Quasi-neutral Region Research Articles

Single-charge band-to-band tunneling via multiple-dopant clusters in nanoscale Si Esaki diodes

The electrostatic potential of p+-n+ junctions, as in Esaki (tunnel) diodes, originates from the Coulomb potentials of ionized dopants in the depletion-layer, but it has been modeled so far based on uniform space-charge regions, ignoring the discrete and random dopant distribution. This model can explain well the band-to-band tunneling (BTBT) between the opposite bands of the quasineutral regions (conduction band in the n+-region and valence band in the p+-region). In this letter, we show that a BTBT transport model should contain the mechanism of tunneling via “inherent” localized bandgap-states, created by dopant-induced potential fluctuation, which becomes detectable as a parallel transport mechanism in nanoscale Esaki diodes. This is manifested by the observation of single-charge (SC) BTBT at 5.5 K in nanoscale Si Esaki diodes. Numerical analysis of nanoscale p+-n+ junctions with random dopant-atom distributions suggests that SC-BTBT is mediated by a potential dip created by a number of dopants “clustered” near each other, i.e., by a multiple-dopant cluster.

Potential-induced degradation: Recombination behavior, temperature coefficients and mismatch losses in crystalline silicon photovoltaic power plant

In recent years, potential-induced degradation (PID) has gained considerable attention due to the significant negative impact on output power of photovoltaic power plant. In this paper, the recombination behavior of PID-affected solar modules dismounted from the photovoltaic power plant was investigated by the two-diode model of photovoltaic module. The mechanism of PID shunting, which is caused by sodium decorated stacking faults across n+-p junction, was further verified by analyzing changes in the quasi-neutral region and the space-charge region for PID-affected p-type crystalline silicon solar modules. It was found that the depletion region recombination increases with the extent of PID. The electroluminescence images also revealed that there exist strong ohmic shunts for PID-affected modules. Then the fundamental temperature coefficients of PID-affected solar modules were investigated. It was found that the increased depletion region recombination leads to a larger temperature coefficient of maximum power for PID-affected solar modules. The result was also verified by the thermography images of PID-affected modules. Finally, the relative mismatch losses (RML) of PID-affected module strings were analyzed. The results revealed that the RML of PID-affected module strings increase as the root-mean-square error (RMSE) of current at maximum power point progressed for the first time.

On the importance of the junction temperature on obtaining the charge transport parameters of a solar cell

The charge transport parameters (CTP) of a solar cell are extremely important as they provide the information required to do the fine tuning of its design and associated technology. This work demonstrates the necessity of knowing the junction temperature to extract them, other way their values might be wrong, misleading the device improvement efforts. Here the junction temperature and the CTP are extracted utilizing a dedicated structure similar to a bipolar junction transistor which allows to screen and separate the recombination current in the space charge region of the solar cell junction from that due to the injection-diffusion-recombination of minority carriers in its quasi-neutral regions. The additional pn junction is located beneath that of the solar cell, in such a way that the minority carriers injected into its base are able to reach the additional junction. Biasing the solar cell junction without biasing the additional one the neat separation of those currents is achieved. From the diffusion current reaching the additional junction the cell temperature is extracted as well as its diffusion saturation current. Then, with these two parameters the remaining ones are straightly obtained. Moreover, the current separation allows the cross verification of the extracted parameters. Additionally, the involved theory, model of Shockley, allows an estimation of the diffusion length and life time of the minority carriers in the solar cell base region. The method, applied to the n-GaInP/p-GaAs heterojunction solar cell shows that under the measurement conditions the true junction temperature was T0 = 296.343 K, the studied structure at this temperature has a diffusion saturation current of J0D = 3.9 × 10−19 A cm−2 and a recombination current density of J0R = 1.9 × 10−12 A cm−2, diffusion length and life time of minority carriers in the base region of Ln = 0.46 μm and τn = 1.2 × 10−10 s, respectively. These saturation current values compared with those obtained through the conventional mathematical fit are orders of magnitude different.

