Rahul Dewan - Jacobs University

8. SUMMARY AND OUTLOOK
to half of the grating period.
In the next chapter the solar cell texture was modified to include triangular texture
at its interfaces. A larger domain of period of the surface texture was also sweeped,
where the optics changed from the domain of effective refractive index region to geometrical
optics region. Due to better incoupling in the effective refractive index region,
the solar cells with periods smaller than 200 nm showed improved response in the
blue part of the spectrum (wavelength 300-500 nm). And enhanced red and infrared
current (wavelength 700-1100 nm) was achieved when the triangular texture was in
the range of the incident longer wavelengths. The solar cell with triangular texture
showed enhanced absorption when the opening angle of the texture was equal or close
to 90 ◦ .
Taking advantage of the fact that this study looked at periodic surface texture, the
optical simulations were also performed using rigorous coupled wave analysis (RCWA).
By making some simple assumptions, this method provided a much faster computation
for the two-dimensional structures. Both the line grating and triangular texture were
investigated and compared to the short circuit current of 13 mA/cm 2 for a solar cell
on a smooth substrate, the short circuit current was increased by around 60% for the
textured solar cells. The approach based on RCWA was further utilized to analyze line
scans of randomly textured zinc-oxide substrates.
In the final chapter of the results, solar cells deposited on three-dimensional pyramids
were simulated. Along with the period and height of the square based pyramids,
the thickness of the absorber layer was also varied. Compared to the two-dimensional
surface textured solar cells, the maximum short circuit current increased by almost
2 mA/cm 2 for pyramid textured solar cells. This enhancement occurs due to the additional
diffraction plane arising from a three-dimensional texture. By adapting the
Yablonovitch limit for the simulated model system, the maximum short circuit current
of the solar cells was calculated from an optically optimized perspective. The calculated
quantum efficiencies were compared with the maximum possible absorption in
the solar cells and the dominant optical losses for thin (500 nm) and thick (2500 nm)
solar cells were identified. As the solar cells gets thinner, reduction of reflection losses
become more crucial where the incident light should ideally be traveling along the
planes of the solar cell interfaces after being diffracted at the front or back grating
structure. With regards to reducing parasitic absorption losses, the idea of utilizing
textured glass substrate and double textured transparent conductive oxide layer were
integrated into the solar cell. It was shown that these techniques reduce the parasitic
losses in the front transparent conductive oxide and the absorberd energy from that
layer can be transferred to the i-layer of the silicon diode.
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