Publications by Michael Johnston


A low viscosity, low boiling point, clean solvent system for the rapid crystallisation of highly specular perovskite films

Energy and Environmental Science Royal Society of Chemistry 10 (2016) 145-152

N Noel, SN Habisreutinger, B Wenger, MT Klug, MT Hörantner, MB Johnston, RJ Nicholas, DT Moore, HJ Snaith

Perovskite-based photovoltaics have, in recent years, become poised to revolutionise the solar industry. While there have been many approaches taken to the deposition of this material, one-step spin-coating remains the simplest and most widely used method in research laboratories. Although spin-coating is not recognised as the ideal manufacturing methodology, it represents a starting point from which more scalable deposition methods, such as slot-dye coating or ink-jet printing can be developed. Here, we introduce a new, low-boiling point, low viscosity solvent system that enables rapid, room temperature crystallisation of methylammonium lead triiodide perovskite films, without the use of strongly coordinating aprotic solvents. Through the use of this solvent, we produce dense, pinhole free films with uniform coverage, high specularity, and enhanced optoelectronic properties. We fabricate devices and achieve stabilised power conversion efficiencies of over 18% for films which have been annealed at 100 °C, and over 17% for films which have been dried under vacuum and have undergone no thermal processing. This deposition technique allows uniform coating on substrate areas of up to 125 cm2, showing tremendous promise for the fabrication of large area, high efficiency, solution processed devices, and represents a critical step towards industrial upscaling and large area printing of perovskite solar cells.


Electron-phonon coupling in hybrid lead halide perovskites

Nature Communications Nature Publishing Group 7 (2016) 11755

A Wright, C Verdi, RL Milot, GE Eperon, MA Pérez-Osorio, HJ Snaith, F Giustino, M Johnston, L Herz

Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here we investigate the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, HC(NH2)2 PbI3, HC(NH2)2 PbBr3, CH3NH3PbI3 and CH3 NH3 PbBr3, and discover that scattering from longitudinal optical phonons via the Fröhlich interaction is the dominant source of electron-phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3 meV, and Fröhlich coupling constants of ∼40 and 60 meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic band-structure picture for describing charge carriers in hybrid perovskites.


Charge-carrier dynamics in 2D hybrid metal-halide perovskites

Nano letters American Chemical Society 16 (2016) 7001-7007

R Milot, RJ Sutton, GE Eperon, AA Haghighirad, J Martinez Hardigree, L Miranda, HJ Snaith, MB Johnston, L Herz

Hybrid metal halide perovskites are promising new materials for use in solar cells, however, their chemical stability in the presence of moisture remains a significant drawback. Quasi two-dimensional perovskites that incorporate hydrophobic organic interlayers offer improved resistance to degradation by moisture, currently still at the cost of overall cell efficiency. To elucidate the factors affecting the optoelectronic properties of these materials, we have investigated the charge transport properties and crystallographic orientation of mixed methylammonium (MA)/phenylethylammonium (PEA) lead iodide thin films as a function of MA:PEA and thus the thickness of the 'encapsulated' MA lead halide layers. We find that monomolecular charge-carrier recombination rates first decrease with increasing PEA fraction, most likely as a result of trap passivation, but then increase significantly as excitonic effects begin to dominate for thin confined layers. Bimolecular and Auger recombination rate constants are found to be sensitive to changes in electronic confinement, which alters the density of states for electronic transitions. We demonstrate that effective charge-carrier mobilities remain remarkably high (near 10 cm2/Vs) for intermediate PEA content and are enhanced for preferential orientation of the conducting lead-iodide layers along the probing electric field. The tradeoff between trap reduction, electronic confinement and layer orientation leads to calculated charge-carrier diffusion lengths reaching a maximum of 2.5 µm for intermediate PEA content (50%).