The plasma-wall transition with collisions and an oblique magnetic field: Reversal of potential drops at grazing incidences

The plasma-wall transition is studied by using 1d3V particle-in-cell simulations in the case of a one dimensional plasma bounded by two absorbing walls separated by 200 Debye lengths (λd). A constant and oblique magnetic field is applied to the system, with an amplitude such that r < λd < R, where r and R are the electron and ion Larmor radii, respectively. Collisions with neutrals are taken into account and modelled by an energy conservative operator, which randomly reorients ion and electron velocities. The plasma-wall transition (PWT) is shown to depend on both the angle of incidence of the magnetic field with respect to the wall, θ, and on the ion mean-free-path to Larmor radius ratio, λci/R. In the very low collisionality regime (λci≫R) and for a large angle of incidence, the PWT consists of the classical tri-layer structure (Debye sheath/Chodura sheath/pre-sheath) from the wall towards the center of the plasma. The drops of potential within different regions are well consistent with already published models. However, when sin θ≤R/λci or with the ordering λci<R, collisions cannot be neglected, leading to the disappearance of the Chodura sheath. In this case, a collisional model yields analytic expressions for the potential drop in the quasi-neutral region and explains, in qualitative and quantitative agreement with the simulation results, its reversal below a critical angle derived in this paper, a regime possibly met in the scrape-off layers of tokamaks. It is further shown that the potential drop in the Debye sheath slightly varies with the collisionality for λci≫R. However, it tends to decrease with λci in the high collisionality regime, until the Debye sheath finally vanishes.

Spatially Resolved Recombination Analysis of CuInxGa1-xSe2 Absorbers With Alkali Postdeposition Treatments

In this contribution, we probe spatial variations in charge-carrier recombination in CuInxGa1-x Se2 (CIGS) absorbers grown on soda–lime glass (SLG) and alkali-free sapphire substrates with NaF and KF postdeposition treatments (PDTs). Temperature-and illumination-dependent device measurements are used to track interface recombination and recombination in the quasi-neutral region. The analysis of these data reveals that the benefit of alkali PDTs depends on the substrate: interface recombination is reduced in devices grown on sapphire substrates, whereas recombination in the quasi-neutral regions is reduced in devices grown on SLG substrates. Cathodoluminescence (CL) spectrum imaging is used to study the spatial distribution of recombination with respect to the grain structure. The grain-boundary CL contrast is similar in films with no PDT, NaF PDT, or KF PDT. A reduced grain-boundary contrast is observed with a NaF + KF PDT; however, suggesting a reduced recombination strength at the grain boundaries (GBs) for combined NaF + KF treatment. CL spectra indicate band tailing, consistent with the fluctuating potential model. Fluctuating potentials are believed to reduce open-circuit voltage, but their spatial distribution has not been studied. Here, CL spectrum imaging data are used to generate maps of the root-mean-square value of the potential energy fluctuations—γ. These maps reveal a bimodal γ distribution for all samples: γ is generally in the range ∼15–50 meV or ∼100–180 meV. The higher γ range is more significantly affected by the PDTs; after the PDTs, it is strongly correlated with GBs. The lower γ range is correlated with higher emission intensity regions, typically grain interiors, and increases in area fraction after the PDTs. These results demonstrate how spatially resolved luminescence and device characterization measurements can be used to monitor changes in recombination in CIGS films and photovoltaic devices. Such measurements can complement empirical device optimization and help improve device performance.

A path-finding toward high-efficiency penternary Cu(In,Ga)(Se,S)2 thin film solar module

The optimal p-n junction structure in a state-of-the-art Cu(In,Ga)(Se,S)2 thin-film solar module technology is investigated. For co-optimization design and path-finding, a TCAD model is developed with experimental samples. The engineerable parameters, i.e., FGa, GGIavg, and CdS thickness, are demonstrated to play a critical role in determining the p-n junction properties such as dark current characteristics Jdark(V), voltage-dependent photocurrent, localized carrier collection efficiency, and interface carrier transportation. We show the optimal Ga-grading is determined by a trade-off between the recombination loss in space charge region and the photo-carrier collection in quasi-neutral region. The optimal CdS thickness is determined by a trade-off between carrier collection efficiency, short-circuit current (JSC) loss, and Jdark(V), which depends on varied Ga-profiles. Overall, thin CdS (≦10 nm) is preferred to reduce the JSC loss in accumulated Ga-profiles, while thicker CdS is preferred to enhance the carrier collection efficiency in flatter Ga-profiles. The band alignment effect on varied Cu(In,Ga)(Se,S)2/CdS junctions is also investigated. It is found sulfur-incorporation can suppress the VOC saturation behavior at wide bandgap. For CIGSeS absorber with SS = 20% and DP =15%, the maximum VOC of 780 mV can be achieved by co-optimized Ga-profile. Furthermore, varied Ga-profiles and CdS buffer layers are explored for pathfinding. An optimal p-n junction structure shows a relative +40% efficiency improvement from 15.5% to 21.9%. This work shows the efficiency headroom of reported CIGSeS thin-film solar module technology through co-optimized CIGSeS composition gradient and buffer layer.

