Solution-processed light-emitting diodes of monolayers of colloidal quantum wells

Abstract

Semiconductor colloidal quantum wells (CQWs) make an exciting quasi-2D class of nanocrystals thanks to their unique properties emerging from their atomically-flat verticallythin geometry including highly anisotropic optical transition dipole moment (TDM), giant oscillator strength, and extraordinarily high absorption cross-section. This regular shape of the CQWs enables them to assemble on the surface of a liquid with a high degree of packing, which creates an almost fully uniform monolayer film with the desired orientation. Nearly all of the emission in a single CQW, again because of its geometry, comes from in-plane (IP) transition dipole moments (TDMs). Thus, an assembled film of such CQWs with an all-facedown orientation may show a highly anisotropic and directional emission. Using such a film in an emissive layer (EML) of an electroluminescent device, in this thesis work our intention is therefore to substantially boost photon outcoupling efficiency in a light-emitting device of oriented monolayer of CQWs. Thus far, research efforts have been conducted to investigate the properties of the deposited film of CQWs with self-assembly. However, in all of the previous studies, the property of CQW monolayer has been investigated only in a passive film; therefore, investigation of the self-assembled film in an active device is lacking. In this thesis, we developed and demonstrated an all-solution-processed colloidal quantum well light-emitting diodes (CQW-LEDs) using a single all-face-down oriented self-assembled monolayer (SAM) film of CQWs that enables a high level of IP TDMs of 92%. This film significantly enhances the outcoupling efficiency of LEDs from 22% (of standard randomlyoriented emitters) to 34% (of face-down oriented emitters). As a result of the outcoupling efficiency increased by 1.55 times, along with the enhanced charge injection and reduced reabsorption in the case of using a single SAM of CQWs, the external quantum efficiency reaches a record high level of 18.1% for the solution-processed type of CQW-LEDs, putting their efficiency performance on par with those of the hybrid organic-inorganic evaporationbased CQW-LEDs and all other best solution-processed LEDs. This EQE value is 65% higher than that in the devices fabricated by spin-coating with the same procedure and structure. Also, the SAM-CQW-LED architecture enables a high maximum brightness of 19,800 cd/m2 with a long operational lifetime of 247 h at 100 cd/m2, as well as a stable saturated deep-red emission (651 nm) with a low turn-on voltage of 1.7 eV and a high J90 of 99.58 mA/cm2. These findings indicate the effectiveness of oriented self-assembly of CQWs as electrically-driven emissive layers in improving outcoupling and external quantum efficiencies in the CQW-LEDs.