Design principles from hybrid perovskites for next-generation photovoltaic materials

Presentation at the Advances in Photovoltaics meeting at the Institute of Physics, London

Transcript

1. Prof. Aron Walsh Department of Chemistry University of Bath, UK From 1st October 2016: Imperial College London wmd-group.github.io @lonepair IOP – Advances in Photovoltaics
2. Chemistry à Physics à Materials Trinity College Dublin, Ireland B.A. and Ph.D. in Computational Chemistry National Renewable Energy Laboratory, USA Department of Energy, Solar Energy Research Centre University College London, UK Marie Curie Intra-European Fellow University of Bath, UK Royal Society University Research Fellow
3. Fundamentals of Solar Energy Electromagnetism (light) Quantum Electrodynamics (light/matter interaction) Solid State Physics (band theory) Solid State Chemistry (optimising materials) Soft Matter Physics (glasses and polymers) Statistical Mechanics (defects and disorder) Thermodynamics (device operation and limits) Electrical Engineering (devices, systems and grids) Marketing and Politics (realise solar energy)
4. Unique Materials Science 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 Dynamic “self-healing” ability Halide perovskites exhibit disorder over multiple length and time scales
5. Talk Outline 1. First-Principles Materials Modelling 2. Emerging Photovoltaic Materials 3. Hybrid Halide Perovskites 4. Challenges and Outlook
6. (1929) Equations Too Difficult to Solve
7. (2016) Massively Parallel Computing UK’s Archer is #50 – we need sustained investment! Top500.org (Supercomputer Ranking)
8. First-Principles Materials Modelling Structure Properties William Hamilton (Dublin, 1805) Hamiltonian (ions and electrons) William Bragg (Wigton, 1862) X-ray Diffraction (unit cells) Physical Chemistry (stimuli) Neville Mott (Leeds, 1905) Input: Output:
9. Rephrase the Many-Body Problem Multi-component solids may have 100s of atoms and 1000s of electrons in a single unit cell Source: F. Bechstedt – Many-body Approach to Electronic Excitations (2015)
10. Density Functional Theory (DFT) Core Electrons all-electron pseudopotential frozen-core Hamiltonian non-relativistic scalar-relativistic spin-orbit coupling Periodicity 0D (molecules) 1D (wires) 2D (surfaces) 3D (crystals) Electron Spin restricted unrestricted non-collinear Basis Set plane waves numerical orbitals analytical functions Functional beyond…….. hybrid-GGA meta-GGA GGA LDA QMC GW RPA TD-DFT Kohn-Sham DFT (Physical Review, 1965)
11. Access Full Range of Properties Input Chemical Structure or Composition Output Total Energy + Electronic Structure Structure atomic forces equilibrium coordinates phonons thermal conductivity elastic constants Thermodynamics internal energy (U) enthalpy (H) free energy (G) activation energies (ΔE) Electron Energies density of states band structure effective mass tensors electron distribution magnetism Excitations transition intensities absorption spectra dielectric functions spectroscopy
12. Beyond T = 0 K is Now Possible Influence of T on structure and properties Phonon lifetimes for accurate spectra and heat transport Phys. Rev. B 89, 205203 (2014); Phys. Rev. Lett. 117, 075502 (2016) PbTe thermal expansion Phonon softening Thermal conductivity
13. #OpenData Computational community: Make raw I/O files available in addition to custom tools. Valuable for the community and now mandated by the UK research councils. https://github.com/WMD-group Our approach: GitHub Software developments (and writing papers) Mendeley Extended reading lists (free Endnote replacement) NoMaD EU materials data initiative (complement Materials Project) Reproducible science: share raw data!
