Photoferroelectric and Photopiezoelectric Properties of Organometal Halide Perovskites

Ferroelectric Organic-Inorganic Hybrid Ammonium Halogenobismuthate(III) for Piezoelectric Energy Harvesting

Halogenobismuthate(III) compounds are of recent interest because of their low toxicity and distinct electrical properties. The utility of these materials as ferroelectrics for piezoelectric energy harvesters is still in its early stages. Herein, we report a hybrid ammonium halogenobismuthate(III) [BPBrDMA]2·[BiBr5], crystallizing in a ferroelectrically active polar noncentrosymmetric Pna21 space group. Its noncentrosymmetric structure was confirmed by the detection of the second harmonic generation response. The ferroelectric P-E hysteresis loop measurements on the thin film sample of [BPBrDMA]2·[BiBr5] gave a saturation polarization (Ps) of 5.72 μC cm-2. The piezoresponse force microscopy analysis confirmed its ferroelectric and piezoelectric nature, showing characteristic domain structures and signature hysteresis and butterfly loops. The piezoelectric energy harvesting attributes of [BPBrDMA]2·[BiBr5] were further probed on its polylactic acid (PLA) composites. The 15 wt % [BPBrDMA]2·[BiBr5]-PLA polymer composite resulted in a high output voltage of 26.2 V and power density of 15.47 μW cm-2. The energy harvested from this device was further utilized for charging a 10 μF capacitor within 3 min.

Enhanced Piezoelectricity of MAPbI3 by the Introduction of MXene and Its Utilization in Boosting High-Performance Photodetectors

Recently, perovskite photodetectors (PDs) are risen to prominence due to substantial research interest. Beyond merely tweaking the composition of materials, a cutting-edge advancement lies in leveraging the innate piezoelectric polarization properties of perovskites themselves. Here, the investigation shows utilizing Ti3C2Tx, a typical MXene, as an intermediate layer for significantly boosting the piezoelectric property of MAPbI3 thin films. This improvement is primarily attributed to the enhanced polarization of the methylammonium (MA+) groups within MAPbI3, induced by the OH groups present in Ti3C2Tx. A flexible PD based on the MAPbI3/MXene heterostructure is then fabricated. The new device is sensitive to a wide range of wavelengths, displays greatly enhanced performance owing to the piezo-phototronic coupling. Moreover, the device is endowed with a greatly reduced response time, down to millisecond level, through the pyro-phototronic effect. The characterization shows applying a -1.2% compressive strain on the PD leads to a remarkable 102% increase in the common photocurrent, and a 76% increase in the pyro-phototronic current. The present work reveals how the emerging piezo-phototronic and pyro-phototronic effects can be employed to design high-performance flexible perovskite PDs.

Piezo-Acoustic Resistive Switching Behaviors in High-Performance Organic-Inorganic Hybrid Perovskite Memristors

Memristors are regarded as promising candidates for breaking the problems including high off-chip memory access delays and the hash rate cost of frequent data moving induced by algorithms for data-intensive applications of existing computational systems. Recently, organic-inorganic halide perovskites (OIHPs) have been recognized as exceptionally favorable materials for memristors due to ease of preparation, excellent electrical conductivity, and structural flexibility. However, research on OIHP-based memristors focuses on modulating resistive switching (RS) performance through electric fields, resulting in difficulties in moving away from complex external circuits and wire connections. Here, a multilayer memristor has been constructed with eutectic gallium and indium (EGaIn)/ MAPbI3 /poly(3,4-ethylenedioxythiophene): poly(4-styrenesulphonate) (PEDOT: PSS)/indium tin oxide (ITO) structure, which exhibits reproducible and reliable bipolar RS with low SET/RESET voltages, stable endurance, ultrahigh average ON/OFF ratio, and excellent retention. Importantly, based on ion migration activated by sound-driven piezoelectric effects, the device exhibits a stable acoustic response with an average ON/OFF ratio greater than 103 , thus realizing non-contact, multi-signal, and far-field control in RS modulation. This study provides a single-structure multifunctional memristor as an integrated architecture for sensing, data storage, and computing.

