Abstract
Controlling the thickness of quantum dot (QD) films is difficult using existing film formation techniques, which employ pre-ligand-exchanged PbS QD inks, because of several issues: 1) poor colloidal stability, 2) use of high-boiling-point solvents for QD dispersion, and 3) limitations associated with one-step deposition. Herein, we suggest a new protocol for QD film deposition using electrical double-layered PbS QD inks, prepared by solution-phase ligand exchange using methyl ammonium lead iodide (MAPbI3). The films are deposited by the supersonic spraying technique, which facilitates the rapid evaporation of the solvent and the subsequent deposition of the PbS QD ink without requiring a post-deposition annealing treatment for solvent removal. The film thickness could be readily controlled by varying the number of spraying sweeps made across the substrate. This spray deposition process yields high-quality n-type QD films quickly (within 1 min) while minimizing the amount of the PbS QD ink used to less than 5 mg for one device (300-nm-thick absorbing layer, 2.5 × 2.5 cm2). Further, the formation of an additional p-layer by treatment with mercaptopropionic acid allows for facile hole extraction from the QD films, resulting in a power conversion efficiency of 3.7% under 1.5 AM illumination.
Highlights
QDs synthesized via colloidal routes are capped with long-chain organic ligands that act as an insulating layer; this prevents their direct use in electronic devices because of the weak interparticle coupling resulting from the insulating layer[7]
We propose a new method for the deposition of conductive PbS QD films using PbS QD inks by a supersonic spray-coating method
We used a PbS QD ink stabilized with MAPbI3, which are colloidally stable under ambient conditions, as determined through dynamic light scattering (DLS) measurements and absorbance spectra analysis
Summary
Hyekyoung Choi[1,2], Jong-Gun Lee[3], Xuan Dung Mai[1], Matthew C. A strategy has been developed wherein the native ligands are replaced with shorter ligands in the solution phase itself; this strategy allows for the direct deposition of the QD solution onto the substrate, yielding conductive films without requiring a solid-state ligand exchange[9, 11, 15,16,17,18,19] These QDs with short ligands, whose dispersions are termed as QD inks, do not exhibit colloidal stability, making it difficult to fabricate high-quality QD films by controlling certain parameters, such as the film thickness[15, 20]. The photovoltaic cells produced from the PbS QD ink by using this coating approach exhibit a power conversion efficiency (PCE) of 3.7%
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