Hybrid perovskites for photovoltaics: charge-carrier recombination, diffusion, and radiative efficiencies

Accounts of chemical research American Chemical Society 49 (2016) 146-154

M Johnston, L Herz

Photovoltaic (PV) devices that harvest the energy provided by the sun have great potential as renewable energy sources, yet uptake has been hampered by the increased cost of solar electricity compared with fossil fuels. Hybrid metal halide perovskites have recently emerged as low-cost active materials in PV cells with power conversion efficiencies now exceeding 20%. Rapid progress has been achieved over only a few years through improvements in materials processing and device design. In addition, hybrid perovskites appear to be good light emitters under certain conditions, raising the prospect of applications in low-cost light-emitting diodes and lasers. Further optimization of such hybrid perovskite devices now needs to be supported by a better understanding of how light is converted into electrical currents and vice versa. This Account provides an overview of charge-carrier recombination and mobility mechanisms encountered in such materials. Optical-pump-terahertz-probe (OPTP) photoconductivity spectroscopy is an ideal tool here, because it allows the dynamics of mobile charge carriers inside the perovskite to be monitored following excitation with a short laser pulse whose photon energy falls into the range of the solar spectrum. We first review our insights gained from transient OPTP and photoluminescence spectroscopy on the mechanisms dominating charge-carrier recombination in these materials. We discuss that mono-molecular charge-recombination predominantly originates from trapping of charges, with trap depths being relatively shallow (tens of millielectronvolts) for hybrid lead iodide perovskites. Bimolecular recombination arises from direct band-to-band electron-hole recombination and is found to be in significant violation of the simple Langevin model. Auger recombination exhibits links with electronic band structure, in accordance with its requirement for energy and momentum conservation for all charges involved. We further discuss charge-carrier mobility values extracted from OPTP measurements and their dependence on perovskite composition and morphology. The significance of the reviewed charge-carrier recombination and mobility parameters is subsequently evaluated in terms of the charge-carrier diffusion lengths and radiative efficiencies that may be obtained for such hybrid perovskites. We particularly focus on calculating such quantities in the limit of ultra-low trap-related recombination, which has not yet been demonstrated but could be reached through further advances in material processing. We find that for thin films of hybrid lead iodide perovskites with typical charge-carrier mobilities of ∼30cm(2)/(V s), charge-carrier diffusion lengths at solar (AM1.5) irradiation are unlikely to exceed ∼10 μm even if all trap-related recombination is eliminated. We further examine the radiative efficiency for hybrid lead halide perovskite films and show that if high efficiencies are to be obtained for intermediate charge-carrier densities (n ≈ 10(14) cm(-3)) trap-related recombination lifetimes will have to be enhanced well into the microsecond range.


Increased Photoconductivity Lifetime in GaAs Nanowires by Controlled n-Type and p-Type Doping.

ACS nano 10 (2016) 4219-4227

JL Boland, A Casadei, G Tütüncüoglu, F Matteini, CL Davies, F Jabeen, HJ Joyce, LM Herz, A Fontcuberta I Morral, MB Johnston

Controlled doping of GaAs nanowires is crucial for the development of nanowire-based electronic and optoelectronic devices. Here, we present a noncontact method based on time-resolved terahertz photoconductivity for assessing n- and p-type doping efficiency in nanowires. Using this technique, we measure extrinsic electron and hole concentrations in excess of 10(18) cm(-3) for GaAs nanowires with n-type and p-type doped shells. Furthermore, we show that controlled doping can significantly increase the photoconductivity lifetime of GaAs nanowires by over an order of magnitude: from 0.13 ns in undoped nanowires to 3.8 and 2.5 ns in n-doped and p-doped nanowires, respectively. Thus, controlled doping can be used to reduce the effects of parasitic surface recombination in optoelectronic nanowire devices, which is promising for nanowire devices, such as solar cells and nanowire lasers.


Perovskite-perovskite tandem photovoltaics with optimized bandgaps

Science American Association for the Advancement of Science (2016)

GE Eperon, T Leijtens, KA Bush, R Prasanna, T Green, JT-W Wang, DP McMeekin, G Volonakis, RL Milot, R May, A Palmstrom, DJ Slotcavage, RA Belisle, JB Patel, ES Parrott, RJ Sutton, W Ma, F Moghadam, B Conings, A Babayigit, H-G Boyen, LM Herz, MB Johnston, MD McGehee, HJ Snaith

