On-wafer investigation of SiC and Si3N4 inclusions in multicrystalline Si grown by directional solidification

Commercial multicrystalline Si (mc-Si) wafers containing SiC and Si₃N₄ inclusions and wire-sawing defects on their surfaces were collected from an mc-Si wafer manufacturer. The mc-Si, used for solar cells, was grown using industrial directional solidification systems. The technique of controlled etching was applied to these mc-Si wafers to dissolve a certain amount of silicon from the surface of each wafer and to partially expose SiC and Si₃N₄ inclusions inside these wafers to allow for direct observation. The physical presence and morphologies of the SiC and Si₃N₄ inclusions within the mc-Si wafers were investigated using scanning electron microscopy. SiC inclusions were composed of SiC particles of different sizes, and they were usually present as clusters embedded within the mc-Si wafers. Si₃N₄ inclusions were present as rods distributed within the mc-Si wafers. It has been shown that the presence of SiC particles is responsible for the formation of the wire-sawing defects, while Si₃N₄ particles are readily sawed across without introducing wire-sawing defects during the wire-sawing process. This work will provide an important base-line for further investigation on how these inclusions affect the photovoltaic performance of mc-Si solar cells.     

Hard inclusions and their detrimental effects on the wire sawing process of multicrystalline silicon

We studied commercial multicrystalline silicon (mc-Si) wafers having wire sawing-related defects on their surfaces. SiC and Si₃N₄ inclusions were identified in these defected areas, and they were highly localized. SiC inclusions were present in the form of clusters embedded in mc-Si wafer. These inclusions introduced a stress concentration into mc-Si wafer. The wire sawing-related defects on the wafer surfaces are arrays of sawing ridges. SiC inclusions were the main cause for these sawing ridges, while the effect of Si₃N₄ was not notable. A model was proposed to explain the formation of the sawing ridges on the surfaces of mc-Si wafers.     

Antireflecting–passivating dielectric films on crystalline silicon solar cells for space applications

Antireflecting–passivating TiO₂–SiO₂ double layers on crystalline silicon (Si) were optimized and characterized for space solar cells applications. In the numeric optimization, the MgF₂–glass–adhesive–TiO₂–SiO₂–Si structure was considered. In order to fabricate the TiO₂–SiO₂ double layer, titanium films were deposited on Si wafers in a vacuum chamber, and then, the sample was annealed in oxygen at high temperatures. Glasses with evaporated MgF₂ thin films were bonded to the TiO₂–SiO₂–Si samples so as to obtain the complete structure. A gain of up to 23.5% in the maximum power is demonstrated for simulated c-Si solar cells using the optimized structure. Characterization of the TiO₂–SiO₂–Si structure using transmission electron microscopy (TEM) and X-ray reflectivity (XRR) as well as optical characterization are presented.     

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.     

Enhancement of photovoltaic properties of multicrystalline silicon solar cells by combination of buried metallic contacts and thin porous silicon

Photovoltaic properties of buried metallic contacts (BMCs) with and without application of a front porous silicon (PS) layer on multicrystalline silicon (mc-Si) solar cells were investigated. A Chemical Vapor Etching (CVE) method was used to perform front PS layer and BMCs of mc-Si solar cells. Good electrical performance for the mc-Si solar cells was observed after combination of BMCs and thin PS films. As a result the current-voltage (I-V) characteristics and the internal quantum efficiency (IQE) were improved, and the effective minority carrier diffusion length (Ln) increases from 75 to 110 μm after BMCs achievement. The reflectivity was reduced to 8% in the 450-950 nm wavelength range. This simple and low cost technology induces a 12% conversion efficiency (surface area = 3.2 cm²). The obtained results indicate that the BMCs improve charge carrier collection while the PS layer passivates the front surface.     

