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Beyond Perovskites: Chemical Principles for Next-Generation Solar Energy Materials

Aron Walsh
September 18, 2018

Beyond Perovskites: Chemical Principles for Next-Generation Solar Energy Materials

Invited lecture at the Royal Society Discussion meeting on "Energy Materials for a Low Carbon Future"

Aron Walsh

September 18, 2018
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  1. Energy Materials for a Low Carbon Future (Royal Society)
    Beyond Perovskites: Chemical
    Principles for Next-Generation
    Solar Energy Materials
    Prof. Aron Walsh
    Department of Materials,
    Imperial College London
    Materials Design Group: https://wmd-group.github.io @lonepair

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  2. Thin-Film Solar Cells
    Cu(In,Ga)Se2
    CdTe
    From indirect (Si) to direct bandgap
    semiconductors for enhanced light absorption
    Light absorbing layer is a p-type semiconductor

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  3. From Materials to Devices
    Existing technologies benefited from decades of
    materials and device optimisation
    Complex chemical processes
    in CdTe solar cells:
    • Cu diffusion
    • CdCl2
    annealing
    • Cd(S,Te) formation
    • (Cd,Zn)S formation
    http://www.nrel.gov/pv/cadmium-telluride-solar-cells.html
    Cu
    S,Te mixing
    Cd,Zn mixing

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  4. Lack of Structural Diversity
    Few materials have produced single-junction solar
    cells with >20% light-to-electricity conversion
    Face-centred cubic semiconductors: Si, GaAs, CdTe, CuInS2
    Crystallographic unit cell of chalcopyrite
    [CuFeS2
    mineral and Cu(In,Ga)(S,Se)2
    ]
    Cu/In ions
    ordered in
    (201) planes
    All ions with
    tetrahedral
    coordination

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  5. Challenge for Emerging Solar Cells
    GaAs efficiency is close to the theoretical limit,
    while Si is cheap. New materials must perform!
    Metal
    oxides
    Cu2
    O
    (6%)
    Bi2
    FeCrO6
    (8%)
    Co3
    O4
    (<1%)
    Metal
    sulphides
    SnS
    (5%)
    Cu2
    ZnSnS4
    (13%)
    FeS2
    (3%)
    Metal
    halides
    CsSnI3
    (10%)
    CH3
    NH3
    PbI3
    (23%)
    BiI3
    (1%)
    Examples of materials studied for thin-film photovoltaics
    (% sunlight to electricity conversion in photovoltaic devices)
    M. A. Green et al, Solar cell efficiency tables (version 52)

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  6. Talk Outline: Beyond Perovskites
    A. 21st Century Materials Modelling
    B. Metal Halide Perovskites
    C. Outlook for Materials Design

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  7. Chemistry on Computers
    “The underlying physical laws for the whole of
    chemistry are completely known” Dirac (1929)
    Boys et al, Nature 4544, 1207 (1956)
    EDSAC: Electronic delay storage
    automatic calculator
    o 3,000 vacuum tubes
    o Power consumption: 11 kW
    o Calculations on H2
    , H2
    O, BH radical

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  8. Supercomputers (1017 FLOPS)
    Top500.org ranking of public installations

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  9. Thousands of Interacting Electrons
    Density Functional Theory (DFT):
    Hohenberg & Kohn (1964); Kohn & Sham (1965)
    Source: F. Bechstedt – Many-body Approach to Electronic Excitations (2015)

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  10. First-Principles Modelling in 2018
    Remove Approximations
    length and times scales
    electron-electron interactions
    electron-phonon interactions
    phonon-phonon interactions
    Accurate Solid-State Properties
    effective mass to carrier mobility
    phonon frequencies to lifetimes
    ground to excited states
    defects and disorder in crystals

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  11. Machine Learning Chemistry
    Statistical algorithms that learn from training data
    and build a model to make predictions
    17k Machine Learning publications on Web of Science (08.18)

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  12. Machine Learning Chemistry
    Decision
    trees
    “Chemical robot”

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  13. Evolution of Materials Modelling
    CZTS Unstable
    CZTS Stable
    Quick-start machine learning guide: K. T. Butler et al, Nature 559, 547 (2018)
    Potential for combining first-principles predictions with
    experiments and databases for data-driven materials discovery

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  14. Share your Research Data
    https://github.com/WMD-group/hot-carrier-cooling
    Global movement of #openscience for
    transparency, reproducibility, and data-sharing

