Methylammonium Lead Iodide Perovskite Research Articles

(Invited) Halide Perovskite Thin Films via Alternate Laser Deposition: Change in the Physical Properties with the Layered Structures

We have formed methylammonium lead iodide (MAPbI3) perovskite by alternating irradiation of PbI2 and CH3NH3I (MAI) with an infrared laser in an ultra-high vacuum to elucidate the growth mechanism and realize structural control of MAPbI3 [1]. In this study, we found that MAPbI3 contains both tetragonal and orthorhombic phases, and the ratio of these phases systematically changes depending on the thickness of alternating layers of PbI2 film.In the sample preparation, the PbI2 and MAI powder targets were alternately irradiated with an infrared laser (λ = 808 nm) to convert them into molecular beams, and then alternately deposited on a synthetic fused silica substrate at room temperature. The deposition rate of PbI2 was controlled to be about 0.1 nm/s using a quartz crystal microbalance thickness monitor. The deposition amount of MAI was adjusted by changing the deposition time while the evaporation pressure was fixed at 4×10-4 Pa. Four different multilayer samples consisting of [PbI2/MAI] × n (n = 1, 2, 5, 10) were prepared under the condition that the total thickness of PbI2 was 300 nm.By analyzing the crystal structures by X-ray diffraction measurements, the prepared samples were found to have the following two unique features: (1) In the samples with n = 1, unreacted PbI2 was present in addition to MAPbI3, while in the samples with n = 2, 5, and 10, only MAPbI3 was observed, indicating a complete solid-phase reaction. (2) The full width at half maximum and peak position of the diffraction peak for MAPbI3 at 2θ = 14.0° ~ 14.1° were different among the samples, and thus we performed the analysis by peak separation. As a result, it was determined that the tetragonal (002) and orthorhombic (020) diffraction peaks were included.Defining r tetra and r ortho as the ratio of the tetragonal and orthorhombic diffraction peak areas to the total peak area, it was found that r tetra : r ortho changes significantly with the thickness of the PbI2 layer. Specifically, r tetra : r ortho =[0.64 : 0.36], [0.39 : 0.61], [0.20 : 0.80], and [0.25 : 0.75] for n =1, 2, 5, and 10, respectively. It is noteworthy that the orthorhombic MAPbI3, which usually appears below 161 K due to the phase transition [2], is contained in the MAPbI3 film formed at room temperature, and that the mixing ratio of tetragonal and orthorhombic phases varies systematically with the thickness of the alternating deposited PbI2 films.[1] K. Kawashima et al., Sci. and Tech. of Adv. Mater. 18, 307 (2017).[2] N. Onoda-Yamamuro et al., J. Phys. Chem. Solids 51, 1383 (1990).

(Invited) Surface Ligands for Methylammonium Lead Iodide Perovskites and Their Impact on Interfacial Energetics, Material Stability, and Photovoltaic Performance

Surface ligand treatment of organic metal halide perovskites can improve material and device stability, alter interfacial energetics, and increase perovskite solar cell (PSC) performance. Unlike in traditional inorganic semiconductors such as silicon, surface ligands often penetrate into the perovskite crystal and result in the formation of reduced dimensionality structures (where the 3D perovskite framework is broken into 2D sheets of varying thickness). Such ligand penetration can have a large impact on the surface energetics and material stability; thus, it is important to understand how surface ligand structure impacts ligand penetration. Using angle-dependent x-ray photoelectron spectroscopy (XPS) and x-ray diffraction we show that surface ligands with ammonium and phosphonic acid binding groups often penetrate into MAPbI3. This ligand penetration can significantly influence surface energetics, as probed with ultraviolet and inverse photoelectron spectroscopy measurements. The observed changes in surface energetics upon surface ligand treatment have a large impact on the PSC performance, with better photovoltaic performance arising for ligands that result in more favorable energy landscapes for electron transfer from MAPbI3 to the electron transport layer, C60. For a series of ammonium containing ligands, we find that both the cross-sectional area and extent of ligand fluorination impact the penetration of the surface ligand into the perovskite. Furthermore, surface ligands with larger cross-sectional areas result in improved MAPbI3 stability.

