Methylammonium Lead Triiodide Perovskite Research Articles

Two-step preparation of methylammonium lead triiodide perovskite film via electrospray deposition of methylammonium iodide solution for solar cell applications

Solution-processable perovskite solar cells (PSCs) have the potential to revolutionize solar cell technology by enabling low power generation costs via low-cost device fabrication. However, most existing research regarding PSCs relies on the spin-coating method, which is not conducive to large-area film deposition. Therefore, the development of an alternative deposition method for perovskite films has become increasingly important for commercialization, for which electrospray deposition is a promising technique. This study investigates the two-step preparation of methylammonium lead triiodide (MAPbI3) perovskite films via the electrospray deposition of a methylammonium iodide (MAI) solution on a spin-coated PbI2 film. The gradual conversion of PbI2 to MAPbI3 with increasing MAI deposition time was revealed, accompanied by an increase in the size of the perovskite crystals. In addition, PSCs were successfully fabricated by electrospraying MAI, achieving a considerable power conversion efficiency of 7.86 % at the optimal MAI deposition time.

Spin–Orbit Coupling Notably Retards Non-radiative Electron–Hole Recombination in Methylammonium Lead Triiodide Perovskites

The giant spin-orbit coupling (SOC) of a heavy lead element significantly extends charge carrier lifetimes of lead halide perovskites (LHPs). The physical mechanism remains unclear and requires a quantum dynamics perspective. Taking methylammonium lead iodide (MAPbI3) as a prototypical system and using non-adiabatic molecular dynamics combined with 1/2 electron correction, we show that SOC notably reduces the non-radiative electron-hole (e-h) recombination by decreasing the non-adiabatic coupling (NAC) primarily as a result of SOC decreasing the e-h wave function overlap by reshaping the electron and hole wave functions. Second, SOC causes spin mismatch subject to spin-mixed states, which further decreases NAC. The charge carrier lifetime is about 3-fold longer in the present of SOC relative to the absence of SOC. Our study generates the fundamental understanding of SOC minimizing non-radiative charge and energy losses in LHPs.

Bright future by enhancing the stability of methylammonium lead triiodide perovskites thin films through Rb, Cs and Li as dopants

This paper deals with the doping of methylammonium lead iodide (MAPbI3) with small amounts of rubidium (Rb), lithium (Li) and caesium (Cs) to improve the crystallisation of MAPbI3 films. Doping significantly increases the peak the (110) diffraction peak, which is related to enhanced crystallisation. The peak located at 148 cm−1 dominates all the Raman spectra in the doped MAPbI3, which is characterised by a central vibrational band corresponding to the symmetric vibration of the anion sublattice. The microstructures of these improved films are characterised by large grain sizes and a uniform surface morphology with no pinholes. Consequently, these films are suitable for high-performance optoelectronic devices. The influence of Li, Rb, and Cs doping on the photophysical properties of MAPbI3 perovskite films is investigated by a photoluminescence analysis. Doping with 5% Li cations increases photoluminescence emissions due to the reduction in the density of trap states. These findings provide fundamental insights into the crystallisation process of MAPbI3 and offer new approaches for developing stable and high-performance devices based on MAPbI3.

Remarkable performance recovery in highly defective perovskite solar cells by photo-oxidation.

Exposure to environmental factors is generally expected to cause degradation in perovskite films and solar cells. Herein, we show that films with certain defect profiles can display the opposite effect, healing upon exposure to oxygen under illumination. We tune the iodine content of methylammonium lead triiodide perovskite from understoichiometric to overstoichiometric and expose them to oxygen and light prior to the addition of the top layers of the device, thereby examining the defect dependence of their photooxidative response in the absence of storage-related chemical processes. The contrast between the photovoltaic properties of the cells with different defects is stark. Understoichiometric samples indeed degrade, demonstrating performance at 33% of their untreated counterparts, while stoichiometric samples maintain their performance levels. Surprisingly, overstoichiometric samples, which show low current density and strong reverse hysteresis when untreated, heal to maximum performance levels (the same as untreated, stoichiometric samples) upon the photooxidative treatment. A similar, albeit smaller-scale, effect is observed for triple cation and methylammonium-free compositions, demonstrating the general application of this treatment to state-of-the-art compositions. We examine the reasons behind this response by a suite of characterization techniques, finding that the performance changes coincide with microstructural decay at the crystal surface, reorientation of the bulk crystal structure for the understoichiometric cells, and a decrease in the iodine-to-lead ratio of all films. These results indicate that defect engineering is a powerful tool to manipulate the stability of perovskite solar cells.

