Excess Minority Carrier Density Research Articles

Effect of Wavelength on the Photocurrent and Photovoltage of Vertical Parallel Silicon Solar Cells under Steady State Condition

The effect of wavelength on the photocurrent and photovoltage of a vertical parallel junction solar cell under steady state condition was theoretically analyzed. Based on the diffusion equation, the excess minority carrier's density is expressed; both photocurrent density and photovoltage determine the junction recombination velocity and the wavelengths. The excess minority carrier density, the photocurrent density, and the photovoltage were calculated and plotted. The objective of this work is to show the effects of solar cell junction recombination velocity and the wavelengths on these electrical parameters. We have shown the effect of wavelength on initiating the short-circuit (Sfsc) and limited the open circuit (Sfoc) of the solar cell through the junction recombination velocity. The determination of Sfoc obtained from the curve of photovoltage versus Sf, and the determination of Sfsc is through from the curve of the photocurrent density according to Sf. The improvement of the solar cells limits the recombination of the excess minority carrier's photogenerated. Parameters and the control of the technological process of manufacturing are significant during the conception of these devices. Several methods of characterization in static or quasi-static mode, as well as in transient state and dynamic frequency mode have been developed for the determination of one or several parameters (1, 2). In this regard, we resolve continuity equation about relative to excess minority carriers in the base.

The effect of illumination power density on carbon defect configuration in silicon doped GaN

A study of efficacy of point defect reduction via Fermi level control during growth of GaN:Si as a function of above bandgap illumination power density and hence excess minority carrier density is presented. Electrical characterization revealed an almost two-fold increase in carrier concentration and a three-fold increase in mobility by increasing the illumination power density from 0 to 1 W cm−2, corroborating a decrease in compensation and ionic impurity scattering. The effect was further supported by the photoluminescence studies, which showed a monotonic decrease in yellow luminescence (attributed to CN) as a function of illumination power density. Secondary ion mass spectroscopy studies showed no effect of illumination on the total incorporation of Si or C. Thus, it is concluded that Fermi level management changed the configuration of the C impurity as the CN−1 configuration became energetically less favorable due to excess minority carriers.

Point defect reduction in wide bandgap semiconductors by defect quasi Fermi level control

A theoretical framework for a general approach to reduce point defect density in materials via control of defect quasi Fermi level (dQFL) is presented. The control of dQFL is achieved via excess minority carrier generation. General guidelines for controlling dQFL that lead to a significant reduction in compensating point defects in any doped material is proposed. The framework introduces and incorporates the effects of various factors that control the efficacy of the defect reduction process such as defect level, defect formation energy, bandgap, and excess minority carrier density. Modified formation energy diagrams are proposed, which illustrate the effect of the quasi Fermi level control on the defect formation energies. These formation energy diagrams provide powerful tools to determine the feasibility and requirements to produce the desired reduction in specified point defects. An experimental study of the effect of excess minority carriers on point defect incorporation in GaN and AlGaN shows an excellent quantitative agreement with the theoretical predictions. Illumination at energies larger than the bandgap is employed as a means to generate excess minority carriers. The case studies with CN in Si doped GaN, H and VN in Mg doped GaN and VM-2ON in Si doped Al0.65Ga0.35N revealed a significant reduction in impurities in agreement with the proposed theory. Since compensating point defects control the material performance (this is particularly challenging in wide and ultra wide bandgap materials), dQFL control is a highly promising technique with wide scope and may be utilized to improve the properties of various materials systems and performance of devices based upon them.

Effect of incidence angle on the electrical parameters of vertical parallel junction silicon solar cell under frequency domain

The effect of incidence angle on the electrical parameters of vertical parallel silicon solar cell under frequency domain was theoretically analyzed. Based on the diffusion-recombination equation, the expression of excess minority carrier density in the base was established according to the modulation frequency and the illumination incidence angle. The excess minority carrier density, the photocurrent density, the photo voltage, series resistance, shunt resistance, electric power and the space charge region capacitance were calculated and plotted. The objective of this work was to show the effects of solar cell modulation frequency and the illumination incidence angle on these electrical parameters, electric power and space charge region capacitance. Plots of solar cell’s electric power with the junction recombination velocity gave the maximum solar cell’s electric power; Pm. Influence of various parameters of incidence angles on the solar cell’s electric power was also studied.

