A High-Performance UVA Photodetector Based on Polycrystalline Perovskite MAPbCl3/TiO2 Nanorods Heterojunctions.

The application of TiO2 nanorods in the field of ultraviolet (UV) photodetectors is hindered by a high dark current, which is attributed to crystal surface defects and intrinsic excitation by carrier thermal diffusion. Here, a photodetector based on polycrystalline perovskite MAPbCl3/TiO2 nanorods heterojunctions has been fabricated to overcome the shortcoming. The structure was composed of horizontal MAPbCl3 polycrystalline and vertically aligned TiO2 nanorods array. Many localized depletion regions at the MAPbCl3/TiO2 interface can reduce the dark current. The TiO2/MAPbCl3 detector shows high performance including a high ratio of light-dark current of about six orders of magnitude, which is much larger than that of the TiO2 detector. This study indicates the potential in the TiO2/MAPbCl3 heterojunction to fabricate high-performance UV detectors.

Structural Phase Transitions and Thermal Degradation Process of MAPbCl3 Single Crystals Studied by Raman and Brillouin Scattering.

Raman spectroscopy was applied to MAPbCl3 single crystals in a wide frequency range from 10 to 3500 cm-1 over a broad temperature range from -196 °C to 200 °C including both two structural phase transitions and a thermal degradation range. Low-frequency lattice modes of MAPbCl3 were revealed for the first time, which showed discontinuous anomalies along with the change in the number of Raman modes at the transition points of -114 °C and -110 °C. Several Raman modes related to the C-N stretching and MA rocking modes in addition to the lattice modes displayed temperature dependences similar to those of MAPbBr3 in both Raman shifts and half widths, indicating that the MA cation arrangement and H-halide bond interactions behave similarly in both systems during the phase transition. The substantial increase in the half widths of nearly all Raman modes especially suggests that the dynamic disorder caused by the free rotational motions of MA cations induces significant anharmonicity in the lattice and thus, reduces the phonon lifetimes. High-temperature Raman and Brillouin scattering measurements showed that the spectral features changed drastically at ~200 °C where the thermal decomposition of MAPbCl3 into PbCl2 began. This result exhibits that combined Raman and Brillouin spectroscopic techniques can be a useful tool in monitoring temperature-induced or temporal changes in lead-based halide perovskite materials.

Optimized photoelectric characteristics of MAPbCl3and MAPbBr3composite perovskite single crystal heterojunction photodetector.

MAPbBr3single crystal (SC) thin layer was successfully grown on MAPbCl3SC substrate to form perovskite SC heterojunction. Planar structure electrodes are deposited by thermal evaporation on the surfaces of MAPbCl3, MAPbBr3, and SCs heterojunction, respectively to evaluate their photoelectric performance. The SC heterojunction device exhibits excellent unidirectional conductivity in the voltage-current curves. Meanwhile, the current-time curves prove that SC heterojunction devices can effectively utilize the advantages of MAPbCl3and MAPbBr3, possessing relatively low dark current (∼300 nA), which is comparable to the dark current of MAPbCl3, but very high photocurrent (∼3500 nA), which is equivalent to the photocurrent of MAPbBr3. Rather than the photocurrent overshot and decay occurring at the exposure of light illumination in the MAPbBr3device, the photocurrent is extremely stable without overshot and decay in the SC heterojunction device. The light-to-dark ratio of the SC heterojunction device is twice that of MAPbCl3device and three times that of MAPbBr3device. Furthermore, the detectivity of the heterojunction device reaches as high as∼7×1011 Jones, an order of magnitude higher than MAPbCl3and MAPbBr3. The excellent characteristics of SC heterojunction further expand the practical application prospect of perovskite materials.

Preparation of Micron-sized Methylamine-PbCl3 perovskite grains by controlling phase transition engineering for selective Ultraviolet-harvesting transparent photovoltaics.

Selective ultraviolet-harvesting transparent perovskite solar cells (T-PSCs) have attracted great interest because of their high transmittance and unique photovoltaic properties, especially in the fields of smart windows for power generation and building glass. However, owing to the unsatisfactory solubility of PbCl2 in most conventional solvents, preparing transparent methylammonium lead chloride (MAPbCl3) films with high quality and sufficient thickness by conventional methods poses a substantial challenge for their application deployment in T-PSCs. In this work, two novel strategies based on an ion-exchange procedure for controlling phase transition engineering (CPTE) are proposed. For CPTE-2, an optimized cubic phase MAPbCl3 film with a large grain size and high full coverage is prepared by transforming the tetragonal phase MAPbI3 precursor into the cubic phase MAPbCl3. Establishing relevant models based on crystal parameters investigates the formation mechanism of this high-quality MAPbCl3 film. Accordingly, the resultant T-PSCs exhibit remarkable film quality and micron-sized grains and reach an optimum efficiency of 0.33% (JSC = 0.66 mA cm-2, VOC = 1.14 V, and FF = 43.72%).

Methylammonium lead chloride: A sensitive sample for an accurate NMR thermometer.

A new solid-state nuclear magnetic resonance (NMR) thermometry sample is proposed. The 207Pb NMR chemical shift of a lead halide perovskite, methylammonium lead chloride (MAPbCl3) is very sensitive to temperature, 0.905±0.010ppmK-1. The response to temperature is linear over a wide temperature range, from its tetragonal to cubic phase transition at 178K to >410K, making it an ideal standard for temperature calibrations in this range. Because the 207Pb NMR lineshape for MAPbCl3 appears symmetric, the sample is ideal for calibration of variable temperature NMR data acquired for spinning or non-spinning samples. A frequency-ratio method is proposed for referencing 207Pb chemical shifts, based on the 1H and 13C frequencies of the methylammonium cation, which are used asan internal standard. Finally, this new NMR thermometer has been used to measure the degree of frictional heating asa function of spinning frequency for a series of MAS rotors ranging in outer diameter from 1.3 to 7.0mm. As expected, the largest diameter rotors are more susceptible to frictional heating, but lower diameter rotors are subjected to higher frictional heating temperatures as they are typically spun at much higher spinning frequencies.