Highly photo-stable CsPbI₃ perovskite quantum dots via thiol ligand exchange and their polymer film application

Abstract

Highly photo-stable CsPbI₃ perovskite quantum dots (PeQDs) were prepared using thiol as an exchanged ligand. Among the CsPbX₃ PeQDs, CsPbI₃ with common capping ligands such as oleic acid (OA) and oleylamine (OLA) was particularly unstable and the optical properties were readily degraded because of its structural instability. To overcome these problems, thiol (SH) as an exchanged ligand was used to stabilize the structure and could maintain the structural and optical properties. The synthesized CsPbI₃ exhibited highly improved photo-stability under ultraviolet (365 nm) irradiation by ligand exchange from OA and OLA to thiol. As an application, perovskite films were fabricated using the PeQDs and a cyclic olefin copolymer (COC). The thiol-treated PeQD films exhibited higher stability than the untreated films under irradiation with a blue light-emitting diode (LED).

Introduction

Bulk perovskite structure with the composition of ABX₃ have attracted much attention firstly, as solar cells based on these structures are easy to manufacture and have higher efficiency than conventional solar cells [1], [2], [3], [4]. Then, quantum confinement effect was revealed as the size of perovskite structures became smaller, as a result, their luminescence characteristics were also noted. Highly dispersible colloidal all inorganic perovskite CsPbX₃ nanocrystals (PeQDs) with emission covering the full spectrum were prepared by using a hot-injection method, and much progress in the research on these highly promising optoelectronic materials has been achieved [5]. PeQDs can be easily and rapidly synthesized, and their emission can be controlled to span the entire visible range (410–700 nm) through halide exchange (Cl, Br, I) in solution [6], [7]. PeQDs show high quantum yields (QYs) of 40–90% with a narrow full-width-at-half-maximum (FWHM) of 20–45 nm, which is similar to that of CdSe quantum dots [8], [9]. Also, it has a wide color gamut reaching about 90 % of BT.2020 [10]. The narrow FWHM results in a wide color gamut, making PeQDs attractive as promising materials for optoelectronic applications such as light emitting diodes (LEDs) [11], [12], [13], photodetector [14], [15], [16] and lasers [17], [18], [19].

However, perovskite structures suffer from serious stability problems due to their ionic characteristics. Among the PeQDs, red-emitting CsPbI₃ QDs with the α-phase structure are particularly unstable and it is difficult to maintain the structural stability of these species under light, moisture, or heat conditions because iodine has a large ionic radius, weak ionic bonds, and low stability [20]. Generally, oleic acid (OA) and oleylamine (OLA) are used as ligands for surface passivation of QDs; however, it is difficult to stabilize the ionic perovskite QDs due to the formation of weak surface bonds [21]. Recently, several methods have been introduced to solve these problems, such as using different types of ligands [22], new precursors [23], or other mixing metals [24], [25].

In this study, the surface OA and OLA ligands were partially exchanged with thiol after synthesis of the PeQDs. The use of thiol as a ligand has previously been reported. Firstly, Alivisatos et al. synthesized PeQDs with the chemical composition Cs₄PbBr₆ by using OLA and 1,3-propanedithiol [26]. Ruan et al. reported the preparation of stable, large (∼100 nm) perovskite particles in polar solvents such as ethanol via repeated surface treatment using 1-octanethiol (OT) and PbBr₂ toluene solution [27]. Generally, previous studies on thiol as a ligand have concentrated on changing the crystal structure or size of the resulting QDs; however, there are few reports on improvements in the stability of PeQDs in relation to the luminescence properties. Herein, focus is placed on the stability of the CsPbI₃ PeQDs. It is known that the α-phase of these QDs, which is the only light emitting structure of CsPbI₃, is particularly inferior in stability. We found that some thiols can attach to the surface of CsPbI₃ because of their high affinity for Pb²⁺, thereby stabilizing CsPbI₃. Thus, the α-phase structure could be maintained and the photoluminescence (PL) could persist for an extended period. When ligand exchange by 1-dodecanethiol (DDT) was performed, the relative quantum yield of 46% was maintained over 120 h under UV (365 nm) irradiation. Further, a thermal aging test after ligand exchange using different thiols (including OT, DDT, and 1-octadecanethiol (ODT)) showed that the CsPbI₃ PeQDs with OT were the most stable. Scheme 1 showed schematic illustration of proceeded experiment of thiol ligand exchanged CsPbI₃ PeQDs. Finally, QD films were fabricated by using the improved PeQDs and a cyclic olefin copolymer (COC), and highly improved optical properties were achieved compared to those of the film from the pristine PeQDs when irradiated with a 15 V back-light unit (BLU).