Route For Improving The Stability Of Perovskite CH₃NH₃PbI₃ And The Performance Of Their Corresponding Thin Film Devices

In organic-inorganic hybrid semiconductor materials have attracted many attention,because the complementary advantages can be achieved by adjusting the interfaces between organic and inorganic components.The organometal halide perovskite materials have become a hot topic for scientists and have made great progress in optoelectronic devices due to their high absorption coefficient,tunable band gap,long carrier diffusion lifetime,simple preparation process and so forth.For organic-inorganic hybrid semiconductors,the stability and high-quality films directly determine the long-term stability,performance and reproducibility of the optoelectronic devices.Herein,the dissociation processes of MAPbI3 and CdI2(AD)were investigated by thermal analysis,in situ powder X-ray diffraction measurement(XRD),as well as in situ Fourier transformation infrared(FTIR)spectroscopy at different atmospheric and pressures.Furthermore,we compared the structure and morphology of MAPbI3 films prepared by different methods and effect of film quality on the photodetectors.On this basis,we had prepared ultrasensitive photodetectors.In addition,we prepared MAPbI3 thin film gas sensors and investigated their gas-sensing properties under both ambient and high pressures.In order to improve the performance and long-term stability of the devices,the following works were carried out:1,Investigation on the stability of MAPbIb by in-situ FTIR spectroscopiesHerein,we investigated the thermal decomposition process of MAPbIb under different oxygen concentrations and pressures using an in-situ FTIR spectroscopies.It was found that MAPbIb perovskite was rather sensitive to oxygen.It began to decompose at 150 ? when 1%(volume fraction)oxygen was used in the experiments and dissociation temperature of this perovskite declined to 100 ? under 21%oxygen environment.It was quite interesting that its decomposition temperature strikingly increased to 250 ? when it was heated in pure nitrogen of atmospheric pressure.It was rather surprising that the decomposition temperature of MAPbI3 further increased to 270 ? in a nitrogen atmosphere of 4.0 MPa.This phenomenon revealed that isolating oxygen and rising pressure can effectively improve the stability of the MAPbI3 materials.2,Preparation of the MAPbI3 films and its gas-sensing propertiesA room-temperature NO2 gas sensor with quick response and high selectivity was successfully fabricated using an MAPbI3 thin film by one-step spin-coating method.It presented a high response 3.3 even under the detection concentration of NO2 as low as 1 ppm.Its average response and recovery times were only-5 s and-25 s at room temperature and atmospheric pressure,respectively,exhibiting its quick-responsive character.Interestingly,the sensitivity of the MAPbI3-based NO2 sensor was strikingly improved under high pressure gas conditions.The MAPbI3 film sensor still exhibited rather good selectivity and quick response and recovery times of 11?25 s and 20?60 s under high pressures.Additionally,theoretical simulation of the adsorption of NO2 molecules on the(110)surface of perovskite was conducted to illustrate the response mechanism of the gas sensor.3,The preparation of high-quality MAPbI3 films and its photosensitive propertiesWe demonstrated a pressure-assisted space-confined solvent-engineering technique(PSS)to grow highly oriented,pinhole free thin films of MAPbI3 with large-scale crystalline grains,high smoothness and crystalline fusion on grain boundaries.The perovskite films with excellent features dramatically reduced recombination loss and then improved the transport property of charge carriers in the film.Compared with the conventional annealing films,carrier lifetime of the MAPbI3 films prepared by PSS strategy was improved by two orders,as high as?2.01 ?s(?2).Consequently,the photodetector devices,based on the high-quality MAPbI3 films,exhibited high photocurrent(105 ?A under 671 nm laser with a power density of 20.6 mW/cm2 at 10 V),an ultrahigh Ilight/Idark ratio(>2.2 ×104 under an incident light of 20.6 mW/cm2),quick response speed(-23 ?s)and,especially,good stability.These excellent performances indicated that the high-quality films will be potential candidates in other MAPbI3-based photoelectric devices.4,Investigation on the stability of Cdl2(AD)hybrid semiconductor by in-situ methodsThe thermal dissociation process and mechanism of organic-inorganic hybrid semiconductor CdI2(AD)were investigated by in-situ FTIR and XRD spectroscopies.The off-plane vibration modes of acridine rings with ?-? stacks were greatly suppressed before the decomposition temperature of CdI2(AD),while the in-plane ones were the dominant modes.The interactions between acridine weakened with temperature rising,resulting in the off-plane vibration modes appearing.It began to decompose at?190?under atmosphere pressure in N2 atmosphere.The released gaseous products and remaining products were acridine and CdI2,respectively.Furthermore,its dissociation temperature increased by?30? in a nitrogen atmosphere of 4.0 MPa.It was quite interesting that the dissociation process of CdI2(AD)hybrid crystals were greatly suppressed under even higher pressures.The decomposition temperature of CdI2(AD)further increased to 300? under 250 MPa.These results will be very helpful for the thermal annealing treatment of organic-inorganic hybrid semiconductor film devices.