Achieving Efficient and Stable Perovskite Solar Cells in Ambient Air Through Non‐Halide Engineering

AbstractThe realization of highly efficient perovskite solar cells (PSCs) in ambient air is considered to be advantageous for low‐cost commercial manufacturing. However, it is fundamentally difficult to achieve comparable device performance to that obtained in an inert atmosphere, especially when the ambient humidity is high. Here, an effective precursor engineering that simultaneously employs non‐halide lead acetate and lead thiocyanate lead sources for fabricating high‐quality methylammonium lead iodide perovskite films in ambient air with enhanced moisture tolerance, is reported. The presence of Ac– and SCN– ions not only enables the facile formation of homogeneous and highly crystalized perovskite films, but also directs the uniform growth of the crystals along the (110) direction. Accordingly, a 20.55% efficiency is demonstrated, one of the best results for air‐processed MAPbI3 PSCs, which is also the highest value achieved with non‐halide lead sources. Furthermore, the unencapsulated device shows fivefold prolonged air stability (3600 h) compared to the conventional PbI2‐based PSC. Together with the use of non‐toxic antisolvent, this strategy is fully compatible with ambient air operation and thus of great potential for practical applications.

Low-field onset of Wannier-Stark localization in a polycrystalline hybrid organic inorganic perovskite

Methylammonium lead iodide perovskite (MAPbI3) is renowned for an impressive power conversion efficiency rise and cost-effective fabrication for photovoltaics. In this work, we demonstrate that polycrystalline MAPbI3s undergo drastic changes in optical properties at moderate field strengths with an ultrafast response time, via transient Wannier Stark localization. The distinct band structure of this material - the large lattice periodicity, the narrow electronic energy bandwidths, and the coincidence of these two along the same high-symmetry direction – enables relatively weak fields to bring this material into the Wannier Stark regime. Its polycrystalline nature is not detrimental to the optical switching performance of the material, since the least dispersive direction of the band structure dominates the contribution to the optical response, which favors low-cost fabrication. Together with the outstanding photophysical properties of MAPbI3, this finding highlights the great potential of this material in ultrafast light modulation and novel photonic applications.

Additive effects of methyl ammonium bromide or formamidinium bromide in methylammonium lead iodide perovskite solar cells using decaphenylcyclopentasilane

Additive effects of methyl ammonium bromide or formamidinium bromide in methylammonium lead iodide perovskite solar cells using decaphenylcyclopentasilane

Optimization of optoelectrical properties during synthesizing methylammonium lead iodide perovskites via a two-step dry process

We investigated the defect states and optoelectrical properties of methylammonium lead iodide (MAPbI3) perovskite films synthesized via a two-step dry process. To synthesize MAPbI3 thin films, PbI2 thin films were deposited via rf magnetron sputtering and were subsequently exposed to methylammonium iodide (MAI) vapor to synthesize MAPbI3 thin films. By using deep level transient spectroscopy and space-charge-limited current methods, the defect density of the perovskite films was proven to be reduced by about 6 times as the reaction temperature increased from 90 to 110 °C. At a high reaction temperature of 130 °C, the deformation process could occur, resulting in a rising trap density within the perovskite film. Along with a significant decline in defect density, the photoresponsivity of films fabricated at 110 °C were higher by about 2.05 and 18.76 times more than those of samples prepared at 100 and 130 °C, respectively. An excellent detectivity of 1.17 × 1013 Jones was observed from the sample synthesized at 110 °C, demonstrating the superiority of a two-step dry process.

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.

