Sustainable Centre for Chemical Technologies Prof. Aron Walsh Department of Chemistry University of Bath, UK Adjunct Professor Yonsei University, Korea a.walsh@bath.ac.uk; @lonepair Emerging materials for solar energy: herzenbergite, kesterite, perovskite and beyond… Spring EMRS 2015 – Invited Talk

Emerging Photovoltaic Materials Record thin-film GaAs device with 29% solar-to- electricity conversion is close to theoretical limit Metal Oxides • Cu 2 O – 6% • Bi 2 FeCrO 6 – 8% Metal Sulfides • SnS – 5% • Cu 2 ZnSn(S,Se) 4 – 13% Metal Halides • CsSnI 3 – 10% • CH 3 NH 3 PbI 3 – 20% High-efficiency (21%) CdTe thin-film solar cell after 30 years of optimisation Complex chemical processes

Solar Energy Materials by Design? Combinatorial Problem > 10100 materials Type & ratio of elements, and their arrangement in space A. Walsh, Nature Chemistry 7, 274 (2015) approach, but origin-of-life chemists still 52, 5845–5847 (2013). or new functionality hemical bond, advances in synthetic chemistry, and large-scale computation, ality. From a pool of 400 unknown compositions, 15 new compounds have structures and properties. Structural prediction Property simulation Targeted synthesis Chemical input Figure 1 | A modular materials design procedure, where an initial selection of chemical elements is subject to a series of optimization and screening steps. Each step may involve prediction of the crystal

An Ideal Photovoltaic Material • No expensive or toxic elements. • Direct optical band gap 1.0 – 1.5 eV. • Semiconductor with low carrier concentrations. • Tolerant to impurities and microstructure. • Chemically stable at interfaces. • Workfunction matched to standard contacts. • Easy to deposit and scale-up production. • Lattice matched to wide-gap absorbers (e.g. ZnSe) for low cost multi-junction devices. What factors are accessible from first-principles?

Descriptor: Optical Absorption Yu and Zunger, Phys. Rev. Lett. 108, 068701 (2012) The integrated absorption spectrum is more important than the band gap

Descriptor: Thermodynamics A. J. Jackson and A. Walsh, J. Mater. Chem. A. 2, 7829 (2014) Cu 2 ZnSnS 4 à Cu 2 S + ZnS + SnS + ½S 2 (g) Gibbs free energy: processing window CZTS Unstable CZTS Stable

Descriptor: Lattice Deformation T influences structure and properties Electron-phonon coupling localises carriers and changes E g Phys. Rev. B 89, 205203 (2014); phonopy.sourceforge.net PbTe thermal expansion Phonons

Descriptor: Workfunctions Acc. Chem. Res. 47, 364 (2014); JACS 136, 2703 (2014) • Approaches for dense and porous frameworks • Separate bulk and surface contributions Solid-state ionisation potentials

Descriptor: Lattice Defects Advanced Materials 25, 1522 (2013) Defects can create and annihilate carriers 0.5 1 1.5 2 2.5 Element Ratio 1e+14 1e+16 1e+18 1e+20 Defect Density 0.1 0.2 0.3 Fermi Energy -0.5 -0.4 -0.3 -0.2 -0.1 0 P Cu (eV) 1e+14 1e+16 1e+18 Hole Density V Cu +Zn Cu Cu Zn Cu Zn - V Cu - Cu Zn Sn Zn Sn +2Zn Cu 2Cu Zn +Sn Zn 2.5 0.5 1 1.5 2 2.5 Element Ratio 1e+14 1e+16 1e+18 1e+20 1e+22 Defect Density 0 0.05 0.1 0.15 Fermi Energy -0.4 -0.3 -0.2 -0 P Cu (eV) 1e+16 1e+17 1e+18 Hole Density V Cu +Zn Cu V Cu - Cu Zn Sn Zn Sn +2Zn Cu V Cu 2.5 (b) Cu2ZnSnS4 (a) Cu2ZnSnS4 (d) Cu2ZnSnSe4 (e) Cu2ZnSnSe4 Cu2 ZnSnS4 Neutral Defect Clusters

Descriptor: Ferroelectricty Polar domains in CH 3 NH 3 PbI 3 separate e- / h+ Regions of high (red) and low (blue) electrostatic potential Nano Letters 14, 2584 (2014); APL Materials 2, 081506 (2014) Spatial separation of conduction channels

Talk Outline 1. Photovoltaic Descriptors From first-principles materials modelling 2. Kesterite & Herzenbergite From quaternary to binary semiconductors 3. Perovskite & Beyond What can we learn from MAPI?

