Unraveling the Implications of Macromolecular Orientation on the Planar and Vertical Charge Transport in Organic Electronic Devices

Abstract

The role of film morphology of conjugated polymers (CPs) and their macromolecular conformation with respect to the substrate in governing charge transport and overall device performances has been a matter of debate. The nature of the macromolecular orientation of CPs in thin films has a pivotal role in controlling the performance of planar and sandwich devices. Thin-film characterizations unveiled that spin-coated films are isotropic with mixed edge-on/face-on conformation, whereas unidirectional floating film transfer method (UFTM) films are anisotropic with edge-on-oriented micrometer-sized ordered domains. To unravel the explicit role of molecular orientation on device performance, two types of devices, organic Schottky diodes (OSDs) for out-of-plane carrier transport and organic field-effect transistors (OFETs) for in-plane charge transport, were fabricated and characterized. Isotropic and relatively low crystalline spin-coated films exhibited superior transport characteristics in the out-ofplane direction than UFTM films irrespective of the crystallinity. On the other hand, edge-on-oriented UFTM thin films demonstrated remarkably enhanced in-plane charge transport with more than two orders of magnitude of enhancement in the fieldeffect mobility. The nature of molecular orientation of different films is significantly correlated with the performance of the OSDs and OFETs.

Facile Fabrication and Characterization of Oriented and Multilayer Thin Films of Solution Processable Conjugated Polymer

“…From the perusal of out‐of‐plane and in‐plane XRD results, it is evident that the UFTM thin films are dominantly edge‐on oriented as well as ordered and crystalline. [ 32 ] Further, the Debye–Scherrer equation for the first diffraction peak (100) at 2 θ = 5.3°, which was the same for all the multilayers, was utilized to evaluate the interplanar d‐spacing and coherent length of domains. [ 38 ] The interplanar d‐spacing for all the multilayer thin films was 1.67 nm, whereas the coherent length of the domains was found to be 23.51 nm, 23.35 nm, and 23.27 nm for 4, 6, and 20 layers, respectively.…”

“…Here, the incident X-rays undergo total external reflection with the grazing incident angle (ω) with respect to the thin-film surface lower than the critical angle (ω c ), i.e., ω < ω c . [30][31][32] Thus, evanescent wave was present within the refracting media that travels parallel to the surface. In this mode, the rapid decrease of evanescent field amplitude resulted in enhanced surface sensitivity of X-rays, with a few nanometers of penetration depths.…”

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

“…Further, it is well known that edge‐on dominated thin films are highly suitable for the in‐plane charge transport. [ 25 ] From the perusal of Figure 3a, the lamellar (100) peak corresponding to the alkyl stacking in out‐of‐plane XRD for both 4% and 9% exists at 2 θ = 5.25°. The further alkyl stacking peaks can also be seen at the same value of 2 θ for both the thinner and thicker films.…”

The role of solvents in the microstructure and charge transport of semiconducting polymer films prepared at the air–liquid interface

“…4 Moreover, the conformation of SCPs in the films, such as edge-on or face-on, and the device structure of FETs (precisely, near the gate–dielectric interface) are also well proven to be crucial in governing the charge transport. 8–15…”

“…4 Moreover, the conformation of SCPs in the films, such as edge-on or face-on, and the device structure of FETs (precisely, near the gate-dielectric interface) are also well proven to be crucial in governing the charge transport. [8][9][10][11][12][13][14][15] Unidirectional alignment of SCPs in thin films is beneficial for attaining higher FET mobility (m) because carrier transport is high along the conjugation direction. 5 Various solutionprocessable methods have been reported for fabricating thin films of SCPs; however, only a few of them render macroscopic alignment of SCPs in resultant films in FETs.…”