An intrinsic electrical conductivity study of perovskite powders MAPbX3 (X = I, Br, Cl) to investigate its effect on their photovoltaic performance

An investigation into the intrinsic electrical conductivity of perovskite powders MAPbX3, where X represents iodine (I), bromine (Br), or chlorine (Cl), was conducted to explore its impact on their photovoltaic performance. Results revealed that MAPbCl3 demonstrated light absorption ability in the ultraviolet and visible regions, while MAPbBr3 showed capacity for light absorption at longer wavelengths in the visible spectrum. On the other hand, MAPbI3 exhibited good absorption at longer wavelengths, indicating its ability to absorb light in the near-infrared region. The optical bandgap of each perovskite was determined to be 2.90 eV for MAPbCl3, 2.20 eV for MAPbBr3, and 1.47 eV for MAPbI3. The electrical conductivities of these powders were measured in-plane using the four-probe method and through-plane by electrochemical impedance spectroscopy (EIS). Electrochemical impedance spectroscopy (EIS) studies revealed a significant change in the conductivity of the MAPbI3 perovskite at temperatures between 80 °C and 100 °C. This change could be attributed to structural modifications induced when the temperature exceeds these values. The through-plane conductivity changed from 3 × 10-8 S cm-1 at 60 °C to approximately 6 × 10-5 S cm-1 at 120 °C and around 2 × 10-3 S cm-1 at 200 °C. Meanwhile, the sheet conductivity (in-plane conductivity) measurements performed at ambient temperature reveal that sheet conductivities are 489 × 103 S m-1, 486 × 103 S m-1 and 510 × 103 S m-1 for MAPbBr3, MAPbCl3 and MAPbI3, respectively. This study provides valuable insights for optimizing the performance of perovskite solar cells. Understanding how dopants influence the electrical conductivity and photovoltaic properties of the perovskite material, this work will enable researchers to design and engineer more efficient and stable solar cell devices based on MAPbX3 perovskites.

Stable and High-Efficiency Perovskite Solar Cells Using Effective Additive Ytterbium Fluoride

With better light utilization, larger tolerance factor, and higher power conversion efficiency (PCE), the HC(NH2 )2 + (FA)-based perovskite is proven superior to the popular CH3 NH3 + (MA)- and Cs-based halide perovskites in solar cell applications. Unfortunately, limited by intrinsic defects within the FA-based perovskite films, the perovskite films can be easily transformed into a yellow δ-phase at room temperature in the fabrication process, a troublesome challenge for its further development. Here, ytterbium fluoride (YbF3 ) is introduced into the perovskite precursor for three objectives. First of all, the partial substitution of Yb3+ for Pb2+ in the perovskite lattice increases the tolerance factor of the perovskite lattice and facilitates the formation of the α phase. Second, YbF3 and DMSO in the solvent form a Lewis acid complex YbF3 ·DMSO, which can passivate the perovskite film, reduce defects, and improve device stability. Consequently, the YbF3 modified Perovskite solar cell exhibits a champion conversion efficiency of 24.53% and still maintains 90% of its initial efficiency after 60 days of air exposure under 30% relative humidity.

Surface-Passivated Single-Crystal Micro-Plates for Efficient Perovskite Solar Cells

Perovskite solar cells (PeSCs) prepared with single crystals (SCs) ideally exhibit higher power conversion efficiencies (PCEs) because they possess a lower density of structural imperfection and superior charge transport. However, the density of the surface defects on the SCs is still very high, thereby inevitably affecting the device performance. Herein, perovskite single-crystal micro-plates were grown on a hole-transporting material, poly[bis(4-phenyl)(2,4,6-trimethylphenyl) amine], through a space-limited inverse temperature crystallization method. The surfaces of the as-prepared SCs were passivated using trioctylphosphine oxide (TOPO) during the device fabrication to alleviate the impact of surface defects. The PCE values are averagely improved from 11.90 ± 0.30% to 14.76 ± 0.65% after the surface passivation; the champion device even exhibits a PCE of 15.65%. The results from photoluminescence and hole-only devices reveal that TOPO treatments effectively reduce the number of surface defects on the single crystals, thereby improving the photovoltaic performance. The surface passivation also inhibits the hysteresis behavior due to the lower defect density. Finally, the TOPO treatment also improves the stability of the single-crystal PeSCs, presumably due to the hydrophobic long alkyl chains. Thus, this work provides an effective approach to achieving high efficiencies of single-crystal PeSCs.

