Organic−inorganic hybrid halide perovskites have emerged recently as highly promising materials for optoelectronics such as photovoltaics and photodetectors. A unique feature of these materials is ion diffusion that directly impacts the optoelectronic process by affecting the charge transport and trapping. In order to shed light on the ionic diffusion behavior, the hybrid perovskites MAPbI 3 and MAPbI 3 with minor doping of phenyl-C61-butyric acid methyl-ester (MAPbI 3 -PCBM) thin-film capacitors were investigated in the presence of steady and dynamic visible illumination of different intensities. Light-induced capacitance, which increases monotonically with the increase of light intensity, was observed in the low-frequency range below 300 kHz of the electric field on both while differing quantitatively. Specifically, the large light-induced capacitance in the MAPbI 3 capacitors can be obtained in the MAPbI 3 -PCBM ones in the dark. In addition, the increase of capacitance with light intensity is much less in the latter with electron trapping induced by PCBM. This result has revealed that the light-induced capacitance in MAPbI 3 capacitors can be ascribed to the contribution of the additional charges across the capacitors associated with ionic diffusion activated by the illumination and that the effects on the capacitance will remain after the illumination is turned off due to residual photoexcited electrons trapped in the MAPbI 3 -PCBM sample.

Halide perovskites intrinsically contain a large amount of point defects. The interaction of these defects with photocarriers, photons, and lattice distortion remains a complex and unresolved issue. We found that for halide perovskite films with excess halide vacancies, the Fermi level can be shifted by as much as 0.7 eV upon light illumination. These defects can trap photocarriers for hours after the light illumination is turned off. The enormous light-induced Fermi level shift and the prolonged electron trapping are explained by the capturing of photocarriers by halide vacancies at the surface of the perovskite film. The formation of this defect−photocarrier complex can result in lattice deformation and an energy shift in the defect state. The whole process is akin to polaron formation at a defect site. Our data also suggest that these trapped carriers increase the electrical polarizability of the lattice, presumably by enhancing the defect migration rate.

Continuous device downsizing and circuit complexity have motivated atomic-scale tuning of memristors. Herein, we report atomically tunable Pd/M1/M2/Al ultrathin (<2.5 nm M1/M2 bilayer oxide thickness) memristors using in vacuo atomic layer deposition by controlled insertion of MgO atomic layers into pristine Al2O3 atomic layer stacks guided by theory predicted Fermi energy lowering leading to a higher high state resistance (HRS) and a reduction of oxygen vacancy formation energy. Excitingly, memristors with HRS and on/off ratio increasing exponentially with M1/M2 thickness in the range 1.2–2.4 nm have been obtained, illustrating tunneling mechanism and tunable on/off ratio in the range of 10–104. Further dynamic tunability of on/off ratio by electric field is possible by designing of the atomic M2 layer and M1/M2 interface. This result probes ways in the design of memristors with atomically tunable performance parameters.

Recently, disordered spinel MgAl2O4 as insulating tunnel barriers for perpendicular magnetic tunnel junctions has attracted interest due to their observed high tunneling magnetoresistance (TMR) and excellent voltage response. Motivated by this, we report the first success in the synthesis of ultrathin films (0.33–4.29 nm) of MgAl2O4 using in vacuo atomic layer deposition (ALD) on Fe and Al electrodes. The electronic properties of samples were evaluated using in situ scanning tunneling spectroscopy. Intriguingly, the sequence of the ALD Al2O3 and ALD MgO was found to dramatically impact the electronic structure of the ALD MgAl2O4, which may be attributed to the different initial adsorption mechanisms of ALD MgO and ALD Al2O3, as revealed in the molecular dynamics simulation. The optimum sequence for the first unit cell (or supercycle) of MgAl2O4 is two ALD Al2O3 cycles followed by one ALD MgO cycle. At three supercycles (0.99 nm), a much higher conduction band minimum (CBM) of 1.71 eV was observed, in contrast to 1.58 or 1.45 eV, which were observed when beginning the supercycles with 1 cycle of Al2O3 (0.11 nm) followed by 1 cycle of MgO (0.11 nm) or only 1 cycle of MgO, respectively. Decreasing the number of supercycles from 3 (∼0.99 nm) to 1 supercycle (∼0.33 nm) resulted in a monotonic decrease in CBM from 1.71 to 1.49 eV, showing some frustration of growth during earlier atomic layer deposition cycles. Additionally, growth on a Fe layer showed a moderate CBM of 1.25 eV. Nevertheless, the observed CBM in the ultrathin ALD MgAl2O4 greatly exceeds that of thermally oxidized AlO x barriers (∼0.6 eV) and is similar to that of high-quality ALD-grown Al2O3 (∼1.7 eV) and MgO grown with an Al2O3 seed layer (∼1.50 eV) of comparable total thickness in the ultrathin range. The high CBM values are indicative of a low defect concentration in the ultrathin ALD MgAl2O4, which is supported by a high dielectric constant of 8.85 (comparable to that of the crystalline MgAl2O4 bulk) observed for a 4.3 nm thick ALD MgAl2O4 film capacitor.