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Excimer Laser Crystallised Thin Film Silicon for Photovoltaics
Renewable energy has gained wider importance with the ever-depleting conventional energy sources. Photovoltaics commonly known as solar cells have been an attractive option in this regard, but their high cost has prevented their widespread application. Silicon has been the most widely used photovoltaic material for a few decades. Thin film solar cells have become an attractive alternative for the conventional bulk crystalline solar cells since they are cheaper to produce. However, the thin film format of silicon, Hydrogenated Amorphous Silicon (a-Si:H), has its drawbacks in photovoltaic applications. Their efficiency has been comparatively low and the cells degrade in performance after long exposure to solar radiation.
Therefore, a more crystalline thin film technology has long been sought after to combine the advantages of thin film technology and crystalline silicon. Many technologies have been suggested and investigated for this need. Excimer Laser crystallisation of Hydrogenated Amorphous Silicon promises good potential in this regard
Excimer lasers operate in the ultra violet (UV) range with small pulse duration of the order of 20 -30 ns. They are specifically preferred in the case of a-Si:H due to high UV absorption coefficient of a-Si:H. Almost all the pulse energy from the excimer laser is absorbed by a-Si:H within a few nanometres into the material, without harming the underlying substrate. This enables the use of cheaper substrate materials such as glass. The technology has been investigated extensively for applications in TFT, resulting in a broad knowledge base.
In ATI, a Lambda Physik Krypton Fluoride excimer laser unit is utilised with custom made annealing apparatus for crystallisation of a-Si:H. The unit has a 25 ns pulse with energy up to 1 Joule per pulse.
The project mainly investigates the photoconductive properties of excimer laser crystallised silicon thin films on glass. Simple photovoltaic devices have been fabricated and currently being investigated. Investigations are done on material properties as well as electrical/photoconductive properties. A number of parameters affect the crystallisation process and role of Hydrogen in the resulting nano-crystalline materials has been given greater emphasis.Copyright 2004 - UniS