Electrical and Electronic Engineering - Research Publications
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ItemNo Preview AvailableRoom Temperature Bias-Selectable, Dual-Band Infrared Detectors Based on Lead Sulfide Colloidal Quantum Dots and Black Phosphorus(AMER CHEMICAL SOC, 2023-06-15)A single photodetector capable of switching its peak spectral photoresponse between two wavelength bands is highly useful, particularly for the infrared (IR) bands in applications such as remote sensing, object identification, and chemical sensing. Technologies exist for achieving dual-band IR detection with bulk III-V and II-VI materials, but the high cost and complexity as well as the necessity for active cooling associated with some of these technologies preclude their widespread adoption. In this study, we leverage the advantages of low-dimensional materials to demonstrate a bias-selectable dual-band IR detector that operates at room temperature by using lead sulfide colloidal quantum dots and black phosphorus nanosheets. By switching between zero and forward bias, these detectors switch peak photosensitive ranges between the mid- and short-wave IR bands with room temperature detectivities of 5 × 109 and 1.6 × 1011 cm Hz1/2 W-1, respectively. To the best of our knowledge, these are the highest reported room temperature values for low-dimensional material dual-band IR detectors to date. Unlike conventional bias-selectable detectors, which utilize a set of back-to-back photodiodes, we demonstrate that under zero/forward bias conditions the device's operation mode instead changes between a photodiode and a phototransistor, allowing additional functionalities that the conventional structure cannot provide.
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ItemNo Preview AvailableFlexible Vanadium Dioxide Photodetectors for Visible to Longwave Infrared Detection at Room Temperature ((press release associated article should be online on 21.06.2023))(WILEY-V C H VERLAG GMBH, 2023-10-13)Abstract Flexible optoelectronics is a rapidly growing field, with a wide range of potential applications. From wearable sensors to bendable solar cells, curved displays, and curved focal plane arrays, the possibilities are endless. The criticality of flexible photodetectors for many of these applications is acknowledged, however, devices that are demonstrated thus far are limited in their spectral range. In this study, flexible photodetectors are demonstrated using a VOx nanoparticle ink, with an extremely broad operating wavelength range of 0.4 to 20 µm. This ink is synthesized using a simple and scalable wet‐chemical process. These photodetectors operate at room temperature and exhibit minimal variance in performance even when bent at angles of up to 100 ° at a bend radius of 6.4 mm. In addition, rigorous strain testing of 100 bend and release cycles revealed a photoresponse with a standard deviation of only 0.55%. This combination of mechanical flexibility, wide spectral response, and ease of fabrication makes these devices highly desirable for a wide range of applications, including low‐cost wearable sensors and hyperspectral imaging systems.
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ItemNo Preview AvailableLong Duration Persistent Photocurrent in 3 nm Thin Doped Indium Oxide for Integrated Light Sensing and In-Sensor Neuromorphic Computation(WILEY-V C H VERLAG GMBH, 2023-09)Abstract Miniaturization and energy consumption by computational systems remain major challenges to address. Optoelectronics based synaptic and light sensing provide an exciting platform for neuromorphic processing and vision applications offering several advantages. It is highly desirable to achieve single‐element image sensors that allow reception of information and execution of in‐memory computing processes while maintaining memory for much longer durations without the need for frequent electrical or optical rehearsals. In this work, ultra‐thin (<3 nm) doped indium oxide (In2O3) layers are engineered to demonstrate a monolithic two‐terminal ultraviolet (UV) sensing and processing system with long optical state retention operating at 50 mV. This endows features of several conductance states within the persistent photocurrent window that are harnessed to show learning capabilities and significantly reduce the number of rehearsals. The atomically thin sheets are implemented as a focal plane array (FPA) for UV spectrum based proof‐of‐concept vision system capable of pattern recognition and memorization required for imaging and detection applications. This integrated light sensing and memory system is deployed to illustrate capabilities for real‐time, in‐sensor memorization, and recognition tasks. This study provides an important template to engineer miniaturized and low operating voltage neuromorphic platforms across the light spectrum based on application demand.
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ItemNo Preview AvailableA Predictive Model for Monolayer-Selective Metal-Mediated MoS2 Exfoliation Incorporating Electrostatics(WILEY, 2024-01)Abstract The metal‐mediated exfoliation (MME) method enables monolayer‐selective exfoliation of van der Waals (vdW) crystals, improving the efficacy of large monolayer production. Previous physical models explaining monolayer‐selective MME propose that the main contributors to monolayer‐selectivity are vdW crystal/metal surface binding energy and/or vdW crystal layer strain resulting from lattice mismatch. However, the performance of some metals for MME is inconsistent with these models. Here, a new model is proposed using MoS2 as a representative vdW crystal. The model explains how the MoS2/metal interface electrostatics, in combination with strain, determines monolayer‐selectivity of MME by modulating the MoS2 interlayer energy. Monolayer MoS2/metal interfaces are characterized using in situ Raman spectroscopy and density functional theory calculations to estimate the electrostatics and strain of MoS2 in contact with different metals. The model successfully demonstrates the dependence of MME monolayer‐selectivity on the MoS2/metal interface electrostatics and highlights the significance of electrostatics in nanomaterial vdW interactions.