The Effect of Short Chain Carboxylic Acids as Additives on the Crystallization of Methylammonium Lead Triiodide (MAPI)

Due to their exceptional properties, the study of hybrid perovskite (HyP) structures and applications dominate current photovoltaic prospects. Methylammonium lead tri-iodide perovskite (MAPI) is the model compound of the HyP class of materials that, in a few years, achieved, in photovoltaics, a power conversion efficiency of 25%. The attention on HyP has recently moved to large single crystals as emerging candidates for photovoltaic application because of their improved stability and optoelectronic properties compared to polycrystalline films. To control the quality and symmetry of the large MAPI single crystals, we proposed an original method that consisted of adding short-chain carboxylic acids to the inverse temperature crystallization (ICT) of MAPI in γ-butyrolactone (GBL). The crystals were characterized by single-crystal X-ray diffraction (SC-XRD), X-ray powder diffraction (XRPD) and Raman spectroscopy. Based on SC-XRD analysis, MAPI crystals grown using acetic and trifluoroacetic acids adopt a tetragonal symmetry “I4cm”. MAPI grown in the presence of formic acid turned out to crystallize in the orthorhombic “Fmmm” space group demonstrating the acid’s effect on the crystallization of MAPI.

Iodide and charge migration at defective surfaces of methylammonium lead triiodide perovskites: The role of hydrogen bonding

Methylammonium lead triiodide perovskite (CH3NH3PbI3) solar cells have exhibited impressive potential in photovoltaic applications, encouraging further work to improve operating stability. Ionic migration has been proposed to be a possible cause of the current–voltage hysteresis which is responsible for the instability of the CH3NH3PbI3 solar cells. Motivated by the variation of the surface geometries of the CH3NH3PbI3 material when going from the pristine to defective surfaces with the iodine vacancies, we adopt the density functional theory method to study the iodide migration along different paths at the defective surfaces followed by employing the time-dependent density functional theory method to calculate the light-induced electronic states along the paths. We find that the iodide movement is largely tuned by the reorientation and rotation of the organic cations with the disruption and formation of the hydrogen bonding during the iodide migration. Besides, the migration of the light-induced charge carriers varies along the paths with different orientations of the organic cations. We attribute the complicated dynamics of the mobile iodide ions and the associated trap states to the hydrogen bonding at the surfaces, providing crucial guidance for improving the stability and charge carrier lifetime in the CH3NH3PbI3 and other metal halide perovskites.

Core-twisted tetrachloroperylenediimide additives improve the crystallinity of perovskites to provide efficient perovskite solar cells

Defects at the grain boundaries and surfaces of metal-halide perovskite films function as non-radiative recombination centers that decrease the performance and/or stability of perovskite solar cells (PSCs). The incorporation of additives into perovskite films can improve their quality and, thereby, provide high-performance PSCs. In this study, we introduced core-twisted tetrachloroperylene diimide (ClPDI) derivatives (n-type small molecules) into the lead iodide precursor solution and used a two-step deposition method to prepare methylammonium lead triiodide (MAPbI3) perovskite films. We examined the effects of the terminal functional groups of the ClPDI derivatives—n-butyl (ClPDI-C4), dimethylaminopropyl (ClPDI-C3DMeA), and aminopropyl (ClPDI-DPA) — on their ability to control the crystallization rate and modulate the film morphology. The power conversion efficiency of the PSC prepared without an additive (15.88%) improved to 16.88 and 18.77% after incorporation of ClPDI-C4 and ClPDI-DPA, respectively, because the strong interactions between these additives and the perovskite passivated defects, slowed the rate of perovskite crystal growth, and increased the grain size. Thus, the use of ClPDI additives in perovskite films can effectively enhance both the efficiency and stability of PSCs.

Recharging upconversion: revealing rubrene's replacement.