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  15. Open-Source Community PV Software
    http://www.solcore.solar; https://github.com/dalonsoa/solcore5

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  16. Talk Outline: Beyond Perovskites
    A. 21st Century Materials Modelling
    B. Metal Halide Perovskites
    C. Outlook for Materials Design

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  17. 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)
    (2012) – Planar thin-film solar cell (Snaith)
    Inorganic
    CsPbI3
    Hybrid
    CH3
    NH3
    PbI3
    or MAPI

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  18. Why Halide 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
    • Dielectric screening: carrier separation (weak
    excitons) and transport (low scattering rates)
    • Slow e-h recombination: low losses, large VOC
    o Polar nano-domains – carrier separation
    o Relativistic effects – spin-orbit coupling
    o Phonon scattering – limit non-radiative events

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  19. 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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  20. Intersection of Hard and Soft Matter
    Jarvist Frost
    CH3
    NH3
    PbI3
    3D periodic boundary
    (80–640 atoms)
    25 fs per frame
    0.5 fs timestep
    based on PBEsol
    forces at T=300K
    “As soft as jelly”
    J. M. Frost, K. T. Butler, A. Walsh, APL Mater. 2, 081506 (2014)
    Combination of density functional theory, GW
    theory, lattice dynamics, molecular dynamics,
    classical Monte Carlo, continuum device models

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  21. Static Dielectric Response
    Standard Inorganic Dielectric
    Organic-Inorganic Dielectrics
    Microstructure
    Conductivity
    Contacts
    Lattice dynamics
    Optical response
    Stat. mechanics
    Sum of:
    Key role in charge carrier generation and lifetimes

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  22. Static Dielectric Response
    High-frequency
    from
    QSGW+SOC
    Low-frequency
    from harmonic
    phonons
    (DFT/PBEsol)
    With Mark van
    Schilfgaarde

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  23. “Giant Dielectric Constant”
    JPCM 20, 191001 (2008)
    JPCL 5, 2390 (2014)
    J. M. Frost and A. Walsh, Acc. Chem. Res. 49, 528 (2016)

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  24. “Giant Dielectric Constant”
    JPCM 20, 191001 (2008)
    JPCL 5, 2390 (2014)
    J. M. Frost and A. Walsh, Acc. Chem. Res. 49, 528 (2016)

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  25. Bananas are Lossy Dielectrics
    J. F. Scott, J. Phys. Conden. Matter 20, 2 (2007)
    Note: the analogy is limited. Mixed ionic-electronic transport in perovskites
    gives rise to more complex Hebb-Wagner polarization (thanks Joachim Maier)

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  26. Electrons as Large Polarons
    Fröhlich electron-lattice interaction
    ! =
    1
    2
    1
    %&

    1
    %(
    )
    ℏ+
    2,∗+

    .
    /
    GaAs = 0.1
    CdTe = 0.3
    CH3
    NH3
    PbI3
    = 2.4
    SrTiO3
    = 3.8
    Frost, Butler, Walsh, APL Mater. 2, 081506 (2014); Acc. Chem. Res. 49, 528 (2016)

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  27. Mixed Ion–Electron Conductors
    Current-voltage hysteresis
    [Snaith et al, JPCL (2014); Unger et al, EES (2014)]
    Rapid chemical conversion between halides
    [Pellet et al, CoM (2015); Eperon et al, MH (2015)]
    Photoinduced phase separation
    [Hoke et al, CS (2015); Yoon et al, ACS-EL (2016)]
    Electric field induced phase separation
    [Xiao et al, NatM (2015); Yuan et al, AEM (2016)]
    Photo-stimulated ionic conductivity
    [Yang et al, AChemie (2015); Kim et al, NatM (2018)]

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  28. Mixed Ion–Electron Conductors
    Nature Comm. 6, 8497 (2015); ACS Energy Lett. 3, 1983 (2018)
    Reservoir of charged point defects (site vacancies)
    in thermodynamic equilibrium: V-
    MA
    , V2-
    Pb
    , V+
    I
    A. Walsh et al, Angewandte Chemie 54, 1791 (2015)
    Figure 3. Iodide ion vacancy migration from DFT calculations (a) Calculated migration
    Vacancy Ea
    (eV)
    I- 0.6
    CH3
    NH3
    + 0.8
    Pb2+ 2.3
    D ~ 10-12cm2s-1
    at T = 300 K
    [PBEsol/DFT in 768 atom supercell
    with nudged-elastic band]

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  29. Mean free path
    of each phonon
    Taming Ions in Halide Perovskites
    Phonon glass
    Phys. Rev. B 94,
    220301 (2016)
    Stimuli that can activate ionic conductivity:
    electrical; chemical; optical; mechanical; thermal
    Walsh and Stranks, ACS Energy Lett. 3, 1983 (2018)
    Lattice thermal conductivity is ultra-low – do mobile ions transport heat?

