Research group meeting talk.

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?