Tuning Molecular Conformations to Enhance Spontaneous Orientation Polarization in Organic Thin Films

Abstract

Three isomeric derivatives of 2,2′,2″-(1,3,5-benzinetriyl)-tris­(1-phenyl-1-H-benzimidazole) (TPBi) bearing ethyl groups on the N-phenyl moieties were synthesized to elucidate the effects of intramolecular interactions on spontaneous orientation polarization (SOP) in thin films. The films of the TPBi derivatives displayed enhanced SOP with a surface potential change of up to 1.8 times that for TPBi, and the p-substituted derivative exhibited the largest potential change reported to date (+141.0 mV/nm). Density functional theory calculations and single-crystal structure analysis suggest that the introduction of the ethyl groups switched the stable molecular conformation from C 1 to C 3 symmetry. Through analysis of the structural anisotropy in the films by spectral ellipsometry and two-dimensional (2D) grazing-incidence wide-angle X-ray scattering, we conclude that the conformational change of the molecules was the major factor underlying the SOP enhancement.

Synthesis of 2,5,8-Tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b′:5,6-b″] Trithiophenes and Their Spontaneous Orientation Polarization in Thin Films

“…In the case of TPBi and its derivatives, the magnitude of the SOP can be reasonably explained based on the current model with similar face-up/face-down ratios of about 1.5 (Table S3). Therefore, we speculate that the in-plane PDM of the C 1 conformations may not be the main contributor to the SOP.…”

“…As in the previous results, a diffraction peak was observed with a periodicity of 1.14 nm along the q z direction for the TPBi film, indicating a disordered layered structure in the direction normal to the film plane. 22,31 The TPBTT and m-ethyl-TPBTT films also showed diffraction with periodicities of 1.25 and 1.19 nm, respectively, along the q z direction. Compared with TPBi, the larger periodicity was ascribed to the larger molecular size, although the diffraction intensity was much lower, indicating the weaker order of the layered structures in the films.…”

“…In our previous study, we introduced ethyl groups onto the N-phenyl moieties of TPBi to examine the effects of substitution on SOP. 22 Ethyl substitutions changed the stable molecular conformations, increasing the magnitude of the molecular PDM in the z-component. Consequently, the SOP of the films with ethyl derivatives increased by up to 1.8 times compared to that of the TPBi film.…”

Charge-Carrier Dynamics and Exciton-Polaron Quenching Studied Using Simultaneous Observations of Displacement Current and Photoluminescence Intensity

“…From the energy diagram for the device (Figure b), it is plausible that Ir­(ppy) 3 may capture holes in the TPBi host and that the energy barrier formed at the Ir­(ppy) 3 /TCTA interface is responsible for the high resistance to the hole extraction process. , If Ir­(ppy) 3 instead captured holes in the EML, a similar energy barrier would also be formed in the CBP host device; however, the experimental results indicate no significant barrier in this case (Figure a). A possible reason for this host-dependent behavior is the dipolar-disorder effect, ,, since CBP is nonpolar, but TPBi is a polar molecule (PDM: 2–10 D). ,, The random electric potential due to the PDM broadens the density of states (DOS), and the tail states of the broad DOS capture holes; thus, the effective barrier height formed at the EML/TCTA interface is higher in the TPBi host device than in the CBP host device.…”

“…A possible reason for this host-dependent behavior is the dipolar-disorder effect, 7,41,42 since CBP is nonpolar, but TPBi is a polar molecule (PDM: 2−10 D). 3,43,44 The random electric potential due to the PDM broadens the density of states (DOS), and the tail states of the broad DOS capture holes; thus, the effective barrier height formed at the EML/TCTA interface is higher in the TPBi host device than in the CBP host device.…”