Multi-junction solar photovoltaics are proven to deliver the highest performance of any solar cell architecture, making them ideally suited for deployment in an increasingly efficiency driven solar industry. Conventional multi-junction cells reach up to 45% efficiency, but are so costly to manufacture that they are only currently useful for space and solar concentrator photovoltaics. Here, we demonstrate the first four and two-terminal perovskite-perovskite tandem solar cells with ideally matched bandgaps. We develop an infrared absorbing 1.2eV bandgap perovskite, FA0.75Cs0.25Sn0.5Pb0.5I3, which is capable of delivering 13.6% efficiency. By combining this material with a wider bandgap FA0.83Cs0.17Pb(I0.5Br0.5)3 material, we reach initial monolithic two terminal tandem efficiencies of 14.0 % with over 1.75 V open circuitvoltage. We also make mechanically stacked four terminal tandem cells and obtain 18.1 % efficiency for small cells, and 16.0 % efficiency for 1cm^2 cells. Crucially, we find that our infrared absorbing perovskite cells exhibit excellent thermal and atmospheric stability, unprecedented for Sn based perovskites. This device architecture and materials set will enable “all perovskite” thin film solar cells to reach the highest efficiencies in the long term at the lowest costs, delivering a viable photovoltaic technology to supplant fossil fuels.


A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells

Science American Association for the Advancement of Science 351 (2015) 151-155

AA Haghighirad, DP McMeekin, G Sadoughi, W Rehman, GE Eperon, M Saliba, MT Horanter, N Sakai, L Korte, B Rech, MB Johnston, LM Herz, HJ Snaith

Metal halide perovskite photovoltaic cells could potentially boost the efficiency of commercial silicon photovoltaic modules from ∼20 toward 30% when used in tandem architectures. An optimum perovskite cell optical band gap of ~1.75 electron volts (eV) can be achieved by varying halide composition, but to date, such materials have had poor photostability and thermal stability. Here we present a highly crystalline and compositionally photostable material, [HC(NH2)2](0.83)Cs(0.17)Pb(I(0.6)Br(0.4))3, with an optical band gap of ~1.74 eV, and we fabricated perovskite cells that reached open-circuit voltages of 1.2 volts and power conversion efficiency of over 17% on small areas and 14.7% on 0.715 cm(2) cells. By combining these perovskite cells with a 19%-efficient silicon cell, we demonstrated the feasibility of achieving >25%-efficient four-terminal tandem cells.


Enhanced UV-light stability of planar heterojunction perovskite solar cells with caesium bromide interface modification

Energy & Environmental Science Royal Society of Chemistry 9 (2016) 490-498

W Li, W Zhang, S Van Reenen, RJ Sutton, J Fan, AA Haghighirad, M Johnston, L Wang, HJ Snaith

© 2016 The Royal Society of Chemistry. Interfacial engineering has been shown to play a vital role in boosting the performance of perovskite solar cells in the past few years. Here we demonstrate that caesium bromide (CsBr), as an interfacial modifier between the electron collection layer and the CH3NH3PbI3-xClx absorber layer, can effectively enhance the stability of planar heterojunction devices under ultra violet (UV) light soaking. Additionally, the device performance is improved due to the alleviated defects at the perovskite-titania heterojunction and enhanced electron extraction.


Efficient perovskite solar cells by metal ion doping

ENERGY & ENVIRONMENTAL SCIENCE 9 (2016) 2892-2901

JT-W Wang, Z Wang, S Pathak, W Zhang, DW deQuilettes, F Wisnivesky-Rocca-Rivarola, J Huang, PK Nayak, JB Patel, HAM Yusof, Y Vaynzof, R Zhu, I Ramirez, J Zhang, C Ducati, C Grovenor, MB Johnston, DS Ginger, RJ Nicholas, HJ Snaith


Radiative Monomolecular Recombination Boosts Amplified Spontaneous Emission in HC(NH2)2SnI3 Perovskite Films.

Journal of Physical Chemistry Letters American Chemical Society 7 (2016) 4178-4184

RL Milot, GE Eperon, T Green, HJ Snaith, MB Johnston, L Herz

Hybrid metal-halide perovskites have potential as cost-effective gain media for laser technology because of their superior optoelectronic properties. Although lead-halide perovskites have been most widely studied to date, tin-based perovskites have been proposed as a less toxic alternative. In this Letter, we show that amplified spontaneous emission (ASE) in formamidinium tin triiodide (FASnI3) thin films is supported by an observed radiative monomolecular charge recombination pathway deriving from its unintentional doping. Such a radiative component will be active even at the lowest charge-carrier densities, opening a pathway for ultralow light-emission thresholds. Using time-resolved THz photoconductivity analysis, we further show that the material has an unprecedentedly high charge-carrier mobility of 22 cm(2) V(-1) s(-1) favoring efficient transport. In addition, FASnI3 exhibits strong radiative bimolecular recombination and Auger rates that are over an order of magnitude lower than for lead-halide perovskites. In combination, these properties reveal that tin-halide perovskites are highly suited to light-emitting devices.


