Improved phosphorous gettering of multicrystalline silicon

The gettering during an emitter diffusion in multicrystalline silicon is improved by adding a low-temperature tail to the standard diffusion. The tail keeps the emitter sheet resistance within the usable range for solar cells. An increase in minority carrier lifetime by a factor ten is obtained. Decrease in recombination activity of grain boundaries and decrease in interstitial iron concentration are mainly responsible for this improved lifetime. The proposed mechanisms for this improvement are: reduction of size of precipitates because of the longer duration, possibly assisted by beneficial changes in thermodynamics and kinetics of the gettering because of the low temperature.     

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.     

烧结曲线对多晶PERC太阳电池光致衰减效应的影响

针对多晶PERC太阳电池其较大的光衰效应会影响功率输出的问题,研究烧结曲线对多晶PERC太阳电池光致衰减效应的影响。在常规烧结曲线1的基础上通过改变烧结曲线峰值温度位置得到优化烧结曲线2和优化烧结曲线3,然后将双面沉积Al₂O₃/SiNx叠层钝化膜的寿命片和丝网印刷后的多晶PERC电池分别采用不同烧结曲线热处理,最后将样品在70℃、800 W/m²环境下进行45 h光衰处理。结果发现经过烧结曲线1～曲线3处理后的寿命片少子寿命衰减率分别为63%、42%和23%,多晶PERC太阳电池转换效率的衰减率分别为6.46%、3.55%和2.30%,光衰处理后的多晶PERC电池的EL测试结果显示烧结曲线1亮度最小,曲线2次之,曲线3最大。以上结果表明,仅通过烧结炉的烧结曲线优化就可以很明显地减小多晶PERC太阳电池的光致衰减幅度,可为探究抑制多晶PERC太阳电池光致衰减效应的方法提供一种全新的思路。     

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.     

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.     

Characteristics of structural defects in the 240 kg silicon ingot grown by directional solidification process

Multi-crystalline silicon for solar cell over single crystalline silicon is its capability of using cheaper raw material. Since cheaper material contains harmful metal impurities, gettering technology has to be applied to silicon wafers to reduce the metal content in the crystal. Low dislocation density in the 240 kg multi-crystalline silicon crystal provides the strong possibility of gettering for the low cost silicon solar cell. Saw damage induced during the slicing process of multi-crystalline silicon ingot was confirmed to generate dislocation loops which can be employed for extrinsic gettering.     

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.     

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.     

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。     

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.     

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.     

Minority carrier lifetime imaging of silicon wafers calibrated by quasi-steady-state photoluminescence

Reliable process control or predictions of solar cell efficiencies from minority carrier lifetimes on silicon wafers require precise lifetime measurements. For inhomogeneous material quality this implies the necessity of adequately averaged spatially resolved lifetime measurements. Materials such as, e.g. multicrystalline upgraded metallurgical grade silicon frequently feature relatively low lifetimes, high trap densities, and several material parameters (charge carrier mobilities and net dopant concentration) that are not straightforwardly predictable or measurable. As this may substantially compromise conventional lifetime measurements, we present a solely luminescence based lifetime imaging technique, which requires virtually no a priori information about material parameters. Our approach is based on a calibration of a wafer's photoluminescence image through a precise lifetime determination of a part of this wafer via quasi-steady-state photoluminescence. Carrier mobilities, net dopant concentration, and surface morphology leave the determination of lifetime virtually unaffected, the injection dependence of lifetime is properly taken into account, and lifetimes down to the timescale of a microsecond can be reliably measured.     

Trends in industrial silicon solar cell processes

The performance and technology of industrial silicon solar cells have improved considerably in recent years. Conversion efficiencies exceeding 18% are reproducibly obtained by cost-effective technologies on large area Cz-silicon. The performance of multicrystalline silicon cells is closing-in at 17.2%. Improved material casting techniques, a refined technology, and efficient in-process material improvement techniques are found to be the major causes behind such advancement. The trend to towards thinner substrates leads to considerable material cost reduction while yielding better performance. The major processing technologies and steps are critically discussed in this article, keeping in mind the priorities of today's PV industry: cost, and environmental issues. The future trends of the technology are outlined.     

A new energy efficient, environment friendly and high productive texturization process of industrial multicrystalline silicon solar cells

A new texturization process based on a uniform, isotropic and slow removal of silicon, using a composition of sodium hydroxide (NaOH) and sodium hypochlorite (NaOCl) solution at an elevated temperature is developed recently for multicrystalline silicon solar cells. This process is applied in optimized condition in regular industrial production line and it immediately replaces the old popular industrial process of texturization using a combination of NaOH solution, alcoholic NaOH solution and hydrochloric acid solution in different steps at a higher temperature. Also the gain in solar cell efficiency at global AM1.5 spectrum, 1 SUN intensity condition is nearly 10% in final value. In addition, it has become finally an energy efficient and environment friendly texturization process for large area multicrystalline silicon solar cells for commercial use. In this paper the cost effectiveness and environment friendly aspects of the proposed process have been studied in detail along with the surface texture analysis of wafers with SEM and AFM micrographs to substantiate the reasons behind the above facts.     

Overview of solar cell technologies and results on high efficiency multicrystalline silicon substrates

Fabrication technologies for multicrystalline silicon (mc-Si) solar cells have advanced in recent years with efficiencies of mc-Si cells exceeding 18%. Intense efforts have been made at laboratory level to improve process technology, growth methods, and material improvement techniques to deliver better devices at lower cost. Deeper understanding of the physics and optics of the device led to improved device design. This provided a fruitful feedback to the industrial sector. Both screenprinting and buried-contact technologies yield cells of high performance. An increasingly large amount of research activity is also focussed on the fabrication of thin solar cells on cheap substrates such as glass, ceramic, or low quality silicon. Success of these efforts is expected to lead to high efficiency devices at much lower costs. Efforts are also being put on low thermal budget processing of solar cells based on rapid thermal annealing.     

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.     

Electrical properties of multicrystalline silicon produced by electromagnetic casting process: Degradation and improvement

The electrical properties of boron-doped multicrystalline silicon for photovoltaic applications, elaborated by the cold crucible pulling process, are studied by the photoconductivity decay method and the electron beam-induced current measurement technique. The bulk lifetime mapping of the minority carriers in the as-grown silicon wafers is drawn up using both the techniques. Moreover, the consequence of phosphorus doping on the recombination properties of extended defects are studied using the EBIC measurements. Two different treatments are investigated in order to improve the electrical properties of the as-grown silicon wafers: (a) thermal phosphorus diffusion, for which the gettering efficiency is determined by the different treatment parameters; (b) remote plasma hydrogen passivation which leads to increase of the minority carrier lifetime.     

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.     

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.     

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.