Relationship between trapping density and open circuit voltage in multicrystalline silicon solar cells

Minority carrier trapping frequently exists in solar grade multicrystalline silicon. At low illumination levels, the effect of trapping centers on open circuit voltage of multicrystalline silicon solar cells is dependent on the trap density and illumination level. In this paper, the relation between trapping density and open circuit voltage of multicrystalline silicon solar cells at different illumination levels is studied by a series of experiments. The experimental evidence suggests that the effect of trapping on open circuit voltage of multicrystalline silicon solar cells is obvious at carrier injection levels equal to and below the trap density, the trapping effect of multicrystalline silicon can be reflected by measuring open circuit voltage at low illumination levels, instead of complicated lifetime measurements, and some multicrystalline silicon solar cells with higher trap densities have higher open-circuit voltages at weak illumination levels. The measurement and analysis of the trapping effect is a relative tool to diagnose the quality of multicrystalline silicon, so a new method is presented to analyze relative quality of multicrystalline silicon by measuring open circuit voltage at weak illumination levels.     

Texturization of multicrystalline silicon by wet chemical etching for silicon solar cells

Two kinds of surface texturization of mc-Si obtained by wet chemical etching are investigated in view of implementation in the solar cell processing. The first one was the acid texturization of saw damage on the surface of multicrystalline silicon (mc-Si). The second one was macro-porous texturization prepared by double-step chemical etching after KOH saw damage layer was previously removed.Both methods of texturization are realized by chemical etching in HF-HNO₃-H₂O with different additives. Macro-porous texturization allows to obtain effective reflectivity (R_eff) in the range 9–20% from bare mc-Si. This R_eff value depends on the time of second step etching that causes porous structure modification. The internal quantum efficiency (IQE) of cells with this kind of texturization has possibility to reach better conversion efficiency than the standard mc-Si solar cells. However, low shunt resistance depends on morphology of porous layer and it is the main factor which can reduce open circuit voltage and conversion efficiency of cells.The effective reflectivity is about 17% for acid texturized mc-Si wafer. The investigation of surface morphology by scanning electron microscopy (SEM) revealed that the dislocations are appearing during chemical etching and they can reduce open circuit voltage. The density of the dislocations can be reduced by controlling depth of etching and optimisation of acid solution.     

Correlation between spatially resolved solar cell efficiency and carrier lifetime of multicrystalline silicon

The correlation between the spatially resolved carrier lifetime of multicrystalline silicon and the spatially resolved monochromatic solar cell efficiency is investigated by means of microwave-detected photoconductance decay (MW-PCD) measurements and illuminated lock-in thermography (ILIT). Local monochromatic solar cell efficiencies are determined from ILIT measurements under short-circuit conditions and at the maximum power point of the cell. The resulting efficiency images are compared with efficiency images obtained from MW-PCD lifetime images of unprocessed neighbouring wafers using PC1D simulations. We observe a qualitative correlation between the measured and the simulated efficiency images. Areas with reduced efficiency are found in the same locations using both methods. However, the dynamic range in the monochromatic efficiency is larger for the images obtained from ILIT measurements. Possible explanations for this difference are a change in carrier lifetime during cell processing and varying lifetimes on microscopic scales, leading to averaging faults in the lifetime images.     

Properties of plasma enhanced chemical vapor deposited silicon nitride for the application in multicrystalline silicon solar cells

Hydrogenated films of silicon nitride (SiNx:H) were investigated by varying the deposition condition in plasma enhanced chemical vapor deposition (PECVD) reactor and annealing condition in infrared (IR) heated belt furnace to find the optimized condition for the application in multicrystalline silicon solar cells. By varying the gas ratio (ammonia to silane), the silicon nitride films of refractive indices 1.85-2.45 were obtained. Despite the poor deposition rate, silicon wafer with the film deposited at 450 °C showed the best minority carrier lifetime. The film deposited with the gases ratio of 0.57 showed the best peak of carrier lifetime at the annealing temperature of 800 °C. The performance parameters of cells fabricated by varying co-firing peak temperature also showed the best values at 800 °C. The multicrystalline silicon (mc-Si) solar cells fabricated in conventional industrial production line applying the optimized film deposition and annealing conditions on large area substrate (125 mm × 125 mm) was found to have the conversion efficiency of 15%.     

