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.     

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.     

Recovery of minority carrier lifetime in low-cost multicrystalline silicon

Lifetime of minority carriers has been widely identified to be the key material parameter determining the conversion efficiency of pn-junction silicon solar cells. Impurities and defects in the silicon crystal lattice reduce the charge carrier lifetime and thus limit the performance of the solar cells. Removal of impurities by silicon material purification is often contradictory with low cost production of photovoltaic devices. In this paper, we present experimental results of an efficient gettering technique which can be applied to low cost processing of multicrystalline silicon solar cells without any additional process steps or compromises with optimal device design parameters. This technique is based on well-known phosphorous gettering. We have discovered that if the silicon wafers are kept in the furnace after the emitter diffusion at the 700°C, significant improvement in the lifetime will take place. At this temperature the properties of the pn-junction remain unaffected meanwhile many lifetime killers are still mobile. The time needed for this temperature program can be easily modified in order to respond to the material quality variations in substrates originating from different parts of multicrystalline ingot. Better control of lifetime can lead to higher degree of starting material utilization.     

A study of electrical uniformity for monolithic polycrystalline silicon solar cells

The aim of this work is to investigate the electrical uniformity of monolithic polycrystalline silicon solar cells prepared by various process techniques. By a series of experiments such as P and Al impurity gettering and silicon nitride passivation, a new conclusion is that the application of P and Al gettering as well as silicon nitride passivation enhances the electrical uniformity of small area solar cells diced from the same polycrystalline silicon solar cells, even if impurity gettering is not effective when the dislocation density is above a threshold value of about 10⁶ cm⁻². The experiments give us some hints that when we cut large area polycrystalline silicon solar cells into small pieces needed for application, we should modify production process slightly.     

Impact of thermal processes on multi-crystalline silicon

Fabrication of modern multi-crystalline silicon solar cells involves multiple processes that are thermally intensive. These include emitter diffusion, thermal oxidation and firing of the metal contacts. This paper illustrates the variation and potential effects upon recombination in the wafers due to these thermal processes. The use of light emitter diffusions more compatible with selective emitter designs had a more detrimental effect on the bulk lifetime of the silicon than that of heavier diffusions compatible with a homogenous emitter design and screen-printed contacts. This was primarily due to a reduced effectiveness of gettering for the light emitter. This reduction in lifetime could be mitigated through the use of a dedicated gettering process applied before emitter diffusion. Thermal oxidations could greatly improve surface passivation in the intragrain regions, with the higher temperatures yielding the highest quality surface passivation. However, the higher temperatures also led to an increase in bulk recombination either due to a reduced effectiveness of gettering, or due to the presence of a thicker oxide layer, which may interrupt hydrogen passivation. The effects of fast firing were separated into thermal effects and hydrogenation effects. While hydrogen can passivate defects hence improving the performance, thermal effects during fast firing can dissolve precipitating impurities such as iron or de-getter impurities hence lower the performance, leading to a poisoning of the intra-grain regions.     

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.     

Enhancing solar cell efficiency by using spectral converters

Planar converters containing quantum dots as wavelength-shifting moieties on top of a multi-crystalline silicon and an amorphous silicon solar cell were studied. The highly efficient quantum dots are to shift the wavelengths where the spectral response of the solar cell is low to wavelengths where the spectral response is high, in order to improve the conversion efficiency of the solar cell. It was calculated that quantum dots with an emission at 603 nm increase the multi-crystalline solar cell short-circuit current by nearly 10%. Simulation results for planar converters on hydrogenated amorphous silicon solar cells show no beneficial effects, due to the high spectral response at low wavelength.     

Classification of shunting mechanisms in crystalline silicon solar cells

The efficiency of a solar cell is given by its average electrical parameters. On inhomogeneous materials and especially on large-area solar cells the inhomogeneity of the short circuit current, the open circuit voltage and the fill factor are important factors to reach high and stable efficiencies and may limit the overall performance of the device.A locally increased dark forward current (shunt) reduces the fill factor and the open circuit voltage of the whole cell. The inhomogeneity of the forward current in a solar cell can be measured using lock-in thermography. The quantitative and voltage-dependent evaluation of these thermographic investigations of various solar cell types on mono- or multi-crystalline silicon enables the classification of the different shunting mechanisms found. By further microscopic investigations the physical reasons for the increased dark forward currents can be determined.It turns out that a high density of crystallographic defects like dislocation tangles or microdefects can be responsible for an increased dark forward current. Unexpectedly, grain boundaries in solar cells on multicrystalline silicon do not show any measurable influence on the local dark forward current. In most cases shunts caused by process-induced defects are dominating the current–voltage characteristic at the maximum power point of the solar cell. In commercial solar cells shunts at the edges are most important, followed by shunts beyond the grid lines.     

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.     

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.     

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.     

Formation of a low reflective surface on crystalline silicon solar cells by chemical treatment using Ag electrodes as the catalyst

A new method was developed for making a porous silicon layer as an anti-reflective coating on the top of crystalline silicon solar cells. The porous silicon layer was formed in a mixed solution of H₂O₂ and HF by using screen-printed Ag front electrodes as the catalyst. With the help of the catalytic effect, porous silicon layers were formed by treatment in a solution chemically milder than conventional solutions. The multi-crystalline silicon solar cell covered with the porous silicon layer showed a surface reflectance below 15% in a wavelength region of 400–800 nm.     

