Redox Activity of Cation Lone-pairs in Multi-Component Semiconductors

2019 EMRS Spring meeting

Transcript

1. Redox Activity of Cation Lone-pairs in Multi-Component Semiconductors Sunghyun Kim and Aron Walsh Dept. of Materials, Imperial College London, UK sunghyun.kim@imperial.ac.uk | frssp.github.io | frssp | 0000-0001-5072-6801 2019 – EMRS Spring Meeting
2. Acknowledgement 2 Prof. Aron Walsh Dr. Ji-Sang Park Dr. Samantha N. Hood
3. Stereochemistry of post-transition metal oxides 3 A. Walsh, D.J. Payne, R.G. Egdell, and G.W. Watson, Chem. Soc. Rev. 40, 4455 (2011) Rocksalt MgO Litharge PbO What happens when lone-pairs are formed in defects? Mg2+: [Ne] 3s0 Pb2+: [Xe] 4f14 5d10 6s2 .. Light-to-electricity conversion efficiency of solar cells
4. Efficiency of Solar Cell 4 J. Appl. Phys. 51, R1 (1980) Phys. Rev. B 90, 035211 (2014) J. Appl. Phys. 32, 510 (1961) Shockley-Queisser limit for a single junction solar cell Reversible Heat engine Irreversible processes in a single junction solar cell
5. SQ model: Well… It’s not working well… 5 Science 352, aad4424 (2016) There must be something wrong with the SQ model or materials.
6. Nonradiative recombination mediated by defects 6 Defects can kill the photoexcited carriers and the performance of solar cells Park, J.-S., Kim, S., Xie, Z. & Walsh, A., Nat. Rev. Mater. 3, 194 (2018)
7. To save the electrons and holes 7 Who killed the electrons? How many are them? How lethal are they? Defect species Defect concentrations Carrier capture cross-sections
8. Two Common Characteristics of “Killer” Centers Deep level Large lattice relaxation “… So-called killer centers, with fast nonradiative transitions, … we list four examples: … 2. Defect with favorable vibrational properties, that is, with large-amplitude modes promoting the transitions, and large-energy modes to take up the electronic energy …” - A. M. Stoneham in Defects and Defect Processes in nonmetallic Solids 8 Which defects exhibit both deep levels and large lattice relaxation? Park, J.-S., Kim, S., Xie, Z. & Walsh, A., Nat. Rev. Mater. 3, 194 (2018)
9. Redox Activity of Cation Lone-pairs in defects Large lattice relaxation Inert-pair effect: ineffective screening by d and f orbitals The large ionization energy for ns orbitals leads to a deep donor levels. Deep level [Kr] 4d10 5S0 5p0 R = 71 pm [Kr] 4d10 5S2 5p0 R = 112 pm The reduction and oxidation may leads to a large change in the structure of defect (similar to that in the bulk phase). Sn(IV) Sn(II) The defects involving the oxidation and reduction of lone-pairs may act as killer centers
10. Post-transition metal compound: Kesterite The champion efficiency of 12.6% has been achieved in 2013. Si Cd Te Cu Ga S Cu Zn Sn S 2− 2+ 3+ 4+ 2+ 1+
11. 11 Talk Outline A. What’s wrong with lone-pairs? A. Activation energy – Lone-pairs introduce deep donor levels! B. Capture cross section – Lone-pairs capture carriers too fast! C. Concentration – We can’t remove them! B. Chemical tuning o Ge alloying o H/Alkali metal (co)doping o Ag alloying
12. Redox Activity of Cation Lone-pairs in defects 12 Deep level Large lattice relaxation 1+ 2+ 4+ 1+ Conduction band 1+ 2+ 1+ 2+ e− e− Sn(IV): 5s05p0 (CZTS, SnS2 ) Sn(II): 5s25p0 (SnS) Sn(IV) Sn(II) Zn Cu Cu Cu Cu Zn The lone-pair formation in a defect would lead to both deep level and large lattice relaxation. Kim, S. et al., ACS Energy Lett. 3, 496 (2018)
13. Lone-pairs in VS , VS -CuZn and SnZn V S 1+ V S 1+ Cu Zn 1− Sn Zn 1+ (V S -Cu Zn )0 V S 1+ Sn Zn 1+ (a) (b) (c) Defect wave functions are well localized around Sn 5s orbitals. 13 J. Mater. Chem. A 7, 2686 (2019)
14. Deep Donor Levels of VS, SnZn , and Complexes 14 0.0 0.5 1.0 1.5 VCu (0/−) CuZn (0/−) ZnCu (+/0) SnZn (2+/+) SnZn (+/0) VS (+/0) VS -CuZn (+/0) SnZn -CuZn (+/0) SnZn -CuZn (0/−) Energy (eV)