First-Principles Study on Out-of-Plane Piezoelectricity of Self-Assembled Ammonia Layers Confined in Two Vertically Stacked Graphene Oxide Nanosheets

Two-dimensional (2D) piezoelectric materials have attracted widespread attention due to their increasingly important niche applications in flexible nanoscale devices. The water-wetted graphene oxide papers exhibit scalable out-of-plane piezoelectricity induced by the hydrogen-bonded network within, and this system can be treated as a potential 2D piezoelectric candidate for future device applications. It triggered our interest to search for more 2D piezoelectric hydrogen-bonded networks. Ammonia (NH3) isoelectronic with water is introduced to generate NH3-wetted graphene oxide papers and realize their out-of-plane piezoelectricity. Their structures and piezoelectricity are investigated using first-principles calculations. They reveal ultrahigh piezoelectricity, compared to the best reported 2D materials. Their piezoelectricity is tuned by varying oxygen-containing functional groups in GO plates, confined NH3 layers, or orientations of NH3 molecules, and it could be applied to fabrication of ammonia sensors. Our study not only enriches the family of 2D piezoelectric nanosystems but also inspires their future experimental exploration.

Stabilization of photoactive phases for perovskite photovoltaics

Interest in photovoltaics (PVs) based on Earth-abundant halide perovskites has increased markedly in recent years owing to the remarkable properties of these materials and their suitability for energy-efficient and scalable solution processing. Formamidinium lead triiodide (FAPbI₃)-rich perovskite absorbers have emerged as the frontrunners for commercialization, but commercial success is reliant on the stability meeting the highest industrial standards and the photoactive FAPbI₃ phase suffers from instabilities that lead to degradation - an effect that is accelerated under working conditions. Here, we critically assess the current understanding of these phase instabilities and summarize the approaches for stabilizing the desired phases, covering aspects from fundamental research to device engineering. We subsequently analyse the remaining challenges for state-of-the-art perovskite PVs and demonstrate the opportunities to enhance phase stability with ongoing materials discovery and in operando analysis. Finally, we propose future directions towards upscaling perovskite modules, multijunction PVs and other potential applications.

Molecular Design of a Metal-Nitrosyl Ferroelectric with Reversible Photoisomerization

The development of photo-responsive ferroelectrics whose polarization may be remotely controlled by optical means is of fundamental importance for basic research and technological applications. Herein, we report the design and synthesis of a new metal-nitrosyl ferroelectric crystal (DMA)(PIP)[Fe(CN)₅(NO)] (1) (DMA = dimethylammonium, PIP = piperidinium) with potential phototunable polarization via a dual-organic-cation molecular design strategy. Compared to the parent non-ferroelectric (MA)₂[Fe(CN)₅(NO)] (MA = methylammonium) material with a phase transition at 207 K, the introduction of larger dual organic cations both lowers the crystal symmetry affording robust ferroelectricity and increases the energy barrier of molecular motions, endowing 1 with a large polarization of up to 7.6 μC cm^-2 and a high Curie temperature (T_c) of 316 K. Infrared spectroscopy shows that the reversible photoisomerization of the nitrosyl ligand is accomplished by light irradiation. Specifically, the ground state with the N-bound nitrosyl ligand conformation can be reversibly switched to both the metastable state I (MSI) with isonitrosyl conformation and the metastable state II (MSII) with side-on nitrosyl conformation. Quantum chemistry calculations suggest that the photoisomerization significantly changes the dipole moment of the [Fe(CN)₅(NO)]^2- anion, thus leading to three ferroelectric states with different values of macroscopic polarization. Such optical accessibility and controllability of different ferroelectric states via photoinduced nitrosyl linkage isomerization open up a new and attractive route to optically controllable macroscopic polarization.

Ferroelectricity in Hybrid Perovskites

Ferroelectric ceramics such as PbZr_xTi_1-xO₃ (PZT) are widely applied in many fields, from medical to aerospace, because of their dielectric, piezoelectric, and pyroelectric properties. In the past few years, hybrid organic-inorganic halide perovskites have gradually attracted attention for their optical and electronic properties, including ferroelectricity, and for their low fabrication costs. In this Review, we first describe techniques that are used to quantify ferroelectric figures of merit of a material. We then discuss ferroelectricity in hybrid perovskites, starting from controversies in methylammonium iodoplumbate perovskites and then focusing on low-dimensional perovskites that offer an unambiguous platform to obtain ferroelectricity. Finally, we provide examples of the application of perovskite ferroelectrics in solar cells, LEDs, and X-ray detectors. We conclude that the vast structure-property tunability makes low-dimensional hybrid perovskites promising, but they have yet to offer ferroelectric figures of merit (e.g., saturated polarization) and thermal stability (e.g., Curie temperature) competitive with those of conventional oxide perovskite ferroelectric materials.

