Defect‐Free Block Copolymer‐Based Distributed Bragg Reflector for Optimized Light Harvesting in BiVO ₄ Photoanodes

Abstract Near‐bandgap photon recycling via distributed Bragg reflector (DBR) offers a promising strategy for maximizing light‐harvesting efficiency of photoelectrochemical (PEC) cells with the minimum charge recombination. However, most DBR films require alternating deposition of inorganic layers via high‐vacuum equipment. Herein, all‐solution‐based DBR films, integrated via block copolymer (BCP) self‐assembly, are precisely tailored for various target wavelengths in the visible range. Using interfacial engineering of symmetric‐confinement, lamella‐forming polystyrene‐ b ‐poly(2‐vinylpyridine) (PS‐ b ‐P2VP) is self‐assembled into a defect‐free multilayer nanostructure oriented parallel to the substrates. When a weak hydroxy acid, lactic acid (LA), is incorporated into the 2VP units, the P2VP domains are favorably swollen and protonated; this causes the lamellar structures to become more asymmetric, whereas uniaxially sharp interfaces between the lamellar microdomains are retained. The reflected wavelength ( λ m ) for the BCP‐based DBR films is cooperatively tuned by controlling the cross‐linking density in the P2VP domains immediately before the LA incorporation. The narrow‐width, high‐reflectance BCP‐based DBR films are integrated into the rear of the BiVO 4 photoanodes to scrutinize the optimal λ m for the best PEC performance. A remarkable increase in the current density is attributed to the redshift of the effective working wavelengths in the BiVO 4 photoanode during the photon‐recycling process.