Improvement of multicrystalline silicon wafer solar cells by post-fabrication wet-chemical etching in phosphoric acid

In this study, we have improved electrical characteristics such as the efficiency (η) and the fill factor (FF) of finished multicrystalline silicon (mc-Si) solar cells by using a new chemical treatment with a hot phosphoric (H₃PO₄) acidic solution. These mc-Si solar cells were made by a standard industrial process with screen-printed contacts and a silicon nitride (SiN) antireflection coating. We have deposited SiN thin layer (80 nm) on p-type mc-Si substrate by the mean of plasma enhanced chemical vapour deposition (PECVD) technique. The reactive gases used as precursors inside PECVD chamber are a mixture of silane (SiH₄) and ammonia (NH₃) at a temperature of 380°C. The developed H₃PO₄ chemical surface treatment has improved η from 5·4 to 7·7% and FF from 50·4 to 70·8%, this means a relative increase of up to 40% from the initial values of η and FF. In order to explain these improvements, physical (AFM, EDX), chemical (FTIR) and optical (spectrophotometer) analyses were done.     

Efficiency improvement of multicrystalline silicon solar cells after surface and grain boundaries passivation using vanadium oxide

The aim of this work is to investigate the effect of vanadium oxide deposition onto the front surface of multicrystalline silicon (mc-Si) substrat, without any additional cost in the fabrication process and leading to an efficient surface and grain boundaries (GBs) passivation that have not been reported before. The lowest reflectance of mc-Si coated with vanadium oxide film of 9% was achieved by annealing the deposited film at 600 °C. Vanadium pentoxide (V₂O₅) were thermally evaporated onto the surface of mc-Si substrates, followed by a short annealing duration at a temperature ranging between 600 °C and 800 °C, under O₂ atmosphere. The chemical composition of the films was analyzed by means of Fourier transform infrared spectroscopy (FTIR). Surface and cross-section morphology were determined by atomic force microscope (AFM) and a scanning electron microscope (SEM), respectively. The deposited vanadium oxide thin films make the possibility of combining in one processing step an antireflection coating deposition along with efficient surface state passivation, as compared to a reference wafer. Silicon solar cells based on untreated and treated mc-Si wafers were achieved. We showed that mc-silicon solar cells, subjected to the above treatment, have better short circuit currents and open-circuit voltages than those made from untreated wafers. Thus, the efficiency of obtained solar cells has been improved.     

Rapid processing of low-cost, high-efficiency silicon solar cells

Rapid and potentially low-cost processing techniques are analyzed and applied toward the fabrication of high-efficiency Si solar cells. (i) A technology that can simultaneously form the phosphorus emitter, boron BSF, andin situ oxide in a single high-temperature furnace step or: simultaneously diffused, textured, and AR coated process (STAR) is presented. (ii) A high quality screen-printed (SP) contact methodology is developed that results in fill factors of 0·785–0·790 on monocrystalline Si. (iii) Aluminum back surface field (Al-BSF) formation is studied in detail to establish the process conditions that result in optimal BSF action. (iv) Screen-printing of Al conductor paste and rapid thermal processing (RTP) are integrated into the BSF procedure, and effective recombination velocities (S _eff) as low as 200 cm/s are demonstrated on 2·3 Ω-cm Si with this rapid thermal processing of screen-printed contacts, Al alloyed BSF processes. (v) A novel passivation scheme consisting of a dielectric stack (plasma silicon nitride on top of a rapid thermal oxide) is developed to reduce the surface recombination velocity (S) to ≈ 10 cm/s at the 1·3 Ω-cm Si surface. The important feature of this stack passivation scheme is its ability to withstand a high-temperature anneal (700–850°C) without degradation in surface recombination velocity. This feature is critical for most current commercial processes that utilize SP contact firing. (vi) Finally, the individual processes are integrated to form high-efficiency, manufacturable devices. Solar cell efficiencies of 17% and >19% are achieved on FZ Si with SP and evaporated (photolithography) contacts, respectively.     

