An overview of silicon ribbon growth technology

A silicon ribbon growth method, String Ribbon, is discussed and compared with the two other vertical ribbon technologies. Manufacturing advances in production of 300 μm String Ribbon are described along with characterization of this ribbon, particularly dislocation distribution. Progress on the growth of 100 μm ribbon and in the making of higher efficiency cells are detailed.     

Ribbon Si solar cells with efficiencies over 18% by hydrogenation of defects

We have fabricated 4 cm² solar cells on String Ribbon Si wafers and edge-defined film-fed grown (EFG) Si wafers with using a combination of laboratory and industrial processes. The highest efficiency on String Ribbon Si wafer is 17.8% with an open circuit voltage (V_oc) of 620 mV, a short circuit current density (J_sc) of 36.8 mA/cm² and a fill factor (FF) of 0.78. The maximum efficiency on EFG Si is 18.2% with a V_oc of 620 mV, a J_sc of 37.5 mA/cm² and a FF of 0.78. These are the most efficient ribbon Si devices made to date, demonstrating the high quality of the processed Si ribbon and its potential for industrial cells. Co-firing of SiN_x and Al by rapid thermal processing was used to boost the minority carrier lifetime of bulk Si from 3–5 μs to 70–100 μs. Photolithography-defined front contacts were used to achieve low shading losses and low contact resistance with a good blue response. The effects of firing temperature and time were studied to understand the trade-off between hydrogen retention and Al-doped back surface field (Al-BSF) formation. Excellent bulk defect hydrogenation and high-quality thick Al-BSF formation was achieved in a very short time (1 s) at firing temperatures of 740–750 °C. It was found that the bulk lifetime decreases at annealing temperatures above 750 °C or annealing time above 1 s due to dissociation of hydrogenated defects.     

Ribbon growth on substrate (RGS) silicon solar cells with microwave-induced remote hydrogen plasma passivation and efficiencies exceeding 11%

For the first time efficiencies above 11% for solar cells (4 cm²) based on Bayer ribbon growth on substrate (RGS) crystalline silicon have been demonstrated including mechanical V-structuring of the front surface, aluminum-gettering, microwave-induced remote hydrogen plasma (MIRHP) passivation and PECVD SiN/SiO₂ double-layer antireflection coating. MIRHP alone resulted in absolute improvements in the open-circuit voltage of 27 mV, in the short-circuit current density of 2.8 mA cm⁻² and in the cell efficiency of 1.9% leading to an open-circuit voltage of 538 mV and an efficiency of 11.1%.     

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.     

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.     

Space solar cells—tradeoff analysis

This paper summarizes the study that had the objective to tradeoff space solar cells and solar array designs to determine the best choice of solar cell and array technology that would be more beneficial in terms of mass, area and cost for different types of space missions. Space solar cells, which are commercially now available in the market and to be available in the near future, were considered for this trade study. Four solar array designs: rigid, flexible, thin film flexible and concentrator solar arrays were considered for assessment. Performance of the solar cells along with solar array designs were studied for two types of space missions: geo synchronous orbit (GEO) and low earth orbit (LEO) spacecraft. The Solar array designs assumed were to provide 15 kW power for 15 years mission life in GEO and 5 kW power for 5 years mission life in LEO altitudes. To perform tradeoff analysis a spread sheet model was developed that calculates the size, mass and estimates the cost of solar arrays based on different solar cell and array technologies for given set of mission requirements. Comparative performance metrics (W/kg, W/m², kg/m², and $/W) were calculated for all solar arrays studied and compared, at the solar array subsystem level and also at the spacecraft system level. The trade analysis results show that high-efficiency multijunction solar cells bring lot of cost advantages for both types of missions. The trade study also show that thin film solar cells with moderate efficiency with ultra lightweight flexible array design may become competitive with well-established single crystalline solar cell technologies in the future.     