One of the major limitations of solid-state perovskite-sensitized photon upconversion to date is that the only annihilator successfully paired with the perovskite sensitizer has been rubrene, raising the question of whether this appraoch of triplet sensitization is universal or limited in scope. Additionally, the inherent energetic mismatch between the perovskite bandgap and the rubrene triplet energy has restricted the apparent anti-Stokes shift achievable in the upconversion process. To increase the apparent anti-Stokes shift for upconversion processes, anthracene derivates are of particular interest due to their higher triplet energies. Here, we demonstrate successful sensitization of the triplet state of 1-chloro-9,10-bis(phenylethynyl)anthracene using the established formamidinium methylammonium lead triiodide perovskite FA0.85MA0.15PbI3, resulting in upconverted emission at 550 nm under 780 nm excitation. We draw a direct comparison to rubrene to unravel the underlying differences in the upconversion processes.

The role of solvents in the formation of methylammonium lead triiodide perovskite

Metal halide perovskites (MHPs) are gaining increasing attention as low-cost, high-performance semiconductors for optoelectronics. In particular, their solution processing is compatible with the large-scale manufacturing of thin-film devices, including solar cells and light-emitting diodes. Understanding the coordination chemistry in precursor-solvent solution and atomistic mechanisms of film formation is of great importance for optimizing the optoelectronic properties of the final films. Using the methylammonium lead triiodide (MAPbI3) as an example, we study the complex evolution of the molecular species from the solution to the initial stage of the crystallization by using a combination of density functional theory (DFT) calculations and ab-initio molecular dynamics (AIMD) simulations. We focus on the widely employed solvents DMSO and DMF, analyze the structures and energies of the iodoplumbate complexes in the form of simple complex of [PbImLn]2−m and polymeric iodoplumbates of ([PbImLn]2−m)x. Based on the calculated formation enthalpies, we propose reaction schemes of MAPbI3 formation in DMSO, DMF and DMF-DMSO binary solvent and explain the advantages of the binary solvent. We highlight the important role of NH…O hydrogen bonds in the formation of iodoplumbates monomers. Our calculations indicate unbalanced reaction energies at several elementary reaction steps in either DMF (formation of [PbI4Ln]2− being highly favourable) or DMSO (formation of [PbI5Ln]3− being retarded). Mixing a small amount of DMSO in DMF gives rise to a better balance in the energies and, therefore, potentially better equilibria in the overall crystallization process and better quality of the final perovskite films.

Performance analysis of carbon-based perovskite solar cells by graphene oxide as hole transport layer: Experimental and numerical simulation

This study compares the experimental photovoltaic performance of methylammonium lead triiodide perovskite solar cell (PSC) containing graphene oxide (GO) and its numerical modelling using Solar Cell Capacitance Simulator-One Dimensional (SCAPS-1D) simulation software. The simulated data from the SCAPS-1D and the experimental results are subsequently compared and then used as a basis to further optimize the solar cell performance. The effect of active layer thickness, hole transport layer (HTL) thickness, the interface defect density, and parasitic resistances (RS and RSH) was specifically investigated to understand what factors should be optimized to boost the device performance. An optimum PCE of 16.51% was achieved using SCAPS-1D simulation. Our work could be used to predict the crucial parameters in optimizing PSCs with GO as HTL. Additionally, the simulation work also provides useful information in selecting material parameters for developing highly efficient low-cost photovoltaic devices in the near future.

Cl-doped MAPbI3 Perovskite Single Crystal Inducing a High Thermoelectric Effect

Recently, the methylammonium lead triiodide perovskite (MAPbI3) has shown great development potential in the solar cell field, and its efficiency as a perovskite solar cell (PCE) has reached 25.2%. However, poor stability is still a problem that restricts its wide application. Compared with the polycrystalline film material of MAPbI3 perovskite, the single crystal MAPbI3 material is much more stable. In addition, it possesses the advantages of ultra-low thermal conductivity and easily regulated electrical conductivity, so this kind of single crystal material has also a broad application prospect in thermoelectric fields. In this letter, we show that the chlorine (Cl) ion-doped MAPbI3 perovskite single crystal was an effective method, which greatly improved the carrier concentration and electrical conductivity of the MAPbI3 single crystal. When the dopant proportion of I:Cl was 100:1, we obtained the highest power factor (PF) of 2.526 µW/m K2 at 30 °C for MAPbI3 single crystal. These results provide an experimental basis for the transformation and application of this kind of perovskite single crystal materials in high efficiency thermoelectric fields.

