Surface texturing of large area multicrystalline silicon solar cells using reactive ion etching method

A reactive ion etching method has been applied to form a surface texture of multicrystalline silicon solar cells in order to reduce the surface reflectance. This surface texture has a pyramid-like shape, and aspect ratio of which can be easily controlled by the gas flow ratio.15 cm × 15 cm multicrystalline silicon solar cells have been fabricated using this texturing method and maximum conversion efficiency of 17.1% has been achieved.     

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.     

Rie-texturing of multicrystalline silicon solar cells

We developed a maskless plasma texturing technique for multicrystalline silicon cells using reactive ion etching that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to extremely low levels. Internal quantum efficiencies as high as those on planar cells have been obtained, boosting cell currents and efficiencies by up to 7% on evaporated metal and 4% on screen-printed cells.     

Reduced light reflection of textured multicrystalline silicon via NPD for solar cells applications

Texturing by negative potential dissolution (NPD) process of p-type multicrystalline silicon for solar cells application is reported. The effect of the negative potential, KOH concentration, and texturing time of cast multicrystalline silicon was studied. Rapid texturing of multicrystalline silicon was achieved in a time-frame of 2 min with the application of negative potential of −30 V and the use of optimal alkaline concentration of 32 wt%. While texturing process in these optimal NPD conditions results in a step-free morphology, necessary in solar cells contacts printing, light reflection was reduced to minimal values, as well.     

Characterization of multicrystalline silicon wafers by non-invasive measurements

Non-invasive transient photoconductance measurements of large grain multicrystalline silicon wafers (ρ=1 Ω cm) are presented. It is shown that the surfaces of untreated wafers can be characterized as infinite sinks for excess charge carriers. The value 24.5 cm² s⁻¹ for the minority carrier diffusion constant was determined in all samples. So in untreated wafers, surface recombination yields a known contribution to the decay time measured and the volume lifetime can be determined. Application of these measurements as a standard characterization of multicrystalline silicon wafers is discussed.     

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.     

The infrared processing in multicrystalline silicon solar cell low-cost technology

New directions in photovoltaics depend very often on financial possibilities and new equipment. In this paper, we present the modification of a standard screen-printing technology by using an infrared (IR) furnace for forming a n⁺/p structure with phosphorus-doped silica paste on 100 cm² multicrystalline silicon wafers. The solar cells were fabricated on 300 μm thick 1 Ω cm p-type multicrystalline Bayer silicon. The average results for 100 cm² multicrystalline silicon solar cells are: I_sc=2589 mA, V_oc=599 mV, FF=0.74, E_ff=11.5%. The cross-sections of the contacts metallized in the IR furnace, as determined by scanning electron microscopy, and the phosphorus profile measured by an electrochemical profiler are shown. IR processing offers many advantages, such as a small overall thermal budget, low power and time consumption, in terms of a cost-effective technology for the continuous preparation of solar cells.     

Multicrystalline silicon solar cells with porous silicon emitter

A review of the application of porous silicon (PS) in multicrystalline silicon solar cell processes is given. The different PS formation processes, structural and optical properties of PS are discussed from the viewpoint of photovoltaics. Special attention is given to the use of PS as an antireflection coating in simplified processing schemes and for simple selective emitter processes as well as to its light trapping and surface passivating capabilities. The optimization of a PS selective emitter formation results in a 14.1% efficiency mc-Si cell processed without texturization, surface passivation or additional ARC deposition. The implementation of a PS selective emitter into an industrially compatible screenprinted solar cell process is made by both the chemical and electrochemical method of PS formation. Different kinds of multicrystalline silicon materials and solar cell processes are used. An efficiency of 13.2% is achieved on a 25 cm² mc-Si solar cell using the electrochemical technique while the efficiencies in between 12% and 13% are reached for very large (100–164 cm²) commercial mc-Si cells with a PS emitter formed by chemical method.     

High-efficiency low-cost integral screen-printing multicrystalline silicon solar cells

This paper describes how the efficiency and throughput of industrial screen-printed multi-Si solar cells can be increased far beyond the state-of-the-art production cells. Implementation of novel processes of isotropic texturing, shallow emitter or single diffusion selective emitter, combined with screen-printed metallization fired through a PECVD SiN_x ARC layer, have been described. Novel dedicated fabrication equipment for emitter diffusion and a PECVD SiN_x deposition system are developed and implemented thereby removing the processing bottlenecks linked to the diffusion and bulk passivation processes. Several types of back-contacted solar cells with improved visual appeal required for building integrated photovoltaic (BIPV) application have been developed.     

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.     

Large area multicrystalline silicon solar cells in high volume production environment—history, status, new processes, technology transfer issues

Cast multicrystalline silicon (mc-Si) solar cell technology, accounted for nearly 41% of all the PV modules manufactured worldwide in 2000. Since 1995 the use of cast mc-Si as a substrate has increased every year and that increase is expected to accelerate in the coming years as the PV market grows further. This impressive growth has been enabled by several factors—wafer suppliers, improvements in casting technology, sawing technology and cell process technology. In this paper the enabling factors will be discussed. The new processes used to enhance the efficiency of the cast multicrystalline silicon solar cells and the criteria for technology transfer will also be discussed.     