Investigating the effect of applied bias on methylammonium lead iodide perovskite by electrical and positron annihilation spectroscopic studies

The formation of point defects in the crystal lattice of organic–inorganic hybrid perovskites is detrimental to the long-term stability of the perovskite-based solar cell. The observed hysteresis in organolead halide perovskite solar cell during the photocurrent measurement is believed to be the consequence of such defect sites. In the present work, we are reporting the formation of reversible and/or irreversible lattice defects in methylammonium lead iodide perovskite depending on the magnitude of bias voltage studied by electrical measurements and positron annihilation spectroscopy (PAS). Current vs voltage (I–V) measurements support the formation of p/n junction at the metal/semiconductor interfaces, suggesting the defects-driven ion migration under the applied bias. In-situ PAS has elucidated the nature of vacancy sites formed due to the application of bias voltage. In the lower biased voltage (<1 V), the formation of lattice defects is reversible, while beyond a threshold voltage of 1.2 V, the defects become permanent as observed by the PAS.

Truth and Myth of Phase Coexistence in Methylammonium Lead Iodide Perovskite Thin Film via Transmission Electron Microscopy.

Hybrid organic-inorganic perovskites have attracted substantial interest as the most favorable prospective material for efficient photovoltaic and optoelectronic devices. However, their extreme sensitivity to electron beam radiation makes it difficult to obtain their intrinsic structure by transmission electron microscopy and can even lead to significant misidentifications. In 2018, the coexistence of methylammonium lead iodide (MAPbI3 ) in the cubic and tetragonal phase using electron microscopy and electron diffraction techniques was reported in article "Self-Organized Superlattice and Phase Coexistence inside Thin Film Organometal Halide Perovskite". Herein, however, that claim is challenged by comparing their experimental data to simulated diffraction patterns and arguing that their perovskite samples may have been damaged due to excessive electron beam irradiation. Consequently, true phase coexistence was not observed in that previously reported work, rather merely the decomposition products of MAPbI3 .

Simultaneously Enhancing Efficiency and Stability of Perovskite Solar Cells Through Crystal Cross-Linking Using Fluorophenylboronic Acid.

Organic-inorganic metal halide perovskites are regarded as one of the most promising candidates in the photovoltaic field, but simultaneous realization of high efficiency and long-term stability is still challenging. Here, a one-step solution-processing strategy is demonstrated for preparing efficient and stable inverted methylammonium lead iodide (MAPbI3 ) perovskite solar cells (PSCs) by incorporating a series of organic molecule dopants of fluorophenylboronic acids (F-PBAs) into perovskite films. Studies have shown that the F-PBA dopant acts as a cross-linker between neighboring perovskite grains through hydrogen bonds and coordination bonds between F-PBA and perovskite structures, yielding high-quality perovskite crystalline films with both improved crystallinity and reduced defect densities. Benefiting from the repaired grain boundaries of MAPbI3 with the organic cross-linker, the inverted PSCs exhibit a remarkably enhanced performance from 16.4% to approximately 20%. Meanwhile, the F-PBA doped devices exhibit enhanced moisture/thermal/light stability, and specially retain 80% of their initial power conversion efficiencies after more than two weeks under AM 1.5G one-sun illumination. This work highlights the impressive advantages of the perovskite crystal cross-linking strategy using organic molecules with strong intermolecular interactions, providing an efficient route to prepare high-performance and stable planar PSCs.

Spectroscopic Perception of Trap States on the Performance of Methylammonium and Formamidinium Lead Iodide Perovskite Solar Cells.

To enhance the efficiency and stability of the organic-inorganic hybrid perovskite (OIHP) solar cells, doping has been demonstrated as a straightforward method. Nevertheless, the perception of trap states regulated by doping and their effects on the performance of solar cells is not in-depth. Herein, typical OIHPs (CH3 NH3 PbI3 and Cs0.05 FA0.85 MA0.10 Pb(I0.97 Br0.03 )3 ) doped with RbI are employed to expound the doping mechanism in affecting the efficiency of devices. Systematic spectroscopic characterizations indicate that doping significantly influences the photocarrier dynamics via directly regulating the trap states. The results indicate that doping would reduce the trap density by passivating defects and induce extra trapping centers. This directly manipulates the transient transport of the photocarriers and finally influences the output of devices. The optimization of solar cell performance requires the tradeoff of competitive relation between the passivation and introduction of trapping centers. The results provide the spectroscopic perception on how doping concentration affects trap density, carrier dynamics, transport behavior, and ultimately the parameters of devices. It provides a straightforward guidance to the design and optimization of OIHP-based solar cells.

