Authors: Yiran Jiang
Source: IDEALS (University of Illinois Urbana-Champaign) (article, 2018-01-18)
Field: Materials Science — Materials Chemistry

Surface and interface engineering in quantum dot and double-heterojunction nanorod light-emitting diodes Colloidal quantum dots (QDs) have shown to be a promising class of materials for the next generation displays. State-of-the-art QD-LEDs that utilize CdSe-based spherical core/shell (C/S) QDs as the electroluminescent layer have shown efficiencies and brightness that rival the best organic LEDs. One of the biggest challenges on developing these high performance QD-LEDs is the charge balance in these devices. Since the valence band edges of CdSe and the typical shell materials (CdS or ZnS) are significantly lower than those of available organic hole transport materials, the band alignment in most CdSe-based QD-LEDs usually favor electron injection over hole injection. This difference of energy barriers for electron and holes introduce difficulties for charge balance, which essentially affects the radiative carrier recombination, and the device performance. For example, while nanocrystal-based LEDs are showing high efficiencies, the maximum efficiencies are often observed in a relatively low-current, low-brightness regime (i.e., only one to a few hundred cd/m2) and the efficiency droop leads to less-than ideal performance at high luminance conditions useful for many applications. External quantum efficiency (EQE) at luminance levels relevant for high-end displays and general lighting (~1,000 and ~5,000 cd/m2, respectively) is modest at best. Therefore, promoting charge balance is critical for improving the performance of QD-LEDs. \nThe first approach taken in my work for improving charge balance is to modify the band alignment in the devices to provide a better match of energy barriers for electron and hole injection. Self-assembled monolayer (SAM) is utilized to modify the ITO electrodes. The SAMs can modify the surface and the work function of ITO, facilitate hole transport into the device and therefore improve charge balance in DHNR-LEDs. Extremely bright DHNR-LEDs with maximum luminance over 100,000 cd/m2 are demonstrated. Furthermore, maximum efficiencies appear at high luminance conditions that can be achieved at very low bias and current density (e.g., 3.1 V and 53 mA/cm2 at ~10,000 cd/m2, corresponding to EQE = 10.7 %, current efficiency = 21.7 cd/A, and luminous power efficacy = 19.5 lm/W). Despite the fact that DHNRs have only about half the photoluminescence quantum yield of C/S QDs, these achieved efficiencies at high luminance conditions are comparable to or surpass those demonstrated by the state-of-the-art QD-LEDs. Through further optimization of the thickness of DHNR films, the EQE of the SAM-based DHNR-LED is improved to about 16%, confirmed the effect of SAM on improving hole transport and charge balance.\nIn addition to the excellent electroluminescent properties, the double heterojunction structure in the DHNRs also allows charges to be extracted from the DHNR-LEDs, making them light-responsive LEDs. The photovoltaic response of the DHNR-LEDs provides sufficient photocurrent for photodetection, while also enabling energy scavenging and harvesting functions. An enhanced photovoltaic response would benefit both photodetection and energy harvesting purposes, but it is challenging to improve photovoltaic response while maintaining high LED performance. Surface modification of nanocrystals is demonstrated as an effective method to simultaneously improve both the photocurrent and the luminance in the DHNR-LEDs. The benzenethiol ligands applied for ligand exchange could reduce the interparticle spacing in the DHNR films, improving electrical conductivity of the films, and facilitating charge transport in the devices. Furthermore, it provides additional energy state between the valence band edge of the hole transport material and that of the semiconductor nanocrystals, facilitating hole injection and extraction in the devices. Through a simple ligand exchange process, the short circuit current, power conversion efficiency and maximum luminance of the DHNR-LEDs are shown to improve 29.3%, 33.3% and 29.0% respectively. \nWith the development of this exciting class of materials, great interest has been focused on the scalable approaches to QD-LED fabrication. Dry means of patterning QD films and fabricating QD-LEDs have been studied. Instead of the commonly used polydimethylsiloxane (PDMS) elastomeric stamps, two different types of materials, shape memory polymer (SMP) and photoresist (PR), are utilized for patterning the QD films. The SMP can switch its elastic modulus as a function of temperature. At elevated temperature above Tg, the structured SMP stamps form conformal contact with the QD films, while at temperature below Tg the SMP exhibit large pull-off forces, enabling rate-independent patterning of the QD films. Photolithographically patterned PR stamps are also explored for patterning QD films. The glassy state of the PR provides rigidity that ensures normal stress to be more evenly distributed on the stamp with less stress concentration at corners and edges, and also affords higher work of adhesion to the QD film than the PDMS stamps. These two characteristics provide a higher pull-off force between the PR and the QD film, which leads to higher patterning yields and spatial resolution that is on par with the conventional photolithography. Optoelectronics ; Light-emitting diode ; Quantum dot ; Heterojunction ; Nanorod ; Materials science ; Diode ; Double heterostructure ; Interface (matter) ; Nanotechnology ; Semiconductor laser theory ; Composite material Quantum Dots Synthesis And Properties ; Semiconductor Quantum Structures and Devices ; Nanowire Synthesis and Applications