Carrier Generation Rate Research Articles

Non-steady state transport of charge carriers. An approach based on invariant embedding method

In this work, we report on a model that describes the microscopic electrical transport as a transmission problem using the invariant embedding technique. Analytical expressions for the transport coefficients under non-steady-state conditions are derived allowing us to calculate carrier concentration and time-dependent conductivity. Employing measurable magnitudes, our theoretical results allow us to determine defect concentrations, carrier generation rates, cross sections of recombination, and capture by traps. This model can be employed to study the conduction processes of semiconductors and test their band and defect structure. In particular, time-dependent photoconductivity measurements of a ZnO microwire have been well fitted using our model indicating a relevant role of intrinsic point defects in this material.

Performance improvement of ultrasonic spray deposited polymer solar cell through droplet boundary reduction assisted by acoustic substrate vibration

This study demonstrates the performance improvement of ultrasonic spray deposited bulk heterojunction type polymer solar cells through droplet boundary reduction assisted by acoustic substrate vibration of varying frequencies between 0–20 kHz. The optimum performance was achieved at 15 kHz of applied frequency, where ∼68% improvement in short-circuit current density and ∼85% improvement in overall cell efficiency were observed compared to the reference devices fabricated on stationary substrates. The performance enhancement is mainly attributed to the improved film morphology due to uniform and homogenous droplet spreading and coalescence under the influence of acoustic vibration. Systematic improvement was observed until 15 kHz when smooth films with significantly reduced droplet boundaries were observed with surface roughness around 10 nm only. However, beyond this point, higher frequencies were found to have detrimental effect on film formation. Significant improvement was observed for every cell parameter for 15 kHz samples. Almost ∼16% enhancement in carrier generation rate and ∼46% enhancement in exciton dissociation probability were observed, as estimated from the photo-current analysis. Urbach energy estimation reveals that the films, prepared at 15 kHz substrate vibration, forms less amount of band edge localized defect states (Eu (no vibration) = 161 meV and Eu (15 kHz) = 120 meV), resulting into reduced non-radiative recombination and better performances. The presented approach opens up new pathways for uniform and scalable thin film growth through acoustic substrate vibration assisted ultrasonic spray deposition technique, which would be beneficial for large scale industrial organic photovoltaic production.

Cu-Ag alloy for engineering properties and applications based on the LSPR of metal nanoparticles.

Efficient generation of high-energy hot carriers from the localized surface plasmon resonance (LSPR) of noble metal (Ag, Au and Cu) nanoparticles is fundamental to many applications based on LSPR, such as photovoltaics and photocatalysis. Theoretically, intra- and inter-band electron transitions in metal nanoparticles are two important channels for the non-radiative decay of LSPR, which determine the generation rate and energy of hot carriers. Therefore, on the basis of first-principles calculations and Drude theory, in this work we explore the potential role of alloying Ag with Cu in modulating the generation rate and energy of hot carriers by studying the intra- and inter-band electron transitions in Cu, Ag and Cu–Ag alloys. It is meaningful to find that the d-sp inter-band electron transition rates are notably increased in Cu–Ag alloys. In particular, the inter-band electron transition rates of Cu0.5Ag0.5 become larger than that of single Cu and Ag across the whole energy range between 1.5 and 3.2 eV. In contrast, intra-band electron transition rates of Cu–Ag alloys become smaller than that of single Cu and Ag. Because the intra-band electron transitions mainly contribute to the resistive loss in metals, which finally results in a thermal effect rather than high-energy hot carriers, the reduction of intra-band electron transitions in Cu–Ag alloy is beneficial for the transforming the energy absorbed by LSPR into high-energy hot carriers through other non-radiative channels. These results indicate that alloying of Ag and Cu can effectively improve the generation rates of high-energy hot carriers through the inter-band electron transition, but decrease the resistive loss through intra-band transition of electrons, which should be used as a guide in optimizing the non-radiative decay processes of LSPR.

Internal quantum efficiency measurements in InAs/GaSb superlattices for midinfrared emitters

The internal quantum efficiency of a series of InAs/GaSb superlattices was investigated as a function of carrier generation rate through variable excitation, quasicontinuous-wave photoluminescence measurements. GaSb thicknesses were varied to maximize the internal quantum efficiency for midwave infrared emission. Internal quantum efficiencies were determined from measurements of the photoluminescence radiance and extraction efficiencies computed within a two-slab model. The peak internal quantum efficiencies varied from 15% to 29% at 77 K, which is in good agreement with expectations from InAs/GaSb superlattice light-emitting diode performance.