Effect of structural phase transition on charge-carrier lifetimes and defects in CH3NH3SnI3 perovskite

Journal of Physical Chemistry Letters American Chemical Society 7 (2016) 1321-1326

ES Parrott, RL Milot, T Stergiopoulos, HJ Snaith, MB Johnston, L Herz

Methylammonium tin triiodide (MASnI3) has been successfully employed in lead-free perovskite solar cells, but overall power-conversion efficiencies are still significantly lower than for lead-based perovskites. Here we present photoluminescence (PL) spectra and time-resolved PL from 8 to 295 K and find a marked improvement in carrier lifetime and a substantial reduction in PL line width below ∼110 K, indicating that the cause of the hindered performance is activated at the orthorhombic to tetragonal phase transition. Our measurements therefore suggest that targeted structural change may be capable of tailoring the relative energy level alignment of defects (e.g., tin vacancies) to reduce the background dopant density and improve charge extraction. In addition, we observe for the first time an above-gap emission feature that may arise from higher-lying interband transitions, raising the prospect of excess energy harvesting.


Structured organic–inorganic perovskite toward a distributed feedback laser

Advanced Materials Wiley 28 (2015) 923-929

M Saliba, S Wood, J Patel, P Nayak, J Huang, J Alexander-Webber, B Wenger, S Stranks, M Hörantner, J Wang, R Nicholas, L Herz, M Johnston, S Morris, H Snaith, M Riede

A general strategy for the in-plane structuring of organic-inorganic perovskite films is presented. The method is used to fabricate an industrially relevant distributed feedback (DFB) cavity, which is a critical step towards all-electrially pumped injection laser diodes. This approach opens the prospects of perovskite materials for much improved optical control in LEDs, solar cells and also toward applications as optical devices.


A review of the electrical properties of semiconductor nanowires: Insights gained from terahertz conductivity spectroscopy

Semiconductor Science and Technology Institute of Physics 31 (2016)

HJ Joyce, JL Boland, CL Davies, SA Baig, MB Johnston

Accurately measuring and controlling the electrical properties of semiconductor nanowires is of paramount importance in the development of novel nanowire-based devices. In light of this, terahertz conductivity spectroscopy has emerged as an ideal non-contact technique for probing nanowire electrical conductivity and is showing tremendous value in the targeted development of nanowire devices. THz spectroscopic measurements of nanowires enable charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities to be measured with high accuracy and high throughput in a contact-free fashion. This review spans seminal and recent studies of the electronic properties of nanowires using terahertz spectroscopy. A didactic description of terahertz time-domain spectroscopy, optical pump–terahertz probe spectroscopy, and their application to nanowires is included. We review a variety of technologically important nanowire materials, including GaAs, InAs, InP, GaN and InN nanowires, Si and Ge nanowires, ZnO nanowires, nanowire heterostructures, doped nanowires and modulation-doped nanowires. Finally, we discuss how terahertz measurements are guiding the development of nanowire-based devices, with the example of single-nanowire photoconductive terahertz receivers.


Broadband Phase-Sensitive Single InP Nanowire Photoconductive Terahertz Detectors

Nano Letters American Chemical Society 16 (2016) 4925-4931

K Peng, P Parkinson, JL Boland, Q Gao, YC Wenas, CL Davies, Z Li, L Fu, M Johnston, HH Tan, C Jagadish

Terahertz time-domain spectroscopy (THz-TDS) has emerged as a powerful tool for materials characterization and imaging. A trend toward size reduction, higher component integration, and performance improvement for advanced THz-TDS systems is of increasing interest. The use of single semiconducting nanowires for terahertz (THz) detection is a nascent field that has great potential to realize future highly integrated THz systems. In order to develop such components, optimized material optoelectronic properties and careful device design are necessary. Here, we present antenna-optimized photoconductive detectors based on single InP nanowires with superior properties of high carrier mobility (∼1260 cm2 V-1 s-1) and low dark current (∼10 pA), which exhibit excellent sensitivity and broadband performance. We demonstrate that these nanowire THz detectors can provide high quality time-domain spectra for materials characterization in a THz-TDS system, a critical step toward future application in advanced THz-TDS system with high spectral and spatial resolution.


