Open-circuit voltage optimisation of amorphous silicon thin film solar cells for flexible photovoltaics
Amorphous silicon (a-Si:H) solar cells can be deposited in the substrate or n-i-p configuration, which allows the use of flexible substrates such as plastic sheets or metallic foils. The authors describe processes compatible with poly-ethylene-naphtalate (PEN) plastic foils and focus on a cell design that avoids Voc losses on textured substrates. They introduce an amorphous carbide n layer (n-SiC) with a high band gap and show that this layer yields high open circuit voltage (Voc) independent of the substrate micro-texture. Initial efficiencies of 9 % on glass coated with hot silver and 8.8 % on plastic substrate are achieved. Finally, the beneficial effect of the improved Voc of the a-Si:H is incorporated into micromorph tandem cells fabricated on flexible substrates. The experiments with a n-SiC layer in the a-Si:H top cells result in the fabrication of micromorph tandem cells with initial efficiency of 10.9 % on a low-cost PEN substrate, for an a-Si:H top cell thickness of 270 nm.