pubs.acs.org/NanoLett
Multicolored Light-Emitting Diodes Based on All-Quantum-Dot Multilayer Films Using Layer-by-Layer Assembly Method Wan Ki Bae,†,‡,| Jeonghun Kwak,‡,| Jaehoon Lim,† Donggu Lee,‡ Min Ki Nam,§ Kookheon Char,*,† Changhee Lee,*,‡ and Seonghoon Lee*,§ †
Intelligent Hybrids Research Center, School of Chemical and Biological Engineering, The WCU Program of Chemical Convergence for Energy and Environment, ‡ School of Electrical Engineering and Computer Science, Inter-university Semiconductor Research Center (ISRC), and § School of Chemistry, NANO Systems Institute, National Core Research Center, Seoul National University, Seoul 151-747, Korea ABSTRACT A systematic analysis of the exciton-recombination zone within all-quantum dot (QD) multilayer films prepared by a layer-by-layer assembly method was made, using sensing QD layers in QD-based light-emitting diodes (QLEDs). Large area practical multicolored colloidal QLEDs were also demonstrated by patterning and placing variously colored QDs (red, orange, yellow-green, and green) in the exciton-recombination zone. KEYWORDS Quantum dots, light-emitting diodes, layer-by-layer assembly, exciton recombination zone
C
layer films,24-26 but these devices have shown only low current density and brightness (maximum brightness ∼1.54 cd m-2) due to the poor carrier conductivity of the intervening polymer. In contrast to these previous attempts, in the present study, we demonstrate multicolored QLEDs with improved device performance (600-fold higher maximum brightness compared with QLED employing QD/polyelectrolyte multilayer films24-26) based on all-QD multilayer films. Through the first systematic engineering of all-QD multilayer films, the exciton-recombination zone within the QLEDs was investigated, and multicolored QLEDs were demonstrated on the common device platform by patterning and placing variously colored QDs (red, orange, and green) in the exciton-recombination zone.
olloidal quantum dot (QD)-based light-emitting diodes (QLEDs) have the advantageous features of a narrow emission bandwidth, a wide emission spectral window in the visible region, and a low-cost synthesis based on a solution process.1-10 Ever since the first QLED has been demonstrated,1 rapid progress has been made in device performances through multilateral approaches, for instance, designing efficient and stable materials or understanding the underlying device physics.9,10 However, for the realization of full-color displays and lighting applications with QLEDs, the development of a fabrication process for the deposition of homogeneous and uniform QD layers over a large area with patterning capability is still necessary. The layer-by-layer assembly method is considered to be one of the most promising and practicable technologies for building up nanosized objects such as polymers,11 nanoparticles,12 nanowires,13 nanosheets,14 and biomolecules15 on various substrates16,17 in desired configurations. In contrast to conventional deposition processes, the layer-by-layer assembly employs specific interaction forces (i.e., electrostatic interactions,18 hydrogen bonding,19 or covalent bonding20) between each deposited layer and therefore provides uniform and homogeneous nanostructured films with welldefined internal structures over large areas along with the capability of hybridization with patterning techniques.21-23 Previously, the layer-by-layer assembly method has been employed for QLEDs comprising QD/polyelectrolyte multi-
All-QD multilayer films were fabricated by a layer-by-layer assembly method using electrostatic interactions between each layer through the sequential deposition of oppositely charged QDs onto the substrates (Figure 1a). Pristine QDs (capped with oleic acid) were surface-modified with cysteamine (CAm) or mercaptopropionic acid (MPA) and dispersed in water (pH 6 or pH 8) to endow positive (-NH3+) or negative (-COO-) charges, respectively, on the surfaces of the QDs. To obtain homogeneous and uniform QD multilayer films with well-defined internal structures, a spinassisted layer-by-layer assembly method27 was employed to flatten each layer by air-shear force and centrifugal force during the spinning process. The maximum coverage of each QD layer, obtained by increasing the concentration of the QD dispersion, was 70% of the theoretical maximum value (a hexagonally packed monolayer; see Supporting Information). The (QD-CAm/QD-MPA)n multilayer films showed linear growth behavior as a function of the number
* To whom correspondence should be addressed:
[email protected],
[email protected], and
[email protected]. |
These authors equally contributed to this work. Received for review: 01/18/2010 Published on Web: 05/26/2010 © 2010 American Chemical Society
2368
DOI: 10.1021/nl100168s | Nano Lett. 2010, 10, 2368–2373
FIGURE 1. Preparation of all-QD multilayer films and QLEDs based on all-QD multilayer films. (a) Schematic for the preparation of all-QD multilayer films based on spin-assisted layer-by-layer assembly by sequentially depositing oppositely charged QDs (blue QDs and red QDs represent negatively charged QDs (QD-MPA) and positively charged QDs (QD-CAm), respectively). (b) Growth behavior of all-QD multilayer films as a function of the number of QD bilayers ((QD-CAm/QD-MPA)n) determined by ellipsometry. (c) Device structure and (d) the energyband diagram (right) of a QLED comprising ITO/PAH (anode), all-QD multilayer films, TPBi (ETL), and LiF/Al (cathode).
of bilayers (Figure 1b), which indicates the successful deposition of each layer. The prepared multilayer films possessed a flat and uniform surface morphology (rms roughness ∼4 nm; see Figure S3 in Supporting Information), but minor defects still existed, i.e., small vacancies between adjacent QDs or partial aggregates of QDs, originating from the spherical geometry of the QDs and imperfections in the QD layering. A device structure of ITO/poly(allylamine hydrochloride) (PAH) (2 nm)/(QD-MPA/QD-CAm)n/2,2′,2′′-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) (40 nm)/LiF (0.5 nm)/Al (100 nm) was adopted for the QLEDs (Figure 1c). A 2 nm PAH layer was employed in the device for two reasons: to avoid probable exciton quenching within the QDs resulting from direct contact with the metal (ITO) and to guarantee a high-density deposition of QDs on the ITO by ensuring the surface charge density of ITO. A TPBi layer was chosen not only because it is a good electron-transporting layer (holeblocking layer) but because it also emits almost no electroluminescence, which guarantees the color purity of the EL spectra, as they would be mostly from the contributions of the QD multilayer films. Parts a and b of Figure 2 show the current-voltageluminance (I-V-L) characteristics and external quantum efficiency of the QLEDs as a function of the number of (QDMPA/QD-CAm)n bilayers. Here n ) 1 means a QD-MPA/QDCam bilayer. QLEDs prepared with a QD monolayer (n ) 0.5) showed low device efficiency due to the leakage current © 2010 American Chemical Society
originating from the direct junction of the TPBi layer and the metal (ITO/PAH) through vacancies in the QD film. In contrast, QLEDs with QD multilayer films thicker than three bilayers (n ) 3) showed a higher efficiency but low current density and brightness. The QLED comprising 2.5 QD bilayers exhibited the best performance in terms of brightness (450 cd m-2 at 50 mA cm-2, with a maximum brightness of 800 cd m-2) and device efficiency (0.3% at 50 mA cm-2). This is a significantly improved value (600-fold higher maximum brightness) in the respects of the requirements for practical applications compared with previous QLEDs comprising QD/polyelectrolyte multilayer films.24-26 Figure 2d displays the EL spectra of QLEDs comprising 2.5 bilayers of green and red QDs. Both QLEDs emitted Gaussian-shaped spectra (with the EL λmax at 512 and 624 nm for green and red QLEDs, respectively) with narrow spectral bandwidths (fwhm
