Terahertz Wave Absorption Property of all Mixed Organic-Inorganic Hybrid Perovskite Thin Film MA(Sn, Pb)(Br, I)3 Fabricated by Sequential Vacuum Evaporation Method.

All mixed hybrid perovskite (MA(Sn, Pb)(Br,I)3) thin film was fabricated by sequential vacuum evaporation method. To optimize the first layer with PbBr2 and SnI2, we performed different annealing treatments. Further, MA(Sn, Pb)(Br, I)3 thin film was synthesized on the optimized first layer by evaporating MAI and post-annealing. The formed hybrid perovskite thin film exhibited absorptions at 1.0 and 1.7 THz with small absorbance (<10%). Moreover, no chemical and structural defect-incorporated absorption was found. In this study, the possibility of changing terahertz absorption frequency through the mixture of metal cations (Sn+ and Pb+) and halogen anions (Br- and I-) was verified.

Surface Degradation Mechanism on CH3NH3PbBr3 Hybrid Perovskite Single Crystal by a Grazing E-Beam Irradiation.

To start a step such as some realization of minimized and integrated devices, it requires simply understanding the surface status of hybrid perovskite on the e-beam irradiation because many commercial semiconductor devices are performed with a surface patterning process using e-beam or etching gas. The surface status of CH3NH3PbBr3 (MAPbBr3) single crystal was studied after a grazing e-beam irradiation in an ultra-high vacuum. The prepared hybrid perovskite single crystal was irradiated by the 3 degree-grazing e-beam with energy of 15 kV for 10 min using a reflection high-electron energy diffraction technique. The e-beam irradiation on the MAPbBr3 hybrid perovskite single crystal induced the deformation from MAPbBr3 into MABr, Br2, and Pb on the surface. The gas phases of MABr and Br2 are depleted from the surface and the Pb element has remained on the surface. As a result of the e-beam irradiation, it formed a polycrystalline-like phase and Pb metal particles on the surface, respectively.

Strong Linear Correlation between CH3NH2 Molecular Defect and THz-Wave Absorption in CH3NH3PbI3 Hybrid Perovskite Thin Film.

To control the density of a CH3NH2 molecular defect, which strongly contributed to a significant THz-wave absorption property in the CH3NH3PbI3 hybrid perovskite thin film formed by the sequential vacuum evaporation method, we performed post-annealing processes with various temperatures and times. In the thin film after post-annealing at 110 °C for 45 min, the density of the CH3NH2 molecular defect was minimized, and CH3NH3I and PbI2 disappeared in the thin film after the post-annealing process at 150 °C for 30 min. However, the density of the CH3NH2 molecular defect increased. Moreover, the THz-wave absorption property for each thin film was obtained using a THz time-domain spectroscopy to understand the correlation between the density of a molecular defect and the THz-wave oscillation strength at 1.6 THz, which originated in the molecular defect-incorporated hybrid perovskite structure. There is a strong linear correlation between the oscillator strength of a significant THz-wave absorption at 1.6 THz and the CH3NH2 molecular defect density.

Clean interface without any intermixed state between ultra-thin P3 polymer and CH3NH3PbI3 hybrid perovskite thin film.

Hole transport layers (HTL) are crucial materials to improve the power conversion efficiency in organohalide hybrid perovskite-based solar-cell applications. Two important physical properties are required in HTL materials: good hole mobility and air-protection. After HTL solution-based deposition, an intermixed chemical state at the interface between HTL and hybrid perovskite is key to confirming the physical property of HTL. We performed high-resolution x-ray photoelectron spectroscopy to investigate the chemical states at the interface between an ultra-thin P3 polymer and CH3NH3PbI3 hybrid perovskite thin film. At the interface, we found no apparent intermixed chemical state. Furthermore, we confirmed that the P3 HTL with the ultra-thin layer (7 nm) protected the hybrid perovskite material against air-exposure for 2 weeks.

Significant THz absorption in CH3NH2 molecular defect-incorporated organic-inorganic hybrid perovskite thin film.

