Flexible NIR-transparent perovskite solar cells for all-thin-film tandem photovoltaic devices

The possibility of growing perovskite solar cells on flexible substrates can be seen as an exciting opportunity, allowing high throughput roll-to-roll manufacturing with a low embodied energy, and creating new applications in buildings, vehicles, portable electronics and internet-of-things based devices. Flexible perovskite solar cells have previously been developed on polymer substrates such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) which are vulnerable to the ingress of moisture. Here we report the development of flexible perovskite solar cells grown on a typical transparent front sheet which is generally used to encapsulate flexible Cu(In,Ga)Se2 (CIGS) solar cells. This type of substrate displays an ultra-low water vapor transmission rate and good UV blocking properties. Perovskite solar cells, grown on such flexible front sheets coated with a highly transparent conducting ZnO:Al (AZO) electrode and vacuum processed ZnO/C60 electron transport multilayer, yield 13.2% and 10.9% stabilized efficiencies for areas of 0.15 cm2 and 1.03 cm2, respectively. The substitution of an opaque rear contact with the transparent electrode enables the realization of flexible NIR-transparent perovskite solar cells with efficiencies above 12%. These devices display an average transmittance of 78% between 800 and 1000 nm and enable the development of 4-terminal polycrystalline all-thin-film flexible perovskite/CIGS tandem devices. In a first proof of concept 18.2% efficiency is obtained.