Potential of amorphous silicon for solar cells

This paper reviews recent developments in the field of amorphous-silicon-based thin-film solar cells and discusses potentials for further improvements. Creative efforts in materials research, device physics, and process engineering have led to highly efficient solar cells based on amorphous hydrogenated silicon. Sophisticated multijunction solar cell designs make use of its unique material properties and strongly suppress light induced degradation. Texture-etching of sputtered ZnO:Al films is presented as a novel technique to design optimized light trapping schemes for silicon thin-film solar cells in both p-i-n and n-i-p device structure. Necessary efforts will be discussed to close the efficiency gap between the highest stabilized efficiencies demonstrated on lab scale and efficiencies achieved in production. In case of a-Si:H/a-Si:H stacked cells prepared on glass substrates, significant reduction of process-related losses and the development of superior TCO substrates on large areas promise distinctly higher module efficiencies. A discussion of future perspectives comprises the potential of new deposition techniques and concepts combining the advantages of amorphous and crystalline silicon thin-film solar cells. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 14 June 1999     

Light trapping effect in silicon wafers with anodically etched surfaces

Received: 10 May 1996/Accepted: 19 November 1996     

Carrier frequency-dependent intrinsic parameters in an ion-implanted silicon photo-MESFET

Carrier frequency-dependent intrinsic parameters of an ion-implanted silicon photo-MESFET have been analysed theoretically. The internal gate source capacitanceC _gs is found to increase with increasing carrier frequency under the normally OFF condition and the change is small under the normally ON condition. Also, the internal drain-source resistanceR _ds increases with frequency at a fixed flux density and wavelength of operation. The ion-implanted photo-MESFET could become useful as optically controlled switching device in digital circuits.     

High performance a-Si solar cells and new fabrication methods for a-Si solar cells

The super chamber, a separated UHV reaction-chamber system has been developed. A conversion efficiency of 11.7% was obtained for an a-Si solar cell using a high-quality i-layer deposited by the super chamber, and a p-layer fabricated by a photo-CVD method.As a new material, amorphous superlattice-structure films were fabricated by the photo-CVD method for the first time. Superlattice structure p-layer a-Si solar cells were fabricated, and a conversion efficiency of 10.5% was obtained.For the fabrication of integrated type a-Si solar cell modules, a laser pattering method was investigated. A thermal analysis of the multilayer structure was done. It was confirmed that selective scribing for a-Si, TCO and metal film is possible by controlling the laser power density. Recently developed a-Si solar power generation systems and a-Si solar cell roofing tiles are also described.     

Thin-film poly-Si solar cells on glass substrate fabricated at low temperature

The performances of thin-film poly-Si solar cells with a thickness of less than 5 μm on a glass substrate have been investigated. The cell of glass/back reflector/n-i-p type Si/ITO is well characterized by the structure of naturally surface texture and enhanced absorption with a back reflector (STAR), where the active i-type poly-Si layer was fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 μm demonstrated an intrinsic efficiency of 10.7% (aperture 10.1%), an open-circuit voltage of 0.539 V and a short-current density of 25.8 mA/cm² as independently confirmed by Japan Quality Assurance. No light-induced degradation is observed. The optical and transport properties of poly-Si cells are summarized. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 7 July 1999     

High-temperature processing of crystalline silicon thin-film solar cells

The crystalline silicon thin-film solar cell combines, in principle, the advantages of crystalline silicon wafer-based solar cells and of thin-film solar cell technologies. Its efficiency potential is the highest of all thin-film cells. In the “high-temperature approach” thin silicon layers are deposited on substrates that withstand processing temperatures higher than 1000 °C. The basic features of the high-temperature crystalline silicon thin-film cell technology are described and some important results are discussed. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Reliability studies on MIS solar cells

Research on Cr/oxide/p-type Si solar cells has produced a 12.2% efficiency on 2 cm² area. Reliability studies have been conducted to determine if degradation occurs during use in an extreme environment. Several cells with A/R coatings and encapsulation have been tested with degradation occurring in some cases after 2 years of use. One cell without an A/R coating was used for 1+1/2 years with degradation occurring only after a crack had appeared. Dark I–V curves, photovoltaic data, and Auger analysis show a quite stable situation to exist. A stable MIS solar cell depends on selection of the insulator, deposition rate of the metal, elimination of moisture, selection of the A/R coating, bonding techniques, and choice of encapsulant. This work points to an optimistic view towards the fabrication of stable terrestrial MIS solar cells. Research supported by the NSF-RANN Program with technical supervision by the Department of Energy.     

