Why hybrid halide perovskites keep me awake at night Sustainable Centre for Chemical Technologies Prof. Aron Walsh Department of Chemistry University of Bath, UK Adjunct Professor Yonsei University, Korea a.walsh@bath.ac.uk Spring EMRS 2015 – EU40 Talk

Solar Cells are Challenging… 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 (systems and grids) Marketing and Politics (realise solar energy)

…Especially Hybrid Perovskites Crystal Structures (phase transitions) Dynamic Disorder (molecular rotations) Ferroelectricity (molecular dipoles) Ionic Conductivity (anion transport) Electrical Conductivity (p-type and n-type) Chemical Stability (breakdown in air) Carrier Stability (free carriers and excitons) Surfaces & Interfaces (poorly defined) Hysteresis (what efficiency to report)

More Questions than Answers “It was a time when average people could make outstanding contributions” P. A. M. Dirac

Talk Outline I. University of Bath Research Group & Techniques II. Metal-Organic Frameworks The Hybrid Advantage III. Hybrid Perovskites a. Crystal Structure b. Electronic Structure c. Solar Cells

Personal Background Trinity College Dublin, Ireland B.A. and Ph.D. in Computational Chemistry National Renewable Energy Laboratory, USA DOE Solar Energy Research Centre University College London, UK Marie Curie Intra-European Fellow University of Bath, UK Professor & Royal Society University Research Fellow

University of Bath Best UK Campus University (Times 2014)

Group Research Focus Materials Characterisation Bulk physical and chemical properties. Chemical Reactions Catalysis; lattice defects; redox chemistry. Materials Engineering Beneficial dopants or alloys. Substrate & Device Effects Interfacial & strain phenomena. Amorphisation Conduction states in InGaZnO4 Hybrid Network Photochromic MIL-125 Classical, Quantum & Multi-Scale Materials Modelling Techniques

Walsh Research Group (2015) Solar Energy Materials • Kesterites • Perovskites • Metal Oxides Metal-Organic Frameworks • Electroactive • Photoactive • Ferroelectric Metastable States • Temperature • Light • Pressure Technique Development • Materials Screening • Multi-scale Methods • Thermodynamics

Talk Outline I. University of Bath Research Group & Techniques II. Metal-Organic Frameworks The Hybrid Advantage III. Hybrid Perovskites a. Crystal Structure b. Electronic Structure c. Solar Cells

Inorganic – Organic Materials Chemical and structural diversity 0 1 2 3 0 Molecular complexes O0I0 Hybrid chains O0I1 Hybrid layers O0I2 Hybrid framework O0I3 1 Coordination polymer O1I0 Mixed layers O1I1 Mixed framework O1I2 2 Coordination layer O2I0 Mixed framework O2I1 3 Coordination framework O3I0 Rule: n+m ≤ 3 Metal Ligand Dimensionality of Inorganic Connectivity (In) I – O – I Connectivity (Om) From Cheetham et al, ChemComm 4780 (2006)

Functional Metal-Organic Frameworks Review: C. H. Hendon, D. Tiana and A. Walsh, PCCP 14, 13120 (2012) Semiconducting frameworks: Energy conversion and storage; next-generation optoelectronics. Exploit chemical freedom of MOFs beyond gas storage and catalysis ERC Starting Grant Proposal (2011)

MOFs: Band Gap Engineering OH O HO O R R = H, NH2 , OH, CH3 , Cl bdc-R OH O HO O NH2 NH2 bdc-(NH2 )2 From UV to Visible Response in MIL-125 C. H. Hendon et al, JACS 135 10942 (2013) Valence Band Conduction Band Chris Experimental validation from Collège de France (Caroline Mellot-Draznieks)

