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15 May 2014 Direct pulsed laser interference texturing for light trapping in a-Si:H/μc-Si:H tandem solar cells
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We present results on direct pulsed laser interference texturing for the fabrication of diffraction gratings in ZnO:Al layers. Micro gratings of 20 micron diameter with a groove period of 860 nm have been written using single pulses of a 355 nm picosecond laser using a home-built two-beam interference setup. The groove depth depends on the local laser intensity, and reaches up to 120 nm. At too high pulse energies, the grooves vanish due to surface melting of the ZnO. The fast scanning stage and the high repetition rate laser of a laser scribe system have been used to write grating textures of several cm2 in ZnO:Al films with a surface coverage of about 80%. A typical laser written grating texture in a ZnO:Al film showed a haze value of about 9% at 700nm. The total transmission of the film was not lowered compared to the film before texturing, while the sheet resistance increased moderately by 15%. A-Si:H/μc-Si:H solar cells with laser textured ZnO:Al front contact layers so far reach an efficiency of 10% and current densities of 11.0 mA/cm2, and 11.2 mA/cm2 for top and bottom cell, respectively. This is an increase of 16% for the bottom cell current as compared to reference cells on planar ZnO:Al. The voltage of the laser textured cells is not reduced compared to the reference cell when slightly overlapping laser pulses of reduced pulse energy are applied. This method allows to write textures in ZnO:Al films that e.g. have been deposited with strongly varying deposition conditions, or cannot be texture etched in HCl. The method can be improved further by using 2D periodic patterns and optimizing the groove pitch, and may be applicable also to other solar cell technologies.
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S. Ring, S. Neubert, F. Ruske, B. Stannowski, F. Fink, and R. Schlatmann "Direct pulsed laser interference texturing for light trapping in a-Si:H/μc-Si:H tandem solar cells", Proc. SPIE 9140, Photonics for Solar Energy Systems V, 914015 (15 May 2014);

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