Stuart Wenham

Elucidation of the layer exchange mechanism in the formation of polycrystalline silicon by aluminum-induced crystallization

Aluminum-induced crystallization of amorphous silicon is studied as a promising low-temperature alternative to solid-phase and laser crystallization. Its advantages for the formation of polycrystalline silicon on foreign substrates are the possible usage of simple techniques, such as thermal evaporation and dc magnetron sputtering deposition, and relatively short processing times in the range of 1 h. The overall process of the Al and Si layer exchange during annealing at temperatures below the eutectic temperature of 577 °C is investigated by various microscopy techniques. It is shown that the ratio of the Al and a-Si layer thicknesses is vitally important for the formation of continuous polycrystalline silicon films on glass substrates. The grain size of these films is dependent on the annealing temperature and evidence is given that grain sizes of 20 μm and more can be achieved. The poly-Si films are described as solid solutions containing 3×1019 cm−3 Al atoms as solute. Only a fraction of the solute is...

Aluminum-induced crystallization of amorphous silicon on glass substrates above and below the eutectic temperature

The achievement of high-quality continuous polycrystalline silicon (poly-Si) layers onto glass substrates by using aluminum-induced crystallization is reported. The crystallization behavior of dc sputtered amorphous silicon on glass induced by an Al interface layer has been investigated above and below the eutectic temperature of 577 °C. Secondary electron micrographs in combination with energy-dispersive x-ray microanalysis show that annealing below this temperature leads to the juxtaposed Al and Si layers exchanging places. The newly formed poly-Si layer is fully crystallized and of good crystalline quality, according to Raman spectroscopy and transmission electron microscopy investigations. At 500 °C, the time needed to crystallize a 500-nm-thick Si layer is as short as 30 min. By annealing above the eutectic temperatures, layer exchange is not as pronounced and the newly formed Al layer is found to contain a network of crystallized Si.

Silicon solar cells

The key attributes for achieving high-efficiency crystalline silicon solar cells are identified and historical developments leading to their realization discussed. Despite the achievement of laboratory cells with performance approaching the theoretical limit, commercial cell designs need to evolve significantly to realize their potential. In particular, the development of cell structures and processes that facilitate entirely activated device volumes in conjunction with well-passivated metal contacts a nd front and rear surfaces is essential (and yet not overly challenging) to achieve commercial devices of 20% efficiency from solar-grade substrates. The inevitable trend towards thinner substrates will force manufacturers to evolve their designs in this direction or else suffer substantial performance loss. Eventually, a thin-film technology will likely dominate, with thin-film crystalline silicon cells being a serious candidate. Present commercial techniques and processes are in general unsuitable for t hin-film fabrication, with even greater importance placed on the achievement of devices with entirely activated volumes (diffusion lengths much greater than device thicknesses), well-passivated metal contacts and surfaces and the important inclusion of li ght trapping. The recent achievement of 21.5% efficiency on a thin crystalline silicon cell (less than 50 μm thick) adds credibility to the pursuit of crystalline silicon in thin films, with a key attribute of this laboratory cell being its extremely good light trapping that nullifies the long-term criticism of crystalline silicon regarding its poor absorption properties and correspondingly perceived inability to achieve high-performance thin-film devices. For low-cost, low-quality polycrystalline sil icon material, the parallel-multijunction cell structure may provide a mechanism for achieving entirely activated cell volumes with the potential to achieve reasonable efficiencies at low cost over the next decade.

24% efficient perl silicon solar cell: Recent improvements in high efficiency silicon cell research

Abstract Recent research upon high efficiency passivated emitter, rear locally-diffused (PERL) cells has resulted in a considerable improvement in the energy conversion efficiencies of silicon solar cells up to 24.0% under the standard global solar spectrum. Under monochromatic light, energy conversion efficiency of 46.3% for 1.04 μm wavelength light has been measured. These efficiencies are the highest ever reported for a silicon device. This progress has been achieved by a combination of several mechanisms. One is the reduction of recombination at the cell front surface by improved passivation of the silicon/silicon dioxide interface. Resistive losses in the cell have been reduced by a double-plating process which increases the thickness for the coarse cell metallization features. The reflective losses have been reduced by the application of a double layer anti-reflection (DLAR) coating. Another advantage of DLAR coating is that it will give an additional 3% higher current density than SiO 2 single layer anti-reflection (SLAR) coated cells when encapsulated into modules. Earlier modifications in the cell mask design have also contributed to this improvement, and will also be discussed in this paper.

