Structural color-tunable mesoporous bragg stack layers based on graft copolymer self-assembly for high-efficiency solid-state dye-sensitized solar cells

We present a facile fabrication route for structural color-tunable mesoporous Bragg stack (BS) layers based on the self-assembly of a cost-effective graft copolymer. The mesoporous BS layers are prepared through the alternating deposition of organized mesoporous-TiO₂ (OM-TiO₂) and -SiO₂ (OM-SiO₂) films on the non-conducting side of the counter electrode in dye-sensitized solar cells (DSSCs). The OM layers with controlled porosity, pore size, and refractive index are templated with amphiphilic graft copolymers consisting of poly(vinyl chloride) backbones and poly(oxyethylene methacrylate) side chains, i.e., PVC-g-POEM. The morphology and properties of the structural color-tunable mesoporous BS-functionalized electrodes are characterized using energy filtered transmission electron microscopy (EF-TEM), field emission-scanning electron microscopy (FE-SEM), spectroscopic ellipsometry, and reflectance spectroscopy. The solid-state DSSCs (ssDSSCs) based on a structural color-tunable mesoporous BS counter electrode with a single-component solid electrolyte show an energy conversion efficiency (η) of 7.1%, which is much greater than that of conventional nanocrystalline TiO₂-based cells and one of the highest values for N719 dye-based ssDSSCs. The enhancement of η is due to the enhancement of current density (J_sc), attributed to the improved light harvesting properties without considerable decrease in fill factor (FF) or open-circuit voltage (V_oc), as confirmed by incident photon-to-electron conversion efficiency (IPCE) and electrochemical impedance spectroscopy (EIS).