Engineered photocatalytic fuel cell with oxygen vacancies-rich rGO/BiO1−xI as photoanode and biomass-derived N-doped carbon as cathode: Promotion of reactive oxygen species production via Fe2+/Fe3+ redox

A visible-light-driven photocatalytic fuel cell system integrating electro-Fenton process (EF-PFC), comprised of oxygen vacancies-rich rGO/BiO1−xI photoanode and biomass-derived N-doped carbon (BNC) cathode, was developed to eliminate organic pollutant and generate electricity synchronously. The structure and properties of rGO/BiO1−xI were characterized by means of XRD, SEM, TEM, solid-state EPR, FTIR, Raman, XPS, UV-vis DRS and PL spectra. The photoelectrochemical activity of rGO/BiO1−xI was characterized by Mott-Schottky plots, photocurrent responses and EIS. The experimental results indicated that the introduced oxygen vacancies and graphene modification not only promoted the light-harvesting ability but also greatly improved the generation and transfer rate of charge carriers. Most importanly, BNC, acted as cathode, could efficiently reduce oxygen to H2O2, thus leading to more ·OH production in the presence of Fe2+. Simultaneously, the photogenerated electron could efficiently accelerate Fe2+/Fe3+ redox cycling for continuous Fenton reaction and increasing electricity generation in synergistic EF-PFC system. Using formic acid as sacrificial fuel, approximately 68% removal efficiency was reached in rGO/BiO1−xI-BNC EF-PFC system under visible light irradiation for 240 min while the photocurrent density (Jsc) and the maximum power density (Pmax) were measured to be 133.18 μA cm−2 and 17.50 μW cm−2. Furthermore, the radicals trappingexperiments, hydrogen peroxide quantitative studies and ESR tests clearly revealed the mechanism of enhanced performance in rGO/BiO1−xI-BNC EF-PFC system. Therefore, this engineered rGO/BiO1−xI-BNC EF-PFC system with the cooperation of Fe2+/Fe3+ redox is a promising means to eliminate organic pollutants and generate electricity synchronously.

Engineering the Charge Transport Properties of Resonant Silicon Nanoparticles in Perovskite Solar Cells

Resonant semiconductor nanoparticles (NPs) that improve both light trapping and scattering have recently emerged as an additional tool for enhancing the efficiency of perovskite solar cells. Among the various types of nanostructures, silicon NPs, which support Mie modes and have lower losses compared with metallic particles with plasmon resonances, exhibit the best improvement for standard methylammonium lead iodide (MAPbI3)‐based solar cells. Herein, not only the optical problem of solar cell optimization with silicon nanoantennas is studied, but also the effects related to charge carrier transport in the presence of NPs are considered. In particular, it is theoretically shown that the silicon nanoantennas can be further optimized by p‐doping. The experimental verification is conducted for MAPbI3‐based solar cells by p‐doped silicon NPs in a hole transport layer (Spiro‐OMeTAD). The improved generation rate of charge carriers and hole transport through the doped silicon NPs leads to improved efficiency of the device.

Modeling of high-efficiency colloidal quantum dot solar cells

The uses of solar energy instead of fossil fuels to supply the global energy demand demonstrate the importance of developing solar cells. Among all solar cells, colloidal quantum dot solar cells have attracted particular attention due to their easy fabrication, size control, low cost, and flexibility. The depleted heterostructure solar cell is introduced and simulated using quantum dots of CdS and TiO2 layers. Then, the Schrodinger equation is solved in the spherical polar coordinate and using the obtained eigenfunctions and eigenvalues, the absorption coefficient of other structural parameters are obtained by finite-difference time-domain method. Then solving Maxwell and Poisson equations using the electric field emitted from sunlight radiation, the generation rates of carriers and the current density and other characteristics of the solar cell based on introduced structure are obtained. For studied structures, the obtained optimum results are J sc ≈ 15 mA / cm2 and η ≈ 7 % . The obtained values are relatively good in comparison with the experimental results for similar materials.

