EMRS Meeting 2017
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Presentation on sulfosalt candidate photoferroic materials: screening procedure and optoelectronic properties.
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EMRS Meeting 2017
- 1. Centre for Sustainable Chemical Technologies Candidate photoferroic absorber materials for solar cells from naturally occurring minerals: enargite, stephanite and bournonite Suzanne K. Wallace, Katrine L. Svane, William P. Huhn, Tong Zhu, David B. Mitzi, Volker Blum, Aron Walsh* EMRS Spring Meeting 2017 24th May 2017
- 2. Centre for Sustainable Chemical TechnologiesPhotoferroic materials for solar cells Bulk photovoltaic effect (BPE) photocurrents measured in single crystals (p-n or p-i-n junctions not needed) Anomalous photovoltaic effect (APE) measured photovoltages >> band gap trieste.nffa.eu/areas/theory/ferroelectric-properties/ photoferroic = photoactive ferroelectric K. T. Butler, J. M. Frost and A. Walsh, Energy Environ. Sci., 2015, 8, 838–848.
- 3. Centre for Sustainable Chemical Technologies 1. Ferroelectric domains Enhanced carrier separation [1] 2. Large dielectric constant Defect-tolerant carrier transport [2] 3. APE in MAPI Measured photovoltage of ~12 V [3] CH3NH3PbI3 (MAPI) methylammonium lead iodide Design principles inspired by MAPI [1] J. M. Frost et al, Nano Letters, 2014, 14, 2584–2590 [2] R. E. Brandt et al. MRS Commun., 2015, 5, 265–275 [3] Yuan et al. Science Advances, 2017, vol. 3, no. 3, e1602164 DOI:10.1038/nature12509
- 4. Centre for Sustainable Chemical Technologies ~200 naturally occurring minerals
- 5. Centre for Sustainable Chemical TechnologiesChemically stable ~200 naturally occurring minerals
- 6. Centre for Sustainable Chemical Technologies ~200 naturally occurring minerals Chemically stable Dark streak colour Eg in visible range
- 7. Centre for Sustainable Chemical Technologies ~200 naturally occurring minerals Chemically stable Dark streak colour Eg in visible range Polar crystal structure Potential ferroelectric Candidate photoferroic minerals
- 8. Centre for Sustainable Chemical TechnologiesCandidate photoferroics: Sulfosalt minerals Cu Ag Pb Sb S As Enargite Cu3AsS4 Stephanite Ag5SbS4 Bournonite CuPbSbS3
- 9. Centre for Sustainable Chemical TechnologiesCandidate photoferroics: Sulfosalt minerals Cu Ag Pb Sb S As Enargite Cu3AsS4 Stephanite Ag5SbS4 Bournonite CuPbSbS3
- 10. Centre for Sustainable Chemical TechnologiesPV design principles
- 11. Centre for Sustainable Chemical Technologies 1. Magnitude of the band gap Sunlight-matched (~ 1.0-1.7 eV) 2. Strength of optical absorption • Direct band gap • Abrupt of absorption edge • c.f. SLME metric [4] 3. Light charge-carrier effective-mass Better carrier mobility and long diffusion length [4] Yu, L.; Zunger, A. Phys. Rev. Lett. 2012, 108 (6), 68701 DOI: 10.1126/science.aad4424 Standard PV design principles
- 12. Centre for Sustainable Chemical Technologies 4. Rashba splitting: may reduce radiative recombination rate and contribute to long carrier lifetimes in MAPI [5, 6]. [5] P. Azarhoosh et al, APL Materials, 2016, 4, 091501 [6] Wang et al, Energy Environ. Sci., 2017, 10, 509-515 [7] R. E. Brandt et al, MRS Communications, 2015, 5(2), 1–11 5. Possible indicators of defect tolerance • Active ns2 lone pairs: character of band extrema implies shallow defects are likely [7] • Large dielectric constant: enhanced charge screening [7] More novel PV design principles
- 13. Centre for Sustainable Chemical TechnologiesDefect tolerance 1. Shallow defects • Reduction in SRH e--h+ recombination • Possible link to character of band extrema [7-9] Host material with antibonding upper VB and bonding lower CB Dangling bond defects repelled into continuum bands instead of band gap 2. Reduced scattering • Carrier transport less hindered by presence of charged defects • Long diffusion lengths in defective materials • Linked to large 𝜀 [7] e- DOI: 10.1021/jz5001787 [7] R. E. Brandt et al, MRS Communications, 2015, 5(2), 1–11 [8] S. B. Zhang et al, Phys.Rev. B, 1998, 57, 9642–9656 [9] A. Zakutayev et al, Phys. Chem. Lett., 2014, 5, 1117–1125
- 14. Centre for Sustainable Chemical TechnologiesComputational methods • HSE06+SOC • Default ‘tight’ settings for basis set Geometry optimisation • Lattice parameters fixed to high-quality XRD data from the ICSD • Internal coordinates relaxed to within a tolerance of 1x10-3 eV/ Å • 4x4x4 gamma-centred k-point grid Band structure calculations • 8x8x8 k-point grid Optical dielectric function, 𝜺 𝝎 • Random phase approximation • Bournonite: 8x8x8 k-point grid • Enargite and stephanite: 10x10x10 k-point grid Absorption co-efficient, 𝜶(𝝎) All-electron electronic structure code Numeric atom-centred orbital basis sets
- 15. Centre for Sustainable Chemical TechnologiesOptoelectronic properties for PV Enargite Cu3AsS4 Stephanite Ag5SbS4 Bournonite CuPbSbS3 1.24 eV 1.37 eV 1.42 eV 1.59 eV Enargite Stephanite Bournonite me cond 0.21 0.33 0.45 mh cond 0.49 0.86 0.94
