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

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

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)

Join the Journal Club – Mendeley Open group: add your own papers!

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

Crystal Structure: CH 3 NH 3 PbI 3 M. Weller et al, Chem. Commun. 51, 4180 (2015) High-resolution powder neutron diffraction

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

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

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)

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)

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)

[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)

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

Dielectric (Polarisation) Response Standard Inorganic Semiconductors Static dielectric constant Optical dielectric constant Lattice polarisation Source: Handbook of Photovoltaics (Wiley, 2002)

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

Rotational Polarisation Standard Inorganic Semiconductors Static dielectric constant Optical dielectric constant Lattice polarisation Organic-Inorganic Semiconductors Dipolar molecular rotation

Rotational Polarisation: Old News

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

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

“Giant Dielectric Constant” Accounts of Chemical Research 49, 528 (2016) JPCL 5, 2390 (2014)

“Giant Dielectric Constant” Accounts of Chemical Research 49, 528 (2016) JPCM 20, 191001 (2008) JPCL 5, 2390 (2014)

Bananas are Lossy Dielectrics J. F. Scott, J. Phys. Conden. Matter 20, 2 (2007) Response dominated by ion transport

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

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

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

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)

Band Gap Engineering: Halides K. T. Butler et al, Materials Horizons 2, 228 (2015) Lighter halides: deeper ionisation potentials

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)

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)

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

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

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+

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

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

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)

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

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