On the thermodynamic limit of photovoltaic energy conversion

An infinite stack ofp—n junctions with smoothly varying bandgap from ∞ to 0 is considered. AnI —V characteristic is derived, which is more correct than the classical exponential characteristic. It is shown that open-circuit operation is a reversible process and leads to the Carnot efficiency, if one defines the efficiency in the way that is usual in the theory of thermodynamic engines. If instead one uses the definition of efficiency usual in photovoltaics, open-circuit mode gives rise to zero efficiency. Then operation at maximum efficiency equals operation at maximum power and is not reversible.     

Effect of signal-modulated optical radiation on the characteristics of a Modfet

The effect of signal-modulated optical radiation on the characteristics of a GaAlAs/GaAs MODFET has been studied analytically. It is found that the offset voltage increases with modulation frequency and the effect of frequency is negligible above 5 MHz. The drain-source current decreases with increase in signal frequency at a constant radiation flux density, doping concentration and drain-source voltage. Studies on sheet concentration and transconductance also show that the signal frequency has a significant effect upto a certain modulation frequency (5 MHz) above which the effect of frequency is insignificant.     

Electrical behaviour of thermally diffused silicon solar cells submitted to rapid annealing

Conventional thermally diffused silicon solar cells have been submitted to subsequent rapid thermal annealing (1–10 s) in the temperature range 600–1000 °C, in order to investigate the effects of such treatments on the electrical behaviour. It appears that a decrease of the minority carrier diffusion length in the base region of the device is produced due to the generation of microscopic defects (as measured by DLTS) induced by the rapid high temperature cycling.     

The influence of tunneling effects on the efficiency of heterojunction solar cells

A theoretical model is developed which presents the transport properties through the space charge region of ap ⁺ n heterojunction solar cell, whereby not only recombination through interface states but also tunneling through potential barriers is taken into account. It is investigated whether tunneling can give rise to optimum heterojunction structures which have better efficiencies that without tunneling. It is found that only if the strongly doped semiconductor has an optimum bandgap and the weakly doped semiconductor a larger bandgap, tunneling can make the structure optimum. In all other cases of optimum structures, tunneling deteriorates the efficiency. Work supported by the Energy R. D. programmes of the Commission of European Communities and the Belgian Ministry of Science.     

Polycrystalline silicon on glass for thin-film solar cells

Although most solar cell modules to date have been based on crystalline or polycrystalline wafers, these may be too material intensive and hence always too expensive to reach the very low costs required for large-scale impact of photovoltaics on the energy scene. Polycrystalline silicon on glass (CSG) solar cell technology was developed to address this difficulty as well as perceived fundamental difficulties with other thin-film technologies. The aim was to combine the advantages of standard silicon wafer-based technology, namely ruggedness, durability, good electronic properties and environmental soundness with the advantages of thin-films, specifically low material use, large monolithic construction and a desirable glass superstrate configuration. The challenge has been to match the different preferred processing temperatures of silicon and glass and to obtain strong solar absorption in notoriously weakly-absorbing silicon of only 1–2 micron thickness. A rugged, durable silicon thin-film technology has been developed with amongst the lowest manufacturing cost of these contenders and confirmed efficiency for small pilot line modules already in the 10–11% energy conversion efficiency range, on the path to 12–13%.     

XPS surface analysis of monocrystalline silicon solar cells for manufacturing control

X-ray photoelectron spectroscopy (XPS) has been applied to surfaces of silicon wafers in the different stages of the assembly line for large-scale monocrystalline silicon solar cell manufacturing (ISOFOTON, Malaga, Spain). XPS results have shown that a considerable amount of carbon is present on the pyramidal-textured monocrystalline silicon surface. This amount decreases slightly but is still present after the process of phosphor diffusion (p-n junction), as well as after subsequent calcination in humid air for SiO₂ film formation (passivation). This amount of carbon may be buried during the process of CVD coating an anti-reflection TiO₂ film. After calcination of the film in order to obtain the TiO₂ rutile phase, an even higher amount of carbon is detected on the TiO₂ anti-reflection coating surface. This indicates that not all organics from the tetra-isopropile ortho-titanate (TPT) precursor were released from the film. Furthermore, in this case phosphor is found in excess on the SiO₂ wafer surface (dead layer) and also on the rutile TiO₂ surface, indicating that an extra phosphor diffusion from the bulk silicon through the TiO₂ film has taken place during calcination. These results demonstrate how thermal treatments applied in the solar cell manufacturing assembly line can influence and may change the intended compositional distribution. These treatments may also introduce defects that act as recombination centres for charge carriers in the solar cell device. Received: 13 September 2000 / Accepted: 10 January 2001 / Published online: 3 May 2001     

