Shockley Read Hall Research Articles

Surface Lattice Perturbation of Electron Transport Layer Reducing Oxygen Vacancies for Positive Photovoltaic Effect

For carbon‐based fully printable mesoscopic perovskite solar cells (FP‐PSCs), due to the serious interfacial defects formed in the uncontrollable crystallization process, modifying the interface between perovskite and electron transport layer is an effective way to enhance their photovoltaic performance. Herein, ultrathin ZrO2 is deposited on the mesoporous TiO2 surface by using spray pyrolysis, and Zr4+ intercalates into the TiO2 surface lattice and works together with Ti4+ and O2− ions. Thanks to this surface lattice perturbation of Zr4+, the reduction of surface oxygen vacancies of TiO2 (electron transport layer) decreases the density of defective states at the TiO2–perovskite interface inhibiting the Shockley–Read–Hall recombination (nonradiative recombination) in the charge cross‐interface transfer. Furthermore, both the open‐circuit voltage and short‐circuit current density are improved significantly. Based on perovskite Cs0.05(FA0.92MA0.08)0.95Pb(I0.92Br0.08)3 for carbon‐based FP‐PSCs, a high power conversion efficiency of 17.81% is obtained. It provides a novel idea and technology for efficient interfacial modification of the electron transport layer for FP‐PSCs.

Design Principles for High QE HgCdTe Infrared Photodetectors for eSWIR Applications

In this paper, we study the limiting mechanisms and design criteria of HgCdTe photodetectors for extended shortwave infrared applications with ultra-high quantum efficiency (QE) in both n-on-p and p-on-n technologies. Numerical and analytical models are employed in order to study the possibility of achieving ultra-high QE eSWIR detectors for the operational wavelengths of approximately 2.0 μm, and our study shows that by proper design of absorber layer and doping density, such a detector can be engineered. Furthermore, we demonstrate that the Shockley–Read–Hall (SRH) lifetime, absorber layer doping density and absorber layer thickness all have an impact on the quantum efficiency whether the detector is used as a small-area pixel element in a focal plane array or as a discrete large-area detector for sensing applications.

COMSOL Simulation of Heat Distribution in InGaN Solar Cells: Coupled Optical-Electrical-Thermal 3-D Analysis

Thermal distribution in solar cells has been rarely investigated despite it significant impact on the performance. The current contribution presents a COMSOL Multiphysics 3-D analysis of the electrical and optical photogeneration properties in relation with the heat distribution in InGaN solar cell. For this simulation, we have coupled the “Semiconductor Module”, the “Heat Transfer Module for Solids,” and the “Wave Optics Module” allowing us to calculate the Shockley–Read–Hall heating, the total heat flux, the Joule heating the carrier’s concentration, the electric field, and the temperature dissipation in the InGaN solar cell structure. Despite the fact that the achievements of InGaN solar cells are still mostly at the state of laboratory studies, the current contribution presenting original results on coupled phenomena occurring in the cells makes it possible to highlight new possible guidelines for an improve of their efficiency.

Modeling the time-dependence of the photoreflectance spectroscopy on ultra-shallow As+ ion implanted silicon

Using a high throughput Photoreflectance (PR) spectroscopy setup, a significant temporal behavior of the PR spectroscopy from a wide range of silicon (Si) wafers is observed, induced by optical heating effect. The PR intensity and the built-in electric field enhance on a time scale of several minutes. A diffusion process-based time-dependent model is developed and validated on shallow Arsenic (As+) ion implanted Si layers. A theoretical form of the temporal built-in electric field is determined and used to fit the experimental PR intensity. Modeling the PR temporal effect enables the real time monitoring of As+ diffusion and the measurement of the diffusivity as a function of low implant energy (E: 3 – 7 keV) from which the lattice temperature is derived experimentally. Furthermore, the As+ Shockley-Read-Hall (SRH) carrier lifetime at 300 K is found to be strongly dependent over the same experimental E range. Using the SRH theory, defects formed by As+ implant are studied and related trap density (Ns) are measured versus E assuming unchanged As+ capture cross section.

