LETTER pubs.acs.org/NanoLett
High-Efficiency Silicon Nanocrystal Light-Emitting Devices Kai-Yuan Cheng,† Rebecca Anthony,‡ Uwe R. Kortshagen,‡ and Russell J. Holmes*,† †
Department of Chemical Engineering and Materials Science and ‡Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States
bS Supporting Information ABSTRACT: We demonstrate highly efficient electroluminescence from silicon nanocrystals (SiNCs). In an optimized nanocrystal-organic light-emitting device, peak external quantum efficiencies of up to 8.6% can be realized with emission originating solely from the SiNCs. The high efficiencies reported here demonstrate for the first time that with an appropriate choice of device architecture it is possible to achieve highly efficient electroluminescence from nanocrystals of an indirect band gap semiconductor. KEYWORDS: Silicon nanocrystal, hybrid nanocrystal-OLEDs, electroluminescence
S
pectral tunability and excellent photoluminescence efficiency are the attributes of colloidal semiconductor nanocrystals13 that have been exploited in hybrid nanocrystal organic light-emitting devices (NC-OLEDs).314 Such architectures typically contain organic layers to facilitate charge carrier transport. Interest in NCOLEDs for display15,16 and solid-state lighting11 applications has grown significantly in recent years. Nanocrystals are attractive for their size-tunable emission properties,1 their compatibility with solution processing, and for recent demonstrations of good device external quantum efficiencies (ηEQE).2,3 To date, attention has been paid almost exclusively to studies of group IIVI, IIIV, and IVVI semiconductor nanocrystals.3 Of particular note are IIVI and IVVI nanocrystals, which have been used to demonstrate tunable electroluminescence (EL) across both the visible and infrared portions of the electromagnetic spectrum.414 Recently, light-emitting devices based on CdSe/ZnS coreshell nanocrystals have been demonstrated with a high ηEQE of ∼2.7% for EL at a wavelength of λ∼600 nm.14 Architectures comprising inorganic transport layers have also been proposed, however, their performance remains lower than in corresponding hybrid structures.17 While much interest has been focused on the study of IIVI and IVVI colloidal semiconductor nanocrystals, less emphasis has been placed on group IV systems, including silicon. While bulk silicon is characterized by an indirect band gap, silicon nanocrystals (SiNCs) with diameters less than 5 nm have been shown to be capable of exceptionally high photoluminescence (PL) efficiencies (ηPL).18,19 Silicon nanocrystals are also attractive for their potentially low toxicity and high natural abundance. Consequently, there have been attempts to integrate SiNCs into hybrid light-emitting devices.20,21 To date, however, device efficiencies have been significantly lower than those obtained using IIVI counterparts and have not reflected the reported large ηPL values. In this study, SiNCs are synthesized using a nonthermal plasma18 and nanocrystal surfaces are functionalized with ligands of r 2011 American Chemical Society
1-dodecene, permitting ηPL ∼ 4060% in uniformly dispersed solution.19 The nanocrystals have diameters of 5 and 3 nm, corresponding to peak emission wavelengths of 853 and 777 nm (Figure 1), and ηPL values of 45 and 43%, respectively. Devices were constructed on glass slides coated with a 150 nm thick layer of indiumtinoxide (ITO), which serves as a transparent anode. A 30 nm thick layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT/PSS) was spun-cast (3000 rpm, 30 s) onto the ITO as a hole-injection layer and was baked at 150 °C for 90 min, followed by a 25 nm thick hole-transport layer (HTL) of cross-linked poly(N,N0 -bis(4-butylphenyl)-N, N 0 -bis(phenyl)benzidine (poly-TPD). The poly-TPD was dissolved in chloroform (5 mg/mL) and cross-linked after spin-coating (8000 rpm, 90 s) by exposure to ultraviolet light (λ = 254 nm, 1.7 mW/cm2) for 80 min.22,23 The emissive layer consisted of a 53 nm thick layer of SiNCs spun-cast from a solution of chloroform (20 mg/mL, 2000 rpm, 90 s) on top of poly-TPD and baked at 64 °C for 1 h. The emissive layer was capped with a 20 nm thick electron-transport layer (ETL) consisting of tris-(8-hydroxyquinolinato) aluminum (Alq3) deposited by vacuum thermal sublimation. The device cathode