Tetragonal CH3NH3PbI3 Research Articles

Facile synthesis of highly stable and efficient MAPbI3 perovskite quantum dots: Spectroscopic characterization and defect analysis

The hybrid methylammonium-based perovskite quantum dots (PQDs) have attracted tremendous attraction for display devices due to their low cost, high luminescence, and color purity. The large-scale production of PQDs is constrained due to the use of toxic and hazardous solvents such as dimethylformamide (DMF) in the synthesis process. Here, a novel approach to prepare PQDs using an environment-friendly solvent, gamma-valerolactone (GVL), is proposed. The synthesized PQDs were found to be uniform with less defect states as compared to the conventionally used DMF solvents, confirmed by different characterizing techniques. The stability of GVL-based PQDs was also found to be improved and proved the efficiency of the present method. Photoluminescence quantum yield (PLQY) of 98 % and single tetragonal CH3NH3PbI3 phase of GVL-derived sample retained over 15 days while the decomposition and degradation were observed in the DMF-derived sample within a short duration. A protocol for the preparation of CH3NH3PbI3 PQDs with less intrinsic defects and uniform optoelectronic properties is presented and paves the way for further modifications and applications.

Nuclear Quantum Effects Prolong Charge Carrier Lifetimes in Hybrid Organic-Inorganic Perovskites.

Hybrid organic-inorganic perovskites (HOIPs) contain light hydrogen atoms that exhibit significant nuclear quantum effects (NQEs). We demonstrate that NQEs have a strong effect on HOIP geometry and electron-vibrational dynamics at both low and ambient temperatures, even though charges in HOIPs reside on heavy elements. By combining ring-polymer molecular dynamics (MD) and ab initio MD with nonadiabatic MD and time-dependent density functional theory and focusing on the most studied tetragonal CH3NH3PbI3, we show that NQEs increase the disorder and thermal fluctuations through coupling of the light inorganic cations to the heavy inorganic lattice. The additional disorder induces charge localization and decreases electron-hole interactions. As a result, the nonradiative carrier lifetimes are extended by a factor of 3 at 160 K and 1/3 at 330 K. The radiative lifetimes are increased by 40% at both temperatures. The fundamental band gap decreases by 0.10 and 0.03 eV at 160 and 330 K, respectively. By enhancing atomic motions and introducing new vibrational modes, NQEs strengthen electron-vibrational interactions. Decoherence, determined by elastic scattering, accelerates almost by a factor of 2 due to NQEs. However, the nonadiabatic coupling, driving nonradiative electron-hole recombination, decreases because it is more sensitive to structural distortions than atomic motions in HOIPs. This study demonstrates, for the first time, that NQEs should be considered to achieve an accurate understanding of geometry evolution and charge carrier dynamics in HOIPs and provides important fundamental insights for the design of HOIPs and related materials for optoelectronic applications.

Optoelectronic functionality and photovoltaic performance of Sr-doped tetragonal CH3NH3PbI3: A first-principles study

The electronic, optical and transport properties, and photovoltaic performances of tetragonal CH3NH3PbI3 with various Sr substitutions for Pb (3.12%, 6.25%, 12.5% and 25%) have been systematically investigated using the first principles method. It is discovered that the 25% Sr content not only reduces the band gap by over 10% to harvest the near-infrared phonons, but also substantially promotes the charge transport through lowering the carrier effective mass and enhancing the carrier mobility. Simultaneously, CH3NH3Pb0·75Sr0·25I3 is conservatively estimated to have a power conversion efficiency of about 19%, showing the improved photovoltaic performance. Further, the phonon-dispersion and formation-energy calculations reveal that CH3NH3Pb0·75Sr0·25I3 is of high dynamic and structural stabilities. The overall improvements in optoelectronics and photovoltaics make CH3NH3Pb0·75Sr0·25I3 promising as an effective optical absorber of solar cells. In contrast, the influences of lower Sr contents are slight. This work provides a theoretical perspective in optimizing the optoelectronic and photovoltaic applications of CH3NH3PbI3.

