Generation Rate Of Charge Carriers Research Articles

Investigation of FABr treatment's potential for triple-cation multi-halide perovskite solar cells

In order to preserve the effectual potential of triple-cation coupled with multi-halide perovskites (PVTs), the phase-related conflict amongst different precursors has to be resolved. There arises the necessity of post-growth passivation treatments, especially using formamidinium bromide (FABr) upon such PVT films. In this research, a methodical comparison assessment is performed on triple-cation, multi-halide (CsFAMAPbIBr)-based PVT solar cells (PSCs) with and without FABr passivation using numerical study. A meticulous estimation of all the performance parameters of both PSCs was implemented in accordance with transformations in thickness, effective density, and acceptor density of the PVT absorber layer together with the charge transport layer thickness variations. The investigation revealed that the charge carrier generation rate remained the same after FABr passivation, however,the recombination rate was lowered by ∼ 32 % (from 7.27 × 1021 to 4.92 × 1021 cm-3s−1). The analysis discerned that at minimum valence band effective density of PVT absorber: 1 × 1017 cm−3, FABr-treated CsFAMAPbIBr-based PSC ensures the highest power conversion efficiency of 24.62 % against its un-passivated correspondent of 22.25 %. Further, this investigation imparts the remarkableness of FABr passivation treatment in regulating the overall performance of triple-cation and multi-halide-based PSCs.

Simulation and numerical modeling of CuO films thickness influence on the efficiency of graphite/CuO/Ni solar cells

AbstractCopper oxide is one of the original semiconductor materials employed for solar cells in the early 19th century before Silicon solar cells became popular due to their abundant availability, and eco‐friendly nature. The optoelectronic parameters signify its huge potential in solar cell devices, though it's far from the theoretically predicted performance, provides tremendous scope to improve the solar cell performance by forming different heterojunctions. In this study, we investigated the copper oxide's (CuO) potential as an active layer in thin‐film solar cells theoretically with a new structure consisting of a Glass/ITO/Graphite/CuO/Ni. Furthermore, the charge carrier's generation rate and theoretical thresholds for photovoltaic device efficiency were determined for varying active layer thicknesses by employing a normalized light intensity equivalent to that of the AM1.5 spectrum. The optimized performance of the simulated structure by considering realistic optical parameters of the solar cell was ~24%, obtained for the 500 nm CuO films. The performed theoretical work can help to employ CuO and boost the performance of solar cells experimentally.

Improved optical performance of CH3NH3PbI3 perovskite-germanium heterostructure photodetectors assisted by a periodic nanosphere array

The methylammonium lead iodide (CH3NH3PbI3)-germanium (Ge) heterojunction with a layer of periodic nanospheres was numerically studied to achieve improved performance over broadband from 300 nm to 1600 nm by the 3D finite element method (FEM). Under AM 1.5 g illumination, the total absorbed power increased 14% in heterojunction photodetectors assisted by an Al2O3 antireflection (AR) array, after optimizing the thickness of perovskite and Ge, as well as the radius, period and material type (metal or dielectric permittivity) of nanosphere array by genetic algorithm, with reference to a corresponding device without nanospheres. The enhanced optical properties were further elaborated and demonstrated by comparatively analyzing broadband absorptance, electric field distributions, absorbed power distributions and the optical generation rate of charge carriers in the two photodetector models. The proposed perovskite-Ge heterojunction with spheres shows great promise for optoelectronic devices.

A comprehensive photovoltaic study on tungsten disulfide (WS2) buffer layer based CdTe solar cell

Transition metal di-chalcogenides (TMCDs)-Tungsten disulfide (WS2) exhibit excellent optoelectronic properties such as suitable bandgap, high absorption coefficient, good conductivity, high carrier mobility, etc. to be used as a photovoltaic material for thin-film solar cells. In the present work, we have replaced the traditional buffer CdS and ITO/ZnO window layer in CdTe solar cells with the non-toxic, earth-abundant WS2 buffer and SnO2 window layer, respectively. The SCAPS-1D solar simulator is used to investigate the potentiality of WS2 as buffer material in CdTe solar cells. This numerical study provides a comparison of the performances between the proposed structure: SnO2/WS2/CdTe/Au and the baseline structure: ITO/ZnO/CdS/CdTe/Au. The impacts of the charge carrier generation rate, spectral response, current-voltage characteristics, bulk defect density, defect density at buffer/absorber interface, operating temperature, and capacitance-voltage characteristics on the solar cell performance parameters have also been analyzed. The tolerance level of defect density in WS2 bulk and WS2/CdTe interface are found to be 1017 cm−3 and 1012 cm−3, respectively. The temperature study reveals the poor structural robustness and thermal stability of the proposed cell. The conversion efficiency of the proposed cell has found to be 20.55% at the optimized device structure. Nevertheles, these findings may provide an insight to fabricate viable, environment friendly, and inexpensive CdTe thin-film solar cells.

