A simple and efficient process for fabricating high efficiency polycrystalline silicon ribbon solar cells

The aim of this work is to clarify the potential of the low cost polycrystalline silicon String Ribbon for fabricating high efficiency solar cells with gettering and passivation techniques. The application of P and Al gettering as well as SiO₂ and H passivation schemes enhanced the material quality and boosted the efficiency of the solar cells. A cell efficiency above 15% has been achieved using a simple fabrication process.     

Application of phosphorus diffusion gettering process on upgraded metallurgical grade Si wafers and solar cells

Although phosphorus (P) diffusion gettering process has been wildly used to improve the performance of Si solar cells in photovoltaic technology, it is a new attempt to apply P diffusion gettering process to upgraded metallurgical grade silicon (UMG-Si) wafers with the purity of 99.999%. In this paper, improvements on the electrical properties of UMG-Si wafers and solar cells were investigated with the application of P diffusion gettering process. To enhance the improvements, the gettering parameters were optimized on the aspects of gettering temperature, gettering duration and POCl₃ flow rate, respectively. As we expected, the electrical properties of both multicrystalline Si (multi-Si) and monocrystalline Si (mono-Si) wafers were significantly improved. The average minority carrier lifetime increased from 0.35 μs to nearly about 2.7 μs for multi-Si wafers and from 4.21 μs to 5.75 μs for mono-Si wafers, respectively. Accordingly, the average conversion efficiency of the UMG-Si solar cells increased from 5.69% to 7.03% for multi-Si solar cells (without surface texturization) and from 13.55% to 14.55% for mono-Si solar cells, respectively. The impurity concentrations of as-grown and P-gettered UMG-Si wafers were determined quantitively so that the mechanism of P diffusion gettering process on UMG-Si wafers and solar cells could be further understood. The results show that application of P diffusion gettering process has a great potential to improve the electrical properties of UMG-Si wafers and thus the conversion efficiencies of UMG-Si solar cells.     

P/A1 co-gettering effectiveness in various polycrystalline silicon

The various polycrystalline silicon materials (cast ingots, ribbons) which are commercially available for the solar cells manufacturing differ very much among themselves due to the different growth processes. The resulting microstructure and impurity content will influence differently the material characteristics during thermal treatments inherent to the device manufacturing. As the gettering efficiency depends on the kind of polycrystalline material, the variations observed in the optimal gettering conditions or passivation will be discussed. In this paper, we compare the performances of various types of polycrystalline silicon upon classical and rapid thermal-process-induced co-diffusion of phosphorus and aluminium. We show that a large bulk minority carrier diffusion length enhancement occurs in the case of co-diffusion when compared to the separate diffusion of phosphorus and aluminium.     

Passivation and gettering of defective crystalline silicon solar cell

Different polycrystalline silicon and single-crystalline silicon with dislocations were used for passivation and gettering processes. These materials have defects and more impurity in the crystal. The dominant increase of electronic performance was found for wafers with more defects by using a different casting method. The wafers of single crystalline silicon with dislocations also have higher increase of efficiency of cell in comparison with that wafer without dislocations during oxide passivation processes used. POCl₃ was used for gettering processes. Single-crystal wafer with or without dislocations was used for comparison of gettering.     

Solar cells prepared on multicrystalline silicon subjected to new gettering and passivation treatments

New combined gettering and passivating procedures for solar cells prepared from multicrystalline silicon (mc-Si) have been considered. Passivation has been performed by (i) diamond-like carbon films deposition onto front or rear side of the wafers with following annealing, or (ii) hydrogen plasma treatments. Gettering region has been formed by deposition of Al film on specially prepared Si with developed surface. The advantages of such a gettering process in comparison with traditional gettering with Al are demonstrated. The improving influence of the treatments on diffusion length in mc-Si and efficiency of prepared solar cells have been found out. Physical mechanisms responsible for the observed effects of gettering and passivation are discussed.     

Large-size multi-crystalline silicon solar cells with honeycomb textured surface and point-contacted rear toward industrial production

In this paper, we present a multi-crystalline solar cell with hexagonally aligned hemispherical concaves, which is known as honeycomb textured structure, for an anti-reflecting structure. The emitter and the rear surface were passivated by silicon nitride, which is known as passivated emitter and rear (PERC) structure. The texture was fabricated by laser-patterning of silicon nitride film on a wafer and wet chemical etching of the wafer beneath the silicon nitride film through the patterned holes. This process succeeded in substituting the lithographic process usually used for fabricating honeycomb textured structure in small area. After the texturing process, solar cells were fabricated by utilizing conventional fabrication techniques, i.e. phosphorus diffusion in tube furnace, deposition of anti-reflection film and rear passivation film by chemical vapor deposition, front and rear electrodes formation by screen printing, and contact formation by furnace. By adding relatively small complicating process to conventional production process, conversion efficiency of 19.1% was achieved with mc-Si solar cells of over 200 cm² in size. The efficiency was independently confirmed by National Institute of Advanced Industrial Science and Technology (AIST).     

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.     

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.     

High-density inductively coupled plasma chemical vapor deposition of silicon nitride for solar cell application

The silicon nitride films were deposited by means of high-density inductively coupled plasma chemical vapor deposition in a planar coil reactor. The process gases used were pure nitrogen and a mixture of silane and helium. Passivated by silicon nitride, solar cells show efficiency above 13%. Strong H-atom release from the growing SiN film and Si–N bond healing are responsible for the improved electrical and passivation properties of SiN film. This paper presents the optimal refractive index of SiN for single layer antireflection coating as well as double layer antireflection coating in solar cell applications.     

