Study of the composition of hydrogenated silicon nitride SiNx:H for efficient surface and bulk passivation of silicon

This work is a contribution towards the understanding of the properties of hydrogenated silicon nitride (SiN_x:H) that lead to efficient surface and bulk passivation of the silicon substrate. Considering the deposition system used (low-frequency plasma-enhanced chemical vapour deposition (PECVD)), we report very low values of surface recombination velocity S_eff. As-deposited Si-rich SiN_x:H leads to the best results (n-type Si: S_eff=4 cm/s - p-type Si: S_eff=14 cm/s). After annealing, the surface passivation quality is drastically deteriorated for Si-rich SiN_x:H whereas it is lightly improved for low refractive index SiN_x:H (n∼2-2.1). The chemical analysis of the layers highlighted a high hydrogen concentration, regardless the SiN_x:H stoichiometry. However, the involved H-bond types as well as the hydrogen desorption kinetics are strongly dependent on the SiN_x:H composition. Furthermore, “N-rich” SiN_x:H appears to be denser and thermally more stable than Si-rich SiN_x:H. When subjected to a high-temperature treatment, such a layer is believed to induce the release of hydrogen in its atomic form, which consequently leads to an efficient passivation of surface and bulk defects of the Si substrate. The results are discussed and compared with the literature data reported for the different configurations of PECVD reactors.     

Optimization of PECVD SiN:H films for silicon solar cells

We have grown silicon nitride (SiN:H) thin films on silicon and glass by the Plasma Enhanced Chemical Vapor Deposition (PECVD) Method at low temperature in order to study their electro-optical properties and correlate these properties to the chemical composition of the layers, so that optimum films may be achieved for silicon solar cells. By varying the silane to ammonia ratio in the plasma gas we have been able to modify the index of refraction, the optical band gap and the silicon surface state passivation properties of the films. From this information we have determined that the optimum silane to ammonia ratio, with other constant parameters in our system, should be 20/65. Our results indicate that the mid-gap surface state density in silicon can be reduced down to 10¹⁰ cm⁻² eV⁻¹ when this optimum (silane to ammonia) ratio is used for depositing SiN:H layers. We have confirmed this optimal ratio by making quantum efficiency measurements on silicon solar cells having their emitter passivated with SiN:H layers deposited with different silane to ammonia ratios. A great reduction of the surface recombination velocity was achieved, as observed from the internal quantum efficiency measurements, for cells with optimal SiN:H layers as compared to those with non-optimum SiN:H layers.     

Low temperature silicon oxide and nitride for surface passivation of silicon solar cells

Surface passivation at low processing temperature becomes an important topic for crystalline and multicrystalline silicon solar cells. In this work, silicon oxide (250°C) and silicon nitride (300°C) have been developed by Photo-CVD and PECVD technique respectively. Effects of deposition parameters on the optoelectronic and structural properties of the films have been investigated. Interface-trap density (D_it) and fixed charge density (Qf) have been estimated by high frequency (1 MHz) capacitance-voltage measurement on Metal–Insulator–Silicon structure (CV-MIS). The effect of silicon oxide and silicon nitride on the performance of c-Si solar cells have been studied.     

Highest-quality surface passivation of low-resistivity p-type silicon using stoichiometric PECVD silicon nitride

The surface passivation properties of silicon nitride (SiN) films fabricated by high-frequency direct plasma-enhanced chemical vapour deposition (PECVD) on low-resistivity (1 Ω cm) p-type silicon solar cell substrates have been investigated. The process gases used were ammonia and a mixture of silane and nitrogen. In order to find the optimum set of SiN deposition parameters, a large number of carrier lifetime test structures were prepared under different deposition conditions. The optimised deposition parameters resulted in outstandingly low surface recombination velocities (SRVs) below 10 cm/s. Interestingly, we find the lowest SRVs for stoichiometric SiN films, as indicated by a refractive index of 1.9. In former studies similarly low SRVs had only been obtained for silicon-rich SiN films. The fundamentally different passivation behaviour of our SiN films is attributed to the addition of nitrogen to the process gases.     

UVCVD silicon nitride passivation and ARC layers for multicrystalline solar cells

This work intends to investigate the effectiveness of silicon nitride layers (SiN_x : H) deposited by photochemical vapor deposition (UVCVD) for antireflection and passivation purposes when applied to electromagnetically casted silicon solar cells (EMC). Effective reflectivity of 10.8% is achieved, as well as 66% increase of minority carrier lifetime.     

