MAPbBr3 Thin Films Research Articles

Low temperature exciton lifetime enhancement of MAPbBr3 studied by broadband terahertz spectroscopy

Over the past few years, lead halide perovskites have demonstrated significant potential in the field of solar cells, light-emitting diodes, and field-effect transistors. We investigated the transient photoconductivity and temperature-induced phase transitions in MAPbBr3 thin films, using ultrabroadband optical pump-terahertz probe spectroscopy with varying excitation fluences and temperature conditions. As the temperature drops from 293 to 123 K, MAPbBr3 undergoes two phase transitions, resulting in a significant extension of exciton lifetime in the tetragonal phase near 150 K, where MAPbBr3 exhibits a longer exciton lifetime. Our research contributes to a better understanding of the temperature-induced phase transition mechanism of MAPbBr3, enhancing its potential applications in optoelectronic devices.

Band Gap Renormalization at Different Symmetry Points in Perovskites.

Using ultrafast broad-band transient absorption (TA) spectroscopy of photoexcited MAPbBr3 thin films with probe continua in the visible and the mid- to deep-ultraviolet (UV) ranges, we capture the ultrafast renormalization at the fundamental gap at the R symmetry point of the Brillouin zone (BZ) and a higher energy gap at the M symmetry point. Advanced global lifetime analysis and lifetime density distribution analysis are applied to extract quantitative information. Our work confirms the similarity of the response at both high-symmetry points, which indicates a band edge renormalization that rises within the instrument response function (IRF, ∼250 fs) and decays in ca. 400-600 fs, undergoing an energy red shift of 90-150 meV. The reported time scale corresponds to the decay of free carriers into neutral excitons. The ability to monitor different high-symmetry points in photoexcited perovskites opens exciting prospects for the characterization of a large class of materials and for photonic applications.

Disentangling Thermal from Electronic Contributions in the Spectral Response of Photoexcited Perovskite Materials.

Disentangling electronic and thermal effects in photoexcited perovskite materials is crucial for photovoltaic and optoelectronic applications but remains a challenge due to their intertwined nature in both the time and energy domains. In this study, we employed temperature-dependent variable-angle spectroscopic ellipsometry, density functional theory calculations, and broadband transient absorption spectroscopy spanning the visible to mid-to-deep-ultraviolet (UV) ranges on MAPbBr3 thin films. The use of deep-UV detection opens a new spectral window that enables the exploration of high-energy excitations at various symmetry points within the Brillouin zone, facilitating an understanding of the ultrafast responses of the UV bands and the underlying mechanisms governing them. Our investigation reveals that the photoinduced spectral features remarkably resemble those generated by pure lattice heating, and we disentangle the relative thermal and electronic contributions and their evolutions at different delay times using combinations of decay-associated spectra and temperature-induced differential absorption. The results demonstrate that the photoinduced transients possess a significant thermal origin and cannot be attributed solely to electronic effects. Following photoexcitation, as carriers (electrons and holes) transfer their energy to the lattice, the thermal contribution increases from ∼15% at 1 ps to ∼55% at 500 ps and subsequently decreases to ∼35-50% at 1 ns. These findings elucidate the intricate energy exchange between charge carriers and the lattice in photoexcited perovskite materials and provide insights into the limited utilization efficiency of photogenerated charge carriers.

Morphology-dependent optical properties of a 3D CH3NH3PbBr3 based metal − organic framework

More recently, Metal–Organic Frameworks (MOFs) have displayed extensive promise in the fields of energy storage, solar cells, catalysis, super capacitors, and optoelectronic devices. This work investigates the nonlinear thermo-optical responses of CH3NH3PbBr3 (MAPbBr3) and mix MAPbBr3 with UiO-66-NH2 (MAPbBr3 + MOF) thin films using the Z-scan technique with different incident power of laser ranging from 10 mW to 50 mW. The results show that the third-order nonlinear optical coefficients of MAPbBr3 + MOF thin films were improved obviously than MAPbBr3 thin films. Furthermore, nonlinear optical properties of MAPbBr3 + MOF thin film is evaluated to be improved from 12.5 × 10-8 to 38.2 × 10-8 by decreasing the input power of laser from 50 mW to 10 mW. This treatment repeated for MAPbBr3 thin film, as well. Considering Kubelka-Munk equation, energy gap) Eg for the MAPbBr3 and MAPbBr3 + MOF thin films deposited on glass film is calculated to be 2.565 and 2.530 eV. Due to analyze the optical characterization of the synthesized samples, optical reflection, transmission and absorption were studied. Kramers-Kronig relations and the reflectance data were used for calculation of complex refractive index and dielectric constant. The transverse and the longitudinal optical phonon modes of MAPbBr3 and MAPbBr3 + MOF thin films locate at about 548 and 598 nm, and 590 and 640 nm, respectively. This research opens a new window for wide scale optoelectronic devices in Q-switching for the generation of pulsed lasers including MAPbBr3 with UiO-66-NH2 by detecting the remarkable nonlinear optical characteristics in lower incident power.

