Single-step-fabricated perovskite quantum dot photovoltaic absorbers enabled by surface ligand manipulation

Lead halide perovskite colloidal quantum dots (PQDs) are receiving great interest in emerging photovoltaics because of their excellent photovoltaic properties and the room-temperature processability without a thermal annealing step. Conductive thick PQD absorbers reported to date have been fabricated via multiple-step layer-by-layer deposition based on solid-state ligand exchange; however, this approach requiring a lot of processing time and cost is not suitable for the mass production. Thus, a single-step fabrication approach of conductive thick PQD absorbers should be devised. Herein, we demonstrate that conductive thick CsPbI₃-PQD absorbers can be fabricated via a single-step process based on the surface ligand manipulation and employed in efficient PQD solar cells. We find that the conventional ethyl acetate-based post-treatment significantly removes long-chain ligands of the unexchanged PQDs (UN-PQDs) and cause film delamination of thick UN-PQD solids because of drastic volume shrinkage. Thus, we employ the methyl acetate-based post-treatment using phenethylammonium acetate (PEAOAc) to replace both long-chain oleate and oleylammonium within thick UN-PQD solids with short-chain PEA and OAc ligands without film delamination. To further reduce long-chain ligands within the resultant PQD solids, we also employ the PQDs prepared via a solution-phase ligand exchange (SPLE-PQDs) using the phenethylammonium iodide. Furthermore, we perform various spectroscopic measurements, including Fourier-transform infrared, nuclear magnetic resonance, and X-ray photoelectron spectroscopy, to quantitatively analyze the surface chemistry and ligands of PQDs. Consequently, CsPbI₃-PQD solar cells, fabricated via a single-step process using SPLE-PQDs and PEAOAc post-treatment, show improved power conversion efficiency (13.7%) compared to that of the UN-PQD device (12.1%).