Materials Research Society (Fall Meeting 2018) Lone Pair Engineering for Multi- Functional Polar Semiconductors Materials Design Group: https://wmd-group.github.io @lonepair Prof. Aron Walsh Imperial College London, UK Yonsei University, Korea

Lone Pairs in Functional Materials Unique properties from combination of off-centric coordination environments and high polarisability Piezoelectric, Ferroelectric, Multiferroic, Ion Transport, Gas Sensing, Photocatalytic, Photovoltaic, Thermoelectric SnSe Pb3 O4 BiFeO3

Lone Pair Engineering A. Electronic and Crystal Structure B. Carrier Capture and Recombination

Inert Pair Effect Unusual coordination environments of certain post-transition metals are linked to stereochemical activity of an s2 lone pair N. V. Sidgwick (1929); L. E. Orgel, J. Chem. Soc. 0, 3815 (1959) “s2–p0 mixing results in an instability with respect to antisymmetric distortions”

Oxidation State Competition Heavy post-transition metals can adopt the group oxidation state N or N-2 Oxidation states and ionicity, Nature Materials 17, 958 (2018) 5s0p0: Sn(IV)O2 Wide bandgap n-type semiconductor 5s2p0: Sn(II)O Small bandgap p-type semiconductor

Stereochemical Activity For a metal in the same oxidation state, the lone pair may be active or inactive: anion effect A. Walsh and G. W. Watson, J. Sol. Stat. Chem. 178, 1422 (2005) 6s2p0: Pb(II)O Layered litharge structure 6s2p0: Pb(II)S Rocksalt structure Self-consistent electron density (DFT/PBE) [from my PhD thesis !]

Lone Pair Electronic Structure Metal s2 band lies deep in the valence band with anti-bonding combination close to the Fermi level A. Walsh and G. W. Watson, J. Sol. Stat. Chem. 178, 1422 (2005) Partial electron density slices (E relative to highest occupied band) (Pb 6s + O 2p) O 2p (Pb 6s + O 2p)* - 7 eV - 3 eV - 1 eV Pb O O O O

Lone Pair Electronic Structure Validation using a range of X-ray spectroscopies (XPS, XES, XAS) led by Russ Edgell (Oxford) D. J. Payne et al, Phys. Rev. Lett. 96, 157403 (2006) DFT/PBE PbO – valence band O 2p O 2p Pb 6s (Pb 6s + O 2p) (Pb 6s + O 2p)* Note: Interaction strength depends on metal s–anion p separation

Band Engineering of Metal Oxides Conduction Band – Metal s0 or p0 Valence Band – Oxygen 2p6 Eg IP

Band Engineering of Metal Oxides Conduction Band – Metal s0 or p0 Valence Band – Oxygen 2p6 Lone pair bonding Lone pair anti-bonding Eg IP Lone pair effects: 1. Reduce bandgap 2. Lower ionisation potential 3. Enhance conductivity (lower m*h )

Band Engineering of Metal Oxides Conduction Band – Metal s0 or p0 Valence Band – Oxygen 2p6 Eg EA

Band Engineering of Metal Oxides Conduction Band – Metal s0 or p0 Valence Band – Oxygen 2p6 Eg EA Metal d0

Band Engineering of Metal Oxides Conduction Band – Metal s0 or p0 Valence Band – Oxygen 2p6 Lone pair bonding Lone pair anti-bonding Metal d0 s2 and d0 cations: (Sn, Sb, Pb, Bi)(Ti, W, V, Nb, Ta)Ox

New Materials for Solar Fuels P. Kamat, J. Phys. Chem. Lett 6, 1917 (2015)

P. Kamat, J. Phys. Chem. Lett 6, 1917 (2015) New Materials for Solar Fuels

Photoactive Oxide: Bi3+V5+O4 A. Walsh et al, Chemistry of Materials 21, 547 (2009) Bonding Anti-bonding

Photoactive Oxide: Sn2+ 2 Ti4+O4 L. A. Burton et al, J. Solid State Chemistry 196, 157 (2012)

Lone Pairs in Bi(III) Oxyhalides Chemistry of Materials 28, 1980 (2016); Chemical Science 7, 4832 (2016) Stable and active BiOBr photocatalyst (with Parkin and Scanlon groups, UCL)

