Emerging materials for solar energy: matlockite, kesterite, perovskite, and beyond...

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Emerging materials for solar energy: matlockite, kesterite, perovskite, and beyond… Prof. Aron Walsh Department of Materials Imperial College London, UK https://wmd-group.github.io @lonepair

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Solar Electricity and Fuel Electricity Solar Cells Chemical Energy Solar Fuels High efficiency (20 – 50%) Low efficiency (< 10%) Physics (electron–hole separation) is easier than chemistry (oxidation/reduction reactions) Fusion Reactor 174,000 Terawatts reaches the Earth’s surface

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Champion (Unstable) Water Splitting

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Many Photovoltaic Technologies A. Polman et al, Science 352, 307 (2016) High performance “Established” Fundamental research Theoretical limit for single-junction cell

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Challenge for Emerging Solar Cells GaAs efficiency is close to the theoretical limit, while Si is cheap. New systems must perform! Metal oxides Cu2 O (6%) Bi2 FeCrO6 (8%) Co3 O4 (<1%) Metal sulfides SnS (5%) Cu2 ZnSn(S,Se)4 (14%) Sb2 S3 (8%) Metal halides CsSnI3 (10%) CH3 NH3 PbI3 (22%) CH(NH2 )2 PbI3 (20%) Examples of materials studied for thin-film photovoltaics

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Talk Outline: Perovskites & Beyond A. Perovskites (ABX3 ) – what can we learn? B. Kesterites (A2 BCX4 ) – danger, low voltage! C. Tin Sulfides (Ax Bx ) – two elements, many problems. D. Beyond (Ax By Xz ) – exploring new materials.

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Hybrid Organic–Inorganic Perovskites Brief History (1958) – Photoconductivity in CsPbI3 (Møller) (1978) – Synthesis of CH3 NH3 PbI3 (Weber) (1994) – Metallic transition in CH3 NH3 SnI3 (Mitzi) (2009) – Perovskite dye cell (Miyasaka) (2013) – Planar thin-film solar cell (Snaith) Inorganic CsPbI3 Hybrid CH3 NH3 PbI3 or MAPI

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> 20% Efficient Perovskites from Korea

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Why Hybrid Perovskites? Essentials for Solar Cells • Strong optical absorption (Eg ~ 1.6 eV) • Light electron and hole masses (conductive) • Easy to synthesise (cheap and scalable) Advanced Features • Large dielectric constants: carrier separation (weak excitons) and transport (low scattering) • Slow e-h recombination: low losses, large VOC o Relativistic effects – spin-orbit coupling o Polar domains – dynamic fluctuations

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Perovskites: Model vs Reality Plastic crystal behaviour probed by Quasi-Elastic Neutron Scattering (P. Barnes, DOI: 10.1038/ncomms8124); 2D IR Spectroscopy (A. Bakulin, DOI: 10.1021/acs.jpclett.5b01555); Inelastic X-ray Scattering (S. Billinge, DOI: 10.1021/acsenergylett.6b00381) with simulations

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Phase Transitions in CH3 NH3 PbI3 M. Weller et al, Chem. Commun. 51, 4180 (2015)

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Phase Transitions in CH3 NH3 PbI3 A. D. Wright et al, Nature Comm. 7, 11755 (2016) Dynamic disorder Static disorder

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Dynamic Processes in Perovskites Faster (fs) Slower (ps) Electrons and Holes Effective semiconductors Lattice Vibrations Symmetry breaking and carrier separation Molecular Rotations Large static dielectric constant Ions and Charged Defects “Self healing” and hysteresis What is moving in perovskite solar cells? Acc. Chem. Res. 49, 528 (2016)

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Kesterite Quaternary Semiconductors 2 4 2 1×1×2 zincblende superlattice First Cu2 ZnSn(S,Se)4 (CZTS) solar cell 1988; 12.6% by IBM (2014); 13.8% report from DGIST (2016) High-thoughput Density Functional Theory: Phys. Rev. B 79, 165211 (2009)

