Silicon feedstock for the multi-crystalline photovoltaic industry

During the last 5 years the PV industry continues to experience a strong economic growth between 15% and 30% per year. Multi-crystalline silicon became the preferred material for PV production with a share of more than 50% of the shipped PV modules world-wide. For the first time, the available quantity of the classical silicon feedstock sources for the PV industry—electronic grade silicon rejects from the silicon and microelectronics industry—is close to be not sufficient to satisfy the requirements of the PV industry. From this situation arises the need to develop short- and long-term solutions to guarantee a sustainable supply of the PV industry with suitable silicon feedstock at acceptable costs.This paper presents a possible route for short- and long-term solutions to provide solar grade (SoG) silicon feedstock for the PV industry. On a short-term basis a twofold solution is proposed: (i) reduction of silicon consumption by reducing the wafer thickness and the introduction of recycling scenarios for silicon waste produced by the PV industry, (ii) introduction of very low-resistivity silicon (0.1 Ω cm).On long term, a route towards the establishment of a SoG silicon production based on widely available metallurgical grade silicon is proposed. This route includes the development of suitable purification techniques. First results that allowed to lower the impurity tolerances for SoG silicon are presented. The introduction of silicon feedstock with higher impurity concentrations which show a tendency to interact with crystal defects and lead to a degradation of the material performance also requires passivation concepts to achieve highly performing solar cells.     

Direct patterned etching of silicon dioxide and silicon nitride dielectric layers by inkjet printing

An inkjet method for the direct patterned etching of silicon dioxide and silicon nitride dielectric is described. The method involves fewer steps, lower chemical usage and generates less hazardous chemical waste than existing resist-based patterning methods (e.g., photolithography), which employ immersion etching. Holes of diameter 40–50 μm and grooves 50–60 μm wide were etched in 300 nm silicon dioxide layers. Grooves were also etched in 75 nm silicon nitride layers formed on textured silicon surfaces. The resulting patterned dielectric layers were used to facilitate masked etching, local diffusions and metal contacting of underlying silicon for solar cell applications.     

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.     

Photovoltaic materials, past, present, future

This paper traces briefly the history of this photovoltaic materials and it tries to look at possible future scenarios. A large part of the paper is concerned with silicon although from solid-state physics we know that silicon is not the ideal material for photovoltaic conversion. From the first solar cell developed at Bell Laboratories in 1954 photovoltaics was dominated by silicon. The reasons for this dominating position are investigated. Crystalline silicon today has a market share of 86% which is almost equally distributed between single crystal and cast silicon. Amorphous silicon has another 13%. The main endeavor is to reduce cost. Present trends in the crystalline field are reviewed. The conventional technology still has significant potential for cost reduction but this comes only with increasing volume. A problem to be solved is the supply of solar-grade silicon material. Other future possibilities include thin film crystalline silicon on different substrates. Because of the low absorption coefficient of silicon light trapping is required. True thin film materials need only 1–2 μm of material. Amorphous silicon, copper indium diselenide (CIS) and CdTe are hopeful approaches for very cost-effective solar cells. Some other, more speculative materials and concepts are described at the end of this paper     

Low-cost texturization of large-area crystalline silicon solar cells using hydrazine mono-hydrate for industrial use

Reduction in optical losses in mono-crystalline silicon solar cells by surface texturing is one of the important issues of modern silicon photovoltaics. In order to achieve good uniformity in pyramidal structures on the silicon surface, a mixture of sodium hydroxide (NaOH) or potassium hydroxide (KOH) and isopropyl alcohol (IPA) is generally used during texturization of mono-crystalline silicon solar cell. However, due to the high cost of IPA, there is always a search for alternate chemical which plays the same role as IPA during texturization for industrial solar cell production. For a better texturization, the interfacial energy between silicon and ionized electrolyte of chemical solution should be reduced to achieve sufficient wettability of the silicon surface, which will enhance the pyramid nucleation. In this work, we have investigated the role of hydrazine mono-hydrate as a surface-active additive, which supplies OH⁻ ions after its dissociation. Our process cuts down the IPA consumption during texturing without any loss in uniformity of textured pyramids. We are the first to report the novel idea to add hydrazine mono-hydrate in NaOH solution for texturing mono-crystalline silicon surface to fabricate solar cells with more than 85% yield in the efficiency range of 14.5–15.3%.     

