Texturisation of multicrystalline silicon solar cells by RIE and plasma etching

Texturing by reactive ion etching (RIE) is demonstrated as an attractive technical solution for lowering of reflectance of multicrystalline silicon solar cells. A suitable sequence of processes is developed to combine the advantage of reactive ion etching with “natural lithography” based on colloidal masks. The RIE single-wafer texturisation is driven to an industrial applicable batch process by plasma etching with a gain in efficiency of 0.3% absolute.     

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.     

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.     

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.     

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.     

Novel process of grain boundary metallisation on mc Si solar cells

The electronic properties of multicrystalline silicon are heavily influenced by impurities concentrated along the grain boundaries that increase the recombination activity near the crystallite borders. Dopants can also diffuse preferentially down the grain boundaries, which leads to a low resistance path down the grain. These and other effects decrease the efficiency of multicrystalline silicon solar cells. Additionally, the efficiency is lowered by the shading of areas of silicon by metallisation lines due to the reduction of the active conversion area of the cell. We present a new way to combine the grain boundaries and the front contact grid with the aim to improve the efficiency of multicrystalline silicon solar cells. A first approach has been developed to produce multicrystalline silicon solar cells with a front contact metallisation following the grain boundaries: The different grain boundaries of a multicrystalline silicon wafer are detected by optical scanning of the wafer surface. Together with the emitter sheet resistivity this image serves as an input to calculate a net of finger lines that follow the grain boundaries wherever possible. Onto these detected grain boundaries the metallisation is performed by evaporative deposition of copper and photolithography. We report on the successful implementation of such a grid on 100×100 mm² wafers.     

Surface and bulk-passivated large area multicrystalline silicon solar cells

We have investigated the surface and bulk passivation technique on large-area multicrystalline silicon solar cells, a large open-circuit voltage has been obtained for cells oxidized to passivate the surface and hydrogen annealed after deposition of silicon nitride film on both surfaces by plasma CVD method (P---SiN) to passivate the bulk. The texture surface like pyramid structure on multicrystalline silicon surface has been obtained uniformly using reactive ion etching (RIE) method. Combining these RIE method and passivation schemes, the conversion efficiency of 17.1% is obtained on 15 cm × 15 cm multicrystalline silicon solar cell. Phosphorus diffusion, BSF formation, passivation technique and contact metallization for low-cost process sequence are also described in this paper.     

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.     

Antireflective porous layer formation on multicrystalline silicon by metal particle enhanced HF etching

Antireflection of silicon (Si) surface is one key technology for the manufacture of efficient solar cells. Metal particle enhanced HF etching is applied to produce uniform antireflecting porous layer on multicrystalline Si wafers that cannot be uniformly texturized by anisotropic etching with an alkaline solution. Fine platinum (Pt) particles are deposited on multicrystalline n-Si wafers by electroless displacement reaction in a hexachloroplatinic acid solution containing HF. Both macroporous and luminescent microporous layers are uniformly formed by immersing the Pt-particle-deposited multicrystalline Si wafers in a HF solution. The reflectance of the wafers is reduced from 30% to 6% by the formation of porous layer. The photocurrent density of photoelectrochemical solar cells using porous multicrystalline n-Si has a 25% higher value than non-porous Si cells.     

Minority carrier lifetime in plasma-textured silicon wafers for solar cells

In this work a comparison between plasma-induced defects by two different SF₆ texturing techniques, reactive ion etching (RIE) and high-density plasma (HDP) is presented. It is found that without any defect-removal etching (DRE), the minority carrier lifetime is the highest for the HDP technique. After DRE, the minority carrier lifetime rises as high as 750 μs for both RIE- and HDP-textured wafers at an excess carrier density of 1×10¹⁵ cm⁻³. The measured lifetimes correspond to an implied one-sun open-circuit voltage of around 680 mV compared to about 640 mV before DRE for the HDP-textured wafers. FZ silicon 1 0 0 wafers were used in this study. We also noted that in the RIE process, the induced defect density was significantly lower for wafers etched at 300 K than those etched at 173 K.     