Electric-Field-Induced Ion Migration Behavior in Methylammonium Lead Iodide Perovskite.

Ionic movement inside organometal halide perovskites (OMHP) materials has been widely reported to be linked with stability issues in the perovskite-based optoelectronic devices. However, the dynamic processes of the ionic movement and how they influence the devices are still not well-understood. In this work, we applied an external electric field to the CH3NH3PbI3 crystal and simultaneously monitored the PL behaviors. Two successive PL responses were observed in the same location of the crystal. First, an irreversible PL quenching was observed caused by the photo-annealing effect under an electric field accompanied by a permanent morphology change. The annealed area also showed reversible PL variation, which was attributed to the activation-deactivation of the radiative recombination centers induced by the migration of the iodine ions. Such results can help us gain a deep insight into how the ionic movements in OMHPs influence the performance of the perovskite-based optoelectronic devices under working conditions.

Improved optical properties of perovskite solar cells by introducing Ag nanopartices and ITO AR layers

Embedded noble metal nanostructures and surface anti-reflection (AR) layers affect the optical properties of methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells significantly. Herein, by employing a combined finite element method and genetic algorithm approach, we report five different types of CH3NH3PbI3 perovskite solar cells by introducing embedded Ag nanoparticles within the CH3NH3PbI3 layer and/or top ITO cylinder grating as an AR layer. The maximum photocurrent was optimized to reach 23.56 mA/cm2, which was 1.09/1.17 times higher than Tran’s report/ flat cases. It is also comparable with values (23.6 mA/cm2) reported in the literature. The calculations of the electric field and charge carrier generation rate of the optimized solar cell further confirms this improvement than flat cases. It attributes to the synergistic effect of the embedded Ag nanoparticles and ITO AR layer. The results obtained herein hold great promise for future boosting the optical efficiency of perovskite solar cells.

Benzotriazole derivative inhibits nonradiative recombination and improves the UV-stability of inverted MAPbI3 perovskite solar cells

Suppressing the nonradiative recombination in the bulk and surface of perovskite film is highly desired to improve the power conversion efficiency (PCE) and stability of halide perovskite solar cells (PSCs). In this study, a benzotriazole derivative (6-chloro-1-hydroxybenzotriazole, Cl-HOBT) is applied to improve the crystallinity and reduce the trap density of methylammonium lead iodide (MAPbI3) perovskite film. Meanwhile, incorporation of Cl-HOBT elongates the photoluminescence carrier lifetime and charge-recombination lifetime, implying the trap-assisted nonradiative recombination is greatly suppressed. Besides, the improved energy level alignment and enhanced built-in potential are conducive to the charge carrier separation and transfer process with Cl-HOBT. Consequently, a PCE of 20.27% and an open-circuit voltage (Voc) of 1.09 V are achieved for the inverted MAPbI3 PSCs, along with an 85% maintaining of the initial PCE under stored at relative humidity of 20% for 500 h. Furthermore, the existence of Cl-HOBT could inhibit the formation of Pb0 defect under prolonged UV illumination to retard the degradation of perovskite film. It is believed that this study paves a novel path for the realization of high-efficiency PSCs with UV-stability.

Electrical Conductivity of Halide Perovskites Follows Expectations from Classical Defect Chemistry

AbstractDefect chemistry is key to understand electrical properties of many functional materials including halide perovskites. However, expectations from defect chemistry about the dependency of defect densities on the doping regime of halide perovskites have not yet been clearly confirmed experimentally. Here, we measure the electrical conductivity of the model halide perovskite Methylammonium Lead Iodide over a wide range of iodine partial pressures. We find that with iodine partial pressure the electrical conductivity changes with different slopes in a double‐logarithmic representation, indicating changes of the perovskite conductivity mechanism. Considering differences in the mobilities of the various defect species, we derive expectations about the dependence of the total conductivity of the perovskite as a function of iodine partial pressure. We find iodine vacancies dominating the conductivity at low iodine partial pressures, whereas at higher partial pressures holes govern the total conductivity. We find the concentration of iodide vacancies to be ∼1018 cm−3, in good agreement with the expected values based on intrinsic ionic disorder. Finally, for the case that the intrinsic ionic disorder concentration is overestimated, we elucidate the possibility to explain the conductivity profile by impurity induced acceptor doping in the perovskite. Thus, our work will allow to develop a more fundamental understanding about the electrical properties of halide perovskites.

