Theory of Ionization Potentials and Electron Affinities in Semiconductors

Presentation for the Scanlon Materials Theory Group meeting

Transcript

1. E-mail: alex.ganose.10@ucl.ac.uk @alexganose Theory of Ionization Potentials and Electron Affinities in Semiconductors Alex M. Ganose & Department of Chemistry, University College London Diamond Light Source Ltd.
2. Definitions (i) 2
3. Definitions (ii) IP: Energy required to remove an electron from the VBM to infinitely far away (vacuum) EA: Energy required to move an electron from infinitely far away to the CBM
4. Definitions (ii) IP: Energy required to remove an electron from the VBM to infinitely far away (vacuum) EA: Energy required to move an electron from infinitely far away to the CBM Workfunction: Energy required to move an electron from the Fermi level to infinitely far away (sample dependent)
5. Why do we care? • Carrier doping limits • Catalysis and photocatalysis • Band offsets between semiconductors determines electron transport
6. Why do we care? – Example • Alignment between absorber, TCO and HTM controls maximum obtainable voltage. • Careful choice of contact materials essential.
7. • What happens if I don’t set the VBM to 0 eV? • In planewave DFT, eigenvalues defined w.r.t the background electrostatic potential • Eigenvalues are not absolute energies WTF – just use the eigenvalues? 7
8. Bulk band alignments (i) • Can easily plot electrostatic potential (MacroDensity) • But need somewhere to call the vacuum level https://github.com/WMD-group/MacroDensity
9. Bulk band alignments (ii) • Trick is to introduce a portion of vacuum into the cell by making a slab (non-polar) • 20 Å slab & 20 Å vacuum; no cell relaxation
10. Bulk band alignments (iii) • Easy to calculate IP and EA by comparing to bulk eigenvalues and core states:
11. Bulk band alignments (iii) • Easy to calculate IP and EA by comparing to bulk eigenvalues and core states:
12. Example: BiSI and BiSeI • Potential solar absorbers; devices show ~0.25 % PCE • Low Voc a result of poor band alignment A. Ganose, K. T. Butler, A. Walsh, and D. O. Scanlon, J. Mater. Chem. A (2016), 4, 2060–2068
13. Limitations of bulk band alignments • In practice, IP and EA are determined by surface termination. • Spilling of charge leads to a surface dipole • Is unique for each surface
14. Surface alignments
15. Surface alignments • By letting the cell relax, the surface IP can be calculated in a similar way as before • Recent paper suggests using the macroscopic average • Should fully include surface dependency Y. Kumagai, K. T. Butler, A. Walsh, and F. Oba, PRB (2017), 95, 125309
16. Conclusion 16 • IP and EA determine fundamental device properties • Bulk band alignment easy to calculate; surface alignment more expensive Note: Experimentally available via X-ray photoelectron spectroscopy (XPS) or Kelvin probe microscopy Warning: results are only as good as your DFT Functional
17. Acknowledgements 17 People: – Dr. David Scanlon – Christopher Savory – Dr. Keith Butler Funding: – M3S, UCL – EPSRC – Diamond Light Source Ltd. Computing resources: – MCC for access to ARCHER