Perovskite solar cells with atomically coherent interlayers on SnO2 electrodes.

In perovskite solar cells, the interfaces between the perovskite and charge-transporting layers contain high concentrations of defects (about 100 times that within the perovskite layer), specifically, deep-level defects, which substantially reduce the power conversion efficiency of the devices1-3. Recent efforts to reduce these interfacial defects have focused mainly on surface passivation4-6. However, passivating the perovskite surface that interfaces with the electron-transporting layer is difficult, because the surface-treatment agents on the electron-transporting layer may dissolve while coating the perovskite thin film. Alternatively, interfacial defects may not be a concern if a coherent interface could be formed between the electron-transporting and perovskite layers. Here we report the formation of an interlayer between a SnO2 electron-transporting layer and a halide perovskite light-absorbing layer, achieved by coupling Cl-bonded SnO2 with a Cl-containing perovskite precursor. This interlayer has atomically coherent features, which enhance charge extraction and transport from the perovskite layer, and fewer interfacial defects. The existence of such a coherent interlayer allowed us to fabricate perovskite solar cells with a power conversion efficiency of 25.8 per cent (certified 25.5 per cent)under standard illumination. Furthermore, unencapsulated devices maintained about 90 per cent of their initial efficiency even after continuous light exposure for 500 hours. Our findings provide guidelines for designing defect-minimizing interfaces between metal halide perovskites and electron-transporting layers.

Defect States in InP/InGaAs/InP Heterostructures by Current-Voltage Characteristics and Deep Level Transient Spectroscopy.

We studied defect states in In0.53Ga0.47As/InP heterojunctions with interface control by group V atoms during metalorganic chemical vapor (MOCVD) deposition. From deep level transient spectroscopy (DLTS) measurements, two defects with activation energies of 0.28 eV (E1) and 0.15 eV (E2) below the conduction band edge, were observed. The defect density of E1 for In0.53Ga0.47As/InP heterojunctions with an addition of As and P atoms was about 1.5 times higher than that of the heterojunction added P atom only. From the temperature dependence of current- voltage characteristics, the thermal activation energies of In0.53Ga0.47As/InP of heterojunctions were estimated to be 0.27 and 0.25 eV, respectively. It appeared that the reverse light current for In0.53Ga0.47As/InP heterojunction added P atom increased only by illumination of a 940 nm-LED light source. These results imply that only the P addition at the interface can enhance the quality of InGaAs/InP heterojunction.

Structural properties of indium tin oxide thin films by glancing angle deposition method.

We have studied the structural and optical properties of indium tin oxide (ITO) films deposited on sapphire substrates by electron beam evaporator with glancing angle deposition method. The ITO films were grown with different deposition angles of 0 degrees, 30 degrees, 45 degrees, 60 degrees at fixed deposition rate of 3 angstroms/s and with deposition rates of 2 angstroms/s, 3 angstroms/s, and 4angstroms/s at deposition angle of 45 degrees, respectively. From analysis of ellipsometry measurements, it appears that the void fraction of the films increased and their refractive indices decreased from 2.18 to 1.38 at the wavelength of 500 as increasing the deposition angle. The refractive index in the wavelength ranges of 550 nm-800 nm also depends on the deposition rates. Transmittance of ITO film with 235-nm-thickness grown at 60 degrees was covered about 20-80%, and then it was increased in visible wavelength range with increase of deposition angle.

Effects of Thermal Annealing on Optical and Microscopic Ferromagnetic Properties in InZnP:Ag Nano-Rods.

InZnP:Ag nano-rods fabricated by the ion milling method were thermally annealed in the 250~350 °C temperature range and investigated the optimum thermal annealing conditions to further understand the mutual correlation between the optical properties and the microscopic magnetic properties. The formation of InZnP:Ag nano-rods was determined from transmission electron microscopy (TEM), total reflectivity and Raman scattering analyses. The downward shifts of peak position for LO and TO modes in the Raman spectrum are indicative of the production of Ag ion-induced strain during the annealing process of the InZnP:Ag nano-rod samples. The appearance of two emission peaks of both (A0 X) and (e, Ag) in the PL spectrum indicated that acceptor states by Ag diffusion are visible due to the effective incorporation of Ag-creating acceptor states. The binding energy between the acceptor and the exciton measured as a function of temperature was found to be 21.2 meV for the sample annealed at 300 °C. The noticeable MFM image contrast and the clear change in the MFM phase with the scanning distance indicate the formation of the ferromagnetic spin coupling interaction on the surface of InZnP:Ag nano-rods by Ag diffusion. This study suggests that the InZnP:Ag nano-rods should be a potential candidate for the application of spintronic devices.

