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Vol. 1153 (2009). 1. Introduction. Thin-film solar cells based on microcrystalline silicon (µc-Si:H) or amorphous silicon (a-Si:H) .... illumination (λ = 400nm) photons get absorbed within the first 200 nm of the silicon absorber layer. .... of 800 nm.
Light trapping in thin-film silicon solar cells with submicron surface texture Rahul Dewan,1 Marko Marinkovic,1 Rodrigo Noriega2, Sujay Phadke2, Alberto Salleo2 and Dietmar Knipp1,* 1

School of Engineering and Science, Electronic Devices and Nanophotonics Laboratory, Jacobs University Bremen, 28759 Bremen, Germany Department of Materials Science and Engineering, Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, USA *[email protected]

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Abstract: The influence of nano textured front contacts on the optical wave propagation within microcrystalline thin-film silicon solar cell was investigated. Periodic triangular gratings were integrated in solar cells and the influence of the profile dimensions on the quantum efficiency and the short circuit current was studied. A Finite Difference Time Domain approach was used to rigorously solve the Maxwell’s equations in two dimensions. By studying the influence of the period and height of the triangular profile, the design of the structures were optimized to achieve higher short circuit currents and quantum efficiencies. Enhancement of the short circuit current in the blue part of the spectrum is achieved for small triangular periods (P 3 µm); the short circuit current drops as well. Light is only diffracted at small diffraction angles, which can be seen by the grating equation

P ⋅ n ⋅ sin (θ m ) = m ⋅ λ

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(C) 2009 OSA

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Received 9 Sep 2009; revised 2 Oct 2009; accepted 3 Oct 2009; published 2 Dec 2009

7 December 2009 / Vol. 17, No. 25 / OPTICS EXPRESS 23064

where P is the period of the grating, n denotes the refractive index of the propagating media after diffraction, m specifies the diffraction order and θ m being the diffraction angle. With an increasing period, the diffraction angle for the integrated triangular grating is reduced. Henceforth the diffracted orders do not interfere with diffracted orders from neighboring unit cells. The constructive interference of the diffracted orders is essential for effectively trapping light inside the absorber layer. Thus the short circuit current for such large texture periods converges towards that of a solar cell on a flat substrate. If the period of the triangular grating is comparable to the incident wavelengths the diffraction angles are large enough to propagate into their neighboring unit cell. Diffracted waves can thus interfere constructively within the thin absorber layer contributing to the higher short circuit current that is observed for periods of 700 nm and 900 nm. The higher current values are obtained for structures with triangular profile heights of 300 nm and above. For the entire spectrum (300 nm – 1100 nm) the short circuit current was calculated to be 12.3 mA/cm2, 15.33 mA/cm2 and 18.9 mA/cm2 for the solar cell on smooth substrate, with triangular structures of period 100 nm and 900 nm, respectively. The latter, where the period is in the range of the incoming wavelengths, has the highest short circuit current because it best utilizes the diffraction and scattering of light. The enhancement comes from light trapping of the longer wavelengths. On the other hand, with a texture period of 100 nm, the small gain in short circuit current is due to its better incoupling of the shorter wavelengths into the cell. 4. Summary

Light trapping and incoupling of light in thin-film microcrystalline silicon solar cells with periodic triangular profiles was investigated. Compared to that of a solar cell on a smooth substrate with 1 µm thick absorber layer, the short circuit current and quantum efficiency are enhanced with the introduction of triangular texturing. The degree of enhancement highly depends on the period of the triangular profile unit cell. When periods of the texture are smaller than the incident wavelength, enhancement in the shorter wavelengths (300 – 500 nm) of the spectrum comes as a result of better incoupling of the light into the absorber layer. For longer wavelengths (700 – 1100 nm) the short circuit current is distinctly enhanced if the period of the triangular profile unit cell is in the range of the incident wavelength. Along with the period, combination of the period and height of the triangular unit cell is also important. Optimal dimensions were found to be for combinations of the triangular profile where the opening angle of the triangle is equal or close to 90°. The total short circuit current for the entire spectrum is increased from 12.3 mA/cm2 by 60% up to 20 mA/cm2 for the best case triangular profile (period of 700 nm and height of 500 nm). Acknowledgements

The authors would like to acknowledge the Institute of Energy Research (IEF-5) Photovoltaics, Research Center Jülich. Specifically, the authors thank C. Haase for providing optical data of the thin films. Furthermore, the authors like to acknowledge H. Stiebig from Malibu Solar for helpful discussions. A. S., R. N. and S. P. acknowledge financial support from the Global Climate Energy Project at Stanford and the Department of Energy Solar America Initiative (grant #DE-FG36-08GO18005).

#116966 - $15.00 USD

(C) 2009 OSA

Received 9 Sep 2009; revised 2 Oct 2009; accepted 3 Oct 2009; published 2 Dec 2009

7 December 2009 / Vol. 17, No. 25 / OPTICS EXPRESS 23065