Carrier Diffusion Length Research Articles

О методике определения диффузионных длин неосновных носителей заряда в неоднородно легированных слоя

The paper considers a method for determining the diffusion lengths of non-basic charge carriers based on the analysis of the spectral characteristics of a planar electron-hole transition. The thickness of the illuminated area was reduced by chemical etching. By changing the signal for radiation in the region of strong absorption, the effective length of the tightening of non-basic charge carriers was determined. With removal of layers, a change in the electrical conductivity of the illuminated area was recorded. This made it possible to construct the doping profile and determine the intensity of the built-in electric field. The obtained results made it possible to calculate the diffusion length of the non-basic charge carriers. An example of implementation of the described method for an InSb-based photodiode is given. The proposed technique can be effectively used to determine small values of diffusion lengths of inhomogeneously alloyed layers.

Non-steady-state photo-EMF in a periodically poled MgO : LiNbO-=SUB=-3-=/SUB=- crystal

The diffusion mode of the non-steady-state photo-EMF excitation is investigated in periodically poled MgO : LiNbO3 crystal at light wavelength λ=457 nm. The signal excitation is carried out along the polar axis of the crystal; the dependencies of the signal amplitude versus the phase modulation frequency, intensity and spatial frequency of the interference pattern are studied in this geometry. The analysis of these dependencies allows determining the photoelectric parameters of the material --- the type, value and relaxation time of the photoconductivity, as well as the diffusion length of charge carriers. Keywords: non-steady-state photo-EMF, periodic domain struc ture, lithium niobate.

Нестационарная фотоэдс в периодически поляризованном кристалле MgO : LiNbO-=SUB=-3-=/SUB=-

The diffusion mode of the non-steady-state photo-EMF excitation is investigated in periodically poled MgO:LiNbO₃ crystal at light wavelength λ=457 nm. The signal excitation is carried out along the polar axis of the crystal; the dependencies of the signal amplitude versus the phase modulation frequency, intensity and spatial frequency of the interference pattern are studied in this geometry. The analysis of these dependencies allows determining the photoelectric parameters of the material – the type, value and relaxation time of the photoconductivity, as well as the diffusion length of charge carriers.

Twist angle can expand charge carrier diffusion length in bilayer black phosphorus: ab initio quantum dynamics

Focusing on bilayer black phosphorus (BP) and performing nonadiabatic molecular dynamic (NA-MD) simulations, we demonstrate that appropriate twist angles can significantly improve the charge carrier lifetime and diffusion length.

Fabrication and characterization of high-resolution 4H-SiC epitaxial radiation detectors for challenging reactor dosimetry environments

Reactor dosimetry environments require radiation detectors that are capable of operating at high temperatures in extremely high neutron and gamma-ray dose rates. Silicon carbide (SiC) is one of the most promising wide bandgap semiconductors (3.27 eV) for harsh environment applications due to its radiation hardness, high breakdown voltage, high electron saturation velocity, and high thermal conductivity. In this paper, we summarize the prospect of Schottky barrier radiation detectors, fabricated on highly crystalline low-defect detector-grade n-type 4H-SiC epitaxial layers with thickness ranging from 20 to 250 lm, for harsh environment applications. A comprehensive discussion on the characterization of the parameters that influence the energy resolution has been included. The usage of electrical and radiation spectroscopic measurements for characterizing the junction and rectification properties, minority carrier diffusion lengths, and energy resolution has been elaborated. Characterization of crucial factors that limit the energy resolution of the detectors such as charge trap centers using thermally stimulated transient techniques is summarized. Finally, the effect of neutron fluence on the performance of the 4H-SiC detectors is discussed.