Porous silicon Bragg mirrors on single- and multi-crystalline silicon for solar cells

Porous silicon Bragg mirrors at back-side of single crystalline and multicrystalline silicon solar cell were numerically simulated by transfer matrix method. It allows to choose the optimal parameters of porous stack of bi-layers (indexes of refraction, number of bi-layers) when the increase of photon absorption in 900–1050 nm spectral region is achieved. Application of Bragg mirrors at back-side of single crystalline solar cell can improve the efficiency on more than 0.8% in absolute for 200 μm both-side textured thickness wafer. The simulated results were compared with characteristics of Bragg mirrors fabricated by electrochemical etching of single- and multi-crystalline silicon. It is shown that despite the natural crystallites disorientation the efficient Bragg mirrors can be fabricated on multicrystalline silicon wafers in such way. Maximum measured reflectivity for Bragg mirrors on multicrystalline substrate achieves approximately 62%, whereas for single crystalline silicon the reflectivity in maximum is more than 90%.     

SOLAR CELLS WITH 15.6% EFFICIENCY ON MULTICRYSTALLINE SILICON,USING IMPURITY GETTERING,BACK SURFACE FIELD AND EMITTER PASSIVATION

Three features have been combined to raise the efficiency of solar cells made on industrial multicrystalline silicon wafers: 1) reduction of bulk recombination by a special gettering process, 2) reduction of back recombination by using a p/p ⁺ junction, 3) reduction of front recombination by emitter back-etching and passivation.

A conversion efficiency of 15.6% has been achieved on 2 × 2 cm² solar cells. Spectral response measurements are used to identify the role of each processing parameter.     

Multicrystalline silicon wafers prepared from upgraded metallurgical feedstock

A solution to the problem of the shortage of silicon feedstock used to grow multicrystalline ingots can be the production of a feedstock obtained by the direct purification of upgraded metallurgical silicon by means of a plasma torch. It is found that the dopant concentrations in the material manufactured following this metallurgical route are in the 10¹⁷ cm⁻³ range. Minority carrier diffusion lengths L_n are close to 35 μm in the raw wafers and increases up to 120 μm after the wafers go through the standard processing steps needed to make solar cells: phosphorus diffusion, aluminium–silicon alloying and hydrogenation by deposition of a hydrogen-rich silicon nitride layer followed by an annealing. L_n values are limited by the presence of residual metallic impurities, mainly slow diffusers like aluminium, and also by the high doping level.     

Dependence of the recombination in thin-film Si solar cells grown by ion-assisted deposition on the crystallographic orientation of the substrate

One promising strategy for achieving high-quality polycrystalline silicon thin-film solar cells on glass is based on low-temperature ion-assisted deposition for epitaxial thickening of a thin, large-grained seeding layer on glass. The crystal growth on the seeding layer is influenced by various factors, amongst which the crystal orientation of the grains plays a substantial role. In this paper we investigate how the electronic properties of solar cells grown on “ideal” seeding layers (Si wafers) are influenced by the crystallographic orientation of the substrate. The Si wafers are heavily doped p-type, ensuring that their contribution to the photogenerated current is small. The films grown on (1 0 0)-oriented Si substrates have a very low density of structural defects, while the films grown on (1 1 1)-oriented Si substrates display a high density of twin defects. The electronic properties of the thin-film solar cells were investigated by means of open-circuit voltage measurements as a function of the incident light intensity. The (1 0 0)-oriented diodes consistently exhibit a higher V_oc than the (1 1 1)-oriented diodes throughout the entire illumination range from 10⁻³ to 10³ Suns. We determine 7 μm as the bulk minority carrier diffusion length of the as-grown (1 0 0)-oriented Si film. A lower bound of 3 μm was found for the bulk minority carrier diffusion length in the as-grown (1 1 1)-oriented Si film. The performances of both types of solar cells were improved by hydrogenation in an ammonia plasma. At voltages around the 1-Sun maximum power point the improvement is due to a reduction of non-ideal current mechanisms. The diffusion length of the (1 0 0) diode remains unaffected by hydrogenation while the lower bound of the diffusion length of the (1 1 1) diode improves to 10 μm.     