COMSOL simulation of non-radiative recombination heat and joule heat in CZTSSe thin film solar cells

A detailed coupled simulation model has been developed to analyze the impact of defect, voltage, and temperature on heat generation rate in CZTSSe thin film solar cells. This has been simulated by mapping the non-radiative recombination heat and Joule heat due to increased defect and voltage across the bulk of this solar cell. Both non-radiative recombination heat and Joule heat depend on defect density and voltage. The non-radiative recombination in solar cells includes the Auger recombination and the Shockley-Read-Hall (SRH) recombination due to mid-gap defects. Increasing the defect density from 1012 cm−3 to 1018 cm−3 increased the heat generation from 105 W/m3 to 109 W/m3. Also, voltage is an effective parameter in heat generation in solar cells. Increasing the voltage from 0 to 0.6 V (near the open-circuit voltage of the cell) has flattened out the profile of non-radiative recombination heat across the cell and the spike of heat generation at the junction has significantly reduced. In any case, the heat generation (due to defect or voltage increment) shows a maximum peak at CZTSSe/CdS junction and a smaller increase at (Mo(S,Se)2/CZTSSe) interface. Finally, the non-radiative recombination heat was calculated at both room temperature (initial run) and at secondary temperature distribution (coupled run).

Role of built-in potential over ETL/perovskite interface on the performance of HTL-free perovskite solar cells

Lead-free, Sn-based perovskite solar cells (PvSCs) have piqued the interest of researchers and their commercialization. Hole transport layer (HTL)-free PvSCs are low-cost and easy to fabricate. The built-in potential of a photovoltaic (PV) device plays an essential role in the separation of light-generated charge carriers. In this simulation work, we have investigated the built-in potential at the interface of electron transport layer (ETL) and Formamidinium tin iodide (FASnI3) with the help of Mott-Schottky plot for various types of ETLs. We have measured how the built-in potential was affected by thickness, doping density, and defect density of FASnI3 perovskite absorber layer and found that the built-in potential of ETL/FASnI3 interface had a significant impact on the device performance parameters. The built-in potential greatly affects the Shockley-Read-Hall (SRH) recombination mechanism. In this simulation, the material parameters of each layer have been optimized and obtained an optimized PCE of more than 16% for all devices. It is anticipated that the results of this study have great potential to understand the role of built-in potential in n-p junction PV devices and consequently device optimization of not only for HTL-free FASnI3-based PvSCs but also for other single n-p junction PvSCs. However, the fabrication of HTL-free FASnI3-based PvSCs is still a challenging task due to the oxidation of Sn2+ to Sn4+.

Quantum efficiency of InGaN–GaN multi-quantum well solar cells: Experimental characterization and modeling

InGaN-based multi-quantum well (MQW) solar cells are promising devices for photovoltaics (e.g., for tandem solar cells and concentrator systems), space applications, and wireless power transfer. In order to improve the efficiency of these devices, the factors limiting their efficiency and stability must be investigated in detail. Due to the complexity of a MQW structure, compared with a simple pn junction, modeling the spectral response of these solar cells is not straightforward, and ad hoc methodologies must be implemented. In this paper, we propose a model, based on material parameters and closed-formula equations, that describes the shape of the quantum efficiency of InGaN/GaN MQW solar cells, by taking into account the layer thickness, the temperature dependence of the absorption coefficient, and quantum confinement effects. We demonstrate (i) that the proposed model can effectively reproduce the spectral response of the cells; in addition, (ii) we prove that the bulk p-GaN layer absorbs radiation, but the carriers photogenerated in this region do not significantly contribute to device current. Finally, we show that (iii) by increasing the temperature, there is a redshift of the absorption edge due to bandgap narrowing, which can be described by Varshni law and is taken into account by the model, and a lowering in the extraction efficiency due to the increase in recombination (mostly Shockley–Read–Hall) inside the quantum wells, which is also visible by decreasing light intensity.