Resistance Switching Effect of Memory Device Based on All-Inorganic Cspbbri2 Perovskite

In this study, the CsPbBrI₂ perovskite film was prepared by the preparation of the sol-gel and the spin-coating method, and the cubic lattice was stabilized by introducing Br⁺ into the CsPbI₃ film, which solved the problem of instability of the traditional perovskite phase. Based on the CsPbBrI₂ perovskite film, the Ag/CsPbBrI₂/ITO memory device with a resistance switching effect was prepared. The morphology and phase compositions of the film were analyzed by scanning electron microscope and X-ray diffraction. The non-volatile and repeatable resistance switching effect of the Ag/CsPbBrI₂/ITO memory device was measured under open-air conditions. The experimental results show that the surface of the CsPbBrI₂ perovskite film is uniform and dense, and the Ag/CsPbBrI₂/ITO memory device has an order of magnitude resistance-on-off ratio after 500 cycles of cyclic voltage. This study shows that Ag/CsPbBrI₂/ITO memory devices based on CsPbBrI₂ perovskite films have potential applications in the field of non-volatile memory devices. At the same time, the transient properties of the CsPbBrI₂ film that can quickly dissolve in deionized water make it potentially useful in short-period data storage units and implantable electronic devices with human or environmental sensors.

Formamidinium Lead Iodide Perovskite Films with Polyvinylpyrrolidone Additive for Active Layer in Perovskite Solar Cells, Enhanced Stability and Electrical Conductivity

In this paper, we studied the influence of polyvinylpyrrolidone (PVP) as a stabilization additive on optical and electrical properties of perovskite formamidinium lead iodide (FAPI) polycrystalline thin films on ZnO nanorods (ZNR). FAPI (as an active layer) was deposited from a single solution on ZNR (low temperature processed electron transport layer) using a one-step method with the inclusion of an anti-solvent. The role of PVP in the formation of the active layer was investigated by scanning electron microscopy and contact angle measurements to observe the effect on morphology, while X-ray diffraction was used as a method to study the stability of the film in an ambient environment. The effect of the PVP additive on the optical and electrical properties of the perovskite thin films was studied via photoluminescence, UV-Vis measurements, and electrical impedance spectroscopy. We have demonstrated that PVP inclusion in solution-processed perovskite FAPI thin films prevents the degradation of the film in an ambient atmosphere after aging for 2 months. The inclusion of the PVP also improves the infiltration of FAPI perovskite into ZnO nanostructures, increases electrical conductivity and radiative recombination of the photo-generated charge carriers. These results show promising information for promoting PVP stabilized FAPI perovskites for the new generation of photovoltaic devices.

Solution-Processed Mixed-Dimensional Hybrid Perovskite/Carbon Nanotube Electronics

Benefiting from their extraordinary physical properties, methylammonium lead halide perovskites (PVKs) have attracted significant attention in optoelectronics. However, the PVK-based devices suffer from low carrier mobility and high operation voltage. Here, we utilize sorted semiconducting single-walled carbon nanotubes (95% s-SWCNTs) to enhance the performance of thin-film transistors (TFTs) based on the mixed-cation perovskite (MA_1-xFA_x)Pb(I_1-xBr_x)₃, enabling mixed-dimensional solution-processed electronics with high mobility (32.25 cm²/(V s)) and low voltage (∼3 V) operation. The resulting mixed-dimensional PVK/SWCNT TFTs possess ON/OFF ratios on the order of 10⁷, enabling the fabrication of high-gain inverters.

Tunable relativistic quasiparticle electronic and excitonic behavior of the FAPb(I1−xBrx)3 alloy

The quasiparticle and excitonic properties of mixed FAPb(I_1−xBr_x)₃ 0 ≤ x ≤ 1 alloys are studied. We show that Br-doping provides an efficient and controllable way to tune the band gap and optical properties, beneficial for material design of high performance tandem solar cells.