MOFs: Magnetic and Electrical Control O O O O O O O O 1y-coo 2y-coo R = COO-, OH, Me, Et, CCH, (-)-menthol, NO2 , Cl, (S)-CFClBr, (S)-CH(CH3 )Ph O O O O ny-cooR ny-ben R R n n R R Chemical Science 4, 4278 (2013); ChemComm 50, 13990 (2014) earch efforts in catalysis,1–4 and recently in electrically popular applications, MOFs haviour such as disorder,12 15 rly interesting as ordered pose opportunity for data magnetosensing.16 Reports ore frequent;17–21 notable ng an antiferromagnetically wn in Fig. 1b)22 and many o an increasing number of cally designed to achieve oic behaviour.25–27 ploit magnetism in hybrid example, magnetic frame- donor heterocycles30 with rogress, critical tempera- hile the understanding of well developed, for hybrid ciples for obtaining high 33 s can be described by the a dimer with two unpaired electrons can have two spin configura- tions, where J = (ES À ET )/2 is the energy difference between the singlet (S) and the triplet (T) states. If J 4 0 the parallel spin configuration (ferromagnetic state, FM) is favoured. Conversely, the anti-parallel (antiferromagnetic state, AFM) spin configu- ration is favoured if J o 0. For example, cupric acetate dihydrate (Fig. 1a), a molecular analogue of the HKUST-1 hybrid frame- work, is an organometallic molecule with a range of accessible spin configurations.34,35 Fig. 1 The Cu–Cu paddlewheel exhibits an antiferromagnetic (open-shell singlet) magnetic ground-state in both the single molecule cupric acetate dihydrate, a, and the periodic metal–organic framework, HKUST-1, b. Schematic spin alignments are shown with arrows. Applications for magnetic and semiconducting MOFs Chris

MOFs: Strain Engineering ACS Applied Materials & Interfaces 136 2793 (2014) Exploit high hybrid flexibility: response to external pressure or internal absorbates Keith

2015: Year of Semiconducting MOFs Million-Fold Electrical Conductivity Enhancement in Fe2 (DEBDC) versus Mn2 (DEBDC) (E = S, O) Lei Sun,† Christopher H. Hendon,‡ Mikael A. Minier,† Aron Walsh,‡ and Mircea Dincă*,† † Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ‡ Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom * S Supporting Information ABSTRACT: Reaction of FeCl2 and H4 DSBDC (2,5- disulfhydrylbenzene-1,4-dicarboxylic acid) leads to the formation of Fe2 (DSBDC), an analogue of M2 (DOBDC) (MOF-74, DOBDC4− = 2,5-dihydroxybenzene-1,4-dicarb- oxylate). The bulk electrical conductivity values of both Fe2 (DSBDC) and Fe2 (DOBDC) are ∼6 orders of magnitude higher than those of the Mn2+ analogues, Mn2 (DEBDC) (E = O, S). Because the metals are of the same formal oxidation state, the increase in conductivity is attributed to the loosely bound Fe2+ β-spin electron. These results provide important insight for the rational design of conductive metal−organic frameworks, highlighting in particular the advantages of iron for synthesizing such materials. replacing Mn2+ with Fe2+ leads to a million-fold enhancement in electrical conductivity, a considerably more pronounced effect than substituting bridging O atoms with less electronegative S atoms. [Fe2 (DSBDC)(DMF)2 ]·x(DMF) was isolated as dark red- purple crystals after heating a degassed and dry solution of H4 DSBDC and anhydrous FeCl2 in N,N-dimethylformamide (DMF) at 140 °C under an N2 atmosphere for 24 h, and washing with additional DMF. Single-crystal X-ray diffraction analysis of Fe2 (DSBDC)(DMF)·x(DMF) revealed an asymmetric unit containing one Fe atom coordinated by three carboxylate groups, two thiophenoxide groups, and one DMF molecule. The sulfur atoms are coordinated in trans fashion to the Fe2+ atom, with Fe−S bond lengths of 2.444(2) and 2.446(2) Å. This indicates that both S atoms interact with the same d orbital of Fe2+, an important orbital symmetry requirement for efficient 12 Communication pubs.acs.org/JACS This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Cation-Dependent Intrinsic Electrical Conductivity in Isostructural Tetrathiafulvalene-Based Microporous Metal−Organic Frameworks Sarah S. Park,† Eric R. Hontz,† Lei Sun,† Christopher H. Hendon,‡ Aron Walsh,‡ Troy Van Voorhis,† and Mircea Dincă*,† † Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ‡ Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom Communication pubs.acs.org/JACS

Talk Outline I. University of Bath Research Group & Techniques II. Metal-Organic Frameworks The Hybrid Advantage III. Hybrid Perovskites a. Crystal Structure b. Electronic Structure c. Solar Cells

Perovskite Crystal structure of the mineral CaTiO 3 A BX 3 Lev Perovski (Russia, 1839)

