Studies On Organic Light-Emitting Devices Based On Polymer Anode

Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)(PEDOT:PSS) is a polymermaterial which has high conductivity and transmittance in the visible light region. Itswork function of5.1eV can facilitate hole injection that enhances the carrier balanceof OLEDs. The methods of depositing a PEDOT: PSS film include spin-coating (SC),gravure printing (GP), inkjet printing (IJP), etc. Screen printing (SP) is a simple,low-cost technique, and it can deposit a polymer film with various patterns. We usedthe method of screen printing to fabricate PEDOT: PSS anode on glass substrate,which sheet resistance was300/□. We also used this polymer anode to fabricateyellow OLEDs with the structure glass/anodes/MoO3(5nm)/NPB(40nm)/CBP：PO-01(8%,30nm)/Bphen(20nm)/LiF(0.5nm)/Mg：Ag(10：1,150nm), anodes werePEODT:PSS, PEDOT:PSS with external ITO and ITO, respectively. At the samevoltage, the current densities of devices with PEDOT:PSS anode were lower than ofdevice with ITO anode due to the higher sheet resistance of PEDOT:PSS. However, theluminance and power efficiencies of device with anode of PEDOT:PSS with externalITO were higher than of device with ITO anode. The maximum luminance efficienciesof devices with PEDOT:PSS with external ITO and ITO were24.9and20.9cd/A,respectively. The maximum power efficiencies of devices with PEDOT:PSS withexternal ITO and ITO were19.0and16.7lm/W, respectively. It’s showed that theexternal ITO of PEDOT:PSS anode played an important role of enhancing the holeinjection. It also showed that the polymer anode fabricated by using screen-printingmethod was feasible for OLED anode.Next we used the method of screen printing to fabricate PEDOT: PSS anode onflexible PET substrate. The screen-printed PEDOT:PSS layer had a thickness of1.5μm, and the average optical transmittance decreased slightly from82%for the purePET substrate to74%for PEDOT: PSS/PET in the visible light region. Then, thebending test was carried out to explore the flexibility of the anodes. The initial sheet resistances of the PEDOT: PSS anodes on the PET substrate was296/□. After700bending cycles, the sheet resistance of the PEDOT: PSS anode increased only to1.8times the initial value.Next we used the prepared flexible PEDOT:PSS anode to fabricate flexible greenOLEDs with the structure PET/anode/MoO3(5nm)/NPB(30nm)/3DTAPBP(10nm)/POAPF：Ir(ppy)3(10%,30nm)/TpPyPB(30nm)/LiF(0.5nm)/Mg：Ag(10：1,150nm), anodes were PEDOT:PSS and ITO, respectively. The turn-on voltage of deviceswith PEDOT:PSS and ITO anodes were2.4V and2.2V, respectively. At the samevoltage, the current density and luminance of device with PEDOT:PSS anode werelower than of device with ITO anode. The maximum luminance efficiencies of deviceswith PEDOT:PSS and ITO anodes were35.1and38.0cd/A, respectively.Finally white FOLEDs with the structure PET/anode/MoO3(5nm)/NPB(30nm)/3DTAPBP(10nm)/Ir(BT)2acac (0.1nm)/POAPF:10wt%Firpic (30nm)/Ir(BT)2acac (0.1nm)/TpPyPB (30nm)/LiF (0.5nm)/Mg:Ag were fabricated. Theanodes were PEDOT: PSS (EL-P3145)(device A), PEDOT: PSS(EL-P3145)/PEDOT: PSS(P AI4083)(device B) and ITO (device C), respectively. Thebrightness of device B was the highest at the same current density. For example, thebrightness values were1590cd/m~2,2380cd/m~2and1550cd/m~2for devices A, B, andC, respectively, at10mA/cm~2. The maximum luminance efficiencies of devices A, Band C were24.8cd/A,26.7cd/A and18.6cd/A. The maximum power efficiencies ofdevices A, B and C were11.1lm/W,12.2lm/W and14.8lm/W, respectively. In orderto explore the flexibility of white OLEDs, bending test of the FOLED was carried out.Devices B and C were bent outwards after100bending cycles at a bending radius of20mm. Unfortunately, many cracks were observed for device C after100bendingcycles, and there was almost no light emission. After100bending cycles, themaximum luminance efficiency of device B decreased from26.7to17.2cd/A, but thepower efficiency could also reach5.0lm/W at a high brightness of1000cd/m~2.