Texturing of large area multi-crystalline silicon wafers through different chemical approaches for solar cell fabrication

Surface texturing of silicon can reduce the reflectance of incident light and hence increase the conversion efficiency of solar cells. Comparatively lesser concentrated (10%) standard alkaline (NaOH/KOH) solution does not give good textured multi-crystalline silicon (mc-Si) surface, which could give satisfactory open-circuit voltage. This is due to grain-boundary delineation with steps formed between successive grains of different orientations. In this work an attempt has been made to obtain a well-textured mc-Si surface through three different approaches. The first two are with two different types of acid solutions and the third with concentrated alkaline NaOH. Solutions of HF–HNO₃–CH₃COOH/H₂O system with different concentrations of HF and HNO₃ were used for texturing. The results on the effect of texturing of these three solutions on the surface morphology of very large area (125 mm×125 mm) mc-Si wafer as well as on the performance parameters of solar cell are presented in this paper. Attempts have been made to study extensively the surface morphologies of mc-Si wafers in two effective regions of the isoetch curves of the HF:HNO₃:diluent's system. Also we studied the reflectance, uniformity, spectral response, short-circuit current, open-circuit voltage, fill factor and dark current–voltage of the cells fabricated using wafers textured with the three different methods. Short-circuit current of the solar cells fabricated using acid-textured wafers were measured to be in the range of 4.93 A. This value is 0.37 and 0.14 A higher than the short-circuit current values measured in the cells fabricated with isotextured and alkaline-textured wafers, respectively.     

Double porous silicon layer on multi-crystalline Si for photovoltaic application

Double porous silicon (d-PS) layers formed by acid chemical etching on a top surface of n⁺/p multi-crystalline silicon solar cells were investigated with the aim to improve the performance of standard screen-printed silicon solar cells. First a macro-porous layer is formed on mc-Si. The role of this layer is texturization of surface. Next, the cells have been manufactured using standard technology based on screen-printing metallization. Finally, a second mezo-porous layer in n⁺ emitter of cell has been produced. The role of this PS layer is to serve as an antireflection coating. In this way, we have obtained d-PS layers on these solar cells. The paper present observation of d-PS microstructure with SEM as well as measurements of its effective reflectance at the level of 2.5% in the 400–1000 nm length wave range. The efficiency of the solar cells with this structure is about 12%.     

Effects of spectral variation on the device performance of copper indium diselenide and multi-crystalline silicon photovoltaic modules

We present results of an experimental investigation of the effects of the daily spectral variation on the device performance of copper indium diselenide and multi-crystalline silicon photovoltaic modules. Such investigations are of importance in characterization of photovoltaic devices. The investigation centres on the analysis of outdoor solar spectral measurements carried out at 10 min intervals on clear-sky days. We have shown that the shift in the solar spectrum towards infrared has a negative impact on the device performance of both modules. The spectral bands in the visible region contribute more to the short circuit current than the bands in the infrared region while the ultraviolet region contributes least. The quantitative effects of the spectral variation on the performance of the two photovoltaic modules are reflected on their respective device performance parameters. The decrease in the visible and the increase in infrared of the late afternoon spectra in each case account for the decreased current collection and hence power and efficiency of both modules.     

Growth of multi-crystalline silicon ingot by improved directional solidification process based on numerical simulation

A multi-crystalline silicon (Si) ingot was simulated and grown using the improved directional solidification (DS) process. Numerical simulations were performed with two different cooling paths and two different coolant flow rates in order to demonstrate the thermal characteristics in the improved DS furnace during the crystal growth. The temperature distributions in the furnace and locally (at the silicon ingot) were predicted as a function of time. From these result, a multi-crystalline Si ingot weighing 300 kg was grown within 40 h using the improved DS process. The Si ingot had a grain size that was larger than 5 mm, and the structure of the ingot was in the form of vertical columns. From the analysis results, the Si ingot exhibited a resistivity below 2 Ω cm and a life time above 3 μs.     

Solar grade silicon feedstock supply for PV industry

For the growth of the photovoltaic industry, it seems inevitable to create an independent feedstock supply with solar grade high-purity silicon, because the availability of the hitherto used by-products in the electronic silicon industry is limited and will not increase reasonably within the next 10–15 years.One of the results of the study “Present and likely future bottlenecks analysis for a sustainable photovoltaic policy” carried out by Bayer and Life, within EPIA, sponsored by the European Commission, within the ALTENER programme, shows that the feedstock used to date, based on electronic grade silicon Scraps is already limiting the PV market expansion even if a true shortage is not expected before 2004–2005 according to a “low growing PV market scenario”. This conclusion implies that a new silicon feedstock not depending on electronic grade silicon production chain must be available on the market from the years 2004 to 2005.Bayer evaluated different ways for the production of solar grade silicon with a capacity of upto 5000 mt/yr. By calculating different routes, it seems possible to install a 5000 t plant by an investment of 110 Mio. Euro and to sell silicon feedstock with a price level of 12–15 Euro/kg. Bayer has been developing a suitable process and the new owner, the Deutsche Solar GmbH, is considering what effort is necessary for a pilot production plant.The feedstock situation is discussed under the aspect of availability of different materials and the efforts under consideration by different parties.     

