Progress and outlook on electron injection in inverted organic light-emitting diodes

With the rapid development of science and technology, the traditional cathode ray tube (CRT) that was invented at the end of the 20th century has revealed significant deficiencies such as high power consumption and large volume. In the second-generation flat panel display technology, i.e.. liquid crystal display (LCD) technology, deficiencies related to high cost and relatively mediocre response speed still exist. Users' dissatisfaction with LCDs subsequently led to the invention of organic light-emitting diode (OLED) displays. They were introduced to the public with wide-angle vision, high color contrast, fast response speed, low power consumption, and low cost. Most commercial active matrix OLEDs use low-temperature polysilicon (LTPS) thin-film transistor (TFT) backplanes. Although LTPS TFT can be used as a p-type transistor owing to its high carrier mobility, high availability, and high stability, it has problems of complex processing and uniformity being difficult to guarantee, which affect the display performance of OLEDs screens. Therefore. to pursue high resolution and stable display, research on inverted OLEDs (iOLEDs) is vital. The bottom cathodes of iOLEDs are connected to the drains of n-type TFTs for realizing better integrated circuits. In addition_ indium tin oxide (ITO) has a high surface work function (approximately 4.8 eV), and for most organic electron transport layer (ETL) materials, the lowest unoccupied molecular orbital (LUMO) is 2.5-3.5 eV. Therefore, for the structure of inverted bottom cathode OLEDs, there is an extremely high electron injection barrier (approximately 2 eV at the ITO/ETL interface), which becomes the main factor hindering the realization of high-performance iOLEDs. Based on two injection theories, i.e., Richardson-Schottky (RS) thermal injection and Fowler-Nordheim (FN) quantum tunneling, three methods have been introduced to achieve highperformance iOLEDs: (1) The most conventional method is barrier regulation, which involves the insertion of a low-workfunction material at the ITO/ETL interface to reduce the electron injection barrier. and then coating of the surface with highly active metal materials such as Mg- and Li-doped ETL or with metal oxides as the interfacial dipole layer. However, metals with high activity and low work functions are easily degraded in water and oxygen environments, thereby affecting device stability. Moreover. metal oxides have advantages such as good mechanical and electrical stability, low cost, visible light transparency, excellent environmental stability, and good charge-transport characteristics. Barrier regulation has developed into a mainstream modification method. (2) The use of doping technology enhances electron injection to ensure that the injection barrier is reduced or does not change significantly. On the later, an electron injection layer (EIL) prepared by the doping method generates ohmic contacts by tunneling through the thin barrier formed by the space charge layer, which can achieve very effective carrier injection in organic semiconductor devices, allowing the device to reach a very low operating voltage while maintaining high quantum efficiency. (3) The construction of micro- and nano-sized structures at the interface based on the interfacial dipole layer or doping system enables electron tunneling under certain conditions, thereby improving injection. Finally, a brief view of the development trend of iOLEDs is presented.