Effective Recombination Velocity Research Articles

Modified laser‐fired contact process for efficient PERC solar cells

AbstractA laser‐fired contact (LFC) process is one of the techniques for making local electrical contacts at the rear side of passivated emitter and rear cell (PERC) solar cells. In the LFC process, opening of the passivated dielectric layers and alloying of Si and Al need to be made in a single step laser process. For this reason, the LFC process is accompanied by the loss of Al and the laser damage to the Si wafer. In this study, we present a novel multistep LFC process combining the conventional LFC and laser‐induced forward transfer (LIFT) processes. The modified LFC scheme we proposed consists of three steps: (a) opening of the passivation layers and partial alloying of Al‐Si, (b) additional deposition of Al on the local contact holes, and (c) post laser firing of the transferred Al. Applying the modified LFC process to the PERC cells of 1.0 cm2 of area, we demonstrate the effective recombination velocity of the laser‐processed wafers can be remarkably reduced while maintaining the low contact resistance. The best of the PERC solar cell fabricated by the modified LFC process exhibited an efficiency of 19.5% while the conventional LFC‐PERC cell showed 18.6%. The efficiency gains of the modified LFC‐PERC cells was largely contributed by the enhanced open circuit voltage (Voc) and fill factor (FF).

Material Parameters and Perspectives for Efficiency Improvements in Perovskite Solar Cells Obtained by Analytical Modeling

Current perovskite solar cells reach high efficiencies with inexpensive materials and preparation methods. However, in order to understand the current values and obtain even higher efficiencies, the fundamental loss mechanisms of perovskite solar cells must be elucidated and predicted by appropriate models. Here, we adapt an analytical, drift-diffusion model for p-i-n solar cells and obtain accurate fits to measured current–voltage characteristics of planar hysteresis-free perovskite solar cells, covering a range of illumination intensities and perovskite thicknesses. Our results give values of carrier recombination lifetimes above 1 μ s, low effective recombination velocities at the interfaces between the perovskite layer and the surrounding layers around 1000 cm/s, built-in voltages that are slightly below the open-circuit voltages of around 1.05 V, and carrier mobilities around $\text{0.1} \text{cm}^{2}{\text{/Vs}}$ . The obtained parameters indicate that interface and bulk recombination are competing mechanisms, none of which can be ruled out in the case of the studied cells. Furthermore, we find that increasing the built-in voltage can significantly improve efficiency in p-i-n cells, while the implied relatively long diffusion length encourages the investigation of pn-type structures as ideal perovskite solar cell junctions.

Modeling of silicon solar cells performances by MATLAB

In this work, the programming tool MATLAB was used to predict the performances of single-crystal silicon solar cells. Two particular structures were studied, namely the conventional cell of N + P type and the cell with a Back surface field (BSF) layer of the N + PP + type. The parameters of these cells (the tension of open circuit, the current of short circuit and internal quantum efficiency (IQE)) were estimated with a good accuracy. Back surface field silicon ( i.e. N + PP + ) structures were found to have better characteristics than the conventional solar cells. We noticed that the effective recombination velocity of the minority carriers could be minimized by simple optimization of the doping concentration in the back layer. The results were in agreement with literature.

OPTIMIZATION OF MULTIJUNCTION SOLAR CELL BY WAFER RAY TRACER FOR DEVELOPMENT OF HIGH PHOTOGENERATED CURRENT

Optical losses limit the excess carriers generation in absorber part of multijuction (MJ) solar cell. The generation of excess carriers is directly proportional to photogenerated current solar cell. Therefore, reduction of optical losses is fundamentally important for improving the power conversion efficiency. Thickness of layers strongly influences the performance of MJ solar cell. In this study we simulated a MJ solar cell of Air/ZnO/SiC/c-Si/a-Si(n)/Al structure using Wafer Ray Tracer (WRT) simulation software and optimized the thicknesses of the layers for photogenerated current. The simulation result shows that without SiC layer, only 57.48% of incident light is absorbed and generates 26.85 mA/cm2 photogenerated current in solar cell. A 70 nm thickness of optimized SiC layer is increasing the light absorption 22.16% and photogenerated current 38.54%. Result shows that there is no transmission of light through the absorber layer. The MJ solar cell without Back Surface Field (BSF) layer of a-Si(n) shows photogenerated current of 37.05 mA/cm2 which can be improved to 37.24 mA/cm2 with a 100 nm thickness of a-Si(n). The c-Si absorber layer shows highest absorptance within 500 nm-1000 nm wavelength of light spectrum with 100 nm thickness of aSi(n). An a-Si(n) BSF layer at the back surface minimizes the effective back-surface recombination velocity and improves the collection probability of minority carriers of solar cell. Furthermore a 100 nm Al rear contact improves the photogenerated current of MJ solar cell to 37.25 mA/cm2. An Al rear contact layer improves the mechanical strength of c-Si absorber layer. The electrical property of Al improves the excess carriers’ collection probability of MJ solar cell.