14. Talk Outline 1. First-Principles Materials Modelling 2. Emerging Photovoltaic Materials 3. Hybrid Halide Perovskites 4. Challenges and Outlook
15. Emerging Photovoltaic Materials GaAs efficiency is close to theoretical limit, while Si is cheap. New materials must perform! Metal Oxides • Cu 2 O – 6% • Bi 2 FeCrO 6 – 8% Metal Sulfides • SnS – 5% • Cu 2 ZnSn(S,Se) 4 – 13% Metal Halides • CsSnI 3 – 10% • CH 3 NH 3 PbI 3 – 22% High-efficiency (22%) CdTe thin-film solar cell took 40 years of optimisation Complex chemical processes
16. An Ideal Photovoltaic Material • No expensive or toxic elements • Direct optical band gap 1.0 – 1.5 eV (825 nm) • Easy to deposit and scale-up production • Semiconductor with low carrier concentrations • Tolerant to impurities and microstructure • Chemically stable at interfaces • Workfunction matched to standard contacts • Lattice matched to wide-gap absorbers (e.g. ZnSe) for low cost multi-junction devices
17. New Compounds (from Somerset)
18. Multi-Component Semiconductors Multernary Materials Screening • Build database of plausible (stoichiometric) materials. • Assess structural, electronic and thermodynamic properties. • Screen & tailor for specific applications. 2 4 2
19. Quaternary Semiconductors Photovoltaic Absorbers Cu 2 ZnSnS 4 , Cu 2 ZnSnSe 4 and Cu 2 ZnGeS 4 Applied Physics Letters 94 041903 (2009) Spin-transport Materials ZnSiAl 2 As 4 , CdGeAl 2 As 4 and CuAlCd 2 Se 4 Applied Physics Letters 95 052102 (2010) Topological Insulators Cu 2 HgPbSe 4 , Cu 2 CdPbSe 4 and Ag 2 HgPbSe 4 Physical Review B 83 245202 (2011)
20. Kesterite Kesterite: Cu 2 (Zn,Fe)SnS 4 [Yakutia, Russia]
21. Cu 2 ZnSn(S,Se) 4 (13% Record Efficiency) Advanced Energy Materials 2, 400 (2012) • Crystal structure: kesterite vs stannite vs disordered • Band gaps: vary with S/Se composition • Phase stability: disproportionation into secondary phases • Lattice defects: origin of electrons and holes
22. Cu 2 ZnSn(S,Se) 4 (13% Record Efficiency) Large recombination losses – origin and solutions remains an open question Advanced Energy Materials 6, 1502276 (2016)
23. Herzenbergite: SnS [Oruro, Bolivia]
24. SnS Solar Cells (0.3 to 4.4% in 20 years) Influence of the Anion on Lone Pair Formation in Sn(II) Monochalcogenides: A DFT Study Aron Walsh and Graeme W. Watson* Department of Chemistry, UniVersity of Dublin, Trinity College, Dublin 2, Ireland ReceiVed: April 8, 2005; In Final Form: August 8, 2005 The electronic structure of SnO, SnS, SnSe, and SnTe in the rocksalt, litharge, and herzenbergite structures has been calculated using density functional theory. Comparison of the distorted and undistorted structures allows for an explanation of the unusual experimentally observed structural transitions seen along the Sn(II) monochalcogenides. Analysis of the electronic structure shows a strong anion dependence of the Sn(II) lone pair, with the Sn(5s) and Sn(5p) states too far apart to couple directly. However, the interaction of Sn(5s) with anion states of appropriate energy produce a filled antibonding Sn(5s)-anion p combination which allows coupling of Sn(5s) and Sn(5p) to occur, resulting in a sterically active asymmetric density on Sn. While the interaction between Sn(5s) and O(2p) is strong, interactions of Sn with S, Se, and Te become gradually weaker, resulting in less high energy 5s states and hence weaker lone pairs. The stability of the distorted structures relative to the symmetric structures of higher coordination is thereby reduced, which induces the change from highly distorted litharge SnO to highly symmetric rocksalt SnTe seen along the series. 18868 J. Phys. Chem. B 2005, 109, 18868-18875
