Defect tolerance
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1. The document discusses predicting the defect tolerance of materials for thin-film photovoltaics from electronic structure calculations. It outlines several key properties for thin-film PV materials including a direct bandgap and strong optical absorption. 2. Defect tolerance is described as having shallow defects that do not enhance recombination and high carrier mobility such that transport is not hindered by charged defects. Design principles for defect tolerance include large dielectric constants, low effective masses, ordered defect structures, and defect-tolerant electronic structures. 3. The electronic structure principles propose that materials with antibonding valence band maxima and bonding conduction band minima will have defect states repelled into continuum bands rather than the band gap, increasing tolerance
![Design principles for defect-tolerance (cont.)
3. Ordered defect structures (ODS)
• High concentration of defects
• Periodic repetition in the lattice creates ODS
• Isolated acceptor and donor levels within band gap, but can combine to
form neutral aggregates
• E.g. [2V-
Cu + In2+
Cu] in CuInSe2
• … Although I’m not so sure how we would predict or tune this!](https://image.slidesharecdn.com/defect-tolerance-170916154324/85/Defect-tolerance-13-320.jpg)
Defect tolerance
- 1. Predicting defect tolerance from electronic structure calculations? WMD Group Meeting Suzanne Wallace 27th March 2017
- 2. Motivation Thin-film photovoltaics • Cheap and cheerful solution processing • Large-scale deployment Can we predict which materials are likely to make good thin-film PV devices? ... Before we waste too much time with trial and error!
- 3. Key Properties forThin Film PV 1. Direct, sunlight-matched optical band gap 2. Strong optical absorption (relative to Si) 3. Defect-tolerance?
- 4. Key Properties forThin Film PV 1. Direct, sunlight-matched optical band gap 2. Strong optical absorption (relative to Si) 3. Defect-tolerance?
- 5. Defect tolerance (for PV) 1. Shallow defects e--h+ recombination not enhanced by presence of deep level defects 2. High carrier mobility Carrier transport not hindered by presence of charged defects DOI: 10.1002/aenm.201100630 e-
- 6. Defect tolerance (for PV) 1. Shallow defects e--h+ recombination not enhanced by presence of deep level defects 2. High carrier mobility Carrier transport not hindered by presence of charged defects Riley Brandt’s definition:The extrinsic, intrinsic or structural defects that form have minimal effect on 𝜇 and 𝜏. DOI: 10.1002/aenm.201100630 e-
- 7. Evidence of defect tolerance (or sensitivity) • Tolerance: long carrier lifetimes of solution processed MAPI • Sensitivity: sub-band gap recombination from PL spectra of CZTS* Can we say anything about the likely defect- tolerance of a material from electronic structure calculations of the perfect, bulk material? Design principles for new PV materials? DOI: 10.1063/1.4820250 * Assuming the explanation lies in the absorber layer!
- 8. Key Papers 1. Most recently: Aron and Alex Zunger’s review ‘Lessons learned from classic semiconductor defectology: Instilling defect tolerance in new compounds’ 2. Studies on specific materials • Comparing Cu3N to GaN: A. Zakutayev, C. M. Caskey, A. N. Fioretti, D. S. Ginley, J.Vidal,V. Stevanovic, E.Tea and S. Lany, J. Phys.Chem. Lett., 2014, 5, 1117–1125. • Original concept for CIS: S. B. Zhang, S.-H.Wei, A. Zunger and H. Katayama-Yoshida, Phys. Rev. B, 1998, 57, 9642–9656. • Discussion of defect-tolerance in MAPI: R. E. Brandt,V. Stevanovi, D. S. Ginley andT. Buonassisi, MRS Communications, 2015, 5, 265–275. 3. High-throughput study: Pandey, M., Rasmussen, F. A., Kuhar, K., Olsen,T., Jacobsen, K.W., &Thygesen, K. S. (2016). Nano Letters, 16(4), 2234–2239
- 9. Key Papers 1. Most recently: Aron and Alex Zunger’s review ‘Lessons learned from classic semiconductor defectology: Instilling defect tolerance in new compounds’ 2. Studies on specific materials • Comparing Cu3N to GaN: A. Zakutayev, C. M. Caskey, A. N. Fioretti, D. S. Ginley, J.Vidal,V. Stevanovic, E.Tea and S. Lany, J. Phys.Chem. Lett., 2014, 5, 1117–1125. • Original concept for CIS: S. B. Zhang, S.-H.Wei, A. Zunger and H. Katayama-Yoshida, Phys. Rev. B, 1998, 57, 9642–9656. • Discussion of defect-tolerance in MAPI: R. E. Brandt,V. Stevanovi, D. S. Ginley andT. Buonassisi, MRS Communications, 2015, 5, 265–275. 3. High-throughput study: Pandey, M., Rasmussen, F. A., Kuhar, K., Olsen,T., Jacobsen, K.W., &Thygesen, K. S. (2016). Nano Letters, 16(4), 2234–2239
- 10. Types of defects 1. The good: conductivity-promoting ‘doping’ defects (shallow level) Can create free carriers 2. The bad: ‘killer’ defects (deep level, charge recombination centres) Enhance e--h+ recombination, quench all transport 3. The ugly: charge scattering defects Reduce mobility
- 11. Types of defects 1. The good: conductivity-promoting ‘doping’ defects (shallow level) Can create free carriers 2. The bad: ‘killer’ defects (deep level, charge recombination centres) Enhance e--h+ recombination, quench all transport 3. The ugly: charge scattering defects Reduce mobility Beneficial Terrible Pretty bad
- 12. Design principles for defect-tolerance Maximizing collection of photoexcited charge carriers! 1. Large 𝜺 Enhance screening of charged defects by host material to reduce their influence on photoexcited charge carriers 2. Low m* 1. Favours free carriers for high mobility and electrical conductivity 2. With higher m* carrier transport is more like polaron hopping Slow transport is associated with thermal energy losses e-
- 13. Design principles for defect-tolerance (cont.) 3. Ordered defect structures (ODS) • High concentration of defects • Periodic repetition in the lattice creates ODS • Isolated acceptor and donor levels within band gap, but can combine to form neutral aggregates • E.g. [2V- Cu + In2+ Cu] in CuInSe2 • … Although I’m not so sure how we would predict or tune this!