Gettering of impurities in solar silicon

Electrical properties of crystalline silicon wafers used for photovoltaïcs are degraded by metallic impurity atoms. Such atoms are introduced during the crystal growth or during the processing steps needed to prepare solar cells. External gettering treatments such as phosphorus diffusion from a POCl₃ source or Al–Si alloying are needed to restore or to improve the bulk electrical properties of the material. Monocrystalline wafers can be easily ugraded by such treatments. In multicrystalline silicon wafers, external gettering by phosphorus diffusion, as well as by Al–Si alloying are efficient, provided the temperature does not exceed 900°C. Longer treatments (2–4 h) are needed in order to increase the minority carrier diffusion length beyond the wafer thickness. So longer times are necessary to dissolve metallic atom containing precipitates. However, if the major part of the wafer is neatly improved, some regions containing dislocation tangles are poorly modified. In such regions, impurities could be involved in the formation of silicates which cannot be dissolved during the gettering treatment. Nevertheless, external gettering treatments are able to clean efficiently single crystalline and multicrystalline silicon wafers, provided the oxygen concentration and the defect density are not too high and are homogeneously distributed.     

Reactive ion etching (RIE) technique for application in crystalline silicon solar cells

Saw damage removal (SDR) and texturing by conventional wet chemical processes with alkali solution etch about 20 micron of silicon wafer on both sides, resulting in thin wafers with which solar cell processing is difficult. Reactive ion etching (RIE) for silicon surface texturing is very effective in reducing surface reflectance of thin crystalline silicon wafers by trapping the light of longer wavelength. High efficiency solar cells were fabricated during this study using optimized RIE. Saw damage removal (SDR) with acidic mixture followed by RIE-texturing showed the decrease in silicon loss by ∼67% and ∼70% compared to conventional SDR and texturing by alkaline solution. Also, the crystalline silicon solar cells fabricated by using RIE-texturing showed conversion efficiency as high as 16.7% and 16.1% compared with 16.2%, which was obtained in the case of the cell fabricated with SDR and texturing with NaOH solution.     

Automatic saw-mark detection in multicrystalline solar wafer images

This paper presents a method of automatic defect inspection for the photovoltaic industry, with a special focus on multicrystalline solar wafers. It presents a machine vision-based scheme to automatically detect saw-mark defects in solar wafer surfaces. A saw-mark defect is a severe flaw that occurs when a silicon ingot is cut into wafers. Early detection of saw-mark defects in the wafer cutting process can reduce material waste and improve production yields. A multicrystalline solar wafer surface presents random shapes, sizes, and orientations of crystal grains in the surface, making the automatic detection of saw-mark defects extremely difficult. The proposed saw-mark detection scheme involves two main procedures: (1) Fourier image reconstruction to remove the multi-grain background of a solar wafer image and (2) a line detection process in the reconstructed image to locate saw-marks. The Fourier transform (FT) is used to eliminate crystal grain patterns and results in a non-textured surface in the reconstructed image. Since a saw-mark is presented horizontally in the sliced wafer, vertical scan lines in the reconstructed image are individually evaluated by a line detection process. A pixel far away from the line sought can then be effectively identified as a defect point. Experimental results show that the proposed method can effectively detect various saw-mark defects, specifically black lines, white lines, and impurities in multicrystalline solar wafers.     

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.     

n–p Junction formation in p-type silicon by hydrogen ion implantation

Hydrogen ion implantations at an energy of 250 keV and a dose of 3×10¹⁶ cm⁻² were applied to float zone, Czochralski grown silicon wafers and to multicrystalline samples. It was found that after annealing at 350°C<T<550°C for 1 h a n–p junction is formed and a photovoltaic behaviour is observed. Spectral responses show that the photocurrent in the near infrared part of the spectrum is comparable to that given by a standard silicon solar cell. The depth of the junction is about 2 μm and C–V measurements show that the junction is graduated. Hydrogen plasma immersion leads to similar results. The conversion of p- to n-type silicon is explained by the formation of shallow donor levels associated to a high concentration of hydrogen.     

Defect recognition and impurity detection techniques in crystalline silicon for solar cells

The efficiency of multicrystalline solar cells is limited by defects and impurities, which include grain boundaries, dislocations, and transition metals. The density of these defects often varies from grain to grain. “Bad grains” with low minority carrier diffusion length generate low open circuit voltage and shunt the “good grains” with high minority carrier diffusion length, thus reducing the overall cell efficiency. It was found that it is more likely to find transition metal clusters in “bad grains” than in “good grains”, and that gettering is not efficient in improving the areas of low diffusion length. The primary objective of materials research in photovoltaics is identification of these lifetime-limiting defects. In this article we summarize the current state of understanding of lifetime-limiting defects in solar cells, summarize the advantages and limitations of traditional analytical tools and discuss novel emerging techniques, including X-ray fluorescence microprobe, X-ray absorption spectromicroscopy, and X-ray beam-induced current