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  30. Talk Outline: Beyond Perovskites
    A. 21st Century Materials Modelling
    B. Metal Halide Perovskites
    C. Outlook for Materials Design

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  31. Solar Absorber 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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  32. Solar Absorber 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
    What can we reliably calculate
    using materials modelling?

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  33. Does it Absorb Sunlight?
    Simple descriptor: Eg
    . Advanced: optical absorption
    and detailed-balance for a thin-film
    SLME metric: Yu and Zunger, Phys. Rev. Lett. 108, 068701 (2012)
    Detailed balance: Blank et al, Phys. Rev. App. 8, 024032 (2017)
    Materials Chemistry
    Structure/Composition
    o Orbital character
    o Band widths
    o Band degeneracy
    o Selection rules

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  34. Is it Stable?
    Experiment (Scragg)
    Kesterite Unstable
    CZTS Stable
    Simple descriptor: ΔHf
    . Advanced: secondary
    phases under realistic growth conditions (ΔGf
    )
    Materials Chemistry
    Structure/Composition
    o Choice of elements
    o Stoichiometry
    o Synthetic routes
    o Metastability
    Kesterites: Jackson and Walsh, J. Mat. Chem A 2, 7829 (2014)
    Perovskites: Zhang et al, Chin. Phys. Lett. 3, 036104 (2018) [arXiv 2014]
    Cu2
    ZnSnS4
    à
    Cu2
    S + ZnS + SnS +S(g)

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  35. Does it Conduct (p–i–n)?
    CZTS Stable
    Simple descriptor: me
    * and ΔH(q,q’) for key
    defects. Advanced: self-consistent EF
    analysis
    Materials Chemistry
    Structure/Composition
    o Band energies
    o Growth conditions
    o (Co-)dopants
    o Solid-solutions
    Analysis of SnS: Y. Kumagai et al, Phys. Rev. Appl. 6, 014009 (2016)
    Defect review: J. Park et al, Nature Rev. Mater. 3, 194 (2018)
    Self-consistent
    defect cycle

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  36. Will Carriers Survive?
    Defect tolerance : Walsh and Zunger, Nature Mater. 16, 964 (2017)
    Defect review: J. Park et al, Nature Rev. Mater. 3, 194 (2018)
    Simple descriptor: ΔH(q,q’) point defect levels.
    Advanced: prediction of carrier capture rates
    Materials Chemistry
    Structure/Composition
    o Redox active ions
    o Lattice vibrations
    o Passivation
    o Post-processing

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  37. How to Extract Charge?
    CZTS Unstable
    CZTS Stable
    Simple descriptor: ɸ (workfunction). Advanced:
    structure matching and interfacial effects
    Materials Chemistry
    Structure/Composition
    o Atomic levels
    o Coordination
    o Reactivity
    o Interface layers
    QM/MM band energies: D. O. Scanlon et al, Nature Mater. 12, 798 (2013)
    ELS for perovskites: K. T. Butler et al, J. Mater. Chem. C 4, 1129 (2016)
    Electronic matching
    Lattice strain
    Site overlap
    for A
    find B with
    low Schottky barrier &
    small lattice mismatch &
    high atomic overlap

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  38. Progress Beyond Perovskites
    Ganose, Savory and Scanlon, Chem. Commun. 53, 20 (2017)

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  39. Conclusions: Beyond Perovskites
    • First-principles modelling is having impact
    on emerging photovoltaic technologies
    • Many simulation approaches required to
    describe length and time scales
    • Developments needed for quantitative
    predictions to support materials discovery
    Slides: https://speakerdeck.com/aronwalsh
    Thanks to past and present group members including Federico
    Brivio, Jarvist Frost, Jonathan Skelton, Lucy Whalley and
    collaboration network incl. P. Barnes, M. Toney, S. Stranks

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  40. Final Note: Chemist vs Machine
    Test your skills against a machine learning
    algorithm at predicting if a material is insulating
    Wolverton group: http://palestrina.northwestern.edu/metal-detection

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