Two-dimensional LBIC and internal quantum efficiency investigations of porous silicon-based gettering procedure in multicrystalline silicon

In this work, a porous silicon-based gettering technique was applied to multicrystalline silicon (mc-Si) wafers. Porous silicon (PS) was formed by the stain-etching technique and was used as a sacrificial layer for efficient external purification technique. The gettering procedure consists of achieving a PS/mc-Si/PS structure that undergoes a heat treatment at 900 °C for 90 min in an infrared furnace under a N₂ ambient. After removing the PS layers, mc-Si solar cells were realized. The effect of the gettering procedure was evaluated by means of the laser beam-induced current (LBIC) mapping, the internal quantum efficiency (IQE) mapping and the dark current-voltage (I-V) characteristic. Consequently, LBIC and IQE images show an enhancement of the gettered sample as compared to a reference untreated one. The serial resistance and the shunt resistance carried out from the dark I-V curves confirm this gettering-related solar cell improvement.     

Texturing of large area multi-crystalline silicon wafers through different chemical approaches for solar cell fabrication

Surface texturing of silicon can reduce the reflectance of incident light and hence increase the conversion efficiency of solar cells. Comparatively lesser concentrated (10%) standard alkaline (NaOH/KOH) solution does not give good textured multi-crystalline silicon (mc-Si) surface, which could give satisfactory open-circuit voltage. This is due to grain-boundary delineation with steps formed between successive grains of different orientations. In this work an attempt has been made to obtain a well-textured mc-Si surface through three different approaches. The first two are with two different types of acid solutions and the third with concentrated alkaline NaOH. Solutions of HF–HNO₃–CH₃COOH/H₂O system with different concentrations of HF and HNO₃ were used for texturing. The results on the effect of texturing of these three solutions on the surface morphology of very large area (125 mm×125 mm) mc-Si wafer as well as on the performance parameters of solar cell are presented in this paper. Attempts have been made to study extensively the surface morphologies of mc-Si wafers in two effective regions of the isoetch curves of the HF:HNO₃:diluent's system. Also we studied the reflectance, uniformity, spectral response, short-circuit current, open-circuit voltage, fill factor and dark current–voltage of the cells fabricated using wafers textured with the three different methods. Short-circuit current of the solar cells fabricated using acid-textured wafers were measured to be in the range of 4.93 A. This value is 0.37 and 0.14 A higher than the short-circuit current values measured in the cells fabricated with isotextured and alkaline-textured wafers, respectively.     

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.     

Damage-free reactive ion etch for high-efficiency large-area multi-crystalline silicon solar cells

Texturing of silicon (Si) wafer surface is a key to enhance light absorption and improve the solar cell performance. While alkaline texturing of single-crystalline Si (sc-Si) wafers was well established, no chemical solution has been successfully developed for multi-crystalline Si (mc-Si) wafers. Reactive-ion-etch (RIE) is a promising technique for effective texturing of both sc-Si and mc-Si wafers, regardless of crystallographic characteristics, and more suitable for thin wafers. However, due to the use of plasma source generated by high power, the wafer surface gets a physical damage during the processing, which requires an additional subsequent damage-removal wet processing. In this work, we developed a damage-free RIE texturing for mc-Si solar cells. An improved self-masking RIE texturing process, developed in this study, produced ∼0.7% absolute efficiency gain on 156×156 mm² mc-Si cells, where the gas ratio and the plasma power density were keys to mitigate the plasma-induced-damage during the RIE processing while maintaining decent surface reflectance. In the self-masking RIE texturing, a mixture of SF₆/Cl₂/O₂ gases was found to significantly affect the surface morphology uniformity and reflectance, where an optimal etch depth was found to be 200-400 nm. We achieved J_sc gain of ∼1.3 mA/cm² while maintaining decent FFs of ∼0.78 without a V_oc loss after optimization of firing conditions.     

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.     

SiO2 passivation layer grown by liquid phase deposition for silicon solar cell application

Surface passivation is one of the primary requirements for high efficient silicon solar cells. Though the current existed passivation techniques are effective, expensive equipments are required. In this paper, a comprehensive understanding of the SiO₂ passivation layer grown by liquid phase deposition (LPD) was presented, which was cost-effective and very simple. It was found that the post-annealing process could significantly enhance the passivation effect of the LPD SiO₂ film. Besides, it was revealed that both chemical passivation and field-effect passivation mechanisms played important roles in outstanding passivation effect of the LPD SiO₂ film through analyzing the minority carrier lifetime and the surface recombination velocity of n-type and p-type silicon wafers. Although the deposition parameters had little influence on the passivation effect, they affected the deposition rate. Therefore, appropriate deposition parameters should be carefully chosen based on the compromise of the deposition rate and fabrication cost. By utilizing the LPD SiO₂ film as surface passivation layer, a 19.5%-efficient silicon solar cell on a large-scale wafer (156 mm × 156 mm) was fabricated.     

