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Solar energy with a twist: dynamic disorder in hybrid perovskites

Aron Walsh
July 01, 2016

Solar energy with a twist: dynamic disorder in hybrid perovskites

Invited presentation at HOPV16 (June 2016 in Swansea, Wales) on hybrid halide perovskite solar cells

Aron Walsh

July 01, 2016
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  1. Prof. Aron Walsh Department of Chemistry University of Bath, UK

    Materials Science Yonsei University, Korea wmd-group.github.io @lonepair Art by Piers Barnes DOI: 10.1039/C6CP03474H HOPV16 – Invited Talk
  2. What is Moving in Perovskite PV? Faster (fs) Slower (ps)

    Electrons and Holes Drift and diffusion of carriers Lattice Vibrations Phonons: organic and inorganic units Molecular Rotations Reorientation of MA+ or FA+ Ions and Defects Transport of charged species
  3. What is Moving in Perovskite PV? Acc. Chem. Res. 49,

    528 (2016) Status: We now understand timescale of processes from a combination of simulation (DFT/MD/MC) and experiment (neutron diffraction, scattering & 2D-vibrational spectroscopy)
  4. Talk Outline Materials Devices 1. Crystal Structure Local symmetry breaking

    2. Dielectric Response Frequency and temperature 3. Band Gap Engineering A, B and X in ABX 3 4. Recombination Electrons and holes
  5. Crystal Structure: CH 3 NH 3 PbI 3 M. Weller

    et al, Chem. Commun. 51, 4180 (2015) High-resolution powder neutron diffraction
  6. Temperature-Driven Disorder (Entropy) J. M. Frost et al, APL Materials

    2, 081506 (2014) Antiferroelectic à Random alignment of MA+ 0K – Order 300K – Disorder StarryNight Monte Carlo Code https://github.com/WMD-group/StarryNight
  7. Inorganic Sublattice Histogram of I distribution 300 K dynamics (PBEsol/DFT)

    Thermal ellipsoids Occupation of harmonic phonon modes (Debye-Waller) Thermal motion of anions https://www.youtube.com/watch?v=K_-rsop0n5A
  8. Soft Phonon Modes: Octahedral Tilts M ½½0 R ½½½ Predicted

    for MAPI: Phys. Rev. B 92, 144308 (2015) Confirmed: IXS phonons (Billinge Group, 2016) Vibrational Brillouin zone boundary instabilities cause dynamic cage tilting Harmonic phonon eigenvectors (Phonopy with ascii-phonons)
  9. Local Symmetry Breaking Direct observation of local distortions A. N.

    Beecher et al, Under Review (2016); arXiv:1606.09267 Experiments by group of Simon Billinge (Columbia University)
  10. Local Symmetry Breaking Confirmation from inelastic X-ray scattering A. N.

    Beecher et al, Under Review (2016); arXiv:1606.09267 Anharmonic phonons double-well potential (lattice dynamics) Short range Distorted low symmetry Long range Effective high symmetry (pair distribution functions)
  11. [CH(NH 2 ) 2 ]PbI 3 Neutron Diffraction J. Phys.

    Chem. Lett. 6, 3209 (2015) Black phase of FAPbI 3 is cubic perovskite (not trigonal as previously reported) FA+ similar rotational disorder to MA+ τFA+ = 2 ps (0.5 THz); τMA+ = 3 ps (0.3 THz)
  12. Talk Outline Materials Devices 1. Crystal Structure Local symmetry breaking

    2. Dielectric Response Frequency and temperature 3. Band Gap Engineering A, B and X in ABX 3 4. Recombination Electrons and holes
  13. Dielectric (Polarisation) Response Standard Inorganic Semiconductors Static dielectric constant Optical

    dielectric constant Lattice polarisation Source: Handbook of Photovoltaics (Wiley, 2002)
  14. Dielectric Calculation of CH 3 NH 3 PbI 3 TO

    TO TO LO LO LO Effective Dielectric Constant Frequency (THz) “TO” modes “LO” modes APL Materials 1, 042111 (2013); Phys. Rev. B 92, 144308 (2015) Sum over phonon eigenmodes with Born effective charges
  15. Rotational Polarisation Standard Inorganic Semiconductors Static dielectric constant Optical dielectric

    constant Lattice polarisation Organic-Inorganic Semiconductors Dipolar molecular rotation
  16. Rotational Polarisation Described by Kirkwood-Fröhlich equation (dielectric response of dipolar

    liquid) Dipoles frozen Dipoles active Orthorhombic Tetragonal M. Maeda et al, J. Phys. Soc. Jap. 66, 1508 (1997) Herbert Fröhlich
  17. Dielectric Response of CH 3 NH 3 PbI 3 •

    These refer to a bulk response that excludes microstructure / conductivity / contact effects • The static dielectric response is larger than common inorganic absorber materials
  18. Bananas are Lossy Dielectrics J. F. Scott, J. Phys. Conden.

    Matter 20, 2 (2007) Response dominated by ion transport
  19. Mixed Ionic-Electronic Conductors Ange. Chemie 54, 1791 (2015); Nature Comm.