Characteristics Extraction of Fully Symmetric GAA and Top-Gate CNTFETs with 6 nm Channel Length

High-compatible applications for top-gate and gate-all-around (GAA) carbon nanotube field-effect transistors (CNTFETs) are presented. The geometrical specifications of these CNTFETs are evaluated and their effects on the characteristics are highlighted. The channel length is considered 6 nm, while the drain, source, and spacer are symmetric with channel length. The evaluations are performed by the COMSOL Multiphysics, and the Shockley-Read-Hall (SRH) recombination model is used to analyze doping, electron, and hole distributions. The drain and source wells are considered with donor doping of ND0 = 1 × 1021 cm−3 and acceptor doping of NA = 1 × 1019 cm−3, then, the current-voltage (I–V) characteristics are extracted. Also, the CNTs with 1.12 nm are placed accurately through the channel, and the terminals are constructed with graphene. The I–V curves show that the threshold voltage for the top-gate and GAA are 0.23 V and 0.21 V, respectively. Besides, the short channel effect (SCE) is reduced which is confirmed by a subthreshold swing (SS) of 62 mV/dec for the top-gate and 58 mV/dec for the GAA. Moreover, drain-induced barrier lowering (DIBL) and ION/IOFF ratio parameters are studied to investigate scaled-down devices. The discussed structures are compared by the figure of merit (FoM) of ΔVDIBLSS/(ION/IOFF), which shows more desirable and better channel control. The temperature variations show that there is no dramatic increase in the leakage current, which proves that symmetric structure is reliable in short-channel devices.

Electroluminescence explored internal behavior of carriers in InGaAsP single-junction solar cell

The internal behaviors of carriers in InGaAsP single-junction solar cell are investigated by using electroluminescence (EL) measurements. Two emission peaks can be observed in current-dependent electroluminescence spectra at low temperatures, and carrier localization exists for both peaks under low excitation. The trends of power index α extracted from excitation-dependent EL spectra at different temperatures imply that there exists a competition between Shockley–Read–Hall recombination and Auger recombination. Auger recombination becomes dominant at high temperatures, which is probably responsible for the lower current density of InGaAsP solar cell. Besides, the anomalous “S-shape” tendency with the temperature of band-edge peak position can be attributed to potential fluctuation and carrier redistribution, demonstrating delocalization, transfer, and redistribution of carriers in the continuum band-edge. Furthermore, the strong reduction of activation energy at high excitations indicates that electrons and holes escaped independently, and the faster-escaping carriers are holes.

Recombination-induced voltage-dependent photocurrent collection loss in CdTe thin film solar cell

Recently, the efficiency of CdTe thin film solar cell has been improved by using new type of window layer Mg x Zn1−x O (MZO). However, it is hard to achieve such a high efficiency as expected. In this report a comparative study is carried out between the MZO/CdTe and CdS/CdTe solar cells to investigate the factors affecting the device performance of MZO/CdTe solar cells. The efficiency loss quantified by voltage-dependent photocurrent collection efficiency (η C(V′)) is 3.89% for MZO/CdTe and 1.53% for CdS/CdTe solar cells. The higher efficiency loss for the MZO/CdTe solar cell is induced by more severe carrier recombination at the MZO/CdTe p–n junction interface and in CdTe bulk region than that for the CdS/CdTe solar cell. Activation energy (E a) of the reverse saturation current of the MZO/CdTe and CdS/CdTe solar cells are found to be 1.08 eV and 1.36 eV, respectively. These values indicate that for the CdS/CdTe solar cell the carrier recombination is dominated by bulk Shockley–Read–Hall (SRH) recombination and for the MZO/CdTe solar cell the carrier recombination is dominated by the p–n junction interface recombination. It is found that the tunneling-enhanced interface recombination is also involved in carrier recombination in the MZO/CdTe solar cell. This work demonstrates the poor device performance of the MZO/CdTe solar cell is induced by more severe interface and bulk recombination than that of the CdS/CdTe solar cell.