Crystal Structure Variation A. F. Wells, Structural Inorganic Chemistry (5th Edition, 1984) Group 14 Group 15 Group 16 Metal Oxides 5s2 SnO: Litharge Layered, 4 coordinate Sb2 O3 : Valentinite Double O-Sb-O chains TeO2 : Tellurite, Layered, 4 coordinate 6s2 PbO: Litharge Layered, 4 coordinate Bi2 O3 : ⍺,β," Highly distorted coordination PoO2 : Flourite No lone pair activity Metal Chalcogenides 5s2 SnS/SnSe Distorted black P structure Sb2 S3 /Sb2 Se3 Sibnite structure with Sb2 X3 sheets TeS2 /TeSe2 Unknown 6s2 PbS/PbSe Rocksalt with no static lone pair activity Bi2 S3 /Bi2 Se3 Sibnite/layered structure PoS2 /PoSe2 Unknown

Polar Semiconductor: Bi2 WO6 “fluorite” (Bi2 O2 )2+ “perovskite” (WO4 )2- −100 −80 −60 −40 −20 0 20 40 0 20 40 60 80 100 Polarization [µC/cm2] Distortion [%] 61 µC/cm2 along <100> Berry phase polarization (PBEsol/PAW/VASP) Lattice distortion from parent Fmmm Aurivillius phase – Low temperature polar (P21 ab) and high-temperature non-polar (C2/m) structures

Polar Semiconductor: Bi2 WO6 Scalable thin-film deposition using aerosol– assisted chemical vapor deposition (AA-CVD) Experiments by Andreas Kafizas and team (ICL, Chemistry) Analysis and optimisation of photoactivity is on-going

Lone Pair Engineering A. Electronic and Crystal Structure B. Carrier Capture and Recombination

SRH: Shockley & Read, Phys. Rev. 87, 835 (1952); Hall, Phys. Rev. 87, 387 (1952) Carrier Capture in Optoelectronics SRH analysis: mid-gap defects are most active Beyond: defects levels are not fixed, but vary with the charge state. Non- radiative recombination is a multi-level phonon-emission process

Structural relaxation (electron-phonon coupling) is a critical component of carrier capture Non-Radiative Capture Process Q = configuration coordinate [change in local structure with charge state] Huang & Rhys, Proc. RS 204, 406 (1950); Henry & Lang, Phys. Rev. 15, 989 (1977) Radiative recombination [Defect luminescence] Defect in charge states E1 and E2 Non-radiative recombination [Phonon emission]

Traits of a Carrier Killer A. M. Stoneham, Rep. Prog. Phys. 44 1251 (1981) What characteristics give rise to efficient carrier trapping and recombination? 1. Cascade: Closely-spaced bound electronic states (e.g. Ni in GaP) 2. Resonance: Favourable vibrational states (e.g. C2 in Si) 3. Coupling: strong electron-lattice interaction (e.g. vacancies)

Lone Pairs Enhance Carrier Capture S. Kim et al, ACS Energy Letters 3, 496 (2018); arXiv 1810.11259 (2018) In Sn(IV) containing semiconductors, electron capture to Sn(III) and Sn(II) is efficient Multi-valency: Carrier localisation and large amplitude lattice distortion Lone pair state of Sn associated with S loss in Cu2 ZnSnS4 solar cells

Lone Pairs Enhance Carrier Capture Sunghyun Kim (Unpublished) Strong “lone pair” recombination centres – limit photovoltaic efficiency, but other applications? Photothermal catalysis, switches, memory… SRH limit S-Q limit JSC 28.97 mA/cm2 28.97 mA/cm2 VOC 0.80 V 1.20 V FF 85.10% 89.99% Efficiency 20.1% 31.0% Lone pair effect on theoretical Cu2 ZnSn(IV)S4 solar cell efficiency CdS CZTS d(1+/0) d(2+/1+) 2 1 0 1 Energy (eV) 200 100 0 Position (nm) ϵ(2+/1+) E F ϵ(1+/0) 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) Giant capture cross-section traps Lone-pair related defects

Conclusions: Solid-State Lone Pairs • Band engineering: bandgaps, band energies, conductivity (effective masses) • No global understanding of structure control, e.g. polar transitions, emphanisis (symmetry breaking), or carrier capture Slides: https://speakerdeck.com/aronwalsh Thanks to group, in particular Kazuki Morita, Sunghyun Kim, Liam Harnett; collaborators including Andreas Kafizas, James Durrant (ICL), Russ Egdell (Oxford), Ram Seshadri (UCSB)