Poling effect on the electrostrictive and piezoelectric response in CH3NH3PbI3 single crystals

Methylammonium lead triiodide perovskites (MAPbI3) have demonstrated a large electrostrictive strain up to 1%, which was theoretically proved to be related to the additional defects under applied bias voltages. However, as this effect was detected only at a low frequency, we explored the electrostrictive response at a higher frequency (4 kHz) by using ac poling at 1 Hz. In such conditions, the electrostrictive coefficient was evaluated at –45.7 nm2 V−2, i.e., quite larger than the results under dc poling or in unpoled samples. The occurrence of a high-frequency electrostrictive effect may be correlated with the defects that rise in the ac poled samples from the ion migration under reverse bias as it was also demonstrated from the shift of the dielectric loss peak in the considered systems. In parallel to the electrostrictive effect, the piezoelectric response shows a drastic increase after the ac and dc poling due to the lattice distortion induced by the applied electric field. Based on the above approach, the achieved electrostrictive and piezoelectric responses at 4 kHz in MAPbI3 may open opportunities in the area of actuators, energy-harvesting, and micro-electromechanical systems.

Elucidating the Role of Ion Migration and Band Bending in Perovskite Solar Cell Function at Grain Boundaries via Multimodal Nanoscale Mapping

AbstractAlthough hybrid organic–inorganic halide perovskite solar cells have achieved extraordinary improvements over the past few years, questions remain about the role of grain boundaries. This article reports on the nanoscale point‐by‐point current–voltage mapping of photovoltaic characteristics in inverted methylammonium lead triiodide perovskite solar cells. These measurements reveal an increased open‐circuit voltage and shunt resistance, along with a suppressed short‐circuit photocurrent at grain boundaries and nearby regions. Support from nanoscale ionic strain and surface potential mapping suggests that local ion accumulation and downward band bending can facilitate charge separation, but hinder charge collection at grain boundaries.

Use of Sodium Diethyldithiocarbamate to Enhance the Open‐Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells

The incorporation of additives to modulate the properties of metal halide perovskite thin films has become a successful approach in improving the power conversion efficiency of perovskite‐based solar cells. Herein, the beneficial use of sodium diethyldithiocarbamate (NaDEDTC) as processing agent in improving the open‐circuit voltage of methylammonium lead triiodide perovskite solar cells is reported. DEDTC reduces the rate of perovskite crystallization. Absorption and emission spectra show that the optical bandgap of the perovskite films remain essentially unchanged and X‐ray diffraction reveals the formation of preferentially oriented crystallites independent of the use of DEDTC. The use of DEDTC, however, results in a decrease in nonradiative decay as inferred from a two order of magnitude increase in electroluminescence efficiency, explaining the increased open‐circuit voltage. Fourier‐transform infrared spectroscopy, nuclear magnetic resonance, and X‐ray photoelectron spectroscopy show that the DEDTC ligand is not present after the film processing. Therefore, DEDTC modulates film formation but is not incorporated in the perovskite or present as a surface ligand. Sodium ions, on the contrary, are incorporated in the perovskite layer.

Influence of lead (II) chloride and/or lead (II) bromide entrance on the efficiency and stability of methyl ammonium lead triiodide perovskite solar cell: Comparative study of the halide composition and substitution percentage

In this research, various amount of PbBr2 and PbCl2 in some parts of PbI2 in the methyl ammonium lead triiodide (MAPbI3) perovskite composition, named [MAI (PbI2)1–Y (PbX2) Y; X: Br, Cl, and Y: 3%, 5%, and 10%] were substituted by the one-step solution method. The crystallographic structure of the as-prepared perovskite films was investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). A comparative evaluation about the influence of the lead halide type and substitution percentage on the optical and electronic properties of the prepared perovskites in the perovskite solar cells (PSCs) were done using current density-voltage curves (J-V), electrochemical impedance spectroscopy (EIS), and photoluminescence (PL) techniques. The best photovoltaic performance and stability of fabricated MAI (PbI2)1–Y (PbBr2) Y-based and MAI (PbI2)1–Y (PbCl2) Y-based cells were obtained at 5% PbBr2 and 3% PbCl2 doping, respectively. The MAI (PbI2)1–Y (PbBr2) Y-based PSCs showed higher photovoltaic and stability performances than MAI (PbI2)1–Y (PbCl2) Y-based cells. The highest power conversion efficiency (PCE) of 17.14% with an open-circuit voltage (Voc) of 1.06 V, a short-circuit photocurrent density (Jsc) of 22.61 mA/cm2 and a fill factor (FF) of 0.72 was achieved for the MAI (PbI2)1–Y (PbBr2) Y-based PSC at the Y = 5%.