Influence of stain etching on low minority carrier lifetime areas of multicrystalline silicon for solar cells

In this work the use of HF/HNO₃ solutions for texturing silicon-based solar cell substrates by stain etching and the influence of texturing on minority carrier lifetimes are studied. Stain etching is currently used to decrease the reflectance and, subsequently improve the photogenerated current of the cells, but also produces nanostructures on the silicon surface. In the textured samples it has been observed that an improvement on the minority carrier lifetime with respect to the samples treated with a conventional saw damage etching process is produced on grain boundaries and defects, and the origin of this effect has been discussed.     

Spectroscopic ellipsometry characterization of SiNx antireflection films on textured multicrystalline and monocrystalline silicon solar cells

We present a spectroscopic ellipsometry study of silicon nitride based antireflection films deposited on chemically textured multi- and monocrystalline silicon wafers. The ellipsometric parameters were measured from the near infrared to the ultra violet spectral region. We report the effective thickness and complex index of refraction parameters of the antireflection films from all studied surfaces, regardless of their microscopic morphology. We report on a method to make ellipsometric measurements of the effective optical constants and thickness parameters of thin films deposited on alkaline etched (100)-oriented monocrystalline silicon. The effect of the texture on the complex index of refraction can be described within an effective medium approximation approach. The optical properties are consistent with those obtained from a series of reference films deposited on flat silicon surfaces.     

The materials characteristic and the efficiency degradation of solar cells from solar grade silicon from a metallurgical process route

The rising conventional energy prices have opened up the market for photovoltaic, but the lack of polycrystalline silicon from the chemical route restricts the growth of crystalline silicon solar cells. Recently there is a trend that produces solar cells by using the newly developed solar grade silicon feedstock from a metallurgical process route. In this article, the chemical components of solar grade silicon feedstock are analyzed. The single crystalline silicon solar cells from 100% solar grade silicon feedstock from a metallurgical process route are investigated. The outdoor performance of solar modules encapsulated by such cells is reported. The experimental evidence suggests that such solar cells can achieve the average efficiency higher than 14% on single crystalline silicon wafers. However, the efficiency degradation of solar cells under natural sunlight is significant, and the electrical uniformity of small cells diced from the whole cell is too bad. The metal impurities, oxygen, carbon, and their complexes influence the performance stabilization. The article proves that the module made by such cells has a big cell mismatch loss than normal cells made by electronic grade silicon, even if these cells come from the same sort. And the operating temperature of the cells of the modules is 15–22 °C higher than normal modules under the same conditions. The solar grade silicon feedstock from a metallurgical process route has to be improved farther in order to be used in photovoltaic industry.     

固化过程中模具与复合材料构件相互作用分析

通过在模具与复合材料之间引入剪切层的方法, 建立预报相互作用下构件变形的有限元模拟程序, 模拟结果与文献中的实验结果基本吻合。同时建立了剪切层性能参数预报模型, 经实验与模拟结果的对比证明, 该模型能够对不同工艺条件下及不同结构尺寸的复合材料构件的固化变形进行较为准确地模拟。      

A new single-component low-cost emitter etch-back process for silicon wafer solar cells

A way of achieving lightly doped emitter is a combination of a heavy emitter diffusion and emitter etch back, which has an added advantage of phosphorous diffusion gettering. However, this chemical emitter etch-back process must fulfil some critical requirements, e.g. cost-effectiveness, near-conformal Si etching even after deep emitter etch back, controlled Si etch rate, post-etch clean Si surface and lowest safety issues in chemical handling and drainage. In this work, we report a new low-cost (less than 1 US Cents/wafer), single-chemical, non-acidic, high-throughput emitter etch-back process for tube-diffused emitters for crystalline Si wafers. This process uses only sodium hypochlorite solution at 80 °C as the Si etchant. This process is versatile with its applications on phosphorous and boron tube-diffused monocrystalline Si and phosphorous tube-diffused multicrystalline Si wafers. The preparation, usage and drainage of this highly diluted solution are easy and safe. The Si etching process leads to excellent spatial uniformity over large-area Si wafers (243 cm²). With deep etch back resulting in a change of sheet resistance by ~60 Ω/sq, the standard deviation value changes by only 2.7%. High surface conformity in the etch-back surface is evident from reflectance studies. Quasi-steady-state photoconductance and photoluminescence imaging are used to demonstrate improved electrical parameters of the etch-back wafers.     

