Exploration Of IR Transmitting Materials And Study Of Degradation Mechanism Of Alq₃-based OLEDs:Insight From First-Principles Calculations

Infrared transmitting materials have got broad applications in industrial, scientific, medical and military field. Especially, as critical components for infrared sensing and detection systems, long-wave infrared（8-12 μm） transmitting window materials with broader covering regions play significant roles in aeronautic space science, medical research and electronic science. However, there is limited optional alternative available for durablelong-wave infrared transmitting materials, which seriously prohibits their applications as external windows and domes in aerobat. Moreover, from the experimental view, to find the potential candidates in experiment according to material genome project is a huge trail-and-error engineering project. Thus, it urges a more efficient and economic way to deal with such kind of task of systematically exploring and searching for durable LWIR transmitting materials.Furthermore, tris（8-hydroxyquinoline）aluminum（III）（Alq₃） small molecule-based organic light-emitting diode（OLED） has become a prominent candidate to achieve flat panel displays of the new generation owing to its wide ranges of advantages. However, the degradation phenomenon and poor stability of Alq₃-based OLEDs have been the major impediment for the development of economically feasible and durable devices for commercial application. Thus, it is of great significance to clarify the origin of instability of Alq₃ and provide a molecule design guideline and scheme for improving the stability of devices for OLED long-term development.In order to solve both problems mentioned above, first-principles calculations based on density functional theory and VASP modeling softwares have been carried out in our work. The degradation phenomenon and the origin of instability of the facial Alq₃-based blue luminescent OLEDs have been illustrated and discussed from atomic level. From the interactions between the models of facial isomer of Alq₃ and H2O/cathode metal layers, it is indicated that the strong attracting effect of oxygen hole region from the three oxygen atoms in the same side is the origin of instability of facial Alq₃. Then a design guideline by substituting C7 position with electron withdrawing groups is proposed to protect Alq₃ from water molecules and other cathode metal atoms. Moreover, the large-scale systematic searching has been made for potential AxBy form（x:y=1:1, 1:2, 1:3, 2:1, 3:1, 2:3, 3:2） long-wave infrared window materials compatible with both mechanical strength and infrared transmitting performance properties. Two hundred and fifty-three stable potential IR materials are selected by evaluating the phonon dispersion. Furthermore, two key descriptors for the optical（Long-wave transmitting cutoff） and the mechanical performance（bulk modulus）, have been theoretically defined and compared with the experimental data. Besides, the performance map has been illustrated and it showsthat there is always a tradeoff between gaining a high LWIR transmission and losing a desirable mechanical property- the larger bulk modulus, the smaller the long-wave infrared tramsmitting cutoff. Finally, seven long-wave infrared（8-12μm） transmitting window materials with both the suitable mechanical and optical performances have been discovered, namely, TiSe, TiS, MgS, CdF2, HgF2, CdO and SrO. Especially, the performances of TiS and CdF2 are comparable to that of the current commercially used ZnS at high temperature. Last but not the least, possible ranges of infrared transmission for halogen, chalcogen and nitrogen compounds have been summarized respectively to guide further exploration for infrared transmitting materials in experiment.