On the capacitance of crystalline silicon solar cells in steady state

In this work, an analytical approach is presented for modeling the capacitance of crystalline silicon solar cells. Based on a one-dimensional modeling of the cell, the excess minority carrier density, the photovoltage, and the capacitance are calculated. The motivation of this work are two-fold: to show base doping density and illumination effects on the capacitance of silicon solar cells, and to propose a determination technique for both dark capacitance and base doping density from C-V characteristics.

Experimental study on scan imaging of silicon wafer by laser-induced photocarrier radiometry

In this paper, the modulated laser-induced excess minority carrier density wave (CDW) of bipolar semiconductor is developed. An analytical expression of CDW in frequency domain is introduced. The numerical simulations are carried out to analyze the effects of minority carrier transport parameters (minority carrier lifetime, diffusion coefficient, and two surfaces recombination velocities) on response of laser-induced photocarrier radiometry (PCR) signal to frequency for semiconductor silicon wafer. The PCR amplitude increases with increasing the minority carrier lifetime, and decreased with increasing the carrier diffusion coefficient and surface recombination velocity. In contrast, the PCR phase lag decreases with increasing the minority carrier lifetime, and increases with increasing the carrier diffusion coefficient and surface recombination velocity. The silicon (Si) wafer with an artificial defect (mechanical scratch) on the surface is experimentally investigated by PCR scanning image system. The distribution maps of the minority carrier transport parameters are obtained by best-fitting method which is based on the carrier density wave analytical expression, and the influences of the artificial defect on carrier transport parameters are discussed in detail. The experimental results indicate that the surface recombination velocity and carrier diffusivity at artificial damaged location are dramatically increased compared with those in the healthy region. The carrier bulk lifetime of whole Si wafer is obtained to be about 38.33 μs by PCR scanning image measurements. Simultaneously, quasi steady-state photoconductance (QSSPC) method is used to measure the carrier effective lifetime of Si wafer, and it is about 33.85 μs. Therefore, the carrier bulk lifetime of Si wafer by PCR scanning image measurement is in good agreement with the QSSPC measurement. However, QSSPC measurement could obtain only the carrier effective lifetime of Si wafer. Furthermore, PCR scanning image measurement can be employed to measure the carrier transport parameters with high resolution in comparison with QSSPC measurement, and to evaluate the localized imperfection.

New Approach to Determine Vertical Parallel Junction Silicon Solar Cell Parameters

The aim of this work is to study the electrical behavior of a vertical parallel junction silicon solar cell by mean of impedance spectroscopy. By solving diffusion-recombination equation, the excess minority carrier density expression was established and the photocurrent and the photo voltage were deduced. We plotted Nyquist diagram of the solar cell impedance and gave corresponding electrical circuits. We then introduced impedance spectroscopy method to access for the first time to some electrical parameters of a vertical junction solar cell (series resistance, parallel resistance, capacitance and inductance).

Influence of Temperature on the Electrical Parameters of a Vertical Parallel Junction Silicon Solar Cell under Polychromatic Illumination in Steady State

This study presents a theoretical study of a vertical parallel junction solar cell under multispectral illumination in steady state. Based on the diffusion equation, the excess minority carrier's density is expressed and both photocurrent density and photovoltage are determined. For all these parameters we showed the effect of external temperature with respect to the operating point of the solar cell through the junction recombination velocity.

INVESTIGATION ON SILICON SOLAR CELL CAPACITANCE AND ITS DEPENDENCE ON BOTH TEMPERATURE AND INCIDENCE ANGLE

The aim of this work is to investigate a theoretical study of a vertical junction silicon solar cell capacitance under monochromatic illumination. By solving the continuity equation and using a one dimensional model in frequency modulation, we derive the analytical expressions of both excess minority carrier density and photovoltage. Based on these expressions, the solar cell capacitance was calculated; we then exhibited the effects of both temperature and incidence angle on the solar cell capacitance.

Incidence angle and spectral effects on vertical junction silicon solar cell capacitance

The aim of this work is to present a theoretical study of a vertical junction silicon solar cell under monochromatic illumination. By solving the continuity equation and using a one-dimensional model in frequency modulation, we derive the analytical expressions of both excess minority carrier density and photovoltage. Based on these expressions, the solar cell capacitance was calculated; we then exhibited the effects of both illumination wavelength and incidence angle on the solar cell capacitance.

