The Fundamental Research Of Top-emitting Organic Light-emitting Devices Based On Silicon

Organic Light-emitting Devices (OLEDs) have attracted intensiveinterests in the fields of science and industry for the merits of light weight ,low cost , broad visual angle , fast response speed , active emitting , highbrightness , high efficiency , availability for full color display , etc . Theresearch works in this field gained rapid development especially since 1987,C.W.Tang for the first time reported the high brightness OLEDs at lowoperating voltage . In recent ten years , OLEDs have become a project on thecutting-edge of scientific research that relates to many intercrossed branchesof science and advanced technology. Most of Organic Light-emitting Devices(OLEDs) use glass as the substrate, and all the structures of the devices aresubstrate emitting so far. Currently, Organic Light-emitting Devices (OLEDs)have been converted into developing top-emitting organic light-emittingdevices (OLEDs) based on silicon. It is very significant to develop OLEDsbased on silicon, with the result making use of mature electric circuit of siliconas the substrate to realize the valid integration of OLEDs and driver circuit .An efficient top-emitting organic light-emitting devices (TOLEDs) basedon silicon can be realized as follows: Firstly, a low turn-on voltage. That amaterial with a lower Ionization Potential than that of hole-transportingmaterials (NPB) is used as a hole-injection layer is an effective way toenhance the brightness at a low voltage. So we chose m-MTDATA as the holeinjection material by comparing with all kinds of hole injection materials, andwe prove that the luminance and current density of devices with m-MTDATAas the hole-transporting layer are higher than those without m-MTDATA.Secondly, a stable semi-transparent cathode with a high transparence is crucialfor TOLEDs. Ag, with the highest conductivity and minimum absorption inthe visible-light region, has a stable performance in air. Together with aLiF/Al bilayer inserted between organic materials and cathode Ag, an effectivecathode is obtained. Therefore, we adopted LiF /Al /Ag as a semi-transparentcathode for TOLEDs, which ensured the injection of electron and theconductivity and transmission of the cathode. Thirdly, the high reflectivity ofthe bottom anode is essential for achieving a high efficiency in a TOLEDbased on silicon. Although Ag has highest conductivity and strongestreflectivity in the visible-light region, it is not considered as an ideal anode forTOLED, because of its low workfunction (-4.3eV). However, Ag2O, reportedas P-type semiconductor, whose energy level matches with the HOMO levelof NPB. UV ozone treatment is a simple way to induce thin Ag2O film on Agsurface. Therefore, we used surface-modified Ag as anode and LiF/Al/Ag ascathode to prepare TOLEDs based on silicon using Alq as the emitting layer.The luminance of the device at 4V is 10cd/m2, and reaches 14090cd/m2 at 14V.The highest efficiency is 1.7cd/A at 8V.The performance of the device issuperior to that of the controlled device that used Ag-only as anode (withoutAg2O).The transparency of this combined cathode Al/Ag is poor because of alarge absorption of Al and a strong reflection at the interface of Ag/air due tolarge refractive index mismatches. So the cathode structure was improved byusing a semitransparent multilayer cathode structure of Al (2.5 nm)/Ag (0.5nm)/Al (1 nm)/Ag (0.5 nm)/Al (0.5 nm) /Ag (11 nm) which can improve lightoutcoupling.In addition, Alq was utilized as an anti-reflective layer on themulti-layer semitransparent cathode to improve the transparency of TOLEDs.By adjusting the thickness of Alq in a range of 0-120nm, we conclude that the78-nm-thick Alq3 layer overlaid on the cathode lead to an excellenttransmission of 70 %. With this cathode structure and an additive 78 nm Alq3film as the top-capping layer, the luminance was enhanced by a factor of1.9–3.1 (the bias from 7 to 21 V) comparing with conventional cathodestructure Al (4 nm)/Ag (12 nm). The improvement originates from a gradualchange of index of refraction and extinction coefficient in the alloylikecathode and from reduction of interface reflection of Ag/air by overlaying atop-capping Alq3 layer onto multilayer cathode. Besides, photoluminescenceof the top Alq3 layer was helpful to improve device luminance.