Microcavity Top-Emitting Organic Light-Emitting Devices

Organic light-emitting devices (OLEDs) have become the active field of research because of their potential application of flat panel display. As passive matrix driven pixels and small size display mature, active matrix driven pixels and big size display have attracted more and more attention. The conventional bottom-emitting OLEDs (BOLEDs) structure usually include a transparent substrate and a transparent indium-tin-oxide(ITO) bottom electrode. Sacrificial aperture ratios of pixels will occur when incorporating BOLEDs onto opaque substrates, amorphous silicon and organic thin-film transistors(TFT). For display applications active matrix driven pixels are required and a good way to accomplish this is to make top-emitting OLEDs (TOLEDs). This would allow OLEDs to be incorporated onto opaque substrates such as Si wafers and active-matrix backplanes with complicated pixel circuits of TFT for higher display quality yet without sacrificing aperture ratios of pixels. TOLEDs have been subjects of intensive studies recently.In this dissertation, main study is on blue and white BOLEDs, TOLEDs, RGB tricolor produced by microcavity TOLEDs, and optimization of efficiency of TOLEDs, and so on. Efficient TOLED using MoO_x modified Ag as the effective hole-injection anode is demonstrated for the first time. The main contents include:1) In the study of BOLEDs, firstly, four blue BOLEDs based on ADN doped with amino-substituted distyrylarylene derivatives (BCzVB), tetra-butylperylene (TBPe), amino-substituted distyrylarylene derivatives (BCzVBi) and p-bis(p-N, N-diphenylaminostyryl)benzene (DSA-Ph) are fabricated. The current efficiencies of these four devices are 2.8cd/A, 5cd/A, 2.03cd/A and 8cd/A, respectively. Commission Internationale d'Eclairage (CIE) coordinates are (x=0.146, y=0.165), (x=0.136,y=0.222), (x=0.164,y=0.146) and (x=0.153, y=0.306), respectively. From the view point of color purity and the response of efficiency vs current density, It seems that BczVB is a promising dopant for blue OLED based on AND, and DSA-Ph is more suitable for white OLED. Secondly, white OLED is constructed by sandwiching an exciton blocking layer TBPi between the emitting layer and the electron injection layer. The device structure is ITO/CuPc/J0503/JBEM(p):DCJT/TPBi/Alq/LiF/Al. It exhibits an efficiency of 3.42cd/A at a current density 4mA/cm², a maximum luminance of 11000cd/m² at 16V, and the CIE coordinates(x=0.34, y=0.36). The device with exciton confined layer shows a 60% higher efficiency than that of corresponding device without TBPi. The enhancement of efficiency is attributed to high density of exciton due to confinement effect. Finally, based on multi-emitting layers Alq:DCJTB/TBADN:TBPe/Alq:C545, a white OLEDs with stable color saturation is achieved. It shows an efficiency of 3.78cd/A and the CIE coordinates (x=0.345,y=0.323 ) at a current density 200mA/cm².2) In the study of TOLEDs, firstly, semi-transparent metal cathode with a structure of LiF(lnm)/Al(10nm)/Ag(2nnm) is achieved. The transmission of the semitransparent cathode is about 30%. Color tunable TOLEDs with Ag(100nm)/ITO(x nm) as reflector anode and LiF/Al/Ag as semitransparent cathode are fabricated. With variation of ITO thickness, different emission colors from bluish green to orange can be obtained from the devices with a single Alq₃ emitter. When the thickness of ITO is 210nm, the Alq₃ based TOLED exhibits a narrowed Electroluminescent(EL) peak at 536nm with a full width at half maximun(FWHM) of 22nm and an efficiency of 1.77cd/A. Secondly, with TBADN:TBPe as emitting layer, highly saturated blue TOLED with CIE_x,y(0.141,0.049) is demonstrated by adjusting the thickness of ITO. When the thickness of ITO is 155nm, TBADN:TBPe based TOLED exhibits a narrowed EL peak at 464nm. Finally, with Alq:DCJTB/TBADN:TBPe/Alq:C545 as white light emitting layer, RGB tricolor TOLEDs are achieved by adjusting the thickness of ITO. The peak wavelengths are 476nm, 539nm and 601nm; CIE coordinates are (0.133,0.201), (0.335,0.567) and (0.513,0.360); FWHMs are 30nm, 48nm and 70nm for blue, green and red device, respectively.3) In order to obtain efficient TOLEDs, firstly, Alq₃ is used as capping layer to improve the transmission of cathode. The transmission of cathode LiF(1nm)/Al(10nm)/Ag(2nm)/Alq₃(45nm) is about 50%, which is 67% higher than that of LiF(lnm)/Al(10nm)/Ag(2nm) without capping layer. And the transmission varies little with visible spectrum. For instance, the transmittances at 460nm, 552nm and 620nm are 46.8%, 45.3% and 49.8%, respectively. Secondly, efficient TOLED using MoO_x modified Ag as the effective hole-injection anode is demonstrated. With Alq₃ as emitting layer, LiF/Al/Ag/Alq₃ as semitransparent cathode, the Ag/MoO_x based TOLED shows a tune-on voltage of 2.67V, and a maximum current efficiency of 7.27cd/A, which are much better than those (3.92V, 6.12cd/A) obtained from Ag/AgO_x based TOLED, and those (5.25V, 3.5cd/A) obtained from corresponding BOLED. Contact potential difference measurement study shows that the work function of Ag/MoO_x is higher than those of Ag/Ag₂O and ozone-treated ITO, leading to a stronger hole injection. Finally, the influences of different thickness of organic stack on the performances of TOLEDs are invested. When the thickness of organic stack is 115nm, the optimized TOLED exhibits an efficiency of 7.44cd/A, which is much greater than that obtained from the corresponding BOLED. Based on the EL spectra simulation of TOLED with different thickness of organic stack, suitable resonant mode index (m=2) is confirmed. A fairly good match between the calculated value and the experimental data is achieved.