Reversible, Fast, and Wide-Range Oxygen Sensor Based on Nanostructured Organometal Halide Perovskite

Gas Sensing Properties of Perovskite Decorated Graphene at Room Temperature

This paper explores the gas sensing properties of graphene nanolayers decorated with lead halide perovskite (CH3NH3PbBr3) nanocrystals to detect toxic gases such as ammonia (NH3) and nitrogen dioxide (NO2). A chemical-sensitive semiconductor film based on graphene has been achieved, being decorated with CH3NH3PbBr3 perovskite (MAPbBr3) nanocrystals (NCs) synthesized, and characterized by several techniques, such as field emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. Reversible responses were obtained towards NO2 and NH3 at room temperature, demonstrating an enhanced sensitivity when the graphene is decorated by MAPbBr3 NCs. Furthermore, the effect of ambient moisture was extensively studied, showing that the use of perovskite NCs in gas sensors can become a promising alternative to other gas sensitive materials, due to the protective character of graphene, resulting from its high hydrophobicity. Besides, a gas sensing mechanism is proposed to understand the effects of MAPbBr3 sensing properties.

Large scale quantum dynamics investigations on the sensing mechanism of H2O, acetone, NO2 and O3 adsorption on the (MA)2Pb(SCN)2I2 surface

The instability of organometal halide perovskites still remains a key obstacle restricting their practical application in gas sensing research. The first step in gas sensing using a semiconductor material is the recognition of a target gas through gas-solid interaction. In the current work, the adsorption mechanisms of MAPbI3-H2O, (MA)2Pb(SCN)2I2-H2O, (MA)2Pb(SCN)2I2-CH3COCH3, (MA)2Pb(SCN)2I2-NO2 and (MA)2Pb(SCN)2I2-O3 have been investigated by large-scale quantum dynamics simulations. The structural changes of the perovskite skeleton, the adsorption energy, and the charge transfer between the semiconductor material and the gas molecules have been analysed. The suitability and effectiveness of quantum dynamics simulations in adsorption mechanism research are firstly validated by comparing the humidity sensing mechanisms of MAPbI3 and (MA)2Pb(SCN)2I2. Different sensing mechanisms of (MA)2Pb(SCN)2I2 to gases with different oxidising properties have been proposed. These sensing mechanisms hopefully lay a foundation for the development of novel perovskite gas sensing materials with enhanced stability, high sensitivity, and high selectivity.

Giant Humidity Effect on Hybrid Halide Perovskite Microstripes: Reversibility and Sensing Mechanism

Despite the exceptional performance of hybrid perovskites in photovoltaics, their susceptibility to ambient factors, particularly humidity, gives rise to the well-recognized stability issue. In the present work, microstripes of CH₃NH₃PbI₃ are fabricated on flexible substrates, and they exhibit much larger response to relative humidity (RH) levels than continuous films and single crystals. The resistance of microstripes decreases by four orders of magnitude on changing the RH level from 10 to 95%. Fast response and recovery time of 100 and 500 ms, respectively, are recorded. Because bulk diffusion and defect trapping are much slower processes, our result indicates a surface-dictated mechanism related to hydrate formation and electron donation. In addition, water uptake behavior of perovskites is studied for the first time, which correlates well with the resistance decrease of the CH₃NH₃PbI₃ microstripes. Furthermore, we report that the photoresponse decreases with increasing humidity, and at the 85% RH level, the perovskite device is not photoresponsive anymore. Our work underscores patterned structures as a new platform to investigate the interaction of hybrid perovskites with ambient factors and reveals the importance of the humidity effect on optoelectronic performance.

Ligand-free all-inorganic metal halide nanocubes for fast, ultra-sensitive and self-powered ozone sensors

Ligand-free all-inorganic lead halide nanocubes have been investigated as ozone sensing materials operating at room temperature. It is found that the nanocubes, crystallined in the orthorhombic CsPbBr₃ structure, can operate at room temperature, be self-powered and exhibit high sensitivity and remarkable repeatability. More importantly, they demonstrate higher sensitivity (54% in 187 ppb) and faster response and recovery times compared to hybrid lead mixed halide perovskite (CH₃NH₃PbI_3-x Cl _x ) layers, which is the only lead halide perovskite material tested for ozone sensing, to date. Following the exposure to an ozone environment, the stoichiometry and the morphology of the nanocubes remain unaltered. The facile and easy fabrication process together with the high responsivity and stability to the ozone environment makes the bare CsPbBr₃ nanocubes a promising material for sensing applications. The sensing properties of the nanoparticulate metal halides presented here provide new exciting opportunities towards engineering reliable and cheap sensing elements for room-temperature operated and self-powered sensors.