Shallow Iodine Defects Accelerate the Degradation of α-Phase Formamidinium Perovskite

Summary Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI3) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects ( I i ) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI3-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of I i and the more effective passivation of iodine vacancies ( V I ) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing.

Addressing the Stability of Lead Halide Perovskites

The rapid rise in efficiencies of lead halide perovskite solar cells has hastened the case for commercialization. One of the key barriers, however, is stability of devices. In this issue of Joule, Zhou et al. studied the degradation mechanism of a FA-Cs mixed cation perovskite-based solar cell with experimental and modeling techniques to show that phase separation into FA-rich and Cs-rich compositions account for the observed degradation.

Crystalline Nature of Colloids in Methylammonium Lead Halide Perovskite Precursor Inks Revealed by Cryo-Electron Microscopy.

Metal halide perovskites have generated interest across many fields for the impressive optoelectronic properties achievable in films produced using facile solution-processing techniques. Previous research has revealed the colloidal nature of perovskite precursor inks and established a relationship between the colloid distribution and the film optoelectronic quality. Yet, the identity of colloids remains unknown, hindering our understanding of their role in perovskite crystallization. Here, we investigate precursor inks of the prototypical methylammonium lead triiodide perovskite using cryo-electron microscopy (cryo-EM) and show, for the first time, that the colloids are neither amorphous nor undissolved lead iodide, as previously speculated, but are a crystalline, non-perovskite material. We identify this as a perovskite precursor phase and discuss this as a potential means to understanding the role of chloride in processing. This work establishes cryo-EM as a viable technique to elucidate the nature of colloids in perovskite inks, a vital step toward a fundamental understanding of thin-film crystallization.

Switching between spectral broadening and narrowing of the exciton absorption band of a CH3NH3PbI3 film on altering the polarity of an applied electric field

Electric field-induced broadening and narrowing of an exciton absorption band have been observed to depend on the polarity of the applied electric field for methylammonium lead triiodide perovskite (MAPbI3) as a thin film sandwiched between a conducting film of fluorine-doped tin oxide (FTO) and an insulating film of poly(methyl methacrylate) (PMMA). The width of the absorption band increased or decreased when the direction of the applied field was toward FTO or PMMA, respectively, indicating that the spectral broadening and narrowing become switchable on altering the direction of the applied electric field. When titanium oxide (TiO2) in a compact layer was introduced between the MAPbI3 and FTO layers, the linear electric field effect that depended on the polarity was either not observed or decreased significantly although the quadratic field effects on the exciton absorption band were similarly observed with or without the TiO2 layer.

First-Principles Investigation of the Thermal Degradation Mechanisms of Methylammonium Lead Triiodide Perovskite

The poor stability of organometallic halide perovskites, especially CH3NH3PbI3 (MAPbI3), in high-temperature environments is one of the challenges retarding its wider applicability. A better understanding of how MAPbI3 perovskite degrades thermally will allow us to develop strategies to improve its “intrinsic” stability. In this study, we employed first-principles density functional theory to obtain detailed thermal degradation energy landscapes for MAPbI3. We focused on studying two reaction pathways: P1, for proton abstraction from methylammonium (MA) to iodine, and P2, for substitution (SN2) of an iodide anion (I–, as nucleophile) at the carbon atom of MA to yield iodomethane (CH3I) and ammonia (NH3). Our calculations provided kinetic data, allowing us to assess the “intrinsic” (in)stability of MAPbI3 under heat stress and to make sense of the seemingly contradictory experimental results. The I– species is more reactive for reaction P1 but not for P2. The energy of the reaction along pathway P1 increases monotonically with respect to proton abstraction. The reaction along pathway P2 is a two-step process, with a CH3–I–Pb–X intermediate formed in the crystal in conjunction with the release of NH3 gas. Our results suggest that the method of preparation (e.g., iodine-deficient conditions) and the morphology (e.g., improved crystallinity) of MAPbI3 could both influence its thermal stability.