High-Responsivity Planar Photodetector Based on Methylammonium Lead Bromide Perovskite Thin Film

We have fabricated planar interdigitated photodetectors exhibiting high responsivity. These detectors are based on thin layers of methylammonium lead bromide (MAPbBr3) at 90 nm thickness. MAPbBr3 thin films were first characterized on glass (borosilicate) substrates using absorption and photoluminescence measurements showing a high absorption edge at 521 nm and strong emission at 530 nm, as expected. MAPbBr3 thin films were then deposited on top of interdigitated electrodes, hence producing planar photodetectors with responsivity up to 0.4 A/W. Such higher performances were attributed to the interdigitated design, low crack density (0.05 µm−2), and lower resistivity (20 MΩ.cm) compared to MAPbBr3 single crystal. Therefore, this work highlights MAPbBr3 thin films as very promising for photodetection applications.

Low-Temperature Emission Dynamics of Methylammonium Lead Bromide Hybrid Perovskite Thin Films at the Sub-Micrometer Scale.

We study the low-temperature (T = 4.7 K) emission dynamics of a thin film of methylammonium lead bromide (MAPbBr3), prepared via the anti-solvent method. Using intensity-dependent (over 5 decades) hyperspectral microscopy under quasi-resonant (532 nm) continuous wave excitation, we revealed spatial inhomogeneities in the thin film emission. This was drastically different at the band-edge (∼550 nm, sharp peaks) than in the emission tail (∼568 nm, continuum of emission). We are able to observe regions of the film at the micrometer scale where emission is dominated by excitons, in between regions of trap emission. Varying the density of absorbed photons by the MAPbBr3 thin films, two-color fluorescence lifetime imaging microscopy unraveled the emission dynamics: a fast, resolution-limited (∼200 ps) monoexponential tangled with a stretched exponential decay. We associate the first to the relaxation of excitons and the latter to trap emission dynamics. The obtained stretching exponents can be interpreted as the result of a two-dimensional electron diffusion process: Förster resonant transfer mechanism. Furthermore, the non-vanishing fast monoexponential component even in the tail of the MAPbBr3 emission indicates the subsistence of localized excitons. Finally, we estimate the density of traps in MAPbBr3 thin films prepared using the anti-solvent method at n∼1017 cm-3.

Band-gap-engineered mixed-halide perovskite films through vacuum-based deposition approach

Mixed-halide perovskites offer the important characteristic of band-gap tunability, which is essential for tandem solar cells. Herein, the single-source vapor deposition method is reported for MAPbBr2Cl1 (MA (methyl ammonium) = CH3NH3), MAPbBr1.5Cl1.5 and MAPbBr1Cl2 perovskite films utilizing the corresponding mixed-halide perovskite nanoparticles as a source for thermal evaporation. Further, band-gap tuning in vacuum-deposited MAPbBr3 thin films is performed through MACl treatment with successful band-gap tuning from 2.37 to 2.48 eV. The limitation of chlorine (Cl) incorporation into the perovskite structure at room temperature is tackled by MACl treatment of vapor-deposited MAPbBr2Cl1, MAPbBr1.5Cl1.5 and MAPbBr1Cl2 films. The structural properties of MAPbBr3−x Cl x films show a continuous shift in the X-ray diffraction peak toward a higher 2θ value, confirming chlorine incorporation into the perovskite structure. Band-gap tuning in MAPbBr3−x Cl x films, from 2.37 to 2.86 eV, is confirmed by absorption spectroscopy. The mixed-halide perovskite films show an improved microstructure with MACl treatment, as observed from the increased grain size of the films. Further, emission studies show good stability of MAPbBr3−x Cl x films against phase segregation under continuous exposure to 325 nm wavelength illumination of 100 mW/cm2 for more than 60 min. The diminished phase segregation might help in advancing the application of mixed-halide perovskites for reliable optoelectronic devices.