Fundamental understanding and implementation of Al-enhanced PECVD SiNx hydrogenation in silicon ribbons

A low-cost, manufacturable defect gettering and passivation treatment, involving simultaneous anneal of a PECVD SiN_x film and a screen-printed Al layer, is found to improve the lifetime in Si ribbon materials from 1–10 μs to over 20 μs. Our results indicate that the optimum anneal temperature for SiN_x-induced hydrogenation is 700°C for EFG and increases to 825°C when Al is present on the back of the sample. This not only improves the degree of hydrogenation, but also forms an effective back surface field. We propose a three-step physical model, based on our results, in which defect passivation is governed by the release of hydrogen from the SiN_x film due to annealing, the generation of vacancies during Al–Si alloying, and the retention of hydrogen at defect sites due to rapid cooling. Controlled rapid cooling was implemented after the hydrogenation anneal to improve the retention of hydrogen at defect sites by incorporating an RTP contact firing scheme. RTP contact firing improved the performance of ribbon solar cells by 1.3–1.5% absolute when compared to slow, belt furnace contact firing. This enhancement was due to improved back surface recombination velocity, fill factor, and bulk lifetime. Enhanced hydrogenation and rapid heating and cooling resulted in screen-printed Si ribbon cell efficiencies approaching 15%.     

Thin silicon string ribbon

Evergreen Solar is a new solar cell company that is commercializing a silicon sheet growth method known as String Ribbon, a method for forming continuous polycrystalline silicon ribbon directly from the melt. Recently, modifications to the growth environment by means of a tunable afterheater have enabled Evergreen to grow flat, 5.6 cm wide and 100 μ thick ribbon with low enough stress to permit solar cell fabrication. The development work was aided by using finite element analysis with a thermoelastic model. Initial work on forming cells on this material was quite promising. The best solar cell made on the 100 μm thick, p-type silicon ribbon ( = 0.1 Ω cm) was 14.7% efficient. It had a V_oc of 639 mV and a fill factor of 0.798 with a J_sc of 28.8 mA/cm². Significant increases in lifetime during processing were not unusual. With better starting lifetimes from higher bulk resistivity values, PC1D modeling indicated that 16% efficient cells should be readily achievable on this material.     

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.     

Effect of heat treatment on carbon in multicrystalline silicon

Effect of heat treatment on carbon in cast multicrystalline silicon (mc-Si) has been studied by means of Fourier Transmission Infrared Spectroscopy. Carbon is found to be involved in the formation of as-grown precipitates in mc-Si with higher oxygen content. The experimental results reveal that carbon is difficult to precipitate in mc-Si with lower oxygen or higher nitrogen concentration during annealing in the temperature range from 450°C to 1150°C. Carbon can enhance the nucleation of oxygen precipitates at lower temperature (<850°C). Although carbon does not affect the amount of oxygen precipitates at higher temperature (>950°C), it is suggested that carbon diffuses into oxygen precipitates by the enhancement of silicon self-interstitials. The experiments point out that preannealing at 750°C enhances the decrease of substitute carbon concentration during subsequent annealing at 1050°C. Dislocations and grain boundaries in mc-Si do not affect carbon thermal treatment properties.     

The economics of coal supply—The state of the art

With the mounting interest in national energy problems has come intensification of efforts to determine the future economic availability of fuels. Until well into the 1970s, work concentrated on estimation of oil and gas supplies. However, subsequently numerous evaluations of coal supply have emerged. Such studies share with earlier work on oil and gas dependence upon an inadequate statistical base. Energy has been one of many areas in which governmental data gathering has badly lagged behind national information requirements.‡ Nevertheless, the fragmentary work that has been possible suggests that the availability of low cost coal resources may be greatly exaggerated.§ In the present article, this work on coal is reviewed.The discussion begins with an effort to show what is required in principle for a satisfactory supply analysis. Then the key studies are evaluated. Most supply analyses prove to be modifications of work undertaken by the U.S. Bureau of Mines (USBM) Process Evaluation Group (Peg) at Morgantown, West Virginia. Therefore, the Group's work is given special attention.This is followed by a review of alternative models of the determinants of coal-mining costs at a particular time, a note on the handling of cost changes over time and discussion of efforts to assign reserves to cost categories. The study concludes with proposals for further research.     

New concepts for high-efficiency silicon solar cells

This paper gives an overview about recent activities in the industrial application of high-efficiency monocrystalline silicon solar cells. It also presents the latest results achieved at Fraunhofer ISE, especially a new patented process for the formation of back-contact points on dielectrically passivated cells called laser-fired contacts and its application to thin wafers.     

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.     

