Despite its competitive photovoltaic efficiency, the structural transformations of the prototypical hybrid perovskite, methylammonium lead iodide, are facilitated by interactions with polar molecules. Changes in optical and electronic properties upon exposure to ammonia potentially can enable the use of hybrid perovskites in gas-sensing applications. We investigated the effects of ammonia on CHNHPbI by exposing perovskite films to a wide range of vapor pressures. Spectroscopic analyses indicated that ammonium cations replaced the methylammonium cations in the perovskite crystal, thereby resulting in the formation of NHPbI. The transformation of CHNHPbI to NHPbI caused distinct changes in the morphology of the film and its crystalline structure; however, the introduction of CHNH gas reversed these changes. An in-depth understanding of the reversible chemical and structural alterations resulting from exposure to polar molecules can advance the development of hybrid perovskite sensors and provide insight into mechanisms by which perovskites convert due to interactions with polar molecules.
Following the remarkable success of the application of CH3NH3PbI3 perovskites in photovoltaics, a great focus has been placed on their stability to improve their optoelectronic properties and seek commercial production. To gain a better understanding of their thermal stability, we investigated the chemical, morphological, and photovoltaic transformations of CH3NH3PbI3 perovskites under elevated temperatures and various controlled atmospheric conditions (vacuum, 1 mbar O2, and 1 mbar H2O). A temperature-dependent study showed that CH3NH3PbI3 decomposed to PbI2 with the release of CH3NH2 and HI under low-temperature annealing (25–150 °C). Further annealing resulted in the formation of metallic lead (Pb0) under vacuum and Pb oxides and hydroxides under an O2 or H2O pressure. Moreover, the sublimation of Pb-based compounds occurred at temperatures above 150 °C, causing structural changes, which resulted in a decrease in the power conversion efficiency of the solar cell. A time-dependent study showed that, compared with vacuum conditions, the addition of O2 or H2O accelerated the degradation of the perovskite films.
Methylammonium lead iodide perovskite (MAPbI3 ), a prototype material for potentially high-efficient and low-cost organic-inorganic hybrid perovskite solar cells, has been investigated intensively in recent years. A study of low-energy electron-induced transformations in MAPbI3 is presented, performed by combining controlled electron-impact irradiation with X-ray photoelectron spectroscopy and scanning electron microscopy. Changes were observed in both the elemental composition and the morphology of irradiated MAPbI3 thin films as a function of the electron fluence for incident energies from 4.5 to 60 eV. The results show that low-energy electrons can affect structural and chemical properties of MAPbI3 . It is proposed that the transformations are triggered by the interactions with the organic part of the material (methylammonium), resulting in the MAPbI3 decomposition and aggregation of the hydrocarbon layer.
Template-free synthesis of phase pure one-dimensional (1D), single crystalline rutile titania nanorods or wires at low temperature still remains a challenging task due to its complex nature of surface chemistry. In these 1D structures, charge transport is highly favored. To further modify the electrical conductivity and optoelectronic properties of these 1D nanostructures, various methods such as doping of TiO2 with metal and nonmetal and synthesis of branched and hybrid structures are developed. If these hybrid structures can directly synthesize on the substrate, the transport of the electron will improve due to reduced grain boundary and exciton recombination. In this contribution, for the first time, we have simultaneously synthesized 1D-rutile TiO2-multiwalled carbon nanotube (MWCNT) composite film directly grown on fluorine dope conducting oxide (FTO) substrate along with 1D-rutile TiO2-MWCNT composite powder. The as-grown nanorods films were single-crystalline and oriented vertically with respect to the substrate, having an average height of ∼2 μm. The well connected network of TiO2 with MWCNTs was observed through electron microscopy. The composite film shows positive movement of the flat-band edge and increase in charge carrier density. The TiO2-MWCNT composite was successfully used as photoanode in a dye sensitized solar cell (DSSC) and exhibits a 60% increase in energy-conversion efficiency compared with only TiO2 nanorods.
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