Applications of Photoluminescence Imaging to Dopant and Carrier Concentration Measurements of Silicon Wafers

Photoluminescence-based imaging is most commonly used to measure the excess minority carrier density and its corresponding lifetime. By using appropriate surface treatments, this high-resolution imaging technique can also be used for majority carrier concentration determination. The mechanism involves effectively pinning the minority excess carrier density, resulting in a dependence of the photoluminescence intensity on only the majority carrier density. Three suitable surface preparation methods are introduced in this paper: aluminum sputtering, deionized water etching, and mechanical abrasion. Spatially resolved dopant density images determined using this technique are consistent with the images obtained by a well-established technique based on free carrier infrared emission. Three applications of the technique are also presented in this paper, which include imaging of oxygen-related thermal donors, radial dopant density analysis, and the study of donor-related recombination active defects. These applications demonstrate the usefulness of the technique in characterizing silicon materials for photovoltaics.

Spatially Resolved Electroluminescence Imaging of Shunt Sources in Crystalline Silicon Solar Cells

Electroluminescence (EL) under forward bias represents the total excess minority carrier density in cells. In contrast, EL under reverse bias can be detected as hot spots, which are closely related to harmful current shunt paths. In this study, we detected the shunt position using two kinds of EL. Additionally, we analyzed by the positions and origins of shunt sources using electron-beam-induced-current, lock-in thermography, and an electron-probe-micro analyser. We found two kinds of shunt and we detected a defect located in the depletion layer. We proposed shunt models in the depletion layer using the band model.

Effect of Interface States on the Properties of a-Si:H/c-Si Heterojunction Solar Cells Based on Solar Materials

The effect of the interface states on the properties of (p+) a-Si:H/(n) c-Si heterojunction solar cells is studied by a set of simulations. The results show that there is almost no effect on the short-circuit current. At very low interface states, there is almost no effect on the open-circuit voltage VOC and the fill factor FF, and then the conversion efficiency. Although, at high interface states, VOC decreases due to the decrease of the excess minority carrier density at the c-Si neutral region and the increase of the effective interface recombination velocity at the heterojucntion interface, which also results in the decrease of FF. In particular, at very high interface states, the hole transport is limited by the interface potential barrier, which results in S-shaped J–V characteristics and low fill factors. As a result, the conversion efficiency decreases with interface states increasing at high interface states.

On the determination of the emitter saturation current density from lifetime measurements of silicon devices

ABSTRACTContactless photoconductance measurements are commonly used to extract the emitter saturation current density (Joe) for crystalline silicon samples containing an emitter on the surface. We review the physics behind the analysis of Joe and compare the commonly used approximations with more generalised solutions using two‐dimensional device simulations. We quantify errors present in such approximations for different test conditions involving varying illumination conditions and surface properties in samples with the same emitter on both sides. The simulated Joe obtained from the dark hole current from the emitter into the bulk is nearly the same as the simulated Joe determined by photoconductance measurements of the rear diffusion. The simulated Joe at the front emitter is equivalent to that at the rear emitter only when the sample is subject to a nearly constant and flat generation profile. For illumination conditions including visible light, the simulated Joe at the front emitter is smaller than the simulated Joe at the rear emitter. Both Joe at the rear emitter and from the dark hole current in the emitter remain nearly constant over a wide range of base doping densities. The approximations used for the determination of Joe from photoconductance measurements make Joe dependent on the excess minority carrier density. Lifetime measurements demonstrate that, even in high‐quality silicon, Joe should be determined from the analytical solution as a function of excess minority carrier density including Shockley‐Read‐Hall recombination. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of Interface States on the Open-Circuit Volatage in a-Si:H/c-Si Heterojunction Solar Cells

The properties of the a-Si:H/c-Si interface are one of the critical issues for the photovoltaic application. The effects of the interface states on the open-circuit voltage VOC were performed by a set of simulations. VOC decreases with Dit increasing, especially at high values of Dit, since the interface states act as recombination centers to decrease the excess minority carrier density in c-Si. Since the conduction band offset ∆EC can saturate part of interface states, VOC increasing with ∆EC increasing.

