Back Surface Recombination Velocity Research Articles

Approach to determining the limiting recombination mechanism in CdTe-based solar cells

Determining exactly how the performance of a thin-film photovoltaic device is limited by a particular recombination mechanism can be difficult, particularly in the case of CdTe solar cells. As a result, efforts are being made to improve all parts of the device without good knowledge of which improvements are necessary. To understand where the device limitation is, the recombination current densities of at least one on of the interfaces must be known. Here, we present a method to determine which recombination mechanisms is limiting. First, back illuminated quantum efficiency measurements are used to determine the key parameters of the back interface – the back surface recombination velocity and the band bending near the back surface. Once these back interface parameters are determined, the recombination current densities can be calculated for a front illuminated device to determine the limiting mechanism. The validity of the approach is tested using previously reported data.

An Investigation of The J-V Curve for N-AlGaAs/p-GaAs Heterojunction Solar Cell Due to Influence of Surface Recombination Velocity

Theoretical analysis can substantially reduce the time and costs required for developing a specific device such as solar cell, by allowing the designer to take a suitable geometry and doping profile prior to the fabrication stage. In this paper, theoretical analysis of electric field and electric potential within depletion layer of N-AlGaAS/p-GaAs has been made by solving Poisson's equation numerically using finite difference method (FDM) with Fortran program. Continuity equations of charge carriers in quasi- nature regions in N and p-layers were solved numerically to obtain the current density curve (J-V curve). Boundary conditions related to surface recombination velocity were implemented in this analysis. The effect of front and back surface recombination velocity on the excess minority carriers distribution and photocurrent have been studied. The aim of this work is to find the influence of surface recombination velocity in the output of N-AlGaAs/p-GaAs heterojunction solar cell. The analysis showed that, the low surface recombination velocity produce high efficient solar cell.

Derivative of Ac back surface recombination velocity as applied to n+/p/p+ silicon solar cell optimum base thickness determination: Effect of both temperature and frequency

Derivative of Ac back surface recombination velocity as applied to n+/p/p+ silicon solar cell optimum base thickness determination: Effect of both temperature and frequency

Machine learning for enhanced semiconductor characterization from time-resolved photoluminescence

<h2>Summary</h2> To enhance the accuracy and effectiveness of optoelectronic characterization, Bayesian inference has been applied to the statistical analysis of time-resolved photoluminescence (TRPL) data with large-scale graphics-processing unit (GPU)-based simulations of electron dynamics. A simulated TRPL dataset, derived from a CH₃NH₃PbI_3-xCl_x perovskite absorber, was used as a case study. From a power scan, Bayesian inference extracts values of the (ambipolar) carrier mobility, free-carrier density, radiative recombination rate, and the overall lifetime of the nonradiative recombination processes. Analysis of the experimental TRPL data yields similar parameter values. The independent contributions of bulk and surface recombination can be distinguished via the introduction of an additional sample thickness. Further experiments separate the front and back surface recombination velocities and can distinguish the electron and hole mobilities. Ultimately, Bayesian inference enables a significant increase in the information yield from TRPL measurements.

BIFACIAL (N+/P/P+) SILICON SOLAR CELL BASE THICKNESS OPTIMIZATION, WHILE ILLUMINATED BY THE REAR FACE WITH MONOCHROMATIC LIGHT OF SHORTWAVELENGTHS

The monochromatic absorption coefficient of silicon, inducing the depth of penetration of light into the base of the solar cell, is used through back surface recombination velocity expressions, to determine the optimum thickness necessary for the production of a large photocurrent. The absorption-generation-diffusion and recombination (bulk and surface) phenomena are taken into account in determining the optimum thickness of an n+ -p-p+ bifacial solar cell, for it manufacture process optimization.

Diode Factor in Solar Cells with Metastable Defects and Back Contact Recombination

AbstractTo achieve a high fill factor, a small diode factor close to 1 is essential. The optical diode factor determined by photoluminescence is the diode factor from the neutral zone of the solar cell and thus a lower bound for the diode factor. Due to metastable defects transitions, the optical diode factor is higher than 1 even at low excitation. Here, the influence of the backside recombination and the doping level on the optical diode factor are studied. First, photoluminescence and solar cell capacitance simulator (SCAPS) simulations are used to determine the back surface recombination velocity of Cu(In, Ga)Se2 with various back contacts and different doping levels. Then, experimental results and simulations show that both back surface recombination and high doping density reduce the optical diode factor. The back surface recombination reduces the optical diode factor with undesirable extra nonradiative recombination. The smaller value achieved by higher doping can increase quasi‐Fermi level splitting at the same time. The simulations show that the back surface recombination reduces the optical diode factor due to an illumination‐dependent recombination rate. In addition, a higher majority carrier doping reduces the influence of majority carrier gain from metastable defect transitions, thus reducing the optical diode factor.

