Optical study of quantum dot and perovskite light-emitting diodes based on simulation

Abstract

Next-generation displays present many challenges, including wider color gamut, higher stability, and increased energy efficiency. Colloidal quantum dots (QDs) and metal halide perovskite materials have emerged as optoelectronic semiconductors for next-generation displays. Compared with organic light-emitting diodes (OLEDs), quantum dot light-emitting diodes (QLEDs) and perovskite light-emitting diodes (PeLEDs) exhibit tunable emission colors, narrow emission spectra, high quantum yields, and low-cost solution processability. In recent years, intense efforts have been devoted to material quality and charge injection, and quantum yields of QDs and perovskite near 100% have been reported. However, the efficiency of QLEDs and PeLEDs is generally less than ~20% due to optical limitations. To achieve improved optical performance, it is necessary to investigate the optics of these devices. This thesis studies the optics of planar light-emitting diodes and proposes systematic routes for optical analysis and optimization. Considering the microcavity effect, surface plasmon polaritons (SPPs), dipole radiation, and the Purcell effect, we can manipulate the optical properties of QLEDs and PeLEDs. The thesis covers the following topics: 1. Wave optics and a dipole model of planar light-emitting diodes. Beginning with Maxwell's equations, light–matter interactions, reflections, refractions, and interference between multiple layers are presented. A dipole model and optical loss channels facilitate the analysis of a simulation. These serve as prerequisites for optical simulation and optimization. 2. Optical design based on the microcavity effect, SPPs, and the Purcell effect. Microcavities can be used to improve efficiency, purify emission colors, and regulate angular intensity distribution. The methods used to enhance and modulate microcavities are discussed. By restraining the waveguides and SPPs, as well as improving the constructive interference in the air modes, device efficiency can be greatly enhanced. Moreover, the Purcell effect plays a role in enhancing or inhibiting the decay rate of dipoles. 3. Light extraction strategy based on optical tunneling. An internal periodic structure is a feasible way to extract waveguide modes, but it can result in complicated fabrication processes and deteriorate electrical performance. An effective strategy is proposed to extract light from waveguide modes to air modes through optical tunneling. Furthermore, the potential of optical tunneling in low-SPP devices is discussed. 4. Full-color QLEDs based on microcavities. The spectral narrowing phenomenon of microcavities is utilized to fabricate red, green, and blue QLEDs with a single QD layer. With enhanced microcavities and proper spacer thicknesses, the spectral selectivity is shifted, and the full-color tunability of QLEDs is realized. 5. High-efficiency microcavity PeLEDs. It is demonstrated that the microcavity effect enables PeLEDs to achieve a light extraction efficiency of 51.4%, much higher than the 8.7% estimated by classic theory. The improvement is also attributed to reduced surface plasmon loss and a high ratio of horizontal dipoles under the Purcell effect. Furthermore, it is found that efficiency increases as the refractive index of the emitting layer index increases, which is the opposite of the classic model.