Journal article

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

We report on the development of fully flexible microcrystalline and micromorph tandem solar cells directly on low-cost substrates like poly-ethylen-terephtalate (PET) and poly-ethylen-naphtalate (PEN). The cells are deposited in nip or 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 microcrystalline cells with a stable efficiency of 8.7% are achieved with an i-layer thickness of 1.2 mm. In tandem devices we obtain an efficiency of 10.9% (initial) with an open circuit voltage of 1.35V and a fill factor (FF) of 71.5%. These cells are slightlytoplimitedwith11.26and11.46mA/cm2 in the amorphous (270nmthick) and the microcrystalline (1.2 mm thick) sub- cells, respectively. We introduce an intermediate reflector (IR) between the bottom and the top cell because it allows increasing the top cell current without compromising the stability by a thicker absorber. The IRs consist of either an ex-situ ZnO or a low refractive index P-doped silicon– oxygen compound deposited in-situ with a plasma process that is fully compatible with solar cell processing. We demonstrate significant current improvement (up to 8% relative) using both kinds of IRs.


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