Observations on the relationship between structure and electrical performance in silicon ribbon solar cells

A correlation between the presence of growth twins and the conversion efficiencies of solar cells made from silicon ribbons grown by the edge-defined, film-fed growth (EFG) technique has been observed. Ribbon samples with no twins furnish cells of the highest conversion efficiency (～9% air mass zero) whereas those with twins have significantly degraded efficiency.     

Characterization of laser-induced damage in silicon solar cells during selective ablation processes

Selective laser ablation of silicon nitride layers on crystalline silicon wafers was investigated for solar cell fabrication. Laser processing was performed with a nanosecond UV laser at various energy densities ranging from 0.2 to 1.5 J cm^?2. Optical microscopy was used as a simple mean to assess the ablation threshold that was correlated to the temperature at the interface between the silicon nitride coating and the silicon substrate. Minority carrier lifetime measurements were performed using a microwave photo-conductance decay technique. Band to band photoluminescence spectroscopy proved to be a sensitive technique to qualify the laser-induced damage to the silicon substrate. The crystalline structure of silicon seemed to be maintained after silicon nitride ablation as shown by UV reflectivity measurements. Laser parameters corresponding to fluences of around 0.4 J cm^?2 were found to achieve selective ablation of SiN_x without causing detrimental damage to the surrounding material.     

熔析结晶法提纯硅工艺研究进展

随着光伏产业的快速发展,对太阳能级硅原材料的需求不断增加。熔析结晶法作为一种冶金硅提纯的新工艺越来越受到重视。熔析结晶法是利用冶金硅中杂质元素的偏析行为,选择适当的熔析介质,使杂质元素从冶金硅中偏析到熔析介质中,进而获得高纯硅的方法。详细介绍了Al-Si、Sn-Si、Cu-Si、Fe-Si和Ca-Si等熔析体系对冶金硅提纯的研究现状,比较了各种介质体系的优缺点。同时针对熔析结晶法提纯硅存在的问题提出了一些建议。     

Combined power of perovskites and silicon solar cells

The photovoltaic (PV) or solar cells technology can be categorised into two main groups, the wafer‐based and thin‐film based PVs. The wafer‐based PVs include the commonly known crystalline silicon (c‐Si) and gallium arsenide (GaAs) cells. The GaAs cells exhibit higher efficiency compared to crystalline silicon (c‐Si) cells but it is the later that dominates the commercial market. Thin‐film based (2nd Generation) PVs, including cadmium telluride (CdTe), amorphous silicon (a‐Si:H) and copper‐indium‐gallium‐selenide (CIGS), generally absorb light more efficiently than wafer‐based cells and can allow the use of materials in very thin films form. CdTe PVs have proven to be highly efficient but holds only a few percentage share of the market. There is still a need for more R&D before further commercialisation. An emerging and relatively new class of thin‐film based photovoltaics (3rd Generation) technology that has the potential to overcome the current energy conversion efficiencies and performance by making use of novel materials. This class of PVs include organic photovoltaic (OPV), dye‐synthesised solar cells (DSSC), quantum‐dot (QD) and last but not least, the perovskite PV. Perovskite PVs can offer a low cost energy generation solution with the best device conversion efficiencies have shot from lower than 4% in 2009 to more than 21% in 2016. Perovskite based devices can be fabricated using vacuum thermal evaporation or by solution processing of the active layers. Although most recent perovskite solar cells with record efficiencies (>20%) are prepared via solution processing, the early breakthrough in perovskite solar cells was made with vacuum processed perovskites thin films. Vacuum thermal evaporation offers the ability and flexibility to prepare solar cell devices in various configuration. Recent developments in the field of perovskite demonstrates its compatibility with both, first and second generation PV technologies, and is therefore likely to be embraced by the conventional PV industry and make its way into utility‐scale power generation.     

Effect of crucible rotation rate on the growth and macrostructure of multicrystalline silicon

This paper examines the influence of uniform rotation of the crucible-melt-crystal system in a flat-bottom Bridgman geometry on the columnar silicon structure formed by a flat solidification front. We analyze the key features of heat exchange via natural convection using numerical simulation of experimental conditions of the growth of multicrystalline silicon from high-purity metallurgical-grade silicon.