Interface recombination feature in metal–semiconductor junction at high photo-excitation

A theory of the photo-induced electromotive force in a p-type semiconductor accounting for the energy band bending and interface recombination dependence on excitation level is developed. It is shown that at high photo-excitation the effective interface recombination velocity in the metal-semiconductor junction is negligible compared with the volume one, when the surface potential is less than its critical value. The photo-induced electromotive force value is maximal at this condition.

Ultra-thin aluminum oxide as an interface passivation layer for ZnO/p-Si heterojunction solar cells

In this paper, aluminum oxide (Al2O3) grown by a self-limiting atomic layer deposition (ALD) method is suggested as an interface passivation layer (IPL) to decrease the electrical recombination loss at the ZnO/p-Si interface. The reverse leakage current (IR) of ZnO/p-Si heterojunction diodes is reduced after inserting optimized Al2O3 films between ZnO/p-Si. From lifetime measurements, the effective recombination velocity trends consistently with the reverse saturation current (IS) of the devices tested. Also, from temperature-dependent current–voltage (I–V) and diverse physical measurements, the IR is shown to be closely related to free carrier trapping sites (Zni and Vo) near the half value of the difference in the work functions of Si and ZnO. Therefore, the experimental results show that the electrical recombination loss at the ZnO/p-Si interface can be effectively reduced by inserting an optimized Al2O3 IPL between the ZnO and p-Si.

Firing-stable PassDop passivation for screen printed n-type PERL solar cells based on a-SiNx:P

Rear side passivation and local back surface field formation are two of the main technological challenges for n-type PERL silicon solar cells. A promising approach is the PassDop process. This process combines a phosphorous doped passivation layer deposited on the rear side with a subsequent laser process to create both a local contact opening as well as a local back surface field. In this paper we introduce a new layer system based on doped amorphous silicon nitride (the fPassDop process) which is able to passivate the n-type surface after a firing step as typically used for screen printed contacts. After the firing step, an effective recombination velocity <5cm/s can be reached with this layer. The measured sheet resistance is in the range of 60Ω/sq after the laser process. In a first test the fPassDop process is applied to small area solar cells achieving a conversion efficiency of 21.3% (675mV Voc). Additionally, we fabricated large area n-type solar cells with screen printed front side contacts achieving 20.1% efficiency and Voc of 668mV.

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.

Impact of Interface States and Bulk Carrier Lifetime on Photocapacitance of Metal/Insulator/GaN Structure for Ultraviolet Light Detection

The influence of interface state density and bulk carrier lifetime on the dependencies of photocapacitance versus wide range of gate bias (-0.1 to -3 V) and light intensity (109 to 1020 photon cm-2 s-1) was studied for metal/insulator/n-GaN UV light photodetector by means of numerical simulations. The light detection limit and photocapacitance saturation were analyzed in terms of the interface charge and interface Fermi level for electrons and holes and effective interface recombination velocity. It was proven that the excess carrier recombination through interface states is the main reason of photocapacitance signal quenching. It was found that the photodetector can work in various modes (on–off or quantitative light measurement) adjusted by the gate bias. A comparison between experimental data and theoretical capacitance-light intensity characteristics was made. A new method for the determination of the interface state density distribution from capacitance–voltage-light intensity measurements was also proposed.

Advanced analytical model for the effective recombination velocity of locally contacted surfaces

In this paper an analytical model of the effective recombination (Seff) on the back surface of locally contacted solar cell is developed. We justify this approach by first showing that the three-dimensional (3D) problem can be reduced to a one-dimensional (1D) problem by calculating Seff. The values of Seff calculated with our model and two existing models are compared to finite element simulations. A large range of cases from high to low scale structures, including variation in the surface recombination velocity at the passivated area are investigated. The model presented in this paper is in good agreement with finite element simulations. Existing models from literature are also compared to finite element simulations, showing some of their limitations. In addition, we propose a method, not restricted to dark conditions, that simulates the 3D locally contacted structures by means of two 1D simulations.