Experimental evidence of very high open-circuit voltages of inversion–layer silicon solar cells

In this paper the first experimental evidence of the high V_oc-potential of inversion-layer silicon solar cells is given. Minority-carrier lifetime measurements on inversion-layer emitters have been performed and the diffused p–n contact of PN-IL silicon solar cells has been optimized for high open-circuit voltages. PN-IL silicon solar cells with open-circuit voltages of 693 mV have been fabricated on 0.2 and 0.5-Ω cm FZ p-Silicon wafers. These values are the highest ever reported V_oc's for inversion-layer silicon solar cells on p-Silicon. This demonstrates that inversion-layer silicon solar cells exhibit a similar potential for achieving high open-circuit voltages as silicon solar cells with a diffused p–n junction.     

Design and analysis of low-reflection grating microstructures for a solar energy absorber

The problem of calculating the photo-induced current in an n⁺p junction solar cell with a microstructure grating on the top is formulated. For the case where useful absorption regions lie below the grating a closed solution of the problem is obtained, in which the carrier photo-generation rate is calculated using a full-vector electromagnetic simulation technique. Two structures: filled lamellae grating and overlayer, and filled V-groove grating and overlayer are considered. The first structure is optimized with respect to the integral photo-current and the second with respect to reflection loss. Both optimized structures are much less reflective and more efficient than an optimized flat-surface Si–SiO₂ layer cell.     

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.     

Wind energy and the hydrogen economy—review of the technology

The hydrogen economy is an inevitable energy system of the future where the available energy sources (preferably the renewable ones) will be used to generate hydrogen and electricity as energy carriers, which are capable of satisfying all the energy needs of human civilization. The transition to a hydrogen economy may have already begun. This paper presents a review of hydrogen energy technologies, namely technologies for hydrogen production, storage, distribution, and utilization. Possibilities for utilization of wind energy to generate hydrogen are discussed in parallel with possibilities to use hydrogen to enhance wind power competitiveness.     

Performance of Silicon Heterojunction Photovoltaic modules in Qatar climatic conditions

We report on the performance of two cell technologies: Silicon Heterojunction (SHJ) and conventional diffused junction n-type mono-crystalline silicon Photovoltaic (PV) arrays, under a harsh environment condition with high temperature and dust accumulation, typical to Qatar. A comparison of the energy yield and Performance Ratio (PR) at plane of array global irradiance as well as module temperature (T_mod) of the two technologies is presented. The SHJ arrays showed a higher energy yield as compared with the conventional arrays thanks to the higher efficiency of the SHJ. The results showed also that dust accumulated on PV modules may cause a drop in the PR of up to approximately 15% if the module is not cleaned for one month. Scheduled module cleaning or raining will return the PR close to its initial value.     

Spatially resolved investigations of lifetime enhancement in vertically grown, multicrystalline silicon ribbons

The influence of gettering or defect passivation steps on recombination activity in the vertically grown, multicrystalline ribbon materials edge-defined film-fed growth and string ribbon silicon has been investigated with the help of photoconductance decay. In contrast to well-known results of integral measurements, spatially resolved lifetime mappings have been obtained by applying microwave detection technique.This aspect of spatial resolution has been found to be indispensable for investigating the impact of different processing steps on material quality in an accurate way. Apart from strong variations in as-grown lifetimes that have been found throughout vertically grown silicon wafers, this is due to areas of comparable starting lifetimes which have been revealed to react very differently to applied processing steps. After processing, some of them reach minority charge carrier lifetimes of more than 300 μs whereas others just show values of a few microseconds. As a consequence, the results of integral measurements strongly depend on the nature of areas incorporated in the specific sample. An impression of the corresponding uncertainties inherent to integral measurements has been obtained by statistical evaluation of spatially resolved lifetime data.     

Highest-quality surface passivation of low-resistivity p-type silicon using stoichiometric PECVD silicon nitride

The surface passivation properties of silicon nitride (SiN) films fabricated by high-frequency direct plasma-enhanced chemical vapour deposition (PECVD) on low-resistivity (1 Ω cm) p-type silicon solar cell substrates have been investigated. The process gases used were ammonia and a mixture of silane and nitrogen. In order to find the optimum set of SiN deposition parameters, a large number of carrier lifetime test structures were prepared under different deposition conditions. The optimised deposition parameters resulted in outstandingly low surface recombination velocities (SRVs) below 10 cm/s. Interestingly, we find the lowest SRVs for stoichiometric SiN films, as indicated by a refractive index of 1.9. In former studies similarly low SRVs had only been obtained for silicon-rich SiN films. The fundamentally different passivation behaviour of our SiN films is attributed to the addition of nitrogen to the process gases.