Strain Tuning via Larger Cation and Anion Codoping for Efficient and Stable Antimony-Based Solar Cells.

Strain induced by lattice distortion is one of the key factors that affect the photovoltaic performance via increasing defect densities. The unsatisfied power conversion efficiencies (PCEs) of solar cells based on antimony chalcogenides (Sb-Chs) are owing to their photoexcited carriers being self-trapped by the distortion of Sb2S3 lattice. However, strain behavior in Sb-Chs-based solar cells has not been investigated. Here, strain tuning in Sb-Chs is demonstrated by simultaneously replacing Sb and S with larger Bi and I ions, respectively. Bi/I codoped Sb2S3 cells are fabricated using poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-enzothiadiazole)] as the hole-transporting layer. Codoping reduced the bandgap and rendered a bigger tension strain (1.76 × 10-4) to a relatively smaller compression strain (-1.29 × 10-4). The 2.5 mol% BiI3 doped Sb2S3 cell presented lower trap state energy level than the Sb2S3 cell; moreover, this doping amount effectively passivated the trap states. This codoping shows a similar trend even in the low bandgap Sb2(SxSe1-x)3 cell, resulting in 7.05% PCE under the standard illumination conditions (100 mW cm-2), which is one of the top efficiencies in solution processing Sb2(SxSe1-x)3 solar cells. Furthermore, the doped cells present higher humidity, thermal and photo stability. This study provides a new strategy for stable Pb-free solar cells.

Impact of strain relaxation on performance of α-formamidinium lead iodide perovskite solar cells.

High-efficiency lead halide perovskite solar cells (PSCs) have been fabricated with α-phase formamidinium lead iodide (FAPbI3) stabilized with multiple cations. The alloyed cations greatly affect the bandgap, carrier dynamics, and stability, as well as lattice strain that creates unwanted carrier trap sites. We substituted cesium (Cs) and methylenediammonium (MDA) cations in FA sites of FAPbI3 and found that 0.03 mol fraction of both MDA and Cs cations lowered lattice strain, which increased carrier lifetime and reduced Urbach energy and defect concentration. The best-performing PSC exhibited power conversion efficiency >25% under 100 milliwatt per square centimeter AM 1.5G illumination (24.4% certified efficiency). Unencapsulated devices maintained >80% of their initial efficiency after 1300 hours in the dark at 85°C.

Electronic Transport Mechanism for Schottky Diodes Formed by Au/HVPE a-Plane GaN Templates Grown via In Situ GaN Nanodot Formation.

We investigate the electrical characteristics of Schottky contacts for an Au/hydride vapor phase epitaxy (HVPE) a-plane GaN template grown via in situ GaN nanodot formation. Although the Schottky diodes present excellent rectifying characteristics, their Schottky barrier height and ideality factor are highly dependent upon temperature variation. The relationship between the barrier height, ideality factor, and conventional Richardson plot reveals that the Schottky diodes exhibit an inhomogeneous barrier height, attributed to the interface states between the metal and a-plane GaN film and to point defects within the a-plane GaN layers grown via in situ nanodot formation. Also, we confirm that the current transport mechanism of HVPE a-plane GaN Schottky diodes grown via in situ nanodot formation prefers a thermionic field emission model rather than a thermionic emission (TE) one, implying that Poole⁻Frenkel emission dominates the conduction mechanism over the entire range of measured temperatures. The deep-level transient spectroscopy (DLTS) results prove the presence of noninteracting point-defect-assisted tunneling, which plays an important role in the transport mechanism. These electrical characteristics indicate that this method possesses a great throughput advantage for various applications, compared with Schottky contact to a-plane GaN grown using other methods. We expect that HVPE a-plane GaN Schottky diodes supported by in situ nanodot formation will open further opportunities for the development of nonpolar GaN-based high-performance devices.

Electronic states of deep trap levels in a-plane GaN templates grown on r-plane sapphire by HVPE.