New Compounds (from Somerset)

Multi-component Semiconductors Multernary Materials Screening • Build database of plausible (stoichiometric) materials. • Assess structural, electronic and thermodynamic properties. • Screen & tailor for specific applications. 2 4 2

Quaternary Semiconductors Photovoltaic Absorbers Cu 2 ZnSnS 4 , Cu 2 ZnSnSe 4 and Cu 2 ZnGeS 4 Applied Physics Letters 94 041903 (2009) Spin-transport Materials ZnSiAl 2 As 4 , CdGeAl 2 As 4 and CuAlCd 2 Se 4 Applied Physics Letters 95 052102 (2010) Topological Insulators Cu 2 HgPbSe 4 , Cu 2 CdPbSe 4 and Ag 2 HgPbSe 4 Physical Review B 83 245202 (2011)

Kesterite Kesterite: Cu 2 (Zn,Fe)SnS 4 [Yakutia, Russia]

Cu 2 ZnSn(S,Se) 4 (13% Record Efficiency) Advanced Energy Materials 2, 400 (2012) • Crystal structure (kesterite vs stannite vs disordered) • Band gaps (as a function of S/Se composition) • Phase stability (disproportionation into secondary phases) • Lattice defects (origin of electrons and holes)

Phonons in Cu 2 ZnSnS 4 (Tue, 11:30) J. M. Skelton et al, APL Materials 3, 041102 (2015)

Disorder in Cu 2 ZnSnS 4 (Mon, 14:30) Is the low V OC an intrinsic limitation of these materials?

Herzenbergite: SnS [Oruro, Bolivia]

SnS Solar Cells (0.3 to 4.4% in 20 years) Influence of the Anion on Lone Pair Formation in Sn(II) Monochalcogenides: A DFT Study Aron Walsh and Graeme W. Watson* Department of Chemistry, UniVersity of Dublin, Trinity College, Dublin 2, Ireland ReceiVed: April 8, 2005; In Final Form: August 8, 2005 The electronic structure of SnO, SnS, SnSe, and SnTe in the rocksalt, litharge, and herzenbergite structures has been calculated using density functional theory. Comparison of the distorted and undistorted structures allows for an explanation of the unusual experimentally observed structural transitions seen along the Sn(II) monochalcogenides. Analysis of the electronic structure shows a strong anion dependence of the Sn(II) lone pair, with the Sn(5s) and Sn(5p) states too far apart to couple directly. However, the interaction of Sn(5s) with anion states of appropriate energy produce a filled antibonding Sn(5s)-anion p combination which allows coupling of Sn(5s) and Sn(5p) to occur, resulting in a sterically active asymmetric density on Sn. While the interaction between Sn(5s) and O(2p) is strong, interactions of Sn with S, Se, and Te become gradually weaker, resulting in less high energy 5s states and hence weaker lone pairs. The stability of the distorted structures relative to the symmetric structures of higher coordination is thereby reduced, which induces the change from highly distorted litharge SnO to highly symmetric rocksalt SnTe seen along the series. 18868 J. Phys. Chem. B 2005, 109, 18868-18875

