Transparent, Fluorophore-Doped Cellulose Nanocrystal Films Prepared from Crop Residue: Superior Radiative Cooler and Organic Photodetector

Carbon-neutral nanomaterials derived from lignocellulosic biomass remain the most preferred choice for lowering down the fossil-fuel-based energy consumption for indoor cooling and metal mining purposes. In this work, the synthetic methodologies for yielding a transparent, conducting cellulose nanocrystal (CNC) film from a highly abundant crop residue, rice straw (Oryza sp. Stems), have been presented. The self-assembly of CNC generates a predominant structural green color. Even the encapsulation of organic fluorophores in CNC films retains a chiral nematic order and a photonic band gap. The highly crystalline nature of the hydroxyl groups present in CNC regulates the excited-state dynamics of Rhodamine B encapsulated in the film. Nevertheless, the transreflective property of the CNC film has been demonstrated owing to its low solar light absorption and high mid-infrared (MIR) emissivity at the atmospheric transmission window (8–13 μm). The CNC film can bring down the temperature of the insulated box by ∼6 °C kept under solar simulator illumination, suggesting an effective radiative cooler. Further, the amine functionalization of CNC has resulted in a remarkable increase in current and mobility, improving the film's conductivity by several folds. The photoresponsivity of the RhB-encapsulated-amine-functionalized CNC photodetector shows a peak response of ∼6.3 mA/W at ∼665 nm under zero bias. The external quantum efficiency of fabricated devices is about 1%. The CNC derived from the crop residue has multidimensional applications ranging from passive cooling to organic electronics.