The valid strong THz absorption at 1.58 THz was probed in the organic-inorganic hybrid perovskite thin film, CH3NH3PbI3, fabricated by sequential vacuum evaporation method. In usual solution-based methods such as 2-step solution and antisolvent, we observed the relatively weak two main absorption peaks at 0.95 and 1.87 THz. The measured absorption spectrum is analyzed by density-functional theory calculations. The modes at 0.95 and 1.87 THz are assigned to the Pb-I vibrations of the inorganic components in the tetragonal phase. By contrast, the origin of the 1.58 THz absorption is due to the structural deformation of Pb-I bonding at the grain boundary incorporated with a CH3NH2 molecular defect.

Surface Instability of Sn-Based Hybrid Perovskite Thin Film, CH3NH3SnI3: The Origin of Its Material Instability.

To understand the instability of Sn-based perovskite, CH3NH3SnI3, photoelectron spectroscopy with synchrotron radiation and theoretical calculations, such as density functional theory and ab initio molecular dynamics, were performed. Findings from this experimental and theoretical study highlight the crucial changes of surface-chemical states, the broken chemical bondings in Sn-I, and the depletion of a CH3-NH3+ cation on the surface region. The material instability origin of Sn-based perovskite can be explained by the chemical state instability in the surface.

Substantial improvement of perovskite solar cells stability by pinhole-free hole transport layer with doping engineering.

We fabricated perovskite solar cells using a triple-layer of n-type doped, intrinsic, and p-type doped 2,2',7,7'-tetrakis(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) (n-i-p) as hole transport layer (HTL) by vacuum evaporation. The doping concentration for n-type doped spiro-OMeTAD was optimized to adjust the highest occupied molecular orbital of spiro-OMeTAD to match the valence band maximum of perovskite for efficient hole extraction while maintaining a high open circuit voltage. Time-dependent solar cell performance measurements revealed significantly improved air stability for perovskite solar cells with the n-i-p structured spiro-OMeTAD HTL showing sustained efficiencies even after 840 h of air exposure.

Flat-lying semiconductor-insulator interfacial layer in DNTT thin films.

The molecular order of organic semiconductors at the gate dielectric is the most critical factor determining carrier mobility in thin film transistors since the conducting channel forms at the dielectric interface. Despite its fundamental importance, this semiconductor-insulator interface is not well understood, primarily because it is buried within the device. We fabricated dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) thin film transistors by thermal evaporation in vacuum onto substrates held at different temperatures and systematically correlated the extracted charge mobility to the crystal grain size and crystal orientation. As a result, we identify a molecular layer of flat-lying DNTT molecules at the semiconductor-insulator interface. It is likely that such a layer might form in other material systems as well, and could be one of the factors reducing charge transport. Controlling this interfacial flat-lying layer may raise the ultimate possible device performance for thin film devices.

Self-assembled nanowires with giant Rashba split bands.

We investigated Pt-induced nanowires on the Si(110) surface using scanning tunneling microscopy (STM) and angle-resolved photoemission. High resolution STM images show a well-ordered nanowire array of 1.6 nm width and 2.7 nm separation. Angle-resolved photoemission reveals fully occupied one-dimensional (1D) bands with a Rashba-type split dispersion. Local dI/dV spectra further indicate well-confined 1D electron channels on the nanowires, whose density of states characteristics are consistent with the Rashba-type band splitting. The observed energy and momentum splitting of the bands are among the largest ever reported for Rashba systems, suggesting the Pt-Si nanowire as a unique 1D giant Rashba system. This self-assembled nanowire can be exploited for silicon-based spintronics devices as well as the quest for Majorana fermions.

Surface property change of graphene using nitrogen ion.

We introduced nitrogen ions to modify the graphene surface and its property changes were investigated. A graphene layer grown on 6H-SiC(0001) was irradiated with 100 eV nitrogen ions. Surface property changes were studied using photoemission spectroscopy (PES), near edge x-ray adsorption spectroscopy (NEXAFS), and atomic force microscopy(AFM). N 1s core level spectra show that three kinds of nitrogen species, nitrogen gas, graphite-like and pyridine-like nitrogen were induced on the nitrogen ion implanted graphene surface.