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 [1]. 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 [2]. 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 [3]. 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 [4]. With textured surfaces, the incoming sunlight (or specifically photons) will not be bounced-off the surface straight away due to reflection effects as what would happen to planar or untextured surfaces. Instead, the photons will be made bouncing back and forth inside the absorbing layer multiple of times hence increasing the chance for the high energy photons to generate electron-hole pairs, increasing the output current and efficiency of the solar cells [5].