Enhancement of Sub-Bandgap Absorption in X-Si Solar Cells by Alloying with Germanium

Abstract

A method of incorporating a Germanium alloyed layer within a thin film crystalline silicon solar cell is described. There is an increasing awareness that thin film crystalline silicon is a potential contender for a future cost effective photovoltaic technology. This technology capitalises on the well proven performance advantages of crystalline silicon while at the same time offering much reduced material costs. The weak absorption by silicon of the longer wavelength components of sunlight necessitates effective light trapping for thin film cells to maintain high efficiency levels while the increased proximity of the collecting junction to the cell surfaces requires good surface passivation. Effective light trapping and surface passivation are both well established techniques as demonstrated in the present generation of high efficiency X-Si devices. This study examines a means of extending the potential of thin-film X-Si devices by enhancing sub-bandgap absorption. This is achieved by incorporating a Ge-alloyed region of reduced bandgap within the cell. In this region longer wavelength photons of sub-bandgap energy are absorbed giving the potential for increased cell current. Low temperature liquid phase epitaxy is used for the fabrication of these devices, allowing the growth of alloyed regions of good crystalline quality and only a few microns in thickness.