Synergistic Control of Molecular Self-Assembly and Orientation to Enhance the Performance of Organic Field-Effect Transistors Utilizing Solution-Processable Organic Semiconductors
Authors: Pandey Shyam S , Sharma Shubham , Gaurav Kumar Vivek , Nagamatsu Shuichi 
Published: December 2024

Introduction: Fast development of solution-processable organic semiconductors and their utilization as active components of organic electronic devices has gained momentum in the recent past aiming towards the practical realization of low-cost flexible and printable electronics. The past decade has witnessed significant scientific efforts to improve the charge transport characteristics of organic semiconductors by tuning their chemical structure or by varying the film fabrication techniques to enhance the macromolecular ordering. Due to the quasi-one-dimensional nature of organic conjugated polymers (CPs), their backbone orientation and self-assembly have been extensively explored to improve the optoelectronic characteristics of organic electronic devices. Organic electronic devices quite often need multilayer fabrication of devise components, where control of their morphology along with the various interfaces critically controls the overall performance of the devices. Therefore, there is an urge for the facile fabrication of large-area uniform films with controllable film morphology and minimal interference to the underlying layers. Our group has developed and improvised a novel method of thin film fabrication known as the floating-film transfer method (FTM), which provides not only large-area uniform thin films but also these films exhibit directional molecular orientation. Control of molecular orientation is highly desired to control the extent and directionality (vertical or in-plane) of the charge transport, which needs to be amicably utilized to harness the full potentiality of the device performance under investigation.

Results and Discussion:

The uniqueness of the FTM lies in 1st of all the fabrication of floating films of solution-processable CPs on an orthogonal liquid substrate followed by its transfer on any desired substrate by stamping, which has been schematically represented in Fig.1(a). Thanks to the isolation of film fabrication and transfer, it is possible to fabricate multilayers of the same or different CPs without having any adverse effect on the underlying layers, which is one of the intriguing issues for thin film fabrication using solution processing. We have demonstrated that controlling the parameters for thin film fabrication using NR-P3HT under FTM led to not only macroscopically oriented thin films but also there was a remarkable enhancement in the field effect mobility (>100 times) of organic field-effect transistors (OFETs) as compared to their spin-coated thin film device counterparts. One of the well-studied CPs, PBTTT-C14 possesses a rigid-rod-like polymeric backbone with liquid-crystalline behaviour, which is rather difficult to orient, and has been reported to impart enhanced carrier mobility when its thin film was annealed at its liquid-crystalline temperature of 180oC. Utilizing highly edge-on oriented thin films of this CP processed by FTM, we have not only demonstrated a very high optical dichroism of >10 but also an OFET mobility of 1.24 cm2/Vs, which is one of the highest reported mobility for this class of CPs as shown in Fig. 1(b). Despite the enhancement in solubility as a function of increasing alkyl-chain length in the P3AT class of CPs, there is a drastic fall in carrier mobility up to 4-5 orders of magnitude. We have recently demonstrated that FTM is capable of solving this issue amicably, where the detrimental influence of alkyl chain length for planer charge carrier transport was not observed making the freedom for the selection of any of P3ATs for OFET applications. Fabrication of a large-area ≈40 cm2 film with uniform orientation was recently reported for poly(3,3‴-dialkylquaterthiophene) (PQT) using FTM. An array of bottom-gate top contact OFETs fabricated along the length of a single large-area (≈15 × 2.5 cm2) thin film demonstrated the average field-effect mobility of 0.03 cm2/V s with a very narrow standard deviation of 12%. It has been found that the pre-conceived notion of the orientation of the FTM thin films is perpendicular to the direction of the flow of thin films is not completely true and the viscous force of not only the liquid substrate but also the CP solution plays a vital role in determining the direction of orientation as shown schematically in Fig. 1(c). Based on combined theoretical and experimental approaches, we have demonstrated the control of the extent and direction of molecular orientation using DPP-TTT, a rigid rod-like donor-acceptor type CP, which is rather difficult to orient owing to its molecular rigidity. Harnessing the synergy of orientation and molecular orientation under FTM using DPP-TTT, a very high field-effect mobility of 12.4 cm2V-1s-1 was recently demonstrated, which is one of the highest values reported for solution-processable organic semiconducting polymers. Results pertaining to the film fabrication by FTM, orientation mechanism, film uniformity and anisotropy along with the application of FTM-processed films towards their suitability of OFETs will be discussed in detail.

Figure 1