Doping MAPbBr3 hybrid perovskites with CdSe/CdZnS quantum dots: from emissive thin films to hybrid single-photon sources.

We report the first doping of crystalline methyl-ammonium lead bromide perovskite (MAPbBr3) films with CdSe/CdZnS core/shell quantum dots (QDs), using a soft-chemistry approach that preserves their high quantum yield and other remarkable luminescence properties. Our approach produces MAPbBr3 films of around 100 nm thickness, doped at volume ratios between 0.01 and 1% with colloidal CdSe/CdZnS QDs whose organic ligands were exchanged with halide ions to allow for close contact between the QDs and the perovskite matrix. Ensemble photoluminescence (PL) measurements demonstrate the retained emission of the QDs after incorporation into the MAPbBr3 matrix. Photoluminescence excitation (PLE) spectra exhibit signatures of wavelength-dependent coupling between the CdSe/CdZnS QDs and the MAPbBr3 matrix, i.e., a transfer of charges from matrix to QD, which increases the QD luminescence by up to 150%, or from QD to matrix. Spatially-resolved PL experiments reveal a strong correlation between the positions of QDs and an enhancement of the PL signal of the matrix. Lifetime imaging of the doped films furthermore shows that the emission lifetime of MAPbBr3 is slower in the vicinity of QDs, which, in combination with the increased PL signal of the matrix, suggests that QDs can act as local nucleation seeds that improve the crystallinity of MAPbBr3, thus boosting its emission quantum yield. Luminescence antibunching measurements provide clear evidence of single-photon emission from individual QDs in perovskite. Finally, the analysis of blinking statistics indicates an improvement of the photostability of individual QDs in perovskite as compared to bare CdSe/CdZnS QDs. At high CdSe/CdZnS QD doping levels, this work thus opens a route to hybrid solar concentrators for visible-light harvesting and hybrid-based LEDs, while a low degree of doping could yield hybrid single-photon sources than can be embedded in field-effect devices for single-charge control, which would allow the construction of nanophotonic devices via low-cost solution-processing techniques as an alternative to solid-state quantum dots.

Effect of solvents and annealing treatment on the properties of the methylammonium lead tribromide perovskite thin films

Solution-processed hybrid organic–inorganic perovskites from organoammonium halide and lead halide precursors form effective active layers for optoelectronics devices. The solvent plays a decisive factor on the preparation of active layer for perovskite application. In this report, the properties of methylammonium lead tribromide (MAPbBr3) thin films were studied using various solvents. The solvents used were dimethylformamide (DMF), dimethyl sulfoxide (DMSO), a mixture of DMF + IPA (Isopropanol). The photoluminescence and absorption spectra show a good solubility of the perovskite content in the mixed solvent than that with DMF or DMSO which is attributed to the good crystallization of content in the film. Furthermore, the effect of annealing treatment on the structural and optical properties of MAPbBr3 thin films prepared with DMF + IPA was investigated. The MAPbB3 film annealed at 100 °C has considerably improved the dissociation of the charges resulting from the good interaction of the solvent with the perovskite material and the good perovskite film quality.

Effects of ITO Substrate Hydrophobicity on Crystallization and Properties of MAPbBr3 Single-Crystal Thin Films.

Fabricating perovskite single-crystal thin films (SCTFs) in controllable manner is the major challenge for the promising potential applications in optoelectronic devices. Although modifying the substrate surface is frequently used to realize the controlled growth of perovskite SCTFs, it is still unclear how the substrate condition affects the crystallization process. In this work, we systemically investigated the effects of the surface hydrophobicity of indium tin oxide substrates on the crystallization process of MAPbBr3 SCTFs prepared by the space-confined method. Comprehensive characterizations show that the surface morphology and crystallinity of SCTFs are improved, and the defect density is reduced when increasing the substrate hydrophobicity. The best MAPbBr3 thin film obtained has a full width at half-height of the rocking curve of the (001) crystal plane of 0.044°. The mechanism of the substrate hydrophobicity on the crystal growth is also discussed. These results will provide guidance to the controllable growth of high-quality SCTFs for perovskite SCTF devices.