Very high efficiency silicon solar cells-science and technology

Although it has been close to 60 years since the first operational silicon solar cell was demonstrated, the last 15 years have seen large improvements in the technology, with the best confirmed cell efficiency improved by over 50 %. The main drivers have been improved electrical and optical design of the cells. Improvements in the former area include improved passivation of contact and surface regions of the cells and a reduction in the volume of heavily doped material within the cell. Optically, reduced reflection and improved trapping of light within the cell have had a large impact. Such features have increased silicon cell efficiency to a recently confirmed value of 24.7%. Over recent years, good progress has been made in transferring some of the corresponding design improvements into commercial product with commercial cells of 17-18% efficiency now commercially available, record values of a mere 15 years ago. The theory supporting these improvements in bulk cell efficiency shows that thin layers of silicon, only a micron or so in thickness, should be capable of comparably high efficiency.

Aluminium-induced crystallisation of silicon on glass for thin-film solar cells

Aluminium-induced crystallisation of amorphous silicon is studied for the formation of continuous polycrystalline silicon thin-films on low-temperature glass substrates. It is shown to be a promising alternative to laser crystallisation and solid-phase crystallisation. Silicon grain sizes of larger than 10 μm are achieved at temperatures of around 475°C within annealing times as short as 1 h. The Al doping concentration of the poly-Si films depends on the annealing temperature, as revealed by Hall effect measurements. A poly-Si/Al/glass structure presented here can serve as a seeding layer for the epitaxial growth of polycrystalline silicon thin-film solar cells, or possibly as the base material with the back contact incorporated.

Polycrystalline silicon thin films on glass by aluminum-induced crystallization

This work focuses on the development and characterization of device quality thin-film crystalline silicon layers directly onto low-temperature glass. The material requirements and crystallographic quality necessary for high-performance device fabrication are studied and discussed. The processing technique investigated is aluminum-induced crystallization (AIC) of sputtered amorphous silicon on Al-coated glass substrates. Electron and ion beam microscopy are employed to study the crystallization process and the structure of the continuous polycrystalline silicon layer. The formation of this layer is accompanied by the juxtaposed layers of Al and Si films exchanging places during annealing. The grain sizes of the poly-Si material are many times larger than the films thickness. Raman and thin-film X-ray diffraction measurements verify the good crystalline quality of the Si layers. The electrical properties are investigated by temperature dependent Hall effect measurements. They show that the electrical transport is governed by the properties within the crystallites rather than the grain boundaries. The specific advantages of AIC are: (1) its simplicity and industrial relevance, particularly for the processes of sputter deposition and thermal evaporation, (2) it requires only low-temperature processing at 500/spl deg/C, (3) its short processing times, and (4) its ability to produce polycrystalline material with good crystallographic and electrical properties. These advantages make the poly-Si material formed by AIC highly interesting and suitable for subsequent device fabrication such as for poly-Si thin-film solar cells.

Novel parallel multijunction solar cell

A novel parallel multijunction solar cell is described which allows fundamental radiative recombination limits upon photovoltaic cell energy conversion efficiency to be approached more closely in good quality material than possible in a single‐junction device. However, the real advantage arises for poor quality material with the structure shown to be particularly tolerant of both impurity and grain boundary effects. A novel implementation approach is described for potentially high‐performance, low‐cost, thin‐film polycrystalline silicon solar cells.

20 000 PERL silicon cells for the ‘1996 World Solar Challenge’ solar car race

High-efficiency large-area PERL (passivated emitter, rear locally diffused) cells have been produced at the University of New South Wales in a sufficient quantity to supply three cars for the 1996 World Solar Challenge (WSC) solar car race. Almost 20 000 cells were fabricated, with designated illumination area efficiency ranging up to 24%. A flat-plate module made from 50 such cells fabricated early in the production process and having an average efficiency of just over 23% has demonstrated a record efficiency of 22.7%. The energy conversion efficiency of a typical cell fabricated late in the production process was subsequently measured as 23.7% at Sandia National Laboratories under standard test conditions (1 kW m−2, global AM1.5 spectrum at 25°C) based on the designated illumination area of 21.6 cm2. Hondas Dream and Aisin Seikis Aisol III were two vehicles using these PERL cells, and were placed first and third, respectively, in the race. Honda also set a new record by reaching Adelaide in 4 days with an impressive average speed of 90 km h−1 over the 3010-km course.

16.7% efficient, laser textured, buried contact polycrystalline silicon solar cell

In the evolution of single‐crystal silicon solar cells, crystallographic texturing of the cell surface using anisotropic etches was an important step in improving cell performance. Unfortunately, this technique is ineffective for polycrystalline silicon solar cells due to generally unfavorable grain orientation. This limitation has been overcome by employing a new laser texturing technique on the top surface of a laser grooved, buried top contact polycrystalline silicon solar cell. This approach has demonstrated an independently confirmed 16.7% (air mass 1.5, 25 °C) efficiency for a 10.5 cm2 polycrystalline silicon solar cell, the highest efficiency ever reported for a polycrystalline cell of this size.