Defect passivation of industrial multicrystalline solar cells based on PECVD silicon nitride

Low surface recombination velocity and significant improvements in bulk quality are key issues for efficiency improvements of solar cells based on a large variety of multicrystalline silicon materials. It has been proven that PECVD silicon nitride layers provide excellent surface and bulk passivation and their deposition processes can be executed with a high throughput as required by the PV industry. The paper discusses the various deposition techniques of PECVD silicon nitride layers and also gives results on material and device properties characterisation. Furthermore the paper focuses on the benefits achieved from the passivation properties of PECVD SiN_x layers on the multi-Si solar cells performance. This paper takes a closer look at the interaction between bulk passivation of multi-Si by PECVD SiN_x and the alloying process when forming an Al-BSF layer. Experiments on state-of-the-art multicrystalline silicon solar cells have shown an enhanced passivation effect if the creation of the alloy and the sintering of a silicon nitride layer (to free hydrogen from its bonds) happen simultaneously. The enhanced passivation is very beneficial for multicrystalline silicon, especially if the defect density is high, but it poses processing problems when considering thin (<200 μm) cells.     

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.     

SiNx deposited by in-line PECVD for multi-crystalline silicon solar cells

SiN_x:H anti-reflective coating (ARC) layers were successfully grown by an in-line plasma enhanced chemical vapor deposition (PECVD) system with an extremely high throughput. Film thickness and refractive index of the as-grown samples were evaluated as functions of growth parameters, such as growth pressure, total gas flow rate, radio frequency (RF) power and SiH₄ to NH₃ gas ratio. It was found that we could achieve high quality films with proper growth conditions and proper post-deposition annealing.     

Ribbon growth on substrate (RGS) silicon solar cells with microwave-induced remote hydrogen plasma passivation and efficiencies exceeding 11%

For the first time efficiencies above 11% for solar cells (4 cm²) based on Bayer ribbon growth on substrate (RGS) crystalline silicon have been demonstrated including mechanical V-structuring of the front surface, aluminum-gettering, microwave-induced remote hydrogen plasma (MIRHP) passivation and PECVD SiN/SiO₂ double-layer antireflection coating. MIRHP alone resulted in absolute improvements in the open-circuit voltage of 27 mV, in the short-circuit current density of 2.8 mA cm⁻² and in the cell efficiency of 1.9% leading to an open-circuit voltage of 538 mV and an efficiency of 11.1%.     

Spatially resolved investigations of lifetime enhancement in vertically grown, multicrystalline silicon ribbons

The influence of gettering or defect passivation steps on recombination activity in the vertically grown, multicrystalline ribbon materials edge-defined film-fed growth and string ribbon silicon has been investigated with the help of photoconductance decay. In contrast to well-known results of integral measurements, spatially resolved lifetime mappings have been obtained by applying microwave detection technique.This aspect of spatial resolution has been found to be indispensable for investigating the impact of different processing steps on material quality in an accurate way. Apart from strong variations in as-grown lifetimes that have been found throughout vertically grown silicon wafers, this is due to areas of comparable starting lifetimes which have been revealed to react very differently to applied processing steps. After processing, some of them reach minority charge carrier lifetimes of more than 300 μs whereas others just show values of a few microseconds. As a consequence, the results of integral measurements strongly depend on the nature of areas incorporated in the specific sample. An impression of the corresponding uncertainties inherent to integral measurements has been obtained by statistical evaluation of spatially resolved lifetime data.     

Texturization of multicrystalline silicon wafers for solar cells by chemical treatment using metallic catalyst

A new etching method for texturing multicrystalline p-type Si wafers for solar cells was developed. In this method, we used platinum or silver particles as the catalysts, which were loaded on the wafers by means of the electroless-plating technique. After deposition of the catalysts, the wafers were etched and textured in HF solution, to which in some cases chemical oxidants were added. The solar cells (4 cm²) manufactured from the textured wafers showed efficiency as high as 16.6%, which was about 1% (absolute) higher than that of the cells made from the wafers treated by the conventional alkaline method.     

Hydrogen passivation of defects in EFG ribbon silicon

To enhance the bulk lifetime of multicrystalline silicon material, gettering of impurities and hydrogen passivation of defects are investigated. In edge-defined film-fed grown (EFG) ribbon silicon, an aluminium-enhanced hydrogenation of defects by silicon nitride has been reported. On thin wafers, the formation of a full area aluminium back surface field will lead to wafer bending due to different thermal expansion coefficients of aluminium and silicon. To circumvent this problem, remote plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SiN_x) as passivation scheme for the front and rear surface is proposed. In this work, the bulk passivation by hydrogenation is investigated using two different hydrogen passivation techniques: (i) passivation in a remote hydrogen plasma and (ii) passivation due to a post-deposition anneal of remote PECVD-SiN_x in a lamp-heated conveyor belt furnace. Measurements of the bulk lifetime show that the lifetime improvement due to remote hydrogen plasma passivation degrades under illumination with white light. In contrast, the hydrogen passivation by a post-deposition SiN_x anneal is only effective if a phosphorous-doped emitter is present below the SiN_x layer during the hydrogenation. This lifetime improvement is stable under illumination.     