Spatial distribution of minority-carrier lifetime and local concentration of impurities in multicrystalline silicon solar cells

Spatial distribution of minority-carrier lifetime (τ) in multicrystalline silicon solar cells was investigated. By mapping of τ, a wide distribution in a higher-efficiency cell and a narrow distribution in a lower-efficiency cell was found, respectively. Based on the relation between τ and the density of grain boundaries, spatial distribution of impurities was characterized by secondary ion mass spectrometry. To clarify the spatial profile near grain boundaries, a method of changing detection area was carried out. Iron (Fe) segregation within 100 μm around a grain boundary was observed. In the lower-efficiency cell, a measurable amount of Fe was contained in a crystalline area as well.     

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.     

Comparison of multicrystalline silicon surfaces after wet chemical etching and hydrogen plasma treatment: application to heterojunction solar cells

The modifications of the surface and subsurface properties of p-type multicrystalline silicon (mc-Si) after wet chemical etching and hydrogen plasma treatment were investigated. A simple heterojunction (HJ) solar cell structure consisting of front grids/ITO/(n)a-Si:H/(p)mc-Si/Al was used for investigating the conversion efficiency. It is found that the optimized wet chemical etching and cleaning processes as a last technological step before the deposition of the a-Si:H emitter are more favorable to HJ solar cells fabrication than the hydrogenation. Solar cells on p-type mc-Si were prepared without high-efficiency features (point contacts, back surface field). They exhibited efficiencies up to 13% for a cell area of 1 cm² and 12% for a cell area of 39 cm².     

Large-area multicrystalline silicon solar cell fabrication using reactive ion etching (RIE)

Surface texturing of crystalline silicon wafer improves the conversion efficiency of solar cells by the enhancement in antireflection property and light trapping. Compared to antireflection coating, it is a more permanent and effective scheme. Wet texturing with the chemicals such as alkali (NaOH, KOH) or acid (HF, HNO₃, CH₃COOH) is too difficult for thinner wafer to apply due to a large amount of silicon loss. However, Plasma surface texturing using Reactive Ion Etching (RIE) can be effective in reducing the surface reflectance with low silicon loss. In this study, we have fabricated a large-area (156×156 mm) multicrystalline silicon (mc-Si) solar cell by mask less surface texturing using a SF₆/O₂ reactive ion etching. We have accomplished texturing with RIE by reducing silicon loss by almost half of that in wet texturing process. By optimizing the processing steps, we achieved conversion efficiency, open circuit voltage, short circuit current density, and fill factor as high as 16.1%, 619 mV, 33.5 mA/cm², and 77.7%, respectively. This study establishes that it is possible to fabricate the thin multicrystalline silicon solar cells of low cost and high efficiency using surface texturing by RIE.     

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.     

High-efficiency multicrystalline silicon solar cells by liquid phase-epitaxy

Thin-film silicon cells produced on crystalline silicon substrates have the potential to achieve high cell efficiencies at low cost. We have used a modified liquid-phase epitaxy growth process to produce very smooth, high-quality silicon films on multicrystalline silicon substrates. Photoconductivity decay measurements indicate that the minority carrier lifetimes in these layers are at least 10 μs, sufficient to achieve cell efficiencies in excess of 16%. This efficiency potential is confirmed in small area cells, which have displayed efficiencies up to 15.4%. Further improvements up to 17% efficiency are possible in the short term, even without the introduction of any light-trapping schemes into the device structure.     

Two-dimensional LBIC and Internal-Quantum-Efficiency investigations of grooved grain boundaries in multicrystalline silicon solar cells

Grain boundaries (GBs) in multicrystalline silicon (mc-Si) degrade the solar cells performances. To enhance the conversion efficiency of photovoltaic cells, preferential grooving was achieved in GBs in order to reduce their area and then their highly recombination effect. For this purpose, a chemical etching method in HF/HNO₃ solution was used. In this study, we show how grooved GB enables deep penetration of phosphorus and metallic contacts, which lead to a P-Al-co-gettering of undesired impurities. As a result, we observe a decrease in the recombination activity in regions close to the GBs as compared to ungrooved sample; this was investigated by the two-dimensional Light-Beam-Induced-Current (LBIC) imaging. In addition, we carried out a two dimensional representation of the Internal-Quantum-Efficiency (IQE), where images show an improvement of the electrical activity in grooved GBs.     

Progress in the metallisation of n-type multicrystalline silicon solar cells

A study on the optimisation of front contacts of n-type multicrystalline silicon solar cells is presented. In this study the same cell processing was applied to two types of wafers: electronic grade (EG-Si) and metallurgic grade (MG-Si) silicon. The contact firing temperature was optimised, by measuring the contact resistivity of the front and back contacts for different firing temperatures. The front contacts were improved by deposing silver using an electrochemical process. The solar cells were characterised before and after the silver deposition. For all cells processed the line resistance was reduced by over 90% after the silver deposition. After the contact improvement, EG-Si cells showed an absolute efficiency improvement of 2.6%, but MG-Si cells suffered a reduction on the cell efficiency, an effect related to parasitic shunting existent in these cells.     

A TEM study of SiC particles and filaments precipitated in multicrystalline Si for solar cells

The microstructure of SiC particles and SiC filament-type precipitates found in block-cast multicrystalline Si was studied in detail by transmission electron microscopy (TEM). TEM investigations showed that the SiC particles are single crystalline and the SiC filaments are microcrystalline. Both types of precipitates consist of cubic SiC. However, a high density of planar defects was found in the filaments. Very wavy and rough interface between SiC filaments and silicon (Si) was revealed by high-resolution TEM. In addition, SiC filaments do not show a special orientation relationship with respect to the Si matrix. The growth mechanisms of SiC precipitates are discussed. Finally, the influence of SiC inclusions in terms of device performance is considered.