Polymeric rheology modifier allows single-step coating of perovskite ink for highly efficient and stable solar cells

Developing environmental friendly solution-processed hybrid perovskite films, with a compelling combination of reproducibility and moisture stability, is a highly desirable prospect to foster hybrid perovskite solar cells uptake in a fiercely competitive high-tech market. Herein, starch biopolymer is exploited as a rheological modifier to tailor the viscosity of perovskite precursor solutions, in order to allow the development of stable inks that can be straightforwardly deposited in a single coating step at mild-temperature, without the use of toxic solvents, to obtain uniform perovskite thin films. Importantly, the as conceived methylammonium lead iodide (MAPbI3) perovskite-starch composite film, integrated in a fully solution processed planar solar cell architecture, featured high power conversion efficiency of 17.2%, which is the highest reported for polymer-perovskite blends, improved moisture stability, more than 800 h stored in humid environment (50%), and resistance to bending stress in flexible devices. Moreover, it is worth to highlight how transmittance and thickness of the composite films can be simply adjusted by varying the perovskite inks viscosity with starch, envisioning the future implementation of such hybrid perovskite composites in printing technology and in different optoelectronic devices.