  Paper offers a low-cost and widely available substrate for electronics. It
posses alternative characteristics to silicon, as it shows low density and
high-flexibility, together with biodegradability. Solution processable
materials, such as hybrid perovskites, also present light and flexible
features, together with a huge tunability of the material composition with
varying optical properties. In this study, we combine paper substrates with
halide-mixed perovskites for the creation of low-cost and easy-to-fabricate
perovskite-on-paper photodetectors with a broadband-tunable spectral response.
From the bandgap tunability of halide-mixed perovskites we create
photodetectors with a cut-off spectral onset that ranges from the NIR to the
green, by increasing the bromide content on MAPb(I$_{1-x}$Br$_x$)$_3$
perovskite alloys. The devices show a fast and efficient response. The best
performances are observed for the pure I and Br perovskite compositions, with a
maximum responsivity of 376 mA/W on the MAPbBr$_3$ device. This study provides
an example of the wide range of possibilities that the combination of solution
processable materials with paper substrates offer for the development of
low-cost, biodegradable and easy-to-fabricate devices.
