Vertical Distribution of Molecular Orientation and Its Implication on Charge Transport in Floating Films of Conjugated Polymers

Abstract

Fabrication of uniform and oriented thin films of conjugated polymers (CPs) with controlled thickness is essential for the implementation of organic devices. Over the years, unidirectional floating film transfer method (UFTM) has emerged as a facile technique for the fabrication of oriented thin films but its distribution of orientation in vertical (thickness) direction has not been explored yet. To investigate this, thin films of nonregioregular poly(3‐hexylthiophene) are fabricated using UFTM with different concentrations of CPs to obtain thickness variation from 11 to 47 nm. Thin films of 4% and 9% polymer solution have similar dichroic ratio of 5.5 but have an appreciable difference in film thickness of 12 and 47 nm where thicker films are disordered as probed by polarized absorption spectra. Organic field‐effect transistors fabricated using thinner films (12 nm) exhibit a saturated charge carrier mobility of 2.3 × 10−3cm2 V−1 s−1, an order of magnitude higher than its thicker (47 nm) counterpart. This difference in device performance can be attributed to the fact that charge transport is principally governed by the top polymer surface at air–polymer interface, which is found to be more disordered owing to the vertical distribution of orientation.

Clarifying the Dominant Role of Crystallinity and Molecular Orientation in Differently Processed Thin Films Regioregular poly(3-Hexylthiophene)

“…After the initial studies to investigate the effect of crystallinity and molecular orientation of CP thin films of RR-P3HTs fabricated using UFTM, dip-coating, and spin-coating; OFETs were fabricated in the BGTC device architecture as shown in Figure 1(B), for investigating anisotropic charge transport. In the previous reports by our group, it has already been reported that thin film fabrication by UFTM is oriented edge-on, which facilitates the facile in-plane charge transport [44,45]. In addition to this, Xue et al [46] reported that RR-P3HT dip-coated films are edge-on oriented.…”

“…To circumvent these issues, thin film fabrication techniques such as dip coating [20] can be used, which can not only harness the enhanced molecular self-assembly of CPs but also solve the huge material wastage in the case of spin-coating. At the same time, the unidirectional floating-film transfer method (UFTM) [21], developed and improvised by our research group provides large area, uniform, anisotropic, and edge-on oriented thin film with the least material wastage. To access the quality of thin films fabricated by various methods in depth; optical, microstructural and electrical characterizations are very crucial.…”

Clarifying the Dominant Role of Crystallinity and Molecular Orientation in Differently Processed Thin Films of Regioregular Poly(3-hexylthiophene)

“…To circumvent these issues, thin film fabrication techniques such as dip coating [ 21 ] can be used, which can not only harness the enhanced molecular self-assembly of CPs but also solve the huge material wastage in the case of spin coating. At the same time, the unidirectional floating-film transfer method (UFTM) [ 22 ] developed and improvised by our research group provides large area, uniform, anisotropic, and edge-on oriented thin films with the least material wastage. To access the quality of thin films fabricated by various methods in depth, optical, microstructural, and electrical characterizations are very crucial.…”