A GGA + vdW study on electronic properties and optoelectronic functionality of Cd-doped tetragonal CH3NH3PbI3 for photovoltaics

The electronic properties and optoelectronic functionalities of tetragonal CH 3 NH 3 PbI 3 doped with different concentrations (6.25%, 12.5% and 25%) of Cd have been investigated using the the generalized gradient approximation with the correction of van der Waals interactions (GGA + vdW). First, we examine the dependence of band gap on the Cd doping concentration, and find that the increased Cd content narrows the band gap up to 1.41 eV, extending the photoabsorption wavelength of CH 3 NH 3 PbI 3 into the range of near-infrared spectrum. Furthermore, the electron mobility is found to be enhanced by 28%, which is beneficial for separating and collecting the photo-excited carriers. The effects of band-gap reduction and electron-mobility enhancement jointly contribute to the promotion in optoelectronics of CH 3 NH 3 PbI 3 . The detailed analyses on the thermodynamic stabilities of doped structures have been made through examining their phonon dispersions and decomposition energies. This work has implications in offering guidance for broadening the photovoltaic applications of CH 3 NH 3 PbI 3 .

Effect of chlorobenzene on the optical and structural properties of CH3NH3PbI3:DMF perovskite films

Methylammonium lead iodide perovskites (MAPbI3) thin films were prepared by changing the molarity concentration of methylammonium iodide and lead iodide in the N,N- dimethylformamide solvent, with and without chlorobenzene (CB). Rietveld refinement analysis revealed that all prepared samples belong to tetragonal CH3NH3PbI3 phase (sp. gr. I4/mcm). The effect of MAPbI3 concentration on the lattice parameters, crystallite size, lattice microstrain and dislocation density of the prepared films with and without CB was investigated in detail. The FESEM images of perovskite films indicated the presence of pinholes in each sample, and the number of pinholes reduced as CB added to MAPbI3. Generally, the transmission and reflection spectra decreased as the amount of MAPbI3 increased in the films; also, both are reduced as CB added to films. The value of the reflectivity over measured wavelengths changed from about 1% to about 18% and 12% in the case of films without and with CB, respectively. Without CB, the highest absorbance intensity was obtained for 1.3 M films, while with CB the 1.5 M film has the highest absorbance. The number of bandgaps appeared in different samples depending on the concentration of MAPbI3 and CB. The refractive index at 500 nm for (1, 1.3 and 1.5 M) MAPbI3 films without CB are 1.6, 1.44, 1.43 and 1.31 respectively, where its value in the corresponding films with CB are 1.34, 1.3, 1.3 and 1.28, respectively. The effect of MAPbI3 concentration and the existence of CB during the film preparation on the extinction coefficient, refractive index, dielectric constant, the photoluminescence peak position, and PL intensity for the different films were explored.

General Decomposition Pathway of Organic-Inorganic Hybrid Perovskites through an Intermediate Superstructure and its Suppression Mechanism.

Organic-inorganic hybrid perovskites (OIHPs) have generated considerable excitement due to their promising photovoltaic performance. However, the commercialization of perovskite solar cells (PSCs) is still plagued by the structural degradation of the OIHPs. Here, the decomposition mechanism of OIHPs under electron beam irradiation is investigated via transmission electron microscopy, and a general decomposition pathway for both tetragonal CH3 NH3 PbI3 and cubic CH3 NH3 PbBr3 is uncovered through an intermediate superstructure state of CH3 NH3 PbX2.5 , X = I, Br, with ordered vacancies into final lead halides. Such decomposition can be suppressed via carbon coating by stabilization of the perovskite structure framework. These findings reveal the general degradation pathway of OIHPs and suggest an effective strategy to suppress it, and the atomistic insight learnt may be useful for improving the stability of PSCs.

CH3NH3PbI3 thin films prepared by hot-casting technique in the air: growth mechanism, trap states and relating solar cells

Tetragonal CH3NH3PbI3 perovskite thin films with large crystallite sizes were successfully fabricated under atmospheric air using a one-step hot-casting technique. The casting temperature governed structural and optical properties of the prepared films. The energy gaps of the hot-casted films changed with changing casting temperature due to the variation of Urbach energy. The hot-casted perovskite thin films had superior structural stability to that of the two-step method films. However, the hot-casted perovskite films contained trap states as suggested by additional emissions other than bimolecular recombination in photoluminescence spectra. The origins of these trap states were believed to be attributed to the presence of iodine vacancies (VI), iodine interstitial sites (Ii) and methylammonium ion vacancies (VMA) in the prepared films. The fabricated perovskite solar cells showed that at low casting temperatures the power conversion efficiencies were relatively higher than the higher ones, this was attributed to their lower non-radiative recombination activities.

Experimental Phonon Dispersion and Lifetimes of Tetragonal CH3NH3PbI3 Perovskite Crystals.

