NANO LETTERS
Efficient CdSe/CdS Quantum Dot Light-Emitting Diodes Using a Thermally Polymerized Hole Transport Layer
2006 Vol. 6, No. 3 463-467
Jialong Zhao,† Julie A. Bardecker,‡ Andrea M. Munro,† Michelle S. Liu,‡ Yuhua Niu,‡ I-Kang Ding,† Jingdong Luo,‡ Baoquan Chen,‡ Alex K.-Y. Jen,*,†,‡ and David S. Ginger*,† Department of Chemistry, UniVersity of Washington, Box 351700, Seattle, Washington 98195-1700, and Department of Materials Science and Engineering, UniVersity of Washington, Box 352120, Seattle, Washington 98195-2120 Received December 7, 2005; Revised Manuscript Received January 13, 2006
ABSTRACT We report multilayer nanocrystal quantum dot light-emitting diodes (QD-LEDs) fabricated by spin-coating a monolayer of colloidal CdSe/CdS nanocrystals on top of thermally polymerized solvent-resistant hole-transport layers (HTLs). We obtain high-quality QD layers of controlled thickness (down to submonolayer) simply by spin-coating QD solutions directly onto the HTL. The resulting QD-LEDs exhibit narrow (∼30 nm, fwhm) electroluminescence from the QDs with virtually no emission from the organic matrix at any voltage. Using multiple spin-on HTLs improves the external quantum efficiency of the QD-LEDs to ∼0.8% at a brightness of 100 cd/m2 (with a maximum brightness over 1000 cd/m2). We conclude that QD-LEDs could be made more efficient by further optimization of the organic semiconductors.
Size-tunable band-gaps, high photoluminescence quantum efficiencies (QEs), good photostability, narrow emission line widths, large spin-orbit coupling, and compatibility with solution processing methods make colloidal semiconductor quantum dots (QDs) promising chromophores for use in optoelectronic devices such as light-emitting diodes (LEDs)1-4 and solar cells.5,6 Early QD-LEDs were fabricated by depositing thick CdSe QD layers on a semiconducting poly(p-phenylene vinylene) (PPV) layer1 or by doping QDs into a poly(vinylcarbazole) (PVK) matrix.2 Although efficiencies improved with the introduction of core/shell QDs, early devices exhibited both low external quantum efficiencies (EQE) (0.0005%-0.2%) and significant emission from the organic host polymers.1,2,7-9 Following the lessons from organic LEDs, it seems clear that multilayer device configurations are a promising route for the study of hybrid organic/ inorganic QD-LEDs, as such configurations allow the independent optimization of materials for charge injection, transport, and emission. Along these lines, Bulovic et al. described a hybrid organic/inorganic multilayer QD-LED structure with an EQE of ∼0.5% at 190 cd/m2 made using a phase separation process that sandwiches a monolayer of QDs between organic * Corresponding authors: Alex K.-Y. Jen: voice, (206) 543-2626; fax, (206) 543-3100; e-mail,
[email protected]. David S. Ginger: voice, (206) 685-2331; fax, (206) 685-8665; e-mail,
[email protected]. † Department of Chemistry. ‡ Department of Materials Science and Engineering. 10.1021/nl052417e CCC: $33.50 Published on Web 02/01/2006
© 2006 American Chemical Society
electron and hole transport layers.3 It is hypothesized that the thin QD layer helps mitigate the impact of the low QD carrier mobilities10 while the sandwich structure helps balance carrier injection. However, at high brightness, even these devices can exhibit significant emission from the organic matrix.3,11 In addition, the elegant phase separation process and resulting device performance can be sensitive to the surface properties of QDs, QD size distribution, organic host matrix, and processing conditions.11,12 Multilayer QD- and quantum-rod LEDs have also been made by spin-coating nanoparticle layers onto spin-coated hole transport layers (HTLs) using solvents that do not dissolve the underlying HTLs.13,14 For instance, QDs with hydrophilic surfaces were deposited on PVK by spin coating from aqueous solution. However, using incompatible solvents for the QD and HTLs can lead to significant dewetting and aggregation of the QDs.13 For applications ranging from efficient LEDs to the study of electroluminescence and charging of single chromophores, it is desirable to be able to disperse uniform, nonaggregated QD layers of any thickness from a submonolayer to a multilayer onto a compatible organic HTL that does not exhibit significant background electroluminescence. In this Letter, we describe the fabrication of QD-LEDs incorporating uniform CdSe/CdS core/shell QD layers by spin-coating the QDs from chloroform solution onto a hydrophobic thermally cross-linked HTL, polystyrene (PS)-
Figure 1. Absorption (dashed line), photoluminescence (dotted line), and electroluminescence spectra (solid line) for CdSe/CdS QD-LEDs with a structure ITO/PS-TPD-PFCB (30 nm)/CdSe QDs/TPBI (40 nm)/Ca (30 nm)/Ag (120 nm) at a voltage of 6.0 V. Structures of the multilayered CdSe/CdS QD-LEDs and cross-linked PS-TPD-PFCB are shown in the inset of this figure.