CO2 and H2O reduction by solar thermochemical looping using SnO2/SnO redox reactions: Thermogravimetric analysis

The thermochemical dissociation of CO₂ and H₂O from reactive SnO nanopowders is studied via thermogravimetry analysis. SnO is first produced by solar thermal dissociation of SnO₂ using concentrated solar radiation as the high-temperature energy source. The process targets the production of CO and H₂ in separate reactions using SnO as the oxygen carrier and the syngas can be further processed to various synthetic liquid fuels. The global process thus converts and upgrades H₂O and captured CO₂ feedstock into solar chemical fuels from high-temperature solar heat only, since the intermediate oxide is not consumed but recycled in the overall process. The objective of the study was the kinetic characterization of the H₂O and CO₂ reduction reactions using reactive SnO nanopowders synthesized in a high-temperature solar chemical reactor. SnO conversion up to 88% was measured during H₂O reduction at 973 K and an activation energy of 51 ± 7 kJ/mol was identified in the temperature range of 798-923 K. Regarding CO₂ reduction, a higher temperature was required to reach similar SnO conversion (88% at 1073 K) and the activation energy was found to be 88 ± 7 kJ/mol in the range of 973-1173 K with a CO₂ reaction order of 0.96. The SnO conversion and the reaction rate were improved when increasing the temperature or the reacting gas mole fraction. Using active SnO nanopowders thus allowed for efficient and rapid fuel production kinetics from H₂O and CO₂.     

Present status and future of crystalline silicon solar cells in Japan

Crystalline silicon solar cells show promise for further improvement of cell efficiency and cost reduction by developing process technologies for large-area, thin and high-efficiency cells and manufacturing technologies for cells and modules with high yield and high productivity.In this paper, Japanese activities on crystalline Si wafers and solar cells are presented. Based on our research results from crystalline Si materials and solar cells, key issues for further development of crystalline Si materials and solar cells will be discussed together with recent progress in the field. According to the Japanese PV2030 road map, by the year 2030 we will have to realize efficiencies of 22% for module and 25% for cell technologies into industrial mass production, to reduce the wafer thickness to 50–100 μm, and to reduce electricity cost from 50 Japanese Yen/kWh to 7 Yen/kWh in order to increase the market size by another 100–1000 times.     

CdO/Cu2O solar cells by chemical deposition

CdO and Cu₂O thin films have been grown on glass substrates by chemical deposition method. Optical transmittances of the CdO and Cu₂O thin films have been measured as 60–70% and 3–8%, respectively in 400–900 nm range at room temperature. Bandgaps of the CdO and Cu₂O thin films were calculated as 2.3 and 2.1 eV respectively from the optical transmission curves. The X-ray diffraction spectra showed that films are polycrystalline. Their resistivity, as measured by Van der Pauw method yielded 10⁻²–10⁻³ Ω cm for CdO and approximately 10³ Ω cm for Cu₂O. CdO/Cu₂O solar cells were made by using CdO and Cu₂O thin films. Open circuit voltages and short circuit currents of these solar cells were measured by silver paste contacts and were found to be between 1–8 mV and 1–4 μA.     

Tri-layer antireflection coatings (SiO2/SiO2–TiO2/TiO2) for silicon solar cells using a sol–gel technique

Antireflection coatings (ARCs) have become one of the key issues for mass production of Si solar cells. They are generally performed by vacuum processes such as thermal evaporation, reactive sputtering, and plasma-enhanced chemical vapor deposition. In this work, a sol–gel method has been demonstrated to prepare the ARCs for the non-textured monocrystalline Si solar cells. The spin-coated TiO₂ single-layer, SiO₂/TiO₂ double-layer and SiO₂/SiO₂–TiO₂/TiO₂ triple-layer ARCs were deposited on the Si solar cells and they showed good uniformity in thickness. The measured average optical reflectance (400–1000 nm) was about 9.3, 6.2 and 3.2% for the single-layer, double-layer and triple-layer ARCs, respectively. Good correlation between theoretical and experimental data was obtained. Under a triple-layer ARC condition, a 39% improvement in the efficiency of the monocrystalline Si solar cell was achieved. These indicate that the sol–gel ARC process has high potential for low-cost solar cell fabrication.     

