Article pubs.acs.org/cm
Performance Improvement of Quantum Dot-Light-Emitting Diodes Enabled by an Alloyed ZnMgO Nanoparticle Electron Transport Layer Jong-Hoon Kim,† Chang-Yeol Han,† Ki-Heon Lee,† Ki-Seok An,‡ Wooseok Song,‡ Jiwan Kim,§ Min Suk Oh,§ Young Rag Do,¶ and Heesun Yang*,† †
Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea Thin Film Materials Research Group, Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea § Flexible Display Research Center, Korea Electronics Technology Institute, Bundang-gu, Seongnam-si, Gyeonggi-do 463-816, Korea ¶ Department of Chemistry, Kookmin University, Seoul 136-702, Korea ‡
S Supporting Information *
ABSTRACT: Since the introduction of inorganic ZnO, typically in the form of nanoparticles (NPs), as an electron transport layer (ETL) material, the device performance of electrically driven colloidal quantum dot-light-emitting diodes (QLEDs), in particular, with either Cd-based II−VI or non-Cd-based III−V (e.g., InP) quantum dot (QD) visible-emitters, has been rapidly improved. In the present work, three Zn1−xMgxO (x = 0, 0.05, 0.1) NPs that possess different electronic energy levels are applied as ETLs of solution-processed, multilayered I−III−VI type QLEDs that consist of a Cu−In−S, Cu−In−Ga−S, or Zn−Cu−In−S QD emitting layer (EML) plus a common organic hole transport layer of poly(9-vinlycarbazole). The luminance and efficiency of those QLEDs are found to be strongly dependent on the type of ZnMgO NP ETL, resulting in the substantial improvements by means of alloyed ZnMgO ETL versus pure ZnO one. Ultraviolet photoelectron and absorption spectroscopic measurements on a series of ZnMgO NP films reveal that their conduction band minimum (CBM) levels are systematically closer to the vacuum level with increasing Mg content. Therefore, such beneficial effects of alloyed NPs on QLED performance are primarily ascribed to the reduced electron injection barrier between ETL and QD EML that is enabled by the upshift of their CBM levels.
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INTRODUCTION Colloidal semiconductor nanocrystals (quantum dots, QDs) possess a high degree of flexibility in tuning the fluorescence wavelength through controlling size and introducing alloying/ doping. In addition, owing to the continual synthetic advancement, fluorescence of QDs has become satisfactorily efficient from a standpoint of practical device application. Thus, QDs have been strongly regarded as alternative active materials to rare earth-doped bulk phosphors and fluorescent/phosphorescent polymers for the fabrication to light-emitting devices. Recently, color-conversion-based white QD-light-emitting diodes (LEDs), where cadmium (Cd)-containing II−VI QDs of green and red emitters are integrated with a blue-pumping LED chip, as highly color-reproducible display backlighting sources, have been commercialized. Parallel with the above color-conversion QD device, the electrically driven QD-lightemitting diode (QLED) has been intensively investigated as a postorganic light-emitting diode (OLED) for the last two decades. QLED is typically based on the multilayered configuration consisting of a hole injection layer (HIL), a hole transport layer (HTL), a QD emitting layer (EML), and an electron transport layer (ETL). Keeping the energy levels of QDs versus surrounding charge transport layers (CTLs) in mind, various © 2014 American Chemical Society
HTL/ETL combinations have been attempted to improve the device performance.1−11 For instance, Sun et al. introduced organic HTL and ETL of poly(N,N′-bis(4-butylphenyl)-N,N′bis(phenyl)benzidine) (poly-TPD) and tris(8-hydroxyquinoline) aluminum (Alq3), respectively, for the fabrication of green, yellow, orange, and red QLEDs, generating maximum values of 9064 cd/m2 in luminance and 2.8 cd/A in current efficiency from a red device.1 Unlike the above organic CTLs, sputterdeposited CTLs of p-type NiO as HTL and n-type zinc tin oxide as ETL were also applied, but these all-inorganic devices exhibited a low external quantum efficiency (EQE) of