S-Methylthiouronium Improves the Photostability of Methylammonium Lead Iodide Perovskites

<i>S</i>-Methylthiouronium Improves the Photostability of Methylammonium Lead Iodide Perovskites

Single 3.3 fs multiple plate compression light source in ultrafast transient absorption spectroscopy

Ultrafast transient absorption spectroscopy is a powerful tool to reveal excited state dynamics in various materials. Conventionally, probe pulses are generated via bulk supercontinuum generation or (noncollinear) optical parametric amplifiers whilst pump pulses are generated separately using (noncollinear) optical parametric amplifiers. These systems are limited by either their spectral density, stability, spectral range, and/or temporal compressibility. Recently, a new intense broadband light source is being developed, the multi-plate compression, which promises to overcome these limitations. In this paper, we analyze the supercontinuum generated by a single Multiple Plate Compression system to set a benchmark for its use in the field of ultrafast pump-probe spectroscopy. We have compressed the supercontinuum to 3.3 fs using chirp mirrors alone, making it an excellent candidate for pump-probe experiments requiring high temporal resolution. Furthermore, the single light source can be used to generate both probe and pump pulses due to its high spectral density (>14.5 nJ/nm) between 490 and 890 nm. The intensity has an average shot-to-shot relative standard deviation of 4.6 % over 490 to 890 nm, calculated over 2,000 sequential shots. By using only 1,000 shot pairs, a Delta T/T noise level of 2.6times 10^{-4} RMS is achieved. Finally, as a proof of concept, the transient absorption spectrum of a methylammonium lead iodide perovskite film is taken, showing great signal to noise with only 1,000 shot pairs. These results show great potential for the employment of this technique in other spectroscopic techniques such as coherent multidimensional spectroscopy.

Enhanced perovskite electronic properties via A-site cation engineering

Organic-inorganic halide perovskites have emerged as excellent candidates for low-cost photovoltaics and optoelectronics. While the predominant recent trend in designing perovskites for efficient and stable solar cells has been to mix different A-site cations, the role of A-site cations is still limited to tune the lattice and bandgap of perovskites. Herein we compare the optoelectronic properties of acetamidinum (Ace) and guanidinium (Gua) mixed methylammonium lead iodide perovskites and shed a light on the hidden role of A-site cation on the carrier mobility of mixed-cation lead iodide perovskites. The cations do not affect the bandgap of the perovskites since the orbitals from Ace and Gua do not contribute to the band edges of the material. However, the mobility of the Ace mixed perovskite is significantly enhanced to be an order of magnitude higher than that of the pristine perovskite. We apply the Ace mixed perovskite in hole-conductor-free printable mesoscopic perovskite solar cells and obtain a stabilized PCE of over 18% (certified 17.7%), which is the highest certified efficiency so far.

Co‐Evaporated Perovskite Light‐Emitting Transistor Operating at Room Temperature

AbstractSolution‐processed hybrid organic–inorganic perovskite light‐emitting transistors (PeLETs) suffer from low brightness and environmental instability induced by temperature‐activated trapping, ionic motion, and polarization effects, which so far have been hindering the realization of devices operating at room temperature. Here high quality thermally co‐evaporated methylammonium lead iodide perovskite films are employed to minimize ionic motion and significantly improve the electroluminescence characteristics of the perovskite active layer, enabling stable device operation up to ≈ 310 K. The demonstration of PeLETs operating at and beyond room‐temperature paves the way for practical applications in the rapidly evolving area of solid‐state lighting, active‐matrix displays, and visible light communications.

Effect of CdS thin film on the performance of methylammonium lead iodide perovskite solar cell

Effect of CdS thin film on the performance of methylammonium lead iodide perovskite solar cell