Electronic structure analysis of nano-lens arrays

Even though a lot of light extraction methods have been developed for efficient organic light-emitting diodes, there are very few reports on mass producible dry processes. Furthermore, those utilizing dry methods require additional processes (for example, masking and surface treatment), as all vacuum deposition equipment has been developed to produce planar films. Without the additional processes, the organic nano-lens array (NLA, a capping layer with a curved surface) formed by vacuum deposition that causes the spontaneous crystallization of π-conjugated organic molecules and a resulting increase in surface tension, has been recently introduced to achieve high mass producibility and high light extraction efficiency of top emitting organic light-emitting diodes. To further understand the high performance due to NLA, it is necessary to perform an extensive analysis of its electronic structure, as π-conjugated organic materials are significantly affected by frontier orbitals such as the highest occupied molecular orbital and the lowest unoccupied molecular orbital levels. Here, X-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments on NLA and planar samples indicate that NLA has a distinctly different electronic structure compared to the planar and has larger absolute value of the difference between molecular average tilt angle and magic angle, indicating the crystallization of NLA. Very interestingly, the total-electron-yield nitrogen K-edge NEXAFS spectra indicate that there is an additional strong peak at a photon energy of 399.8 eV in NLA, which is a strong evidence for the crystallization, thus achieving the high performance by means of the increased refractive index.