高效湿处理磷光有机发光二极管

I. INTRODUCTION  

Phosphorescent organic light emitting diodes (OLEDs)  have drawn considerable attention because of their ability to  harvest both singlet and triplet excitons simultaneously through  intersystem crossing, approaching a near 100% internal  quantum efficiency.1  Mostly, highly efficient phosphorescent  OLEDs have been fabricated through high vacuum thermal  evaporation by following multilayer device architecture.2-3 The  additional layers like charge-transport and carrier-blocking can  help balance charge injection to the emissive layer and enhance  device efficiency. To make cost-effective and large-area rollto-roll fabrication, wet-processable phosphorescent OLEDs  with higher efficiencies are in extreme demand.4-5

In this work, high efficiency green phosphorescent OLED was fabricated by using spin-coating to deposit hole-injection  layer, poly(ethylenedioxythiophene):poly(styrenesulfonate)  (PEDOT:PSS) and emissive layer. The resultant OLED device  exhibited a power efficiency of 46 lm/W and current efficiency  of 58 cd/A at 100 cd/m2 . Further, 4,4,4-tris(N-carbazolyl)- triphenylamine (TCTA) and N,N-dicarbazolyl-3,5-benzene  (mCP) layer were deposited onto hole-injection layer  PEDOT:PSS through spin-coating in the individual devices.  The resultant devices showed significant performance  improvement.

II. EXPERIMENTAL  

A. Device Fabrication  

The schematic energy-level diagrams of OLEDs are shown  in Figure 1. All the devices were fabricated on pre-cleaned  glass substrate coated with a 125 nm indium tin oxide (ITO)  layer. The fabrication process included first spin-coating an  aqueous solution of PEDOT:PSS at 4000 rpm for 20 s on ITO  anode layer. The resulting hole-injection layer was baked at  150 °C for 40 mins. The emissive layer consisted of a host  4,4,NN-dicarbazolebiphenyl (CBP) doped with green dye  tris(2phenyl-pyridine) iridium (Ir(ppy)3). The emissive layer  solution was prepared in tetrahydrofuran at temperature of 40  °C for 0.5 h with stirring. The resulting solution was then spincoated at 2500 rpm for 20s under nitrogen. Thereafter, the  electron-transporting layer TPBi, the electron injection layer  LiF, and the cathode Al, were deposited by thermal  evaporation in a vacuum chamber at the base pressure of 10-4 Torr (Device A). For Device B and C, an additional holetransport layer of TCTA and mCP spin coated at 2500 rpm for  20s onto PEDOT:PSS, respectively. The resulting holetransport layers of TCTA and mCP were baked at temperature  60 °C and 50 °C for 30 mins, respectively to remove residual  solvent before the deposition of the emissive layer.

B. Device Characterization  

All the resultant devices were measured under atmospheric  condition. The current density-voltage and luminance (J-V-L)  characteristics of the resultant devices were measured through  a Keithley2400 electrometer with Minolta CS-100A  luminance-meter, while the spectrum and CIE color chromatic  coordinates were measured by using PR-655spectrascan  spectroradiometer.

Fig1

III. RESULTS AND DISCUSSION 

Table 1 compares the performance of the wet-processed  green phosphorescent OLEDs without (Device A) and with an  additional hole-transport layer (Devices B and C). Initially,  Device A exhibited a power efficiency of 46 lm/W, and a  current efficiency of 58 cd/A, and an external quantum  efficiency of 15.7% at 100 cd/m2 . The employment of the small  molecule host material, CBP, could satisfy the requirements for  high efficiency phosphorescent OLED. First, the triplet energy  of CBP (2.55 eV) is higher than that of green emitter, Ir(ppy)3,  which is 2.4 eV, avoiding the quenching of triplet exciton of  the emitter by the host. Subsequently, host CBP makes an  efficient energy transfer route from host to the triplet emitter,  resulting in a high power efficiency. Secondly, CBP has  bipolar charge-transport characteristics.13 To achieve a high  recombination rate in the desired emitting layer, a balanced  carrier-injection to the emissive layer is hence necessary. The  inherent bipolar characteristic of CBP balanced the hole- and  electron-transport to the emissive layer, improving  recombination rate and also decrease the efficiency roll-off.  Thirdly, a good solubility of host CBP and green emitter  Ir(ppy)3 in the solvent, tetrahydrofuran, ensuring a smooth film  thickness and also help prevent non-uniformity resulting from  unwanted segregation of host or emitters materials.