Butylamine-Catalyzed Synthesis of Nanocrystal Inks Enables Efficient Infrared CQD Solar Cells.

The best-performing colloidal-quantum-dot (CQD) photovoltaic devices suffer from charge recombination within the quasi-neutral region near the back hole-extracting junction. Graded architectures, which provide a widened depletion region at the back junction of device, could overcome this challenge. However, since today's best materials are processed using solvents that lack orthogonality, these architectures have not yet been implemented using the best-performing CQD solids. Here, a new CQD ink that is stable in nonpolar solvents is developed via a neutral donor ligand that functions as a phase-transfer catalyst. This enables the realization of an efficient graded architecture that, with an engineered band-alignment at the back junction, improves the built-in field and charge extraction. As a result, optimized IR CQD solar cells (Eg ≈ 1.3 eV) exhibiting a power conversion efficiency (PCE) of 12.3% are reported. The strategy is applied to small-bandgap (1 eV) IR CQDs to augment the performance of perovskite and crystalline silicon (cSi) 4-terminal tandem solar cells. The devices show the highest PCE addition achieved using a solution-processed active layer: a value of +5% when illuminated through a 1.58 eV bandgap perovskite front filter, providing a pathway to exceed PCEs of 23% in 4T tandem configurations with IR CQD PVs.

Electrostatics of two-dimensional lateral junctions

The increasing technological control of two-dimensional (2D) materials has allowed the demonstration of 2D lateral junctions exhibiting unique properties that might serve as the basis for a new generation of 2D electronic and optoelectronic devices. Notably, the chemically doped MoS2 homojunction, the WSe2-MoS2 monolayer and MoS2 monolayer/multilayer heterojunctions, have been demonstrated. Here we report the investigation of 2D lateral junction electrostatics, which differs from the bulk case because of the weaker screening, producing a much longer transition region between the space-charge region and the quasi-neutral region, making inappropriate the use of the complete-depletion region approximation. For such a purpose we have developed a method based on the conformal mapping technique to solve the 2D electrostatics, widely applicable to every kind of junctions, giving accurate results for even large asymmetric charge distribution scenarios.

Analysis of Back-Contact Interface Recombination in Thin-Film Solar Cells

The open-circuit voltage (VOC) in a generic TCO/buffer/absorber/back-contact thin-film solar cell device is a key parameter in the recombination analysis. In particular, VOC is sensitively influenced by the interface recombination at the buffer/absorber front interface and at the absorber/back-contact interface. This paper reports the temperature, excitation light intensity, and wavelength-dependent open-circuit voltage analysis to separate and quantify recombination rates in solar cells at the front and back interfaces, in the depletion regions, and in the quasi-neutral region. The wavelength-dependent VOC analysis is exploited to extract the absorber/back-contact recombination coefficient. The experimentally observed results are verified using SCAPS-1D (one dimensional-a solar cell capacitance simulator) simulation.