Environmental Gating and Galvanic Effects in Single Crystals of Organic-Inorganic Halide Perovskites

Understanding the impact of environmental gaseous on the surface of organometal halide perovskites (OMHPs) couples to the electronic and ionic transport is critically important. Here, we explore the transport behavior and origins of the gas sensitivity in MAPbBr₃ single crystals (SCs) devices using impedance spectroscopy and current relaxation measurements. Strong resistive response occurs when crystals are exposed to different environments. It was shown that SC response to the environment is extremely different at the surface as compared to the bulk due to the disorder surface chemistry. The nonlinear transport properties studied using ultrafast Kelvin probe force microscopy (G-KPFM) to unravel spatio-temporal charge dynamics at SC/electrode interface. The relaxation processes observed in pulse relaxation and G-KPFM measurements along with gas sensitivity of crystals suggest the presence of a triple-phase boundary between environment, electrode, and crystal. Results indicate that the environment is a nontrivial component in the operation of OMHP devices which is reminiscent of fuel cell systems. Furthermore, the triple-phase boundary can play a significant role in the transport properties of OMHPs due to the possibility of the redox processes coupled to the concentration of bulk ionic species. Although instrumental for understanding the device characteristics of perovskites, our studies suggest a new opportunity of coupling the redox chemistry of the Br₂-Br^- pair that defines the bulk ionic conductivity of MAPbBr₃ with the redox chemistry of gaseous (or liquid) environment via a suitable electrocatalytic system to enable new class of energy storage devices and gas sensors.

Perfection of Perovskite Grain Boundary Passivation by Eu-Porphyrin Complex for Overall-Stable Perovskite Solar Cells

The formation of defects at surfaces and grain boundaries (GBs) during the fabrication of solution-processed perovskite film are thought to be responsible for its instability. Herein, Eu-porphyrin complex (Eu-pyP) is directly doped into methylammonium lead triiodide (MAPbI3) precursor, perfectly fabricating 2D (Eu-pyP)0.5MA n -1Pb n I3 n +1 platelets inlaying the GBs of 3D polycrystalline interstices in this protocol. The device based on Eu-pyP doped perovskite film possesses a champion efficiency of 18.2%. More importantly, the doped perovskite solar cells device shows beyond 85% retention of its pristine efficiency value, whereas the pure MAPbI3 device has a rapid drop in efficiency down to 10% within 100 h under 45% humidity at 85 °C in AM 1.5 G. The above acquired perovskite films reveal an unpredictable thermodynamic self-healing ability. Consequently, the findings provide an avenue for defect passivation to synchronously improve resistibility to moisture, heat, and solar light including UV.

Organic Monomolecular Layers Enable Energy-Level Matching for Efficient Hole Transporting Layer Free Inverted Perovskite Solar Cells

High-efficiency hole transport layer free perovskite solar cells (HTL-free PSCs) with economical and simplified device structure can greatly facilitate the commercialization of PSCs. However, eliminating the key HTL in PSCs results usually in a severe efficiency loss and poor carrier transfer due to the energy-level mismatching at the indium tin oxide (ITO)/perovskite interface. In this study, we solve this issue by introducing an organic monomolecular layer (ML) to raise the effective work function of ITO with the assistance of an interface dipole created by Sn-N bonds. The energy-level alignment at the ITO/perovskite interface is optimized with a barrier-free contact, which favors efficient charge transfer and suppressed nonradiative carrier recombination. The HTL-free PSCs based on the ML-modified ITO yield an efficiency of 19.4%, much higher than those of HTL-free PSCs on bare ITO (10.26%), comparable to state-of-the-art PSCs with a HTL. This study provides an in-depth understanding of the mechanism of interfacial energy-level alignment and facilitates the design of advanced interfacial materials for simplified and efficient PSC devices.

Superior Self-Powered Room-Temperature Chemical Sensing with Light-Activated Inorganic Halides Perovskites

Hybrid halide perovskite is one of the promising light absorber and is intensively investigated for many optoelectronic applications. Here, the first prototype of a self-powered inorganic halides perovskite for chemical gas sensing at room temperature under visible-light irradiation is presented. These devices consist of porous network of CsPbBr₃ (CPB) and can generate an open-circuit voltage of 0.87 V under visible-light irradiation, which can be used to detect various concentrations of O₂ and parts per million concentrations of medically relevant volatile organic compounds such as acetone and ethanol with very quick response and recovery time. It is observed that O₂ gas can passivate the surface trap sites in CPB and the ambipolar charge transport in the perovskite layer results in a distinct sensing mechanism compared with established semiconductors with symmetric electrical response to both oxidizing and reducing gases. The platform of CPB-based gas sensor provides new insights for the emerging area of wearable sensors for personalized and preventive medicine.

Solution Processed CH3NH3PbI3-xClx Perovskite Based Self-Powered Ozone Sensing Element Operated at Room Temperature

Hybrid lead halide spin coated perovskite films have been successfully tested as portable, flexible, operated at room temperature, self-powered, and ultrasensitive ozone sensing elements. The electrical resistance of the hybrid lead mixed halide perovskite (CH₃NH₃PbI_3-xCl_x) sensing element, was immediately decreased when exposed to an ozone (O₃) environment and manage to recover its pristine electrical conductivity values within few seconds after the complete removal of ozone gas. The sensing measurements showed different response times at different gas concentrations, good repeatability, ultrahigh sensitivity and fast recovery time. To the best of our knowledge, this is the first time that a lead halide perovskite semiconductor material is demonstrating its sensing properties in an ozone environment. This work shows the potential of hybrid lead halide based perovskites as reliable sensing elements, serving the objectives of environmental control, with important socioeconomic impact.

Surface potential mapping and n-type conductivity in organic–inorganic lead iodide crystals

We demonstrated the electronic structure of CH₃NH₃PbI₃ and its variation on the surface via surface potential and work function distribution.

Defects in metal triiodide perovskite materials towards high-performance solar cells: origin, impact, characterization, and engineering

The progress of defect science in metal triiodide perovskite is critically reviewed, including the origin, impacts, characterization, and engineering.