Bandgap-tunable cesium lead halide perovskites with high thermal stability for efficient solar cells

Advanced Energy Materials 6 (2016) 1502458-

R Sutton, GE Eperon, L Miranda, ES Parrott, BA Kamino, JB Patel, MT Hörantner, MB Johnston, AA Haghighirad, DT Moore, HJ Snaith

Highest reported efficiency cesium lead halide perovskite solar cells are realized by tuning the bandgap and stabilizing the black perovskite phase at lower temperatures. CsPbI2Br is employed in a planar architecture device resulting in 9.8% power conversion efficiency and over 5% stabilized power output. Offering substantially enhanced thermal stability over their organic based counterparts, these results show that all-inorganic perovskites can represent a promising next step for photovoltaic materials.


Formation dynamics of CH3NH3PbI3 Perovskite following two-step layer deposition

Journal of physical chemistry letters American Chemical Society 7 (2016) 96-102

JB Patel, RL Milot, AD Wright, L Herz, M Johnston

Hybrid metal-halide perovskites have emerged as a leading class of semiconductors for optoelectronic devices because of their desirable material properties and versatile fabrication methods. However, little is known about the chemical transformations that occur in the initial stages of perovskite crystal formation. Here we follow the real-time formation dynamics of MAPbI3 from a bilayer of lead iodide (PbI2) and methylammonium iodide (MAI) deposited through a two-step thermal evaporation process. By lowering the substrate temperature during deposition, we are able to initially inhibit intermixing of the two layers. We subsequently use infrared and visible light transmission, X-ray diffraction, and photoluminescence lifetime measurements to reveal the room-temperature transformations that occur in vacuum and ambient air, as MAI diffuses into the PbI2 lattice to form MAPbI3. In vacuum, the transformation to MAPbI3 is incomplete as unreacted MAI is retained in the film. However, exposure to moist air allows for conversion of the unreacted MAI to MAPbI3, demonstrating that moisture is essential in making MAI more mobile and thus aiding perovskite crystallization. These dynamic processes are reflected in the observed charge-carrier lifetimes, which strongly fluctuate during periods of large ion migration but steadily increase with improving crystallinity.


Low ensemble disorder in quantum well tube nanowires

Nanoscale Royal Society of Chemistry 7 (2015) 20531-20538

M Christopher L., P Parkinson, N Jiang, JL Boland, S Conesa-Boj, HH Tan, C Jagadish, L Herz, M Johnston

We have observed very low disorder in high quality quantum well tubes (QWT) in GaAs-Al0.4Ga0.6As core-multishell nanowires. Room-temperature photoluminescence spectra were measured from 150 single nanowires enabling a full statistical analysis of both intra- and inter-nanowire disorder. By modelling individual nanowire spectra, we assigned a quantum well tube thickness, a core disorder parameter and a QWT disorder parameter to each nanowire. A strong correlation was observed between disorder in the GaAs cores and disorder in the GaAs QWTs, which indicates that variations in core morphology effectively propagate to the shell layers. This highlights the importance of high quality core growth prior to shell deposition. Furthermore, variations in QWT thicknesses for different facet directions was found to be a likely cause of intra-wire disorder, highlighting the need for accurate shell growth.


Temperature-dependent charge-carrier dynamics in CH3NH3PbI3 Perovskite thin films

