We are developing photovoltaic cells (solar cells) directly fabricated on flexible substrates such as woven or non-woven textiles. This entails the use of wet or dry (vacuum) coating for conducting layers, and plasma enhanced chemical vapour deposition (PECVD) for the photoactive layers of amorphous and nanocrystalline silicon. Subsequently we need to seal the device from ambient conditions: this hermetic cover may be applied by one or more of a variety of procedures, such as a PECVD polymer.
The crucial formation of a thin-film PIN silicon structure requires substrates to withstand a process temperature of ~200oC for some minutes whilst exposed to a plasma-activated gas mixture: we have shown that polyester is one suitable material. This coating process provides a sequence of N-type, I-type (undoped), and P-type silicon, with the first interface (N / I) controlled so as to reduce the doping carried-over into the undoped layer. The undoped layer is the optically-absorbing layer that produces pairs of mobile charge carriers. The two doped layers provide a built-in electric field across the undoped layer that separates these oppositely-charged carriers. The charges are delivered by the conducting layers to the external electrical load, and one of these layers must be transparent to enable light to enter the device (typically a transparent conducting oxide). Our cells use a bi-layer of metal and conducting polymer for the back-contact to the N-type layer, which provides the conductivity required for effective charge collection and a more flexible but less-conducting layer to bridge any micro-breaks in the metal layer. Whilst the sequence of deposition steps and the process parameters have been determined, the layer thicknesses have not yet been optimised. An additional feature that is available from a textile substrate is the enhancement of optical absorption by non-specular scattering from the textured rear surface: this aspect must also be designed and optimised into the complete cell structure.
We have received Scottish Government SMART funding to Power Textiles Limited.
