Light trapping and optical losses in microcrystalline silicon pin solar cells deposited on surface-textured glass/ZnO substrates

doi:10.1016/j.solmat.2004.02.020

Solar Energy Materials and Solar Cells

Volume 85, Issue 1, 1 January 2005, Pages 1-11

https://doi.org/10.1016/j.solmat.2004.02.020 Get rights and content

Abstract

Influence of front TCO thickness, surface texture and different back reflectors on short-circuit current density and fill factor of thin film silicon solar cells were investigated. For the front TCO studies, we used ZnO layers of different thickness and applied wet chemical etching in diluted HCl. This approach allowed us to adjust ZnO texture and thickness almost independently. Additionally, we used optical modeling to calculate optical absorption losses in every layer. Results show that texture and thickness reduction of front ZnO increase quantum efficiency over the whole spectral range. The major gain is in the red/IR region. However, the higher sheet resistance of the thin ZnO causes a reduction in fill factor. In the back reflector studies, we compared four different back reflectors: ZnO/Ag, Ag, ZnO/Al and Al. ZnO/Ag yielded the best, Al the worst light trapping properties. Furthermore, the Ag back contact turned out to be superior to ZnO/Al for microcrystalline cells. Finally, the smooth ZnO/Ag back contact showed a higher reflectivity than the rough one. We prepared pin cells with rough and smooth ZnO/Ag interface, leaving the roughness of all other interfaces unchanged.

Introduction

In microcrystalline silicon (μc-Si) thin film solar cells one desires to minimize the i-layer thickness to reduce deposition time and costs. Consequently, due to the low-absorption coefficient of μc-Si for long wavelength light, light trapping is extremely important. In the pin cell configuration, efficient light trapping is obtained by combining textured transparent conductive oxide (TCO) films as front contact with highly reflective TCO/metal back reflectors (BR). However, due to the multiple passes of scattered light within the solar cell, light absorption is also enhanced in photovoltaically non-active layers like front TCO and TCO/metal back contact. Therefore, spectral response in the infra-red (IR) region will be sensitive to the TCO/metal reflectivity and the front TCO absorption coefficient. On the other hand, the required low sheet resistance of the TCO calls for thicker or more conductive front TCO layers, both usually increase absorption losses. Hence, front TCO type and thickness must be optimized with respect to the competing requirements of good electrical and optical properties. This paper shows the influence of front TCO thickness, texture and back reflector on short-circuit current density j_SC and fill factor FF of μc-Si pin solar cells. Such a study was made possible by the use of very homogeneous magnetron sputtered ZnO layers, whose thickness and texture can be adjusted almost independently of the bulk material properties. Additionally to this experimental study, we calculated optical absorption losses in all layers within the solar cell. We use our optical model to study four characteristic solar cells deposited on different glass/ZnO substrates.

Section snippets

Experimental

The ZnO films applied as front contacts were prepared by Applied Films on 30×40 cm² glass substrates in an in-line DC-magnetron sputtering system from ceramic targets [1]. These highly conductive and transparent layers (Fig. 1) are very homogeneous. Some of these initially smooth films of various thickness (between 500 and 1500 nm) were texture-etched in diluted (0.5%) HCl yielding surfaces with different roughness and light scattering properties [2], [3]. The etching rates were between 2 and 5

Front TCO study

To demonstrate the etching (texturing) effect, we prepared two cells with ZnO/Ag BR. For these cells the “standard” ZnO/Ag BR with ZnO thickness of ≈100 nm was applied. One cell was deposited on the smooth 500 nm ZnO substrate without etching (Fig. 2a). The second one was prepared on another piece of this ZnO/glass substrate which was etched for 30 s to obtain a textured surface (Fig. 2b). Here the resulting ZnO thickness was 330 nm. Fig. 3 shows the QE of these two cells as well as the (1-R)

Conclusions

Texture-etching of the front ZnO increases quantum efficiency over the whole spectral range. This is partly due to an anti-reflection effect in the blue/UV wavelength region and mostly due to an enhanced light trapping in the red/IR. On other hand better light trapping increases optical absorption in all layers within solar cell. A further improvement in short-circuit current density can be easily achieved by using a thinner front ZnO, which reduces free carrier absorption losses in the red/IR

Acknowledgements

The authors thank M. Ruske from Applied Films GmbH, Alzenau, Germany for supplying glass/ZnO substrates, J. Frystacky from Optics department, IP Prague for polishing of ZnO, W. Appenzeller and G. Schöpe from IPV FZ Jülich for extensive technical assistance and S. Michel from IPV FZ Jülich for the evaporation.

This work was supported by the Bundesministerium für Wirtschaft (BMWi) under contract no. 0329854A and by the European Commission (contract: ENK6-CT-2000-00321).

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