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• Mixed phase samples, e.g. Cu2 ZnSnS4 à Cu2 SnS3 + ZnS • Cation disorder, e.g. Cu-Zn; Cu-Sn; Zn-Sn mixing • Deep level defects, i.e. fast non-radiative recombination • Interface reactions, e.g. MoS2 formation at back contact Challenging for experiment, theory and simulation! Issues for Kesterite Solar Cells Wallace, Mitzi and Walsh, ACS Energy Letters 2, 776 (2017) Champion solar cells suffer from large voltage deficits, e.g. for CZTS (Eg = 1.5 eV), VOC = 0.7 V

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Call for CZTS Support Wallace, Mitzi and Walsh, ACS Energy Letters 2, 776 (2017) A focused research effort could help overcome efficiency bottlenecks

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2017 STARCELL Network (EU-H2020) http://www.starcell.eu A large consortium (led by IREC, Spain) to improve the performance of kesterite photovoltaics

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Tin Sulfide: 0.4% to 4.4% in 23 Years

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Tin Sulfide: Mixed Phases Phase competition: Sn(II) and Sn(IV) compounds L. A. Burton and A. Walsh, J. Phys. Chem. C 116, 24262 (2012) Sn(II) 5s25p0 Sn(IV) 5s05p0 Sterically active lone pair [asymmetric coordination] e.g. SnO, SnS Octahedral or tetrahedral environments e.g. SnO2 , SnS2

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Tin Sulfide: Mixed Phases Phase competition: Sn(II) and Sn(IV) compounds Predicted phase diagrams: Skelton et al, J. Phys. Chem. C 121, 6446 (2017)

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Tin Sulfide: Mixed Phases Phase competition: Sn(II) and Sn(IV) compounds Predicted phase diagrams: Skelton et al, J. Phys. Chem. C 121, 6446 (2017)

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Tin Sulfide: Electronic Challenges Device issues with Fermi level pinning Appl. Phys. Lett. 102, 132111 (2013); Phys. Rev. Appl. 6, 014009 (2016)

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Tin Sulfide: π-SnS New cubic phase can be grown in thin-films using a variety of deposition techniques Golan, Nano Lett 15, 2174 (2015); Skelton et al, APL Materials 5, 036101 (2017) • Previously misidentified as “zincblende” phase • Structure solved on basis of X-ray and electron diffraction • Phonon stable (PBEsol) • Eg = 1.7 eV (HSE06 + SOC) • Chiral: non-linear optics

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Solar Energy Shopping List • Low-cost and non-toxic elements • Direct optical bandgap (1 – 2 eV) • Easy to deposit and scale-up production • Semiconductor with low carrier concentrations • Tolerant to impurities and microstructure • Chemically stable at interfaces • Workfunction matched to electrical contacts

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Computational Screening Descriptors A. M. Ganose et al, Chem. Comm. 53, 20 (2017)

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Computational Screening Descriptors A. M. Ganose et al, Chem. Comm. 53, 20 (2017) BAND GAP OPTICAL ABSORPTION EFFECTIVE MASS DEFECT PHYSICS e-h RECOMBINATION BAND OFFSETS

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V-VI-VII Chalcohalide Semiconductors Energy Environmental Science 8, 838 (2015); APL 108, 112103 (2016) Trivalent cation with monovalent & divalent anions V VI VII Bi O F Sb S Cl Se Br Te I Ferroelectrics Photocatalysts Solar Cell Absorbers Topological Conductors

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Conclusion and Outlook Many new materials being studied for solar energy conversion. Challenge is translation to efficient devices. Theory and simulation can help to identify and overcome bottlenecks. Project Collaborators: Keith Butler, Jarvist Frost, Jonathan Skelton, Lucy Whalley, Ruoxi Yang, Suzy Wallace (ICL); Simon Billinge (Columbia); Mark van Schilfgaarde (Kings); Bruno Erhler (AMOLF) Funding: ERC; EPSRC; Royal Society; Leverhulme Slides: https://speakerdeck.com/aronwalsh

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DGIST CZTSSe Record Certificate

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Screening New Photoactive Materials Sustainability index From Searching over 4 Trillion Compounds… D. W. Davies et al, Chem 1, 617 (2016); https://github.com/WMD-group/SMACT