Doping and ion substitution in colloidal metal halide perovskite nanocrystals

The past decade has witnessed tremendous advances in synthesis of metal halide perovskites and their use for a rich variety of optoelectronics applications. Metal halide perovskite has the general formula ABX3, where A is a monovalent cation (which can be either organic (e.g., CH3NH3+ (MA), CH(NH2)2+ (FA)) or inorganic (e.g., Cs+)), B is a divalent metal cation (usually Pb2+), and X is a halogen anion (Cl-, Br-, I-). Particularly, the photoluminescence (PL) properties of metal halide perovskites have garnered much attention due to the recent rapid development of perovskite nanocrystals. The introduction of capping ligands enables the synthesis of colloidal perovskite nanocrystals which offer new insight into dimension-dependent physical properties compared to their bulk counterparts. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties (e.g., absorption band gap, PL emission, and quantum yield (QY)) and stabilities of perovskite nanocrystals. The doping and ion substitution processes can be performed during or after the synthesis of colloidal nanocrystals by incorporating new A', B', or X' site ions into the A, B, or X sites of ABX3 perovskites. Interestingly, both isovalent and heterovalent doping and ion substitution can be conducted on colloidal perovskite nanocrystals. In this review, the general background of perovskite nanocrystals synthesis is first introduced. The effects of A-site, B-site, and X-site ionic doping and substitution on the optoelectronic properties and stabilities of colloidal metal halide perovskite nanocrystals are then detailed. Finally, possible applications and future research directions of doped and ion-substituted colloidal perovskite nanocrystals are also discussed.

Appraisement of Crystal Expansion in CH3 NH3 PbI3 on Doping: Improved Photovoltaic Properties

Three-dimensional hybrid perovskite materials (CH₃ NH₃ PbI₃ ) suffer from intrinsic instability owing to organic cation evaporation and ion migration. The inclusion of a large organic cation such as guanidinium has been probed to stabilize the structure. This work proposes the inclusion of imidazolium iodide (C₃ N₂ H₅ I) as an organic cation inside the CH₃ NH₃ PbI₃ matrix, as a reservoir to control the spontaneous loss of iodide. The introduction of imidazolium iodide in amounts below 20 % has an impact on the crystallization process but not on the optical properties. It also positively controls non-radiative recombination and improves the open-circuit voltage of the solar cells. The present study paves way for a deeper insight into the limit of multi-dimensional perovskite to further push the performance.

Impact of cesium in methylammonium lead bromide perovskites: insights into the microstructures, stability and photophysical properties

The thermal and moisture instabilities of pure organic lead halide perovskites are the foremost concerns towards the commercialization of perovskite solar cells, which can be avoided by introducing an inorganic cation, such as cesium ion (Cs+) at the A-site of the perovskite crystals. In this report, the impacts of substituted Cs+ cations on the inherent properties such as microstructures, morphology, and photophysics of pure methylammonium lead bromide (MAPbBr3) perovskites have been investigated. Successful formation of mixed MA1-xCsxPbBr3 phases (with 0 ≤ x ≤ 1.0) was predicted from the theoretically calculated tolerance factor, which was further supported by the appearance of sharp diffraction peaks in X-ray diffraction (XRD) patterns without any additional peaks in the whole composition range. Substitution of Cs+ ions brings significant lattice contraction in the parent MAPbBr3 crystal due to the ion size disparity in the ionic radii between MA+ and Cs+ ions. We examine the vibrational signatures of the Raman bands related to the organic MA+ and infer the nature of interactions between the organic moiety and the surrounding inorganic cage as a function of Cs concentration. Raman spectroscopic analysis reveals structural distortion due to the altered H-bonding interaction of the N+-HBr- type between MA+ and the PbBr3- octahedral framework as a function of Cs content, which is responsible for the octahedral tilting in Cs substituted MAPbBr3. We also found hindered rotational motions of MA+ in the octahedral cage of mixed cationic systems, resulting in the orientational ordering of MA in the presence of Cs. These results certainly offer highly ordered mixed phase structures and promote superior thermal stability, as evident from the thermogravimetric analysis. The photoluminescence intensity becomes considerably enhanced at increased substitution levels, which highlights the capability of incorporated Cs+ cations in suppressing non-radiative recombination in a pure MA-based crystal, possibly related to the mitigation of trapping. The substitution of Cs+ with MAPbBr3 allows innovative strategies to improve the proficiency of tandem solar cells by modifying their structural and photophysical properties.

Reset Voltage-Dependent Multilevel Resistive Switching Behavior in CsPb1- xBi xI3 Perovskite-Based Memory Device

All-inorganic CsPb_{1- x}Bi _xI₃ perovskite film was successfully fabricated by incorporating Bi³⁺ in CsPbI₃ to stabilize the cubic lattice. Furthermore, the perovskite film was applied to manufacture a simple Ag/CsPb_{1- x}Bi _xI₃/indium tin oxide (ITO) memory device with a bipolar resistive switching behavior. Nonvolatile, reliable, and reproducible switching properties are demonstrated through retention and endurance test under fully open-air conditions. The memory device also presents highly uniform and long-term stable characteristics. Importantly, by modulating the reset stop voltages, multilevel high-resistance states are observed for the first time in lead halide perovskite memory device. The resistive switching behavior is proposed to explain the formation and partial rupture of conductive multifilament that are dominated by the migration of iodine ions and their corresponding vacancies in perovskite film. This study suggests Ag/CsPb_{1- x}Bi _xI₃/ITO device potential application for multilevel data storage in a nonvolatile memory device.