Anion-Dependent Polarization and Piezoelectric Power Generation in Hybrid Halide MAPbX3 (X = I, Br, and Cl) Thin Films with Out-of-Plane Structural Adjustments

Anion-dependent differences in the electromechanical energy harvesting capability of perovskite halides have not been experimentally demonstrated thus far. Herein, anion-dependent piezoelectricity and bending-driven power generation in high-quality methylammonium lead halide MAPbX₃ (X = I, Br, and Cl) thin films are explored; additionally, anisotropic in situ strain is imposed to improve energy harvesting under tensile bending. After applying the maximum in situ strain of -0.73% for all the halide thin films, the MAPbI₃ thin-film harvester exhibited a peak voltage/current of ≈23.1 V/≈1703 nA as the best values, whereas MAPbBr₃ and MAPbCl₃ demonstrated ≈5.6 V/≈176 nA and ≈3.3 V/≈141 nA, respectively, under identical bending conditions. Apart from apparent ferroelectricity of tetragonal MAPbI₃ , origin of the piezoelectricity in both cubic MAPbBr₃ and MAPbCl₃ is explored as being related to organic-inorganic hydrogen bonding, lattice distortion, and ionic migration, with experimental supports of effective piezoelectric coefficient and grain boundary potential. Conclusively, piezoelectricity of the cubic halides is assumed to be due to their soft polarity modes and relatively low elastic modulus with vacancies contributing to space-charge polarization. In the case of ferroelectric MAPbI₃ , the distortion of PbI₆ octahedra and atomic displacement within each octahedron are quantitatively estimated.

Performance optimization strategies of halide perovskite-based mechanical energy harvesters

Halide perovskites, possessing unique electronic and photovoltaic properties, have been intensively investigated over the past decade. The excellent polarization, piezoelectricity, dielectricity and photoelectricity of halide perovskites provide new opportunities for the applications of mechanical energy harvesting. Although various studies have been conducted to develop halide perovskite-based triboelectric and piezoelectric nanogenerators, strategies for their electrical performance optimization are rarely mentioned. In this review, we systematically introduce the recent research progress of halide perovskite-based mechanical energy harvesters and summarize the different optimization strategies for improving both the piezoelectric and triboelectric output of the devices, bringing some inspiration to guide future material and structure design for halide perovskite-based energy devices. A summary of the current challenges and future perspectives is also presented, offering some possible directions for development in this emerging field.

Efficient Piezoelectric Energy Harvesting from a Discrete Hybrid Bismuth Bromide Ferroelectric Templated by Phosphonium Cation

Bismuth containing hybrid molecular ferroelectrics are receiving tremendous attention in recent years owing to their stable and non-toxic composition. However, these perovskite-like structures are primarily limited to ammonium cations. Herein, we report a new phosphonium based discrete perovskite-like hybrid ferroelectric with a formula [Me(Ph)₃ P]₃ [Bi₂ Br₉ ] (MTPBB) and its mechanical energy harvesting capability. The Polarization-Electric field (P-E) measurements resulted in a well-defined ferroelectric hysteresis loop with a remnant polarization value of 2.1 μC cm^-2 . Piezoresponse force microscopy experiments enabled visualization of the ferroelectric domain structure and evaluation of the piezoelectric strain coefficient (d₃₃ ) for an MTPBB single crystal and thin film sample. Furthermore, flexible devices incorporating MTPBB in polydimethylsiloxane (PDMS) matrix at various concentrations were fabricated and explored for their mechanical energy harvesting properties. The champion device with 20 wt % of MTPBB in PDMS rendered a maximum peak-to-peak open-circuit voltage of 22.9 V and a maximum power density of 7 μW cm^-2 at an optimal load of 4 MΩ. Moreover, the potential of MTPBB-based devices in low power electronics was demonstrated by storing the harvested energy in various electrolytic capacitors.

