Study On Adjusting The Spectra Of Inverted Top-emitting Organic Light-emitting Devices

Organic light-emitting devices (OLEDs) are electric injected double-carriers type display devices. Compared to traditional display technologies, OLED technology is apromising flat panel display technology due to the merits of light weight, broad viewing angle, low cost, high contrast, high brightness, fast response speed, etc. Since C.W.Tang reported the high brightness OLED at low operating voltage for the first time in 1987, The performance of OLEDs have been greatly improved during the last two decades. A variety of high-performance, long-life organic light-emitting materials and devices are being reported. OLEDs are considered to become next-generation mainstream flat panel display technology.In order to utilize the advantages of the large aperture ratio of top-emitting OLED in the field of active matrix displays and the high mobility of n-channel silicon as well as its convenience in manufacturing and integrate OLEDs with the drivers, it's essential to study the principle of inverted top-emtting organic light-emitting devices (ITOLED).The mobility is weak in organic materials, this makes the efficiency of the devices not be very high. Therefore it is necessary to adopt p-type doping or/and n-type doping technologies to improve the ability of the hole or/and electron injection , reduce the device operating voltage and increase the luminous efficiency.In this paper, the ITOLED with the C545T-doped Alq3 as the light-emitting layer, the different thicknesses m-MTDATA:MoOx as the doped hole injection layers is investigated. The structure of the device is Al(60nm)/LiF(1 nm)/ Alq₃(40nm)/ Alq₃:C545T(1%,20nm)/ NPB (5nm)/ m-MTDATA (35nm) m-MTDATA:MoO_x (3:1, X nm)/Ag(22 nm). The devices with the thicknesses of m-MTDATA: MoOx of 10,20,30,40 nm are named device A, device B, device C, device D respectively.As compared, the device with 10nm m-MTDATA:MoO_x has maximum luminance of 2.5 lm/w under 10V forward bias, and maximum brightness of 23000 cd/m² under 350 mA/cm² current density, corresponding to a low turn-on voltage of 5.0 V. Compared to other ITOLEDs, this ITOLED shows reduced turn-on voltage and increased power efficiency. When the doped layer thicknesses change from 10nm to 40nm, the coverage of the device electroluminescence spectra change obviously.To take the advantage of the microcavity effect that exists in the ITOLED, by changing its thickness rather than introducting other materials, we can adjust the electroluminescence spectra of the device ,achieve multi-peak emission and adjust the spectral width by introduction of the doped hole injection layer m-MTDATA:MoOx without influencing the operating voltage. This will be an important guiding which is significant of preparing full-color display devices and white ITOLEDs of high color-rendering index.Because of the top semi-transparent Ag has a certain absorption in visible light and the mismatching refractive index between Ag and air, it leads to the outcoupling characteristics of the devices are not ideal. To solve the problem, depositing the light outcoupling layer m-MTDATA of appropriate thickness above Ag.The structure of the device is Al( 60 nm)/ Alq₃:LiF(10:1, 10 nm)/ Alq₃( 30 nm)/Alq₃: C545T(1%,20nm)/NPB(5nm)/m-MTDATA(35nm)/m-MTDATA:MoO_x(3:1,10nm)/Ag(22nm)/m-MTDATA(Xnm). The devices with the thicknesses of m-MTDATA of 0, 60, 90nm are named device A, device B, device C respectively.The device with 60nm m-MTDATA has maximum brightness of 11140cd/m² and maximum current efficiency of 9.48cd/A when the current density is 117.57mA/cm².This shows that m-MTDATA as the light outcoupling layer plays a very important role in anti-reflection of specific wavelength light. The device with 60nm m-MTDATA also has the largest full-width-half-maximum of the electroluminescence spectra at different voltages. As the angle increases, the shifting range of the spectra of the devices with different outcoupling layer thickness is all within 4nm.The extent of such spectra shift is not obvious and it is conducive to the applications of such OLED in display area.