Investigation of III-V GaP solar cell on silicon substrate

The best solar conversion efficiencies have been reached thanks to multijunction solar based on III-V semiconductors on GaAs or Ge substrates. While displaying high conversion efficiencies, these solar cells suffer from the high cost of such substrates. To benefit from both the low cost and technological maturity of silicon cells, III-V tandem cells on silicon seem a good compromise to overpass the theoretical efficiency limit of the Si single cells. To study the GaP/Si interface effect on the solar cell characteristic, a GaP n-i-p solar cell has been grown on silicon substrate. Two types of electrical contacts configurations have been processed: a top-top configuration in which the current does not see the GaP/Si interface and the top-bottom configuration where the electric current crosses the interface. A comparison of dark I-V, I-V under solar illumination, and EQE measurements on both configurations is performed. The top-bottom contacts configuration shows an EQE a little bit lower than the top-top contact one, likely due to lower carrier diffusion length or recombination at the lower interface. However, the result on the EQE of the top-bottom configuration is encouraging for the future development of the GaP-based/Si tandem solar cells, and any other tandem cell on silicon using GaP as an intermediate selective contact.

The modelling and simulation of perovskite solar cell consisting textile-based electrodes

Perovskite solar cell (PSC) consisting textile-based electrodes has been created novel features for future energy supply. Recently, Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 perovskite has appeared as a favourable candidate for flexible perovskite solar cells. However, a confined performance was acquired when the conventional Spiro-OMeTAD were used as hole transport layer (HTL) material because of suboptimal valance band offset (ΔEV) between Perovskite/HTL layers. In this study, a theoretical drift–diffusion​ modelling is used by finite volume method (FVM) to investigate performance of textile perovskite solar cell with double hole transport layers (HTLs) with graded band gap energies. Copper thiocyanate (CuSCN) and Nickel Oxide (NiO) are replaced as double HTLs, which dramatically enhanced the performance due to optimal upward step by step valance band offsets (ΔEV) between Perovskite/Textile Composite Electrode layers. It is shown that the decreasing of the metal–semiconductor schottky barrier height of electrodes leads to significant increase in fill factor (FF). Also, it is revealed that the enriching of perovskite carrier diffusion length by equipoising of carrier capture cross section, carrier total defect density and carrier thermal velocity substantially promote the overall device efficiency. The short-circuit current (Jsc) of 20.65 mA/cm 2, open-circuit voltage (Voc) of 1.19 V, fill factor (FF) of 73.57%, and power conversion efficiency (PCE) of 18.20% have been obtained by optimization of electrodes properties and perovskite layers. Remarkably, the advantageous of low cost, flexible, and chemical stable conducive textile composite, extends new opportunities in the commercial expansion of highly efficient photovoltaic textile in optoelectronic industry field.

Recent Advances in the Functionalization of Perovskite Solar Cells/Photodetectors

AbstractMetal halide perovskite materials have excellent photoelectric properties, including high absorption coefficient, long charge carrier diffusion length, and low exciton binding energy, making them excellent candidates for photoconversion devices. In the past decade, great progress has been made in improving the performance of perovskite‐based solar cells and photodetectors. However, perovskite devices need to be endowed with more functionality to meet practical application requirements. In this regard, the three major trends in perovskite device development involving flexibility, transparency, and self‐healing have attracted wide interests. This review first summarizes the fundamental design principles of endowing perovskite devices with more functionality. Specifically, the functionalization is realized through elaborate substrate and electrode selection, perovskite layer modification, and device layout. This review presents the recent advances and specific requirements for functionalization of perovskite devices according to the three basic components: substrate, electrode, and perovskite layer. Finally, potential challenges and future research directions in this field are outlined. The aim of this review is not only to shed light on the basic design principles of functionalized perovskite devices, but also to serve as the roadmap for future exploration in perovskite devices.

Recent Progress in Halide Perovskite Nanocrystals for Photocatalytic Hydrogen Evolution.

