Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

Abstract

We report on the optical and structural characterization of rubrene polycrystalline films fabricated from vacuum deposition with various substrate temperatures (Tsub). Depending on Tsub, the role of twisted and planar rubrene conformational isomers on the properties of rubrene films is focused. The temperature (T)-dependent inverse optical transmission (IOT) and photoluminescence (PL) spectra were performed on these rubrene films. The origins of these IOT and PL peaks are explained in terms of the features from twisted and planar rubrene molecules and of the band characteristics from rubrene molecular solid films. Here, two rarely reported weak-peaks at 2.431 and 2.605 eV were observed from IOT spectra, which are associated with planar rubrene. Besides, the T-dependence of optical bandgap deduced from IOT spectra is discussed with respect to Tsub. Together with IOT and PL spectra, for Tsub > 170 °C, the changes in surface morphology and unit cell volume were observed for the first time, and are attributed to the isomeric transformation from twisted to planar rubrenes during the deposition processes. Furthermore, a unified schematic diagram in terms of Frenkel exciton recombination is suggested to explain the origins of the dominant PL peaks performed on these rubrene films at 15 K.

Optical Properties and Stability of Bilayer Rubrene-Alq3 Films Fabricated by Vacuum Deposition

“…This was also confirmed for the VD Rub films in our previous paper [16]. The ratio of twisted and planar isomers in the film is determined by the technological features of the preparation of samples, in particular, by the temperature of a substrate [32].…”

Characterization of coumarin-6 polycrystalline films growth from vacuum deposition at various substrate temperatures

“…The temperature ( T )-dependence of equilibrium vapor pressure from the source powder plays an important role during the vacuum deposition process 20 – 25 . However, the data of vapor pressure over a wide enough T range for some small organic molecules are often difficult to obtain, and based on our literature survey the relevant information for coumarin-6 is not available.…”

“…1 . The CSS zone is a region where the actual vapor pressure exceeds the equilibrium vapor pressure such that the degree of supersaturation is large enough for nucleation and subsequent growth 21 – 25 . Based on the spreading of CSS zone, T sub was varied from 106 to 178 °C, and the deposition period τ = 51 ± 1 min was chosen.…”

“…Figure 8 depicts three IOT and PL spectra for coumarin-6 polycrystalline films fabricated with T sub = 125, 150, and 169 °C. The absorption threshold ( i.e ., E op ) was practically defined at the intersection of two guide lines 25 , one for the transmitted background with hv ≪ E op and the other one for the strong absorption with hv > E op . For the film deposited at T sub = 150 °C, the abrupt increase in IOT spectrum (with smallest band-tail absorption) around E op suggests that this film had a highest crystallinity among other films, and this deduction was consistent with its smallest FWHM detected from Fig.…”

“…The difference between T sou and T sub corresponds to the degree of supersaturation which affects the nucleation and subsequent growth 20 – 26 . The proper values of T sub corresponding to a fixed T sou can be experimentally determined from the concept of constitutional supersaturation (CSS), and the effects of T sub on the size, orientation, and crystallinity of the constituent grains and on the growth rate of various small organic molecular polycrystalline films were reported 21 – 25 .…”