Molecular deposition condition dependent structural and charge transport properties of CBP films

Optimizing organic molecular deposition conditions to improve charge transport is of great importance for applications in OLED. Based on molecular dynamics simulations, 4,4́-bis(Ncarbazolyl)-1,1́-biphenyl (CBP), a linear bipolar host OLED molecule, are deposited with different substrate temperature Tsub and inserting molecule velocity vd. It is found that in the investigated parameter range, both lower substrate temperature and higher initial velocity of inserting molecules are beneficial for obtaining more ordered morphology. Further studies based on kinetic Monte Carlo simulations reveal that the deposited structures with higher molecule order possess higher charge mobilities, which arises from the reduced energetic disorder and improved spatial correlation of site energies. It is also found that the effect of molecular ordering on electron transport is more obvious than that on hole transport. With electrical field of 1.6×105V/cm, the electron and hole mobilities can be enhanced by 100% and 42% when Tsub is increased from 200 K to 300 K (vd = 0.002 Å/fs), respectively. While their mobilities can also be enhanced by 30% and 17% with the increase of vd from 0.002 to 0.008 Å/fs (Tsub = 300 K). The larger enhancement of electron mobility can result in more balanced electron and hole mobilities in CBP, thus improve its bipolar transport characteristic. Our work not only connects the molecular morphology with charge transport properties, but also provides a strategy for improving charge mobility through the optimization of molecular deposition conditions. © 2020 Elsevier B.V.