Tutorial EP05—Emerging Low-Dimensional and Perovskite Light-Emitting Materials and Devices

8:00 am – 9:30 am

Part I: Moungi Bawendi
Introduction to Colloidal Quantum Dots

This lecture will cover the synthesis, basic photophysics and provide a perspective on the applications of colloidal quantum dots. In particular, we will discuss the synthesis of II-VI (CdSe and the like) and III-V (InAs and the like) nanocrystals and core-shell morphologies based on them. We will focus on the photoluminescence properties of quantum dots from a fundamental perspective but also in the context of applications that include displays, QDLEDs, and for biolabeling.

9:30 am – 10:00 am  BREAK

10:00 am – 11:30 am

Part II: Hemamala Karunadasa
Structural Origins of the Luminescence of Layered Halide Perovskites

Upon UV/near UV excitation, layered halide perovskites emit light of various colors: blue, green, yellow, red, and even white. In contrast to many inorganic phosphors that contain extrinsic emissive dopants or surface sites, here, the emission appears to be intrinsic to the bulk material. In this tutorial, I will discuss the structural origins of both the narrow and the broadband emission from 2D hybrid perovskites. I will describe how reducing the dimensionality of the inorganic sublattice dramatically changes the photophysical properties of 3D and 2D halide perovskites―transitioning from materials where sunlight generates free electrons and holes in solar cells to materials where UV light creates strongly correlated electron-hole pairs (excitons) that radiatively recombine in phosphors. I will further describe how 2D perovskites that do not contain obvious emissive sites can emit every color of visible light, affording broadband white light with the high color rendition required for solid-state lighting applications. I will emphasize the different synthetic handles we have for tuning the emission of layered perovskites through structural design of both the organic and the inorganic layers. I will further discuss alternative methods for tuning the emission from these materials, including post-synthetic transformations such as gas-solid halogen exchange reactions and application of gigapascal-scale pressure.

11:30 am – 1:30 pm BREAK

1:30 pm – 3:00 pm

Part III: Tae-Woo Lee
Efficiency Metal Halide Perovskite Light-Emitting Diodes

Metal halide perovskites are emerging high color-purity emitters with low material cost. However, low electroluminescence (EL) efficiency at room temperature is a challenge that should be overcome. In this presentation, various strategies to achieve efficient perovskite light-emitting diodes (PeLEDs) will be presented. The EL efficiency limitations of PeLEDs using the perovskite emitter in forms of (1) 3D crystal structures, (2) quasi-2D crystal structures and (3) nanoparticles (NPs) can be overcome by controlling the confinement of electron-hole pairs and the charge transport in the devices at the same time. First, to improve EL efficiency of PeLEDs based on 3D crystal structures, a self-organized buffer hole injection layer can be introduced to reduce the hole injection barrier and block the exciton quenching at the interface. The high-efficiency methylammonium lead bromide (MAPbBr₃) and CsPbBr₃ PeLEDs were realized based on the buffer hole injection layers and the temperature dependence of EL in the CsPbBr₃ PeLEDs was systematically investigated and related with ion migration, EL quenching pathways and electron-phonon coupling. Furthermore, the formation of metallic lead atoms causes strong exciton quenching, and it was prevented by finely increasing the molar proportion of MABr in MAPbBr₃ solution. Also, the efficiency in PeLEDs can be increased by decreasing MAPbBr₃ grain sizes and consequently improving uniformity and coverage of MAPbBr₃ layers. Chemically inert graphene avoids quenching of excitons by diffused metal atom species from indium tin oxide. Second, quasi-2D perovskites were studied because of the advantages of quasi-2D perovskites such as the enhancement of film quality, exciton confinement and reduced trap density, and quasi-2D PeLEDs with high efficiency and brightness were demonstrated. Finally, perovskite NPs were effectively used in PeLEDs because they can show high luminescence efficiency and high color-purity in both solution states and film states, and high efficiency PeLEDs based on MAPbBr₃ and formamidinium lead bromide (FAPbBr₃) NPs were also fabricated.

3:00 pm – 3:30 pm BREAK

3:30 pm – 5:00 pm

Part IV: Tze-Chien Sum
The Photophysics of Halide Perovskite Emitters

This part will outline the fundamental photophysics of metal halide perovskite emitters with a focus on the model CH₃NH₃PbI₃ system. The presentation will not only include polycrystalline, quasi-2D, nanoparticle perovskite systems but also emerging Pb-free systems (e.g., CsSnI₃). Challenges and opportunities in achieving optical gain from these excellent light emitters will also be discussed.

Instructors

• Moungi Bawendi, Massachusetts Institute of Technology
  
• Hemamala Karunadasa, Stanford University
  
• Tae-Woo Lee, Seoul National University
  
• Tze-Chien Sum, Nanyang Technological University