Rupendra Kumar Sharma, Ph.D.

We achieved a remarkable 3.2 % absolute increase at low illumination (0.01 suns), and a 1.4 % absolute increase at 0.1 and 1.0 sun compared to an STC-optimized cell (optimized experimentally for best efficiency at 1.0 sun). This analysis suggests designing PV modules providing energy production that is slightly better matched to the actual people’s energy needs throughout the day and year.

Zinc oxide (ZnO) is a low-cost and environmentally friendly material with unique optical properties and a variety of nano and microstructures imposing challenges for energy conversion, scintillators, photocatalytic wastewater treatment, electrochemical energy storage, or sensing applications. In this work, the nominally undoped and Al-doped nanocrystalline ZnO thin layers were pulsed laser deposited (PLD) on fused silica glass. The samples were characterized by photothermal deflection (PDS) and photoluminescence (PLS) spectroscopy.

Zinc oxide (ZnO) is a low-cost, environmentally friendly material with unique optical properties and a wide variety of nano- and microstructures, making it attractive for applications in energy conversion, scintillators, photocatalytic wastewater treatment, electrochemical energy storage, and sensing. In this work, nominally undoped and Al-doped nanocrystalline ZnO (AZO) thin films were deposited by pulsed laser deposition (PLD) on commercial gold-based interdigitated electrodes (IDE) on glass substrates. Surface morphology and structure were characterized using Correlative Probe and Electron Microscopy (CPEM), combining Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) to perform spatially correlated measurements.

While all traditional photovoltaic materials are covalent compounds with coordination number 4, currently the most progressive materials - hybrid halide perovskites - have coordination number 6 and are ionic. This can be the key to success, especially regarding surface electronic properties, which are essential for polycrystalline semiconductors. This motivates us to explore completely new materials - cubic lithium thiospinels - and compare them with the former. Li thiospinels are prepared from stoichiometrically mixed precursors as a pellet in a nitrogen-filled glovebox. This tablet serves as a target in Pulsed Laser Deposition (PLD) and a special chamber is constructed that avoids air exposure of the precursor tablet during the loading.

Recent advances in the performance of perovskite solar cells (PSCs) have been strongly linked to advancements in interfacial engineering and the development of charge-selective contact. Self-assembled monolayers (SAMs) play an important role in inverted PSCs due to their distinctive and versatile ability to manipulate chemical and physical interface properties. Here we compared the dipole moments and energy levels of (2-(9H-carbazol-9-yl)ethyl)phosphonic acid (2PACz)(4(3,6Dimethyl-9Hcarbazol-9yl)butyl)phosphonic acid (Me-4PACz), (4-(3,6-Dimethoxy-9H-carbazol-9-yl)butyl)phosphonic acid (MeO-4PACz), and (2-(3,7-Dibromo-10H-phenothiazin-10-yl)ethyl)phosphonic acid (Br-2EPT) molecules and theirimpact on the energy conversion of PSC devices.

Ultrafast UV photodetectors (UV PDs) are crucial components in modern optoelectronics because conventional detectors have reached a bottleneck with low integration, functionalities, and efficiency. Core-shell metal oxide nanobrushes (MOx NBs)-based UV PDs have enhanced the absorption, tunable performance, and good compatibility for diversified applications, including imaging, self-powered systems, remote communications, security, and wearable electronics. Core-shell PDs are developed with complex hierarchical or heterostructured configurations that encapsulate 1D MOx nanowires on 1D nanostructures (NSs) to transport high charge carrier mobility or efficiency by reducing scattering and recombination rates. This review presents a thorough development of MOx core-shell microstructure for the enhancement of detection response and stability with controlled parameters for multifunctional applications. Significant roles of MOx NBs-based UV PDs exploring various growth techniques and complex photodetection mechanisms with their challenges, limitations, and prospects, providing valuable insights for propelling the progression of photodetector technology in this comprehensive review are discussed meticulously. The novelty of MOx NBs-based UV PDs lies in their distinctive brush-like morphology aspect, tunable properties, and improved performance compared to other NSs, for rapid and sensitive response (̴µs-ms) under UV light illumination. The diverse photoresponse parameters and multifunctional applications of UV PDs incorporating MOx NBs are carefully summarized, which will set the roadmap for future photodetector technology

