Investigation of Efficiency Improvement on Silicon Solar Cells Due to Porous Layers
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ABSTRACT Observation of light emission from porous Si has demonstrated that the optical properties of Si can be drastically altered by the quantum size effects. We have investigated the improvement of absorption properties of Si material by forming a porous Si layer. Shallow-junction commercial crystalline as well as polycrystalline Si solar cells without anti-reflective coatings have been processed into porous Si solar cells by a wet chemical etching technique. Our best results have demonstrated more than 15% improvement in short-circuit current with no change in open-circuit voltage. The performance of the porous Si solar cells has been found to be sensitive to the porous layer thickness. The efficiency can be reduced when the porous layer is relatively deep, presumably due to the penetration of pores through the shallow junction. We believe porous Si can be optimized for photovoltaic applications by properly controlling its porosity and thickness.
I. INTRODUCTION Since the first demonstration of visible photoluminescence (PL) from porous Si by Canham [1], there has been a great deal of activity on this subject, including the demonstration of visible electroluminescence [2-6]. Even though the origin of the highly efficient visible PL has not yet been clarified, recent studies indicate that the quantum effect is likely to be the origin of light emission from porous Si [7-9]. Recently, a highly sensitive porous Si based photodetector has also been demonstrated [10]. In this study we investigate the possibility of employing porous Si for photovoltaic applications. Silicon with a bandgap of 1.12eV, is not optimal as a material for the solar cell application. Materials such as GaAs and CdTe with bandgaps nearer to 1.5eV, which is the peak in the solar spectrum through the atmosphere, have higher theoretical and practical efficiencies. However, they have reduced cost competitiveness against conventional power generation systems because these rIl-V and Il-VI compounds are expensive, toxic, complex to manufacture and often rely on hazardous deposition processes, as compared to silicon. Fortunately, porous Si structures increase the bandgap from that of bulk silicon due to the quantum confinement effect.
593 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
For simplicity, we can use a quantum box with infinite potential barrier to model this material system. The energy levels for a particle of effective mass m* confined to a three-dimentional potential box of dimension lxlxl is given by 2
(n +n2+n )h 2-n3
8m *12
where h is Planck's constant and n1 , n2 , and n3 are the quantum numbers. From this equation it can be easily deduced that for dimensions of 50 to 10,A and a particle effective mass of 0.26me (for Si), the ground state energy level varies from 0.174 to 4.35 eV. It is therefore expected that porous Si films with broad Si particle size distribution can absorb a wide range of the radiation spectrum. We have investigated the performance improvement of solar cells due to the formation of a p
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