A shape- and morphology-controlled metal organic framework template for high-efficiency solid-state dye-sensitized solar cells

Abstract

This report provides a facile process to produce shape- and morphology-controlled MIL-125(Ti), a subclass of metal organic frameworks (MOFs) using poly(ethylene glycol) diglycidyl ether (PEGDGE) as a structure directing agent. Upon deliberate calcination, MIL-125(Ti) is converted to mesoporous hierarchical TiO₂ (hier-TiO₂) with an anatase phase, a large surface area and a variety of nanostructures. The morphology changes from 200 nm circular plates to 1 μm bipyramids with increasing PEGDGE amount, indicating the pivotal role of PEGDGE as a shape controller. When the hier-TiO₂ is deposited onto a nanocrystalline TiO₂ (nc-TiO₂) layer as the scattering layer, the dye-sensitized solar cell (DSSC) with a quasi-solid-state polymer electrolyte records a high conversion efficiency (7.1% at 100 mW cm⁻²), which is much higher than that of DSSCs with a nc-TiO₂ layer only (4.6%) or with commercial scattering TiO₂ (cs-TiO₂) on a nc-TiO₂ layer (5.0%). A solid-state DSSC using a single component solid polymer, i.e., poly((1-(4-ethenylphenyl)methyl)-3-butyl-imidazolium iodide) (PEBII), also exhibits an excellent efficiency of up to 8.0%. The improved efficiency results from the pivotal role of the hier-TiO₂ in improving the surface area and light harvesting properties, as demonstrated by N₂ adsorption/desorption isotherm, reflectance spectroscopy, incident photon-to-current efficiency (IPCE), and electrochemical impedance spectroscopy (EIS) analyses.