Photoreduction of CO2 to fuels under sunlight using optical-fiber reactor

An optical-fiber reactor is employed to photocatalytically reduce CO₂ with H₂O to fuels under UVA artificial light and concentrated natural sunlight. The optical fiber is coated with gel-derived TiO₂–SiO₂ mixed oxide-based photocatalysts. Fe atom is found to insert into the TiO₂–SiO₂ lattice during sol–gel process, resulting in the full visible light absorption as well as the effect on product selectivity of the derived catalyst. Under UVA, ethylene is mainly produced on Cu–Fe/TiO₂ catalyst with the quantum yield of 0.0235%, whereas Cu–Fe/TiO₂–SiO₂ catalyst is observed to favor methane production with the quantum yield of 0.05%. Meanwhile, the overall energy efficiency is found to be much higher on Cu–Fe/TiO₂–SiO₂ (0.0182%) than on its Cu–Fe/TiO₂ counterpart (0.0159%). There is only methane evolved over both bare TiO₂–SiO₂ and Cu–Fe/TiO₂–SiO₂ catalysts under natural sunlight with the production rates of 0.177 and 0.279 μmol/g-cat h, respectively. For the former catalyst, the increase in light intensity is not found to compensate the inherent electron–hole recombination in the TiO₂–SiO₂–acac catalyst, whereas the superior photoactivity of Cu–Fe/TiO₂–SiO₂ catalyst under natural sunlight could be ascribed to its full absorption of visible light.     

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.     

Gettering effects by aluminum upon the dark and illuminated I–V characteristics of N–P–P silicon solar cells

Impurity gettering is an essential process step in silicon solar cell technology. A widely used technique to enhance silicon solar cell performance is the deposition of an aluminum layer on the back surface of the cell, followed by a thermal annealing. The aluminum thermal treatment is typically done at temperatures around 600°C for short times (10–30 min). Seeking a new approach of aluminum annealing at the back of silicon solar cells, a systematic study about the effect the above process has on dark and illuminated I–V cell characteristics is reported in this paper. We report results on silicon solar cells where annealing of aluminum was done at two different temperatures (600°C and 800°C), and compare the results for cells with and without aluminum alloying. We have shown that annealing of the aluminum in forming gas at temperatures around 800°C causes improvement of the electrical cell characteristics. We have also made evident that for temperatures below 250 K, the predominant recombination process for our cells is trap-assisted carrier tunneling for both annealing temperatures, but it is less accentuated for cells with annealing of aluminum at 800°C. For temperatures above 250 K, the recombination proceeds through Shockley–Read–Hall trap levels, for cells annealed at both temperatures. Furthermore, it seems from DLTS measurements that there is gettering of iron impurities introduced during the fabrication processes. The transport of impurities from the bulk to the back surface (alloyed with aluminum) reduces the dark current and increases the effective diffusion length as determined from dark I–V characteristics and from spectral response measurements, respectively. All these effects cause a global efficiency improvement for cells where aluminum is annealed at 800°C as compared to conventional cells where the annealing was made at 600°C.     

Low temperature silicon oxide and nitride for surface passivation of silicon solar cells

Surface passivation at low processing temperature becomes an important topic for crystalline and multicrystalline silicon solar cells. In this work, silicon oxide (250°C) and silicon nitride (300°C) have been developed by Photo-CVD and PECVD technique respectively. Effects of deposition parameters on the optoelectronic and structural properties of the films have been investigated. Interface-trap density (D_it) and fixed charge density (Qf) have been estimated by high frequency (1 MHz) capacitance-voltage measurement on Metal–Insulator–Silicon structure (CV-MIS). The effect of silicon oxide and silicon nitride on the performance of c-Si solar cells have been studied.     

ALUMINUM PRODUCTION USING HIGH-TEMPERATURE SOLAR PROCESS HEAT

The primary metals industry is one of the most energy intensive in the manufacturing sector, and is consequently also a major source of climate-altering gases. The replacement of electrolysis or electrothermal processes with direct reduction processes using high-temperature solar process heat may well be economical, especially when the costs of CO₂ emission are included in the analysis. In particular, aluminum production by carbothermal reduction is a very high-temperature, energy-intensive process. The temperature required, in the range 2300–2500 K, is too high for practical process heat addition from combustion sources alone. Only electric-arc furnaces or highly concentrated solar are capable of supplying process heat at these high temperatures. The aluminum industry presents unique opportunities for industrial implementation of solar process heat. Use of high-temperature solar process heat can drastically reduce the emission of climate-altering gases, reduce the reliance on electricity, and make possible a direct thermal route from the ore to metal. Two industrially-researched direct aluminum or aluminum–silicon alloy producing processes, and one process that forms an intermediate AlN compound are proposed for study and demonstration projects for alternative solar-thermal processes to replace the Hall–Héroult process.     

Didactic software for solar cells and materials parameters analysis

The authors present a package of computer applications gathered in a form of virtual laboratory as a package of the virtual exercises to solar cells parameters analysis at a student university laboratory. The applications may also be used to analyze some aspects of silicon materials or solar cell structures as a research tool. The package consists of three blocks of exercises. Each of the blocks solves different tasks and is based on different software platforms (LabWindows, Visual Basic, Java and Flash). The range of the tasks is relatively wide: from analysis of the role of parameters of the material (silicon) on the I–V and spectral curves of a solar cell up to simulation of the method of solar quality silicon parameter testing—evaluation of a value of the minority carriers diffusion length from surface photovoltage spectra.     