Investigation of correlation between open-circuit voltage deficit and carrier recombination rates in Cu(In,Ga)(S,Se)2-based thin-film solar cells

The temperature-illumination-dependent open-circuit voltage (VOC) method is utilized to separately and quantitatively estimate carrier recombination rates at the buffer/absorber interface, in the space-charge region (SCR), and in the quasi-neutral region (QNR) of Cu(In,Ga)(S,Se)2 (CIGSSe)-based thin-film solar cells with various device structures. The correlation between open-circuit voltage deficits (VOC,def) among the carrier recombination rates of the CIGSSe solar cells with a conversion efficiency (η) above 17% is examined. It is revealed that VOC,def is decreased to 0.373 V with the reduced carrier recombination rate at the buffer/absorber interface through the development of device structures. To further decrease VOC,def (for the improved η), the carrier recombination rates in SCR and QNR are essential to be reduced by the further improvement of CIGSSe quality. Consequently, understanding the quantitative carrier recombination rates across the device, estimated from the temperature-illumination-dependent VOC method, is practical to know which part of the solar cell needs to be developed for high η above 20%.

Non-equilibrium synergistic effects in atmospheric pressure plasmas

Non-equilibrium is one of the important features of an atmospheric gas discharge plasma. It involves complicated physical-chemical processes and plays a key role in various actual plasma processing. In this report, a novel complete non-equilibrium model is developed to reveal the non-equilibrium synergistic effects for the atmospheric-pressure low-temperature plasmas (AP-LTPs). It combines a thermal-chemical non-equilibrium fluid model for the quasi-neutral plasma region and a simplified sheath model for the electrode sheath region. The free-burning argon arc is selected as a model system because both the electrical-thermal-chemical equilibrium and non-equilibrium regions are involved simultaneously in this arc plasma system. The modeling results indicate for the first time that it is the strong and synergistic interactions among the mass, momentum and energy transfer processes that determine the self-consistent non-equilibrium characteristics of the AP-LTPs. An energy transfer process related to the non-uniform spatial distributions of the electron-to-heavy-particle temperature ratio has also been discovered for the first time. It has a significant influence for self-consistently predicting the transition region between the “hot” and “cold” equilibrium regions of an AP-LTP system. The modeling results would provide an instructive guidance for predicting and possibly controlling the non-equilibrium particle-energy transportation process in various AP-LTPs in future.

Kinetic features and non-stationary electron trapping in paraxial magnetic nozzles

The paraxial expansion of a collisionless plasma jet into vacuum, guided by a magnetic nozzle, is studied with an Eulerian and non-stationary Vlasov–Poisson solver. Parametric analyzes varying the magnetic field expansion rate, the size of the simulation box, and the electrostatic potential fall are presented. After choosing the potential fall leading to a zero net current beam, the steady states of the simulations exhibit a quasi-neutral region followed by a downstream sheath. The latter, an unavoidable consequence of the finite size of the computational domain, does not affect the quasi-neutral region if the box size is chosen appropriately. The steady state presents a strong decay of the perpendicular temperature of the electrons, whose profile versus the inverse of the magnetic field does not depend on the expansion rate within the quasi-neutral region. As a consequence, the electron distribution function is highly anisotropic downstream. The simulations revealed that the ions reach a higher velocity during the transient than in the steady state and their distribution functions are not far from mono-energetic. The density percentage of the population of electrons trapped during the transient, which is computed self-consistently by the code, is up to 25% of the total electron density in the quasi-neutral region. It is demonstrated that the exact amount depends on the history of the system and the steady state is not unique. Nevertheless, the amount of trapped electrons is smaller than the one assumed heuristically by kinetic stationary theories.

Study of the ionization rate of the released deuterium in vacuum arc discharges with metal deuteride cathodes

The ionization rate of the released deuterium from a metal deuteride cathode in vacuum arc discharges is investigated by both experiments and modeling analysis. Experimental results show that the deuterium ionization rate increases from 2% to 30% with the increasing arc current in the range of 2–100 A. Thus the full ionization assumption, as is widely used in arc plasma simulations, is not satisfied for the released deuterium at low discharge current. According to the modeling results, the neutral-to-ion conversion efficiency for the deuterium traveling across the cathodic spot region can be significantly less than one, due to the fast plasma expansion and rarefaction in the vacuum. In addition, the model also reveals that, unlike the metal atoms which are mainly ionized in the sheath region and flow back to the cathode, the deuterium ionization primarily occurs in the quasi-neutral region and moves towards the anode. Consequently, the cathodic sheath layer acts like a filter that increases the deuterium fraction beyond the sheath region.

