Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 5400−5407
pubs.acs.org/JPCL
Inverted Device Architecture for Enhanced Performance of Flexible Silicon Quantum Dot Light-Emitting Diode ̇ Batu Ghosh,*,†,‡ Hiroyuki Yamada,†,§ Shanmugavel Chinnathambi,∥ Irem Nur Gamze Ö zbilgin,†,⊥ ,†,§,⊥ and Naoto Shirahata*
J. Phys. Chem. Lett. 2018.9:5400-5407. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 09/25/18. For personal use only.
†
International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan ‡ Department of Physics, Triveni Devi Bhalotia College, Raniganj, West Bengal 713383, India § Department of Physics, Chuo University, 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551, Japan ∥ International Center for Young Scientists (ICYS), NIMS, 1-2-1 Sengen, Tsukuba 305-0047, Japan ⊥ Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0814, Japan S Supporting Information *
ABSTRACT: Here we report for the first time highly flexible quantum dot lightemitting diodes (QLEDs), in which a layer of red-emitting colloidal silicon quantum dots (SiQDs) works as the optically active component, by replacing a rigid glass substrate with a thin sheet of polyethylene terephthalate (PET). The enhanced optical performance for electroluminescence (EL) at room temperature in air is achieved by taking advantage of the inverted device structure. Our QLEDs do not exhibit parasitic EL emissions from the neighboring compositional layers or surface states of QDs over a wide range of driving voltages and do not exhibit a shift in the EL peak position as the operational voltage increases. Compared to the previous Si-QLEDs with a conventional device structure, our QLED has a longer device operational lifetime and a long-lived EQE value. The currently obtained brightness (∼5000 cd/m2), the 3.1% external quantum efficiency (EQE), and a turn-on voltage as low as 3.5 V are sufficiently high to encourage further developments of Si-QLEDs.
F
Most QLEDs adopt a conventional (or normal) device structure where indium tin oxide (ITO) and aluminum (Al) work as the anode and the cathode, respectively. These QLEDs have a multilayer structure composed of an electron injection layer (EIL), an electron transportation layer (ETL), an optically active layer, a hole transportation layer (HTL), and a hole injection layer (HIL). Unlike the normal structure, the inverted device structure of QLEDs (iQLEDs) adopts the ITO as the cathode. Recently, the inverted structure has received much attention due to (i) its high compatibility with n-channel field-effect transistors for pixel driver backplanes in activematrix displays,10,11 (ii) the availability of the transparent bottom cathode and a high work function (i.e., air stable) top anode, (iii) a broad range of choice of inorganic crystals working as the ETL (e.g., ZrO2, TiO2, and ZnO)12,13 or HTL (e.g., V2O5, WO3, and MoO3),14,15 which shield organic interlayers from air, and (iv) the availability for EIL/ETL use of ZnO that provides the deep energy level of the highest occupied molecular orbital (HOMO), serving as an efficient hole blocker. Additionally, iQLEDs are advantageous from the viewpoint of processing toward improved device performance.
lexible display technologies are attracting increased market attention due to their potential applications, including portable, foldable, and wearable photoelectronics.1−3 Organic light emitting diodes (OLEDs) take advantage of a selfluminous layer to construct a device multilayer. Reducing its entire thickness yields mechanical flexibility (e.g., bending, stretching, and folding). However, device instabilities upon exposure to air, which are inherent to the insulating nature of organics, hinder operations at high applied voltages.4 Therefore, the flexibility of current commercialized OLEDs is limited by the encapsulation layer to protect from air. Colloidal quantum dots (QDs), which are semiconductor crystals nanostructured in three dimensions, provide advantageous photoluminescence (PL) properties, including color purity, narrower spectra for emission (full-width at halfmaximum, fwhm