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 From Mitzi (1995) to Miyasaka

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 From Mitzi (1995) to Miyasaka 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 Mitzi to Miyasaka (2009) 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 Mitzi to Miyasaka (2009) 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

Talk Outline I. University of Bath Research Group & Techniques II. Metal-Organic Frameworks The Hybrid Advantage III. Hybrid Perovskites a. Crystal Structure b. Electronic Structure c. Solar Cells

CH 3 NH 3 PbI 3 (MAPI for short) Oxidation States CH 3 NH 3 + Pb2+ 3 × I- 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

CH 3 NH 3 PbI 3 (MAPI for short)

CH 3 NH 3 PbI 3 Phase Transitions M. Weller et al, Chem. Commun. 51, 4180 (2015) Orthorhombic Tetragonal Cubic

Methylammonium (CH 3 NH 3 +) A closed shell (18 electron) molecular cation with a large electric dipole J. M. Frost et al, Nano Letters 14, 2584 (2014) Deprotonation (pK a ~ 10): CH 3 NH 3 + à CH 3 NH 2 + H+

Molecular Vibrations of CH 3 NH 3 + CH 3 NH 3 + = 8 atoms = 18 modes (3N-6) (C3v symmetry: 5A1 + 1A2 + 6E irreducible representations) Jarv

Phonon Modes of CH 3 NH 3 PbI 3 F. Brivio et al, ArXiv 1504.07508 (2015) Pseudo-cubic phase = 12 atoms = 36 modes (3N)

Phonon Modes of CH 3 NH 3 PbI 3 Pseudo-cubic phase = 12 atoms = 36 modes (3N) F. Brivio et al, Phys. Rev. B. 92, 144308 (2015)

Vibrational Spectra of CH 3 NH 3 PbI 3 Fed On-going Experiments: Aurelien Leguy Piers Barnes Oli Weber Mark Weller Mariano Compoy Alejandro Goni

https://www.youtube.com/watch?v=PPwSIYLnONY First-principles Dynamics (300 K) “MAPI is as soft as jelly” Jarvist 11.2013 PbI 6 Octahedra MA+ “Salt cellars” (0.1ps mean) Timestep of 0.5×10-15 s

Molecular Orientation from Simulation J. M. Frost et al, APL Materials 2, 081506 (2014) Temperature: 300 K Production: 58 ps Timestep: 0.5 fs

Observation of Molecular Motion Quasi-Elastic Neutron Scattering (QENS) 14 ps residence time Probes 1 – 200 ps (Piers Barnes) 2D Photon Echo (Pump-probe IR) 300 fs / 3 ps processes Probes 0 – 6 ps (Artem Bakulin) Nature Communications 6, 7124 (2015)

Talk Outline I. University of Bath Research Group & Techniques II. Metal-Organic Frameworks The Hybrid Advantage III. Hybrid Perovskites a. Crystal Structure b. Electronic Structure c. Solar Cells

CH 3 NH 3 PbI 3 Band Structure 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 Dresselhaus Splitting (Suppress Recombination) [Molecule breaks centrosymmetry]

“Natural” Valence Band Alignments K. T. Butler et al, Materials Horizons 2, 228 (2015) Similar to other thin-film PV materials Band gap engineering through A, B or X site modification

Free Carriers or Excitons in Devices? Exciton binding from effective mass theory: E b = m*e4 !2ε2 ≈ 45 meV (ε ∞ ) ; 1.6 meV (ε0 ) The situation is different to organic PV where the dielectric constants are small and effective masses are heavy. Semiconducting perovskites are distinct from passive dyes in sensitised cells. Carrier masses & dielectric screening favour free carrier generation J. M. Frost et al, Nano Letters 14, 2584 (2014)

Multi-scale Dielectric Relaxation A. Walsh, J. Phys. Chem. C 119, 5755 (2015)

Multi-scale Dielectric Relaxation A. Walsh, J. Phys. Chem. C 119, 5755 (2015)

“Giant Dielectric Constant”

“Giant Dielectric Constant”

Talk Outline I. University of Bath Research Group & Techniques II. Metal-Organic Frameworks The Hybrid Advantage III. Hybrid Perovskites a. Crystal Structure b. Electronic Structure c. Solar Cells