Advanced Functional Materials Wiley 25 (2015) 6218-6227

RL Milot, GE Eperon, HJ Snaith, M Johnston, L Herz

The photoluminescence, transmittance, charge-carrier recombination dynamics, mobility, and diffusion length of CH3NH3PbI3 are investigated in the temperature range from 8 to 370 K. Profound changes in the optoelectronic properties of this prototypical photovoltaic material are observed across the two structural phase transitions occurring at 160 and 310 K. Drude-like terahertz photoconductivity spectra at all temperatures above 80 K suggest that charge localization effects are absent in this range. The monomolecular charge-carrier recombination rate generally increases with rising temperature, indicating a mechanism dominated by ionized impurity mediated recombination. Deduced activation energies Ea associated with ionization are found to increase markedly from the room-temperature tetragonal (Ea ≈ 20 meV) to the higher-temperature cubic (Ea ≈ 200 meV) phase adopted above 310 K. Conversely, the bimolecular rate constant decreases with rising temperature as charge-carrier mobility declines, while the Auger rate constant is highly phase specific, suggesting a strong dependence on electronic band structure. The charge-carrier diffusion length gradually decreases with rising temperature from about 3 μm at -93 °C to 1.2 μm at 67 °C but remains well above the optical absorption depth in the visible spectrum. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH3NH3PbI3 under typical field conditions. The photoconductivity in CH3NH3PbI3 thin films is investigated from 8 to 370 K across three structural phases. Analysis of the charge-carrier recombination dynamics reveals a variety of starkly differing recombination mechanisms. Evidence of charge-carrier localization is observed only at low temperature. High charge mobility and diffusion length are maintained at high temperature beyond the tetragonal-to-cubic phase transition at ≈310 K.


Temperature-dependent charge-carrier dynamics in CH3NH3PbI3 perovskite thin films

Advanced Functional Materials Wiley 25 (2015) 6218-6227

RL Milot, G Eperon, HJ Snaith, M Johnston, L Herz

The photoluminescence, transmittance, charge-carrier recombination dynamics, mobility, and diffusion length of CH<inf>3</inf>NH<inf>3</inf>PbI<inf>3</inf> are investigated in the temperature range from 8 to 370 K. Profound changes in the optoelectronic properties of this prototypical photovoltaic material are observed across the two structural phase transitions occurring at 160 and 310 K. Drude-like terahertz photoconductivity spectra at all temperatures above 80 K suggest that charge localization effects are absent in this range. The monomolecular charge-carrier recombination rate generally increases with rising temperature, indicating a mechanism dominated by ionized impurity mediated recombination. Deduced activation energies E<inf>a</inf> associated with ionization are found to increase markedly from the room-temperature tetragonal (E<inf>a</inf> ≈ 20 meV) to the higher-temperature cubic (E<inf>a</inf> ≈ 200 meV) phase adopted above 310 K. Conversely, the bimolecular rate constant decreases with rising temperature as charge-carrier mobility declines, while the Auger rate constant is highly phase specific, suggesting a strong dependence on electronic band structure. The charge-carrier diffusion length gradually decreases with rising temperature from about 3 μm at -93 °C to 1.2 μm at 67 °C but remains well above the optical absorption depth in the visible spectrum. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH<inf>3</inf>NH<inf>3</inf>PbI<inf>3</inf> under typical field conditions.


Vibrational properties of the organic inorganic halide perovskite CH3NH3PbI3 from theory and experiment: factor group analysis, first-principles calculations, and low-temperature infrared spectra

Journal Of Physical Chemistry C American Chemical Society 119 (2015) 25703-25718

M-A Perez-Osorio, RL Milot, MR Filip, JB Patel, L Herz, MB Johnston, F Giustino

In this work, we investigate the vibrational properties of the hybrid organic/inorganic halide perovskite MAPbI3 (MA = CH3NH3) in the range 6-3500 cm-1 by combining first-principles density-functional perturbation theory calculations and low-temperature infrared (IR) absorption measurements on evaporated perovskite films. By using a group factor analysis, we establish the symmetry of the normal modes of vibration and predict their IR and Raman activity. We validate our analysis via explicit calculation of the IR intensities. Our calculated spectrum is in good agreement with our measurements. By comparing theory and experiment, we are able to assign most of the features in the IR spectrum. Our analysis shows that the IR spectrum of MAPbI3 can be partitioned into three distinct regions: the internal vibrations of the MA cations (800-3100 cm-1), the cation librations (140-180 cm-1), and the internal vibrations of the PbI3 network (&lt;100 cm-1). The low-frequency region of the IR spectrum is dominated by Pb-I stretching modes of the PbI3 network with Bu symmetry and librational modes of the MA cations. In addition, we find that the largest contributions to the static dielectric constant arise from Pb-I stretching and Pb-I-Pb rocking modes, and that one low-frequency B2u Pb-I stretching mode exhibits a large LO-TO splitting of 50 cm-1.

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