Effective recombination velocity of textured surfaces

Surface texturization is an effective way to enhance the absorption of light for optoelectronic devices but it also aggravates the surface recombination by enlarging the surface area. In order to evaluate the influence of texture structures on the surface recombination, an effective surface recombination velocity is defined which is assumed to have an equivalent recombination effect on a flat surface. Based on numerical and analytical calculation, the dependences of effective surface recombination on the pattern geometry, the surface recombination velocity, and the diffusion length are analyzed.

Modelling c‐Si/SiNx interface recombination by surface damage

AbstractAmorphous silicon nitride (SiNx) layers are widely used for the electronic passivation of surfaces in Si solar cells. However, measurements have consistently shown that the effective recombination velocity Seff increases with decreasing excess carrier density Δn &lt; 1015 cm–3 at acceptor densities below 1017 cm–3. This poor performance is not observed at surfaces diffused with dopants, which requires an additional high temperature process during cell fabrication. Understanding the poor performance at low Δn may therefore provide strate gies for cost reduction in solar cell processing. This paper presents a detailed quantitative analysis of this effect. It is concluded that the lifetime is reduced to a value near 1 μs in a 100–500 nm thin region underneath the SiNx layer. We predict, that to avoid this effect, a mere tenfold reduction of the density of defect states in this region is sufficient. A possible explanation of this effect are defects caused by H‐termination during wafer pretreatment. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Very low surface recombination velocity on p-type c-Si by high-rate plasma-deposited aluminum oxide

Aluminum oxide layers can provide excellent passivation for lowly and highly doped p-type silicon surfaces. Fixed negative charges induce an accumulation layer at the p-type silicon interface, resulting in very effective field-effect passivation. This paper presents highly negatively charged (Qox=−2.1×1012 cm−2) aluminum oxide layers produced using an inline plasma-enhanced chemical vapor deposition system, leading to very low effective recombination velocities (∼10 cm s−1) on low-resistivity p-type substrates. A minimum static deposition rate (100 nm min−1) at least one order of magnitude higher than atomic layer deposition was achieved on a large carrier surfaces (∼1 m2) without significantly reducing the resultant passivation quality.

Analysis of Microwave Absorption Caused by Free Carriers in Silicon

Microwave absorption caused by free carriers was investigated. A 9.35 GHz microwave interferometer was constructed. The transmissivity of 525-µm-thick silicon substrates decreased from 60.4 to 3.8% as the resistivity decreased from 1000 to 4 Ω cm. This characteristic was explained well by a numerical analysis using the free carriers absorption theory. Microwave free carrier photo absorption caused by light-induced carriers was also investigated for p-type silicon samples coated with 100 nm thermally grown SiO2 layers as well as SiOx layers deposited by the vacuum evaporation method. The effective minority carrier lifetime and recombination velocity were analyzed in the case of the photo induced carrier generation with 532 nm light illumination. The effective minority carrier lifetime was increased from 360 to 540 µs and the recombination velocity was decreased from 78 to 30 cm/s by 1.3 ×106 Pa H2O vapor heat treatment at 260 °C for 3 h for light illumination at 0.315 mW/cm2 in the case of the thermally grown SiO2/Si because of the passivation of SiO2/Si interfaces. They were markedly increased from 30 to 380 µs and from 1300 to 60 cm/s, respectively, by the H2O vapor heat treatment in the case of the vacuum-evaporated SiOx/Si.

Recombination at the interface between a metallic precipitate and a semiconductor matrix: Application to the electron-beam-induced-current contrast

A self-consistent calculation of the barrier height and of the effective recombination velocity at the interface between a metallic precipitate and a semiconductor matrix has been performed within the Read–Hall–Schockley framework. The recombination has been investigated for Si, Ge and GaAs. The precipitate size has a dramatic effect on recombination due to the enhancement of the surface charge density on the metallic precipitate when the size decreases. The electron beam induced current (EBIC) contrast of metallic precipitate has been investigated.

