MANUFACTURINGLASERS
Solar cell processing
Solar cells are classified into three generations: Generation I solar cells are made of crystalline silicon where the biggest cost factor is the silicon itself. Commercial solar cells have an efficiency of around 15-20% [2]. Currently, generation I devices account for more than 90% of the market and production in the solar industry. Generation II solar cells have been developed to address energy requirements and production costs of solar cells. Generation II devices are based on thin active layers deposited on a supporting substrate such as glass, ceramics, or foil. Based on the thin layer, there are three successful types of thin-film solar cells: cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon. Typical thin-film cells have efficiencies around 10% for CdTe, 12% for CIGS and 6% for thin-film silicon [3]. Thin-film solar cells account for about 10% of the total market share, with their importance increasing fast. Generation III devices include multi-junction solar cells typically based on Ge, GaAs and GaInP layers. Efficiencies up to 40% [4] have been demonstrated but this comes at the expense of a considerable cost premium. Recently thin film organic cells as well as die sensitized cells have also received some attention [4]. A great engineering effort is underway to lower the cost per kWh of electricity generated by solar cells to
reach cost parity with conventional fossil fuels. In order to achieve this objective, researchers are investigating new cell manufacturing techniques to boost efficiency and lower manufacturing costs. Laser systems and novel optical configurations play a vital role in this endeavor.
Laser for silicon solar cells (generation I) For crystalline silicon (c-Si) solar cells, there are a number of applications where lasers improve the process quality while also improving throughput. Using lasers with pulse durations below 30 ns minimizes thermal affectation as compared to other long-pulse or continuous wave industrial lasers. Ultraviolet (UV) radiation typically leads to increased absorption in the material as compared to longer wavelengths, which permits higher- resolution processing. The choice of laser and wavelength is application dependant and related to the light absorption behaviour of the material and to the geometry required.
Laser Edge Isolation
There are different methods for performing edge isolation on silicon solar cells, for example plasma barrel etching, dry etching, and dispensing of etching paste. Plasma etching of wafer stacks is quite common, but this method is not inline capable and requires the use of undesired
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www.solar-pv-management.com Issue II 2011
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