The plasma-wall transition layers in the presence of collisions with a magnetic field parallel to the wall

The plasma-wall transition is studied by means of a particle-in-cell (PIC) simulation in the configuration of a parallel to the wall magnetic field (B), with collisions between charged particles vs. neutral atoms taken into account. The investigated system consists of a plasma bounded by two absorbing walls separated by 200 electron Debye lengths (λd). The strength of the magnetic field is chosen such as the ratio λ d / r l, with rl being the electron Larmor radius, is smaller or larger than unity. Collisions are modelled with a simple operator that reorients randomly ion or electron velocity, keeping constant the total kinetic energy of both the neutral atom (target) and the incident charged particle. The PIC simulations show that the plasma-wall transition consists in a quasi-neutral region (pre-sheath), from the center of the plasma towards the walls, where the electric potential or electric field profiles are well described by an ambipolar diffusion model, and in a second region at the vicinity of the...

Hybrid 3D model for the interaction of plasma thruster plumes with nearby objects

This paper presents a hybrid particle-in-cell (PIC) fluid approach to model the interaction of a plasma plume with a spacecraft and/or any nearby object. Ions and neutrals are modeled with a PIC approach, while electrons are treated as a fluid. After a first iteration of the code, the domain is split into quasineutral and non-neutral regions, based on non-neutrality criteria, such as the relative charge density and the Debye length-to-cell size ratio. At the material boundaries of the former quasineutral region, a dedicated algorithm ensures that the Bohm condition is met. In the latter non-neutral regions, the electron density and electric potential are obtained by solving the coupled electron momentum balance and Poisson equations. Boundary conditions for both the electric current and potential are finally obtained with a plasma sheath sub-code and an equivalent circuit model. The hybrid code is validated by applying it to a typical plasma plume–spacecraft interaction scenario, and the physics and capabilities of the model are finally discussed.

Influences of donor defect passivation on the performance of Cu(In,Ga)Se2 thin-film solar cell

In this work, the study of donor defect passivation in CIGS solar cell was simulated by wxAMPS software. The effects of near-interface shallow donor defect and bulk donor defect on the performances of solar cell were investigated mainly through changing the state densities and their position distributions. The results show that the high density of the near-interface shallow donor defect state decreases the open circuit voltage. By contrast, the low density of the near-interface shallow donor defect state has the higher open circuit voltage and short-circuit current density. But it easily leads to the CdS conduction band offset increased, then raising carrier recombination rate which decreases the fill factor. For the passivation of the bulk donor defect in the absorption layer, the performances of the CIGS solar cell are much dependent on the position distribution of donor defect state density. Among them, the decrease of the state density in the space charge region can improve the open-circuit voltage. Meanwhile, the reduction of the state density in the quasi-neutral region can increase the short-circuit current density. With the bulk donor defect state density decreased, the conversion efficiency will saturate due to the limitation of the acceptor concentration. When the donor defect state density is low enough, the influence of capture cross section on the device performance is reduced. The conversion efficiencies varying with passivation region were compared from two directions (CdS to Mo and Mo to CdS). It was found the passivation of the donor defect in the transition region where the position from the quasi-neutral region to the space charge region in the absorption was critical to the improvement in the conversion efficiency of the solar cell under a suitable alkali element concentration.

Theory and simulation of anode spots in low pressure plasmas

When electrodes are biased above the plasma potential, electrons accelerated through the associated electron sheath can dramatically increase the ionization rate of neutrals near the electrode surface. It has previously been observed that if the ionization rate is great enough, a double layer separates a luminous high-potential plasma attached to the electrode surface (called an anode spot or fireball) from the bulk plasma. Here, results of the first 2D particle-in-cell simulations of anode spot formation are presented along with a theoretical model describing the formation process. It is found that ionization leads to the build-up of an ion-rich layer adjacent to the electrode, forming a narrow potential well near the electrode surface that traps electrons born from ionization. It is shown that anode spot onset occurs when a quasineutral region is established in the potential well and the density in this region becomes large enough to violate the steady-state Langmuir condition, which is a balance between electron and ion fluxes across the double layer. A model for steady-state properties of the anode spot is also presented, which predicts values for the anode spot size, double layer potential drop, and form of the sheath at the electrode by considering particle, power, and current balance. These predictions are found to be consistent with the presented simulation and previous experiments.