On the influence of light intensity on the limits of applicability of modulated optical signals recovery method

The kinetics of silicon photoconductivity with recombination centers of gold is investigated. If the frequency of light intensity modulation is less than the inverse value of the main charge carriers’ lifetime, the functions describing the dependence of light intensity on time and the dependence of the photocurrent on time agree within some factor. At high frequencies of modulation of light intensity distortions arise. The dependences of light intensity on time and photocurrent on time become different. In this case, the effect of the recombination rate on the function of the variable component of the photocurrent is not significant. Basically, this function is determined by the dependence of the generation rate on time. At high frequencies, it is possible to "restore" the shape of the optical pulse using the electric pulse of the photoresistor. The dependences on the frequency of phase, linear and nonlinear distortions arising in the "restoration" of the dependence of the light intensity on time are obtained. The results are given for different values of the rate of charge carrier generation.

(Invited) Tunable and Efficient Hot Carrier Generation from Plasmonic Au-Pd Nanoalloy Photocatalyst

Noble metals are the most studied plasmonic materials due to their chemical stability, small ohmic losses and high DC conductivity. However, the majority of electronic states in the valence band of these metals require at least 2 eV to excite. Therefore, even Au, which is often used for near-infrared hot carrier applications, has a band structure that is not ideal for this low energy wavelength regime. One possible route to improving the efficiency of hot carrier generation in noble metals at near-infrared wavelengths, is to shift their electronic density of states (EDOS) closer to the Fermi level via alloying with a transition metal. Here, we present the tuning of optical and electronic properties of co-evaporated Au-Pd alloy thin films at different compositions by taking ultraviolet photoelectron spectroscopy (UPS) measurements to represent the EDOS, ellipsometry measurements to show the modulated dielectric function and the calculated plasmonic quality factors, and infrared-pump/THz-probe spectroscopy measurements for studying the dynamics of the hot carrier generation in the near-infrared regime. For comparison, calculated EDOS and hot carrier distributions for these alloys are also shown. Furthermore, we characterized the alloy thin film structural properties by grazing incidence x-ray diffraction (GIXRD). Our GIRXD results show that all Au-Pd alloy thin films are true alloys (single phase). UPS data supports our initial hypothesis that adding Pd to Au considerably increases the EDOS in the near-infrared region. The calculated hot carrier distribution show that adding Pd to Au increases the number of carriers generated, and that the Au-Pd alloys thin films generate hotter holes than electrons. Finally, time-dependent changes in ∆E/E from 1550 nm-pump/ THz-probe spectroscopy illustrate that for Pd and Au-Pd alloys there is a change in the conductivity attributed to contributions arising from both thermal and nonthermal electron distribution, while for Au there is no evidence of transitions under 1550 nm pump-excitation. We hypothesize this is due to a lower generation rate of hot carriers in Au than for Pd and Au-Pd alloys in the near-infrared regime.

Simulation of the Formation of a Cascade of Displacements and Transient Ionization Processes in Silicon Semiconductor Structures under Neutron Exposure

The formation of a disordered defect region in bulk silicon is simulated using the molecular-dynamics method for various energies of a primary recoil atom. Variations in the volume and number of radiation-induced defects in a cluster during its formation are calculated. The generation rates of nonequilibrium carriers and amplitude-temporal dependences of pulses of ionization currents in test Schottky diodes with hyperhigh frequencies are found theoretically.

Detecting ultra-fast and near-infrared pulses based on two-photon absorption in multiple quantum wells

A theoretical framework and the computational infrastructure for optical characterization of a waveguide (WG) photodetector (PD) are presented based on multiples quantum well (MQW) with a rib structure that is able to resolve a light pulse with a temporal width of 10[Formula: see text]fs. Such pulses are limited to the C-band of optical telecommunications. This pulse width is shorter than the temporal resolution limit of a commercial PD, due to the nonlinear phenomenon known as nondegenerate two-photon absorption (ND2PA). The results show the importance of the operating characteristics that affect carrier generation rate (CGR).

Analytical Study of Carrier Generation Rate in Graphene Nanoscroll

Graphene nanoscroll introduced recently is another form of a graphene-based two-dimensional material, which is especially attractive in nanoelectronic applications. As carriers can travel ballistically or semi-ballistically, they can reach high-speed and energy if the channel length is enough. Therefore, they can collide and result in an excessive current called ionisation current. As a result, it is important to study this mechanism carefully. In this paper, we propose an analytical approach to calculate an ionisation coefficient.

