Effect of c-Si 1-x Ge x Thickness Grown by LPCVD on the Performance of Thin-Film a-Si/c-Si 1-x Ge x /c-Si Heterojunction
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Effect of c-Si1-xGex Thickness Grown by LPCVD on the Performance of Thin-Film a-Si/c-Si1-xGex/c-Si Heterojunction Solar Cells Sabina Abdul Hadi1, Pouya Hashemi2, Nicole DiLello2, Ammar Nayfeh1, and Judy L. Hoyt2 1
Microsystems Engineering, Masdar Institute of Science and Technology, P.O. Box 54224, Abu Dhabi, UAE 2
Microsystems Technology Laboratories, MIT, Cambridge, MA 02139, USA
ABSTRACT In this paper the effect of Si1-xGex absorber layer thickness on thin film aSi:H/crystalline-Si1-xGex/c-Si heterojunction solar cells (HIT cells) is studied by simulation and experiment. Cells with 1, 2 and 4 µm-thick epitaxial cap layers of p-type Si0.59Ge0.41 on top of 5 µm Si1-xGex graded buffer layers are fabricated and compared to study the effect of the absorber layer thickness. The results show no change in Voc (0.41V) and that Jsc increases from 17.2 to 18.1 mA/cm2 when the Si0.59Ge0.41 absorber layer thickness is increased from 1 to 4 µm. The effect of thickness on Jsc is also observed for 2 and 4 µm-thick Si and Si0.75Ge0.25 absorber layers. Experiments and simulations show that larger Ge fractions result in a higher magnitude and smaller thickness dependence of Jsc, due to the larger absorption coefficient that increases optical carrier generation in the near surface region for larger Ge contents. INTRODUCTION New materials are being explored to improve the efficiency of solar cells. Incorporating a semiconductor with a smaller bandgap, such as Si1-xGex, to absorb more of the solar spectrum, is one way to increase output current and improve efficiency, especially in thin layers [1]. In addition, Si1-xGex epitaxial layers can be grown at lower temperatures compared to epitaxial Si layers, while maintaining a high crystalline quality. However, using Si1-xGex has two challenges: first the reduction in open circuit voltage (Voc) due to increased dark current from the smaller bandgap, and second the formation of dislocations due to the lattice mismatch with Si. The introduction of a narrower band-gap material can be implemented in either a single junction or tandem cell design [2,3]. Ge/SiGe is an attractive material for multi junction cells when combined with high bandgap III-V material to preserve Voc while maximizing the current and efficiency. In order to maximize the benefits of Si1-xGex in a single junction thin film PV system, a design should allow the p-n junction to reside in the Si1-xGex layer, while maintaining a high Voc. One such design is a heterojunction emitter based solar cell (HIT), in which a large bandgap (1.7 eV) amorphous Si (a-Si) emitter is used to increase Voc. The HIT cell design has been used to fabricate thin film crystalline Si (c-Si) cells [2,3]. Alberi et al. fabricated and simulated a-Si (p)/cSi (n-)/c-Si (n+) solar cells, studying the material quality requirements [4]. This HIT cell design can be used in Si1-xGex based cells to potentially increase the Voc and improve efficiency. Earlier we showed by simulation that SiGe based cells can outperform Si based cells in terms of efficiency for
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