Mono- and tri-crystalline Si for PV application

Crystalline silicon wafers are by far the dominant absorber materials for today's production of solar cells and modules due to their good price/performance relation and their proven environmental stability. These wafers are mainly produced either by a solar-optimized Czochralski (Cz)-growth method yielding crystalline silicon with low defect density (c-Si) or by a directional solidification or a ribbon growth method yielding large grained multi-crystalline (mc-Si) wafers with higher defect density. To further improve the price/performance relation of Cz solar cells, tri-crystalline silicon (tri-Si) is being developed as a high-quality wafer material that combines both the high diffusion length of minority carriers of up to 1300 μm of c-Si and the productivity of mc-Si. More than 1000 μm LID free diffusion length could be reached with specially doped tri-crystals. Due to an increased mechanical stability tri-Si allows both quasi-continuous pulling and thin slicing with higher mechanical yields. This paper reviews the structural, electronic, and mechanical properties of tri-crystalline silicon wafers with respect to c-Si wafers for solar applications. Actual non-textured solar cells processed with a simple cost effective fabrication process exhibit the same cell efficiencies up to 15.9% for both tri-silicon and mono-silicon wafers. With an improved process, up to 18% cell efficiency can be obtained with textured mono-Si.     

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.     

A practical field study of various solar cells on their performance in Malaysia

A practical field study has been carried out with the intention to analyze and compare the performance of various types of commercially available solar panels under Malaysia's weather. Four different types of solar panels, such as mono-crystalline silicon, multi-crystalline silicon, amorphous silicon and copper–indium–diselenide (CIS) solar panels are used for the practical field study. A number of performance related parameters have been collected using data logger over a period of three consecutive days in the hope that this would give some initial information on the real performance of different solar panels. Results show that mono-crystalline silicon and multi-crystalline silicon solar module perform better when they are under hot sun, whereas the CIS and triple junction amorphous silicon solar panel perform better when it is cloudy and has diffused sunshine. Furthermore, the efficiency of crystalline silicon solar panel has been found to drop when the temperature rises higher. This phenomenon does not appear in the CIS and amorphous silicon solar panels, which shows that the performance of CIS and amorphous silicon solar cells are better in terms of power conversion efficiency and overall performance ratio. Better performance of thin film solar cells like amorphous silicon and CIS are observed from the initial results, which draws attention over the selection of solar panels and also may encourage the usage of these in tropical weather like Malaysia.     

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%.     

Multi-wavelength all optical measurement for the characterization of recombination process in thin mc-Si samples

A contactless, all-optical and non-destructive technique for simultaneous measurement of minority carrier recombination lifetime and surface recombination velocity, at low injection level, in multi-crystalline silicon samples is presented. Being contactless and non-destructive with respect to the surface to be analyzed, the method does not need any surface treatment to be applied and therefore is suitable for routine lifetime characterization in solar cell fabrication processes.     

Implications for current regulatory waste toxicity characterisation methods from analysing metal and metalloid leaching from photovoltaic modules

The appropriateness of regulatory methods to characterise the toxicity of photovoltaic (PV) modules was investigated to quantify potential environmental impacts for modules disposed of in landfills. Because solar energy is perceived as a green technology, it is important to ensure that end-of-life issues will not be detrimental to solar energy's success. United States Environmental Protection Agency Method 1311, California waste extraction test, and modified versions of both were performed on a multi-crystalline silicon module and cells and a copper indium gallium diselenide (CIGS) module. Variations in metal leachate concentrations were found with changes in testing parameters. Lead concentrations from the multi-crystalline module ranged from 16.2 to 50.2?mg/L. Cadmium concentrations from the CIGS module ranged from 0.1 to 3.52?mg/L. This raises doubt that regulatory methods can adequately characterise PV modules. The results are useful for developing end-of-life procedures, which is a positive step towards avoiding an e-waste problem and continuing trends of increasing installation and cost reduction in the PV market.     

绒面类型对斜入射光下太阳电池发电能力影响的分析

太阳电池在发电运行时大部分时间处于不同斜入射辐照条件,然而太阳电池及其组件的输出功率参数都是在垂直入射辐照下测量并成为衡量其发电能力的标准。对不同类型绒面的太阳电池,用这样的标准来衡量比较其实际发电运行输出可能会产生出入。通过对金字塔型绒面的单晶硅太阳电池与球窝型绒面的多晶硅太阳电池在斜入射光照时的光反射情况和两种类型组件的实际发电情况进行理论分析和实验测量,得到以下结论：按照现行标准测量结果标称输出性能,多晶硅太阳电池的实际运行发电能力相对于单晶硅太阳电池而言被略为低估了,但低估程度小于3%。一般而言,各种减反射手段所优化的实际是垂直入射辐照条件下的发电输出结果,其实际运行发电效果增益并不如标准测量结果所显示的那么大。     

单晶硅埚底料中石英的分离研究

通过对单晶硅埚底料进行重熔试验,研究重熔后埚底料中石英的分离情况,并分析重熔所得到的硅片中的氧、碳、金属杂质含量,以期探讨埚底料在太阳电池多晶硅硅锭生产中用作原料的可行性.试验发现通过重熔可以将单晶硅埚底料中的石英与硅料分离,重熔后得到的硅片中氧含量比较高,而金属杂质含量基本可达到太阳电池多晶硅锭的生产要求.