Hybrid organic-inorganic perovskites were reported to have ultralow thermal conductivity in recent studies. In this Letter, we report the first experimental phonon dispersion and lifetimes of tetragonal CH3NH3PbI3 single crystals at both 200 and 300 K by high-energy resolution inelastic X-ray scattering, which enables a thorough understanding of the underlying mechanisms for the ultralow thermal conductivity. Notably, we observed unusual and significant phonon dips along the [100] and [110] directions at both temperatures. The ultralow thermal conductivity can be attributed to small group velocities due to ultrasoft acoustic modes and short phonon lifetimes originating from the strong acoustic-optical coupling. We further provided the structural origins for these peculiar phonon features. Moreover, our results and interpretation are consistent with the reported temperature-dependent trend for thermal conductivity of CH3NH3PbI3. Our work offers critical guidelines for accelerating the design and discovery of novel hybrid materials for energy applications including photovoltaics and thermoelectrics.

Structural and Chemical Changes to CH3 NH3 PbI3 Induced by Electron and Gallium Ion Beams.

Organic-inorganic hybrid perovskites, such as CH3 NH3 PbI3, have shown highly promising photovoltaic performance. Electron microscopy (EM) is a powerful tool for studying the crystallography, morphology, interfaces, lattice defects, composition, and charge carrier collection and recombination properties at the nanoscale. Here, the sensitivity of CH3 NH3 PbI3 to electron beam irradiation is examined. CH3 NH3 PbI3 undergoes continuous structural and compositional changes with increasing electron dose, with the total dose, rather than dose rate, being the key operative parameter. Importantly, the first structural change is subtle and easily missed and occurs after an electron dose significantly smaller than that typically applied in conventional EM techniques. The electron dose conditions under which these structural changes occur are identified. With appropriate dose-minimization techniques, electron diffraction patterns can be obtained from pristine material consistent with the tetragonal CH3 NH3 PbI3 phases determined by X-ray diffraction. Radiation damage incurred at liquid nitrogen temperatures and using Ga+ irradiation in a focused ion beam instrument are also examined. Finally, some simple guidelines for how to minimize electron-beam-induced artifacts when using EM to study hybrid perovskite materials are provided.

Growth of Compact CH3NH3PbI3 Thin Films Governed by the Crystallization in PbI2 Matrix for Efficient Planar Perovskite Solar Cells.

As a convenient preparation technique, a two-step method, which is normally done by spin-coating CH3NH3I onto PbI2 film followed by a thermal annealing, is generally used to prepare solution-processed CH3NH3PbI3 films for planar perovskite solar cells. Here, we prepare the compact CH3NH3PbI3 thin films by the two-step method at a low temperature (<80 °C) and investigate the effects of PbI2 crystallization on the structure-property correlation in the CH3NH3PbI3 films. It is found that the importance of the crystallization in PbI2 matrix lies in governing the transition from the (001) plane of trigonal PbI2 to the (002) plane of tetragonal CH3NH3PbI3 in the rapid reaction process for atoms to coordinate into perovskite during spin-coating, which actually determines the morphology and the type of vacancy defects in resulting perovskite; a better crystallized PbI2 film has a much stronger ability to react with CH3NH3I solution and produces larger CH3NH3PbI3 grains with a higher crystallinity. The CH3NH3PbI3/TiO2 planar solar cell derived from a better crystallized PbI2 film exhibits significantly improved performance and stability as the result of the higher crystallinity inside the perovskite film. Moreover, it is demonstrated that the crystalline PbI2 film matrix subjected to the annealing after a slow heating process prior to contacting CH3NH3I solution is more effective for CH3NH3PbI3 formation than that with a direct annealing history. The results in this paper provide a guide for preparing high-quality CH3NH3PbI3 thin films for efficient perovskite solar cells and CH3NH3PbI3 interfacial films over the layers susceptible to temperature.

First-Principles Study of Electron Injection and Defects at the TiO2/CH3NH3PbI3 Interface of Perovskite Solar Cells.