A comprehensive physics based surface potential and drain current model for SiGe channel dual programmable FETs

In the present work, a comprehensive analytical model for surface potential and drain current has been developed for double gate silicon-germanium channel dual programmable FETs (SiGe-DP-FET) which is capable to conduct both in n- as well as p-modes. The developed model accurately predicts the impact of mole fraction (m) variation in Si(1 − m)Ge m channel on various device characteristics such as energy band, surface potential, electric field, drain current, and transconductance. While deriving the surface potential expression, the active SiGe channel is bifurcated in three regions such that regions I and III denote the two gated regions while region II denotes the ungated region. Further, in order to compute the expression for surface potential, 2D Poisson’s equation has been solved in gated regions while linear approximation has been applied in ungated region and subsequently, the drain current expression has been obtained by integrating the band-to-band generation rate of charge carriers across the source side Schottky junctions. Moreover, in order to validate the proposed model, the analytical results are compared and verified with 2D TCAD ATLAS simulated results and it has been observed that the results obtained using proposed model match well with the simulated results.

Tuning the optical properties of GaAs solar cell through triple-layer antireflection coatings

Solar cells must be efficient in order to reduce optical losses. A variety of light-trapping techniques are employed to reduce optical losses, especially those due to reflections. Solar cells are coated with antireflective coatings (ARCs) to reduce reflected light. The optimum triple-layer antireflective coatings (TLARC) on a high index subsrate has been investigated in this paper. An evaluation of the reflectivity of TLARC materials over visible and near-IR wavelength ranges is performed with the suitable substrate material. Moreover, reflectance can be affected by the important parameter of thickness of the antireflection coatings. The reflectance, transmittance, short circuit current density, external quantum efficiency, and charge carrier generation rate are calculated with wavelength for various thickness of ARC. The optimized thickness of d1 = 120 nm, d2 = 167 nm, and d3 = 175 nm produces zero reflectance at wavelengths of 400 nm and 700 nm, respectively. Following our investigations, we conclude that TLARCs MgF2/SiO/ZnSe are the best ARC candidates in a broad wavelength range of 400–1400 nm for GaAs solar cell.

Improved optical properties of perovskite solar cells by introducing Ag nanopartices and ITO AR layers

Embedded noble metal nanostructures and surface anti-reflection (AR) layers affect the optical properties of methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells significantly. Herein, by employing a combined finite element method and genetic algorithm approach, we report five different types of CH3NH3PbI3 perovskite solar cells by introducing embedded Ag nanoparticles within the CH3NH3PbI3 layer and/or top ITO cylinder grating as an AR layer. The maximum photocurrent was optimized to reach 23.56 mA/cm2, which was 1.09/1.17 times higher than Tran’s report/ flat cases. It is also comparable with values (23.6 mA/cm2) reported in the literature. The calculations of the electric field and charge carrier generation rate of the optimized solar cell further confirms this improvement than flat cases. It attributes to the synergistic effect of the embedded Ag nanoparticles and ITO AR layer. The results obtained herein hold great promise for future boosting the optical efficiency of perovskite solar cells.

Enhancing efficiency and stability of inverted structure perovskite solar cells with fullerene C60 doped PC61BM electron transport layer

[6, 6]-phenyl C61butyric acid methyl ester (PC61BM) is a common fullerene derivative widely employed as an electron transport layer in inverted perovskite solar cells. However, major challenges such as interfacial recombination, current leakage and low mobility results in poor photovoltaic performance and stability for PC61BM-based devices. To overcome these issues, we demonstrated, for the first time, doping of the PC61BM with fullerene C60 to tailor the properties of the PC61BM. This doping approach helped to realize superior power conversion efficiency of perovskite solar cell with 17.46% versus pristine device with 14.20%. The C60 doped PC61BM reinforced the surface morphology of the PC61BM layer with reduced roughness and better coverage, up-shift of Fermi Level, higher electrical conductivity, higher maximum charge carrier generation rate, and higher charge collection probability. All these improvements contributed to reduced charge leakage at the interface, reduced charge recombination and promoted efficient charge transfer, transport, and collection. Furthermore, the enhanced hydrophobicity of the C60 doped PC61BM enabled superior ambient device stability compared to the pristine device. This work facilitates a simple and effective route to tailor perovskite solar cell performance towards higher efficiency and better stability.

