CH3NH3PbI3 xClx Research Articles

The Effect of Humidity upon the Crystallization Process of Two-Step Spin-Coated Organic-Inorganic Perovskites.

Moisture is shown to activate the reaction between PbI2 and methylammonium halides. In addition, two activating mechanisms are proposed for the formation of CH3 NH3 PbI3 and CH3 NH3 PbI3-x Clx films from a series of carefully controlled experiments. When these rapidly formed perovskite films are directly fabricated into the devices, poor photovoltaic properties are found, due to heavy surface charge recombination. However, the cell performance can be significantly enhanced to 13.63 % and to over 12 % in the steady state for CH3 NH3 PbI3 and to 15.50 % and over 14 % in the steady state for CH3 NH3 PbI3-x Clx , if the rapidly formed perovskite film is annealed. Thus, it is believed that moisture (below 60 % RH) is not a problem for the fabrication of highly efficient perovskite solar cells.

Charge carrier decay and diffusion in organic-inorganic CH3 NH3 PbI3-x Cl x perovskite based solar cell

In recent years, organic–inorganic lead halides attracted widespread interest, mainly due to their impressive photoconversion properties and low-cost solution processing. In this study, we employed small amplitude transient photovoltage and photocurrent spectroscopy to investigate charge transport and recombination properties of perovskite CH3NH3PbI3–xClx solar cell under realistic light harvesting conditions (<1 sun). Cell structure resembles outlay commonly found in organic photovoltaics, with perovskite absorber being sandwiched between two thin layers of organic polymers. Tested device displayed high power conversion efficiency (10.3%), good fill factor and negligible hysteresis effect. Fundamental device parameters were characterized at various open-circuit voltages (Voc) by examination of small voltage and current perturbations created by the low intensity pulsed laser excitations. The obtained results exhibit long charge carrier lifetimes and fast charge transport over the full range of applied optical bias, as well as remarkable diffusion lengths exceeding 1 μm. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Improving the interfacial contact between CH3NH3PbI3–xClx and Au by LiTFSI solution treatment for efficient photoelectric devices

Organic lead halide compounds with perovskite structure become a promising photovoltaic material for low-cost thin film solar cells in recent years. The property of perovskite/metal interface is a fundamental topic for the effective charge transfer at metal electrodes. In this work, we develop an interface modification method of lithium bis(trifluoromethane sulfonimide) (LiTFSI) solution treatment, which can effectively decrease the charge transfer resistance at the CH3NH3PbI3–xClx/Au interface. After the solution treatment, uniform nanodots are formed at the surface of CH3NH3PbI3–xClx films, and the barrier height at CH3NH3PbI3–xClx/Au interface reduces from 0.51 V to 0.08 V. As a consequence, the efficiency of hole conductor free solar cells with CH3NH3PbI3–xClx harvester increase from 4.0% to 7.6% under one sun condition. It is also found that the hole conductor free perovskite solar cell can work in a photodetector mode, which has the same output properties with phototransistors. After the LiTFSI solution treatment, the sensitivity of this photodetector can be improved for about one time.

CH3NH3PbI3 and CH3NH3PbI3–xClx in Planar or Mesoporous Perovskite Solar Cells: Comprehensive Insight into the Dependence of Performance on Architecture

In perovskite solar cells (PSCs), issues of compatibility between the photoabsorber and the cell architecture arise. In this work, we systematically demonstrated the characteristics of PSCs with an organometal halide, CH3NH3PbI3 or CH3NH3PbI3–xClx, in a planar or mesoporous architecture, and the dependence of the cell photovoltaic performance on the architecture was illustrated in detail. In addition to the inherent photoelectric characteristics, CH3NH3PbI3 and CH3NH3PbI3–xClx also differ in other aspects, such as light absorption, crystallinity, surface coverage, and dissociation of the photogenerated electrons. For PSCs with CH3NH3PbI3, the mesoporous ones gave high power conversion efficiencies (PCE) of up to 14.05%, which is much higher than those of the planar ones (up to 6.76%). For PSCs with CH3NH3PbI3–xClx, the planar and mesoporous devices exhibited PCEs of up to 12.67% and 7.87%, respectively, quite in contrast with the case of CH3NH3PbI3.

Effect of Mesostructured Layer upon Crystalline Properties and Device Performance on Perovskite Solar Cells.