- 14. Design principles for defect-tolerance (cont.) 4. *** Defect-tolerant electronic structure *** • Defect levels more likely to be in the continuum than band gap region (where they matter less) • Scatter free carriers more because they are charged in the band gap region? • Host material with upperVB that is antibonding and lower CB that is bonding • Dangling bond defects would be repelled quantum mechanically into continuum bands instead of band gap • Often the case for materials with • Lone pairs (bonding s-orbital deep inVB) • p-d repulsion (bonding d-band below theVB) DOI: 10.1002/aenm.201100630
- 15. Key Papers 1. Most recently: Aron and Alex Zunger’s review ‘Lessons learned from classic semiconductor defectology: Instilling defect tolerance in new compounds’ 2. Studies on specific materials • Comparing Cu3N to GaN: A. Zakutayev, C. M. Caskey, A. N. Fioretti, D. S. Ginley, J.Vidal,V. Stevanovic, E.Tea and S. Lany, J. Phys.Chem. Lett., 2014, 5, 1117–1125. • Original concept for CIS: S. B. Zhang, S.-H.Wei, A. Zunger and H. Katayama-Yoshida, Phys. Rev. B, 1998, 57, 9642–9656. • Discussion of defect-tolerance in MAPI: R. E. Brandt,V. Stevanovi, D. S. Ginley andT. Buonassisi, MRS Communications, 2015, 5, 265–275. 3. High-throughput study: Pandey, M., Rasmussen, F. A., Kuhar, K., Olsen,T., Jacobsen, K.W., &Thygesen, K. S. (2016). Nano Letters, 16(4), 2234–2239
- 16. ‘DefectTolerant Semiconductors for Solar Energy Conversion’ • AntibondingVBM, bonding CBM • Electronic energy levels of constituent elements are resonant withVBs and CBs • Defect states from removal or insertion of these elements from/ to the host fall into the bands instead of in the band gap Cu3N (tolerant.) vs. GaN (sensitive) Extend argument beyond vacancies and interstitials: Antisites, surfaces and dislocations formed by same bond breaking mechanisms
- 17. Bonding character of band extrema Cu3N (tolerant) • Strong p-d interaction inVB (c.f. CuInSe2) • Pronounced Cu-d/ N-p bonding and antibonding interactions GaN (sensitive) • VB mostly N-p states
- 18. Key Papers 1. Most recently: Aron and Alex Zunger’s review ‘Lessons learned from classic semiconductor defectology: Instilling defect tolerance in new compounds’ 2. Studies on specific materials • Comparing Cu3N to GaN: A. Zakutayev, C. M. Caskey, A. N. Fioretti, D. S. Ginley, J.Vidal,V. Stevanovic, E.Tea and S. Lany, J. Phys.Chem. Lett., 2014, 5, 1117–1125. • Original concept for CIS: S. B. Zhang, S.-H.Wei, A. Zunger and H. Katayama-Yoshida, Phys. Rev. B, 1998, 57, 9642–9656. • Discussion of defect-tolerance in MAPI: R. E. Brandt,V. Stevanovi, D. S. Ginley andT. Buonassisi, MRS Communications, 2015, 5, 265–275. 3. High-throughput study: Pandey, M., Rasmussen, F. A., Kuhar, K., Olsen,T., Jacobsen, K.W., &Thygesen, K. S. (2016). Nano Letters, 16(4), 2234–2239
- 19. ‘Defect physics of the CuInSe2 chalcopyrite semiconductor’ prononced p-d repulsion More anti-bonding character atVBM CuInSe2 (tolerant.) vs. CuIn5Se8 (sensitive)
- 20. Key Papers 1. Most recently: Aron and Alex Zunger’s review ‘Lessons learned from classic semiconductor defectology: Instilling defect tolerance in new compounds’ 2. Studies on specific materials • Comparing Cu3N to GaN: A. Zakutayev, C. M. Caskey, A. N. Fioretti, D. S. Ginley, J.Vidal,V. Stevanovic, E.Tea and S. Lany, J. Phys.Chem. Lett., 2014, 5, 1117–1125. • Original concept for CIS: S. B. Zhang, S.-H.Wei, A. Zunger and H. Katayama-Yoshida, Phys. Rev. B, 1998, 57, 9642–9656. • Discussion of defect-tolerance in MAPI: R. E. Brandt,V. Stevanovi, D. S. Ginley andT. Buonassisi, MRS Communications, 2015, 5, 265–275. 3. High-throughput study: Pandey, M., Rasmussen, F. A., Kuhar, K., Olsen,T., Jacobsen, K.W., &Thygesen, K. S. (2016). Nano Letters, 16(4), 2234–2239
