and high room-temperature mobility (>100 cm 2 V −1 s −1 ). [3,9,10] The density of states in multilayer MoS 2 , which is three times higher than that of a single-layer MoS 2 , provides higher currents in the ballistic limit. [9][10][11] Photoresponsivity (responsivity) is one of the most important figure-of-merits in photodetection applications. As the responsivity of a single-layer MoS 2 phototransistor is limited due to its low light absorption, [3,[12][13][14] multilayer MoS 2 is a more promising light-absorbing and photoelectric-conversion material for highly responsive photodetection applications. Many studies carried out to improve the responsivity have aimed to increase the light absorption in MoS 2 by optical resonance or by integrating a layer of highly absorbing materials such as quantum dots, [15] organic materials, [16][17][18] nanoshells, [19] nanoparticles, [20,21] and ferroelectrics. [22,23] All of these approaches have enhanced the responsivity to a certain degree, but the additional light-absorbing overlayer changed the spectral photoresponsivity and more importantly formed a leakage path.In this study, we demonstrated an enhanced photoresponsivity of a multilayer MoS 2 phototransistor by distributing localized Au/MoS 2 Schottky junctions on the MoS 2 channel without additional light-absorbing overlayer. A simple spherical-lens photolithography (SLP) technique was employed to form a triangular lattice of submicrometer Au disks. The moderately small Au disks did not induce surface plasmon resonances at the wavelengths of interest. They simply provided the Schottky junction with a van der Waals (vdW) gap [24] where the photogenerated holes were captured and more electrons were induced into the channel. [25] In this manner, the intrinsic responsivity of the MoS 2 phototransistor was significantly enhanced in the entire wavelength range. Results and DiscussionWe fabricated conventional multilayer MoS 2 phototransistors. As shown in the schematic in Figure 1a, a local bottom gate was employed to consider gate-controlled carriers only in the MoS 2 channel, excluding any spurious effects. [26] The SiO 2 gate insulator planarizing the gate pattern was formed The photoresponsivity of a MoS 2 phototransistor is limited owing to its low light absorption. Many studies aiming to improve the photoresponsivity have enhanced the light absorption in MoS 2 by optical resonance or by integrating an absorbing layer. However, the light-absorbing overlayer changes the spectral photoresponsivity and forms a leakage path. In this study, an enhanced photoresponsivity of a multilayer MoS 2 phototransistor is obtained by localized Au/MoS 2 Schottky junctions without light-absorbing overlayer. Au disks are formed on the MoS 2 surface using a simple spherical-lens photolithography technique, forming localized Schottky junctions between MoS 2 and Au disks. Photogenerated holes drift to the interface due to the built-in electric field around the Schottky junction and are trapped in the interface states between MoS 2 and Au dis...

In article number 1903085, Jaejin Lee, Hui Joon Park, and co‐workers demonstrate perovskite/GaAs 2‐ and 4‐terminal tandem cells. High performance, stable, wide‐bandgap perovskite photovoltaics (PVs) (1.8–1.9 eV) are developed through a solvent‐controlled process. The tandem architecture is also feasible for a thin‐film flexible PV, which is essential to reduce its cost for commercialization with superior bendability. This approach is expected to improve the usability of GaAs PVs with enhanced efficiency and lower cost for applications where light‐weight and flexibility are critical.

A compact and low-profile photovoltaic (PV) cell with a built-in antenna is proposed for Internet of Things (IoT) applications. The proposed design exploits the gallium arsenide (GaAs)-based PV cell for antenna operation; for this purpose, a hexagonal slot with a trapezoidal perturbation is cut from the active area and bottom contact of the PV cell for resonance. The bottom contact of the PV cell is also used as the ground plane for the antenna. An AC blocking circuit is designed to prevent the flow of RF current toward the PV cell, and a chip inductor is used as an RF choke in the circuit. Thus, a single device simultaneously functions as a PV cell and an antenna. The GaAs PV cell shows a power conversion efficiency (PCE) of 13.25% without antireflection coating, with an open-circuit voltage (Voc), a shortcircuit current density (Jsc), and a fill factor (FF) of 0.963 V, 21.00 mA/cm 2 , and 65.52%, respectively. Furthermore, the optical transparency of the proposed PV-cell antenna is greater than 90%. The complete structure occupies an overall volume of 31.4 mm × 33 mm × 0.639 mm (0.25λo × 0.26λo × 0.0052λo at 2.45 GHz), and the antenna operates in the range of 2.14 to 2.94 GHz, with a gain of 2.8 dBi at 2.45 GHz.INDEX TERMS Compact antenna, integrated antenna, IoT devices, photovoltaic (PV) cell, slot antenna.

This paper presents a compact low-profile slot antenna integrated with a photovoltaic cell. The photovoltaic cell consists of a top metal grid, gallium arsenide substrate, and metallic bottom contact. The metallic bottom contact was used as a ground plane where the slot was etched for resonance. A second substrate was placed under the ground plane, and a 50 Ω microstrip line was printed on its bottom side to excite the slot. A chip inductor was used as a radio frequency (RF) choke in the alternating current blocking circuit to suppress RF current leakage towards the photovoltaic cell. Hence, the proposed antenna has a unique feature of functioning simultaneously as a photovoltaic cell and an antenna. The overall dimensions––25 mm × 31.75 mm × 0.893 mm (0.48λo × 0.61λo × 0.017λo at 5.77 GHz)––of the photovoltaic cell-integrated slot antenna structure can be used effectively with Internet of Things devices.

Thin-film vertical cavity surface emitting lasers (VCSELs) mounted onto heatsinks open up the way toward low-power consumption and high-power operation, enabling them to be widely used for energy saving high-speed optical data communication and three-dimensional sensor applications. There are two conventional VCSEL polarity structures: p-on-n and n-on-p polarity. The former is more preferably used owing to the reduced series resistance of n-type bottom distributed Bragg reflection (DBR) as well as the lower defect densities of n-type GaAs substrates. In this study, the p-on-n structures of thin-film VCSELs, including an etch stop layer and a highly n-doped GaAs ohmic layer, were epitaxially grown in upright order by using low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The p-on-n structures of thin-film VCSELs were transferred onto an aluminum heatsink via a double-transfer technique, allowing the top-emitting thin-film VCSELs to keep the p-on-n polarity with the removal of the GaAs substrate. The threshold current (Ith) and voltage (Vth) of the fabricated top-emitting thin-film VCSELs were 1 mA and 2.8 V, respectively. The optical power was 7.7 mW at a rollover point of 16.1 mA.