| Author: | Zhuang, Lyuchao |
| Title: | Development of metal halide perovskite films for light emitting diode applications |
| Advisors: | Lau, Shu Ping (AP) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Perovskite materials Metal halides Light emitting diodes Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xxi, 162 pages : color illustrations |
| Language: | English |
| Abstract: | Perovskites have been considered good candidates for next-generation semiconductors, which have been widely investigated in many fields, such as solar cells, photodetectors, and light-emitting diodes (LEDs). In addition to the outstanding electrical properties, perovskite materials possess tunable bandgap, high colour purity, and solution processability. However, the main factors hindering emission efficiency improvement are the small exciton binding energy and long exciton diffusion length in 3-dimensional (3D) perovskites. It is recognized that the facile dissociation of excitons will intrinsically restrict the efficiency of perovskite light-emitting diodes (PeLEDs). Furthermore, the grain size reduction generates more grain boundaries, leading to an increase in trap state density. At the same time, the larger grain size of perovskite lowers the possibility of radiative recombination. Thus, balancing the exciton diffusion length and trap state density is essential. Moreover, quasi-2D perovskite with the formula of A′2An−1BnX3n+1 (1 ≤ n ≤ ∞) has been considered an alternative to achieve blue emission due to quantum confinement, where A′ refers to a large organic cation, A stands for a small monovalent cation, B is a divalent metal cation, X represents a halide ion. Further, n refers to the number of [BX6]4- octahedral units. The geometry of quasi-2D perovskite can be seen as cutting the 3D structure along the (100) crystal plane. The solubility difference of various components and lower formation energy for low-n phases (n ≤ 3) account for the undesirable and uncontrollable phase distribution. Inevitably, the energy transfer from low-n phases to large-n (n > 3) phases always follows energy loss, due to the inhomogeneous distribution of energy domains. Furthermore, the insulation nature of organic molecules will hinder the transport of carriers, which will be unfavorable for adjusting the emission spectrum and the device's performance. Firstly, in this thesis, we proposed a doping strategy of potassium bromide (KBr) to enhance the photoluminescence efficiency of CsPbBr3 perovskite films via a zero-dimensional/three-dimensional (0D/3D) Cs4-xKxPbBr6/CsPbBr3 heterostructure construction. Our density functional theory (DFT) calculations have demonstrated the feasibility of 0D/3D carrier confinement heterostructure formation. The unique heterostructure confines the charge in the 3D phase, which inhibits the free charge diffuse to the grain boundary from suffering trap-assisted recombination, leading to the significantly improved photoluminescence quantum yield (PLQY). As a result, the optimized green PeLEDs exhibited a noticeable improvement in efficiency with the EQE from 2.1% to 12.8%, achieving a maximum brightness of 39400 cd m-2. Secondly, we adopted an anti-solvent treatment to modulate the phase distribution for efficient energy transfer. By using different polarity anti-solvents: toluene (TO), diethyl ether (DE), chlorobenzene (CB), anisole (AN), and ethyl acetate (EA), we observed the different phase distribution for n-butylammonium bromide (BABr) and phenethylammonium bromide (PEABr) dual-ligands treated quasi-2D perovskite films. The EA-treated quasi-2D perovskite film has a desirable narrowed phase distribution and presents a flattened energy cascade, which is confirmed by transient absorption (TA). Our first-principles calculations suggested that the EA treatment induces preferential growth of large-n phases and enhances energy transfer due to the vital hydrogen bonding energy ~ -17 kcal/mol. Furthermore, by changing the ratio of BABr and PEABr, we develop efficient sky-blue and green light emission PeLEDs with an improved external quantum efficiency (EQE) ranging from 4.21% to 8.77% and good spectra stability during operation. To further investigate the blue PeLEDs, based on the PEABr treated CsPb(BrxCl1-x)3 perovskite films, we introduced an acetamidinium (AA+) substitution process to reconfigure the orientation of phenyl groups. Using sum frequency generation vibrational spectroscopy (SFG-VS), we demonstrated the relationship between EQE and the phenyl group's orientational disorder. We suppose that this phenyl group's orientation reconfiguration prohibits carrier diffusion, leading to more carrier radiative recombination for light emission rather than annihilating at the grain boundary. Moreover, the incorporation of AA+ cation induced the formation of an additional (PEAxAA1-x)2CsPb2Br7 phase. It is beneficial for eliminating energy loss via a smoother energy transfer pathway, which is proved by the TA kinetics dynamic. Consequently, the fabricated PeLEDs achieved the EQE of 7.48%, and the maximum luminance of 1206.6 cd m-2 is illustrated by the optimized AABr additive. More importantly, the emission peak at 487 nm demonstrates improved stability under different voltage biases. This work provides new direction into the potential underlying mechanism for PeLEDs efficiency and phenyl group orientational disorder. |
| Rights: | All rights reserved |
| Access: | open access |
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