, D. M. (1995) ‘Anion and cation vacancies in CdTe’, Applied Physics A: Materials Science & Processing, 61, pp. 213–215. 2. Emanuelsson, P., Omling, P., Meyer, B. K., Wienecke, M. & Schenk, M. (1993) ‘Identification of the cadmium vacancy in CdTe by electron paramagnetic resonance’, Phys. Rev. B 47, pp. 15578–15580.
5. CdTe: VCd -1 1. Meyer, B. K. and Hofmann, D. M. (1995) ‘Anion and cation vacancies in CdTe’, Applied Physics A: Materials Science & Processing, 61, pp. 213–215. 2. Emanuelsson, P., Omling, P., Meyer, B. K., Wienecke, M. & Schenk, M. (1993) ‘Identification of the cadmium vacancy in CdTe by electron paramagnetic resonance’, Phys. Rev. B 47, pp. 15578–15580.
6. CdTe: VCd -1
7. CdTe: VCd 0 Double hole-polaron? Tetrahedral with delocalised holes?
8. CdTe: VCd 0 0.5 eV lower in energy than the tetrahedral or two-polaron state
9. CdTe: VCd Thermodynamic Behaviour
10. ΔE n 0 Bipolaron a b c CdTe: VCd Carrier Capture Kinetics
11. CdTe: VCd Carrier Capture Kinetics ΔE n 0 Bipolaron a b c
12. CdTe: VCd Carrier Capture Kinetics 1 S. Kim, J. A. Márquez, T. Unold and A. Walsh, Energy Environ. Sci., 2020,13, 1481-1491 2 J. M. Burst, W. K. Metzger et al, Nature Energy, 2016, 1, 1–8
13. Conclusions S. R. Kavanagh, Aron Walsh and D. O. Scanlon ACS Energy Lett. 2021, 6, 4, 1392–1398 DOI: 10.1021/acsenergylett.1c00380 VCd → Single, deep, negative-U, double-acceptor level. VCd –1 → Hole–polaron configuration. Cadmium vacancies facilitate rapid charge-carrier recombination via a 3-step cycle, reducing maximum PV efficiency by >5% for untreated CdTe –> due to tellurium dimerization, metastable configurations, and anharmonic potential energy surfaces. @Kavanagh_Sean_ kavanase sean.kavanagh.19@ucl.ac.uk