Numerous parameters are regulated in the wet chemical oxidation process for TOPCon/POLO solar cell technology to improve silicon oxide passivation (SiO2). Understanding the electronic properties, particularly the lifetime of the carriers and their thickness, requires knowledge of the properties of the surface of crystalline silicon (c-Si), which is subjected to native oxide etching followed by wet chemical oxidation, such as nitric acid or hot water oxidation and various hydrogenation methods. This is tracked with lifetime measurement equipment, and spectral ellipsometry is used to measure the thickness of the oxide layer by using the single sided polished wafers with surface orientation (1 1 1). In addition to the actual values, their time stability is also tracked. Before the hydrogenation step was introduced, the lifetime of the samples was in the order of approximately 0.001 ms, which is less than the bulk lifetime. With the hydrogenation , lifetime increased by more than order of magnitude for relatively long time with no difference between (1 1 1) and (1 0 0) wafers indicating that hydrogenation of the Si/SiO2 interface is performed.

We demonstrate that through careful optimization of contact layer parameters (thickness and doping) of silicon heterojunction cell structure, efficiency at low illumination is improved by almost 8 % while losing only about 4 % at 1 sun illumination, thus potentially increasing the total energy yield in cloudy countries

We suggest a new solar cell loss analysis by using the external quantum efficiency (EQE) measured with sufficiently high sensitivity to also account for defects. Unlike common radiative-limit methods, where the impact of deep defects is ignored by exponential extrapolation of the Urbach absorption edge, our loss analysis considers the full EQE including states below the Urbach edge and uses corrections for band-filling and light trapping. We validate this new metric on a whole range of photovoltaic materials and verify its accuracy by electrical simulations. Any deviations between this newly established metric and experimental open circuit voltage are due to the presence of spatially localized defects and are explained as violations of the assumption of flat quasi-Fermi levels through the device.

Nanorods of erbium-doped zinc oxide (ZnO:Er) were fabricated using a hydrothermal method. One batch was prepared with and another one without constant ultraviolet (UV) irradiation applied during the growth. The nanorods were free-standing (FS) as well as deposited onto a fused silica glass substrate (GS). The goal was to study the atomistic aspects influencing the charge transport of ZnO nanoparticles, especially considering the differences between the FS and GS samples. We focused on the excitons; the intrinsic defects, such as zinc interstitials, zinc vacancies, and related shallow donors; and the conduction electrons. UV irradiation was applied for the first time during the ZnO:Er nanorod growth. This led to almost total exciton and zinc vacancy luminescence reduction, and the number of shallow donors was strongly suppressed in the GS samples. The effect was much less pronounced in the FS rods. Moreover, the exciton emission remained unchanged there. At the same time, the Er3+ content was decreased in the FS particles grown under constant UV irradiation while Er3+ was not detected in the GS particles at all. These phenomena are explained.

Perovskite solar cells (PSCs) have attracted extensive research interest in energy harvesting systems for their superior performance due to high absorption coefficients, long electron-hole diffusion length and ambipolar charge transport. The rapid progress achieved in conversion efficiency and charge transfer stabilities of PSCs is promising to meet future energy demands. Nevertheless, the long-term stability of the absorber and the electron transport layers (ETL) of PSCs is a significant challenge for commercialization. This review focuses on PSCs' stability, efficiency, interface engineering, and durability, particularly emphasising their ZnO-based ETL. Recent progress and challenges in enhancing the stability against moisture-induced, thermal-induced, and UV light-induced degradation have also been discussed critically. The review presents the enhancement in efficiency and stability via tandem configurations and highlights existing challenges and future perspectives.