Theoretical Insight into High‐Efficiency Triple‐Junction Tandem Solar Cells via the Band Engineering of Antimony Chalcogenides

Antimony chalcogenides have become a family of promising photoelectric materials for high‐efficiency solar cells. To date, single‐junction solar cells based on individual antimony selenide or sulfide are dominant and show limited photoelectric conversion efficiency. Therefore, great gaps remain for the multiple junction solar cells. Herein, triple‐junction antimony chalcogenides‐based solar cells are designed and optimized with a theoretical efficiency of 32.98% through band engineering strategies with Sb2S3/Sb2(S0.7Se0.3)3/Sb2Se3 stacking. The optimum Se content of the mid‐cell should be maintained low, i.e., 30% for achieving a low defect density in an absorber layer. Therefore, Sb2(S0.7Se0.3)3‐based mid solar cells have contributed to elevate the external quantum efficiency in triple‐junction devices by the full utilization of the solar spectrum. In a single‐junction solar cell, the bandgap gradient is regulated through the Se content gradient along the depth profile of Sb2(S1−xSex)3. Besides, an increasing Se content profile provides an additional built‐in electric field for boosting hole charge carrier collection. Thus, the high charge carrier generation rate leads to a 17.96% improvement in the conversion efficiency compared with a conventional cell. This work may pave the way to boost the conversion efficiency of antimony chalcogenides‐based solar cells to their theoretical limits.

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.

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.

Estimation of the Maximum Nonequilibrium Charge-Carrier Concentration in GaN Under Electron-Beam Irradiation

An empirical expression describing the lateral distribution of the nonequilibrium charge-carrier generation rate in GaN by a focused electron beam is obtained. The maximum nonequilibrium charge-carrier concentration as a function of the beam energy and diffusion length is calculated using this expression. Comparison of the results with values obtained using previously proposed approximate expressions provides an opportunity to evaluate the applicability of these approximations. Within the study of GaN by the cathodoluminescence and electron-beam-induced-current methods, the possibility to achieve a low injection level is analyzed considering the fact that the diffusion lengths of nonequilibrium charge carriers can be measured correctly only at low injection levels. It is shown that, in spite of the submicron values of the diffusion lengths in GaN, rather low beam currents are required to achieve a low injection level.

Improvement in electrostatic characteristics of doped TFETs by hole layer formation

We present a distinct approach to enhance the performance of physically doped tunnel field-effect transistors (TFETs) based on creation of a layer of positive charge at the semiconductor–insulator interface in the source region. Formation of such a hole layer resolves the issue related to material solubility and improves direct-current (DC) as well as high-frequency figures of merit. To implement this approach, a typical $$P^{+}$$ -I- $$N^{+}$$ -type physically doped TFET structure is considered. Furthermore, a metal electrode with workfunction of 4.53 eV is placed over the heavily doped $$P^{+}$$ source region with a negative supply voltage. The negative voltage at the source electrode attracts holes from the source region and creates a hole layer just below the semiconductor–insulator interface. This phenomenon makes the source–channel junction abrupt and reduces the tunneling barrier width, resulting in higher tunneling generation rate of charge carriers at the source–channel junction. Thus, the proposed device shows 100-fold increased ON-state current and a threshold voltage reduction of 300 mV. Analog/radiofrequency (RF) parameters are also greatly improved compared with the conventional device. Furthermore, optimization of the spacer length ( $$L_\mathrm{SG}$$ ), i.e., the gap between the source and gate electrode, and application of a negative voltage ( $$-V_\mathrm{SE}$$ ) at the source electrode (SE), were applied to achieve the optimum performance. Moreover, device linearity was also analyzed in a comparative manner.

Design of double gate vertical tunnel field effect transistor using HDB and its performance estimation

In this paper, we demonstrate the double gate vertical tunnel field-effect transistor using homo/hetero dielectric buried oxide (HDB) to obtain the optimized device characteristics. In this concern, the existence of double gate, HDB and electrode work-function engineering enhances DC performance and Analog/RF performance. The use of electrostatic doping helps to achieve higher on-current owing to occurrence of higher tunneling generation rate of charge carriers at the source/epitaxial interface. Further, lightly doped drain region and high-k dielectric below channel and drain region are responsible to suppress the ambipolar current. Simulated results clarifies that proposed device have achieved the tremendous performance in terms of driving current capability, steeper subthreshold slope (SS), drain induced barrier lowering (DIBL), hot carrier effects (HCEs) and high frequency parameters for better device reliability.