Recent Advances in Synthesis, Properties, and Applications of Metal Halide Perovskite Nanocrystals/Polymer Nanocomposites

Metal halide perovskite nanocrystals (PNCs) have recently garnered tremendous research interest due to their unique optoelectronic properties and promising applications in photovoltaics and optoelectronics. Metal halide PNCs can be combined with polymers to create nanocomposites that carry an array of advantageous characteristics. The polymer matrix can bestow stability, stretchability, and solution-processability while the PNCs maintain their size-, shape- and composition-dependent optoelectronic properties. As such, these nanocomposites possess great promise for next-generation displays, lighting, sensing, biomedical technologies, and energy conversion. The recent advances in metal halide PNC/polymer nanocomposites are summarized here. First, a variety of synthetic strategies for crafting PNC/polymer nanocomposites are discussed. Second, their array of intriguing properties is examined. Third, the broad range of applications of PNC/polymer nanocomposites is highlighted, including light-emitting diodes (LEDs), lasers, and scintillators. Finally, an outlook on future research directions and challenges in this rapidly evolving field are presented.

Twinning in MAPbI3 at room temperature uncovered through Laue neutron diffraction

The crystal structure of MAPbI3, the signature compound of the hybrid halide perovskites, at room temperature has been a reason for debate and confusion in the past. Part of this confusion may be due to twinning as the material bears a phase transition just above room temperature, which follows a direct group-subgroup relationship and is prone to twinning. Using neutron Laue diffraction, we illustrate the nature of twinning in the room temperature structure of MAPbI3 and explain its origins from a group-theoretical point-of-view.

Unveiling Predominant Air-Stable Organotin Bromide Perovskite toward Mechanical Energy Harvesting

Organotin halide perovskites are developed as an appropriate substitute to replace highly toxic lead-based hybrid perovskites, which are a major concern for the environment as well as for human health. However, instability of the lead-free Sn-based perovskites under ambient conditions has hindered their wider utility in device applications. In this study, we report a predominantly stable lead-free methylammonium tin bromide (MASnBr₃) perovskite that has air stability over 120 days without passivation under ambient conditions. Further, the feasibility of this predominant air-stable MASnBr₃ perovskite for use in the harvesting of mechanical energy is described with the fabrication of an ecofriendly, flexible, and cost-effective piezoelectric generator (PEG) using MASnBr₃-polydimethylsiloxane composite films. The fabricated PEG exhibits high performance along with good mechanical durability and long-term stability. This flexible device reveals a high piezoelectric output voltage of ∼18.8 V, current density of ∼13.76 μA/cm², and power density of ∼74.52 μW/cm² under a periodic applied pressure of 0.5 MPa. Further, the ability of PEG to scavenge energy from various easily accessible biomechanical movements is demonstrated. The energy generated from PEG by finger tapping is stored in a capacitor and is used to power both a stopwatch and a commercial light-emitting diode. These findings offer a new insight to achieve long-term air-stable Sn-based hybrid perovskites, demonstrating the feasibility of using organotin halide perovskites to realize highly efficient, ecofriendly, mechanical energy harvesters with a wide range of utility that includes wearable and portable electronics as well as biomedical devices.

Large electrostrictive response in lead halide perovskites

Lead halide perovskites have demonstrated outstanding performance in photovoltaics, photodetectors, radiation detectors and light-emitting diodes. However, the electromechanical properties, which are the main application of inorganic perovskites, have rarely been explored for lead halide perovskites. Here, we report the discovery of a large electrostrictive response in methylammonium lead triiodide (MAPbI₃) single crystals. Under an electric field of 3.7 V µm^-1, MAPbI₃ shows a large compressive strain of 1%, corresponding to a mechanical energy density of 0.74 J cm^-3, comparable to that of human muscles. The influences of piezoelectricity, thermal expansion, intrinsic electrostrictive effect, Maxwell stress, ferroelectricity, local polar fluctuation and methylammonium cation ordering on this electromechanical response are excluded. We speculate, using density functional theory, that electrostriction of MAPbI₃ probably originates from lattice deformation due to formation of additional defects under applied bias. The discovery of large electrostriction in lead iodide perovskites may lead to new potential applications in actuators, sonar and micro-electromechanical systems and aid the understanding of other field-dependent material properties.