We report on the defect states incorporated in a-plane GaN crystals grown on r-plane sapphire substrates by hydride vapor phase epitaxy (HVPE), using deep level transient spectroscopy (DLTS). Two defect states were observed at 0.2 eV and 0.55 eV below the conduction band minimum with defect densities of 5 × 1012/cm3 and 4.7 × 1013/cm3, respectively. The size of capture cross section, non-linear relation of trap densities from the depth profile, filling pulse width, and PL measurements indicated that the electronic deep trap levels in a-plane GaN on r-plane sapphire by HVPE originated from non-interacting point defects such as NGa, complex defects involving Si, O, or C, and VGa-related centres. Even though the a-plane GaN templates were grown by HVPE with high growth rates, the electronic deep trap characteristics are comparable to those of a-plane GaN layers of high crystal quality grown by MOCVD. This study prove that the growth of a-plane GaN templates on r-plane sapphire by HVPE is a promising method to obtain a-plane GaN layers efficiently and economically without the degradation of electrical characteristics.

Work Function Modification of Tungsten-Doped Indium Oxides Deposited by the Co-Sputtering Method.

We have studied the work function modification of tungsten-doped indium oxides (IWOs) through the co-sputtering of indium oxide (In2O3) and indium tungsten oxide (In2O3 80 wt% + WO3 20 wt%) via a radio frequency (RF) magnetron sputtering system. By controlling the elemental deposition of IWOs, the resultant work functions varied from 4.37 eV to 4.1 eV. The IWO thin films showed excellent properties for application as transparent conducting oxide materials in the region of 0 to 2.43 at.% of tungsten versus the total metal content. The carrier concentration of n-type IWO thin films varied from 8.39 x 10(19) cm(-3) to 8.58 x 10(21) cm(-3), while the resistivity varied from 3.15 x 10(-4) Ωcm to 2.26 x 10(-3) Ωcm. The largest measured optical band gap was 3.82 eV determined at 2.43 at.% of tungsten atoms relative to the total amount of metal atoms, while the smallest optical band gap was 3.6 eV at 4.78 at.% of tungsten. IWO films containing more than 2.43 at.% of tungsten atoms relative to the total number of metal atoms revealed an average transmittance of over 80% within the visible light region.

Transport properties of unrestricted carriers in bridge-channel MoS2 field-effect transistors.

Unsuppressed carrier scattering from the underlying substrate in a layered two-dimensional material system is extensively observed, which degrades significantly the performance of devices. Beyond the material itself, understanding the intrinsic interfacial and surficial properties is an important issue for the analysis of a high-κ/MoS2 heterostructure. Here, we report on the electronic transport properties of bridge-channel MoS2 field-effect transistors fabricated by a contamination-free transfer method. After neglecting all the surrounding perturbations, it is possible to reveal the significant improvement of room-temperature mobility and subthreshold slope. A systematic study on variable-temperature transport measurements has quantified the trap density of states both in free-standing and SiO2-supported MoS2 systems. Compared to the bridge-channel MoS2 devices with an ideal interface, the unsuspended devices have a large amount of band tail states, and then the impact of their electronic states on the device performance is also discussed.

Memory effect by carrier trapping into V3Si nanocrystals among SiO2 layers on multi-layered graphene layer.

We report the electrical characteristics and conduction mechanism of a resistive switching memory device consisting of V3Si nanocrystals embedded in the SiO2 layer on multi-layered graphene. The V3Si nanocrystals with average size of 5 nm were formed between the SiO2 layers by thin film deposition and post-annealing process at 800 degrees C for 5 s. The current values of high (HRS) and low resistance states (LRS) at 1 V were measured to be about 3.26 x 10(-9) A and 3.11 x 10(-8) A, respectively. The ratio of the HRS and LRS after applying sweeping bias of ± 6 V appeared to be about 9.54 at 1 V. The resistance switching could originate from the effect of carrier trap and emission into the V3Si nanocrystals via the tunneling, space charge limited current, and thermionic emission mechanisms controlled by the modulation of the Fermi level of the graphene layer. The V3Si nanocrystals memory device has a strong possibility for the application of nonvolatile memory devices.

Emission enhancement of InGaN/GaN light emitting diode by using Ag nanoparticles.

We have studied the effect of surface plasmon (SP) coupling to enhance emission efficiency of light emitting diode (LED) with multiple quantum wells (MQWs) structure by positioning Ag nanoparticles on the line-arrayed patterns. The line-arrayed patterns were fabricated by photolithography and inductively coupled plasma reactive ion etching process. The Ag nanoparticles were formed by thermal annealing at 300 degrees C and 400 degrees C for 30 min for Ag films with thickness of 10 nm and 15 nm deposited on the patterned LED structures, respectively. The photoluminescence (PL) emission intensity of line-patterned LED with Ag nanoparticles was overall enhanced. According to the spectra of time resolved PL, carrier life times of line-patterned LED with and without Ag nanoparticles appeared about 0.47 and 5.47 ns, respectively.