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.     

Enhancement of photovoltaic properties of multicrystalline silicon solar cells by combination of buried metallic contacts and thin porous silicon

Photovoltaic properties of buried metallic contacts (BMCs) with and without application of a front porous silicon (PS) layer on multicrystalline silicon (mc-Si) solar cells were investigated. A Chemical Vapor Etching (CVE) method was used to perform front PS layer and BMCs of mc-Si solar cells. Good electrical performance for the mc-Si solar cells was observed after combination of BMCs and thin PS films. As a result the current-voltage (I-V) characteristics and the internal quantum efficiency (IQE) were improved, and the effective minority carrier diffusion length (Ln) increases from 75 to 110 μm after BMCs achievement. The reflectivity was reduced to 8% in the 450-950 nm wavelength range. This simple and low cost technology induces a 12% conversion efficiency (surface area = 3.2 cm²). The obtained results indicate that the BMCs improve charge carrier collection while the PS layer passivates the front surface.     

New processes for the production of solar-grade polycrystalline silicon: A review

The global energy consumption is predicted to grow dramatically every year. Higher energy prices and public awareness for the global warming problem have opened up the market for solar cells. The generation of electricity with solar cells is considered to be one of the key technologies of the new century. The impressive growth is mainly based on solar cells made from polycrystalline silicon. This paper reviews the recent advances in chemical and metallurgical routes for photovoltaic (PV) silicon production.     

Requirements of PECVD SiNx:H layers for bulk passivation of mc-Si

Optimization of plasma enhanced chemical vapor deposited hydrogenated silicon nitride (SiN_x:H) towards bulk passivation of multi-crystalline silicon cells has been carried out for both low and high frequency (HF) plasma deposition. Experimental results showed that bulk passivation is not caused by hydrogen incorporation in the top silicon layer during deposition and subsequent diffusion towards the bulk during firing, but that it is released from the SiN_x:H film. We demonstrate that the amount of passivation depends on the SiN_x:H density and its resistance against etching in HF. Optimization of the density, varying deposition temperature and using hydrogen dilution resulted in an optimized passivation.     

Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N⁺/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N⁺/P, 78.5 cm²) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I_sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I–V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.     

Optimization of porous silicon reflectance for silicon photovoltaic cells

The antireflection properties of electrochemically formed porous silicon (PS) layers in the 0.3 μm thick n⁺ emitter of Si p–n⁺ junctions, have been optimized for application to commercial silicon photovoltaic cells. The porosity and thickness of the PS layers are easily adjusted by controlling the electrochemical formation conditions (current density and anodization time). The appropriate PS formation conditions were determined by carrying out a two steps experiment. A first set of samples allowed to determine the optimal porosity and a second one to adjust the thickness of the PS layers, by evaluating the interference features of the reflectance produced by the layers. A PS layer with optimal antireflection coating (ARC) characteristics was obtained in 30% HF in only 3.5 s. The effective reflectance is reduced to 7.3% between 400 and 1150 nm which leads to a gain of up to 33% in the theoretical short circuit current of a p–n⁺ shallow junction compared to a reference junction without a PS layer. The effective reflectance with optimized PS layers is significantly less than that obtained with a classical TiO₂ ARC on a NaOH pretextured multicrystalline surface (11%).     

Mechanical effects of chemical etchings on monocrystalline silicon for photovoltaic use

The mechanical effects of two etching treatments commonly applied on silicon wafers for the PV industry, are considered. The failure characteristics of this material under concentrated load are shown . In both cases, the maximum elongation and sustainable load of the etched wafers were measured to be higher than those of the original sample. The employed experimental procedure and results are presented here and a statistical data analysis substantiates the results observed. An attempt of explanation for this effect is offered based on the removal of a shallow highly defective layer induced by the etching of the material.     

Germanium-doped Czochralski silicon for photovoltaic applications

Germanium (Ge)-doped Czochralski (GCZ) silicon has been grown for photovoltaic (PV) applications. It is found that Ge doping improves the mechanical strength of CZ silicon, resulting in the reduction of breakage during wafer cutting, cell fabrication and module assembly. Boron-oxygen (B-O) defects that lead to the light-induced degradation (LID) of carrier lifetime are effectively suppressed by Ge doping. The decrease in the maximum concentration of B-O defects increases with an increase of Ge concentration. The efficiency of GCZ silicon solar cells and the power output of corresponding PV modules both exhibit smaller loss under sunlight illumination. The current work suggests that GCZ silicon should be potentially a novel substrate for thin solar cells with low LID effect.     