Widegap CH3NH3PbBr3 solar cells for optical wireless power transmission application

The optoelectrical properties of bromide-based perovskite thin films were evaluated to develop an optical wireless power transmission system. CH3NH3PbBr3 (MAPbBr3) films were deposited by the anti-solvent method after substrate preheating. The low-temperature exciton emission and discontinuous peak shift from phase transitions in the emission spectra indicate the high optical quality of MAPbBr3 films on TiO2. The Urbach energy and diffusion length of the MAPbBr3 thin films on TiO2 were 22.7 meV and 1.14 μm, respectively. The films exhibited good solar cell performance under monochromatic light with an efficiency over 20%.

Cooling dynamics of electrons in MAPbBr3 probed in the deep-UV

Transient absorption in the Visible and in the deep-UV is performed on MAPbBr3 thin films with 3.1 eV pump excitation. The UV probe can access higher order transitions in the material exploring different high-symmetry points of the Brillouin zone. Uncorrelated electron-hole pairs are generated within the instrument response function of 150 fs. The photobleaching at 3.3 eV shows that electron cooling happens in ~ 1 ps.

Origin of the Enhanced Photoluminescence Quantum Yield in MAPbBr3 Perovskite with Reduced Crystal Size

Methylammonium lead bromide perovskite (MAPbBr3) has been widely investigated for applications in visible perovskite light-emitting diodes (LEDs). Fine-tuning of the morphology and of the crystal size, from the microscale down to the quantum confinement regime, has been used to increase the photoluminescence quantum yield (PLQY). However, the physical processes underlying the PL emission of this perovskite remain unclear. Here, we elucidate the origin of the PL emission of polycrystalline MAPbBr3 thin films by different spectroscopic techniques. We estimate the exciton binding energy, the reduced exciton effective mass, and the trap density. Moreover, we confirm the coexistence of free carriers and excitons, quantifying their relative population and mutual interaction over a broad range of excitation densities. Finally, the enhanced PLQY upon crystal size reduction to the micro- and nanometer scale in the presence of additives is attributed to favored excitonic recombination together with reduced surface ...

Enhancement of photocurrent extraction and electron injection in dual-functional CH3NH3PbBr3 perovskite-based optoelectronic devices via interfacial engineering

Photocurrent extraction and electron injection in CH3NH3PbBr3 (MAPbBr3) perovskite-based optoelectronic devices are both significantly increased by improving the contact at the PCBM/MAPbBr3 interface with an extended solvent annealing (ESA) process. Photoluminescence quenching and x-ray diffraction experiments show that the ESA not only improves the contact at the PCBM/MAPbBr3 interface but also increases the crystallinity of the MAPbBr3 thin films. The optimized dual-functional PCBM-MAPbBr3 heterojunction based optoelectronic device has a high power conversion efficiency of 4.08% and a bright visible luminescence of 1509 cd m−2. In addition, the modulation speed of the MAPbBr3 based light-emitting diodes is larger than 14 MHz, which indicates that the defect density in the MAPbBr3 thin film can be effectively reduced by using the ESA process.

Hansen theory applied to the identification of nonhazardous solvents for hybrid perovskite thin-films processing

Metal-halide perovskites have become the most studied material for efficient next-generation solar cells, in part because of the possibility of depositing high quality semiconducting perovskites by simple solution-based methods. However, the majority of solvent systems implemented in literature for deposition of lead halide perovskites are hazardous to handle. Investigation of alternatives perovskite processing methods are hence key to safely upscale the perovskite photovoltaic manufacturing.In this manuscript we use the Hansen theory to find suitable nonhazardous solvents to solubilize two lead salts, PbBr2 and PbI2, used to fabricate the corresponding methylammonium (MA) lead halide perovskite films. The method is based on the use of the Hansen solubility parameters (HSPs) of a lead salt and a solvent to provide insight into whether interactions between them are favorable or not. We first studied the solubility properties of each lead salt (PbBr2 and PbI2) by determining experimentally their Hansen solubility spheres. Using these results, computer simulations allow us to determine compatible nonhazardous solvents with these two compounds in terms of solubility. We further investigated the use of a nonhazardous diester blend polar solvent to deposit MAPbI3 and MAPbBr3 thin films.

