Abstract
The crystallization of Si thin-film on glass using continuous-wave diode laser is performed. The effect of various processing parameters including laser power density and scanning speed is investigated in respect to microstructure and crystallographic orientation. Optimal laser power as per scanning speed is required in order to completely melt the entire Si film. When scan speed of 15–100 cm/min is used, large linear grains are formed along the laser scan direction. Laser scan speed over 100 cm/min forms relatively smaller grains that are titled away from the scan direction. Two diode model fitting of Suns -V oc results have shown that solar cells crystallized with scan speed over 100 cm/min are limited by grain boundary recombination (n = 2). EBSD micrograph shows that the most dominant misorientation angle is 60°. Also, there were regions containing high density of twin boundaries up to ~1.2 × 10-8 /cm2 . SiOx capping layer is found to be effective for reducing the required laser power density, as well as changing preferred orientation of the film from ⟨ 110 ⟩ to ⟨ 100 ⟩ in surface normal direction. Cracks are always formed during the crystallization process and found to be reducing solar cell performance significantly.
Highlights
Polycrystalline silicon thin-film solar cells on glass are strong candidate for generation photovoltaic technology as it combines advantages of both wafer Si solar cells technology and thin-film solar cells technology
Commercial polycrystalline silicon thin-film solar cells have been fabricated by CSG Solar which achieved photovoltaic conversion efficiency of 10.4% in 2007 [1]
High density of intragrain defects are generated during the solid phase crystallization (SPC) process and it was found to be lifetime limiting recombination pathway which greatly limit the open-circuit voltage [2]
Summary
Polycrystalline silicon thin-film solar cells on glass are strong candidate for generation photovoltaic technology as it combines advantages of both wafer Si solar cells technology and thin-film solar cells technology. Commercial polycrystalline silicon thin-film solar cells have been fabricated by CSG Solar which achieved photovoltaic conversion efficiency of 10.4% in 2007 [1] In this solar cell, 2 μm thick a-Si thin-films on borosilicate glass were crystallized using solid phase crystallization (SPC) process which produced grain sizes in the range of 1–2 μm. Alternate to SPC, it was reported that continuouswave (CW) diode laser crystallization of Si thin-film on glass can form defect-free grains with very large grains size up to few tenths of millimeters in length [3] Performance potential of this solar cells has shown that photovoltaic conversion efficiency above 13% can be achieved with a diffused homo-junction emitter [4]. Cracks in the film are discussed and their influence on solar cell performance is evaluated
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