Single plasmon hot carrier generation in metallic nanoparticles

Hot carriers produced from the decay of localized surface plasmons in metallic nanoparticles are intensely studied because of their optoelectronic, photovoltaic and photocatalytic applications. From a classical perspective, plasmons are coherent oscillations of the electrons in the nanoparticle, but their quantized nature comes to the fore in the novel field of quantum plasmonics. In this work, we introduce a quantum-mechanical material-specific approach for describing the decay of single quantized plasmons into hot electrons and holes. We find that hot carrier generation rates differ significantly from semiclassical predictions. We also investigate the decay of excitations without plasmonic character and show that their hot carrier rates are comparable to those from the decay of plasmonic excitations for small nanoparticles. Our study provides a rigorous and general foundation for further development of plasmonic hot carrier studies in the plasmonic regime required for the design of ultrasmall devices.

Impact of localization phenomenon and temperature on the photoluminescence spectra of GaSbBi alloys and GaSbBi/GaAs quantum dots

In this work, the low temperature PL spectrum of GaSbBi ternary alloys and GaSbBi/GaAs Quantum Dots (QDs) are investigated using two different mathematical models. The first model based on the Gaussian distribution of the localized states takes into consideration the carrier dynamics associated with bulk and QD systems through the inclusion of different carrier dependent parameters such as carrier generation rate, recapture, thermal escape, radiative and non-radiative recombination times and localization depth to compute and reproduce the temperature dependent of the luminescence keys. The k dot p method realized using a 10 band Hamiltonian is used to calculate the carrier effective masses from the temperature dependent band diagram which serves as an input to reproduce the PL spectra. Two separate expressions of intrinsic carrier concentration for bulk and QD used in this model help to achieve a close agreement between the theoretically calculated results and experimental data. This work imparts a deep level understanding of the behaviour of carriers and optoelectronic parameters in III-V-Bi structures.

Temperature Dependence of the Turn-On Delay Time of High-Power Lasers–Thyristors

The temperature dependence of the turn-on delay of a high-power laser–thyristor based on an AlGaAs/GaAs heterostructure has been studied. It was shown that the main factor responsible for the rise in the turn-on delay with increasing temperature is that the generation rate of excess carriers in the p-type base decreases in this case. The temperature dependence of the generation rate of excess carriers in the p-base of the phototransistor part of the structure is due to the change in the useful part of the spontaneous emission and in the impact-ionization rate in the space-charge region of the collector p-n junction. It was found that at low control currents, the key contribution comes from the impact ionization, which results in that a noticeable rise in the delay time is observed with increasing temperature. At high control currents, an important contribution is made by the change in the useful fraction of the absorbed spontaneous emission, which results in that there is hardly any temperature dependence of the delay time.

The coupling effects of the different carrier generation rate distributions and recombination caused by nanostructures on the all-back-contact ultra-thin silicon solar cell performances

Ultra-thin silicon solar cell is a kind of good cost-effective energy solution. Moreover, nanostructured front surface and all-back contact design have been proposed as the promising methods to make ultra-thin silicon an effective solar irradiance absorber. Different nanostructures result in the different generation rate distributions and surface recombination, even though their corresponding integrated generation rates are the same. In addition, under the effect of carrier lifetime, the different generation rate distributions determine the number of carriers that reach the electrodes. Therefore, the grating nanostructures with three different section shapes are chosen for the investigation of the coupling effects of the different generation rate distributions and recombination caused by nanostructures. After consideration of the effect of the surface enhancement ratio of nanostructure, the deviation of electrical characteristics (fill factor), which is caused by the different generation rate distributions caused by nanostructures with the same integrated generation rate, is 0.20% at most as the effective surface recombination increases. Meanwhile, as the carrier lifetime decreases, the deviation of electrical characteristics (short circuit current) is only 1.80% at maximum and does not change with carrier lifetime. Therefore, it is found that, in all-back-contact ultra-thin silicon solar cell, the different generation rate distributions caused by nanostructures with the same integrated generation rate have little effect on the electrical characteristics, no matter how the effective surface recombination and carrier lifetime change. Based on the conclusion, a simplified method is developed for the electrical characteristics simulations. As the effective surface recombination and carrier lifetime change, the minimum error of electrical characteristics calculated by the simplified method is 0.06%, and the maximum is just 3.10%, which proves its accuracy.

