Functional Perfluorooctanoic Acid for Enhancing Stability of Perovskite QDs
Authors: Lee Ji-Eun , Lee Seung-Jae , Ko Yun-Hyuk , Park Jea-Gun 
Published: February 2020

Recently, metal halide-based perovskite quantum dots (PrQDs) have become highly attractive in many optoelectronic applications, such as photodetectors, photovoltaics, lasers, light emitting diodes (LEDs), due to their outstanding optical and electrical properties.1 In particular, PrQDs have been broadly applied for displays and LEDs due to their wide color wavelength tunability (300-900 nm), narrow full-width half-maximum (FWHM) emission (<30 nm), and high photo-luminescence quantum yield (PL-QY) (>80%), as well as their simple and cost-effective one-pot synthesis process in comparing to inorganic phosphor materials.2,3

However, the low stability of PrQDs still the main hindrance issue that significantly limit the reliability and applicability of their uses in optoelectronic devices.4 It is well-known that the oleic acid (OA) and oleic amine (OAm) ligands are not enough to protect the QDs against moisture and oxygen due to their short chains. In order to overcome this low stability issue of PrQDs, we investigated various carboxylate and polymer materials having long and complex chains for passivation the surface of PrQDs.
Motivated by this idea, we proposed a novel, eﬃcient and simple process method to enhance the stability of PrQDs using systematical ligand engineering with longer and complex carbon chains. Here, we used perfluorooctanoic acid (PFOA) as an alternative for OA and OAm ligands. This proposed ligand was applied directly to the crude solution during the synthesis to passivate the outer surface of PrQDs as explained schematically in Fig 1a. PFOA, which belongs to carboxylate fluorine group, has a strong electron withdrawing group with considerable steric hindrance.5 In other words, PFOA can effectively passivate the surface of PrQDs with eight carbon chains in order to study their effect on the stability of PrQDs.
To synthesize CsPbBr3-XIX QDs (PrQDs) capped with PFOA ligand, 5 mL of octadecene (ODE) and 0.188 mmol of PbX2 (such as PbI2 and PbBr2) were loaded into 25 mL 3-neck flask and then degassed under vacuum for 1 hr at 120 ºC. Later, 0.5 mL of dried OAm and different ratios of OA to PFOA (OA: PFOA). The selected OA: PFOA ratios were 10:0, 7:3, 6:4, 5:5, 4:6, and 3:7. Then, the mixture of OA: PFOA was injected at 120 ºC under N2 gas ambient. After solutionization of a PbX2 salt in ODE, the temperature raised to 165 ºC then Cs-oleate solution (0.4 mL, 0.125 M in ODE) was quickly injected and maintained for 5 sec. later, the reaction mixture was cooled using an ice-water bath. The final crude solution was purified by centrifuging process. For smaller QDs, the synthesis process should be carried out at a reaction temperature of below 160 ºC. A centrifugation of below 0 ºC or/and an addition of tert-butanol to the crude solution (ODE:tBuOH=1:1 by volume) were significantly helpful in achieving a complete precipitation of QDs. After centrifugation, the supernatant discarded and the precipitated particles were redispersed in hexane. The hexane is more suitable for PrQDs, compared to chloroform and toluene, due to its low polarity (0.009).
Both PrQDs uncapped and capped with PFOA ligand showed a cubic shape with high crystallinity and well dispersity, as shown in high-resolution transmission electron microscopy (HR-TEM) images (Figs 2a and b). In addition, the sizes of Pr QDs uncapped and capped with PFOA ligand were 10.2±2.39 and 11.5±2.61nm, respectively.
The PFOA capped PrQDs showed a PL peak shifted to a longer wavelength by 17 nm compared to uncapped PrQDs (from 534 to 551 nm) showing successful passivation of PrQDs (Fig 3a). In addition, The QDs exhibited a broader FWHM ranging from 24.4 to 35.1 nm and higher PL intensity ranging from 32,000 to 15,000 a.u. with increasing the mole fraction of PFOA as shown clearly in Fig. 3b. However, after 7 days the PL intensity of PFOA-capped PrQDs decreased by 15%, while uncapped PrQDs decreased by 57% after the same period (Fig 3d) showing the functionality of PFOA ligands in improving the stability of PrQDs. In our study, we present to conclude our outcomes, the functional PFOA ligands can be a promising candidate material for PrQD-based display and LED applications.

Figure 1