Due to its environmental cleanliness and high energy density, hydrogen has been deemed as a promising alternative to traditional fossil fuels. Photocatalytic water-splitting using semiconductor materials is a good prospect for hydrogen production in terms of renewable solar energy utilization. In recent years, halide perovskite nanocrystals (NCs) are emerging as a new class of fascinating nanomaterial for light harvesting and photocatalytic applications. This is due to their appealing optoelectronic properties, such as optimal band gaps, high absorption coefficient, high carrier mobility, long carrier diffusion length, etc. In this review, recent progress in halide perovskite NCs for photocatalytic hydrogen evolution is summarized. Emphasis is given to the current strategies that enhance the photocatalytic hydrogen production performance of halide perovskite NCs. Some scientific challenges and perspectives for halide perovskite photocatalysts are also proposed and discussed. It is anticipated that this review will provide valuable references for the future development of halide perovskite-based photocatalysts used in highly efficient hydrogen evolution.

Antisolvent Engineering to Enhance Photovoltaic Performance of Methylammonium Bismuth Iodide Solar Cells

High absorption ability and direct bandgap makes lead-based perovskite to acquire high photovoltaic performance. However, lead content in perovskite becomes a double-blade for counterbalancing photovoltaic performance and sustainability. Herein, we develop a methylammonium bismuth iodide (MBI), a perovskite-derivative, to serve as a lead-free light absorber layer. Owing to the short carrier diffusion length of MBI, its film quality is a predominant factor to photovoltaic performance. Several candidates of non-polar solvent are discussed in aspect of their dipole moment and boiling point to reveal the effects of anti-solvent assisted crystallization. Through anti-solvent engineering of toluene, the morphology, crystallinity, and element distribution of MBI films are improved compared with those without toluene treatment. The improved morphology and crystallinity of MBI films promote photovoltaic performance over 3.2 times compared with the one without toluene treatment. The photovoltaic device can achieve 0.26% with minor hysteresis effect, whose hysteresis index reduces from 0.374 to 0.169. This study guides a feasible path for developing MBI photovoltaics.

Diffusion length of non-equilibrium current carriers in nanowire radial p-n junctions: Effect of the curvature

In core-shell nanowire radial p-n junction, spatial (along the radius) distribution of the injected carriers is determined not only by recombination falling of the non-equilibrium carrier concentration but also by specific falling due to cylindrical symmetry of the structure. This forces us to consider an effective diffusion length of non-equilibrium carriers in nanoscale radial structures. This effective diffusion length proves to be larger (up to 25%) than the diffusion length in usual planar p-n junction (made of the same material) under injection from the shell to the core and smaller than it (up to 60%) under injection from the core to the shell.

Elucidating the photovoltaic effect of monoclinic K2SnBr6 by mixed-cation mixed-halide substitution from first-principles calculations

Vacancy-ordered double perovskites (A2BX6) have recently received impressive attention for photovoltaic applications, and the compound K2SnBr6 has desirable features for use in solar cells in such a way that it exhibits direct bandgap behaviour with dispersed band edges. However, the large bandgap value limits its use in higher-efficiency solar cells. Therefore, we have carried out exploratory research by analysing the photovoltaic effect of K2SnBr6 by tuning its bandgap with cation and anion substitutions. We studied the properties of K(2−y)RbySnBr(6−x)I x and K2Sn(1−z)Ti z Br(6−x)I x compounds to explore the photovoltaic effect by gradual substitution of Rb+, Ti4+ and I−1 for K+, Sn4+ and Br−1, respectively. Our density functional calculations in the monoclinic ground state crystal structure with the space group P121 /n revealed that the K(2−y)Rb y SnBr(6−x)I x compounds considered in this study exhibit direct bandgap behaviour with well-dispersed band edges. Moreover, the bandgap value decreases as a function of the Rb and I concentrations. In addition to the low charge carrier effective mass, low excitonic binding energy values and low recombination rate, these compounds exhibit comparatively greater absorption coefficients in the visible range. The charge carrier transport properties, such as carrier mobility, carrier relaxation time, carrier diffusion coefficient and carrier diffusion length, are also seen in higher ranges for these Rb- and I-substituted compounds when compared to the parent compound. In addition, we have calculated the open-circuit voltage, fill factor, short-circuit current and power conversion efficiency for each compound. From the calculations and analysis, we observed that Rb and I substitution in K2SnBr6 increases the photovoltaic effect, and thus K(2−y)Rb y SnBr(6−x)I x compounds may be employed as absorbing layers in higher-efficiency solar cells.