SnS Solar Cells (0.3 to 4.4% in 20 years) Influence of the Anion on Lone Pair Formation in Sn(II) Monochalcogenides: A DFT Study Aron Walsh and Graeme W. Watson* Department of Chemistry, UniVersity of Dublin, Trinity College, Dublin 2, Ireland ReceiVed: April 8, 2005; In Final Form: August 8, 2005 The electronic structure of SnO, SnS, SnSe, and SnTe in the rocksalt, litharge, and herzenbergite structures has been calculated using density functional theory. Comparison of the distorted and undistorted structures allows for an explanation of the unusual experimentally observed structural transitions seen along the Sn(II) monochalcogenides. Analysis of the electronic structure shows a strong anion dependence of the Sn(II) lone pair, with the Sn(5s) and Sn(5p) states too far apart to couple directly. However, the interaction of Sn(5s) with anion states of appropriate energy produce a filled antibonding Sn(5s)-anion p combination which allows coupling of Sn(5s) and Sn(5p) to occur, resulting in a sterically active asymmetric density on Sn. While the interaction between Sn(5s) and O(2p) is strong, interactions of Sn with S, Se, and Te become gradually weaker, resulting in less high energy 5s states and hence weaker lone pairs. The stability of the distorted structures relative to the symmetric structures of higher coordination is thereby reduced, which induces the change from highly distorted litharge SnO to highly symmetric rocksalt SnTe seen along the series. 18868 J. Phys. Chem. B 2005, 109, 18868-18875

Single Crystal Tin Sulfides L. A. Burton et al, Chemistry of Materials 25, 4908 (2013) SnS Sn2 S3 SnS2 Lee

Phases of SnS L. A. Burton and A. Walsh, J. Phys. Chem. C 116, 24262 (2012) Pnma (equilibrium); Cmcm (high T); Fm-3m (high P) Zincblende SnS does not exist (erroneous assignment in Small 2, 368, 2006) • XRD – matches rocksalt diffraction pattern • Simulation – spontaneous amorphisation • Chemistry – Sn(II) disfavours tetrahedrons

Workfunction Matching for SnS L. A. Burton and A. Walsh, Appl. Phys. Lett. 102, 132111 (2013) Sn(II) is distinct from Sn(IV): filled 5s2 orbitals

Workfunction Matching for SnS L. A. Burton and A. Walsh, Appl. Phys. Lett. 102, 132111 (2013) Standard ‘thin-film’ contacts will result in poor performance nction A pre- nction ed sin- ear the ificant HSE06 an for n self- hange. on the in the ational surfa- y-body for the recent bonded ations, display conduction bands of SnS are misaligned, with the valence FIG. 2. Predicted band alignment between SnS (HSE06 ionisation potential) and a range of materials used in thin-film solar cells from Refs. 3, 28, 29, and 32. TABLE I. Candidate electrical contacts for SnS devices based on energy level alignment and element availability. Values for metals are taken from Ref. 33 and values for semiconductor buffers from Refs. 34 and 35. Metallic contacts Workfunction (eV) n-type buffers Workfunctiona (eV) Titanium 4.33 (Zn,Cd)S 2.95 (ZnS) – 3.88 (CdS) Tungsten 4.55 Zn(S,Se) 2.95 (ZnS) – 4.09 (ZnSe) Tin 4.42 Zn(O,S) 3.91 (ZnO) – 2.95 (ZnS) aEquated to reported electron affinities/conduction band edge due to n-type nature. The limiting energies for the semiconductor solid-solutions are taken as those of the parent binary compounds. It should be noted that absolute values can vary according to the particular sample and/or study.

Mixed Phases in SnS Solar Cells L. A. Burton et al, Chemistry of Materials 25, 4908 (2013) SnS2 will be a source of recombination. Isolated Sn2 S3 may be a good absorber. ncy rier ling be pid high e n- ples tals, be d by the 9,80 fide erns SnS As hat this to her et al.58,83 This value agrees well with the experimental observations of Zainal et al.84 The calculated valence band maxima, conduction band minima and vacuum potentials allow us construct a natural band offset diagram for each of the tin sulfide phases, which is shown in Figure 6. The procedure is detailed in ref 19, where Figure 6. Calculated band offset diagram, using the HSE06 functional,

Talk Outline 1. Photovoltaic Descriptors From first-principles materials modelling 2. Kesterite & Herzenbergite From quaternary to binary semiconductors 3. Perovskite & Beyond What can we learn from MAPI?