One of the most fascinating characteristics of perovskite solar cells (PSCs) is the retrieved obtainment of outstanding photovoltaic (PV) performances withstanding important device configuration variations. Here we have analyzed CH3NH3PbI3-xClx in planar or in mesostructured (MS) configurations, employing both titania and alumina scaffolds, fully infiltrated with perovskite material or presenting an overstanding layer. The use of the MS scaffold induces to the perovskite different structural properties, in terms of grain size, preferential orientation, and unit cell volume, in comparison to the ones of the material grown with no constraints, as we have found out by X-ray diffraction analyses. We have studied the effect of the PSC configuration on photoinduced absorption and time-resolved photoluminescence, complementary techniques that allow studying charge photogeneration and recombination. We have estimated electron diffusion length in the considered configurations observing a decrease when the material is confined in the MS scaffold with respect to a planar architecture. However, the presence of perovskite overlayer allows an overall recovering of long diffusion lengths explaining the record PV performances obtained with a device configuration bearing both the mesostructure and a perovskite overlayer. Our results suggest that performance in devices with perovskite overlayer is mainly ruled by the overlayer, whereas the mesoporous layer influences the contact properties.

Chemical and Electronic Structure Characterization of Lead Halide Perovskites and Stability Behavior under Different Exposures—A Photoelectron Spectroscopy Investigation

The past few years, two perovskite materials have attracted much attention in the solar cell community: CH3NH3PbI3 and CH3NH3PbI3–xClx. While these materials are usually characterized using their structure (via X-ray diffraction (XRD)) and performance within solar cell communities, not so much attention has been devoted to their surface chemical composition and, specifically, the surface composition. Photoelectron spectroscopy (PES) can easily fulfill this task, and, in addition to chemical information, PES provides an overall picture of the electronic structure of the perovskite and its relation to mesoporous TiO2 when studied with hard X-rays. In this work, CH3NH3PbI3 and CH3NH3PbI3–xClx have been compared with each other and also to CH3NH3PbCl3, and it appears that, despite very different morphologies and kinetics of formation, the two former materials present a very similar electronic structure and chemical composition (i.e., no chlorine is observed in the final CH3NH3PbI3–xClx materials). Nevertheless, chlorine is very important during the preparation, because it affects the formation of crystalline CH3NH3PbI3. We have also exposed the classical CH3NH3PbI3 to various environments, such as water, temperature, and long-time storage in air and argon, and followed changes of the surface composition with PES. The main result of the different exposures is that the perovskite is decomposed into PbI2, but an important point is that this degradation seems to occur already at 100 °C and is not only related to large humidity. Indeed, even in an inert atmosphere such as argon, a slow degradation to PbI2 is observed. The results obtained are crucial for a better understanding of this material and will help to improve not only the post-conditioning of the cells but also their synthesis.

Perovskite Oxide SrTiO3 as an Efficient Electron Transporter for Hybrid Perovskite Solar Cells

In this work, we explored perovskite oxide SrTiO3 (STO) for the first time as the electron-transporting layer in organolead trihalide perovskite solar cells. The steady-state photoluminescence (PL) quenching and transient absorption experiments revealed efficient photoelectron transfer from CH3NH3PbI3–xClx to STO. Perovskite solar cells with meso-STO exhibit an open circuit voltage of 1.01 V, which is 25% higher than the value of 0.81 V achieved in the control device with the conventional meso-TiO2. In addition, an increase of 17% in the fill factor was achieved by tailoring the thickness of the meso-STO layer. We found that the application of STO leads to uniform perovskite layers with large grains and complete surface coverage, leading to a high shunt resistance and improved performance. These findings suggest STO as a competitive candidate as electron transport material in organometal perovskite solar cells.

Optimized Organometal Halide Perovskite Planar Hybrid Solar Cells via Control of Solvent Evaporation Rate

Organometallic halide perovskite-based solar cells have exhibited rapidly increasing efficiencies through the use of mesoporous composites. The addition of materials used in organic solar cells to perovskite-based solar cells (PSCs) enables the fabrication of low-cost, flexible, low-temperature, solution-processed PSCs. However, obtaining sufficient coverage of the organic layer, usually poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS), with CH3NH3PbI3–xClx films remains difficult in spite of the advances. In this study, we investigated the influence of controlling the solvent evaporation rate on the degree of PEDOT:PSS surface coverage by CH3NH3PbI3–xClx. We determined that an adequately fast spinning speed, drying at room temperature, and stepwise ramp annealing are critical for obtaining optimized planar hybrid perovskite solar cells with an ITO/PEDOT:PSS/CH3NH3PbI3–xClx/PCBM/Al structure and efficiencies of up to 11.8%.