Characterization of interface states in a-Si : H/c-Si heterojunctions by an expression of the theoretical diffusion capacitance

Capacitance spectroscopy under illumination and at a forward bias close to the open-circuit voltage (Voc) has recently been proposed to characterize interface states in a-Si : H/c-Si heterojunctions, and the interface defect density, Dit, is estimated from the simulations of capacitance. In this paper, the theoretical diffusion capacitance, CD, is presented for measurement to directly characterize the interface states. By solving the excess minority carrier density in c-Si when interface states are introduced, the expression of CD is developed as a function of Dit, the excess minority carrier density out of c-Si depletion and diffusion regions, Δn, and the carrier diffusion lengths in c-Si. CD decreases with increasing Dit since the interface states act as recombination centres to decrease the excess carrier density in c-Si. The measurement is sensitive to Dit down to 1010 cm−2 eV−1. Accordingly, in the measurement of Δn and the carrier diffusion lengths in c-Si, the interface states can be characterized directly and accurately by the theoretical diffusion capacitance.

Three dimensional simulated modelling of diffusion capacitance of polycrystalline bifacial silicon solar cell

A three dimensional (3-D) simulated modelling was developed to analyse the excess minority carrier density in the base of a polycrystalline bifacial silicon solar cell. The concept of junction recombination velocity was ado-pted to quantify carrier flow through the junction, and to examine the solar cell diffusion capacitance for three illumination modes (front side, back side and both front and back sides). Plots of diffusion capacitance against grain size, grain boundary recombination velocity, junction recombination velocity and illumination wavelength were used to study the influence of cell parameters on the capacitance. The results indicated that junction and grain boundary recombination velocities played determinant roles, especially, for small grain size and long wav-elength. Hence, high diffusion capacitance was obtained for high junction recombination velocity, large grain size and long wavelength; while small grain size led to increased recombination centers and corresponding decrease in the diffusion capacitance

Passivation study of the amorphous–crystalline silicon interface formed using DC saddle‐field glow discharge

AbstractThe DC saddle‐field (DCSF) glow discharge method was used to deposit intrinsic a‐Si:H onto c‐Si to passivate the c‐Si surface. The effective minority carrier lifetime in the heterostructures as a function of the excess minority carrier density in the c‐Si wafers was measured. The results were then analyzed in the context of recombination associated with interface defect states using three known recombination models. The defect density and the charge density at the interface are inferred. In addition subsequent annealing of the samples was studied. It is shown that for our intrinsic a‐Si:H samples improvements in surface passivation are directly correlated with the reduction of interface defects and not the reduction of minority carrier concentration at the interface due to electric field. We have achieved excellent surface passivation with effective carrier lifetime >4 ms for an intrinsic a‐Si:H sample deposited at a process temperature of 200 °C and thickness of about 30 nm. It is also demonstrated that subsequent annealing, at 240 °C, of the samples which were prepared at process temperatures <240 °C greatly increases the effective lifetime.

3D Study of Bifacial Silicon Solar Cell under Intense Light Concentration and Under External Constant Magnetic Field

This work presents a three-dimensional study of bifacial silicon solar cell under intense light concentration and under constant magnetic field. This approach is based on the resolution of the minority continuity equation, taking into account the distribution of the electric field in the bulk evaluated as a function of both majority and minority carrier densities. In this approach, new analytical expressions of diffusion length, diffusion coefficient and excess minority carrier density was established for front illumination. The effect of magnetic field on excess minority carrier generation, carriers mobility, carriers density and electric parameters profile are then analysed.

New approach of both junction and back surface recombination velocities in a 3D modelling study of a polycrystalline silicon solar cell

A 3D modelling is used to obtain the expression of excess minority carrier density in base region of a polycrystalline silicon solar cell. The concept of the junction recombination velocity S f is used to quantify how excess carrier flow through the junction in actual operating conditions. The plot of the photocurrent density allowed study the influence of the grain boundary recombination velocity and grain size on both, the junction recombination velocity and on the back surface recombination velocity of an n + - p - p + solar cell. This study pointed out the importance of the losses at the back side of solar cell. It's also show that the junction recombination velocity is more important for small grain size with large values of grain boundary recombination velocity.