表面等离子激元与F-P共振耦合平衡钙钛矿太阳能电池有源层内载流子产生速率

表面等离子激元与F-P共振耦合平衡钙钛矿太阳能电池有源层内载流子产生速率

Performance improvement of doped TFET by using plasma formation concept

Formation of abrupt doping profile at tunneling junction for the nanoscale tunnel field effect transistor (TFET) is a critical issue for attaining improved electrical behaviour. The realization of abrupt doping profile is more difficult in the case of physically doped TFETs due to material solubility limit. In this concern, we propose a novel design of TFET. For this, P+ (source)-I (channel)-N (drain) type structure has been considered, wherein a metal electrode is deposited over the source region. In addition to this, a negative voltage is applied to the source electrode (SE). It induces the surface plasma layer of holes in the source region, which is responsible for steepness in the bands at source/channel junction and provides the advantage of higher doping in source region without any addition of the physical impurity. The proposed modification is helpful for achieving steeper band bending at the source/channel interface, which enables higher tunneling generation rate of charge carriers at this interface and overcomes the issue of low ON-state current. Thus, the proposed device shows the increment of 2 decades in drain current and 252 mV reduction in threshold voltage compared with conventional device. The optimization of spacer length (LSG) between source/gate (LSG) and applied negative voltage (Vpg) over source electrode have been performed to obtain optimum drain current and threshold voltage (Vth). Further, for the suppression of ambipolar current, drain region is kept lightly doped, which reduces the ambipolar current up to level of Off state current. Moreover, in the proposed device gate electrode is underlapped for improving RF performance. It also reduces gate to drain capacitances (Cgd) and increases cut-off-frequency (fT), fmax, GBP, TFP. In addition to these, linearity analysis has been performed to validate the applicability of the device.

A New Design Approach of Dopingless Tunnel FET for Enhancement of Device Characteristics

Formation of abrupt tunneling junction for the sub-nanometer tunnel FET (TFET) is crucial for achieving better electrical behavior. This task is more challenging in the case of dopingless TFETs (DL TFETs). In this concern, we propose a novel design of DL TFET, wherein a metallic layer has been placed in the oxide region at the space present between gate and source electrode (used for inducing p+ region) of conventional dopingless n-TFET to overcome the issue of low on-state current ( $\text{I}_{\mathrm{on}}$ ) due to presence of tunneling barrier. Proposed modification is helpful for achieving steeper tunneling junction at the source/channel interface, which enables higher tunneling generation rate of charge carriers at this interface. The optimization for work function of the metal layer (ML) has been performed for improving $\text {I}_{\mathrm{on}}$ , point subthreshold swing and threshold voltage ( $\text {V}_{\text {th}}$ ). Finally, the impact of the ML misalignment from the gate/source terminal and optimization of its length is also presented.

Determination of the radiative efficiency of GaN-based light-emitting diodes via bias dependent resonant photoluminescence

We report a method to determine the radiative efficiency (ηrad) of GaN-based light-emitting diodes using excitation density and bias dependent room temperature photoluminescence (PL) measurements selectively exciting the active region. Considering carrier escape by tunnelling out of the active region, we extrapolate the generation rate of charge carriers from photocurrent measurements under reverse bias. A model describing the recombination of carriers including phase-space filling is then fitted to excitation density dependent PL data obtained under forward bias to extract ηrad. Results show that ηrad vs. carrier density is asymmetric around its maximum due to phase-space filling.

Analytical modeling of organic solar cells including monomolecular recombination and carrier generation calculated by optical transfer matrix method

An analytical model of organic solar cell has been developed including the effect of monomolecular recombination and charge carrier generation rate, simultaneously. The charge carrier generation rate, depending on position and wavelength, obtained from optical transfer matrix method, has been incorporated into electrical transport equation of carriers; this has led to combining optical and electrical phenomena into this model. Charge carrier generation rate profile has been investigated and included to develop the model. The proposed model addresses the propagation of light and the effects of optical phenomena like reflection and interference inside the device. Compared to previous models, this model is an improved version because of considering recombination mechanism and position and wavelength dependent generation rate simultaneously. This analytical model is useful for finding the performance of the organic solar cell device such as current-voltage relation, power-voltage relation, efficiency, etc. avoiding the complexities of numerical calculations. The proposed model has been validated by comparing the results obtained from the model with that of published experimental works. This model may help to analyse organic solar cells and optimize their parameters for improving the performance.

What parameters can be reliably deduced from the current-voltage characteristics of an organic bulk-heterojunction solar cell?

Through the analysis of scales and simplification of the drift-diffusion device model, we have obtained a quantitative description of the mechanisms underlying the current-voltage (j–V) characteristics of organic bulk-heterojunction solar cells. The mechanisms have been resolved into the competition between the photogeneration, recombination, and extraction/injection rates, which determines the bulk charge carrier concentration; and the combined effect of the built-in field and the boundary layers in shaping the electric potential distribution, which determines the bulk field. The relationships between the j–V characteristics and standard model parameters have been captured with analytical expressions and verified through 1-D numerical simulations. We have determined that while the charge carrier generation rate can be reliably extracted with the device model from j–V measurements alone, the effective density of states and built-in potential, and the mobility and recombination prefactor are clustered pairs that can only be decoupled through other characterization techniques.