25. SnS Solar Cells (0.3 to 4.4% in 20 years) Influence of the Anion on Lone Pair Formation in Sn(II) Monochalcogenides: A DFT Study Aron Walsh and Graeme W. Watson* Department of Chemistry, UniVersity of Dublin, Trinity College, Dublin 2, Ireland ReceiVed: April 8, 2005; In Final Form: August 8, 2005 The electronic structure of SnO, SnS, SnSe, and SnTe in the rocksalt, litharge, and herzenbergite structures has been calculated using density functional theory. Comparison of the distorted and undistorted structures allows for an explanation of the unusual experimentally observed structural transitions seen along the Sn(II) monochalcogenides. Analysis of the electronic structure shows a strong anion dependence of the Sn(II) lone pair, with the Sn(5s) and Sn(5p) states too far apart to couple directly. However, the interaction of Sn(5s) with anion states of appropriate energy produce a filled antibonding Sn(5s)-anion p combination which allows coupling of Sn(5s) and Sn(5p) to occur, resulting in a sterically active asymmetric density on Sn. While the interaction between Sn(5s) and O(2p) is strong, interactions of Sn with S, Se, and Te become gradually weaker, resulting in less high energy 5s states and hence weaker lone pairs. The stability of the distorted structures relative to the symmetric structures of higher coordination is thereby reduced, which induces the change from highly distorted litharge SnO to highly symmetric rocksalt SnTe seen along the series. 18868 J. Phys. Chem. B 2005, 109, 18868-18875
26. Single Crystal Tin Sulfides L. A. Burton et al, Chemistry of Materials 25, 4908 (2013) SnS Sn2 S3 SnS2 Lee
27. Workfunction Matching for SnS L. A. Burton and A. Walsh, Appl. Phys. Lett. 102, 132111 (2013) Sn(II) is distinct from Sn(IV): filled 5s2 orbitals
28. Workfunction Matching for SnS L. A. Burton and A. Walsh, Appl. Phys. Lett. 102, 132111 (2013) Standard ‘thin-film’ contacts will limit performance nction A pre- nction ed sin- ear the ificant HSE06 an for n self- hange. on the in the ational surfa- y-body for the recent bonded ations, display conduction bands of SnS are misaligned, with the valence FIG. 2. Predicted band alignment between SnS (HSE06 ionisation potential) and a range of materials used in thin-film solar cells from Refs. 3, 28, 29, and 32. TABLE I. Candidate electrical contacts for SnS devices based on energy level alignment and element availability. Values for metals are taken from Ref. 33 and values for semiconductor buffers from Refs. 34 and 35. Metallic contacts Workfunction (eV) n-type buffers Workfunctiona (eV) Titanium 4.33 (Zn,Cd)S 2.95 (ZnS) – 3.88 (CdS) Tungsten 4.55 Zn(S,Se) 2.95 (ZnS) – 4.09 (ZnSe) Tin 4.42 Zn(O,S) 3.91 (ZnO) – 2.95 (ZnS) aEquated to reported electron affinities/conduction band edge due to n-type nature. The limiting energies for the semiconductor solid-solutions are taken as those of the parent binary compounds. It should be noted that absolute values can vary according to the particular sample and/or study.
29. Mixed Phases in SnS Solar Cells L. A. Burton et al, Chemistry of Materials 25, 4908 (2013) SnS2 will be a source of recombination. Isolated Sn2 S3 may be a good absorber. ncy rier ling be pid high e n- ples tals, be d by the 9,80 fide erns SnS As hat this to her et al.58,83 This value agrees well with the experimental observations of Zainal et al.84 The calculated valence band maxima, conduction band minima and vacuum potentials allow us construct a natural band offset diagram for each of the tin sulfide phases, which is shown in Figure 6. The procedure is detailed in ref 19, where Figure 6. Calculated band offset diagram, using the HSE06 functional,
30. Talk Outline 1. First-Principles Materials Modelling 2. Novel Photovoltaic Materials 3. Hybrid Halide Perovskites 4. Challenges and Outlook
31. 19th Century Science Zeitschrift für Anorganische Chemie 3, 195 (1873)
32. Oxidation states CH 3 NH 3 + Pb2+ 3 × I- Tolerance factor α = 0.91 CH3NH3PbX3, ein Pb(II)-System mit kubischer Perowskitstruktur CH3NH3PbX3, a Pb(II)-System with Cubic Perovskite Structure Dieter Weber Institut für Anorganische Chemie der Universität Stuttgart Z. Naturforsch. 