Energy Harvesting Research: The Road from Single Source to Multisource

Energy harvesting technology may be considered an ultimate solution to replace batteries and provide a long-term power supply for wireless sensor networks. Looking back into its research history, individual energy harvesters for the conversion of single energy sources into electricity are developed first, followed by hybrid counterparts designed for use with multiple energy sources. Very recently, the concept of a truly multisource energy harvester built from only a single piece of material as the energy conversion component is proposed. This review, from the aspect of materials and device configurations, explains in detail a wide scope to give an overview of energy harvesting research. It covers single-source devices including solar, thermal, kinetic and other types of energy harvesters, hybrid energy harvesting configurations for both single and multiple energy sources and single material, and multisource energy harvesters. It also includes the energy conversion principles of photovoltaic, electromagnetic, piezoelectric, triboelectric, electrostatic, electrostrictive, thermoelectric, pyroelectric, magnetostrictive, and dielectric devices. This is one of the most comprehensive reviews conducted to date, focusing on the entire energy harvesting research scene and providing a guide to seeking deeper and more specific research references and resources from every corner of the scientific community.

Enhanced Piezoelectric Response in Hybrid Lead Halide Perovskite Thin Films via Interfacing with Ferroelectric PbZr0.2Ti0.8O3

We report a more than 10-fold enhancement of the piezoelectric coefficient d₃₃ of polycrystalline CH₃NH₃PbI₃ (MAPbI₃) films when interfacing them with ferroelectric PbZr_0.2Ti_0.8O₃ (PZT). Piezoresponse force microscopy (PFM) studies reveal [Formula: see text] values of 0.3-0.4 pm/V for MAPbI₃ deposited on Au, indium tin oxide, and SrTiO₃ surfaces, with small phase angle fluctuating at length scales smaller than the grain size. In sharp contrast, on samples prepared on epitaxial PZT films, we observe large-scale polar domains exhibiting clear, close to 180° PFM phase contrasts, pointing to polar axes along the film normal. By separating the piezoresponse contributions from the MAPbI₃ and PZT layers, we extract a significantly higher [Formula: see text] of ∼4 pm/V, which is attributed to the enhanced alignment of the MA molecular dipoles promoted by the unbalanced surface potential of PZT. We also discuss the effect of the interfacial screening layer on the preferred polar direction.

A Light Harvesting, Self-Powered Monolith Tactile Sensor Based on Electric Field Induced Effects in MAPbI3 Perovskite

Organolead trihalide perovskite MAPbI₃ shows a distinctive combination of properties such as being ferroelectric and semiconducting, with ion migration effects under poling by electric fields. The combination of its ferroelectric and semiconducting nature is used to make a light harvesting, self-powered tactile sensor. This sensor interfaces ZnO nanosheets as a pressure-sensitive drain on the MAPbI₃ film and once poled is operational for at least 72 h with just light illumination. The sensor is monolithic in structure, has linear response till 76 kPa, and is able to operate continuously as the energy harvesting mechanism is decoupled from its pressure sensing mechanism. It has a sensitivity of 0.57 kPa^-1 , which can be modulated by the strength of the poling field. The understanding of these effects in perovskite materials and their application in power source free devices are of significance to a wide array of fields where these materials are being researched and applied.

Exploring Anomalous Polarization Dynamics in Organometallic Halide Perovskites

Organometallic halide perovskites (OMHPs) have attracted broad attention as prospective materials for optoelectronic applications. Among the many anomalous properties of these materials, of special interest are the ferroelectric properties including both classical and relaxor-like components, as a potential origin of slow dynamics, field enhancement, and anomalous mobilities. Here, ferroelectric properties of the three representative OMHPs are explored, including FAPb_x Sn_1-x I₃ (x = 0, x = 0.85) and FA_0.85 MA_0.15 PbI₃ using band excitation piezoresponse force microscopy and contact mode Kelvin probe force microscopy, providing insight into long- and short-range dipole and charge dynamics in these materials and probing ferroelectric density of states. Furthermore, second-harmonic generation in thin films of OMHPs is observed, providing a direct information on the noncentrosymmetric polarization in such materials. Overall, the data provide strong evidence for the presence of ferroelectric domains in these systems; however, the domain dynamics is suppressed by fast ion dynamics. These materials hence present the limit of ferroelectric materials with spontaneous polarization dynamically screened by ionic and electronic carriers.