Concentration enhancement of current density and diffusion length in III–V ternary compound solar cells

Current density and output power of solar cells, respectively, made from different materials combinations: GaAsp(:Zn)/GaAsP(:Te)/GaAs-CVD GaAsp(:Zn)/GaAs(:Te)/GaAs-CVD GaAlAs(:Zn)/GaAs(:Si)-LPE Silicon,p onn (commercial grade) have been compared at increasing light levels, i.e. solar concentrations from 1 sun to 100 suns. A strongly super-linear increase in output (current density) is found for the ternary compound cells in agreement with earlier measurements. The faster rate of increase of the current with concentration in ternary compounds as compared to silicon can be explained by a trap-filling mechanism at higher injection levels. A Gaussian distribution of compensated donor states can explain the superlinear current increase.     

Second stage concentration with tapers for fluorescent solar collectors

Light concentration of fluorescent sheet collectors can be enhanced by providing the output edges with a taper of a higher refractive index with reflecting surfaces. Concentration ratios achievable by this means are computed for two shapes of tapers, one with a plane boundary towards the collector and one with an additional cylindric lens. The general limitation of concentration ratio is given by Liouville's theorem or Abbe's sine condition. Computed second stage concentration ratios are: 1) For plane boundary taper andn ₁=1.5,n ₂=2 and three reflections in the taper:C=1.49, and 2) for the lens-taper combination under the same conditions:C=1.76, nearly reaching the Liouville limit, 1.79.     

Sulphur-doped silicon background-limited infrared photodetectors near 77 K

The photoconductive properties of sulphur-doped extrinsic silicon infrared detectors prepared by closed-tube diffusion techniques have been investigated. Spectral response data show that this material would be suitable for thermal imaging of 3–5 μm radiation, whilst detectivity measurements as a function of temperature indicate that background-limited operation is achievable near liquid nitrogen temperature (77 K).     

Fluorescent solar energy collectors: Operating conditions with diffuse light

The fluorescent energy conversion principle using several sheets of transparent material doped with fluorescent molecules to concentrate radiation is extended to include diffuse radiation. Two cases are treated here: diffuse radiation only and a composite spectrum consisting of 40% direct and 60% diffuse radiation simulating the average illumination of a flat exposure in central Europe. In both cases photovoltaic conversion efficiency is significantly higher than with the AM1 spectrum. This is due to the blue shift and narrow shape of the diffuse spectral distributions. With realistic boundary conditions the theoretical conversion efficiency is 1.56 times higher than for the AM1 case. The highest theoretical conversion efficiency is now 38%.     

Optical management in high‐efficiency thin‐film silicon micromorph solar cells with a silicon oxide based intermediate reflector

In the effort to increase the stable efficiency of thin film silicon micromorph solar cells, a silicon oxide based intermediate reflector (SOIR) layer is deposited in situ between the component cells of the tandem device. The effectiveness of the SOIR layer in increasing the photo‐carrier generation in the a‐Si:H top absorber is compared for p–i–n devices deposited on different rough, highly transparent, front ZnO layers. High haze and low doping level for the front ZnO strongly enhance the current density (J_sc) in the μc‐Si:H bottom cell whereas J_sc in the top cell is influenced by the angular distribution of the transmitted light and by the reflectivity of the SOIR related to different surface roughness. A total J_sc of 26.8 mA/cm² and an initial conversion efficiency of 12.6% are achieved for 1.2 cm² cells with top and bottom cell thicknesses of 300 nm and 3 μm, and without any anti‐reflective coating on the glass. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Epilift technique for Si solar cells

We present an overview of the Epilift technique, which allows the fabrication of single-crystal silicon films, suitable for photovoltaic purposes. Epitaxial layers are grown by liquid phase epitaxy on partially masked, single-crystal silicon substrates. The layers are detached from the substrate by selective chemical or electrochemical etching, allowing the substrate to be re-used. Epilayers grown on (100) substrates display highly textured surfaces as well as narrow overgrowth widths of the epitaxial layer over the oxide, making them particularly suitable for photovoltaic devices. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 1 July 1999