SiOx(C)/SiNx dual-layer anti-reflectance film coating for improved cell efficiency

Light-induced plating (LIP), in which the current driving the metal reduction process is derived from illuminated solar cells, is an attractive technique for solar cell metallization because of its potential simplicity. However, applying the LIP techniques on standard acidic-textured multicrystalline silicon wafers with a silicon nitride-coated surface presents a challenge. The use of a spray-on carbon-doped non-stoichiometric silicon oxide [SiO_x(C)] dielectric film before nickel and silver plating can greatly reduce background plating while helping decrease the reflectance on the front of silicon solar cells. The sprayed dielectric films have low refractive indices of 1.3–1.4, depending on the annealing temperature. Simulation studies show that the SiO_x(C)/SiN_x dual-layer anti-reflective coating has a lower weighted reflectance against an AM 1.5 G spectrum compared with the SiN_x single coating. Finally, the performance of the laser-doped solar cells with a standard SiN_x as an anti-reflectance coating were compared with those with the SiO_x(C)/SiN_x double-layer stack. An efficiency of 16.74% on a large, commercial-grade, p-type, multicrystalline silicon substrate was achieved.     

Surface passivation of crystalline silicon wafer via hydrogen plasma pre-treatment for solar cells

The carrier lifetime of crystalline silicon wafers that were passivated with hydrogenated silicon nitride (SiN_x:H) films using plasma enhanced chemical vapor deposition was investigated in order to study the effects of hydrogen plasma pre-treatment on passivation. The decrease in the native oxide, the dangling bonds and the contamination on the silicon wafer led to an increase in the minority carrier lifetime. The silicon wafer was treated using a wet process, and the SiN_x:H film was deposited on the back surface. Hydrogen plasma was applied to the front surface of the wafer, and the SiN_x:H film was deposited on the hydrogen plasma treated surface using an in-situ process. The SiN_x:H film deposition was carried out at a low temperature (<350 °C) in a direct plasma reactor operated at 13.6 MHz. The surface recombination velocity measurement after the hydrogen plasma pre-treatment and the comparison with the ammonia plasma pre-treatment were made using Fourier transform infrared spectroscopy and secondary ion mass spectrometry measurements. The passivation qualities were measured using quasi-steady-state photoconductance. The hydrogen atom concentration increased at the SiN_x:H/Si interface, and the minority carrier lifetime increased from 36.6 to 75.2 μs. The carbon concentration decreased at the SiN_x:H/Si interfacial region after the hydrogen plasma pre-treatment.     

Low temperature surface passivation for silicon solar cells

Surface passivation at low processing temperatures becomes an important topic for cheap solar cell processing. In this study, we first give a broad overview of the state of the art in this field. Subsequently, the results of a series of mutually related experiments are given about surface passivation with direct Plasma Enhanced Chemical Vapour Deposition (PECVD) of silicon oxide (Si-oxide) and silicon nitride (Si-nitride). Results of harmonically modulated microwave reflection experiments are combined with Capacitance-Voltage measurements on Metal-Insulator-Silicon structures (CV-MIS), accelerated degradation tests and with Secondary Ion Mass Spectrometry (SIMS) and Elastic Recoil Detection (ERD) measurements of hydrogen and deuterium concentrations in the passivating layers. A large positive fixed charge density at the interface is very important for the achieved low surface recombination velocities S. The density of interface states D_it is strongly reduced by post deposition anneals. The lowest values of S are obtained with PECVD of Si-nitride. The surface passivation obtained with Si-nitride is stable under typical operating conditions for solar cells. By using deuterium as a tracer it is shown that hydrogen in the ambient of the post deposition anneal does not play a role in the passivation by Si-nitride. Finally, the results of CV-MIS measurements (Capacitance-Voltage measurements on Metal-Insulator-Silicon structures) on deposited Si-nitride layers are used to calculate effective recombination velocities as a function of the injection level at the surface, using a model that is able to predict the surface recombination velocity S at thermally oxidized silicon surfaces. These results are not in agreement with the measured increase of S at low injection levels.     

