Point Defects and Beyond… Prof. Aron Walsh Imperial College London, UK Yonsei University, Korea The Origins of Electronic Defects in Halide Perovskites nanoGe Online Meetup (April 2020) Defect distributions in a CH3 NH3 PbI3 (MAPI) thin film

Crystal defects that cause rapid non-radiative recombination even in low concentrations* Hunting “Killer Defects” in Solar Cells Which defects are active? How to minimise them? The limit of performance? *A. M. Stoneham, Rep. Prog. Phys. 44, 1251 (1981)

Point Defects in ABX3 Perovskites Point defects are best defined in the non-interacting dilute limit (<<1% concentration) Vacancies VA , VB , VX Interstitials Ai , Bi , Xi Antisites AB , AX , BA , BX , XA , XB *Indicated by various first-principles reports to be deep defects in MAPbI3

Halogens are Redox Active Standard notation for interstitials and antisites is misleading for defects involving halides Homoatomic polyanions I! ", I# ", I$ ", I% ", I& ", I' !"… I() '" Interhalogen anions I! Cl" , ClIBr" , ICl' " Cations I* , I#*, I$* “Advanced Inorganic Chemistry” Cotton and Wilkinson (6th Edition, 1999) Dissociation of iodine complexes under visible light illumination

Different Flavours of Point Defects ! + ℎ" ⇋ # ! + ℎ" ⇋ # + ! ⇋ ! Active for Trapping Defect D can capture a hole Active for Recombination Defect D can capture a hole and an electron ! Electronically Inert Defect D does “nothing” Shallow levels in MAPbI3 are weakly bound L. Whalley et al, J. Chem. Phys 146, 220901 (2017)

Modelling and Experiment First-Principles Calculable Measurement (Free) Energy change ΔU/ΔH/ΔG • Heats of formation and concentrations • Diffusion barriers Defect ionisation level (Optical) Optical absorption; photoluminescence (PL); photoconductivity… Defect ionisation level (Thermal) Deep-level transient spectroscopy; thermally stimulated conductivity… Defect vibrations ⍵(T) • IR / Raman / PL spectra • Diffusion rates • Recombination rates

Pushing Defect Modelling to its Limit Perfect Crystal Population of Point Defects S. Kim et al, Energy Environ. Sci. (2020); DOI: 10.1039/D0EE00291G

Carrier Trapping Hole trapping in V and H centres studied in metal halides since the 1950s L. D. Whalley et al, ACS Energy Letters 2, 2713 (2017) 2I! " + h# → I$ " V centre Self-trapping I! " + I% " + h# → I$ " H centre Interstitial trapping Predicted excited-states TDDFT (PBE0 with SOC) in DALTON2016

Carrier Trapping Structural deformation upon carrier trapping is substantial, resulting in a giant Huang-Rhys factor L. D. Whalley et al, Under Review (2020) I% & I% " ħ = 4.7meV (38/cm) 6.6meV (53/cm) Multiphonon emission: ∆ ħ = 360 phonons

Defect Theory Challenges 1. Chemical kinetics – room temperature solution processing is far from standard high-T equilibrium thermodynamic models 2. Strong relativistic effects – in Pb compounds, SOC renormalises the band gap and dispersion 3. Coupled ionic-electronic transport – standard analysis of deep trap levels breaks down 4. Beyond dilute limit – high concentrations with aggregation and evidence for secondary phases

Where are the Traps? [Led by Sam Stranks] T. A. S. Doherty et al, Nature, In Press (2020) Photoemission electron microscopy (PEEM) of (Cs0.05 FA0.78 MA0.17 )Pb(I0.83 Br0.17 )3 films

Where are the Traps? T. A. S. Doherty et al, Nature, in Press (2020) Discrete trapping regions are observed – evidence for selective hole traps (with TR-PEEM) PEEM PEEM + AFM

Models of Polycrystalline CsPbI3 Grain boundaries are sinks for excess iodine due to strain relief (large crystal relaxation) J. S. Park et al, ACS Energy Letters 4, 1321 (2019) 400 atom model of a Σ5 [130] tilt boundary in CsPbI3 . Formation energy: 0.23 J/m-2 Relative energy of excess iodine, Ii + Accumulation of charged iodine interstitials at the grain boundary (up to 1018 cm-3): linked to relaxation energy (I–I bond length)

Conclusion Defect distributions in halide perovskites are sensitive to many factors, making it hard to extract universal conclusions. Extended defects play a critical role as trap reservoirs and we need more research to truly understand them! Contributions from group and collaborators: Lucy Whalley, Youngkwang Jung, Youngwon Woo, Jacob Wilson, Sunghyun Kim (ICL); Ji-Sang Park (Kyungpook), Sam Stranks and team (Cambridge) Slides: https://speakerdeck.com/aronwalsh @lonepair