High-efficiency and stable piezo-phototronic organic perovskite solar cell

Perovskite materials are regarded as next-generation organic photovoltaic (OPV) materials due to their excellent physical and chemical properties. Recent theoretical and experimental advances also revealed the piezoelectric properties of CH₃NH₃PbI₃ perovskite thin films. In this work, a CH₃NH₃PbI₃ perovskite piezo-phototronic solar cell is studied in theory. The output parameters such as open circuit voltage, current–voltage characteristics, fill factor, power conversion efficiency, and maximum output power with external strains are presented. The coefficient to characterize piezo-phototronic modulation is also calculated for the piezo-phototronic solar cell. With the change of strain, the output performance can be controlled and enhanced. This principle can offer not only a novel and unique approach to produce high-performance, stable perovskite solar cells, but also a principle to design new piezoelectric perovskite optoelectronic devices.

Enhancing the performance of perovskite solar cells with strain based on a piezo-phototronic effect.

Rashba Effect in a Single Colloidal CsPbBr3 Perovskite Nanocrystal Detected by Magneto-Optical Measurements

This study depicts the influence of the Rashba effect on the band-edge exciton processes in all-inorganic CsPbBr₃ perovskite single colloidal nanocrystal (NC). The study is based on magneto-optical measurements carried out at cryogenic temperatures under various magnetic field strengths in which discrete excitonic transitions were detected by linearly and circularly polarized measurements. Interestingly, the experiments show a nonlinear energy splitting between polarized transitions versus magnetic field strength, indicating a crossover between a Rashba effect (at the lowest fields) to a Zeeman effect at fields above 4 T. We postulate that the Rashba effect emanates from a lattice distortion induced by the Cs⁺ motion degree of freedom or due to a surface effect in nanoscale NCs. The unusual magneto-optical properties shown here underscore the importance of the Rashba effect in the implementation of such perovskite materials in various optical and spin-based devices.

A Game Changer: A Multifunctional Perovskite Exhibiting Giant Ferroelectricity and Narrow Bandgap with Potential Application in a Truly Monolithic Multienergy Harvester or Sensor

An ABO₃ -type perovskite solid-solution, (K_0.5 Na_0.5 )NbO₃ (KNN) doped with 2 mol% Ba(Ni_0.5 Nb_0.5 )O_3-δ (BNNO) is reported. Such a composition yields a much narrower bandgap (≈1.6 eV) compared to the parental composition-pure KNN-and other widely used piezoelectric and pyroelectric materials (e.g., Pb(Zr,Ti)O₃ , BaTiO₃ ). Meanwhile, it exhibits the same large piezoelectric coefficient as that of KNN (≈100 pC N^-1 ) and a much larger pyroelectric coefficient (≈130 µC m^-2 K^-1 ) compared to the previously reported narrow-bandgap material (KNbO₃ )_1-x -BNNO_x . The unique combination of these excellent ferroelectric and optical properties opens the door to the development of multisource energy harvesting or multifunctional sensing devices for the simultaneous and efficient conversion of solar, thermal, and kinetic energies into electricity in a single material. Individual and comprehensive characterizations of the optical, ferroelectric, piezoelectric, pyroelectric, and photovoltaic properties are investigated with single and coexisting energy sources. No degrading interaction between ferroelectric and photovoltaic behaviors is observed. This composition may fundamentally change the working principles of state-of-the-art hybrid energy harvesters and sensors, and thus significantly increases the unit-volume energy conversion efficiency and reliability of energy harvesters in ambient environments.