Current-Voltage Hysteresis H. J. Snaith et al, J. Phys. Chem. Lett. 5, 1511 (2014) Operation voltage and history affect recombination: sensitive to J-V scan rate 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -20 -10 0 10 20 FB-SC SC-FB Current density (mAcm-2) Applied Bias (V) Scan rate: 0.15 V/s Direction Jsc (mA/cm2) η (%) Voc (V) FF FB-SC 22.9 15.5 1.05 0.66 SC-FB 22.8 10.2 0.97 0.46 9 of 17 The Journal of Physical Chemistry Letters

Domains of Molecular Dipoles Ferroelectric Hamiltonian (Monte Carlo solver) Regions of high (red) and low (blue) electrostatic potential J. M. Frost et al, APL Materials 2, 081506 (2014) Spatial separation of conduction channels

Impact of Built-in Electric Field Changes in polarisation of perovskite layer: small capacitance and too rapid to explain hysteresis J. M. Frost et al, APL Materials 2, 081506 (2014)

Is CH 3 NH 3 PbI 3 Ferroelectric? No, but complex polarisation behaviour. • Below 165 K, molecular dipoles align in antiferroelectric configuration • Above 165 K, molecular dipoles are disordered; long-range ferroelectric order parameter is never ~1.0. • Radial order parameter has short-range order up to high T; internal electrical fields predicted.

Mixed Ionic-Electronic Conductors Ange. Chemie 54, 1791 (2015); Nature Comm. 6, 7497 (2015) Charged Ion Drift and Diffusion Consistent activation energy from simulations and kinetic measurements of photovoltaic response (chronophotoamperometry by Piers Barnes and Brian O’Regan, ICL)

Hybrid Perovskite Challenges Crystal Structures (three phase transitions) Structural Disorder (molecular rotations) Ferroelectricity (molecular dipoles) Ionic Conductivity (anion transport) Electrical Conductivity (p-type and n-type) Chemical Stability (breakdown in air) Carrier Stability (free carriers and/or excitons) Surfaces & Interfaces (poorly defined) Hysteresis (what efficiency to report)

Conclusion The interesting chemistry and physics of hybrid halide perovskites are worth losing sleep over. Presentation: http://wmd-group.github.io/ References: Mendeley group “Hybrid Perovskite Solar Cells” Group: PV: Adam, Federico, Suzy, Keith, Jarvist; MOFs: Chris, Ruoxi, Jess, Katrine; Metastable: Jonathan, Lora, Clovis Perovskite Collaborators: Shiyou Chen (Fudan); Su-Huai Wei (NREL); David Scanlon (UCL); Piers Barnes (ICL); Mark van Schilfgaarde (KCL); Laurie Peter (Bath)

Group Hybrid Perovskite Publications Principles of chemical bonding and band gap engineering in hybrid organic–inorganic halide perovskites (2015) http://dx.doi.org/10.1021/jp512420b Ferroelectric materials for solar energy conversion: photoferroics revisited (2015) http://dx.doi.org/10.1039/C4EE03523B Self-regulation mechanism for charged point defects in hybrid halide perovskites (2015) http://dx.doi.org/10.1002/anie.201409740 The dynamics of methylammonium ions in hybrid organic– inorganic perovskite solar cells (2015) http://dx.doi.org/10.1038/ncomms8124 Ionic transport in hybrid perovskite solar cells (2015) http://dx.doi.org/10.1038/ncomms8497 Role of microstructure in the electron–hole interaction of hybrid lead halide perovskites (2015) http://dx.doi.org/10.1038/nphoton.2015.151

Group Hybrid Perovskite Publications Structural and electronic properties of hybrid perovskites for high-efficiency thin-film photovoltaics (2013) http://dx.doi.org/10.1063/1.4824147 Atomistic origins of high-performance in hybrid halide perovskite solar cells (2014) http://dx.doi.org/10.1021/nl500390f Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells (2014) http://dx.doi.org/10.1063/1.4890246 Band alignment of the hybrid halide perovskites CH 3 NH 3 PbCl 3 , CH 3 NH 3 PbBr 3 and CH 3 NH 3 PbI 3 (2015) http://dx.doi.org/10.1039/C4MH00174E Assessment of polyanion (BF 4 − and PF 6 −) substitutions in halide perovskites (2015) http://dx.doi.org/10.1039/C4TA05284F