Dependence of the Amplified Spontaneous Emission Threshold in Spirofluorene Thin Films on Molecular Orientation

The dependence of the amplified spontaneous emission threshold (E-th) on molecular orientation in vacuum deposited thin films fabricated from diphenyldiamine- and carbazole-substituted spirofluorene derivatives was investigated. The molecular orientations were modified by changing the substrate temperature during deposition and were almost parallel to the substrate at around 300 K in all of the films. The orientation became random in the film containing diphenyldiamine compound at around 380 K, resulting in decreased optical anisotropy. However, for the carbazole compound, the molecular orientation scarcely changed from 300 to 380 K, while the root-mean-square roughness increased from 0.28 to 0.94 nm. E-th approximately doubled for both the films when the optical anisotropy decreased because of the randomization of molecular orientation or when the surface roughness increased.