Free-standing nanopaper electrode for all-printed super-flexible perovskite solar cells

Abstract

The development of all-printed, flexible solar cells of high efficiency and ultra-low weight will offer advancements for new market entrants. Herein, we report the design and fabrication of all-printed in ambient air, super-flexible perovskite solar cells with approaching 20% power conversion efficiency and extremely low weight of 5.1 g m-2 leading to an unprecedented power-to-weight ratio of 38,470 W Kg-1. This performance advance was achieved through the design of a highly transparent and conducting nanopaper used as a free-standing bottom electrode (FSBE). The FSBE consisted of cellulose nanocrystals grafted with semi-metallic super-reduced polyoxometalate clusters that enabled high conductivities up to 18 S cm-1 combined with transparency >96%. It also acted as a conformal barrier preventing performance loss upon heating at 95 oC under continuous illumination in inert environment; and strong resistance to decomposition when immersed in a mild citric acid water solution for 100 days, which we further exploit to demonstrate full device recyclability. The inherent flexibility of cellulose nanocrystals enabled remarkable flexibility of these cells under 2,000 repeated bending and folding cycles and mechanical strength upon extensive strain up to 20%. Notably, the nanopaper remained unaffected for strains up to 60%. These findings open the door for efficient and lightweight solar cells with a low environmental impact.