Rupendra Kumar Sharma, Ph.D.

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 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.