Application of Solution-Processed Multi-Layer Organic Light-Emitting Diodes Based on Cross-Linkable Small Molecular Hole-Transporting Materials

Comparing with vacuum evaporation, fabricating organic light-emitting diodes (OLEDs) by solution-processing methods has plenty of merits, such as high material utilization, simple equipment and feasibility of low cost large size panels, which has drawn much attention. Small molecule hole-transporting materials (HTMs) are an important part of OLED devices for hole transporting and electron blocking. Cross-linking groups such as styryl, oxetane, etc. are added to the structure to form cross-linkable small molecular hole transporting materials, which are suitable for solution. processed technology. Initiating with the thermal or the ultra-violet irradiation cross-linking curing reaction, the polymer network structure is formed. As applied in the multilayer OLEDs, the inducing of cross-linkable hole-transporting materials can solve the miscibility problem of the interlayers between the emitting material layer(EML) and the hole transporting layer(HTL), then improve the efficiency and stability of the device consequently. In this paper, according to the requirements of the solution process for the materials, many techniques about spin-coating and ink-jet printing are presented, such as the methods of fabrication, the quality of films affected by manual factors and the comparison of different manufactory measures. Afterwards, to summarize their performance in devices, various cross-linkable HTMs are listed and investigated, such as trifluoroethylene group, styrene group, oxetane group, siloxane group and unsaturated ester group. Finally, the developing trend of cross-linkable HTMs and solution process is introduced.