Light-induced picosecond rotational disordering of the inorganic sublattice in hybrid perovskites

Femtosecond resolution electron scattering techniques are applied to resolve the first atomic-scale steps following absorption of a photon in the prototypical hybrid perovskite methylammonium lead iodide. Following above-gap photoexcitation, we directly resolve the transfer of energy from hot carriers to the lattice by recording changes in the mean square atomic displacements on 10-ps time scales. Measurements of the time-dependent pair distribution function show an unexpected broadening of the iodine-iodine correlation function while preserving the Pb-I distance. This indicates the formation of a rotationally disordered halide octahedral structure developing on picosecond time scales. This work shows the important role of light-induced structural deformations within the inorganic sublattice in elucidating the unique optoelectronic functionality exhibited by hybrid perovskites and provides new understanding of hot carrier-lattice interactions, which fundamentally determine solar cell efficiencies.

Anomalous photovoltaic effect in organic-inorganic hybrid perovskite solar cells

Organic-inorganic hybrid perovskites (OIHPs) have been demonstrated to be highly successful photovoltaic materials yielding very-high-efficiency solar cells. We report the room temperature observation of an anomalous photovoltaic (APV) effect in lateral structure OIHP devices manifested by the device's open-circuit voltage (V_OC) that is much larger than the bandgap of OIHPs. The persistent V_OC is proportional to the electrode spacing, resembling that of ferroelectric photovoltaic devices. However, the APV effect in OIHP devices is not caused by ferroelectricity. The APV effect can be explained by the formation of tunneling junctions randomly dispersed in the polycrystalline films, which allows the accumulation of photovoltage at a macroscopic level. The formation of internal tunneling junctions as a result of ion migration is visualized with Kelvin probe force microscopy scanning. This observation points out a new avenue for the formation of large and continuously tunable V_OC without being limited by the materials' bandgap.

Electron-acoustic phonon coupling in single crystal CH3NH3PbI3 perovskites revealed by coherent acoustic phonons

Despite the great amount of attention CH₃NH₃PbI₃ has received for its solar cell application, intrinsic properties of this material are still largely unknown. Mobility of charges is a quintessential property in this aspect; however, there is still no clear understanding of electron transport, as reported values span over three orders of magnitude. Here we develop a method to measure the electron and hole deformation potentials using coherent acoustic phonons generated by femtosecond laser pulses. We apply this method to characterize a CH₃NH₃PbI₃ single crystal. We measure the acoustic phonon properties and characterize electron-acoustic phonon scattering. Then, using the deformation potential theory, we calculate the carrier intrinsic mobility and compare it to the reported experimental and theoretical values. Our results reveal high electron and hole mobilities of 2,800 and 9,400 cm² V⁻¹ s⁻¹, respectively. Comparison with literature values of mobility demonstrates the potential role played by polarons in charge transport in CH₃NH₃PbI₃.

Carrier mobility is a basic semiconductor property. Mante et al., use femtosecond lasers to investigate coherent acoustic phonons and relate their deformation potentials to estimate the intrinsic electron and hole mobilities of CH₃NH₃PbI₃ single crystals to be 2,800 and 9,400 cm² V⁻¹ s⁻¹, respectively.

Piezoelectric scattering limited mobility of hybrid organic-inorganic perovskites CH3NH3PbI3

Carrier mobility is one of the most important parameters for semiconducting materials and their use in optoelectronic devices. Here we report a systematic first principles analysis of the acoustic phonon scattering mechanism that limits the mobility of CH₃NH₃PbI₃ (MAPbI₃) perovskites. Due to the unique hybrid organic-inorganic structure, the mechanical, electronic and transport properties are dominated by the same factor, i.e. the weak interatomic bond and the easy rotation of methylammonium (MA) molecules under strain. Both factors make MAPbI₃ soft. Rotation of MA molecule induces a transverse shift between Pb and I atoms, resulting in a very low deformation potential and a strong piezoelectricity in MAPbI₃. Hence the carrier mobility of pristine MAPbI₃ is limited by the piezoelectric scattering, which is consistent to the form of its temperature dependence. Our calculations suggest that in the pristine limit, a high mobility of about several thousand cm² V⁻¹ S⁻¹ is expected for MAPbI₃.

Tuning the gap of lead-based halide perovskites by introducing superalkali species at the cationic sites of ABX3-type structure

New perovskites are formed by introducing polyatomic yet aprotic superalkali species at the cationic A-sites of the ABX₃ structure.