Purification of Perovskite Quantum Dots Using Low-Dielectric-Constant Washing Solvent “Diglyme” for Highly Efficient Light-Emitting Devices

Abstract

Cesium lead halide (CsPbX, X = Cl, Br, or I) perovskite quantum dots (QDs) are known as ionic nanocrystals, and their optical properties are greatly affected by the washing solvent used during the purification process. Here, we demonstrate the purification process of CsPbBr perovskite QDs using low-dielectric-constant solvents to completely remove impurities, such as the reaction solvent and desorbed ligands. The use of the ether solvent diethylene glycol dimethyl ether (diglyme), having a low dielectric constant of ε = 7.23, as a poor solvent for reprecipitation allowed for multiple wash cycles, which led to high purity and high photoluminescence quantum yield for CsPbBr QDs. The light-emitting device constructed with the CsPbBr QDs and washed twice with diglyme (two-wash) showed a low turn-on voltage of 2.7 V and a peak external quantum efficiency of over 8%. Thus, the purification of perovskite QDs with multiple wash cycles using a low-dielectric-constant solvent is an effective approach for enhancing not only the optical properties but also the efficiency of perovskite quantum dot light-emitting devices.

Lead Halide Perovskite Quantum Dots To Enhance the Power Conversion Efficiency of Organic Solar Cells

“…As observed the characteristic bands of the capping ligands drastically decrease upon washing, being 1,2‐dichloroethane the more effective at stripping the QDs while still maintaining enough ligands to afford sufficient processability in organic solvents. We note that this is not unexpected since high dielectric constant ( ϵ r ) solvents are known to be advantageous when performing the purification of the QDs, and, in fact, ϵ r values for 1,2‐dichloroethane and methyl acetate are 10.45 and 6.7, respectively.…”

Lead Halide Perovskite Quantum Dots To Enhance the Power Conversion Efficiency of Organic Solar Cells

“…As observed the characteristic bands of the capping ligands drastically decrease upon washing, being 1,2-dichloroethane the more effective at stripping the QDs while still maintaining enough ligands to afford sufficient processability in organic solvents. We note that this is not unexpected since high dielectric constant (e r )s olvents are known to be advantageous when performing the purification of the QDs, [35] and, in fact, e r values for 1,2-dichloroethane and methyl acetate are 10.45 and 6.7, respectively. Figure 1c shows the UV/Vis absorption spectra of thin films of the PTB7-Thdonor and PC 71 BM acceptor,and of the colloidal dispersion of CsPbI 3 QDs.U nder these conditions, the overlap between the absorption profiles of the QDs and PC 71 BM is notable,w hile both are complementary to that of PTB7-Th.…”

Lead Halide Perovskite Nanocrystals: Room Temperature Syntheses toward Commercial Viability

“…A general consensus is that the solvent must be aprotic, as protic solvents can promote the extraction of lead ions from the nanocrystal. [43][44][45] The solvent must also be of medium polarity, that is, polar enough to precipitate the NCs easily, but not so polar as to remove bound ligands or degrade the perovskite crystal during short-term exposure. Typically, short esters have been most successful.…”

“…[40][41][42] Furthermore, the one-pot nature of the colloidal synthesis means that the impact of purification processes, which are necessary to remove reaction by-products and excesses, must also be considered. [43][44][45] Beyond the properties of specific ligands, the synthetic methodology employed also influences the commercial attraction of perovskite nanocrystals. Most commonly, high-temperature, air-free "hot-injection" syntheses are employed, which are challenging to scale up.…”