Performance improvement of three terminal heterojunction bipolar transistor based hybrid solar cell using nano-rods

In this research, performance enhancement of three terminal heterojunction bipolar transistor structure based solar cell (HBTSC) is investigated via implementing perovskite layer as emitter absorber region and nanorods within emitter. For this study, a p-n-p hybrid HBTSC structure has been proposed with MAPbI3/CdS/ACZTSe materials for photon absorber layer, and MgF2 for anti-reflection layer (ARL). The purpose of this research is mainly to optimize emitter (MAPbI3) and CdS nanorods thickness of HBTSC structure therefore boosting device efficiency through optoelectronic and electrical simulations. During obtaining the results, the effects of the surface, Shockley–Read–Hall (SRH) and radiative recombination mechanisms has been considered. Two new proposed structures based on planar MAPbI3/CdS/ACZTSe & MAPbI3/CdS nanorods/CdS/ACZTSe perovskite–kesterite based hybrid HBTSCs has been introduced. The maximum efficiency (η) of 23.45% and short-circuit current density (Jsc) of 39.93 mAcm−2 are obtained for planar MAPbI3/CdS/ACZTSe based perovskite–kesterite HBTSCs with open-circuit voltage (Voc) of 760.05 mV and fill factor (FF) of 77.28%, respectively. As for MAPbI3/CdS nanorods/CdS/ACZTSe based hybrid HBTSCs with η of 27.91%, Jsc of 42.03 mAcm−2, Voc of 786.8 mV, and FF of 84.39%, a total improvement of 4.46% is achieved when compared to planar hybrid perovskite–kesterite HBTSCs structure.

N-polar GaN p-n junction diodes with low ideality factors

High-quality N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy. The room-temperature current–voltage characteristics reveal a high on/off current ratio of >1011 at ±4 V and an ideality factor of 1.6. As the temperature increases to 200 °C, the apparent ideality factor gradually approaches 2. At such high temperatures, Shockley–Read–Hall recombination times of 0.32–0.46 ns are estimated. The measured electroluminescence spectrum is dominated by a strong near-band edge emission, while deep level and acceptor-related luminescence is greatly suppressed.

Hydrodynamic and Energy Transport Model-Based Hot-Carrier Effect in GaAs pin Solar Cell

The hot-carrier effect and hot-carrier dynamics in GaAs solar cell device performance were investigated. Hot-carrier solar cells based on the conventional operation principle were simulated based on the detailed balance thermodynamic model and the hydrodynamic energy transportation model. A quasi-equivalence between these two models was demonstrated for the first time. In the simulation, a specially designed GaAs solar cell was used, and an increase in the open-circuit voltage was observed by increasing the hot-carrier energy relaxation time. A detailed analysis was presented regarding the spatial distribution of hot-carrier temperature and its interplay with the electric field and three hot-carrier recombination processes: Auger, Shockley–Read–Hall, and radiative recombinations.

Large‐Area (Ag,Cu)(In,Ga)Se2 Thin‐Film Solar Cells with Increased Bandgap and Reduced Voltage Losses Realized with Bulk Defect Reduction and Front‐Grading of the Absorber Bandgap

The 1.24 eV bandgap, 18.8% power conversion efficiency Ag‐alloyed chalcopyrite (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells are characterized to relate voltage and efficiency improvements to electro‐optical (EO) characteristics. Shockley–Read–Hall recombination center defect density, identified and characterized through deep level transient spectroscopy and time‐resolved photoluminescence (TRPL), is reduced through potassium and copper treatment optimization. Concomitantly, longer minority carrier lifetimes are achieved, which increases open‐circuit voltage (VOC). Near‐conduction band defects associated in earlier studies with light‐induced current instability are also mitigated. Analysis of charge‐carrier dynamics after single‐ and two‐photon excitation is used to separate recombination at the front interface and in the absorber bulk. From TRPL decay simulations, the authors estimate ranges of key solar cell material characteristics: bulk carrier lifetime τbulk = 110–210 ns, charge‐carrier mobility μ = 110–160 cm2 V−1 s−1, and front interface recombination velocity Sfront = 700–1050 cm s−1. This lowest‐reported Sfront for ACIGS absorbers originates from the notched conduction band grading, which also makes the impact of the back interface recombination negligible. It is suggested in the results that solar cell performance enhancements can be made most readily with two distinct strategies: improving device architecture and reducing semiconductor defect densities. Using these approaches, power conversion efficiency in large‐area solar cells is improved by 1.1% absolute.