33 b, 1443-1445 (1978); eingegangen am 21. August 1978 Synthesis, X-ray CH3NH3PbX3 (X = Cl, Br, I) has the cubic perovskite structure with the unit cell parameters a = 5,68 A (X = C1), a = 5,92 A (X = Br) and a = 6,27 A (X = I). With exception of CH3NH3PbCl3 the compounds show intense colour, but there is no significant conductivity under normal conditions. The properties of the system are explained by a "p-resonance-bonding". The synthesis is described. Im System APbX3 (A = einwertiges Kation, X = Cl, Br, I) ist die Perowskitstruktur bislang nur bei Hochtemperaturmodifikationen des Typs CsPbX3 [1, 2] bekannt. Dagegen kristallisiert das Sn(II)-analoge CsSnBr3 [3-5] schon bei Normal- bedingungen im kubischen Perowskitgitter. Ver- mutlich reicht die Größe des Cs+-Kations nicht aus, um in einer Pb(II)-Perowskitstruktur den ihm zur die kubischen Kristalle der Zusam C H 3 N H 3 P M 3 schwarz sind. Die gemisch ten Verbindungen lassen charakteristi zwischenstufen erkennen. So verursacht Substitution von Bromid gegen Chlorid aufhellung, wie die dunkelgelbe CH3NH3PbBr2( 3Clo,7 verdeutlicht. Wi Bromid durch Iodid ersetzt wie im From Weber to Mitzi to Miyasaka
33. S It has been pointed out (26) that this scenario is particularly appealing, because other nonstellar injection models suffer the disadvantage of both having to account for the fractionation through other means and facing the prospect that the remarkable similarity between the cosmic-ray source composition and the composition of solar energetic particles is purely accidental. However, this hypothesis is based entirely on extrapolation of the solar case, because before now it has not been possible to de- termine the abundances of elements in the coronae of other stars. Our failure to detect a similar FIP effect in the corona of Procyon provides evidence that the FIP effect is not a ubiquitous signature of late-type stellar coronae. REFERENCES AND NOTES 1. S. R. Pottasch, Astrophys J. 137, 945 (1963). 2. A. Mogro-Campero and J. A. Simpson, ibid. 171, L5 (1972). 3. A. B. C. Walker, H. R. Rugge, K. Weiss, ibid. 194, 471 (1974). 4. H. R. Rugge and A. B. C. Walker, ibid. 203, L139 (1976). 5. J. H. Parkinson, Astron. Astrophys. 57, 185 (1977). 6. J.-P. Meyer, Astrophys. J. Suppl. Ser. 57, 172 (1985). 7. M. Casse and P. Goret, Astrophys. J. 221, 703 (1978). 8. U. Feldman, Phys. Scr. 46, 202 (1992). 9. S. Bowyer and R. F. Malina, in Extreme Ultraviolet Astronomy, R. F. Malina and S. Bowyer, Eds. (Per- gamon, New York, 1991), p. 94. 10. R. Griffin, Mon. Not. R. Astron. Soc. 155,139 (1971). 11. J. Tomkin and D. L. Lambert, Astrophys. J. 223, 937 (1978). 12. K. Kato and K. Sadakane, Astron. Astrophys. 167, 111 (1986). 13. M. Steffen, Astron. Astrophys. Suppl. Ser. 59, 403 (1985). 14. B. Edvardsson et al., Astron. Astrophys. 275, 101 (1993). 15. J. H. M. M. Schmitt et al., Astrophys. J. 290, 307 (1985). 16. B. Haisch, J. J. Drake, J. H. M. M. Schmitt, ibid. 421, L39 (1994). 17. K. G. Widing and U. Feldman, ibid. 334,1046 (1989). 18. J. H. M. M. Schmitt, B. M. Haisch, J. J. Drake, the referees for pertinent comments, which im- proved the manuscript. J.J.D. was supported by National Aeronautics and Space Administration grant AST91-15090 administered by the Center for Extreme Ultraviolet Astrophysics, University of California. 18 August 1994; accepted 28 December 1994 Conducting Layered Organic-inorganic Halides Containing (1 1 0)-Oriented Perovskite Sheets D. B. Mitzi,* S. Wang, C. A. Feild, C. A. Chess, A. M. Guloy Single crystals of the layered organic-inorganic perovskites, [NH2C(I)=NH2]2(CH3NH3)m Snml3m+2, were prepared by an aqueous solution growth technique. In contrast to the recently discovered family, (C4H9NH3)2(CH3NH3)n_1Snnl3n+1 which consists of (100)- terminated perovskite layers, structure determination reveals an unusual structural class with sets of m (110)-oriented C1-n3NI-13 perovskite sheets separated by iodoforma- midinium cations. Whereas the m = 2 compound is semiconducting with a band gap of 0.33 + 0.05 electron volt, increasing m leads to more metallic character. The ability to control perovskite sheet orientation through the choice of organic cation demonstrates the flexibility provided by organic-inorganic perovskites and adds an important handle for tailoring and understanding lower dimensional transport in layered perovskites. Recent interest in organic-inorganic mul- tilayer perovskites stems from the flexibility to