Enhanced Crystallinity in Organic–Inorganic Lead Halide Perovskites on Mesoporous TiO2 via Disorder–Order Phase Transition

Organic–inorganic halide perovskite (OIHP) compounds are very interesting materials for device application in, for example, solar cells, electro-optics, and electronic circuits. In this report, we investigated OIHPs reported as CH3NH3PbI3–xClx (MAPbI3–xClx) and CH3NH3PbI3 (MAPbI3) prepared from solution on mesoporous TiO2/glass substrates. A long-term conversion from disordered to more crystalline OIHPs was observed for both types of samples from XRD patterns over 5 weeks. The conversion rate to more crystalline OIHPs could be increased by increasing the temperature of the sample. SEM analyses of the two types of OIHPs show remarkably different surface microstructures. The X-ray diffractograms suggest that both samples are dominated by the similar crystal structure, although the preferential orientation for the crystal structure is different. Moreover, the results suggest that the material reported as MAPbI3–xClx is a combination of MAPbI3 and MAPbCl3. The crystal structure and exact nature of the material is important for the understanding and optimization of devices, and the possibility for enhanced crystallinity of the OIHPs shown in this report will therefore be important for further improvement and understanding of the devices.

Thermally Induced Structural Evolution and Performance of Mesoporous Block Copolymer-Directed Alumina Perovskite Solar Cells

Structure control in solution-processed hybrid perovskites is crucial to design and fabricate highly efficient solar cells. Here, we utilize in situ grazing incidence wide-angle X-ray scattering and scanning electron microscopy to investigate the structural evolution and film morphologies of methylammonium lead tri-iodide/chloride (CH3NH3PbI3–xClx) in mesoporous block copolymer derived alumina superstructures during thermal annealing. We show the CH3NH3PbI3–xClx material evolution to be characterized by three distinct structures: a crystalline precursor structure not described previously, a 3D perovskite structure, and a mixture of compounds resulting from degradation. Finally, we demonstrate how understanding the processing parameters provides the foundation needed for optimal perovskite film morphology and coverage, leading to enhanced block copolymer-directed perovskite solar cell performance.

Influence of Thermal Processing Protocol upon the Crystallization and Photovoltaic Performance of Organic–Inorganic Lead Trihalide Perovskites

We investigate the thermally induced morphological and crystalline development of methylammonium lead mixed halide perovskite (CH3NH3PbI3–xClx) thin films and photovoltaic device performance with meso-superstructured and planar heterojunction architectures. We observe that a short rapid thermal annealing at 130 °C leads to the growth of large micron-sized textured perovskite domains and improved the short circuit currents and power conversion efficiencies up to 13.5% for the planar heterojunction perovskite solar cells. This work highlights the criticality of controlling the thin film crystallization mechanism of hybrid perovskite materials for high-performing photovoltaic applications.

Why Lead Methylammonium Tri-Iodide Perovskite-Based Solar Cells Require a Mesoporous Electron Transporting Scaffold (but Not Necessarily a Hole Conductor)

CH3NH3PbI3-based solar cells were characterized with electron beam-induced current (EBIC) and compared to CH3NH3PbI(3-x)Clx ones. A spatial map of charge separation efficiency in working cells shows p-i-n structures for both thin film cells. Effective diffusion lengths, LD, (from EBIC profile) show that holes are extracted significantly more efficiently than electrons in CH3NH3PbI3, explaining why CH3NH3PbI3-based cells require mesoporous electron conductors, while CH3NH3PbI(3-Clx ones, where LD values are comparable for both charge types, do not.

First-Principles Modeling of Mixed Halide Organometal Perovskites for Photovoltaic Applications

We computationally investigate organometal CH3NH3PbX3 and mixed halide CH3NH3PbI2X perovskites (X = Cl, Br, I), which are key materials for high efficiency solid-state solar cells. CH3NH3PbX3 perovskites exhibited the expected absorption blue shift along the I → Br → Cl series. The mixed halide systems surprisingly showed the CH3NH3PbI3 and the CH3NH3PbI2Cl (or CH3NH3PbI3–xClx) perovskites to have similar absorption onset at ∼800 nm wavelength, whereas CH3NH3PbI2Br absorbs light below ∼700 nm. To provide insight into the structural and electronic properties of these materials, in light of their application as solar cell active layers, we perform periodic DFT calculations on the CH3NH3PbX3 and CH3NH3PbI2X perovskites. We find a good agreement between the calculated band structures and the experimental trend of optical band gaps. For the mixed halide perovskites our calculations show the existence of two different types of structures with different electronic properties, whose relative stability varies by v...