On the Colloidal Stability of PbS Quantum Dots Capped with Methylammonium Lead Iodide Ligands

Abstract

Phase-transfer exchange of pristine organic ligands for inorganic ones is essential for the integration of colloidal quantum dots (CQDs) in optoelectronic devices. This method results in a colloidal dispersion (ink) which can be directly deposited by various solution-processable techniques to fabricate conductive films. For PbS CQDs capped with methylammonium lead iodide ligands (MAPbI 3 ), the most commonly employed solvent is butylamine, which enables only a short-term (hours) colloidal stability and thus brings concerns on the possibility of manufacturing CQD devices on a large scale in a reproducible manner. In this work, we studied the stability of alternative inks in two highly polar solvents which impart long-term colloidal stability of CQDs: propylene carbonate (PC) and 2,6-difluoropyridine (DFP). The aging and the loss of the ink’s stability were monitored with optical, structural, and transport measurements. With these solvents, PbS CQDs capped with MAPbI 3 ligands retain colloidal stability for more than 20 months, both in dilute and concentrated dispersions. After 17 months of ink storage, transistors with a maximum linear mobility for electrons of 8.5 × 10 –3 cm 2 /V s are fabricated; this value is 17% of the one obtained with fresh solutions. Our results show that both PC- and DFP-based PbS CQD inks offer the needed shelf life to allow for the development of a CQD device technology.

Scalable PbS Quantum Dot Solar Cell Production by Blade Coating from Stable Inks

“…After ligand exchange and transfer of the PbS QDs to NMF, the first excitonic peak in the absorption spectrum shifted to 1.40 eV (885 nm), with an increased FWHM. The redshift can be explained either by an increase in the effective size of nanocrystals due to the inorganic ligand shell formed on the surface, or by the change in the dielectric permittivity of the solvent, or by the aggregation of the QDs in solution, which does not compromise the colloidal stability …”

“…In our recent publication, we have demonstrated PbS QD inks stability in two different polar solvents, namely, in propylene carbonate and 2,6-difluoropyridine (DFP), with the ink in DFP preserving the electronic properties for over 3 months. DFP is an interesting solvent for QD solar cell fabrication since it is characterized by a high dielectric constant and a relatively low boiling point.…”

Microfluidic Processing of Ligand‐Engineered NiO Nanoparticles for Low‐Temperature Hole‐Transporting Layers in Perovskite Solar Cells

“…For instance, stable dispersions of metal oxide NPs in polar solvents can be achieved using ligands containing organic substitutes, such as carboxylic acids, [42] thiols, or [43] mercaptocarboxylic acids, [44] or inorganic substitutes like hydrazine, [45] chalcogenides (S 2À , Se 2À , Te 2À ), [46] or halides (Cl À , Br À , I À ). [47] Here, we focus on two facile approaches that have been widely applied to different NPs to render them dispersible in polar solvents. The first of these involves a tetrafluoroborate salt, where surface modification of NPs is achieved using BF 4…”

High-Performance Electron-Transport-Layer-Free Quantum Junction Solar Cells with Improved Efficiency Exceeding 10%

“…Colloidal quantum dots (CQDs) with tunable band gap and controllable doping have been recognized as key materials for solution-processable optoelectronics. − In particular, lead sulfide (PbS) colloidal quantum dot solar cells (CQDSCs), capable of harvesting infrared (IR) photons − and enhancing photocurrent by the multi-exciton generation effect, , offer a great potential to break the Shockley–Queisser limitation of photovoltaic technology. Recently, systematic optimization in device structure, , as well as synthesis ,, and surface chemistry post-treatment methods of CQD, − successfully extended the light response range of PbS CQDSCs to near-infrared (NIR) wavelength (1900 nm), , generated a high short-circuit current ( J sc ) of 34 mA/cm 2 , and brought about long-term device stability without encapsulation (>1000 h). , Development of bending-durable and semi-transparent − CQDSCs proved their multi-functional applications, such as in portable and tandem devices . Consequently, CQDSCs are expected to lead performance breakthroughs in single-segment and/or tandem solar cells.…”

“…and CQDs have become a mainstream device structure for quantum dot photovoltaics. , Recently, an advance in surface passivation of PbS CQDs based on a liquid-phase ligand exchange process successfully generated a certified power conversion efficiency (PCE) over 13% in ZnO/PbS CQDSCs . The optimization of ionic ligands, solvents, and additives used in liquid-phase ligand exchange further promoted the colloidal stability of PbS CQDs for reproducible and large-scale manufacture of CQDSCs. However, band alignment at the ZnO/PbS interface was proved to be mismatched due to the intrinsic shallow band energy levels of ZnO, and this issue becomes more serious when the band gap of PbS CQDs shrinks for harvesting NIR light. − Additionally, the strong light absorbance of a wide-band-gap MOETL in the ultraviolet (UV) range reduces the multi-exciton generation effect of CQDs induced by high-energy UV photons.…”