SRH suppressed P-G-I design for very long-wavelength infrared HgCdTe photodiodes.

The very long wavelength infrared (VLWIR, >14 µm) spectral band is an indispensable part of new-generation infrared remote sensing. Mercury cadmium telluride (HgCdTe or MCT) has shown excellent potential across the entire infrared band. However, the dark current, which is extremely sensitive to the technological level and small Cd composition, severely limits the performance of VLWIR HgCdTe photodiodes. In this study, cut-off wavelengths of up to 15 µm for HgCdTe devices with novel P-G-I (including wide bandgap p-type cap layer, grading layer and intrinsic absorption layer) designs have been reported. Compared with a device with a double-layer heterojunction (DLHJ) structure, the designed P-G-I structure successfully reduced dark current by suppressing the Shockley-Read-Hall process. Considering the balance of quantum efficiency and dark current, with the introduction of an approximately 0.8 µm thickness Cd composition grading layer, the device can achieve a high detectivity of up to 2.5×1011 cm Hz1/2 W-1. Experiments show that the P-G-I-T device has a lower dark current and a better SRH process suppressing ability than DLHJ devices, the measured detectivity achieved 8.7×1010 cm Hz1/2 W-1. According to additional research, the trap-assisted tunneling current is the primary component of the dark current. Controlling the trap concentration to as low as 1×1013 cm-3 will be continuous and meaningful work. The proposed study provides guidance for VLWIR HgCdTe photodetectors.

Study of nonlinear effects and self-heating in a silicon microring resonator including a Shockley-Read-Hall model for carrier recombination.

A detailed description of the non-linear effects in silicon is needed when designing ring resonators in the silicon platform. The optical field propagating in the ring waveguide is strongly absorbed due to two-photon-absorption (TPA) and free-carrier-absorption (FCA), which become more prominent with increasing the input power in the ring. We present a new approach for the modelling of non-linear effects in silicon based ring resonators. We have numerically solved the non-linear problem coupling the variation of refractive index and loss due to TPA, FCA , self-heating and Shockley-Read-Hall (SRH) theory for trap-assisted recombination process. The model is validated by reproducing experimental measurements on a ring and a racetrack resonator having different Q-factors and waveguide cross-sections. As a result, we show that the SRH recombination is the origin of the dependence of free carrier lifetime on the power circulating in the ring and how this dependence is affected by the surface trap density and trap energy level. The model is then applied to the calculation of the maximum power that can incident the silicon rings designed for the Si PIC mirror of a hybrid III-V/Si widely tunable laser.

Enhanced p-Type Conductivity of NiOx Films with Divalent Cd Ion Doping for Efficient Inverted Perovskite Solar Cells.

The effect of substitutional metal dopants in NiOx on the structural and electronic structures is of great interest, particularly for increasing the p-type conductivities as a hole transport layer (HTL) applied in perovskite solar cells (PSCs). In this paper, experimental fabrications and density functional theory calculations have been carried out on Cd-doped NiOx films to examine the effect of divalent doping on the electronic and geometric structures of NiOx. The results indicate that divalent Cd dopants reduced the formation energy of the Ni vacancy (VNi) and created more VNi in the films, which enhanced the p-type conductivity of the NiOx films. In addition, Cd doping also deepened the valence band edge, reduced the monomolecular Shockley-Read-Hall (SRH) recombination losses, and promoted hole extraction and transport. Hence, the PSCs with Cd:NiOx HTLs manifest a high efficiency of 20.47%, a high photocurrent density of 23.00 mA cm-2, and a high fill factor of 79.62%, as well as negligible hysteresis and excellent stability. This work illustrates that divalent elements such as Cd, Zn, Co, etc. may be potential dopants to improve the p-type conductivity of the NiOx films for applications in highly efficient and stabilized PSCs.