Anisotropy and polarization dependence of multiphoton charge carrier generation rate in diamond

Anisotropy of multiphoton carrier generation in monocrystalline IIa diamond and its dependence on the polarization state of excitation light (near-infrared few-cycle laser pulses) are investigated using photoluminescence and nonlinear absorption measurements. We observe anisotropy between multiphoton transition rates for light linearly polarized along the \ensuremath{\langle}100\ensuremath{\rangle} and \ensuremath{\langle}110\ensuremath{\rangle} crystallographic directions of diamond and a strong intensity-dependent decrease of the transition rate for circularly polarized light. Measured results are compared with numerical simulations using time-dependent density functional theory and with an analytical model assuming a parabolic two-band system and the Houston function as the time-dependent wave function of the valence and conduction bands.

Efficient wave optics modeling of nanowire solar cells using rigorous coupled-wave analysis.

We investigate the accuracy of rigorous coupled-wave analysis (RCWA) for near-field computations within cylindrical GaAs nanowire solar cells and discover excellent accuracy with low computational cost at long incident wavelengths but poor accuracy at short incident wavelengths. These near fields give the carrier generation rate, and their accurate determination is essential for device modeling. We implement two techniques for increasing the accuracy of the near fields generated by RCWA and give some guidance on parameters required for convergence along with an estimate of their associated computation times. The first improvement removes Gibbs phenomenon artifacts from the RCWA fields, and the second uses the extremely well-converged far-field absorption to rescale the local fields. These improvements allow a computational speedup between 30 and 1000 times for spectrally integrated calculations, depending on the density of the near fields desired. Some spectrally resolved quantities, especially at short wavelengths, remain expensive, but RCWA is still an excellent method for performing those calculations. These improvements open up the possibility of using RCWA for low-cost optical modeling in a full optoelectronic device model of nanowire solar cells.

Hole transport layer influencing the charge carrier dynamics during the degradation of organic solar cells

Here, we demonstrate the effect of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and molybdenum trioxide (MoO3) hole transport layers (HTLs) on degradation of the bulk-heterojunction organic solar cell (OSC) with the combination of two active layers—poly(3-hexylthiophene) and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]. The study is performed for unencapsulated conventional structure OSCs exposed to the ambient dark condition. In the self-life test situation, it is found that PEDOT:PSS based devices show an initial higher short circuit current and subsequent faster degradation with time in comparison to the MoO3 based devices. The effects of these HTLs have been shown in terms of better charge extraction and a decrease in the shunt resistance as well as the bulk resistance of the active layer. The charge carrier generation probability evaluated from photocurrent data shows the dominant impact of active layer degradation rather than the oxidation of a top electrode under the ambient condition. This suggested mechanism is further supported by impedance spectroscopy as well as the evaluated transit time, global mobility, and exciton dissociation probability, establishing that the degradation does not much affect the transport property of the active material. Rather, it affects more the carrier generation rate. The low hole extraction barrier in PEDOT:PSS based devices show small transit time and high global mobility compared to MoO3. It is found that during the degradation process, the bulk resistance of the device significantly increases, which reduces the diffusion current in the device.

Effect of the Near Surface Depletion Layer on Photo-Response of Direct Band Gap Semiconductor

In this study, the effects of the photo-responses of the near surface depletion layer and the deep bulk of gallium antimonide (GaSb) are investigated under different doping levels, injection level and illumination energies. First, the absorption rate of photons is described as a function of illumination energies at different locations inside the sample and as a function of the depth below the illuminated surface for photons with different energies. Then, the doping level dependence of the low injection level radiative, Auger and effective excess carrier lifetimes just under the surface and in the deep bulk are investigated by considering the variation of energy bending at the surface. The variation of the low injection level excess carrier lifetimes with the depth below the illuminated surface for samples with different doping levels is also described. This is followed by the description of the combined effects of doping and illumination energy on the photo-response of the entire bulk at slightly low injection level. Finally, the excess carrier injection level dependence of excess carrier lifetimes under the illuminated surface and deep bulk is also described for samples with different doping levels. Since photon absorption rate is directly related to the free carriers generation rate, the description of the photon absorption rate as functions of illumination energies and the depth below the illuminated surface is found to be the important and factors in the investigation in the photo-response of semiconductors along with the doping and the injection levels. The analysis of the results also shows that, the under surface region is insensitive to the doping level and that of the deep bulk is highly affected by the doping level. The injection level dependence of the photo-response of the under surface region is similar for all samples with different doping. The energy of illumination each photon mainly suppresses the photo-responses of the points situated closer to the penetration depths of each photon below the illuminated surface of the sample. Keywords: photo-response, direct band gap semiconductor, gallium antimonide GaSb DOI : 10.7176/APTA/75-03