Variation in the Photocurrent Response Due to Different Emissive States in Methylammonium Lead Bromide Perovskites

Thin films and crystals of methylammonium lead bromide (MAPbBr3) perovskites have strong photoluminescence (PL). Previous studies have shown that the emission arises from different states. However, the role of these states in the performance of a solar cell has not been reported. We have used photocurrent and photoluminescence microscopies (PCM and PLM) to investigate the correlation between the photocurrent (PC) and the PL behavior in the different regions of MAPbBr3 thin film solar cells. Our results show that the PC and the PL responses from the different regions in the thin film show poor correlation compared to the correlation between those of a high efficiency GaAs solar cell. Furthermore, we establish a relationship between the different emissive states and the PC and the PL responses. Out of the two emissive states at 2.34 and 2.28 eV that have been reported, only the state at 2.34 eV has a dominant contribution to the PC. Our results suggest that the emission at 2.28 eV is related to traps, which...

Effect of the precursor's stoichiometry on the optoelectronic properties of methylammonium lead bromide perovskites

Methylammonium lead bromide (MAPbBr3) perovskites have been widely studied in applications such as lasers and light-emitting diodes, thanks to their favorable bandgap, efficient charge transport, and the possibility of processing by simple solution methods. The film morphology has a large impact on the optical and electronic properties of the material; hence the deposition methods and the type of precursors used are crucial in the preparation of efficient optoelectronic devices. Here we studied the effect of the precursor´s stoichiometry of solution processed MAPbBr3 thin films on their optical and electronic properties. We found a drastic effect of the stoichiometry on the resulting thin film morphology, and suggest the use of excess MA cations to enhance the photo- and electroluminescence of hybrid perovskites.

In-Situ Formed Type I Nanocrystalline Perovskite Film for Highly Efficient Light-Emitting Diode.

Excellent color purity with a tunable band gap renders organic-inorganic halide perovskite highly capable of performing as light-emitting diodes (LEDs). Perovskite nanocrystals show a photoluminescence quantum yield exceeding 90%, which, however, decreases to lower than 20% upon formation of a thin film. The limited photoluminescence quantum yield of a perovskite thin film has been a formidable obstacle for development of highly efficient perovskite LEDs. Here, we report a method for highly luminescent MAPbBr3 (MA = CH3NH3) nanocrystals formed in situ in a thin film based on nonstoichiometric adduct and solvent-vacuum drying approaches. Excess MABr with respect to PbBr2 in precursor solution plays a critical role in inhibiting crystal growth of MAPbBr3, thereby forming nanocrystals and creating type I band alignment with core MAPbBr3 by embedding MAPbBr3 nanocrystals in the unreacted wider band gap MABr. A solvent-vacuum drying process was developed to preserve nanocrystals in the film, which realizes a fast photoluminescence lifetime of 3.9 ns along with negligible trapping processes. Based on a highly luminescent nanocrystalline MAPbBr3 thin film, a highly efficient green LED with a maximum external quantum efficiency of 8.21% and a current efficiency of 34.46 cd/A was demonstrated.

Quantitative Correlation of Perovskite Film Morphology to Light Emitting Diodes Efficiency Parameters

This paper reports quantitative correlation of CH3NH3PbBr3 (MAPbBr3) thin film morphology to light emitting diode efficiency parameters. Sequential (spin coating) deposition is used for highly reproducible and dense film morphology of MAPbBr3 thin-film. In this fabrication process using an orthogonal solvent approach, control of morphology, coverage, thickness, and optical properties in these compact thin-films is demonstrated. Optical studies show direct correlation between morphology to dynamics of photoluminescence (PL) and absolute PL yield. Perovskite light emitting diodes (PeLEDs) are fabricated from these films to find the best ratio of PbBr2 versus MABr for optimal performance. This study demonstrates PeLEDs with high brightness, ≈1050 cd m−2 at 4.7 V (luminance efficiency ≈0.1 cd A−1), for optimal thin-film process with state-of-the-art device performance. This quantitative analysis suggests that these state-of-the-art PeLEDs suffer from poor charge carrier balance (≈2%) and out-coupling efficiency (≈6%). Interestingly, charge carrier balance and PL yield together can explain the change in PeLED efficiency modulation with film morphology. Studies on single carrier devices show that these PeLEDs are electron current dominated and charge carrier balance increases with operating bias voltage.