P-n junction theory in view of excess majority carriers

The classical junction theory is reconstructed in view of “excess majority carriers”. The effects of excess majority carriers, in addition to the excess minority carriers, are described analytically and integrated into the theory. The quantity and distribution of excess majority carriers are presented, which give rise to an “accompanying electric field” and guarantee the space-charge neutrality of each side of the junction. The accompanying electric field is figured out analytically, and the contributions of majority carriers are clarified quantitatively, which enables the current continuity in either side of the junction. Furthermore, at any position in the quasi-neutral region, the various current components are solved in closed form and the net current is obtained in a straightforward way. The reconstructed theory provides a more thorough understanding of junction operation with physical conciseness.

ZnSnP2 solar cell with (Cd,Zn)S buffer layer: Analysis of recombination rates

A ZnSnP2 bulk crystal was prepared by flux method and cut into a ZnSnP2 wafer with thickness of 200µm, which was utilized as the absorber of a ZnSnP2 solar cell with a Al/ZnO:Al/ZnO/(Cd,Zn)S buffer/ZnSnP2 wafer/Cu structure. A 3.44%-efficient ZnSnP2 solar cell was obtained. Moreover, we assessed performance of the ZnSnP2 solar cell concentrating on main loss mechanisms limiting device efficiency. Based on investigations of the majority carrier barrier at ZnSnP2 and the Cu back electrode, the barrier height is estimated to be 59meV, which could limit the fill factor of the solar cell and should be decreased. In addition, voltage-independent recombination rates at the buffer/absorber interface (Ri0), in the space-charge region (Rd0), and in the quasi-neutral region (Rb0) of the ZnSnP2 solar cell were determined from illumination dependence of the open-circuit voltage. It was found that Rd0 is 2.65×1013cm−2s−1, much high than Ri0 + Rb0 of 1.2×109cm−2s−1. The high Rd0 was attributed to the mechanical surface polish of the ZnSnP2 wafer. It is thought that Rb0 should be lower than Ri0 because of the large ZnSnP2 grain and non-optimized buffer/absorber interface. These findings suggested that Ri0 and Rd0 should be further decreased to enhance photovoltaic performance.

Dark Currents in a Fully-Depleted LWIR HgCdTe P-on-n Heterojunction: Analytical and Numerical Simulations

In this paper, we use the theory of Evans and Landsberg, which is a generalization of the Shockley–Read–Hall recombination statistics in the space charge region (SCR), to include effects of Auger and radiative recombination processes that are also of origin in the SCR. Using analytical expressions for the current density, we calculate the total dark current density for a variety of conditions. Contributions include radiative and Auger transitions of origin in both the quasi-neutral region and the SCR. Numerical simulations are used to assess the nature of the limitations associated with the analytical calculation in the n-extrinsic region ( $$N_{\rm d} \gg n_{\rm i}$$ , where $$N_{\rm d}$$ is the doping concentration and $$n_{\rm i}$$ is the intrinsic carrier concentration), and to extend the calculations to operating temperatures in the intrinsic region ( $$n_{\rm i} \gg N_{\rm d}$$ ). Major findings include the observation that in a fully depleted $$P^+n$$ double-layer planar hetero-structure, at a reverse bias voltage sufficiently high to suppress the Auger process, SRH centers are not limiting, and the dark current is due to radiative transitions of origin in the n-side SCR. From the numerical simulations, while the Auger recombination rate changes drastically with varying the carrier concentration (such as applying reverse bias), the radiative recombination rate remains nearly invariant to varying the carrier concentration, and, as such, does not appreciably change with increasing reverse bias. Using the theory of van Roosbroeck and Shockley, the radiative recombination rate is obtained by integrating the measured optical absorption coefficient over all photon energies. Hence, the theory links the measured absorption coefficient to the measured dark current density for conditions in which the dominant current component is due to radiative recombination. Finally, the numerical simulations reveal, in both the n-extrinsic and intrinsic operating regions, that, under sufficient conditions, the detector is radiatively limited.