Heterojunction carrier selective contacts for solar cells have gained great attention because of the ability of these contacts to efficiently collect majority carriers while hindering the recombination of minority carriers, thus resulting in the highest reported voltage among crystalline silicon technologies. The electrode work-function, doping and thickness of the doped layer remain the key parameters for governing the cell performance. Recently, we have studied the requirements for carrier selective contacts under various illumination levels and reported logarithmic dependence of these parameters. In this work, we define a new metric for describing the ability of a contact to collect the carriers and named it as contact strength. We have developed an analytical approach for contact strength of hole selective contacts which represents the requirements on doping, thickness of the contact layer, and electrode work-function for a given illumination. First, the numerical model is calibrated with the experimental data for a wide range of illumination (0.01 Sun – 1.0 Sun) and then simulations have been fitted with analytical model. For the selective contact layer, we observe that only the total charge, rather than thickness and doping individually, matters. The work-function of the top electrode also contributes strongly to contact strength and can in principle substitute doped layer. This insightful metric will guide the solar cell technologists to better understand the carrier selective contacts and to maximize the cell performance not only at standard test conditions (STC), but also at low light illuminations.

Molybdenum oxide is an intensively studied material, thanks to its high bandgap, high work function, and potentially also photochromism, plasmonic properties, and layered structure. In this contribution, we employ Pulsed Laser Deposition (PLD) from stoichiometric MoO3 and metal Mo target at temperature range of 25 °C – 500 °C and oxygen pressure variation of 0.1 mbar – 0.4 mbar to deposit high transparency MoO3 layers. The combination of Photothermal Manuscript File Click here to view linked ReferencesDeflection Spectroscopy (PDS) and Spectral Ellipsometry is applied to accurately track all the optical properties. The X-ray diffraction and Scanning Electron Microscopy (SEM) are used to monitor crystallinity and surface morphology.

The double-gate (DG) junction field-effect transistor (JFET) is a classical electron device, with a simple structure that presents many advantages in terms of not only device fabrication but also its operation. The device has been largely used in low-noise applications, but also more recently, in power electronics. Physics-based compact models for JFETs, contrary to MOSFETs, are, however, scarce. In this paper, an analytical, charge-based model is established for the mobile charges, drain current, and transconductances of symmetric DG JFETs, covering all regions of device operation. The model is unified and continuous from subthreshold to linear and saturation operation and is valid over a large temperature range. This charge-based model constitutes the basis of a full compact model of the DG JFET.

The paper deals with optimization of 1700V of 4H-SiC Junction Barrier Schottky diode parameters.

The ON-state characteristics of a 1.7-kV 4H–SiC junction barrier Schottky diode were studied after 4.5-MeV electron irradiation. Irradiation doses were chosen to cause a light, strong, and full doping compensation of an epitaxial layer. The diodes were characterized using Deep Level Transient Spectroscopy, C–V(T), and I–V measurements without postirradiation annealing. The calibration of model parameters of a device simulator, which reflects the unique defect structure caused by the electron irradiation, was verified up to 2000 kGy. The quantitative agreement between simulation and measurement requires: 1) the Shockley–Read–Hall model with at least two deep levels on the contrary to ion irradiation and 2) a new model for enhanced mobility degradation due to radiation defects. The diode performance at high electron fluences is shown to be limited by the doping compensation at the epitaxial layer.

The paper deals with investigation of deep levels in SiC-Schottky diodes with frequency resolved admittance spectroscopy (FRAS). The results obtained by FRAS are compared with deep level transient spectroscopy measurement.

Physics-based analytical modelling, characterization and TCAD simulations of nanoscale multi-gate field effect transistors (FETs).

The paper deals with simulation and characterization of 4H-SiC JBS diodes irradiated by hydrogen and carbon Ions.

The paper deals with simulation and characterization of 4H-SiC JBS diode irradiated by hydrogen and carbon ions.

The paper deals with the effect of proton and carbon irradiation on 4H-SiC 1700V MPS diode characteristics.

In this article, the effect of local radiation damage on the electrical characteristics of 1700 V 4H-SiC Merged-Pin Schottky (MPS) diode have been investigated. Radiation defects introduced by irradiation with 670 keV protons were placed into the low–doped n-type epi–layer and their influence on diode characteristics were characterized by capacitance DLTS, C-V profiling and I-V measurements. Simulation model laccounting for the effect of proton irradiation was developed, calibrated and used for analysis of underlying effects and temperature dependencies.

This paper deals with the simulation and modeling of the effect of neutron irradiation on 4H-SiC 1700V normally-off JFET.

The paper deals with experimental and simulation studies of 1700V 4H-SiC MPS diodes irradiated with fast light ions.