Hydrogen passivation of defects in multicrystalline silicon solar cells

The knowledge of how hydrogen interacts with defects and impurities in silicon is crucial for the understanding of device performance, especially for solar cells made from disordered silicon wafers. Hydrogen can be introduced in silicon by several techniques, but this paper will be focused on hydrogenation by means of plasma enhanced chemical vapor deposition of hydrogen-rich silicon nitride layer on the surface of the wafer. Passivation effects are observed after annealing and evaluated using minority carrier diffusion length measurements and light-beam-induced current scan maps.It was found that individual intragrain defects are well passivated, while deep levels are transformed into poorly recombining shallow levels at grain boundaries and dislocation clusters. In solar cells, the stability of the hydrogen passivation is much higher with this technique than with other hydrogenation techniques. This is probably due to an encapsulation of hydrogen by the frontwall silicon nitride coating layers and by the backside aluminum film.     

Numerical analysis of field-effect surface passivation for solar cells

In order to investigate the influence of surface potential on the electric characteristics of solar cells, the characteristics of conventional cells and back-contact type high-efficiency silicon cells were analyzed using 2-dimensional numerical simulation, varying the surface electrical potential. The locations where surface electrical potential is controlled are the rear side in conventional cells and the front side in back-contact cells. As a result of the calculations, it was found that field-effect surface passivation yields cell characteristics equivalent to those of a cell with effective surface recombination velocity of 0 cm/s, even if the cell has a poor interface (i.e., D_it > 1.0 × 10¹¹cm⁻²eV⁻¹). It was also found that both the use of a higher resistivity wafer and — especially in p-type substrates — the formation of inversion layers causes the field-effect surface passivation to work the fullest effect. In addition, a computer simulation based on physical-parameter measurements taken from actual materials forecasts that a back-contact cell would realistically be able to exceed 25% efficiency under AM1.5 global, one-sun illumination.     

Passivation of silicon by silicon nitride films

It is shown by contactless transient photoconductivity measurements in the microwave frequency range that Si₃N₄ films are an outstanding passivation of the n-type c-Si surface. Si₃N₄ on n-type Si forms an accumulation layer, which acts as an ideally reflecting potential barrier for minority carriers (holes). Due to the small space charge layer capacitance, minority carrier storage at this interface is very limited.In contrast to the latter measurements on p-type Si wafers covered with Si₃N₄ are characterized by storage of excess charge carriers in the surface depletion layer. The stored charge carriers decay slowly. The minority carriers (electrons) collected at the surface show a reduced mobility.     

Silicon nitride film for solar cells

In this work, our aim was to determine the deposition parameters leading to optimal optical properties of Silicon nitride (SiN) film for photovoltaic application. The deposition was performed in an industrial pulsed direct-PECVD using a gas mixture of NH₃/SiH₄.After defining the optimum deposition parameters, we have chemically evaluated the film quality in BOE solution. Plasma removal of the optimized SiN films from multicrystalline 4-in solar cells allows highlighting and estimating the emitter passivation and ARC effects on the solar cell electrical performance.     

Structure of PECVD Si:H films for solar cell applications

The structure of undoped SiH films and solar cells deposited under different hydrogen concentration and substrate temperatures were studied. The characterization techniques used were XRD, Raman spectroscopy, TEM, optical absorption, and hydrogen effusion. The high concentration films were amorphous in the as-deposited state but crystallized upon annealing at 700°C. Middle and low concentration films were nanocrystalline (nc) and remained nc up to 800°C annealing. A theoretical explanation is given for the stability of these films. Such films, on glass substrates, had optical absorption spectra close to those of amorphous material. The solar cell samples, showed some nc morphology in all-concentration states.     

Fundamental understanding and implementation of Al-enhanced PECVD SiNx hydrogenation in silicon ribbons

A low-cost, manufacturable defect gettering and passivation treatment, involving simultaneous anneal of a PECVD SiN_x film and a screen-printed Al layer, is found to improve the lifetime in Si ribbon materials from 1–10 μs to over 20 μs. Our results indicate that the optimum anneal temperature for SiN_x-induced hydrogenation is 700°C for EFG and increases to 825°C when Al is present on the back of the sample. This not only improves the degree of hydrogenation, but also forms an effective back surface field. We propose a three-step physical model, based on our results, in which defect passivation is governed by the release of hydrogen from the SiN_x film due to annealing, the generation of vacancies during Al–Si alloying, and the retention of hydrogen at defect sites due to rapid cooling. Controlled rapid cooling was implemented after the hydrogenation anneal to improve the retention of hydrogen at defect sites by incorporating an RTP contact firing scheme. RTP contact firing improved the performance of ribbon solar cells by 1.3–1.5% absolute when compared to slow, belt furnace contact firing. This enhancement was due to improved back surface recombination velocity, fill factor, and bulk lifetime. Enhanced hydrogenation and rapid heating and cooling resulted in screen-printed Si ribbon cell efficiencies approaching 15%.     