Antireflection and surface passivation behaviour of SiO2/Si/SiO2 quantum wells on silicon

Quantum wells (QWs) consisting of Si and Si-related materials (such as SiO₂) are of interest for solar cell work because they can possibly be used as a surface passivating antireflection (AR) coating or as the top cell in an all-silicon tandem solar cell. In this study, we fabricate SiO₂/Si/SiO₂ QW layers by RF magnetron sputtering and thermal oxidation. On high-resistivity (300 Ω cm) n-type silicon wafer substrates, the effective surface recombination velocity provided by our SiO₂/Si/SiO₂ QWs is around 4 cm/s for 13 Å Si thickness and 480 cm/s for 150 Å Si thickness. The parasitic optical absorption in the well-passivating QWs is negligible for terrestrial photovoltaic applications. However, they have very poor AR properties on Si wafers and hence would have to be covered by an additional reflection reducing dielectric film.     

Low-temperature contacts through SixNy-antireflection coatings for inverted a-Si:H/c-Si hetero-contact solar cells

High-temperature contact firing of screen printable metal pastes is getting more problematic as silicon wafers used in solar cell production are becoming thinner. Besides, an electronic degradation of the Si_xN_y/c-Si interface occurs at these temperatures especially if the Si_xN_y layer is directly deposited onto high-quality absorbers as on the front side of a-Si:H/c-Si hetero-contact solar cells of inverted geometry. The latter structure has been proposed as an easy producible high-efficiency solar cell. Low-temperature alternatives such as local ablation of Si_xN_y with 355 nm laser radiation are examined with regard to the stability of the electronic quality of passivated areas between the openings in the Si_xN_y layer. Contactless time-resolved microwave conductivity measurements (TRMC) are applied to measure changes in electronic passivation after these treatments. Subsequent galvanic metallization of the openings is optimized for its use as ohmic contacts.     

Substrate to substrate aluminized bonding of 10 cm round silicon substrates for application in solar photovoltaics

Large area silicon solar cells with screen printed contacts have been realized for the first time on 10 cm diameter, p-type, Cz silicon wafers which were bonded to silicon substrates by alloying of a suitably thick screen printed layer of Al on them. In cells made on 300 μm thick wafers without texturization, antireflection coating and passivation of the front surface, the values of the open-circuit voltage (V_oc), the short-circuit current density (J_sc), curve factor (CF) and the efficiency (η) were found to be in the range 572–579 mV, 16–19.2 mA cm⁻², 0.53–0.61 and 5.5–5.89%, respectively, under simulated tungsten halogen light of 100 mW cm⁻² intensity. Using thinner wafers and having optical confinement, surface passivation and effective back surface field, the cell performance would be substantially improved. In fact, an efficiency close to 18% (AM1.5) would be realizable with this approach. Another attractive feature of this approach is that a low-cost silicon substrate could be used at the bottom that would act as support for the thin top surface without disadvantage to the cell performance. In this paper only the principle has been demonstrated experimentally. Possible improvements have been shown by computer simulation.     

Effect of porous silicon on the performances of silicon solar cells during the porous silicon-based gettering procedure

In this work we analyse the effect of porous silicon on the performances of multicrystalline silicon (mc-Si) solar cells during the porous silicon-based gettering procedure. This procedure consists of forming PS layers on both front and back sides of the mc-Si wafers followed by an annealing in an infrared furnace under a controlled atmosphere at different temperatures. Three sets of samples (A, B and C) have been prepared; for samples A and B, the PS films were removed before and after annealing, respectively. In order to optimize the annealing temperature, we measure the defect density at a selected grain boundary (GB) using the dark current–voltage (I–V) characteristics across the GB itself. The annealing temperature was optimized to 1000 °C. The effect of these treatments on the performances of mc-Si solar cells was studied by means of the current–voltage characteristic (at AM 1.5) and the internal quantum efficiency (IQE). The results obtained for cell A and cell B were compared to those obtained on a reference cell (C).     

