ABSTRACT
Recently, much attention has been paid to using nanomaterials for improving the electroluminescent (EL) performance of OLEDs.4-7 Among those nanomaterials, SWCNTs are expected to be used in OLEDs due to their facile synthesis process, excellent mechanical properties, and good carrier-transporting ability. Although SWCNTs
have been reported to improve the EL efficiency when used as polymer:SWCNTs composites, their role in OLEDs is not clear yet. Several possible explanations have been proposed, including a) improving hole injection and transportation8, b) improving the conductivity of polymer films9, c) blocking holes in the polymer composite10, and d) trapping holes of SWCNTs in a hole-conducting polymer.11 However, these explanations are not suitable for an explanation of all the phenomena occurring in the reported literature. We investigated the roles of SWCNTs on the EL performance of OLEDs when mixing them with a hole-conducting material and a light-emitting material, respectively. The effects of SWCNTs in OLEDs have been elucidated. Our findings provide a clear recognition of the role of SWCNTs in OLEDs and will be helpful for improving OLED performance by using SWCNTs. The SWCNTs were synthesized by a laser ablation method12 and were purified by hydrothermal and chemical treatments. The hole injection material poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrene sulfonate) (PSS) (Baytron P 4083) (PEDOT) was acquired from the Bayer Company. The hole-transporting material poly(9-vinylcarbazole) (PVK) and the electron-transporting material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) were purchased from the Sigma Aldrich Company and used as received. The light-emitting conjugated polymer polyfluorence (PFO; purchased from the H. W. Sand Company) and Super-Yellow (purchased from the Merck Company) were used as received. The electrophosphorescent material iridium (III) tris(2-(4-totyl) pyridinato-N,C2) [Ir(mppy)3] was purchased from American Dye Source Inc. To investigate the role of SWCNTs in the performance of the OLEDs, three types of device architecture were fabricated. The first type of devices has the structure of ITO/PEDOT:SWCNT (x wt%, 40 nm)/PFO (80 nm)/Ca (20 nm)/Al (80 nm), where x is 0, 0.001, 0.01, and 0.02, respectively relative to solid PEDOT content. The second one is ITO/PEDOT:SWCNT (0.01 wt% relative to solid PEDOT content, 40 nm)/PVK:PBD:Ir(mppy)3 (69 wt% : 30 wt% : 1 wt%, 60 nm)/Ca (20 nm)/Al (80 nm), which use Ir(mppy)3 as a phosphorescent emissive material. The third one has a structure of ITO/PEDOT (40 0m)/Super-Yellow:SWCNT (x wt%, 80 nm)/Ca (20 nm)/Al (80 nm), where x is 0 and 0.005, respectively. To get the uniform dispersion of SWCNTs in PEDOT, we suspended the SWCNTs in deionized water and ultrasonicated the
SWCNT solution for 30 minutes, and then we mixed the SWCNT solution with the PEDOT solution and deionized water to form the composites at the designed doping concentration. For the SuperYellow:SWCNT composite preparation, SWCNTs dispersed in chloroform were added to the Super-Yellow chloroform solution to give the desired doping concentration. The PEDOT:SWCNTs composites were spin-coated onto an ITO-coated glass substrate, and the thickness of the film was 40 nm. The light-emitting layers for the three types of devices were all formed by spin coating from their chloroform solutions. Finally, the calcium (Ca) and aluminum (Al) electrodes were deposited sequentially by thermal evaporation under the vacuum of 2 × 10−6 Torr, and the deposition rate was typically about 1 Å/s. The thicknesses and morphology of the films were measured by Vecco atomic force microscopy (AFM) using a tapping mode. The resistivity and conductivity of PEDOT:SWCNT composite films on glass substrates were measured by a four-probe method with a Keithley source meter. The current and voltage characteristics of the OLEDs were measured with a Keithley 2400 source meter. The EL spectra of OLEDs were recorded by a HORIBA Jobin Yvon spectrometer. The luminance and efficiency of OLEDs were measured by an OLED testing system calibrated by the National Institute of Standards and Technology. We investigated the roles of SWCNTs in a hole-conducting layer on the performance of OLEDs at first. Figure 5.1a shows the current density-voltage (J-V) characteristics of OLEDs employing PEDOT:SWCNT composites with various doping concentrations under the forward bias. By increasing the SWCNT concentration in PEDOT, the current density of devices becomes higher under the same voltage, which means the hole injection and/or transport is improved by introducing SWCNTs in the PEDOT layer of devices. However, the EL intensity didn’t show a monotonous increase when increasing the SWCNT concentration in PEDOT under the same current density. From Fig. 5.1b we can see, when the SWCNT concentration in PEDOT is no higher than 0.01 wt%, the higher the doping concentration, the stronger the EL intensity. But when the doping concentration of SWCNT reaches 0.02 wt%, it will deteriorate the EL performance of devices and the EL intensity becomes lower than that of the device without SWCNT doping. Our results have some difference from those reported by Woo et al.11 In their case, the doping concentration of SWCNTs in PEDOT is no less than 0.05
wt%. Although they observed the enhancement of current density when increasing the SWCNT doping concentration in PEDOT, which is similar with our case, they only found reduced EL brightness in devices employing PEDOT:SWCNT composites relative to the device without using SWCNTs. As a result, their explanation of SWCNTs as hole traps in a PEDOT film should be reconsidered, since there will be no EL intensity enhancement in our case if the injected holes were initially trapped by SWCNTs in a PEDOT film.