Comparison between SiNx:H and hydrogen passivation of electromagnetically casted multicrystalline silicon material

This work intends to compare two different passivation methods for electromagnetically continuous pulling silicon (EMCP): remote plasma hydrogenation and remote plasma enhanced CVD of SiN followed by high-temperature sintering. All experiments are carried out on textured and non-textured EMCP samples from the same ingot. To check the effect of high-temperature diffusion on EMCP, a n⁺-emitter is formed on one group of the samples using POCl₃ diffusion. Passivation capabilities of both techniques are checked using measurements of minority carrier lifetime by means of microwave photoconductance decay mapping. Solar cells are made to compare lifetime measurement with cell parameters.     

Overview on SiN surface passivation of crystalline silicon solar cells

Silicon nitride (SiN) fabricated by plasma-enhanced chemical vapour deposition (PECVD) is increasingly used within the crystalline silicon (c-Si) photovoltaic industry as it offers the possibility to fabricate a surface and bulk passivating antireflection coating at low temperature (450°C). This article presents an overview on the present status of SiN for industrial as well as laboratory-type c-Si solar cells. Topics covered include the fundamentals of the PECVD technology, the present status of high-throughput PECVD machines for the deposition of SiN onto c-Si wafers, and a review of the fundamental properties of Si–SiN interfaces fabricated by PECVD.     

Low cost surface passivation for p-type mc-Si based on pseudobinary alloys (Al2O3)x(TiO2)1−x

The conventional process for back side passivation with full face Al screen printing layer is not suitable for very thin multicrystalline (mc-Si) solar cells and approaches to new technological processes are searched for. More investigations have been concentrated on local aluminum contacts and passivation coatings with different layers on mc-Si wafers. The aim of this work is to prove that (Al₂O₃)_x(TiO₂)_1−x is one promising candidate to be applied as passivation layer on multicrystalline Si. Investigations were performed on dielectric films of pseudobinary alloy (PBA) (Al₂O₃)_x(TiO₂)_1−x, prepared by chemical solution deposition known initially as sol–gel method. It was determined that their optical, dielectric and electrophysical properties are suitable for applications of these layers as back side surface passivation for thin multicrystalline silicon cells.     

Utilisation of a micro-tip scanning Kelvin probe for non-invasive surface potential mapping of mc-Si solar cells

We have applied a micro-tip Scanning Kelvin Probe to produce high-resolution surface potential maps of silicon nitride (Si₃N₄) coated multi-crystalline Silicon (mc-Si) solar cells in a non-contact, non-invasive fashion. We show this technique highlights two types of defects: localised surface charge and shunts. In the latter case we contrast the non-contact surface potential maps with contact measurements made by the Shuntscan technique.Using a guarded micro-tip with active shield we show for the first time surface potential changes at the mc-Si grain boundaries which are due to different mc-Si polytypes. The high-resolution scanning Kelvin probe (HR-SKP) has a surface potential resolution of <10 mV at a tip diameter <200 μm.     

Effect of hydrogen dilution on intrinsic a-Si:H layer between emitter and Si wafer in silicon heterojunction solar cell

In silicon heterojunction solar cells, a thin intrinsic amorphous-silicon (a-Si:H) buffer layer between a doped emitter and a c-Si wafer is essential to minimize carrier recombination. This study examines the effect of H₂ dilution on the properties of the intrinsic a-Si:H layers deposited on Si wafers by plasma-enhanced chemical vapor deposition. A H₂/SiH₄ ratio of 24 led to improvements in the quality of intrinsic a-Si:H films and in the performance of passivation compared to a-Si:H film without H₂ dilution. A high H₂-dilution ratio, however, degraded the passivation of the a-Si:H film. The Si heterojunction solar cells with an optimal intrinsic a-Si:H layer showed an efficiency of 12.3%.     

Process modeling and optimization of PECVD silicon nitride coated on silicon solar cell using neural networks

The process of plasma enhanced chemical vapor deposition silicon nitride films coated on silicon solar cells as antireflection layers is modeled and optimized using neural networks. This neural network model is built based on the robust design technique with process input–output experimental data. The input parameters selected are as substrate temperature, SiH₄ and NH₃ flow rates, and RF power; while the output parameters are deposition rate, refractive index, and short circuit current. This model can then be applied to predict the input–output relationships of the process. Optimal operating conditions of this process can be determined using this model.