Perovskite: CaTiO 3 [Achmatovsk, Russia]

Hybrid Halide Perovskites Snaith (Oxford) Grätzel (EPFL) Park (SKKU) Seok (KRICT) APL Mater. 1, 042111 (2013); Nano Letters 14, 2484 (2014) A B X3 a (Å) Eg (eV) NH4 + Pb I 6.21 1.38 CH3 NH3 + Pb I 6.29 1.67 CH(NH2 )2 + Pb I 6.34 1.55 (1991) Dye cell à (2015) Perovskite cell [20.1% efficiency] See Mendeley Group “Hybrid Perovskite Solar Cells”

Methylammonium (CH 3 NH 3 +) A closed shell (18 electron) molecular cation with a large electric dipole J. M. Frost et al, Nano Letters 14, 2584 (2014) Deprotonation (pK a ~ 10): CH 3 NH 3 + à CH 3 NH 2 + H+

First-principles Dynamics (300 K) 25 fs per frame (up to 200 ps) J. M. Frost et al, APL Materials 2, 081506 (2014) Jarvist http://dx.doi.org/10.6084/m9.figshare.1061490 ß Focus on one CH 3 NH 3 ion 3D periodic boundary (80 - 640 atoms) “MAPI is as soft as jelly” 11.2013

Observation of Molecular Motion Quasi-Elastic Neutron Scattering (QENS) 14 ps residence time Probes 1 – 200 ps (Piers Barnes) 2D Photon Echo (Pump-probe IR) 300 fs / 3 ps processes Probes 0 – 6 ps (Artem Bakulin) Nature Comm. 6, 7124 (2015); J. Phys. Chem. Lett. 6, 3663 (2015)

Mixed Ionic-Electronic Conductors Ange. Chem. 54, 1791 (2015); Nature Communications (2015) Charged Ion Drift and Diffusion Consistent activation energy from simulations and kinetic measurements of photovoltaic response (chronophotoamperometry by Piers Barnes and Brian O’Regan – ICL)

An Ideal Photovoltaic Material • No expensive or toxic elements. • Direct optical band gap 1.0 – 1.5 eV (825 nm). • Semiconductor with low carrier concentrations. • Tolerant to impurities and microstructure. • Chemically stable at interfaces. • Workfunction matched to standard contacts. • Easy to deposit and scale-up production. • Lattice matched to wide-gap absorbers (e.g. ZnSe) for low cost multi-junction devices. Where to next?

V-VI-VII Chalcohalides K. T. Butler et al, Energy & Environmental Science 8, 838 (2015) Trivalent cation + monovalent and divalent anions A poorly studied class of polar semiconductors including SbSBr, SbSI and SbSeI

V-VI-VII Chalcohalides K. T. Butler et al, Energy & Environmental Science 8, 838 (2015) Band gap control through halide or chalcogenide Predicted high-efficiency junction

Organic-Inorganic Sb Sulfides R. Y. Yang et al, J. Phys. Chem. Lett. 6, 5009 (2015) From Binary to Ternary to Hybrid Materials Optical, dielectric and electronic properties similar to hybrid perovskites, but Pb free Cs 2 Sb 8 S 13

Solar Minerology Stable light absorbing minerals made from earth abundant elements There may be a solar energy solution in your local natural history museum. The next “perovskite”? Mir mine, Russia

Solar Minerology Stable light absorbing minerals made from earth abundant elements There may be a solar energy solution in your local natural history museum The next “perovskite”? Mir mine, Russia

Group E-MRS Talks Symposium D “Emerging Solar” Monday 14:30 “Chemical disorder and inhomogeneity in Cu 2 ZnSnS 4 from multi-scale simulations” Tuesday 11:30 “Phase equilibria of Cu 2 ZnSnS 4 absorber layers in thin-film solar cells” Wednesday 14:50 “Electronics structure optimisation of hybrid antimony sulfides” EU40 Plenary Session (Wednesday at 17:30)

Conclusion There are many emerging solar energy materials. The key challenge is to realise their potential in efficient devices. Predictive materials modelling can help to guide the way. Group Members: PV – Adam, Federico, Suzy, Keith, Jarvist; MOFs – Chris, Ruoxi, Jess, Katrine; Metastable – Jonathan, Lora, Clovis Collaborators: PV – Mark van Schilfgaarde (KCL); Mark Weller (Bath); Piers Barnes and Brian O’Regan (ICL); Xingao Gong (Fudan); David Scanlon (UCL); Su-Huai Wei (NREL); Nam-Gyu Park (SKKU) Funding: