Improvement of Electron Injection by a Ramp-Shape N-Doped Architecture for High-Efficiency Organic Light Emitting Diodes

Abstract

Over the past decades, organic light-emitting diodes (OLEDs) have drawn a lot of attention due to its numerous advantages over light-emitting diodes (LEDs) and liquid crystal display (LCDs). These characteristics make OLED become one of the most promising candidates for commercial applications such as solid state lighting and active matrix organic light-emitting diode (AMOLED) display. However, device properties such as lifetime, efficiency, power consumption are still the main issues needed to overcome for their practical uses. In order to fulfill a highly power-efficient light source or display with OLEDs, the device energy loss during electron–photon conversion should be low. Recent years, there are plenty of approaches proposed to improve the electron or hole injection from the electrodes and reduce the ohmic losses. The most typical way is inserting a chemically p- or n-doped layer between the hole transporting layer (HTL) and anodes or between the electron transporting layer (ETL) and the cathodes, respectively. In our work, we report OLED with a novel n-doped ramp-shape electron injection structure composed of rubidium carbonate (Rb2CO3) doped 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) which is prepared by thermal co-evaporation. We increase the doping concentration from 17.5 mol% to 84 mol% while depositing the electron injection materials approaching the cathode. By increasing the doping concentration, the activation energy of the material decreases as the Fermi level shifting toward the lowest unoccupied molecular orbital (LUMO). Under thermal equilibrium conditions, a ramp-shape architecture is formed and gradually distribute the energy barrier from cathode to electron transport layer in the device therefore achieve significant electron injection improvement. As a result, a OLED with the ramp-shape electron injection structure ITO/NPB (40nm)/Alq3 (40nm)/Rb2CO3:MADN (14nm, 17.5~84 mol%)/LiF (1nm)/Al (140nm) accomplished maximum luminance of 33370 cd/m2 and current efficiency of 5.292 cd/A. By contrast, the device without the ramp-shape architecture (the normal device) ITO/NPB (40nm)/Alq3 (40nm)/LiF (1nm)/Al (140nm) only achieved a maximum luminance of 17030 cd/m2 and current efficiency of 3.41 cd/A. The total improvement (55% by current efficiency and 95% by luminance) made by adding ramp-shape structure to device indicated a remarkable electron injection facilitation and its performance is much more efficient than the device with typical electron injection layer ITO/NPB (40nm)/Alq3 (40nm)/Rb2CO3:MADN (5nm, 64 mol%)/LiF (1nm)/Al (140nm). Figure 1