consisted of a 0.5 nm thick layer of LiF and a 50 nm thick layer of Al. Devices were characterized in air ambient immediately after fabrication. It is important to note that in this work, the polymer HTL is selected for its large energy gap and ability to be cross-linked. One limitation of previous work on silicon NC-OLEDs has been the use of nonorthogonal solvents for the nanocrystal and polymer transport layers leading to redissolving of the polymer layer. The low efficiencies previously observed for silicon NC-OLEDs could also reflect inefficient charge carrier injection and confinement in the Received: January 16, 2011 Published: April 04, 2011 1952
dx.doi.org/10.1021/nl2001692 | Nano Lett. 2011, 11, 1952–1956
Nano Letters
LETTER
Figure 1. Nanocrystal photoluminescence and size. (a) Photoluminescence for silicon nanocrystals chemically passivated with ligands of 1-dodecene in the hydrosilylation solution of 1:5 1-dodecene:mesitylene. The diameters of the nanocrystals shown are 5 nm (solid line) and 3 nm (broken line) with peak emission wavelengths of 853 and 777 nm, respectively. For both spectra, the excitation wavelength is 395 nm. Transmission electron micrographs of silicon nanocrystals having diameters of (b) 3 nm and (c) 5 nm.
SiNC layer.21 The use of optimized, wide energy gap HTL and ETL materials resolve these problems by creating a double heterostructure that permits the confinement of both charge carriers and excitons to the nanocrystal emissive layer. These modifications contribute to a drastic improvement in NC-OLED efficiency and also eliminate parasitic EL originating from the polymer HTL. Using this system, we demonstrate ηEQE values that to our knowledge are more than twice as large as those previously reported for any NCOLED system.7,13,14 High efficiency is observed for SiNCs of varying sizes at EL wavelengths ranging from the red to the infrared. Interestingly, while in most previous studies the organic transport layers are chosen to have the largest possible charge carrier mobility,12 we demonstrate here that while the mobility is important for low voltage operation, it is more critical to engineer an architecture that permits effective charge and exciton confinement in the nanocrystal layer. The EL of devices constructed using both 5 and 3 nm diameter SiNCs was collected as a function of applied current density and is shown in Figure 2a,b. Emission originating from the SiNCs is clearly observed with no emission from the adjacent transport layers. The lack of transport layer emission confirms that charge carriers and excitons are well-confined to the emissive layer. In Figure 2a,b, a blue-shift of the emission peak is observed with increasing current density. This spectral change is reversible with current density and may reflect the size dispersion of the SiNCs and a variation of the charge injection and PL efficiency with particle diameter. Figure 2ce shows device performance for NC-OLEDs based on SiNCs of varying size and PL efficiency, whose corresponding EL is shown in Figure 2a,b. A peak ηEQE of 8.6% is realized for devices containing SiNCs that are 5 nm in diameter, and the efficiency remains >4% to a current density of ∼0.2 mA/cm2. These devices exhibit a turn-on voltage of ∼1.55 V, and a peak optical power output of 0.13 mW/cm2. Here, the turn-on voltage is defined as the voltage leading to an optical power output of ∼5 nW/cm2. Devices constructed using SiNCs that are 3 nm in diameter show a peak ηEQE of 6.7%, and the efficiency remains >4% to a current density of 0.3 mA/cm2. These devices show a turn-on voltage of ∼2.30 V, and a peak optical power output of 0.27 mW/cm2. These efficiencies are for emission in the forward-viewing direction, and the forward-emitted optical
power was measured using a large-area photodetector (see Supporting Information). In order to investigate how the choice of transport layer impacts device efficiency, NC-OLEDs were examined as a function of ETL material. In order to determine the role of the electron mobility in determining device performance, thermally evaporated layers of either 4,40 ,400 -tris-(N-carbazolyl)-triphenlyamine (TCTA) or N, N0 -dicarbazolyl-4-40 -biphenyl (CBP) were chosen for comparison with Alq3. Typically used as an HTL in organic light-emitting devices, TCTA has a very low electron mobility
