Browsing by Subject "Orientation-controlled film"

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Open Access
Monolayer-thick light-sensitive nanocrystal skins of oriented colloidal quantum wells
(2023-05) Bozkaya, Taylan
Colloidal quantum wells (CQWs), a two-dimensional member of semiconductor nanocrystals, featuring very tight vertical quantum confinement, possess giant oscillator strengths. Also, CQWs exhibit remarkably large absorption cross-sections, thanks to their oscillator strengths combined with their laterally large geometries. Additionally, as a powerful tool of fabrication, CQWs lend themselves to be conveniently self-assembled into monolayer-thick films in a single orientation of our choice: either face-down (lying down on their large lateral surfaces and side by side leaving no large gap between them similar to a mosaic pattern) or edge-up (standing up on their thin edges and facing each other in a very dense superstructure formation of repeating chains). In this thesis, to make use of the attractive absorption properties of CQWs and leverage on our ability to construct their orientation-controlled self-assemblies, we show the first account of monolayer-thick light-sensitive nanocrystal skins (LS-NS) that employ self-oriented CQWs as their active absorptive layer. These CQW LS-NS devices operate on the principle of strong optical absorption of the monolayered assembly of CQWs and the subsequent photogenerated potential build-up across their strongly capacitive thin device for sensing in the visible to ultraviolet. Such oriented CQWs in the LS-NS device architecture yield profoundly reduced surface roughness in their monolayer-thick films, essential to high device performance. Here, specifically, we developed and demonstrated two groups of LS-NS devices: one group consisting of all face-down oriented CQWs and the other, of all edge-up ones. We systematically studied their photocharging effect, spectral sensitivity and decay times. We observed in all LS-NS devices that the spectral sensitivity complies with the first (heavy-hole) and second (light hole) excitonic peaks of the absorption of the CQWs. We also found that, as the excitation power is increased, the peak photovoltage readout increases while the sensitivity decreases. The photocharging effect was further observed as the excitation was turned off. Finally, using the edge-up orientation, we identified a profound peak photovoltage signal enhancement. These findings of the thesis indicate that the proposed LS-NS devices of the orientation-controlled CQW monolayers hold great promise for applications in photos-sensing facades over larger surfaces.
Open Access
Solution-processed light-emitting diodes of monolayers of colloidal quantum wells
(2022-06) Dehghanpour Baruj, Hamed
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.
Open Access
Solution-processed/evaporation-based light-emitting diodes of face-down/edge-up oriented colloidal quantum wells
(2023-08) Bozkaya, İklim
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.