use organic layers to tailor magnetic (1, 2), optical (3, 4), thermochromic (5), or structural (6) properties of adjacent non- conducting metal halide perovskite sheets. Typically, these self-assembling structures consist of single (100)-terminated perov- skite sheets alternating with alkylammo- nium bilayers, with the alkyl chains extend- ing into the space between layers and van der Waals interactions between chains holding the layers together. More compli- cated organic cations have also been incor- porated, including those with benzene rings and unsaturated hydrocarbon tails (4, 7). The ability to polymerize the organic layer (7, 8) or to study conformational changes within long-chain alkylammonium bilayers (9) provides further flexibility and interest. layers. Observation of enhanced exciton binding energies in both the lead(II) and tin(II) analogs of these layered perovskites highlight the two-dimensional nature and the effect of dielectric modulation (3, 11). In this report, we discuss the synthesis, structure, and transport properties of a class of conducting layered halides, [NH2C(I) =NH2]2(CH3NH3)mSnml3m+2 (m = 2 to 4), that consists of m CH3NH3SnI3 perovskite layers terminating on a (110) crystallograph- ic plane, rather than on the usual (100) plane. This structure appears to be stabilized by the interposed layers of iodoformami- dinium cations, which orient along the channels provided by the (110) perovskite surfaces. The ability to form either (100)- or (110)-terminated perovskite sheets through the choice of organic cation in the initial crystal growth solution (in this case, bu- - ~I.0 interactions between organic tail groups on or- ganic-inorganic-organic layers induce stacking of the layers to form the alternating, organic- charge-carrying sheet of carrier transport. T sharp x-ray reflections Fig. 1. Schematic of a TFT device structure having a layered organic-inorganic perovskite as the Fig. 2. (A) X-ray diffrac pleted TFT with (C 6 H semiconducting channel electrodes. (B) Represen organic perovskite used R E P O R T S From Weber to Mitzi to Miyasaka
34. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells Akihiro Kojima,† Kenjiro Teshima,‡ Yasuo Shirai,§ and Tsutomu Miyasaka*,†,‡,| Graduate School of Arts and Sciences, The UniVersity of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan, Graduate School of Engineering, Toin UniVersity of Yokohama, and Peccell Technologies, Inc., 1614 Kurogane-cho, Aoba, Yokohama, Kanagawa 225-8502, Japan, and Graduate School of Engineering, Tokyo Polytechnic UniVersity, 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan Received December 9, 2008; Revised Manuscript Received April 1, 2009; E-mail: miyasaka@cc.toin.ac.jp Light-energy conversion by photoelectrochemical cells has been extensively studied in the past 50 years using various combinations of inorganic semiconductors and organic sensitizers.1 Dye-sensitized mesoscopic TiO2 films have been established as high-efficiency photoanodes for solar cells.2 As cost-effective devices, dye- sensitized photovoltaic cells suit vacuum-free printing processes for cell fabrication; such processes enable researchers to design thin, flexible plastic cells by low-temperature TiO2 coating technol- ogy.3 With a thin photovoltaic film, optical management is an important key for harvesting light while ensuring high efficiency. Organic sensitizers often limit light-harvesting ability because of their low absorption coefficients and narrow absorption bands. To overcome this, researchers have examined quantum dots such as CdS,4a,b CdSe,4c-e PbS,4f,g InP,4h and InAs4i for photovoltaic cells in both electrochemical and solid-state structures. Intense band- gap light absorption by these inorganic sensitizers, however, has not allowed high performance in quantum conversion and photo- voltaic generation; significant losses in light utilization and/or charge separation are found at the semiconductor-sensitizer interface. We have studied the photovoltaic function of the organic-inorganic lead halide perovskite compounds CH3 NH3 PbBr3 and