N,N′-diphenyl-N,N′-bis(4-n-butylphenyl)-(1,1′-biphenyl)4,4′-diamine (TPD)-perfluorocyclobutane (PFCB). We study the morphology of the resulting CdSe/CdS QD layers by atomic force microscopy (AFM) and show that this method produces high-quality, easily varied QD films and that (in contrast to most other reported QD-LEDs,1,2,7,8,11,13) the reported method produces QD-LEDs with nearly pure electroluminescence from the QDs with virtually no emission from the organic semiconductor layers. Finally, we demonstrate that the use of multiple solution-processable HTLs, as enabled by the solvent resistance of the cross-linked polymers, can significantly improve the QE and the brightness of these QD-LEDs. Indeed, including a second layer of 4,4′,4′′-tris(N-carbazolyl)triphenylamine bis(vinylbenzyl ether) (TCTA-BVB) spin-coated on top of the PS-TPDPFCB layer yields devices with 0.8% QE at 100 cd/m2 and maximum brightness in excess of 1000 cd/m2 again with pure, narrow emission from only the QDs. To produce the multilayer device structures studied herein (shown in the inset of Figure 1), the cross-linkable HTL, PS-TPD-PFCB (also shown in the inset of Figure 1), was first spin-coated onto a plasma-cleaned indium tin oxide (ITO) slide and then thermally cross-linked by heating to 235 °C for 45-60 min under nitrogen. Jen et al. have developed similar thermally polymerized hole transport materials containing N,N′-diphenyl-N,N′-bis(4-n-butylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) groups for use in highly efficient organic LEDs.15-18 It is well-known that TPD is a good hole transport material,19,20 but TPD has a tendency to recrystallize under operating conditions. Furthermore, TPD is soluble in common organic solvents, making multilayer device fabrication challenging. In contrast, thermally polymerized HTLs possess a high Tg (∼150 °C) and are able to resist interfacial erosion by organic solvents during the 464
spin coating of emissive layers, making them promising candidates for use in robust QD-LED device fabrication. After cross-linking the PS-TPD-PFCB, a layer of CdSe/ CdS QDs with an average diameter of 4.6 nm was spincoated from a chloroform solution with an optical density (OD) of 0.1-4.5 at the first nanoparticle excitonic absorption maximum, with a solution OD of ∼1.5 being found to produce a near monolayer of dots (see below) when spincoating was performed at 1500 rpm (this speed was used for all devices). The CdSe QDs were synthesized in-house using the hot injection CdO precursor route21 with an injection temperature at 280 °C and a growth temperature at 265 °C, and the CdS shells were grown on extracted CdSe cores using the successive ion layer adsorption and reaction (SILAR) process22 at 240 °C. The thickness of all the CdS shells was calculated to be approximately two to three monolayers (MLs). We note that it is very important to thoroughly wash and extract the as-synthesized QDs to remove excess ligand and synthetic byproducts. Postsynthesis, all QD samples were twice washed by dissolution in chloroform and subsequent precipitation with acetone followed by drying under N2. The quantum yield of the CdSe/ CdS particles used in these measurements was reduced from ∼50% to ∼30% due to the washing steps. Following deposition of the QD layer, the electron transport layer, 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI),23 was deposited by thermal evaporation. The devices were then completed by thermal evaporation of the back electrode (Ca (30 nm)/Ag (120 nm)) through a shadow mask. All evaporations were performed under a vacuum better than 4 × 10-6 Torr. Each device pixel had an active area of 3 × 2 or π × 12 mm2. Current-voltage characteristics of the QDLEDs were measured on a Hewlett-Packard 4155B semiconductor parameter analyzer. The electroluminescence emission power was measured using a Newport 2835-C multifunction optical