Colloidal quantum dots (CQDs) can be used to build efficient optoelectronic devices because of their tunable light absorption and emission properties. They are compatible with thin-film manufacturing methods. However, using CDQs in inks is more challenging. CDQ inks need to combine colloidal stability with suitable mechanical properties for printing, while retaining the desired optoelectronic properties of the quantum dots.

Edward H. Sargent, University of Toronto, Canada, and colleagues have overcome these challenges and designed QCD-based inks for inkjet-printed infrared (IR) photodetectors. The researchers created the inks using a quantum dot ligand-exchange process. They first synthesized oleic-acid-capped CQDs and then replaced the oleic-acid ligands with smaller, less insulating ligands such as I₂. The team also varied the solvents and evaluated the resulting inks in terms of their colloidal stability, jetting conditions, and resulting film morphology to find the most suitable candidate for inkjet printing onto an active photodetector surface.

The most effective CQD inks were those composed of PbS QDs in a N-methyl-2-pyrrolidone (NMP) solvent with the addition of 1 wt% n-butylamine (BTA). This low concentration of BTA helps to make the inks colloidally stable and simultaneously avoids the reduction of the QDs’ surface that comes with higher amine concentrations. The resulting QD formulations are easily printable: The rapid evaporation of BTA combined with the slower evaporation of NMP enables the deposition of a smooth thin film of QDs. Inkjet-printed photodetectors made with these inks have the highest specific detectivities reported to date (more than 10¹² Jones in the near-infrared region).

• Stable Colloidal Quantum Dot Inks Enable Inkjet-Printed High-Sensitivity Infrared Photodetectors,
  Rafal Sliz, Marc Lejay, James Z. Fan, Min-Jae Choi, Sachin Kinge, Sjoerd Hoogland, Tapio Fabritius, F. Pelayo García de Arquer, Edward H. Sargent,
  ACS Nano 2019.
  https://doi.org/10.1021/acsnano.9b06125