Surface morphology and optical reflection of thermally evaporated thin film al-doped silicon on plastic substrates for solar cells applications

Abstract

Having known the fact that Silicon (Si) costs about 50% of the overall production costs of a crystalline Si (c-Si) PV module, an obvious alternative for cost reduction is to go for a thinner c-Si, in the order of less than 50μm. But, the major drawback of the thin film c-Si solar cell in this range is its relatively low efficiency compared to its bulk (300μm) c-Si counterpart. This is due to the fact that at a lower Si thickness, the optical absorption of the film becomes poorer being an indirect band gap semiconductor, hence needing both photons and phonons to be involved in near-band gap absorption processes. The poor optical absorption will translate into a lower generation of electron-hole pairs by the incoming photons, thus lowering the output current collected at electrodes. In order to solve this issue, the absorption coefficient of the film has to be increased. An effective light-trapping scheme has to be employed where the film surface has to be textured by any of the known methods; alkaline or acidic chemical texturing, laser annealing or by lithography process. This paper investigates the characteristics of surface morphology and optical reflection of different compositions of aluminium-doped thin films Si (Al/Si ratio), deposited by a simple thermal evaporation process onto a cheap plastic substrates (poly ethylene terephthalate, PET). The hole concentration (p-type) of the resulting films has also been verified by Hall effect measurement to give parallel knowledge on the doping level of the films. From this investigation, it can be observed that the thermally evaporated p-type Si produces self-textured surfaces as illustrated by the Atomic Force Microscopy (AFM). The films exhibit the surface roughness Root Mean Square (RMS) in between 9nm to 12nm as Al/Si ratio increases from 0.1 to 0.4, a very good indication of enhanced light trapping capabilities. The optical reflection of each film was then measured by Filmetics F20 Optical Reflectometer. The results show an increase in surface reflection as the Al dopant concentration increases due to an increase in the number of Al crystallites distributed within the film. This indirectly shows that for a thin film Si doped with Al metal, the light trapping effects introduced by the self-textured surfaces (9nm 12nm) as obtained by the thermal evaporation technique are not good enough to overcome the strong reflection effects from the Al crystallites especially at Al/Si ratio of 0.2 or more.