Orthogonal colloidal quantum dot inks enable efficient multilayer optoelectronic devices

Seungjin Lee,Oleksandr Voznyy,Edward H Sargent,Laxmi Kishore Sagar,F Pelayo García De Arquer,Se-Woong Baek,Bin Chen,Min-Jae Choi,Joshua Wicks,Amin Morteza Najarian,Sjoerd Hoogland,Geetu Sharma,Margherita Biondi,Maral Vafaie

doi:10.1038/s41467-020-18655-7

Abstract

Surface ligands enable control over the dispersibility of colloidal quantum dots (CQDs) via steric and electrostatic stabilization. Today’s device-grade CQD inks have consistently relied on highly polar solvents: this enables facile single-step deposition of multi-hundred-nanometer-thick CQD films; but it prevents the realization of CQD film stacks made up of CQDs having different compositions, since polar solvents redisperse underlying films. Here we introduce aromatic ligands to achieve process-orthogonal CQD inks, and enable thereby multifunctional multilayer CQD solids. We explore the effect of the anchoring group of the aromatic ligand on the solubility of CQD inks in weakly-polar solvents, and find that a judicious selection of the anchoring group induces a dipole that provides additional CQD-solvent interactions. This enables colloidal stability without relying on bulky insulating ligands. We showcase the benefit of this ink as the hole transport layer in CQD optoelectronics, achieving an external quantum efficiency of 84% at 1210 nm.

Highlights

Surface ligands enable control over the dispersibility of colloidal quantum dots (CQDs) via steric and electrostatic stabilization
We sought to use a shorter BT ligand to synthesize a weakly polar CQD ink without the methyl pending group required in previous works; the BT-exchanged PbS CQDs were substantially insoluble in weakly polar solvents including chloroform, dichloromethane, and chlorobenzene (Supplementary Fig. 1a)
We sought to use benzoic acid (BA) ligands to synthesize CQD inks stabilized in weakly polar solvents (Fig. 1a) toward the ultimate goal of efficient charge transport in final CQD films

Summary

Introduction

Surface ligands enable control over the dispersibility of colloidal quantum dots (CQDs) via steric and electrostatic stabilization. High-quality CQD inks based on short ligands, such as lead halides[25,26,27], acetate salts[26,27], and thiols[3,28], have been demonstrated These works have relied on the use of highly polar solvents. This approach stands in the way of the creation of multifunctional multilayer devices—structures in which the different layers incorporate distinct sizes and composition of quantum dots —since highly polar solvents disperse the prior layers For this reason, today’s highest-performing CQD optoelectronic devices rely on a solid-state ligand exchange step[1,3,25,26,27] to construct the final CQD hole transport layer (HTL) atop the main active layer

Methods

Results

Conclusion