Hydrophilic Ligand Exchange Induces Improved Upconversion Nanoparticle Incorporation in Dye-Sensitized Solar Cells with Efficiency Exceeding 8% in a Low-Temperature Fabrication Process

We synthesized core–shell upconversion nanoparticles (UCNPs) with a composition of LiYF 4:Yb 0.2 /Er 0.02 /Ho 0.02 /Tm 0.02 @LiYF 4:Yb 0.2 and incorporated them into the photoanodes of dye-sensitized solar cells (DSSCs) fabricated through a low-temperature process. The as-prepared hydrophobic UCNPs displayed a good protection ability to prevent the ingress of electrolyte but caused cracking in the mesoporous layers. To resolve this issue, we modified the as-prepared UCNPs by performing ligand exchange using 4-aminobenzoic acid (4ABA), transforming the hydrophobic surface into a hydrophilic one. By design of a three-layer structure to prevent electrolyte ingress, the hydrophilic nature of the 4ABA-modified UCNPs enabled better incorporation into the photoanodes. Consequently, the incorporation of 4ABA-UCNPs significantly improved the power conversion efficiency of DSSCs from 6.32 to 8.22%. This enhancement surpassed most reported values in the literature for DSSCs fabricated using a low-temperature process. Importantly, compared to other hydrophilic ligands, the use of 4ABA did not noticeably increase the charge transfer resistance due to its appropriate molecular weight.