A facile and efficient preparation method for the doped light emitting layer in OLEDs: Blade-coated planar source evaporation

Abstract

Although vacuum thermal evaporation (VTE) is a widely-used fabrication method in organic light-emitting diodes (OLED) industry, it has long been suffering from the sophisticated fabrication process, low material utilization, and high costs, which become much significant in the preparation of doped light emitting layer (EML). To overcome those intrinsic drawbacks of VTE, we propose a facile and efficient preparation method for the doped EML, i.e., blade-coated planar source evaporation (BCPSE), which combines the benefits of both solution-based blade-coating and combined planar source evaporation (BCPSE) techniques. BCPSE can simplify the fabrication process, increase the material utilization, and reduce the fabrication time, without degrading the device performance compared to that prepared by VTE. The film morphology and fabrication condition were carefully investigated and optimized, which show that BCPSE can increase the material utilization to 95 % and decrease the preparation time to several seconds, implying a great reduction in the fabrication cost. The validity of BCPSE was further demonstrated in a comparison study of a typical orange phosphorescent OLED, in which the mixed ternary EML comprising iridium-based dopant, n-type host, and p-type host was fabricated by using both VTE and BCPSE techniques. With the BCPSE technique, the device exhibited improved maximum external quantum efficiency (EQEmax) of 7.9 %, maximum current efficiency (CEmax) of 13.7 cd/A, and maximum power efficiency (PEmax) of 12.0 lm/W, which are comparable to the device employing VTE. In addition, both techniques gave the same electroluminescent spectra with the Commission International d'Eclairage (CIE) coordinates of (0.59, 0.38). The proposed method is not only helpful in reducing the fabrication costs while maintaining the device performance, but promising in advanced OLED applications, such as microOLEDs.