With a global market share of about 90%, crystalline silicon is by far the most important photovoltaic technology today. This article reviews the dynamic field of crystalline silicon photovoltaics from a device-engineering perspective. First, it discusses key factors responsible for the success of the classic dopant-diffused silicon homojunction solar cell. Next it analyzes two archetypal high-efficiency device architectures – the interdigitated back-contact silicon cell and the silicon heterojunction cell – both of which have demonstrated power conversion efficiencies greater than 25%. Last, it gives an up-to-date summary of promising recent pathways for further efficiency improvements and cost reduction employing novel carrier-selective passivating contact schemes, as well as tandem multi-junction architectures, in particular those that combine silicon absorbers with organic–inorganic perovskite materials.

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High-efficiency crystalline silicon solar cells: status and perspectives

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...

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

Aluminum-induced crystallization of amorphous silicon (a-Si) is studied using various microscopy techniques and x-ray photoelectron spectroscopy. During the isothermal annealing of subsequently deposited aluminum and a-Si films on glass, a layer exchange process is induced, while a continuous polycrystalline silicon film (poly-Si) on glass is formed within the initial metal layer and therefore displaces it. This crystallization process is conducted at temperatures ranging from 350 °C to 500 °C, significantly below the eutectic temperature of the Si–Al binary system of 577 °C. The results presented focus on the influences of the polycrystalline structure of the evaporated Al, the Si–Al layer sequence, and the interface layer between the Al and Si films on the overall crystallization process. They reveal that the larger the Al grain size of the initial polycrystalline Al layer, the larger the grain size of the final poly-Si film and the slower the entire layer exchange process. It is further shown that the layer sequence, although influencing the speed of the poly-Si formation, has little impact on the overall layer exchange process. Additionally, evidence is given that an Al oxide interface layer separates the continuous poly-Si layer from the Al, independent of the original layer sequence. The analyzed oxide interface layer remains at its position throughout the entire Al and Si layer exchange process. An existing phenomenological model of the diffusion-controlled crystallization during the layer exchange is extended to embrace the role of the parameters discussed in this paper.

Influence of interface and Al structure on layer exchange during aluminum-induced crystallization of amorphous silicon

https://openresearch-repository.anu.edu.au/bitstream/1885/40838/3/review1.pdf

A review of thin-film crystalline silicon for solar cell applications. Part 2: Foreign substrates

Polycrystalline silicon thin-film solar cells: Status and perspectives

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.

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

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

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 film's 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.

Oliver Nast

Papers

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

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

Influence of interface and Al structure on layer exchange during aluminum-induced crystallization of amorphous silicon

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

Polycrystalline silicon thin films on glass by aluminum-induced crystallization