15. Large Lattice Relaxation of VS , VS -CuZn and SnZn The large lattice distortions lead to the small carrier capture barriers. ΔQ offset indicates the degree of lattice relaxation. ΔQ Neutral trap Repulsive trap Giant trap V S -Cu Zn 1+ V S 2+ Sn Zn 1+ Sn Zn 2+ Cu Sn 1− 1000/T (1/K) σn (cm2) 0 2 4 6 8 10 10−30 10−27 10−24 10−21 10−18 10−15 10−12 (a ) (b Configuration Coordinate Capture cross section 15 J. Mater. Chem. A 7, 2686 (2019)
16. Can we remove SnZn 2+? SnZn 2+ - Sn poor - Zn rich - hole poor (n-type) 16
17. Phase diagram: Thermodynamics are cruel Ag 8 SnSe 6 Ag Ag 2 SnS ZnSe SnSe SnSe Se CuS Cu 2 S Cu Cu 2 SnS 3 S Sn ZnS SnS SnS 2 (a) (b) 17 ZnS is too stable with respect to the formation of CZTS. μZn (eV) μCu (eV) μSn (eV) 0 −1 −2 −2 −1 −1 −2 0 0 o Zn-rich Additional Zn forms ZnS. o Sn-poor The Cu-rich secondary phases are conductive. o hole poor (n-type) The acceptor (CuZn ) are too many.
18. 3. Concentrations & Trap Limited Conversion Efficiency 18 T = 300K W = 3µm 32% 23% Wed 29 May 2019 18:30 (Sunghyun Kim) E8.8 Theoretical Maximum Efficiency of Kesterite Solar Cells
19. You Get What You Paid For Cu Zn Sn S ☠ ☠ Killer center Many killer centers Ga 3+ 4+ 2+
20. 20 Talk Outline A. What’s wrong with lone-pairs? A. Activation energy – Lone-pairs introduce deep donor levels! B. Capture cross section – Lone-pairs capture carriers too fast! C. Concentration – We can’t remove them! B. Chemical tuning o Ge alloying o H/Alkali metal (co)doping o Ag alloying
21. Reducing Capture Cross Section: Cu2 ZnGeSe4 - 5 0 5 10 0.0 0.5 1.0 1.5 2.0 Q (amu1/2 Å) Energy (eV) GeVI+e+h GeIII+h GeVI 21 Electron capture by GeZn 2+ Hole capture by GeZn 1+ Se-poor Se-rich Zn-rich Cu-poor Concentration (cm-3) Energy (eV) p0 VZn ZnCu Ge Zn VCu V Se -Cu Zn CuZn EF,p EF,n VCu CuZn VSe GeZn VSe -CuZn EF ZnCu V Se Configuration Coordinate of GeZn (2+/1+) Trap limited SQ limit CZGSe* η 26.6% 32.2% 7.6% *Phys. Status Solidi (a) 215, 1800043 (2018)
22. To p, or not to p, that is the question. 22 High hole concentration → Many recombination centers Low hole concentration → Low p-type conductivity Dilemma & Codoping! '
23. H/Alkali metal Codoping 23 0 0.5 1.0 1.5 0 5 10 15 20 25 30 Current density (mA/cm3) Voltage (V) TLCE w/o doping TLCE w/ doping SQ limit (a) CZTS (b) Doping concentration: 1020 cm−3 Codoping increases the carrier lifetime and efficiency. 32% 23% 25%
24. CuZn is too stable because rCu ≈ rZn : Ag2 ZnSnSe4 24 1014 1020 Energy (eV) EF,p EF,n VCu CuZn SnZn EF Trap limited SQ limit ACZGSe* η 31.0% 33.6% 10.1% Doping concentration (cm−3) *Adv. Energy Mater. 18, 1802540 (2018)
25. Lone-pairs: Stereochemistry of post-transition metal could lead to the formation of ‘killer’ centres 25 Large lattice relaxation Deep level Fermi level pinning Narrow phase space ☠ Carrier lifetime ☠ Power conversion efficiency Chemical Tuning Fermi level control by doping and alloying Alternative elements w\ stable oxidation states Cu Zn Sn S What happens when lone-pairs are formed in defects? Which defects exhibit deep levels and favorable vibrational properties?
26. Making Good Solar Cells: a No-win Scenario. Do not go gentle into that good night Dylan Thomas Do not go gentle into that good night, Old age should burn and rave at close of day; Rage, rage against the dying of the light. Though wise men at their end know dark is right, Because their words had forked no lightning they Do not go gentle into that good night. … 26
27. “I don’t believe in a no-win scenario.” Do not go gentle into that good night Do not go gentle into that good night, Old age should burn and rave at close of day; Rage, rage against the nonradiative recombination. Though wise men at their end know dark is right, Because their words had forked no lightning they Do not go gentle into that good night. … 27