- 16. Centre for Sustainable Chemical TechnologiesRashba splitting for PV Bournonite (CuPbSbS3) - with SOC - without SOC No SOC direct gap SOC Rashba split
- 17. Centre for Sustainable Chemical TechnologiesAbsorption coefficient
- 18. Centre for Sustainable Chemical TechnologiesDefect tolerance from electronic structure Bonding character of VBM Bournonite CuPbSbS3 Enargite Cu3AsS4 Stephanite Ag5SbS4 Cu d-orbital S p-orbital Ag d-orbital S p-orbital Cu d-orbital S p-orbital
- 19. Centre for Sustainable Chemical TechnologiesSpontaneous lattice polarisation Katrine Svane [10] R. Wahl, D. Vogtenhuber and G. Kresse, Phys. Rev. B, 2008, 78, 104116 [11] H. H. Wieder, Phys. Rev., 1955, 99, 1161–1165 [12] I. Grinberg and A. M. Rappe, Phys. Rev. B, 2004, 70, 220101 [10] [11] [12] Computational details • 500 eV cut-off energy • 2x2x2 k-point grid • HSE06 functional
- 20. Centre for Sustainable Chemical TechnologiesConclusions and outlook 1. Optoelectronic properties for PV • Sunlight-matched optical band gap • m* < 1me • Strong absorption • Rashba splitting in bournonite 2. Strong lattice polarisation of enargite and stephanite Next steps: defect-tolerance 1. Calculate defect levels Test hypothesis for tolerance from band extrema character 2. Calculate ionic dielectric constant Large contribution as in MAPI? [13] [13] Brivio et al, APL Materials 1, 042111 (2013) Volker Blum William Huhn Tong Zhu David Mitzi Aron Walsh and Katrine Svane
- 21. Centre for Sustainable Chemical Technologies Defect tolerance from band extrema character • Host material with antibonding upper VB and bonding lower CB Dangling bond defects repelled into continuum bands instead of band gap Associated with: • Lone pairs bonding s-orbital deep in VB • p-d repulsion bonding d-band below the VB Examples: • Pb ns2 electrons in MAPbI3 [5] • Cu 3d-states in CuInSe2 [6] and Cu3N [7] DOI: 10.1021/jz5001787 [5] R. E. Brandt et al, MRS Communications, 2015, 5(2), 1–11 [6] S. B. Zhang et al, Phys.Rev. B, 1998, 57, 9642–9656 [7] A. Zakutayev et al, Phys. Chem. Lett., 2014, 5, 1117–1125 Extend argument beyond vacancies and interstitials: Antisites, surfaces and dislocations formed by same bond breaking mechanisms [7]
- 22. Centre for Sustainable Chemical Technologies Defect tolerance based on electronic structure S. B. Zhang et al, Phys.Rev. B, 1998, 57, 9642–9656 pronounced p-d repulsion More anti-bonding character at VBMCuInSe2 (tolerant.) vs. CuIn5Se8 (sensitive) More bonding character at VBM
- 23. Centre for Sustainable Chemical Technologies p-d repulsion in enargite 1. p-d repulsion in CuInSe2 and enargite (Cu3AsS4) S. B. Zhang et al, Phys.Rev. B, 1998, 57, 9642–9656
- 24. Centre for Sustainable Chemical Technologies Energy barriers to polarisation switching
- 25. Centre for Sustainable Chemical Technologies Defect tolerance from electronic structure Enargite Stephanite Bournonite MAPI <100> [12] 𝜀∞ 𝜀∞ 𝜀∞ 𝜀∞ 𝜀0 xx 5.70 6.01 7.16 6.29 22.39 yy 5.89 5.86 7.24 5.89 27.65 zz 5.91 5.83 7.55 5.75 17.97 [12] Brivio et al. APL Materials 1, 042111 (2013). 3. Dielectric constants compared to MAPI 𝜀 = 𝜀0 + 𝜀∞
- 26. Centre for Sustainable Chemical Technologies Computational Methodologies
- 27. Centre for Sustainable Chemical Technologies Electronic band structures with FHI-aims FHI-aims: all-electron electronic structure code based on numeric atom-centred orbital basis sets Geometry optimization • Initial structures taken from high-quality XRD data from the icsd • Default ‘tight’ settings • HSE06+SOC • 4x4x4 gamma-centred k-point grid • Lattice parameters fixed to unit cell from high quality XRD data • Internal coordinates relaxed to within a tolerance of 1x10-3 eV/ Å Band structure calculations • HSE06+SOC • 8x8x8 k-point grid
- 28. Centre for Sustainable Chemical Technologies Absorption co-efficients with FHI-aims Optical dielectric function, 𝜺(𝝎) • Random phase approximation (RPA) • HSE06+SOC • 10x10x10 k-point grid for enargite and stephanite • 8x8x8 k-point grid for bournonite Absorption co-efficient, 𝜶(𝝎)
- 29. Centre for Sustainable Chemical Technologies Spontaneous lattice polarisation with VASP VASP: plane-wave electronic structure code using PAW pseudopotentials • Using Berry-phase formalism • Polarisation only allowed along z-axis for all 3 materials (based on symmetry) • Only differences in polarisation are meaningful • Optimise +PS structure • Invert to obtain structure with opposite polarisation, -PS • Calculate polarisation difference between two structures, 2PS • To verify that change in polarisation is continuous, calculate P of a number of configurations connecting the +PS and -PS structures VASP calculation settings: • 500 eV cut-off energy for plan-wave basis set • 2x2x2 k-point grid • HSE06 functional K. L. Svane and A. Walsh, The Journal of Physical Chemistry C, 2016.