- 21. ‘Identifying defect-tolerant semiconductors with high minority-carrier lifetimes: beyond hybrid lead halide perovskites’ Defect-tolerance of MAPI: • Large 𝜀 • Small m* and high band dispersion • VBM composed of anti-bonding states because of Pb s2 lone pair But… will only Pb defects be shallow? Is % pDOS at the band extrema an important factor?
- 22. MAPI defect levels Density Functional Calculations of Native Defects in CH3NH3PbI3: Effects of Spin–Orbit Coupling and Self-Interaction Error Mao-Hua Du. The Journal of Physical Chemistry Letters 2015 6 (8), 1461-1466. DOI: 10.1021/acs.jpclett.5b00199 HSE-SOC results suggest Pb-based defects are in continuum bands, but I and MA within band gap?
- 23. Key Papers 1. Most recently: Aron and Alex Zunger’s review ‘Lessons learned from classic semiconductor defectology: Instilling defect tolerance in new compounds’ 2. Studies on specific materials • Comparing Cu3N to GaN: A. Zakutayev, C. M. Caskey, A. N. Fioretti, D. S. Ginley, J.Vidal,V. Stevanovic, E.Tea and S. Lany, J. Phys.Chem. Lett., 2014, 5, 1117–1125. • Original concept for CIS: S. B. Zhang, S.-H.Wei, A. Zunger and H. Katayama-Yoshida, Phys. Rev. B, 1998, 57, 9642–9656. • Discussion of defect-tolerance in MAPI: R. E. Brandt,V. Stevanovi, D. S. Ginley andT. Buonassisi, MRS Communications, 2015, 5, 265–275. 3. High-throughput study: Pandey, M., Rasmussen, F. A., Kuhar, K., Olsen,T., Jacobsen, K.W., &Thygesen, K. S. (2016). Nano Letters, 16(4), 2234–2239
- 24. ‘Defect-Tolerant MonolayerTransition Metal Dichalcogenides’ Generally agrees with hypothesis based on bonding character of band extrema Introduce a descriptor • Normalized orbital overlap (NOO) • Calculated from the projected density of states of the pristine system • Measures the degree of similarity of the conduction and valence band manifolds Limitations: • Only vacancies • Only PBE (due to large number of structures) • Underestimate band gap and hence may predict incorrect defect depth
- 25. Aside • Ionic semiconductors more defect- tolerant than covalent semiconductors? • Comparing electrical conductivity and transparency of covalent and ionic amorphous semiconductors • Deep levels not found in ionic amorphous semiconductor • … although this doesn’t appear to have the same antibonding VBM thing going on! In this case: Si p-states will give deep levels but in ZnO get shallow levels because MO diagram is so skewed?
- 26. Candidate PV materials: 3 sulfosalt minerals Band gap: 1.24 eV 1.59 eV 1.37 eV
- 27. Will they grow up to be thin-film PV devices? • Less cation disorder expected than in CZTS due to less chemical similarity • p-d repulsion in enargite (c.f. CuInSe2) • Wavefunction parity suggests antibondingVBM • Gather experimental measurements related to defect-tolerance • Next… calculate the defects!
- 28. Concluding remarks • It’d be really cool if we could predict defect-tolerance! • But… how much can we infer from hypothesis of defect tolerance based on electronic structure? • Until recently hypothesis mostly based on 5 materials! • Strength of high-throughput study a bit questionable? • Is it dependent upon the type of defect? E.g. MAPI only tolerant to Pb defects? • Real materials contain much more than just point defects! – can we really extend argument to GBs etc.? • How free are we tune materials for defect-tolerance for PV? • Is p-d repulsion and ns2 lone pairs enough?