Apparent “gettering” of the Staebler-Wronski effect in amorphous silicon solar cells

The stability behaviour of intrinsic amorphous silicon materials incorporated in a p⁺-i-n⁺ solar cell structure is considerably different from that observed by electrical characterization methods in individual thin films. This is due to the fundamental difference in Fermi-level position in a single layer compared to the situation occuring in devices. We have employed the differences in the re-equilibration behaviour that have been observed in various intrinsic materials when the Fermi-level is shifted towards the valence band edge, in order to design a cell with a new profiled i-layer which would possess an improved electric field distribution after light soaking compared to cells with a constant i-layer. The contribution of the interface region to the stabilized conversion efficiency is greatly improved, whereas the first 50 nm of the cell structure remain unchanged. Thus, it appears that the Staebler-Wronski effect is gettered away from the junction, much like the impurity gettering concept in crystalline solar cells.     

Study on diffusion barrier layer of silicon-based thin-film solar cells on polyimide substrate

ZnO and Ni films were used as the diffusion barrier layer between Al and n-type μc-Si:H for the hydrogenated amorphous silicon (a-Si:H) solar cells on polyimide (PI) substrate. The electrical, optical and uniformity properties of ZnO or Ni film influence strongly the performance and uniformity of solar cells. The uniformity of the solar cells with ZnO diffusion barrier layer degraded with the increasing thickness of ZnO film. The uniformity of solar cells with Ni diffusion barrier layer was more than 90%, which was generally better than those with ZnO film. A power-to-weight ratio of 200 W/kg was obtained for a-Si:H thin-film solar cell on PI substrate with a size of 14.8 cm².     

多晶硅电池片双面扩散的仿真研究

结合PC1D模拟软件,对减薄至200μm的多晶硅电池片进行双面扩散与背靠背扩散的对比研究。试验表明:双面扩散工艺和背靠背扩散工艺均具有良好的吸杂效果,少子寿命有很大提高,少子扩散长度已大于电池片厚度。双面扩散比背靠背扩散具有更好的吸杂和钝化效果,少子寿命更长。但PC1D软件仿真及试验结果显示,扩散后电池片的转换效率、短路电流、开路电压等电学性能没有显著改善。     

Extended high temperature Al gettering for improvement and homogenization of minority carrier diffusion lengths in multicrystalline Si

Multicrystalline Si for photovoltaic applications is a very inhomogeneous material with localized regions of high dislocation density and large impurity and precipitate concentrations which limit solar cell efficiency by acting as carrier recombination sites. Due to slow dissolution of precipitates in multicrystalline Si, these regions cannot be improved by conventional P and Al gettering treatments for removal of metal impurities which give good results for single crystal Si. It is shown that an extended high temperature Al gettering treatment can improve minority carrier diffusion lengths in these low quality regions and homogenize the electrical properties of multicrystalline Si wafers.     

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.     

Laser enhanced gettering of silicon substrates

One challenge to the use of lightly-doped, high efficiency emitters on multicrystalline silicon wafers is the poor gettering efficiency of the diffusion processes used to fabricate them. With the photovoltaic industry highly reliant on heavily doped phosphorus diffusions as a source of gettering, the transition to selective emitter structures would require new alternative methods of impurity extraction. In this paper, a novel laser based method for gettering is investigated for its impact on commercially available silicon wafers used in the manufacturing of solar cells. Direct comparisons between laser enhanced gettering (LasEG) and lightly-doped emitter diffusion gettering demonstrate a 45% absolute improvement in bulk minority carrier lifetime when using the laser process. Although grain boundaries can be effective gettering sites in multicrystalline wafers, laser processing can substantially improve the performance of both grain boundary sites and intra-grain regions. This improvement is correlated with a factor of 6 further decrease in interstitial iron concentrations. The removal of such impurities from multicrystalline wafers using the laser process can result in intra-grain enhancements in implied open-circuit voltage of up to 40 mV. In instances where specific dopant profiles are required for a diffusion on one surface of a solar cell, and the diffusion process does not enable effective gettering, LasEG may enable improved gettering during the diffusion process.     

Passivation and etching of fine-grained polycrystalline silicon films by hydrogen treatment

Here we investigated the effects of hydrogen treatment on highly defected polycrystalline silicon solar cells in terms of defects passivation and surface etching. The poly-Si films were formed by high-temperature chemical vapour deposition. The hydrogen treatment was carried out through deposition of a-SiN_x:H layer followed by a thermal treatment or by direct hydrogen plasma. The deposition of silicon nitride layers on polysilicon cells led to a slight increase in the open-circuit voltage without damage to the surface. In contrast, after plasma hydrogenation, the results revealed an etching process of the emitter simultaneously with an important increase of the measured open-circuit voltage by a factor 2, reaching 420 mV.     

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.     

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.     

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.     

Latest results on semitransparent POWER silicon solar cells

The paper presents the latest results of the polycrystalline wafer engineering result (POWER) silicon solar cell research (G. Willeke, P. Fath, The POWER silicon solar cell, Proceedings of the 12th EPVSEC, Amsterdam, 1994, pp. 766–768). Mono – as well as bifacially active semitransparent silicon solar cells have been created by forming perpendicularly overlapping grooves on the front and the rear side of a silicon wafer resulting in a regular pattern of holes. The developed very simple manufacturing process is fully compatible with an industrial production and uses POCl₃-tube diffusion, PECVD silicon nitride as single ARC and screen-printing metallization. Maximum efficiencies of η=11.2% for monofacial POWER cells on 0.4 Ω cm Cz material with a transparency of 18.2% and η=12.9% for bifacial cells on 1 Ω cm Cz material with a transparency of 16% have been obtained. Results for multicrystalline (mc) semitransparent mono- and bifacially active silicon solar cells are also presented.