Smoothening intermediate reflecting layer for tandem thin-film silicon solar cells

We introduce the concept of smoothening intermediate reflecting layers (IRLs) in Micromorph thin-film silicon tandem solar cells in the superstrate configuration. The aim of such structures is not only to provide a selective reflection of light for optimal light management in both sub-cells but also to tune the morphology of the surface on which the microcrystalline silicon bottom cell is grown. This novel type of IRL is shown to enable the use of rough front electrodes with excellent light scattering properties in Micromorph devices without impacting the electrical property of the microcrystalline material. A low-index (1.5), highly transparent and insulating UV-curable lacquer is used as IRL. The electrical contact between both sub-cells is ensured by a partial covering of this lacquer layer: the bottoms of the structure are efficiently filled whereas the tips are unveiled by an etching step. This results in an efficient smoothening of the surface of the top cell, validated by a Voc boost of up to 50mV compared to a standard IRL. A strong top cell current increase is also shown, with up to 2.3mA/cm2 (20% relative) current gain compared to a cell with no IRL. Adjusting the volume of lacquer composing this advanced interlayer enables a fine tuning of its opto-electrical properties. The introduction of a smoothening IRL is shown, with simple calculations, to be a key element towards the obtaining of 13.5% stable-efficiency Micromorph devices using present state-of-the-art single junctions.