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (σ_ph), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p–i–n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100 mw/cm²) at room temperature.     

Silicon ribbons and foils—state of the art

Research and development on crystal growth technologies for production of crystalline silicon ribbon have been under way now for three decades. I review here their progress toward establishment of manufacturing capabilities for silicon wafers for photovoltaic applications. I examine technology improvements which are currently being explored for a future generation of low cost solar products based on ribbon wafers, and discuss potential limits of the technology.     

Silicon and solar-grade silicon production by solar dissociation of Si3N4

The temperature required for carbothermal reduction of silica—in the range 2100–2300 °C—is past the upper limit for combustion process heat. It is therefore an interesting candidate for solar–thermal processing. The production of solar-grade silicon from carbothermally produced silicon requires an energy-intensive long-duree high-temperature purification process. We propose here a two-step solar process for the production of silicon from silica: first, a carbothermal reduction in the presence of nitrogen to yield silicon nitride and second, the solar dissociation of the nitride to yield silicon. This last step could be combined with purification of the silicon if solar-grade silicon is the desired end-product. In this paper, we report on experimental results that indicate that silicon nitride can be dissociated to yield silicon with no detectable nitride content.     

Simple, fast and accurate two-diode model for photovoltaic modules

This paper proposes an improved modeling approach for the two-diode model of photovoltaic (PV) module. The main contribution of this work is the simplification of the current equation, in which only four parameters are required, compared to six or more in the previously developed two-diode models. Furthermore the values of the series and parallel resistances are computed using a simple and fast iterative method. To validate the accuracy of the proposed model, six PV modules of different types (multi-crystalline, mono-crystalline and thin-film) from various manufacturers are tested. The performance of the model is evaluated against the popular single diode models. It is found that the proposed model is superior when subjected to irradiance and temperature variations. In particular the model matches very accurately for all important points of the I-V curves, i.e. the peak power, short-circuit current and open circuit voltage. The modeling method is useful for PV power converter designers and circuit simulator developers who require simple, fast yet accurate model for the PV module.     

Radiative energy loss in a polysilicon CVD reactor

This work aims at understanding the energy loss in the polysilicon deposition reactor during the production of solar grade silicon. The radiative heat transfer between the polysilicon rods and the reactor wall in the so-called Siemens reactor is studied in detail in this paper. First, the most commonly used reactor configuration, 36 rods in three rings, is explained, detailing the particular radiation transfer of each rod toward the wall. Based on this analysis, some proposals for diminishing the energy loss are proposed: enlarge the reactor capacity, improve the properties of the reactor wall and introduce thermal shields. The impact of each proposal on the energy savings is quantified. If the reactor capacity is enlarged from 36 to 60 rods, the energy savings would be around 11 kWh per kg of polysilicon produced (kWh kg⁻¹). Increasing the reflectivity of the wall, the savings would be around 17 kWh kg⁻¹. And finally, the potential for cost reduction because of the introduction of thermal shields would be 20 kWh kg⁻¹.     

Price dynamics and market relations in solar photovoltaic silicon feedstock trades

The paper intends to provide empirical evidence on silicon feedstock price dynamics to assess the way that major photovoltaic countries—the United States of America, Germany, Japan and South Korea—are related in trade competition to China. This issue is explored by addressing the following questions: How have feedstock prices transmitted between geographically separate and contractually different markets? How have different country markets been positioned in feedstock trades by contract? What causes the failure of full price transmission or perfect market integration? Based on a careful examination of China's status in feedstock trades, the price series by country and by contract are measured with the series by country and trade pattern over a reasonable period from 2007 to 2010. The dynamic price relations are analyzed with causality, cointegration and equilibrium tests. The results exhibit dominant but differential market relations by contract. The speed and extent of price transmission are substantially higher in spot trades. The failure of perfect market equilibrium is due to the material heterogeneity and contract arrangements. These findings imply that sufficient attention should be paid to the way that different photovoltaic silicon feedstock countries have competed and the factors that work to explain price inefficiency, but not price manipulations.     

High Concentration Silicon Solar Cells

The present state-of-the-art of silicon high concentration solar cells is reviewed. Two cell structures, the silicon point contact cell and the microgrooved cell structure, have demonstrated efficiencies of 28% at 150× and 24.7% at 100×, respectively. The problems associated with operation at high concentration are discussed. It is noted that Auger recombination is the limiting factor in the operation of high efficiency solar cells, affecting not only the open circuit voltage, but having a great impact on the short circuit current as well. With additional design improvements, efficiencies over 30% at concentration appear to be within reach.