Evaluation of the Efficiency of Photoelectrochemical Activity Enhancement for the Nanostructured LaFeO3 Photocathode by Surface Passivation and Co-Catalyst Deposition.

Perovskite-type lanthanum iron oxide, LaFeO3, is a promising photocathode material that can achieve water splitting under visible light. However, the performance of this photoelectrode material is limited by significant electron-hole recombination. In this work, we explore different strategies to optimize the activity of a nanostructured porous LaFeO3 film, which demonstrates enhanced photoelectrocatalytic activity due to the reduced diffusion length of the charge carriers. We found that surface passivation is not an efficient approach for enhancing the photoelectrochemical performance of LaFeO3, as it is sufficiently stable under photoelectrocatalytic conditions. Instead, the deposition of a Pt co-catalyst was shown to be essential for maximizing the photoelectrochemical activity both in hydrogen evolution and oxygen reduction reactions. Illumination-induced band edge unpinning was found to be a major challenge for the further development of LaFeO3 photocathodes for water-splitting applications.

Enhancing optical absorption by reducing the trap states in polymer semiconductor processed for solar cell

Current-voltage characteristics of an organic solar cells can be tuned by tailoring the active layer material’s polymer chain ordering which in turn decides the PV performance. The suppression of exciton recombination, improved charge carrier separation, large carrier diffusion length can be achieved by improving the packing density of the π-π stacking, polymer chain length and chain ordering. The high packing density π-π stacking can be formed through enhancing the percentage of crystallinity of the semiconducting polymer layer. This can be achieved by optimizing the processing parameters. This work gives some insights on the spinning time of active layer (P3HT:PC61BM) on the structural, optical and solar cell performance. P3HT:PC61BM crystalline properties were studied through grazing incidence X-ray diffraction (GI-XRD). Optical absorption of the active layer was studied by the UV–visible spectrometer. The trap density of states of the solar cells processed at various active layer spinning condition is evaluated. From the obtained result it is noted that device fabricated with longer spinning time showed higher power conversion efficiency over shorter spinning duration. These changes in the device performance could be due to solvent evaporation kinetics, chain alignment in the polymer thin film. Further, the mechanism of trap states formation based on the active layer processing condition is explained.

Back-illuminated photoelectrochemical flow cell for efficient CO2 reduction

Photoelectrochemical CO2 reduction reaction flow cells are promising devices to meet the requirements to produce solar fuels at the industrial scale. Photoelectrodes with wide bandgaps do not allow for efficient CO2 reduction at high current densities, while the integration of opaque photoelectrodes with narrow bandgaps in flow cell configurations still remains a challenge. This paper describes the design and fabrication of a back-illuminated Si photoanode promoted PEC flow cell for CO2 reduction reaction. The illumination area and catalytic sites of the Si photoelectrode are decoupled, owing to the effective passivation of defect states that allows for the long minority carrier diffusion length, that surpasses the thickness of the Si substrate. Hence, a solar-to-fuel conversion efficiency of CO of 2.42% and a Faradaic efficiency of 90% using Ag catalysts are achieved. For CO2 to C2+ products, the Faradaic efficiency of 53% and solar-to-fuel of 0.29% are achieved using Cu catalyst in flow cell.