A new strategy for the fabrication of cost-effective silicon solar cells

Fabrication of solar cells with very high efficiencies currently requires extremely complex processing. In order to make photovoltaics an economical large scale source of energy, very high efficiencies have to be achieved by low-cost processing. The innovative approach for the cost-effective production of highly efficient silicon solar cells presented in this paper is characterised by only four simple and environmentally safe large-area fabrication steps. The basic processing sequence consists of: (i) mechanical surface grooving, (ii) simple diffusion or inversion, (iii) shallow angle metal evaporation, and (iv) plasma silicon nitride deposition. Cell design, fabrication techniques and processing sequences for metal-insulator-semiconductor contacted diffused n⁺-p junction (MIS-n⁺p) and MIS-inversion-layer (MIS-IL) silicon solar cells are outlined. The new simple approach turned out to be most successful, as demonstrated by mechanically grooved MIS-n⁺p silicon solar cells with efficiencies above 21% using exclusively aluminium as metallisation.     

Experimental evidence of very high open-circuit voltages of inversion–layer silicon solar cells

In this paper the first experimental evidence of the high V_oc-potential of inversion-layer silicon solar cells is given. Minority-carrier lifetime measurements on inversion-layer emitters have been performed and the diffused p–n contact of PN-IL silicon solar cells has been optimized for high open-circuit voltages. PN-IL silicon solar cells with open-circuit voltages of 693 mV have been fabricated on 0.2 and 0.5-Ω cm FZ p-Silicon wafers. These values are the highest ever reported V_oc's for inversion-layer silicon solar cells on p-Silicon. This demonstrates that inversion-layer silicon solar cells exhibit a similar potential for achieving high open-circuit voltages as silicon solar cells with a diffused p–n junction.     

Recycling of solar cell silicon scraps through filtration, Part I: Experimental investigation

This study investigated the removal of SiC and Si₃N₄ inclusions from top-cut solar cell silicon scraps by filtration with foam filters. Laboratory experiments tested various models for the removal mechanism of inclusions and the efficiency of ceramic foam filters. Inclusions in solar cell silicon top-cut scraps were mainly needle-like Si₃N₄ particles and lumpy SiC inclusions. SiC and Si₃N₄ inclusions sometimes agglomerated as clusters. Si₃N₄ inclusions were usually more than 500 μm long with diameters of ∼20 μm, and SiC inclusions were usually smaller than 500 μm. After filtration, no Si₃N₄ inclusions were found. The remaining inclusions were mainly SiC inclusions smaller than 10 μm. Filters with smaller pores improved the removal of inclusions. It was discovered that contamination of the silicon may occur from components of the filters themselves, especially from binders. The crucible was also a source of contamination. A filtration process that does not produce contamination of its own should be developed before being used in real industrial processes. Mechanisms for the removal of inclusions from silicon through filtration are as follows: (1) cake filtration, for the removal of large Si₃N₄ rods and large SiC inclusions; (2) deep-bed filtration of smaller Si₃N₄ inclusions and most SiC particles; (3) formation of large SiC clusters and bridges across pores, and (4) silicon dissolution into carbon filters, and a subsequent reaction to form layers of SiC.     

Comparative study of different approaches of multicrystalline silicon texturing for solar cell fabrication

Alkali etchant cannot produce uniformly textured surface to generate satisfactory open circuit voltage as well as the efficiency of the multi-crystalline silicon (mc-Si) solar cell due to the unavoidable grain boundary delineation with higher steps formed between successive grains of different orientations during alkali etching of mc-Si. Acid textured surface formed by using chemicals with HNO₃–HF–CH₃COOH combination generally helps to improve the open circuit voltage but always gives lower short circuit current due to high reflectivity. Texturing mc-Si surface without grain boundary delineation is the present key issue of mc-Si research. We report the isotropic texturing with HF–HNO₃–H₂O solution as an easy and reliable process for mc-Si texturing. Isotropic etching with acidic solution includes the formation of meso- and macro-porous structures on mc-Si that helps to minimize the grain-boundary delineation and also lowers the reflectivity of etched surface. The study of surface morphology and reflectivity of different mc-Si etched surfaces has been discussed in this paper. Using our best chemical recipe, we are able to fabricate mc-Si solar cell of 14% conversion efficiency with PECVD AR coating of silicon nitride film. The isotropic texturing approach can be instrumental to achieve high efficiency in mass production using relatively low-cost silicon wafers as starting material with the proper optimization of the fabrication steps.     

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.     

Analysis of multicrystalline silicon solar cells by modified 3-diode equivalent circuit model taking leakage current through periphery into consideration

We proposed a modified 3-diode equivalent circuit model for analysis of multicrystalline silicon (Mc-Si) solar cells. By using this equivalent circuit model, we can precisely evaluate the characteristics of Mc-Si solar cells taking the influence of grain boundaries and large leakage current through the peripheries into consideration and extract electrical properties. The calculated value of current-voltage characteristics for small size (3 mm×3 mm) Mc-Si solar cells using this model completely agreed with the measured value at various cell temperatures. Moreover, the calculated open-circuit voltage (V_oc) obtained by extracted parameters and measured V_oc agreed well.     

Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N⁺/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N⁺/P, 78.5 cm²) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I_sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I–V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.     

Study on hydrogenated silicon nitride for application of high efficiency crystalline silicon solar cells

The hydrogenated silicon nitride films (SiN_x:H) deposited by plasma enhanced chemical vapor deposition (PECVD) technique is commonly used as an antireflection coating as well as surface passivating layer of crystalline silicon solar cells. The refractive indices of SiN_x:H films could be changed by varying the growth gas ratio R(=NH₃/SiH₄+NH₃) and annealing temperature. For optimum SiN_x:H film, the optical and chemical characterization tools by varying the film deposition and annealing condition were employed in this study. Metal-insulator-semiconductor (MIS) devices were fabricated using SiN_x:H as an insulator layer and they were subjected to capacitance-voltage (C-V) and current-voltage (I-V) measurements for electrical characterization. The effect of rapid thermal annealing (RTA) on the surface passivation as well as antireflection properties of the SiN_x:H films deposited at various process conditions were also investigated for the fabrication of low cost and high efficiency silicon solar cells.     

Silicon and solar-grade silicon production by solar dissociation of Si3N4

The temperature required for carbothermal reduction of silica—in the range 2100–2300 °C—is past the upper limit for combustion process heat. It is therefore an interesting candidate for solar–thermal processing. The production of solar-grade silicon from carbothermally produced silicon requires an energy-intensive long-duree high-temperature purification process. We propose here a two-step solar process for the production of silicon from silica: first, a carbothermal reduction in the presence of nitrogen to yield silicon nitride and second, the solar dissociation of the nitride to yield silicon. This last step could be combined with purification of the silicon if solar-grade silicon is the desired end-product. In this paper, we report on experimental results that indicate that silicon nitride can be dissociated to yield silicon with no detectable nitride content.     

Solar preparation of SiOx (x ≈ 1) nanopowders from silicon vaporisation on a ZrO2 pellet. XPS and photoluminescence characterisation

SiO_x nanoparticles were prepared by vaporisation and condensation of melted silicon droplets put on zirconia pellets in a solar reactor at the focus of a 2 kW solar furnace. The size of the grains were nanometric, generally included in the range 20–40 nm, and the O/Si atomic ratio values were close to stoichiometry (O/Si ≈ 1 ± 0.2). XPS, DRX and TEM analyses show that these nanoparticles are amorphous with various silicon chemical environments which can be described as constituted with polysubstituted Si-(O_4−nSi_n) tetrahedral configurations. The estimated oxygen atomic concentrations for these nanoparticles was in good agreement with thermodynamic equilibrium calculations for the system ZrO₂–Si at high temperature. The predominant gaseous species is the SiO molecule. The SiO_x nanoparticles present photoluminescence property similar to those currently reported for electrolytic porous silicon.     

The influence of structural defects on phosphorus diffusion in multicrystalline silicon

We have investigated the diffusion of phosphorous (P) in multicrystalline Silicon (Si) during solar cell emitter formation by secondary ion mass spectrometry (SIMS). From the experimental results, we observe significantly increased in-grain diffusion depths in areas of structural disorder that are not readily observed by the naked eye. We believe that this effect is due to increased concentrations of Si self-interstitials in the areas surrounding the defects, causing an enhanced transient response of elemental P diffusion. In areas adjacent to a grain boundary a slight, but notably smaller, enhancement of the P diffusion depth is observed.     

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.     

Correlating internal stresses, electrical activity and defect structure on the micrometer scale in EFG silicon ribbons

In the present paper, we study the influence of defects through their stress fields on the electrical activity and residual stress states of as-grown edge-defined film-feed (EFG) multicrystalline silicon (mc-Si) ribbons. We apply a combination of micro-Raman spectroscopy, electron beam induced current, defect etching and electron backscatter diffraction techniques that enables us to correlate internal stresses, recombination activity and microstructure on the micrometer scale. The stress fields of defect structures are considered to be too small (several tens of MPa) to influence directly the electrical activity, but they can enhance it via stress-induced accumulation of metallic impurities. It is commonly found that not all recombination-active dislocations on grain boundaries (GBs) and within grains are accompanied by internal stresses. The reason for this is that dislocations interact with each other and tend to locally rearrange in configurations of minimum strain energy in which their stress fields can cancel partially, totally or not at all. The outcome is a nonuniform distribution of electrical activity and internal stresses along the same GB, along different GBs of similar character as well as inside the same grain and inside different grains of similar crystallographic orientations. Our work has implications for developing crystal growth procedures that may lead to reduced internal stresses and consequently to improved electrical quality and mechanical stability of mc-Si materials by means of controlled interaction between structural defects.