Experimental study on the performance of solar Rankine system using supercritical CO2

An experimental study is carried out to investigate the performance of a solar Rankine system using supercritical CO₂ as a working fluid. The testing machine of the solar Rankine system consists of an evacuated solar collector, a pressure relief valve, heat exchangers and CO₂ feed pump, etc. The solar energy powered system can provide electricity output as well as heat supply/refrigeration, etc. The system performance is evaluated based on daily, monthly and yearly experiment data. The results obtained show that heat collection efficiency for the CO₂-based solar collector is measured at 65.0–70.0%. The power generation efficiency is found at 8.78–9.45%, which is higher than the value 8.20% of a solar cell. The result presents a potential future for the solar powered CO₂ Rankine system to be used as distributed energy supply system for buildings or others.     

A simple process to remove boron from metallurgical grade silicon

It is necessary to develop solar grade (SoG) silicon for the photovoltaic industry. A desirable approach is to upgrade metallurgical grade (MG) silicon. The most problematic impurities to remove from MG silicon are B and P. A simple process to remove B from MG silicon has been developed by refining MG silicon in the molten state followed by directional solidification. With this approach, B has been reduced to 0.3 ppma, P to <10 ppma and all other impurities to <0.1 ppma using commercially available, as-received MG silicon. It remains to develop a similar P reduction process so that SoG silicon production from MG silicon can be commercialized. The B-removal process was applied to B overdoped electronic grade silicon, and the resulting material was used for crystal growth. Test solar cells of 12.5–13.4% (1 cm²) efficiency were produced.     

Purification of metallurgical grade silicon by a solar process

The purification of upgraded metallurgical silicon by extraction of boron and phosphorus was experimentally demonstrated using concentrated solar radiation in the temperature range 1550–1700 °C. The process operated with a flow of Ar at reduced pressure (0.05 atm) for elimination of P, and with a flow of H₂O for elimination of B. Impurity content decreased by a factor of 3 after a 50-min solar treatment, yielding Si samples with final average content of 2.1 ppm_w B and 3.2 ppm_w P.     

Solar chemical reactor technology for industrial production of lime

We developed the solar chemical reactor technology to effect the endothermic calcination reaction CaCO₃(s) → CaO(s) + CO₂(g) at 1200–1400 K. The indirect heating 10 kW_th multi-tube rotary kiln prototype processed 1–5 mm limestone particles, producing high purity lime that is not contaminated with combustion by-products. The quality of the solar produced quicklime meets highest industrial standards in terms of reactivity (low, medium, and high) and degree of calcination (exceeding 98%). The reactor’s efficiency, defined as the enthalpy of the calcination reaction at ambient temperature (3184 kJ kg⁻¹) divided by the solar energy input, reached 30–35% for quicklime production rates up to 4 kg h⁻¹. The solar lime reactor prototype operated reliably for more than 100 h at solar flux inputs of about 2000 kW m⁻², withstanding the thermal shocks that occur in solar high temperature applications. By substituting concentrated solar energy for fossil fuels as the source of process heat, one can reduce by 20% the CO₂ emissions in a state-of-the-art lime plant and by 40% in a conventional cement plant. The cost of solar lime produced in a 20 MW_th industrial solar calcination plant is estimated in the range 131–158 $/t, i.e. about 2–3 times the current selling price of conventional lime.     

Role of hydrazine monohydrate during texturization of large-area crystalline silicon solar cell fabrication

Reduction of optical losses in monocrystalline silicon solar cells by surface texturing is one of the important issues of modern silicon photovoltaic. For texturization during commercial monocrystalline silicon solar cell fabrication, a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used in order to achieve good uniformity of pyramidal structure on the silicon surface. The interfacial energy between silicon and electrolyte should be reduced in order to achieve sufficient wettability for the silicon surface which in turn will enhance the pyramid nucleation. In this work, we have investigated the role of hydrazine monohydrate as a surface-active additive, which supplies OH⁻ ions after dissociation. This cuts down the IPA consumption during texturing without any loss of uniformity of textured pyramid. We are probably the first group to report such a novel idea of using hydrazine monohydrate addition in NaOH solution for texturization of solar cell. We were able to fabricate monocrystalline silicon solar cells with more than 85% yield in the range of 14–15% efficiency.