Surface recombination property of silicon wafers determined accurately by self-normalized photocarrier radiometry

Surface recombination has a great impact on the performance of solar cells and photodetectors, and thus is necessary to be accurately determined. In this paper, a self-normalized photocarrier radiometry (PCR) with mean square variance graph is employed to improve the determination accuracy of the surface recombination velocities of silicon wafers and to reduce the influence of initial values on multi-parameter estimations. In this method data obtained from both the illuminated front surface and the illuminated rear surface are employed. The uncertainties of the estimated carrier recombination parameters, especially for the surface recombination velocities are improved significantly. Moreover, the influence of instrumental frequency response (IFR) on the multi-parameter estimation is totally eliminated. Theoretical simulations and experiments are performed to confirm the theoretical predictions. The estimated average uncertainties of the front and back surface recombination velocities for the sample used are approximately ± 7.60% and ± 5.71% by the proposed method, which are much improved than ± 13.65% and >±50% by the conventional method.

Distinguishing bulk and surface recombination in CdTe thin films and solar cells using time-resolved terahertz and photoluminescence spectroscopies

Understanding the nature of recombination and its dependence on defects and interfaces is essential for engineering materials and contacts for a higher open-circuit voltage (Voc) and power conversion efficiency in photovoltaic (PV) devices. Time-resolved photoluminescence (TRPL) has conventionally been used to evaluate recombination, but carrier redistribution often dominates the response at short times. Here, we report on the quantification of carrier dynamics and recombination mechanisms by complementary use of both time-resolved terahertz spectroscopy and TRPL combined with numerical modeling of the continuity equations and Poisson's equation. We have demonstrated this approach using CdTe thin films. A thin-film stack with CdTe fabricated by vapor transport deposition and treated with CdCl2 exhibited a bulk lifetime of 1.7 ± 0.1 ns, a negligible CdTe/CdS interface recombination velocity, and a back surface recombination velocity of 6.3 ± 1.3 × 104 cm/s. In contrast, a film stack without CdCl2 treatment had a bulk lifetime of only 68 ± 12 ps and a higher interface recombination velocity of 4 ± 2 × 108 cm/s. By determining the locus and mechanisms of performance-limiting recombination, we can accelerate the development of thin-film PVs with higher Voc and efficiency. While the method has been demonstrated here using CdTe, it is also applicable to perovskites, Cu(InGa)Se2, Cu2ZnSn(S,Se)4, and emerging technologies.

Efficiency enhancement of WSe2 heterojunction solar cell with CuSCN as a hole transport layer: A numerical simulation approach

In this research work, the tungsten diselenide (WSe2)-based thin-film solar cell with a copper thiocyanate (CuSCN) hole transport layer (HTL) has been designed and studied by using the Solar Cell Capacitance Simulator in One Dimension software program (SCAPS-1D). A comparative numerical study between Al/ITO/CdS/WSe2/Ni and Al/ITO/CdS/WSe2/CuSCN/Ni has been done. The SCAPS-1D simulator has been utilized to investigate photovoltaic parameters such as open circuit voltage, short-circuit current density, fill-factor, power conversion efficiency and quantum efficiency of heterojunction solar cell due to the variation of thickness, doping density, bulk defect density, defect density at buffer/absorber and absorber/HTL interfaces, operating temperature, band alignment and back surface recombination velocity. The conversion efficiency of 17.33% has been determined for the WSe2 solar cell without HTL. On the contrary, the efficiency of the proposed solar cell with CuSCN HTL has been found to be 24.20% at the optimal device structure. These numerical findings of the present study may therefore provide an intuitive approach to fabricate an economically feasible and highly efficient WSe2-based heterojunction thin-film solar cell.

Improvement of quantum efficiency of field-assisted GaN monolayer photocathode

In this paper, in order to explore quantum efficiency of field-assisted GaN monolayer photocathode, uniform-doping and exponential-doping reflection-mode GaN monolayer photocathode model with an external electric field have been established, and quantum efficiency formulas have been derived. We simulate the quantum efficiency of photocathode with uniform-doping and exponential-doping structure under different external electric fields, number of monolayers in emission layer, thickness of buffer layer and interface recombination velocity. The results show that quantum efficiency enhances with increase of external electric field. As the thickness of emission layer increases, the quantum efficiency decline, while as the thickness of buffer layer increases, the quantum efficiency improves. At the same time, an appropriate interface recombination velocity can improve the quantum efficiency of photocathode. When back surface recombination velocity and GaN ML/GaN ML interface recombination velocity are 8 × 106 cm/s and 108 cm/s, respectively, the optimal quantum efficiency can be obtained.

Enabling bifacial thin film devices by developing a back surface field using CuxAlOy

Bifacial solar cells have the potential to increase the energy yield per unit area over traditional monofacial devices without significant added cost, driving $/kWh costs lower and accelerating the adoption of solar photovoltaics. However, the performance of bifacial thin film solar cells significantly lags that achieved by crystalline silicon cells. Here we incorporate wide bandgap CuxAlOy as a back buffer layer for CdTe devices and achieve a backside illuminated device with high current density and high fill factor. Moreover, these values remain nearly constant even as the absorber layer thickness changes, indicating that a fully-depleted device is not required for efficient charge collection. We show that this response is indicative of a back surface field, albeit with a persistent high back surface recombination velocity. By managing electron reflection, we achieved a backside illumination conversion efficiency of 7.1% and bifaciality of 0.55 for a 3.3 µm CdTe device and 8.0% and 0.62 for a 2 µm device. Future improvements can be made by identifying and incorporating a passivation material that reduces the back surface recombination velocity.

AC Back Surface Recombination Velocity in n&amp;lt;sup&amp;gt;＋&amp;lt;/sup&amp;gt;-p-p&amp;lt;sup&amp;gt;＋&amp;lt;/sup&amp;gt; Silicon Solar Cell under Monochromatic Light and Temperature

Excess minority carrier’s diffusion equation in the base of monofaciale silicon solar cell under frequency modulation of monochromatic illumination is resolved. Using conditions at the base limits involving recombination velocities Sf and Sb, respectively at the junction (n＋/p) and back surface (p＋/p), the AC expression of the excess minority carriers’ density δ (T, ω) is determined. The AC density of photocurrent Jph (T, ω) is represented versus recombination velocity at the junction for different values of the temperature. The expression of the AC back surface recombination velocity Sb of minority carriers is deduced depending on the frequency of modulation, temperature, the electronic parameters (D (ω)) and the thickness of the base. Bode and Nyquist diagrams are used to analyze it.

Performance enhancement of Sb2Se3 solar cell using a back surface field layer: A numerical simulation approach

In the present work, antimony selenide (Sb2Se3)-based solar cell with a back surface field (BSF) layer has been designed and studied. The purpose of this research is to improve the performance of the Sb2Se3-based solar cell by introducing low-cost and widely available barium silicide (BaSi2) material as the BSF layer into the basic Sb2Se3-based heterojunction solar structure. A comparative study on the performance of the conventional Sb2Se3 solar cell structure consisting of Al/FTO/CdS/Sb2Se3/Mo and the proposed structure of Al/FTO/CdS/Sb2Se3/BaSi2/Mo is made. The photovoltaic parameters such as open circuit voltage, short-circuit current density, fill-factor, power conversion efficiency, and quantum efficiency of heterojunction structures are analyzed intensively by using the Solar Cell Capacitance Simulator in One Dimension (SCAPS-1D) program. To optimize the device, the thickness of Sb2Se3 absorber layer is varied from 0.1 to 2 μm. In addition, the effects of acceptor ion and bulk defect densities in Sb2Se3 absorber layer, interface defect density of buffer/absorber and absorber/BSF, back surface recombination velocity, operating temperature, and cell resistances on the overall performances are investigated. The thicknesses of BaSi2 BSF and Sb2Se3 absorber layers are optimized to be 0.3 μm and 1 μm, respectively. The efficiency of the proposed solar structure with a thin 1 μm Sb2Se3 absorber layer is obtained to be 29.35%. The simulation results lead to suggest that the BaSi2 material as a BSF layer would be effective to fabricate low cost and high-efficient Sb2Se3-based thin-film solar cells.

Temperature Coefficients and Operating Temperature Verification for Passivated Emitter and Rear Cell Bifacial Silicon Solar Module

In this article, the temperature coefficients of the bifacial passivated emitter and rear cell (PERC) solar module and the full-area aluminum back surface field (Al-BSF) solar module were calculated based on the theoretical model under different module temperatures and irradiances. Meanwhile, the measured temperature coefficients were obtained by testing the performance of bifacial PERC solar module and conventional Al-BSF solar module under different module temperatures and irradiances. The results show that the laboratory measurements are close to the calculated values, and the normalized temperature coefficients of output power for the bifacial PERC solar module are lower than that of the conventional Al-BSF solar module under the same operating conditions. An interesting finding is that the normalized temperature coefficients of short-circuit current for the bifacial PERC solar module are larger than that of the conventional Al-BSF solar module under the same irradiance. This is attributed to lower back surface recombination velocity and higher carrier collection efficiency of PERC solar cells compared with conventional Al-BSF solar cells. Additionally, the operating temperatures of the bifacial PERC solar module and conventional Al-BSF solar module were surveyed outdoor. It is found that the operating temperature of the bifacial PERC solar module is higher than that of the conventional Al-BSF solar module in some cases. However, this tendency is reversed in other cases when these two solar modules work under the same environmental conditions. The conclusion was explained reasonably according to the actual operating conditions of solar modules. The obtained results in this work can provide useful insight into evaluating the field performance of the bifacial PERC solar module.

Back Surface Recombination Velocity Dependent of Absorption Coefficient as Applied to Determine Base Optimum Thickness of an n+/p/p+ Silicon Solar Cell

The monochromatic absorption coefficient of silicon, inducing the light penetration depth into the base of the solar cell, is used to determine the optimum thickness necessary for the production of a large photocurrent. The absorption-generation-diffusion and recombination (bulk and surface) phenomena are taken into account in the excess minority carrier continuity equation. The solution of this equation gives the photocurrent according to absorption and electronic parameters. Then from the obtained short circuit photocurrent expression, excess minority carrier back surface recombination velocity is determined, function of the monochromatic absorption coefficient at a given wavelength. This latter plotted versus base thickness yields the optimum thickness of an n+-p-p+ solar cell, for each wavelength, which is in the range close to the energy band gap of the silicon material. This study provides a tool for improvement solar cell manufacture processes, through the mathematical relationship obtained from the thickness limit according to the absorption coefficient that allows base width optimization.

Lamella Silicon Solar Cell under Both Temperature and Magnetic Field: Width Optimum Determination

This work deals with determining the optimum thickness of the lamella wafer of silicon solar cell. The (p) base region makes up the bulk of the thickness of the wafer. This thickness has always been a factor limiting the performance of the solar cell, as it produces the maximum amount of electrical charges, contributing to the photocurrent. Determining the thickness of the wafer cannot be only mechanical. It takes into account the internal physical mechanisms of generation-diffusion-recombination of excess minority carriers. They are also influenced by external factors such as temperature and magnetic field. Under these conditions, magneto transport equation is required to be applied on excess minority carrier in lamella base silicon solar cell. It yields maximum diffusion coefficient which result on Lorentz law and Umklapp process. Then from photocurrent, back surface recombination velocity expressions are derived, both maximum diffusion coefficient and thickness dependent. The plot of the back surface recombination calibration curves as function of lamella width, leads to its maximum values, trough intercept points. Lamella optimum width is then obtained, both temperature and magnetic field dependent and expressed in relationships to show the required base thickness in the elaboration process.

Assessment of Bulk and Interface Quality for Liquid Phase Crystallized Silicon on Glass

This paper reports on the electrical quality of liquid phase crystallized silicon (LPC-Si) on glass for thin-film solar cell applications. Spatially resolved methods such as light beam induced current (LBIC), microwave photoconductance decay (MWPCD) mapping, and electron backscatter diffraction were used to access the overall material quality, intra-grain quality, surface passivation, and grain boundary (GB) properties. LBIC line scans across GBs were fitted with a model to characterize the recombination behavior of GBs. According to MWPCD measurement, intra-grain bulk carrier lifetimes were estimated to be larger than 4.5 μs for n-type LPC-Si with a doping concentration in the order of 1016 cm−3. Low-angle GBs were found to be strongly recombination active and identified as highly defect-rich regions which spatially extend over a range of 40–60 μm and show a diffusion length of 0.4 μm. Based on absorber quality characterization, the influence of intra-grain quality, heterojunction interface, and GBs/dislocations on the cell performance were separately clarified based on two-dimensional (2-D)-device simulation and a diode model. High back surface recombination velocities of several 105 cm/s are needed to get the best match between simulated and measured open circuit voltage ( V oc), indicating back surface passivation problem. The results showed that V oc losses are not only because of poor back surface passivation but also because of crystal defects such as GBs and dislocation.

Base Thickness Optimization of a (n+-p-p+) Silicon Solar Cell in Static Mode under Irradiation of Charged Particles

In this work, we propose a method to determinate the optimum thickness of a monofacial silicon solar cell under irradiation. The expressions of back surface recombination velocity depending the damage coefficient (kl) and irradiation energy (op) are established. From their plots, base optimum thickness is deduced from the intercept points of the curves. The short-circuit currents Jsc0 and Jsc1 corresponding to the recombination velocity Sb0 and Sb1 are determinated and a correlation between the irradiation energy, the damage coefficient and optimum thickness of the base is established.

Influence of P+-ZnTe back surface contact on photovoltaic performance of ZnTe based solar cells

In order to improve photovoltaic performance of solar cells based on ZnTe thin films two device structures have been proposed and its photovoltaic parameters have been numerically simulated using Solar Cell Capacitance Simulator software. The first one is the ZnO/CdS/ZnTe conventional structure and the second one is the ZnO/CdS/ZnTe/P+-ZnTe structure with a P+-ZnTe layer inserted at the back surface of ZnTe active layer to produce a back surface field effect which could reduce back carrier recombination and thus increase the photovoltaic conversion efficiency of cells. The effect of ZnO, CdS and ZnTe layer thicknesses and the P+-ZnTe added layer and its thickness have been optimized for producing maximum working parameters such as: open-circuit voltage Voc, short-circuit current density Jsc, fill factor FF, photovoltaic conversion efficiency η. The solar cell with ZnTe/P+-ZnTe junction showed remarkably higher conversion efficiency over the conventional solar cell based on ZnTe layer and the conversion efficiency of the ZnO/CdS/ZnTe/P+-ZnTe solar cell was found to be dependent on ZnTe and P+-ZnTe layer thicknesses. The optimization of ZnTe, CdS and ZnTe layers and the inserting of P+-ZnTe back surface layer results in an enhancement of the energy conversion efficiency since its maximum has increased from 10% for ZnO, CdS and ZnTe layer thicknesses of 0.05, 0.08 and 2 µm, respectively to 13.37% when ZnO, CdS, ZnTe and P+-ZnTe layer thicknesses are closed to 0.03, 0.03, 0.5 and 0.1 µm, respectively. Furthermore, the highest calculated output parameters have been Jsc = 9.35 mA/cm2, Voc = 1.81 V, η = 13.37% and FF = 79.05% achieved with ZnO, CdS, ZnTe, and P+-ZnTe layer thicknesses about 0.03, 0.03, 0.5 and 0.1 µm, respectively. Finally, the spectral response in the long-wavelength region for ZnO/CdS/ZnTe solar cells has decreased at the increase of back surface recombination velocity. However, it has exhibited a red shift and showed no dependence of back surface recombination velocity for ZnO/CdS/ZnTe/P + -ZnTe solar cells.

Estimating various losses in c-Si solar cells subjected to partial shading: insights into J–V performance reduction

This article reports the effect of partial shading (PS) on the electrical output of a solar cell using the two-diode model. The reduction in electrical performance parameters induced by various recombination losses has been explained for c-Si solar cells under the effect of PS. The PS mainly affects the short-circuit current density $$({J}_{\mathrm{SC}})$$ and efficiency $$(\eta )$$ of the solar cells. $${J}_{\mathrm{SC }}$$ and $$\eta $$ decrease from 37.84 to 5.48 mA $$\hbox {cm}^{-2}$$ and from 18.31 to 2%, respectively. Among all the energy losses encountered for PS, spatial relaxation and recombination losses are the dominating factors responsible for the reduction in $${J}_{\mathrm{SC}}$$ . PC1D and Griddler simulations have been performed to evaluate the effect of front surface and bulk recombination. The PC1D simulated external quantum efficiency is governed by the front and back surface recombination velocity and carrier life time of the charge carriers under PS. The power loss ( $${P}_{\mathrm{e}}$$ ) of $${\sim }$$ 34% from the emitter region has been determined by resistance analysis in correlation with the recombination in the emitter region of the solar cells under PS.