Comparative Study of the Correlation between Diffusion Length of Charge Carriers and the Performance of CsSnGeI3 Perovskite Solar Cells

Due to their enhanced performance and simplicity in manufacturing, scalability, and versatility, lead-halide perovskite-based solar cells (HPSCs) have received much attention in the domains of energy. Lead is present in nature as a poisonous substance that causes various issues to climate and human health and prevents its further industrialization. Over the past few years, there has been a noticeable interest in exploring some alternative lead-free perovskites. However, owing to some intrinsic losses, the performance that may be achieved from these photovoltaics is not up to standards. Thus, for the purpose of efficiency improvement, a comprehensive simulation is required to comprehend the cause of these losses. In the current research, an investigation into how to employ the promisingly efficient lead-free, all-inorganic cesium tin–germanium iodide (CsSnGeI3) perovskites as the photoactive layer in HPSCs was performed. Results exhibited a high efficiency of 12.95% with a CsSn0.5Ge0.5I3 perovskite thickness of 0.6 μm and a band gap of 1.5 eV at room temperature. High efficiency may be achieved using phenyl-C61-butyric acid methyl ester (PCBM) as an electron transport material because of its favorable energy-level alignment with the perovskite material. The research further tested the perovskite layer thickness and defect density in depth. The results showed that the carrier diffusion lengths have a big effect on how well the HPSC works.

Probing Local Emission Properties in InGaN/GaN Quantum Wells by Scanning Tunneling Luminescence Microscopy

Scanning tunneling electroluminescence (STL) microscopy is performed on a 3 nm‐thick InGaN/GaN quantum well (QW) with [In] = 0.23 such that the main light emission occurs in the green. The technique is used to map the radiative recombination properties at a scale of a few nanometers and correlate the local electroluminescence map with the surface topography simultaneously imaged by scanning tunneling microscopy. While the expected green emission is observed all over the sample, measurements performed on a 500 nm × 500 nm area around a 150 nm‐large and 2.5 nm‐deep hexagonal defect reveal intense emission peaks at higher energies close to the defect edges, features which are not visible in the macrophotoluminescence spectrum of the sample. Via a fitting of the local tunneling electroluminescence spectra, quantitative information on the fluctuations of the intensity, peak energy, width, and phonon replica intensity of the different spectral contributions is obtained, which provides information on carrier localization in the QW. This procedure also indicates that the carrier diffusion length on the probed area of the QW is shorter than 50 nm.

Spectroscopic performance of Ni/4H-SiC and Ti/4H-SiC Schottky barrier diode alpha particle detectors

Advancement in the growth of 4H-SiC with low micropipe densities (∼ 0.11 cm-2) in achieving high pure epitaxial layers, enabled the development of high-resolution 4H-SiC alpha particle Schottky radiation detectors for harsh environments. In particular, the study considers two types of 4H-SiC radiation detectors having Ni and Ti as Schottky contacts. They are fabricated by depositing Ni and Ti on 25 μm thick n-type 4H-SiC by epitaxially growing on 350 μm thick conducting SiC substrates. Electrical characterization and alpha spectral measurements performed on Ni/4H-SiC and Ti/4H-SiC SBDs are reported in this work. The spectral measurements were carried out using 241Am alpha emitting radioactive source. Ni/ 4H-SiC Schottky detector showed a better spectral response with 22.87 keV FWHM (∼ 0.416%) at a reverse bias of 150 V for 5.48 MeV alpha particles while Ti/4H-SiC Schottky detector achieved a resolution of 38.25 keV FWHM (∼ 0.697%) at 170 V reverse bias. This work presented spectral broadening analysis to understand the various factors affecting the energy resolution of the detectors. The extracted charge collection efficiencies (CCEs) are approximately 99% in both the detectors. In addition, polarization effects are not noticed in any of the fabricated detectors. The diffusion length of minority carriers (Lp ) is computed based on the drift-diffusion model by fitting the CCE curve as a function of applied bias, and the values are close to 9 μm and 7 μm for Ni/4H-SiC SBD and Ti/4H-SiC SBD detectors, respectively. Annealing at 400°C for 5 minutes in N2 ambient resulted in resolution of 23.98 keV FWHM (∼ 0.436%) for Ni/4H-SiC SBD detector at -170 V and 36.21 keV FWHM (∼ 0.661%) for Ti/4H-SiC SBD detector at -150 V. Overall Ni/4H-SiC SBD detectors showed superior spectral characteristics and superior resolution when compared to Ti/4H-SiC SBD detectors. However, the Ti/4H-SiC SBD detector fabricated in this work performed better than the previously reported work on a similar device structure. Hence, future work aimed at improving resolution of radiation detectors could also consider Ti/4H-SiC SBDs along with Ni/4H-SiC SBDs.