Progress on Perovskite Materials and Solar Cells with Mixed Cations and Halide Anions

Organic-inorganic halide perovskite materials (e.g., MAPbI₃, FAPbI₃, etc.; where MA = CH₃NH₃⁺, FA = CH(NH₂)₂⁺) have been studied intensively for photovoltaic applications. Major concerns for the commercialization of perovskite photovoltaic technology to take off include lead toxicity, long-term stability, hysteresis, and optimal bandgap. Therefore, there is still need for further exploration of alternative candidates. Elemental composition engineering of MAPbI₃ and FAPbI₃ has been proposed to address the above concerns. Among the best six certified power conversion efficiencies reported by National Renewable Energy Laboratory on perovskite-based solar cells, five are based on mixed perovskites (e.g., MAPbI_1-xBr_x, FA_0.85MA_0.15PbI_2.55Br_0.45, Cs_0.1FA_0.75MA_0.15PbI_2.49Br_0.51). In this paper, we review the recent progress on the synthesis and fundamental aspects of mixed cation and halide perovskites correlating with device performance, long-term stability, and hysteresis. In the outlook, we outline the future research directions based on the reported results as well as related topics that warrant further investigation.

Preparation of CH3NH3PbI3 thin films with tens of micrometer scale at high temperature

The fabrication of high-quality organic-inorganic hybrid halide perovskite layers is the key prerequisite for the realization of high efficient photon energy harvest and electric energy conversion in their related solar cells. In this article, we report a novel fabrication technique of CH3NH3PbI3 layers based on high temperature chemical vapor reaction. CH3NH3PbI3 layers have been prepared by the reaction of PbI2 films which were deposited by pulsed laser deposition, with CH3NH3I vapor at various temperatures from 160 °C to 210 °C. X-ray diffraction patterns confirm the formation of pure phase, and photoluminescence spectra show the strong peak at around 760 nm. Scanning electron microscopy images confirm the significantly increased average grain size from nearly 1 μm at low reaction temperature of 160 °C to more than 10 μm at high reaction temperature of 200 °C. The solar cells were fabricated, and short-circuit current density of 15.75 mA/cm2, open-circuit voltage of 0.49 V and fill factor of 71.66% have been obtained.

Mixed-Organic-Cation (FA)x(MA)1-xPbI3 Planar Perovskite Solar Cells with 16.48% Efficiency via a Low-Pressure Vapor-Assisted Solution Process

Compared to that of methylammonium lead iodide perovskite (MAPbI₃), formamidinium lead iodide perovskite (FAPbI₃) has a smaller energy band gap and greater potential efficiency. To prevent the transformation of α-FAPbI₃ to δ-FAPbI₃, preparation of (FA)_x(MA)_1-xPbI₃ was regarded as an effective route. Usually, the planar (FA)_x(MA)_1-xPbI₃ perovskite solar cells are fabricated by a solution process. Herein, we report a low-pressure vapor-assisted solution process (LP-VASP) for the growth of (FA)_x(MA)_1-xPbI₃ perovskite solar cells that features improved electron transportation, uniform morphology, high power conversion efficiency (PCE), and better crystal stability. In LP-VASP, the (FA)_x(MA)_1-xPbI₃ films were formed by the reaction between the PbI₂ film with FAI and MAI vapor in a very simple vacuum oven. LP-VASP is an inexpensive way to batch-process solar cells, avoiding the repeated deposition solution process for PbI₂ films, and the device had a low cost. We demonstrate that, with an increase in the MAI content, the (101) peak position of FAPbI₃ shifts toward the (110) peak position of MAPbI₃, the (FA)_x(MA)_1-xPbI₃ perovskites are stable, and no decomposition or phase transition is observed after 14 days. The photovoltaic performance was effectively improved by the introduction of MA⁺ with the highest efficiency being 16.48% under conditions of 40 wt % MAI. The carrier lifetime of (FA)_x(MA)_1-xPbI₃ perovskite films is approximately three times longer than that of pure FAPbI₃. Using this process, solar cells with a large area of 1.00 cm² were fabricated with the PCE of 8.0%.

Cesium power: low Cs+levels impart stability to perovskite solar cells

Towards increasing the stability of perovskite solar cells, the addition of Cs⁺is found to be a rational approach.