Bilayer broadband antireflective coating to achieve planar heterojunction perovskite solar cells with 23.9% efficiency

Although perovskite solar cells (PSCs) have achieved encouraging efficiency, the photon loss at the substrate due to light reflection has not been well addressed. Light management is promising to reduce reflection loss and realize higher power conversion efficiency (PCE) of PSCs. Here, a bilayer antireflective coating (ARC) has been designed and coated onto the backside of the glass substrate of (FAPbI 3 ) x -(MAPbBr 3 ) 1 _ x PSCs to enhance photon harvesting and consequently the device efficiency. The bottom layer of the bilayer ARC is made from a silica polymer and the top layer is made from the mixture of hexamethyldisiloxane-modified mesoporous silica nanoparticles and a fluorinated silica polymer. By adjusting the refractive index and the film thickness of each layer according to a two-layer model, enhanced glass trans-mittance in a broadband wavelength range can be reached, with the maximum transmittance increasing from ca. 90% to over 95%. With the bilayer ARC, the maximum short-circuit current density and PCE of (FAPbI 3 ) x (MAPbBr 3 ) 1- x PSCs can be increased from 25.5 mA cm -2 and 22.7% to 26.5 mA cm -2 and 23.9% with negligible changes in fill factor and open-circuit voltage. This work presents a simple yet effective strategy to enhance the efficiency of solar cells employing bilayer antirefective coatings, which can be applied to other types of solar cells.