Analytical Model and Current Gain Enhancement of Polysilicon-Emitter Contact Bipolar Transistors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents an analytical model that simulates the current gain improvement of polysilicon-emitter bipolar transistors based on the effective recombination velocity method. The theoretical framework developed in this paper incorporates the 2-D structure effect of the polysilicon layer with columnar grain boundary, tunneling processes of holes through polysilicon/silicon interface oxide layer, and nonuniform doping concentration and bulk recombination effects in the single-crystal emitter. The study goes on to derive an analytical expression for the base current density from which the analytical expression of the current gain was then derived. The effect of oxide breakup, at the polysilicon/silicon interface, on current gain was also considered. The dependence of the current gain on temperature was analyzed numerically; the results are in good agreement with experimental data. The approach outlined in this paper allows one to avoid many of the unnecessary simplifications inherent in previous works and to clearly assess the relevance of each physical mechanism. </para>

Analysis of the recombination velocity and of the electron beam induced current contrast at the interface between a metallic precipitate and a semiconductor matrix

A self-consistent calculation of the barrier height and of the effective recombination velocity at the interface between a metallic precipitate and a semiconductor matrix has been performed within the Read–Hall–Shockley framework. The procedure has taken account of the bending of the minority carrier quasi-Fermi level across the precipitate space-charge region and in the quasineutral region assuming the rigid displacement of the bands. The interface energy states are considered to have a monoenergetic distribution in two limiting cases including the half-filled-level model. The recombination has been investigated versus the semiconductor parameters (doping concentration, defect concentration.) and the temperature. The precipitate size has a dramatic effect on recombination. This is due to the enhancement of the surface charge density on the metallic precipitate which increases as σch(R)=σ∞ch+A/Rα when the size decreases. However, the surface charge density reaches at small size a limiting value controlled by the defect concentration. The enhancement of the surface charge density tends to reach significant barrier height up to precipitate sizes of 150 Å. The continuity equation around the precipitate has been solved in the case of a low-level electron beam injection in order to determine the electron beam induced current contrast at the interface. The contrast has been investigated versus the size for different defect concentrations, incident beam energies, and for the two kinds of defect distribution.

Low temperature back-surface-field contacts deposited by hot-wire CVD for heterojunction solar cells

The growing interest in using thinner wafers (< 200 μm) requires the development of low temperature passivation strategies for the back contact of heterojunction solar cells. In this work, we investigate low temperature deposited back contacts based on boron-doped amorphous silicon films obtained by Hot-Wire CVD. The influence of the deposition parameters and the use of an intrinsic buffer layer have been considered. The microstructure of the deposited thin films has been comprehensively studied by Spectroscopic Ellipsometry in the UV–visible range. The effective recombination velocity at the back surface has been measured by the Quasi-Steady-State Photoconductance technique. Complete double-side heterojunction solar cells (1 cm 2) have been fabricated and characterized by External Quantum Efficiency and current–voltage measurements. Total-area conversion efficiencies up to 14.5% were achieved in a fully low temperature process (< 200 °C).

Simple analytical solution and efficiency improvement of polysilicon emitter solar cells

A simple analytical model has been developed to simulate the performance of solar cells with polysilicon contact on the front surface. The polysilicon layer with a columnar grain structure is modeled by an effective recombination velocity using a two-dimensional transport equation. A one-dimensional transport equation in the single-crystal emitter is solved, taking into account bulk recombination and non-uniformly doped emitter. Then, simple analytical expressions for the emitter reverse saturation current and light-generated current densities are obtained. The collection of the light-generated carriers in polysilicon layer has been discussed and an analytical solution of the light-generated current is derived. The results show that the polysilicon layer can result in a decrease in emitter reverse saturation current density and an increase in solar cell photovoltaic parameters. In fact, the emitter region should not be treated as a ‘dead layer’ because thin polysilicon layer front surface contact gives an improvement of about 60 mV for the open-circuit voltage, 3.6 mA/cm 2 for the photocurrent, and 3.9% for the cell efficiency.

Grain-boundary recombination in Cu(In,Ga)Se2 solar cells

Two-dimensional simulations are performed to investigate the impact of grain boundaries (GBs) on Cu(In,Ga)Se2 (CIGS) solar-cell performance. Charged defect levels and compositional variations at GBs are considered. Neutral grain boundaries in the CIGS layer are predicted to be most detrimental if they are parallel to the main junction and located within the depletion region. For columnar GBs with a grain size near 1μm, the effective grain-boundary recombination velocity must be less than 104cm∕s to allow for record-efficiency devices. The majority-hole repulsion (additional donors at the GB) and the resulting band bending have a small effect on current collection but substantially lower the open-circuit voltage, and the combined effect is generally a lowering of the solar-cell efficiency. Minority-electron repulsion (additional acceptors at the GB) will partially mitigate GB recombination. A downshift of the valence-band energy, as predicted by the observed Cu depletion at CIGS GBs, can effectively block holes from the GB region and allow efficiencies comparable to GB-free material.