Cobalt-doped zinc oxide nanoparticles as selective absorbers for neutral color semitransparent photovoltaics

Controlling the color of semi-transparent components for building-integrated photovoltaics (BIPV) is critical for broad technology acceptance. Luminescent solar concentrators (LSC), also known as “solar windows”, hold immense potential as seamless BIPV elements. Currently many such devices feature inherent brown, yellow or orange tints originating from absorber electronic structure. In this work, we demonstrate a spectral tailoring strategy to achieve neutral-colored, haze-free devices using cobalt-doped zinc oxide nanoparticles (Co:ZnO NPs) as selective absorbers for LSCs. Here, Co:ZnO NPs serve as complementary absorbers to LSC nanophosphors, such as quantum dots (QDs) or metal nanoclusters (NCs). By maximizing Co 2+ doping level in the zinc oxide nanoparticle while preserving selective absorption characteristics via a hot-injection colloidal synthesis, we engineered a strong 500-700 nm absorption band of tetrahedral Co 2+ that precisely offsets the nanophosphor tint yielding a neutral color. This high doping level minimizes the necessitated NP loading in the selected polymer, effectively preventing NP agglomerates and subsequent haze. Ligand engineering with nonylamine further enabled homogeneous dispersion of Co:ZnO NPs within a hydrophobic polymer matrix. Calculations confirm that Co:ZnO NPs effectively neutralize the color of diverse near-infrared-emitting QDs and metal NCs devices, while preserving a desired >50% average visible transmittance. We experimentally validated this concept on a prototype Au-Cu metal nanocluster LSC (5 × 5 cm 2 ), achieving a neutral color transmittance alongside a stable photovoltaic performance with a minimal penalty on photocurrent output (∼10%). By leveraging the tailored absorption and optical clarity of Co:ZnO NP/polymer nanocomposites, this scalable approach resolves a major aesthetic barrier, advancing the practical deployment of semi-transparent BIPV.