Calculation of Minority Carriers’ Lifetime in HgCdTe Structures by Using the Method Based on Mutual Correlation Between the Concentration of Electrical Particles

We perform a theoretical study of the bulk generation-recombination mechanisms, and the carrier lifetime in n- and p-type long- and mid-wavelength infrared HgCdTe structures at liquid nitrogen (LN) temperature has been done. This article is a continuation of our previous work and presents a method for calculating the excess minority carrier lifetime considering the mutual correlation between the concentration of electrons, holes, ionized impurities, and trap states resulting from the principle of charge conservation. It has been shown that recombination involving shallow impurity states in HgCdTe structures with low intrinsic concentration has a significant impact on minority carriers’ lifetime. This applies to both p- and n-type materials. The results of calculations of the carrier lifetime determined by the interband processes: radiative, Auger 1 and 7, recombination involving impurities, as well as and Shockley–Read–Hall (SRH) mechanisms related to mercury vacancies have been compared with experimental data presented by other authors. By analyzing the published calculations made by American researchers, the values of the overlap integrals <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert {F}_{1}{F}_{2} \vert $ </tex-math></inline-formula> for Auger processes were determined, which allowed to obtain a good compliance of our calculations with the experimental values. In this article, the cross sections for SRH recombination with the participation of mercury vacancies were estimated. These values are lower than those previously presented by other researchers. By verifying the results of the calculations with the experimental data, we confirmed the effectiveness of the proposed method.

Light-induced trap emptying revealed by intensity-dependent quantum efficiency of organic solar cells

Revisiting the intensity-dependent quantum efficiency (IDQE) technique in the context of non-fullerene acceptors, we find that at forward-bias conditions, the response exhibits what seems to be anomalous behavior that is not consistent with light excitation induced trap filling. Analysis based on the Shockley–Read–Hall model leads to the conclusion that the contacts cause the traps to be completely full in the dark. The role of the light excitation is to half-empty the traps, and thus, the “anomalous” behavior is created. By fitting the IDQE at several bias levels, we find that the trapping is consistent with multiphonon capture by a state close to the middle of the gap. As trap-assisted recombination is a significant loss mechanism, it is essential to fully monitor it for indoor applications as well as to cross the single junction 20% power conversion efficiency limit.

Simulation of CZTSSe kesterite thin film photovoltaics with TiO2 or CdS/ZnO: Comparing the electrical and thermal properties

The optical, electrical and thermal properties of two Cu2ZnSn4SxSe4−x (CZTSSe) based thin film solar cells with TiO2 and CdS/ZnO electron transport layer (ETL) were investigated through 2D and 3D mapping simulations. The simulations compared the optical photogeneration rates, current-voltage characteristics of the two cells, Shockley-Read-Hall (SRH) recombination rate and the electric-field distribution at the interface of CZTSSe and ETL materials by solving the coupled drift-diffusion, Poisson, and rate equations. The maximum non-radiative SRH recombination is 2.5 × 1028 1/m3s at CSSs/TiO2 interface which is half of it at CZTSSe/CdS/ZnO interface (5 × 1028 1/m3. s). This is because of the electric-field spike at the CdS/ZnO/ITO interface which enhances the electron passivation at this interface and in turn improves the open-circuit voltage of the CZTSSe/CdS/ZnO based solar cell to 0.55 V. The heat generation in the two cells was also simulated by solving the heat equation coupled with electrical and optical wave equations. SRH recombination heat is higher in CZTSSe/CdS/ZnO cell (1010 W/m3) compared to cell with CZTSSe/TiO2 (0.4 × 1010 W/m3) due to less average heat conductivity of CdS/ZnO. The conductive heat flux is higher at the junction of CZTSSe/TiO2 (5 × 105 W/m2) which is over 10 times more than the heat conduction flux at the junction of CZTSSe/CdS/ZnO (2 × 104 W/m2). The practical outcome of this simulation is that although CdS/ZnO is a better ETL material for electrical characteristics of the cell, the thermal stability of the cell is better with TiO2 layer. This might be a guiding outcome for practical realization of CZTSSe solar cell structures with high thermal stability.