CH3 NH3 PbI3 as visible-light sensitizers in photoelectrochemical cells. In addition to being synthesized from abundant sources (Pb, C, N, and halogen), these perovskite materials have unique optical properties,5 excitonic methylamine in methanol solution followed by recrystallization. Synthesis of CH3 NH3 PbBr3 on the TiO2 surface was carried out by dropping onto the TiO2 film a 20 wt % precursor solution of CH3 NH3 Br and PbBr2 in N,N-dimethylformamide; subsequent film formation was done by spin-coating.8 For CH3 NH3 PbI3 , an 8 wt % precursor solution of CH3 NH3 I and PbI2 in γ-butyrolactone was employed. The liquid precursor film coated on the TiO2 gradually changed color simultaneously with drying, indicating the formation of CH3 NH3 PbX3 in the solid state. A vivid color change from colorless to yellow occurred for CH3 NH3 PbBr3 and from yellowish to black for CH3 NH3 PbI3 . X-ray diffraction analysis (Rigaku RINT- 2500) for CH3 NH3 PbBr3 and CH3 NH3 PbI3 prepared on TiO2 showed that both materials have crystalline structures that can be assigned to the perovskite form. CH3 NH3 PbBr3 gave diffraction peaks at 14.77, 20.97, 29.95, 42.9, and 45.74°, assigned as the (100), (110), (200), (220), and (300) planes, respectively, of a cubic perovskite structure with a lattice constant of 5.9 Å.9 CH3 NH3 PbI3 gave peaks at 14.00 and 28.36° for the (110) and (220) planes, respectively, of a tetragonal perovskite structure with a ) 8.855 Å and c ) 12.659 Å.9 Scanning electron microscopy (SEM) observation of the CH3 NH3 PbBr3 -deposited TiO2 showed nanosized particles (2-3 nm) that existed here and there on the TiO2 and/or CH3 NH3 PbBr3 surface (Figure 1). Published on Web 04/14/2009 From Weber to Mitzi to Miyasaka
35. CH 3 NH 3 PbI 3 – Hybrid Perovskite APL Mater. 1, 042111 (2013); Nano Letters 14, 2484 (2014) A B X3 a (Å) Eg (eV) CH3 NH3 + Pb I 6.36 1.61 CH(NH2 )2 + Pb I 6.32 1.48 CH(NH2 )2 + /CH3 NH3 + Pb I /Br - - (2009) 4% à (2016) 22% light-to-electricity conversion > 2500 Publications. Mendeley Group: “Hybrid Perovskite Solar Cells” Inorganic Hybrid
36. Essentials for Solar Cells • Strong optical absorption (E g ~ 1.5 eV) • Light electron and hole masses (conductive) • Easy to synthesise (cheap and scalable) Why Hybrid Halide Perovskites? Advanced Features • Large dielectric constants: carrier separation (no excitons) and transport (low scattering) • Low recombination rates (low losses; high V OC ): o Rashba effect (reciprocal space separation) o Polar domains (real space separation)
37. Methylammonium (CH 3 NH 3 + or MA+) A closed shell (18 electron) molecular cation with a large electric dipole J. M. Frost et al, Nano Letters 14, 2584 (2014)
38. Crystal Structure: CH 3 NH 3 PbI 3 M. Weller et al, Chem. Commun. 51, 4180 (2015) High-resolution powder neutron diffraction
39. 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
40. Timescales of Molecular Motion Librations Rotations Validated by quasi-elastic neutron scattering (N. Comm 2015) and 2D IR spectra (JPCL 2015) Antiferroelectric < 165 K; paraelectric at 300 K with short-range order Flips between equivalent <100> basins
41. 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
42. Soft Phonon Modes: Octahedral Tilts M ½½0 Predicted for MAPI: Phys. Rev. B 92, 144308 (2015) Confirmed: IXS phonons (Billinge Group, 2016) R ½½½ Vibrational Brillouin zone boundary instabilities cause dynamic cage tilting Harmonic phonon eigenvectors (Phonopy with ascii-phonons)
43. Local Symmetry Breaking A. N. Beecher et al, ACS Energy Letters (2016) Experiments by group of Simon Billinge (Columbia University) Direct observation of local distortions via pair distribution functions
44. Real Space e-h Separation Polar domains: “Ferroelectric highways” Nano Letters 14, 2584 (2014) p-n junction Single domain ferroelectric Nano-domain polarity
45. 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+
46. Real Space e-h Separation 3D drift-diffusion simulations: e-/h+ pathways T. S. Sherkar et al, PCCP 18, 331 (2016) Macroscopic polarisation is not necessary to improve photovoltaic performance!
