Development of micromorph tandem solar 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 A-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 mu m. In tandem devices we obtain an efficiency of 10.9% (initial) with an open circuit voltage of 1.35 V and a fill factor (FF) of 71.5%. These cells are slightly top limited with 11.26 and 11.46 mA/cm(2) in the amorphous (270 nm thick) and the microcrystalline (1.2 mu m 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. (C) 2008 Elsevier B.V. All rights reserved.