Solution-processed/evaporation-based light-emitting diodes of face-down/edge-up oriented colloidal quantum wells

Abstract

Colloidal quantum wells (CQWs) have emerged as a quasi-two-dimensional class of semiconductor nanocrystals with the critical structural properties of being both atomically flat and vertically ultrathin. In these CQWs with atomically accurate thickness control, their extremely strong and precise one-dimensional quantum confinement, defined by few nanometers in sub-nm precision, gives rise to well-controlled anisotropic emission. The emission characteristics are composed by the contributions of transition dipole moments (TDMs), which are by their nature highly anisotropic in CQWs. In-plane TDMs lying in the lateral plane and out-of-plane TDMs along the vertical direction, taken with respect to the plane of a CQW, contribute to the emission characteristics proportionally. These features of CQWs make them highly attractive for use in light-emitting diodes (LEDs). In this thesis, to this end, in LEDs constructed either using all-solution based processing or evaporation, we show face-down and edge-up oriented self-assemblies of CdSe/Cd0.25Zn0.75S core/hot-injection shell (HIS) grown CQWs, along with their Fourier analyses using back focal plane (BFP) imaging. Results show that the out-of-plane TDM distribution for all-face-down oriented CQW film is suppressed 4.1 times, with its in-plane TDM distribution reaching 92%. Thanks to the strong contribution from the in-plane TDMs, the corresponding angularly resolved distribution of luminescence exhibits a highly directional intensity profile for the film of face-down CQWs placed lying on a substrate. Used as an electroluminescent layer, all-face-down oriented CQWs enable extraordinarily large external quantum efficiency (EQE) increased by almost 2 folds compared to that of randomly-oriented CQWs, with EQE reaching 18.1% in the case of face-down orientation, a record high level for solution-processed CQW LEDs. Moreover, in this thesis work, for the edge-up oriented self-assemblies of CQWs creating superstructures in chain, we investigated the distribution of TDMs and discovered length of the chain formation of such stacked CQWs plays an essential role. Extending CQW chains from 50 to 500 nm in length, on average in the film, the out-of-plane TDMs in the long-chained edge-up CQWs placed standing on a substrate is increased 3.5 times in comparison to those of the short-chained edge-up CQWs. The contribution of out-of-plane TDMs in directional emission is also improved via inducing longer chains. In the light of the results of Fourier image analysis, being used as the electroluminescent layer of evaporated LEDs in inverted architecture, all-edge-up oriented CQWs enable 50% enhancement in luminance levels compared to that of randomly-oriented CQWs. Additionally, in comparison to the face-down oriented CQWs used as the electrically driven emissive layer in the same device structure of LEDs, the edge-up oriented CQWs exhibit 60% improvement in charge injection. Such strongly orientation-dependent behavior of CQW layered structures, as exploited in this thesis, encourages further systematic studies on their ensemble optical emission characteristics in both solution-processed and evaporation-based LEDs and promises great potential for LED and other optoelectronic device applications.