With rapid development of science and technology, demands for display panels are increasingly improved, which tend to become lighter and thinner with less power consumed. Accordingly, there emerges organic light emitting diode (OLED) display panel. Compared to conventional liquid crystal display panel, the OLED display panel is self-luminous without backlight module having high energy consumption and, hence, is lighter and thinner with less power consumed. Thus, more and more attention is paid to the OLED display panel.
Depending on different organic light emitting material adopted, OLED in the OLED display panel may be classified into small molecule light emitting diode (SMLED) and polymer light emitting diode (PLED). In comparison, the SMLED may be manufactured with a vacuum thermal evaporator having high cost and not suit for application in large area display panels, while the PLED may be manufactured more easily with reduced costs on equipments and process via wet methods such as solution spin coating or droplet coating, and suit for application in the large area display panels.
A proper device structure is of key importance in improving characteristics of the PLED such as brightness, current efficiency and stability. Referring to FIG. 1, which illustrates a block diagram of a conventional PLED, the PLED mainly includes a hole transport part 1′, an organic light emitting layer 2′ and an electron transport part 3′ stacked sequentially. Principle for the PLED to emit light may be described as follows. Holes injected through the hole transport part 1′ are combined with electrons injected through the electron transport part 3′ in the organic light emitting layer 2′ to generate excitons so as to emit light. Herein, the hole transport part 1′ and the electron transport part 3′ serve mainly to address imbalance between injections of two types of carries, wherein the hole transport part 1′ may include an anode 10′, a hole injection layer (HIL) 11′ and a hole transport layer (HTL) 12′, while the electron transport part 3′ may include a cathode 30′ and an electron transport layer (ETL) 31′, and may further include an electron injection layer (EIL) in certain kinds of PLED (not shown).
Generally speaking, however, organic polymer light emitting material is of partial hole transport type, and hole transport capability of the hole transport part in the PLED is far greater than electron transport capability of the electron transport part therein. Accordingly, during startup of the PLED, an exciton combination region may be generated closer to the electron transport part by holes and electrons. Excitons may be further de-excited at an interface between the electron transport part and the organic light emitting layer or even in the electron transport part, such that spectroscopic redshift may occur in the PLED.