We investigated electron injection rates and vacancy defect properties by performing first-principles calculations on the interface of an anatase-TiO2(001) and a tetragonal CH3NH3PbI3(110) (MAPbI3(110)). We found that the coupling matrix element between the lowest unoccupied molecular orbital of MAPbI3 and the TiO2 conduction band (CB) minimum is negligibly small, the indication being that electron-injection times for low-energy excited states are quite long (more than several tens of picoseconds). We also found that higher-lying CB states coupled more strongly; injection was expected to take place on a femtosecond time scale. Furthermore, we found that vacancy defects in the TiO2 layer produced undesired defect levels that caused hole traps and recombination centers. Whereas most of the vacancy defects in the MAPbI3 layer produced no additional states in the MAPbI3 gap, a Pb vacancy (VPb) at the interface created an energy level below the MAPbI3 CB edge and had a lower energy of formation than the VPb defect in bulk because of the interaction with the TiO2 surface.

Tetragonal CH3NH3PbI3 is ferroelectric

Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI3). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI3 (unlike MAPbBr3) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material's relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity's hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI3, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material's noncentrosymmetry. We note that the material's ferroelectric nature, can, but need not be important in a PV cell at room temperature.

Direct observation of intrinsic twin domains in tetragonal CH3NH3PbI3

Organic–inorganic hybrid perovskites are exciting candidates for next-generation solar cells, with CH3NH3PbI3 being one of the most widely studied. While there have been intense efforts to fabricate and optimize photovoltaic devices using CH3NH3PbI3, critical questions remain regarding the crystal structure that governs its unique properties of the hybrid perovskite material. Here we report unambiguous evidence for crystallographic twin domains in tetragonal CH3NH3PbI3, observed using low-dose transmission electron microscopy and selected area electron diffraction. The domains are around 100–300 nm wide, which disappear/reappear above/below the tetragonal-to-cubic phase transition temperature (approximate 57 °C) in a reversible process that often ‘memorizes’ the scale and orientation of the domains. Since these domains exist within the operational temperature range of solar cells, and have dimensions comparable to the thickness of typical CH3NH3PbI3 films in the solar cells, understanding the twin geometry and orientation is essential for further improving perovskite solar cells.

Charge Carrier Trapping at Surface Defects of Perovskite Solar Cell Absorbers: A First-Principles Study.

The trapping of charge carriers at defects on surfaces or grain boundaries is detrimental for the performance of perovskite solar cells (PSCs). For example, it is the main limiting factor for carrier lifetime. Moreover, it causes hysteresis in the current-voltage curves, which is considered to be a serious issue for PSCs' operation. In this work, types of surface defects responsible for carrier trapping are clarified by a comprehensive first-principles investigation into surface defects of tetragonal CH3NH3PbI3 (MAPbI3). Considering defect formation energetics, it is proposed that a Pb-rich condition is preferred to an I-rich one; however, a moderate condition might possibly be the best choice. Our result paves the way for improving the performance of PSCs through a rational strategy of suppressing carrier trapping at surface defects.

Doping optimization of organic-inorganic hybrid perovskite CH3NH3PbI3 for high thermoelectric efficiency

The newly discovered photovoltaic hybrid perovskite materials have been suggested for thermoelectric applications as they possess very low thermal conductivity and large Seebeck coefficient. However, to achieve a high figure of merit, chemical doping is necessary to increase the electrical conductivity. In the present work, we examined the thermoelectric figure of merit for CH3NH3PbI3 as a function of carrier concentration based on first-principles calculations. For doped semiconductors, the impurity scattering usually plays a dominant role in the charge transport. Both impurity scattering and acoustic phonon scattering have been incorporated in our calculations. We showed that at the impurity concentration of 1018cm−3, the room temperature zT value of tetragonal CH3NH3PbI3 could be optimized to reach unity at the carrier concentration on the same order of magnitude as 1018cm−3. The hole-doped CH3NH3PbI3 exhibits superior thermoelectric property than the electron-doped one, and we propose to engineer the vacancies of organic cations for the enhanced hole concentration and thermoelectric efficiency.

Synergistic Effects of Water and Oxygen Molecule Co-adsorption on (001) Surfaces of Tetragonal CH3NH3PbI3: A First-Principles Study

The poor environmental stability of organometallic halide perovskite solar cells presents a big challenge for its commercialization, which is mainly due to the degradation of perovskite materials in humid air. The role played by water molecules has been extensively studied in the degradation processes, where strong interactions between water molecules and perovskite surfaces are found. Using first-principles simulations, we find that oxygen molecules also have strong interactions with (001) surfaces of tetragonal CH3NH3PbI3 through the formation of a chemical Pb–O bond on the PbI2-terminated surface and a hydrogen bond on the CH3NH3I-terminated surface. The adsorbed oxygen molecules introduce empty states near the Fermi level of the surfaces, which can facilitate charge transfer between the surface and oxygen molecules. Furthermore, when an oxygen molecule is located atop a Pb atom on PbI2-terminated surface, the calculated adsorption energies indicate that the surface is more attractive to water molecule...