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ItemAn ITO-Free Kesterite Solar Cell(WILEY-V C H VERLAG GMBH, 2024-02)Photovoltaic thin film solar cells based on kesterite Cu2 ZnSn(S, Se)4 (CZTSSe) have reached 13.8% sunlight-to-electricity conversion efficiency. However, this efficiency is still far from the Shockley-Queisser radiative limit and is hindered by the significant deficit in open circuit voltage (VOC ). The presence of high-density interface states between the absorber layer and buffer or window layer leads to the recombination of photogenerated carriers, thereby reducing effective carrier collection. To tackle this issue, a new window structure ZnO/AgNW/ZnO/AgNW (ZAZA) comprising layers of ZnO and silver nanowires (AgNWs) is proposed. This structure offers a simple and low-damage processing method, resulting in improved optoelectronic properties and junction quality. The ZAZA-based devices exhibit enhanced VOC due to the higher built-in voltage (Vbi ) and reduced interface recombination compared to the usual indium tin oxide (ITO) based structures. Additionally, improved carrier collection is demonstrated as a result of the shortened collection paths and the more uniform carrier lifetime distribution. These advances enable the fabrication of the first ITO-free CZTSSe solar cells with over 10% efficiency without an anti-reflective coating.
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ItemProgress and Future Prospects of Wide-Bandgap Metal-Compound-Based Passivating Contacts for Silicon Solar Cells(WILEY-V C H VERLAG GMBH, 2022-07)Advanced doped-silicon-layer-based passivating contacts have boosted the power conversion efficiency (PCE) of single-junction crystalline silicon (c-Si) solar cells to over 26%. However, the inevitable parasitic light absorption of the doped silicon layers impedes further PCE improvement. To this end, alternative passivating contacts based on wide-bandgap metal compounds (so-called dopant-free passivating contacts (DFPCs)) have attracted great attention, thanks to their potential merits in terms of parasitic absorption loss, ease-of-deposition, and cost. Intensive research activity has surrounded this topic with significant progress made in recent years. Various electron-selective and hole-selective contacts based on metal compounds have been successfully developed, and a champion PCE of 23.5% has been achieved for a c-Si solar cell with a MoOx -based hole-selective contact. In this work, the fundamentals and development status of DFPCs are reviewed and the challenges and potential solutions for enhancing the carrier selectivity of DFPCs are discussed. Based on comprehensive and in-depth analysis and simulations, the improvement strategies and future prospects for DFPCs design and device implementation are pointed out. By tuning the carrier concentration of the metal compound and the work function of the capping transparent electrode, high PCEs over 26% can be achieved for c-Si solar cells with DFPCs.
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ItemNo Preview AvailableImaging Current Paths in Silicon Photovoltaic Devices with a Quantum Diamond Microscope(AMER PHYSICAL SOC, 2022-07-18)
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ItemGap-Surface Plasmon-Enhanced Photoluminescence of InSe(WILEY-V C H VERLAG GMBH, 2023-05)2D materials, with distinct characteristics compared to their conventional bulk counterparts, have been a popular topic in various optoelectronic research fields. Herein, indium selenide (InSe), a monochalcogenide van der Waals layered semiconductor, which has been studied due to its thickness dependent optical characteristics is explored. For InSe to be used as a versatile light source, enhancing the emission of InSe is required. Here, enhanced photoluminescence (PL) from multi‐layer InSe is demonstrated using a gap plasmon induced between Ag nanocube dimer and an Au substrate. Such plasmonic structures support multiple resonances, one of those overlapping with InSe's band edge PL emission. The calculated Purcell factor shows a 200‐fold increase on the short edge of nanocube dimers. Experimentally, PL enhancement of 6‐fold is demonstrated at room temperature. In addition, a method for determining the thickness of 2D materials via dark‐field spectroscopy using white light illumination is shown. This study paves the way for the incorporation of 2D InSe into nanophotonic structures.
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ItemNo Preview AvailableIntegration of Black Phosphorus Photoconductors with Lithium Niobate on Insulator Photonics(Wiley, 2023-01)Photodetectors formed with layered two-dimensional (2D) materials have shown significant potential for integration with photonic circuits, offering fast, high responsivity and low noise detection over a broad range of optical wavelengths. However, only preliminary trials of this concept have been performed on emerging photonics platforms such as lithium niobate on insulator (LNOI). In this study, a novel architecture consisting of ≈15 nm thick layered black phosphorus (bP) photoconductors draped over LNOI waveguides is demonstrated. The performance of these detectors is studied across the telecom bands at room temperature, and a high extrinsic responsivity of 148 mA W−1 is measured at λ = 1550 nm under low bias conditions (VDS = 0.3 V). The spectral response of the detectors is broad allowing the response of other photonic components, such as fiber-to-chip grating couplers, to be characterized in situ, without need to out-couple the light. Finally, the speed of the bP detectors is found to be beyond our instrumentation, setting 100 ns as an upper-limit rise/fall time, with the actual speed of the bP detector likely to be much faster.
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ItemInfrared modulation via near-room-temperature phase transitions of vanadium oxides & core–shell composites(Royal Society of Chemistry (RSC), 2023)Vanadium oxides (VOx) are highly promising materials for heat retardant coatings, enabled by their insulator-to-metal phase transition (IMT). Currently, this IMT typically occurs at 68 °C, well above room temperature. Here, we develop a dopant-free approach to lower the IMT temperature to ∼40 °C enabling near-room temperature infrared modulation, by simple, solution phase synthesis. This is achieved by both controlling the stoichiometry of the metal oxide and by using a SiO2 shell around the VOx particles, with the difference in thermal expansion coefficient between SiO2 and VOx inducing sufficient strain in the VOx to dramatically lower the IMT temperature. This approach enables the production of a functional solution of suspended VOx nanoparticles with near-room temperature IMT. The combination of near-room temperature IMT and solution phase nanoparticles dramatically increases the ease, scalability, and efficacy of VOx application.