    6, 8497 (2015) Low Schottky Formation Energy ~ 0.15 eV Vacancy E a (eV) CH 3 NH 3 + 0.8 Pb2+ 2.3 I- 0.6 es, ,8] es in m nd in id iodide [MAI; Reaction (2)] or lead i sub-lattices. nil ! V= MA þ V== Pb þ 3V I þ MAPbI3 nil ! V= MA þ V I þ MAI nil ! V== Pb þ 2V I þ PbI 2 of Prof. X. G. Gong Key Laboratory for Computational Physica Surface Physics Laboratory, Fudan Univers Reservoir of charged point defects independent of external chemical potential Figure 3. Iodide ion vacancy migration from DFT calculations (a) Calculated migration
  20. Talk Outline Materials Devices 1. Crystal Structure Local symmetry breaking

    2. Dielectric Response Frequency and temperature 3. Band Gap Engineering A, B and X in ABX 3 4. Recombination Electrons and holes
  21. Electronic Structure of CH 3 NH 3 PbI 3 Configuration:

    PbII [5d106s26p0]; I-I [5p6] F. Brivio et al, Physical Review B 89, 155204 (2014) Relativistic QSGW theory with Mark van Schilfgaarde (KCL) Conduction Band Valence Band
  22. Band Gap Engineering: Principles A site: Usually electronically inactive, but

    can change structure (volume and tilting pattern) B site: Forms lower conduction band (Pb is key for electron affinity and transport) X site: Forms upper valence band (I is key for ionisation potential and hole transport) A. Walsh, J. Phys. Chem. C 119, 5755 (2015)
  23. Band Gap Engineering: Halides K. T. Butler et al, Materials

    Horizons 2, 228 (2015) Lighter halides: deeper ionisation potentials
  24. Band Gap Engineering: Double Metals 2Pb2+ à Ag+ + Bi3+

    (Charge conserving) Conduction network is broken: large effective mass & wider indirect E g Bad news for PV applications! With D. O. Scanlon (UCL)
  25. Bismuth Chalcohalides (BiXY) O S Se Te F BiOF P4/nmm

    Batteries (Eg 4.2 eV) BiSF Unknown BiSeF Unknown BiTeF Unknown Cl BiOCl P4/nmm Photocatalyst (3.4) BiSCl Pnma “Gunn Effect” (1.9) BiSeCl Pnma “Gunn Effect” (1.8) BiTeCl P63 mc Rashba (0.8) Br BiOBr P4/nmm Photocatalyst (2.8) BiSBr Pnma “Gunn Effect” (2.0) BiSeBr Pnma “Gunn Effect” (0.9-1.5) BiTeBr P3-m1 Rashba (0.3-0.5) I BiOI P4/nmm Photocatalyst (2.0) BiSI Pnma “Gunn Effect” (1.8) BiSeI Pnma “Gunn Effect” (1.6) BiTeI P3-m1 Rasha / TI (0.4-0.5) Chalcogenide Halide Pnma P3-m1 Property engineering: 2 degrees of freedom Chem. Mater. 28, 1980 (2016); J. Mater. Chem. A 4, 2060 (2016)
  26. BiSeI – Band Edge Electronic Structure A. M. Ganose et

    al, J. Mater. Chem. A 4, 2060 (2016) Anisotropic bonding: challenge for growth Valence band Conduction band Anion p Metal p
  27. Talk Outline Materials Devices 1. Crystal Structure Local symmetry breaking

    2. Dielectric Response Frequency and temperature 3. Band Gap Engineering A, B and X in ABX 3 4. Recombination Electrons and holes
  28. Real Space e-h Separation Polar networks in CH 3 NH

    3 PbI 3 separate e- / h+ Regions of high (red) and low (blue) electrostatic potential APL Materials 2, 081506 (2014); Nature Photonics 7, 695 (2015) e- h+
  29. Reciprocal Space e-h Separation Symmetry breaking by CH 3 NH

    3 + / tilting Relativistic Rashba splitting of band edges also separates electrons / holes Reduced recombination: Momentum selection rule Recombination modeling by Pooya Azarhoosh (KCL) optically excite thermalise recombine Energy vs k Physical Review B 89, 155204 (2014); arXiv:1604.04500
  30. B: Effect of Light Intensity Changes in radiative recombination rate

    APL Materials (2016); arXiv:1604.04500 Solar cells operate in a different regime to many pump-probe spectroscopies
  31. B: Effect of Light Intensity Changes in radiative recombination rate

    APL Materials (2016); arXiv:1604.04500 Solar cells operate in a different regime to many pump-probe spectroscopies See posters: 3704 by Pooya Azarhoosh (Recombination theory) 3763 by Scott McKechnie (Physical origin)
  32. Conclusion Hybrid perovskites offer a rich solid-state chemistry and physics.

    Some puzzles are solved, but many remain. We need reliable and quantitative data (simulation and experiment) and theories to make progress. Group Members: PV – Lucy, Federico, Suzy, Keith, Youngkwang, Jarvist; MOFs – Chris, Jess, Katrine; Metastability – Jonathan, Lora, Ruoxi, Clovis Collaborators: PV – Mark van Schilfgaarde (KCL); Mark Weller and Saiful Islam (Bath); Piers Barnes and Brian O’Regan (ICL); Shiyou Chen (Fudan); Simon Billinge (Columbia) Slides: https://speakerdeck.com/aronwalsh
  33. Challenges for Halide Perovskites Local Structure – correlation lengths Dynamic

    Disorder – connection to optoelectronics Ionic Conductivity – how to limit Electrical Conductivity – control p-type and n-type Chemical Stability – breakdown with O 2 / H 2 O Surfaces & Interfaces – poorly defined Alloys –thermodynamics and photo-stability Hysteresis – how to eliminate Beyond 3D – 2D (12%) & 1D hybrid perovskites