47. 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); APL Materials 4, 091501 (2016)
48. B: Effect of Light Intensity Changes in radiative recombination rate Solar cells operate in a different regime to many pump-probe spectroscopies APL Materials 4, 091501 (2016)
49. Research Update #1 Rashba splitting has been measured
50. Research Update #2 Hot carriers have been observed
51. Research Update #3 Low dimensional perovskites can be useful
52. 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 and 1D hybrid perovskites
53. Talk Outline 1. First-Principles Materials Modelling 2. Emerging Photovoltaic Materials 3. Hybrid Halide Perovskites 4. Challenges and Outlook
54. An Ideal Photovoltaic Material • No expensive or toxic elements • Direct optical band gap 1.0 – 1.5 eV (825 nm) • Easy to deposit and scale-up production • Semiconductor with low carrier concentrations • Tolerant to impurities and microstructure • Chemically stable at interfaces • Workfunction matched to standard contacts • Lattice matched to wide-gap absorbers What factors are accessible from first-principles?
55. Descriptor: Optical Absorption Yu and Zunger, Phys. Rev. Lett. 108, 068701 (2012) The integrated absorption spectrum is more important than the band gap
56. Descriptor: Workfunctions Acc. Chem. Res. 47, 364 (2014); JACS 136, 2703 (2014) • Approaches for dense and porous frameworks • Separate bulk and surface contributions Solid-state ionisation potentials
57. Descriptor: Thermodynamics A. J. Jackson and A. Walsh, J. Mater. Chem. A. 2, 7829 (2014) Cu 2 ZnSnS 4 à Cu 2 S + ZnS + SnS + ½S 2 (g) Gibbs free energy: processing window CZTS Unstable CZTS Stable
58. Descriptor: Dielectric Function TO TO TO LO LO LO Effective Dielectric Constant Frequency (THz) “TO” phonon modes “LO” phonon modes for MAPbI3 APL Materials 1, 042111 (2013); Phys. Rev. B 92, 144308 (2015) Sum over phonon eigenmodes with Born effective charges
59. Descriptor: Lattice Defects Advanced Materials 25, 1522 (2013) Defects can create and annihilate carriers 0.5 1 1.5 2 2.5 Element Ratio 1e+14 1e+16 1e+18 1e+20 Defect Density 0.1 0.2 0.3 Fermi Energy -0.5 -0.4 -0.3 -0.2 -0.1 0 P Cu (eV) 1e+14 1e+16 1e+18 Hole Density V Cu +Zn Cu Cu Zn Cu Zn - V Cu - Cu Zn Sn Zn Sn +2Zn Cu 2Cu Zn +Sn Zn 2.5 0.5 1 1.5 2 2.5 Element Ratio 1e+14 1e+16 1e+18 1e+20 1e+22 Defect Density 0 0.05 0.1 0.15 Fermi Energy -0.4 -0.3 -0.2 -0 P Cu (eV) 1e+16 1e+17 1e+18 Hole Density V Cu +Zn Cu V Cu - Cu Zn Sn Zn Sn +2Zn Cu V Cu 2.5 (b) Cu2ZnSnS4 (a) Cu2ZnSnS4 (d) Cu2ZnSnSe4 (e) Cu2ZnSnSe4 Cu2 ZnSnS4 Neutral Defect Clusters
60. Descriptor: Available Contacts Computational procedure for assessing and screening materials interfaces ELS Approach Electronic matching Lattice strain Site overlap for A find B with low Schottky barrier & small lattice mismatch & high atomic overlap K. T. Butler et al, J. Mater. Chem. C 4, 1129 (2016) Builds on: J. App. Phys. 55, 378 (1984) New absorbers require new contacts!
61. Beyond Lead Halide Solar Cells A. M. Ganose, D. O. Scanlon, et al, Chem. Comm. (2016)
62. Conclusion There are many emerging solar energy materials. The key challenge is to realise their potential in efficient devices. Predictive materials modelling can help to guide the way. Group Members: Lucy, Dan, Suzy, Federico, Youngkwang, Keith, Jarvist, Ruoxi, Jess, Katrine; Jonathan, Lora Collaborators: Mark van Schilfgaarde (KCL); Piers Barnes and Brian O’Regan (ICL); Xingao Gong (Fudan); David Scanlon (UCL); Su-Huai Wei (Beijing HPC); Simon Billinge (Columbia) Funding: