Boundary Recombination Velocity Research Articles

Unveiling microscopic carrier loss mechanisms in 12% efficient Cu2ZnSnSe4 solar cells

Understanding carrier loss mechanisms at microscopic regions is imperative for the development of high-performance polycrystalline inorganic thin-film solar cells. Despite the progress achieved for kesterite, a promising environmentally benign and earth-abundant thin-film photovoltaic material, the microscopic carrier loss mechanisms and their impact on device performance remain largely unknown. Herein, we unveil these mechanisms in state-of-the-art Cu2ZnSnSe4 (CZTSe) solar cells using a framework that integrates multiple microscopic and macroscopic characterizations with three-dimensional device simulations. The results indicate the CZTSe films have a relatively long intragrain electron lifetime of 10–30 ns and small recombination losses through bandgap and/or electrostatic potential fluctuations. We identify that the effective minority carrier lifetime of CZTSe is dominated by a large grain boundary recombination velocity (~104 cm s−1), which is the major limiting factor of present device performance. These findings and the framework can greatly advance the research of kesterite and other emerging photovoltaic materials.

A theoretical study of radio wave attenuation through a polycrystalline silicon solar cell

One-dimensional study of both electronic and electrical parameters of a silicon solar cell in the presence or not of an electric field, a magnetic field, or an electromagnetic field does not take into account the grain size and the grain boundary recombination velocity. A three-dimensional study, on the contrary, takes those factors into account. However, the three-dimensional study poses the problem of the attenuation of the wave in the grain of the polycrystalline solar cell as well as the issue of finding the expressions of its components. This study aimed to solve these issues by considering radio waves, which are becoming more and more present in our environment via telecommunication masts. We first obtained the expressions of both the electric field and magnetic field in a grain of a polycrystalline silicon solar cell by solving the dispersion equation. Then we investigated the evolution of the radio wave into the grain by analyzing the behavior of the exponential coefficient that appeared in the expressions of both the electric field and the magnetic field. The study has shown that the attenuation of the radio wave can be neglected through the polycrystalline silicon solar grain and by extension through the polycrystalline silicon solar cell.

Diffusion length and grain boundary recombination activity determination by means of induced current methods

The application of induced current methods for a quantitative description of multicrystalline silicon solar cell properties is demonstrated. For the minority carriers' diffusion length (L) and grain boundary recombination velocity (Vs) determination three types of measurements were used. They included the measurement of EBIC signal dependence on electron beam energy and of EBIC and XBIC grain boundary contrast profiles. The L and Vs values obtained by means of minimization the residual function between measured and model induced current curves are presented. The inaccuracy of obtained parameters is discussed for each of three types of measurements.

Two-Dimensional Finite Element Method Analysis Effect of the Recombination Velocity at the Grain Boundaries on the Characteristics of a Polycrystalline Silicon Solar Cell

To take into account the variation of the recombination velocity at the grain boundaries, we present in this paper a new approach of characterization of the solar cells, based on the two dimensional finite element method. The results of this study on a bifacial polycrystalline silicon solar cell, modelled in the rectangular form, highlighting the effects of the boundary recombination velocity (Sgb) on the solar cell electrical parameters. The photogenerated excess carrier’s density, the photocurrent density; the phototovoltage and the current-voltage characteristics are analyzed, namely. A good agreement with the results given in the literature is observed.

Comparison of thin epitaxial film silicon photovoltaics fabricated on monocrystalline and polycrystalline seed layers on glass

AbstractWe fabricate thin epitaxial crystal silicon solar cells on display glass and fused silica substrates overcoated with a silicon seed layer. To confirm the quality of hot‐wire chemical vapor deposition epitaxy, we grow a 2‐µm‐thick absorber on a (100) monocrystalline Si layer transfer seed on display glass and achieve 6.5% efficiency with an open circuit voltage (VOC) of 586 mV without light‐trapping features. This device enables the evaluation of seed layers on display glass. Using polycrystalline seeds formed from amorphous silicon by laser‐induced mixed phase solidification (MPS) and electron beam crystallization, we demonstrate 2.9%, 476 mV (MPS) and 4.1%, 551 mV (electron beam crystallization) solar cells. Grain boundaries likely limit the solar cell grown on the MPS seed layer, and we establish an upper bound for the grain boundary recombination velocity (SGB) of 1.6x104 cm/s. Copyright © 2014 John Wiley & Sons, Ltd.

Explanation of red spectral shifts at CdTe grain boundaries

We use cathodoluminescence spectrum imaging to investigate the nanoscale properties of CdTe thin-films for solar cells deposited by close-spaced sublimation. Luminescence emission is detected (bands) at ∼1.32 eV and ∼1.50 eV, which are consistent with Z- and Y-bands. For the grains in the as-deposited films, there is a significant redshift in the transition energies near the grain boundaries. The high grain boundary recombination velocity and the donor-acceptor pair (DAP) mechanism of the Z-band transition account for the contrast between grain boundaries and the grain interior. By applying DAP theory, we estimate the concentration of the shallow donor species participating in the Z-band transition to be ∼1017 cm−3.

On the diffusion length and grain size homogeneity requirements for efficient thin-film polycrystalline silicon solar cells

We examine the influence of intragrain defects and grain boundaries on the macroscopic performance of a thin film polycrystalline silicon solar cell. In addition, we evaluate the effect of grain size inhomogeneity on the cell performance via circuit simulations. From an analytical study of charge transport in individual grains and homogeneous grain systems, we obtain the grain size and intragrain diffusion length requirements for a desired efficiency. We identify the conditions under which the grain size and the intragrain diffusion length dominate the cell characteristics. In devices with intragrain effective diffusion length Lmono ⩽ 100 µm and grain boundary recombination velocity SGB ⩽ 104 cm s−1, achieving a larger grain size beyond several µm is not crucial. The inhomogeneous distribution circuit simulations show that grain size inhomogeneity is not the main limiting factor in polycrystalline silicon solar cells. This is so even in thin polycrystalline silicon films with a broad grain size distribution such as those made with aluminum-induced crystallization at low annealing temperature. The main reason is that the optimum bias point for grains of different sizes only differ by about ∼50 mV over a fairly wide grain diameter range 0.5–50 µm even when Lmono = 100 µm and SGB = 105 cm s−1.

Cathodoluminescence measurement of grain boundary recombination velocity in vapour grown p-CdTe

A cathodoluminescence (CL) based method for measuring the minority carrier diffusion length (L) and reduced recombination velocity(Sred) of an individual grain boundary is presented. The technique is based on the van Roosbroeck model for the steady-state carrier distribution near a free surface in a semi-infinite solid. Values of L = 0.55±0.03 μm and Sred = 0.23±0.02 are obtained for vapour grown p-type CdTe. The effect of the electron beam generation volume on the accuracy of measurement is also discussed. Intermediate beam voltages (e.g. 15 kV for CdTe) are likely to produce the most reliable results. The method can be used to characterise the electrical activity of grain boundaries in thin-film solar cells.

A contactless method for measuring the recombination velocity of an individual grain boundary in thin-film photovoltaics

A cathodoluminescence-based, contactless method for extracting the bulk minority carrier diffusion length and reduced recombination velocity of an individual grain boundary is applied to vapor grown CdTe epitaxial films. The measured diffusion length was within the range of 0.4–0.6 μm and the grain boundary recombination velocity varied from 500 to 750 cm/s. The technique can be used to investigate the effect of grain boundaries on photovoltaic performance.

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

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.

A Novel Method for the Estimation of Surface Recombination Velocity of Polycrystalline CIGS Solar Cells

The surface recombination velocity of minority carriers at the grain boundaries severely affects the performance of the polycrystalline solar cells. In the present work, the effect of grain size and grain boundary recombination velocity on the short circuit photocurrent of polycrystalline copper indium gallium di‐selenide (CIGS) solar cell is estimated theoretically. The model has been used to determine the surface recombination velocity for CIGS using our own experimental data on carrier generation rate. A good agreement is found between theory and available experimental results.

Application of genetic algorithms for the extraction of electrical parameters of multicrystalline silicon

The application of the genetic algorithms (GAs) concept in measurements science deals with a fitting procedure used for the numerical prediction of physical parameters from an experimental data curve. In this work, GAs were applied for the extraction of electrical parameters of multicrystalline silicon solar cells. The experimental technique used is the light-beam-induced-current (LBIC). From LBIC measurements, we deduced the values of the diffusion length L and the grain boundary recombination velocity Vr of the minority carriers of the multicrystalline silicon wafers using the fitting procedure. The nonlinear fitting procedure is based on the minimization of the standard deviation of the theoretical LBIC profile from the experimental one. However, this criterion is not convex, and using traditional deterministic optimization algorithms leads to local minima solutions. To overcome this problem, the nonlinear least-square minimization technique was computed with the GAs strategy, increasing the probability of obtaining the best minimum value of the cost function in very reasonable time. The results of the proposed algorithm are presented and compared with the Levenberg–Marquardt algorithm and the Gauss–Newton method. The GAs-based numerical technique was found to be a promising and a powerful technique for numerical evaluation of the electrical parameters of silicon solar cells.

Combination of gettering and etching in multicrystalline silicon used in solar cells processing

Undesired impurities can be removed away from multicrystalline silicon (mc-Si) wafers by combining porous silicon (PS) formation and heat treatments. The gettering procedure used in this work is based on the formation of a PS film at both back and front sides of the mc-Si wafers, followed by a heat treatment. The latter was achieved in an infrared furnace at different temperatures and during various periods. We show that when the based material undergoes such a gettering, the electrical properties (short-circuit current, open-circuit voltage, serial and shunt resistances) and the electronic parameters (diffusion length and grain boundary recombination velocity) of the corresponding solar cells can be improved only if some regions of the wafers are etched. Compared to reference cells based on untreated wafers, the diffusion length and grain boundary recombination velocity of solar cells fabricated from gettered and etched samples was improved by about 30% and reduced by a factor of 10, respectively.

High open-circuit voltage values on fine-grained thin-film polysilicon solar cells

Grain boundaries are known to be the main limiting factor for a high performance of polysilicon solar cells. Defects at these grain boundaries serve as recombination centers for minority and majority carriers. Grain boundaries are also known to be paths for enhanced hydrogen diffusion, which results in passivation of part of the defects. In this paper, we show that grain boundaries are also paths for an enhanced phosphorus diffusion that limits the effect of hydrogen passivation. Phosphorus spikes along the grain boundaries enhance the junction area and determine the collection and the recombination volumes. Avoiding this preferential diffusion of phosphorus atoms during emitter formation, we obtained open-circuit voltages (Voc) up to 536mV on polysilicon material with a grain size of only 0.2μm. These high Voc values can only be accounted for by theory if a much smaller grain boundary recombination velocity is assumed than what was previously accepted for p-n junctions on fine-grained polysilicon solar cells.

Structure optimization according to a 3D model applied on epitaxial Si solar cells: A comparative study☆

Abstract A three-dimensional (3D) analytical model based on the Green's Function method, is applied on n + pp + type epitaxial solar illuminated on the front side. This model is implemented through a simulation program in order to determine the device parameters, which optimize the cell's efficiency. A number of cells fabricated on low-cost UMG substrate have been studied and their spectral response is evaluated. The basic design parameter selected for optimization of these cells is the epitaxial layer thickness, through variation of grain size and grain boundary recombination velocity. In addition, 3D spectral response results are compared with experimental data and the 1D model.

Modelling recombination current in polysilicon solar cell grain boundaries

Abstract The recombination current in grain boundaries is theoretically investigated for a polycrystalline silicon solar cell n–p junction operating either in short-circuit or in open-circuit configurations. The analysis is carried out using results performed in earlier works, by means of numerical simulations including the presence of grain boundaries. The variation of grain boundary recombination current density with exciting light wavelength is reported for different cell parameters like grain boundary recombination velocity, base dopant density and base thickness. The objective of the present paper is to understand to what extend the light wavelength and the junction configuration allow an optimal measure of the grain boundary recombination parameter.

Porous silicon-based passivation and gettering in polycrystalline silicon solar cells

In this work, we report on the effect of introducing a superficial porous silicon (PS) layer on the electrical characteristics of polycrystalline silicon solar cells. The PS layer was formed using a vapour etching (VE)-based method. In addition to its known anti-reflecting action, the forming hydrogen-rich PS layer acts as a passivating agent for the surface of the cell. As a result we found an improvement of the I– V characteristics in dark conditions and AM1 illumination. We show that when the formation of a superficial PS layer is followed by a heat treatment, gettering of impurities from the polycrystalline silicon material is possible. After the removal of the PS layer and the formation of the photovoltaic (PV) structure, we observed an increase of the light-beam-induced-current (LBIC) for treatment temperatures not exceeding 900 °C. An improvement of the bulk minority carrier diffusion length and the grain boundary (GB) recombination velocity were observed as the temperature rises, although a global decrease of the LBIC current was observed for temperatures greater than 900 °C.

Effective Diffusion Length of Multicrystalline Solar Cells

Previous definitions of effective diffusion length of a multicrystalline semiconductor are reviewed. A new definition is proposed, which is based on the comparison of the current collected from the base of a multicrystalline solar cell and a single-crystal cell for uniform carrier generation, and holds for any sample thickness. An analytical expression for is obtained from the solution of the three-dimensional equation for the charge collection probability inside a grain with square cross section. This expression is used to numerically study the dependence of on the bulk diffusion length, grain size and grain boundary recombination velocity, and to make a comparison with previous specifications of . The results indicate, in particular, that is the value that would be determined by the surface photovoltage method. The case of a cell with grains of different properties is briefly discussed and an average is defined. It is also shown that can be used to calculate the contribution of the base to the reverse saturation current of the cell.

Quantitative interpretation of electron-beam-induced current grain boundary contrast profiles with application to silicon

An improved method is described for extracting material parameters from an experimental electron-beam-induced current (EBIC) contrast profile across a vertical grain boundary by directly fitting an analytical expression. This allows the least-squares values of the grain boundary recombination velocity and the diffusion length in each grain to be determined without the need for the reduction of the experimental profile to a few integral parameters, as is required in a previously reported method. Greater accuracy of the extracted values is expected since none of the information contained in the experimental contrast data is discarded and a less extensive spatial range of measured data is required than in the commonly used method. Different models of the carrier generation volume are used in the fitting and the effect of the choice of generation model on extracted values is investigated. In common with other EBIC approaches, this method is insensitive to changes in the diffusion length when the collection efficiency is high and diffusion lengths may not be reliably established in those cases.