Development of amorphous silicon solar cell fabricated at low temperature and its performance enhancement using tungsten oxide buffer layer

Abstract

This thesis focuses on amorphous silicon (a-Si) solar cell which is deposited at low temperature around 130°C to use cheap substrate such as polyimide, polyethylene terephthalate, polyethylene naphthalate and polycarbonate. Due to the low doping efficiency of low deposition temperature, a-Si solar cell which is depos-ited at low temperature shows low fill factor and low conversion efficiency. This thesis, also, focuses on per-formance improvement to achieve high conversion efficiency. In order to achieve this object, very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) system was introduced instead of PECVD and solar cell was fabricated by using that PECVD and VHF-PECVD. With VHF-PECVD system, the thinner plasma sheaths and the lower RF-voltage directly related to the sheath potential can be obtained. Due to this, higher electron densities and better SiH4 dissocia-tion in the bulk plasma, as well as increased radical and ion flux onto the growing surface, can be achieved. A consequence of all this is a net increase in the deposition rate and the film quality especially at low tempera-ture. Also, tungsten oxide (WOx) film was developed and solar cell was fabricated by using that film. Using the WOx film as a buffer layer at the interface of n-a-Si/ZnO is a new approach to make amorphous silicon based thin film solar cell because all these methods which improve the solar cell efficiency are concerning the front-side, that is, window-side structures. WOx buffer layer have advantages compared to conventional amorphous silicon (a-Si:H) solar cell which has no WOx buffer layer . Firstly, the optical band gap of the WOx film is 3.35 eV. This wide optical band gap property is very useful for not only a window layer of a-Si solar cells but also a buffer layer of a-Si solar cells. WOx films pro-vides high transparency, which leads to open the way of the window layer and buffer layer. Secondly, the resistance of the materials determines the series r...