Optimization of amorphous silicon thin film solar cells for flexible photovoltaics

We investigate amorphous silicon (a-Si:H) thin film solar cells in the n-i-p or substrate configuration that allows the use of nontransparent and flexible substrates such as metal or plastic foils such as polyethylene- naphtalate (PEN). A substrate texture is used to scatter the light at each interface, which increases the light trapping in the active layer. In the first part, we investigate the relationship between the substrate morphology and the short circuit current, which can be increased by 20% compared to the case of flat substrate. In the second part, we investigate cell designs that avoid open-circuit voltage (Voc) and fill factor (FF) losses that are often observed on textured substrates. We introduce an amorphous silicon carbide n -layer (n-SiC), a buffer layer at the n/i interface, and show that the new cell design yields high Voc and FF on both flat and textured substrates. Furthermore, we investigate the relation between voids or nanocrack formations in the intrinsic layer and the textured substrate. It reveals that the initial growth of the amorphous layer is affected by the doped layer which itself is influenced by the textured substrate. Finally, the beneficial effect of our optical and electrical findings is used to fabricate a-Si:H solar cell on PEN substrate with an initial efficiency of 8.8% for an i -layer thickness of 270 nm. © 2008 American Institute of Physics

Development of micromorph tandem solar cells on flexible low cost plastic substrates

We report on the development of fully flexible micromorph tandem solar cells directly on low cost substrates like PET and PEN. The cells are deposited in nip/nip configuration on the plastic substrate coated with a highly reflecting Ag-ZnO back contact. Light trapping is achieved by combining a periodically textured substrate and a diffusing ZnO front contact. Single junction microcrocystalline cell with a stable efficiency of 8.6 % and 8.4 % are achieved with i-layer thickness of 1.8 and 1.2µm respectively. In tandem devices we obtain an efficiency of 10.9 % (initial) with an open circuit voltage of 1.35 V and a FF of 71.5%. These cells are slightly top limited with 11.26 and 11.46 mA/cm2 in the amorphous (270 nm thick) and the microcrystalline sub-cells (1.2 µm thick), respectively. We introduce an intermediate reflector between the bottom and the top cell because it allows increasing the top cell current without compromising stability by a thicker absorber. We present results on ex-situ ZnO and a new material, a low refractive index P-doped silicon-oxygen compound deposited in-situ by plasma process, in the same reactor as the micromorph cell