Lattice thermal conductivity of organic-inorganic hybrid perovskite CH3NH3PbI3

Great success has been achieved in improving the photovoltaic energy conversion efficiency of the organic-inorganic hybrid perovskite-based solar cells, but with very limited knowledge on the thermal transport in hybrid perovskites, which could affect the device lifetime and stability. Based on the potential field derived from the density functional theory calculations, we studied the lattice thermal conductivity of the hybrid halide perovskite CH3NH3PbI3 using equilibrium molecular dynamics simulations. Temperature-dependent thermal conductivity is reported from 160 K to 400 K, which covers the tetragonal phase (160–330 K) and the pseudocubic phase (&amp;gt;330 K). A very low thermal conductivity (0.59 W/m K) is found in the tetragonal phase at room temperature, whereas a much higher thermal conductivity is found in the pseudocubic phase (1.80 W/m K at 330 K). The low group velocity of acoustic phonons and the strong anharmonicity are found responsible for the relatively low thermal conductivity of the tetragonal CH3NH3PbI3.

The nature of hydrogen-bonding interaction in the prototypic hybrid halide perovskite, tetragonal CH3NH3PbI3.

In spite of the key role of hydrogen bonding in the structural stabilization of the prototypic hybrid halide perovskite, CH3NH3PbI3 (MAPbI3), little progress has been made in our in-depth understanding of the hydrogen-bonding interaction between the MA+-ion and the iodide ions in the PbI6-octahedron network. Herein, we show that there exist two distinct types of the hydrogen-bonding interaction, naming α- and β-modes, in the tetragonal MAPbI3 on the basis of symmetry argument and density-functional theory calculations. The computed Kohn-Sham (K-S) energy difference between these two interaction modes is 45.14 meV per MA-site with the α-interaction mode being responsible for the stable hydrogen-bonding network. The computed bandgap (Eg) is also affected by the hydrogen-bonding mode, with Eg of the α-interaction mode (1.73 eV) being significantly narrower than that of the β-interaction mode (2.03 eV). We have further estimated the individual bonding strength for the ten relevant hydrogen bonds having a bond critical point.

Ab Initio Study of Interaction of Water, Hydroxyl Radicals, and Hydroxide Ions with CH3NH3PbI3 and CH3NH3PbBr3 Surfaces

Although there have been tremendous breakthroughs in perovskite solar cells over the past few years, degradation of perovskite has been a huge problem. Recently, a number of experimental studies have demonstrated that organic–inorganic halide perovskite materials are sensitive to humid air, and several degradation mechanisms have been proposed. However, the decomposition process of perovskites is only partially known and controversial. In this paper, we theoretically study the structures of the tetragonal CH3NH3PbI3 and CH3NH3PbBr3 (110) surfaces and the degradation mechanism using density functional theory calculations both with and without the van der Waals correction. The computed results indicate that the CH3NH3+ (MA) cations preferentially orient with the NH3 group pointing into the surface. This allows the formation of more hydrogen···halide hydrogen bonds between the MA cations and the halides. Moreover, the interactions of water molecules, hydroxyl radicals, and hydroxide ions with the perovskite ...

First-Principles Study of Ion Diffusion in Perovskite Solar Cell Sensitizers.

Hysteresis in current-voltage curves has been an important issue for conversion efficiency evaluation and development of perovskite solar cells (PSCs). In this study, we explored the ion diffusion effects in tetragonal CH3NH3PbI3 (MAPbI3) and trigonal (NH2)2CHPbI3 (FAPbI3) by first-principles calculations. The calculated activation energies of the anionic and cationic vacancy migrations clearly show that I(-) anions in both MAPbI3 and FAPbI3 can easily diffuse with low barriers of ca. 0.45 eV, comparable to that observed in ion-conducting materials. More interestingly, typical MA(+) cations and larger FA(+) cations both have rather low barriers as well, indicating that the cation molecules can migrate in the perovskite sensitizers when a bias voltage is applied. These results can explain the ion displacement scenario recently proposed by experiments. With the dilute diffusion theory, we discuss that smaller vacancy concentrations (higher crystallinity) and replacement of MA(+) with larger cation molecules will be essential for suppressing hysteresis as well as preventing aging behavior of PSC photosensitizers.