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.     

多晶PERC太阳电池的背面与正面结构性能优化

多晶硅太阳电池以其价格低廉的优势成为低成本太阳电池的首选,但其光电转换效率提升空间有限。钝化发射极和背面电池（PERC）技术是当前晶硅太阳电池提效的主要途径。多晶PERC电池结合了多晶硅电池的低成本和PERC电池的高效,是当前多晶硅电池的研究热点。本文研究了多晶PERC电池的背面和正面结构优化与设计,提出了提高多晶PERC电池效率的产业化技术方法。通过在硅片背面用三层SiN_x:H薄膜来代替常规双层SiN_x:H薄膜,在保证优良的背面钝化的同时,使电池长波响应得到改善,电池光电转换效率由20.19% 提升至20.26%。优化多晶PERC电池的背面激光开窗工艺,使多晶电池效率较常规工艺提升0.11%。而在多晶PERC电池的正面叠加选择性发射极技术,可较常规工艺提升电池效率0.10%。综合运用多种提效手段有利于保持多晶PERC电池的竞争力。     

16.4% efficient, thin active layer silicon solar cell grown by liquid phase epitaxy

An energy conversion efficiency of 16.4% is reported for a silicon solar cell of 4.11 cm² total area with a thin active layer of 32 μm grown by liquid phase epitaxy (LPE). This is the highest ever total area efficiency for a cell of this type and is due to a number of improvements over earlier reported results. The thin active layer was grown by LPE on an inactive silicon substrate from an indium solution in a 20% hydrogen/argon forming gas mixture ambient rather than pure hydrogen. Higher current density and efficiency than previously reported for similar cell structures have been achieved by employing microgroove texturing of the front surface, a very shallow (0.25 μm) and high sheet resistivity (220 Ω□) top surface phosphorus diffusion, an optimized ZnS/MgF₂ double layer antireflection coating on top of a 200Å thick, high quality passivation SiO₂ layer, a large aspect ratio (0.45) for the metal contacts, and a graded doping level within the 32 μm thick LPE active layer. The effect of the improved techniques on the cell performance and the properties of the thin active layers are discussed.     

Performance of superstrate multijunction amorphous silicon-based solar cells using optical layers for current management

The performance of multijunction amorphous silicon-based thin film solar cells has been reported using thin layers of TiO₂ and SiO_x acting as refractive index matching optical layers for different interfaces of the superstrate device structure. Improvement of short-circuit current from the sub-cells of a-Si/μc-Si cells is demonstrated with TiO₂ as anti-reflection layer at TCO/Si interface and SiO_x as intermediate-reflector layer between two sub-cells. An initial efficiency of 11.8% is achieved by applying both the TiO₂ and SiO_x optical layers in a-Si/μc-Si solar cell.     

Further option for solar concentrators: Aluminum first surface mirrors

The research and development of aluminum first surface solar mirrors is presented. Two protection films for the aluminum layer are discussed: Si₂O₃ and SiO₂. Two electron guns (e-gun) are used to manufacture aluminum first surface solar mirrors. One, for aluminum evaporation, eliminates or minimizes pinholes observed when aluminum is evaporated with tungsten filaments. The other e-gun allows the evaporation of SiO and SiO₂ without the mirror contamination previously seen due to the air when the chamber was opened. Better adherence between the aluminum film and the Si₂O₃ or SiO₂ is obtained due the use of two electron guns that does not permit the chamber to open. Si₂O₃ is a material obtained by oxidation of SiO by admitting some oxygen into the evaporation chamber (10⁻⁴ Torr). The optimum thickness of the aluminum layer was 1000 Å or higher, about 2500 Å for the Si₂O₃, and 3200 Å for the SiO₂. The specular reflectance of these mirrors is about 0.89. These mirrors were tested in the environmental chamber for accelerated weathering without any important degradation, making them another option for solar concentrators in solar energy applications.