meter. Electroluminescence spectra were recorded with an Oriel InstaSpec IV charge-coupled device camera. Photometric units (cd/m2) were calculated from the forward output power and the electroluminescence spectra of the devices, assuming Lambertian distribution of the electroluminescence emission.24 All device measurements were performed under ambient conditions. Figure 1 shows the absorption, photoluminescence, and device electroluminescence spectra from CdSe/CdS QDLEDs with a single HTL (PS-TPD-PFCB). For the CdSe QD-LEDs with a turn-on voltage of approximately 4.0 V using a thermally polymerized HTL, only a single electroluminescence band is observed at about 610 nm from the CdSe QDs at any forward bias (up to 20.0 V). The emission full width at half-maximum (fwhm) is about 30 nm, similar to the QD photoluminescence spectrum. The electroluminescence peak is slightly red shifted from the photoluminescence peak as has been observed by previous researchers and attributed to Fo¨rster energy transfer and/or the Stark effect.2,25,26 Importantly, we contrast the pure QD electroluminescence observed from these devices with the electroluminescence from the best QD-LEDs that we were able to prepare using the same batch of QDs by spin-coating mixed Nano Lett., Vol. 6, No. 3, 2006
Figure 2. AFM images of CdSe/CdS QD layers. The surface coverages for samples A-D were estimated to be 0.3, 0.7, 1.0, and 1.4 MLs of QDs, respectively, as discussed in the text.
Figure 3. Electroluminescence spectra of CdSe/CdS QD-LEDs with different QD layer thicknesses under forward biases of 6.0, 8.0, 10.0, 12.0, 14.0, and 16.0 V (from bottom to top). The surface coverages for samples A-D were estimated to be 0.3, 0.7, 1.0, and 1.4 MLs, respectively, as discussed in the text.
QD/molecular TPD solutions.3 In the molecular TPD samples, an emission band peaking near 500 nm is observed that we tentatively assign to an exciplex between the TPD and TPBI. The relative intensity of the 500 nm band is strongly dependent on the parameters of spin coating. Such emission from the organic matrix is common to many previously reported QD-LEDs,1-3,7,8,11,13,26 and the elimination of the organic electroluminescence using the simple and robust process of spin-coating QDs onto the organic HTL is thus significant. We hypothesize that the good performance of the multilayer devices reported herein results in part from the formation of uniform layers of QDs as a result of spin-coating hydrophobic dots from organic solvent onto a compatible (and very smooth) hydrophobic surface. This hypothesis is supported by the AFM images in Figure 2. It can be seen that the coverage of the QD layer can be varied from submonolayer with a root mean square (rms) surface roughness of 1.62 nm (Figure 2A) to a more complete monolayer with rms roughness near 0.68 nm (Figure 2B). We note that we cannot resolve individual QDs at high densities via AFM. Nevertheless, at monolayer coverage the QD layer becomes very smooth (rms roughness 0.62 nm as seen in Figure 2C) while for thicker QD layers, the film again becomes rougher (0.96 nm) (Figure 2D). The absorption spectra of the single monolayer of CdSe/CdS QDs on PS-TPD-PFCB (on glass slides) were measured, and their optical density at the band edge (585 nm) is about 0.002. This is in agreement with the expected absorption of a monolayer of QDs based on the extinction coefficient data of ref 27, and we thus assign the film in Figure 2C to a monolayer of QDs. By assuming Figure 2C is about 1 ML (based on the smoothness and OD measurements) we can assign the images from panels A-D of Figure 2 as corresponding to 0.3, 0.7, 1.0, and 1.4 MLs, respectively, by extrapolating from solution concentrations. We note that the formation of such uniform QD layers is also facilitated by the fact that the surface of the polymerized
PS-TPD-PFCB is also very smooth, with an rms surface roughness