Solution-Processed Ternary Tin (II) Alloy as Hole-Transport Layer of Sn-Pb Perovskite Solar Cells for Enhanced Efficiency and Stability.

Tin-lead (Sn-Pb) narrow-bandgap (NBG) perovskites show great potential in both single-junction and all-perovskite tandem solar cells. Sn-Pb perovskite solar cells (PSCs) are still limited by low charge collection efficiency and poor stability. Here, a ternary Sn (II) alloy of SnOCl is reported as the hole-transport material (HTM) with a work function of 4.95eV for Sn-Pb PSCs. The solution-processed SnOCl layer has a texture structure that not only reduces the optical loss of the devices, but also changes grain growth of Sn-Pb perovskites and boosts the carrier diffusion length to 3.63µm. The formation of small perovskite grains at the HTM/perovskite interface is suppressed. These result in an almost constant internal quantum efficiency (IQE) of 96 ± 2% across the absorption spectrum of Sn-Pb perovskites. The SnOCl HTM significantly enhances the stability of Sn-Pb PSCs with 87% of its initial efficiency retained after 1-sun illumination for 1200h, and keeps 85% efficiency under 85°C thermal stress for 1500h. The hybrid HTM further improves the stabilized efficiencies of single-junction Sn-Pb PSCs and all-perovskite tandem solar cells to 23.2% and 25.9%, respectively. This discovery opens an avenue to the multicomponent metal alloys as HTM in PSCs.

Room-temperature nonradiative recombination lifetimes in c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11 ≤ x ≤ 0.21)

Lattice-matched Al1−xInxN / GaN heterostructures with InN mole fraction (x) of 0.18 have attracted considerable interest for use in GaN-based optoelectronic devices. Because the light emission efficiency (ηemission) of Al1−xInxN alloys is far less than that of InxGa1−xN, understanding its causes is essential. For this purpose, room-temperature photoluminescence lifetime (τPLRT), which almost represents the nonradiative recombination lifetime that limits the internal quantum efficiency in low ηemission semiconductors, of c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11≤x≤0.21) was examined. For the epilayers grown on low threading dislocation density (TDD) GaN substrates (≪107cm−2), τPLRT principally decreased with increasing x, indicating a progressive increase in the concentration of nonradiative recombination centers (NRCs), NNRC. One of the probable causes is the growth temperature (Tg) reduction that is indispensable to incorporate more In, because in insufficient Tg regime higher Tg is preferred for enhancing the surface migration of adatoms to decrease the concentrations of vacancies that compose NRCs. The Al1−xInxN epilayers of the same x but grown on high TDD (>108cm−2) GaN-on-sapphire templates exhibited shorter τPLRT. Because the diffusion length of minority carriers was nearly zero in the Al1−xInxN epilayers, the shorter τPLRT indicates higher bulk NNRC in high TDD epilayers. The Al1−xInxN epilayers of considerably rough surface morphologies exhibited spatially inhomogeneous τPLRT, implying that excited carriers recombined everywhere at InN-rich to InN-poor portions, where NNRC were likely lower to higher, respectively, than the average due to the deviations in the surface stoichiometry at various non-c-plane surfaces at a given Tg.