Direct Effect of Dielectric Surface Energy on Carrier Transport in Organic Field-Effect Transistors

High Throughput Characterization of Organic Thin Film Transistors

Automation is vital to accelerating research. In recent years, the application of self-driving labs to materials discovery and device optimization has highlighted many benefits and challenges inherent to these new technologies. Successful automated workflows offer tangible benefits to fundamental science and industrial scale-up by significantly increasing productivity and reproducibility all while enabling entirely new types of experiments. However, it's implemtation is often time-consuming and cost-prohibitive and necessitates establishing multidisciplinary teams that bring together domain-specific knowledge with specific skillsets in computer science and engineering. This perspective article provides a comprehensive overview of how the research group has adopted "hybrid automation" over the last 8 years by using simple automatic electrical testers (autotesters) as a tool to increase productivity and enhance reproducibility in organic thin film transistor (OTFT) research. From wearable and stretchable electronics to next-generation sensors and displays, OTFTs have the potential to be a key technology that will enable new applications from health to aerospace. The combination of materials chemistry, device manufacturing, thin film characterization and electrical engineering makes OTFT research challenging due to the large parameter space created by both diverse material roles and device architectures. Consequently, this research stands to benefit enormously from automation. By leveraging the multidisciplinary team and taking a user-centered design approach in the design and continued improvement of the autotesters, the group has meaningfully increased productivity, explored research avenues impossible with traditional workflows, and developed as scientists and engineers capable of effectively designing and leveraging automation to build the future of their fields to encourage this approach, the files for replicating the infrastructure are included, and questions and potential collaborations are welcomed.

Dissecting the Interplay between Organic Charge-Modulated Field-Effect Transistors and Field-Effect Transistors through Interface Control Engineering

Organic charge-modulated field-effect transistors (OCMFETs) have garnered significant interest as sensing platforms for diverse applications that include biomaterials and chemical sensors owing to their distinct operational principles. This study aims to improve the understanding of driving mechanisms in OCMFETs and optimize their device performance by investigating the correlation between organic field-effect transistors (OFETs) and OCMFETs. By introducing self-assembled monolayers (SAMs) with different functional groups on the AlOx gate dielectric surface, we explored the impact of the surface characteristics on the electrical behavior of both devices. Our results indicate that the dipole moment of the dielectric surface is a critical control variable in the performance correlation between OFET and OCMFET devices, as it directly impacts the generation of the induced floating gate voltage through the control gate voltage. The insights obtained from this study contribute to the understanding of the factors affecting OCMFET performance and emphasize their potential as platforms for diverse sensing systems.

Not Just Surface Energy: The Role of Bis(pentafluorophenoxy) Silicon Phthalocyanine Axial Functionalization and Molecular Orientation on Organic Thin-Film Transistor Performance

Understanding the effect of surface chemistry on the dielectric-semiconductor interface, thin-film morphology, and molecular alignment enables the optimization of organic thin-film transistors (OTFTs). We explored the properties of thin films of bis(pentafluorophenoxy) silicon phthalocyanine (F₁₀-SiPc) evaporated onto silicon dioxide (SiO₂) surfaces modified by self-assembled monolayers (SAMs) of varying surface energies and by weak epitaxy growth (WEG). The total surface energy (γ^tot), dispersive component of the total surface energy (γ^d), and polar component of the total surface energy (γ^p) were calculated using the Owens-Wendt method and related to electron field-effect mobility of devices (μ_e), and it was determined that minimizing γ^p and matching γ^tot yielded films with the largest relative domain sizes and highest resulting μ_e. Subsequent analyses were completed using atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS) to relate surface chemistry to thin-film morphology and molecular order at the surface and semiconductor-dielectric interface, respectively. Films evaporated on n-octyltrichlorosilane (OTS) yielded devices with the highest average μ_e of 7.2 × 10^-2 cm²·V^-1·s^-1 that we attributed to it having both the largest domain length, which were extracted from power spectral density function (PSDF) analysis, and a subset of molecules with a pseudo edge-on orientation relative to the substrate. Films of F₁₀-SiPc with the mean molecular orientation of the π-stacking direction being more edge-on relative to the substrate also generally resulted in OTFTs with a lower average V_T. Unlike conventional MPcs, F₁₀-SiPc films fabricated by WEG experienced no macrocycle in an edge-on configuration. These results demonstrate the critical role of the F₁₀-SiPc axial groups on WEG, molecular orientation, and film morphology as a function of surface chemistry and the choice of SAMs.

Blurred Electrode for Low Contact Resistance in Coplanar Organic Transistors

Inefficient charge injection and transport across the electrode/semiconductor contact edge severely limits the device performance of coplanar organic thin-film transistors (OTFTs). To date, various approaches have been implemented to address the adverse contact problems of coplanar OTFTs. However, these approaches mainly focused on reducing the injection resistance and failed to effectively lower the access resistance. Here, we demonstrate a facile strategy by utilizing the blurring effect during the deposition of metal electrodes, to significantly reduce the access resistance. We find that the transition region formed by the blurring behavior can continuously tune the molecular packing and thin-film growth of organic semiconductors across the contact edge, as well as provide continuously distributed gap states for carrier tunnelling. Based on this versatile strategy, the fabricated dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) coplanar OTFT shows a high field-effect mobility of 6.08 cm² V^-1 s^-1 and a low contact resistance of 2.32 kΩ cm, comparable to the staggered OTFTs fabricated simultaneously. Our work addresses the crucial impediments for further reducing the contact resistance in coplanar OTFTs, which represents a significant step of contact injection engineering in organic devices.

Benzoselenadiazole-Based Conjugated Molecules: Active Switching Layers with Nanofibrous Morphology for Nonvolatile Organic Resistive Memory Devices

In this work, two symmetrical donor-acceptor-donor (D-A-D) type benzoselenadiazole (BSeD)-based π-conjugated molecules were synthesized and employed as an active switching layer for non-volatile data storage applications. BSeD-based derivatives with different donor units attached through common vinylene linkers showed different electrical and optical properties. 4,7-Di((E)-styryl)benzo[c][2,1,3]selenadiazole (DSBSeD) and 4,7-bis((E)-4-methoxystyryl)benzo[c][2,1,3]selenadiazole (DMBSeD) are sandwiched between gallium-doped ZnO (GZO) and metal aluminum electrodes respectively through solution-processed spin-coating method. The solution-processed nanofibrous switching layer containing the DMBSeD-based memory device showed reliable memory characteristics in terms of write and erase operations with low SET voltage than the random-aggregated DSBSeD-based device. The nanofibrous molecular morphology of switching layer overcomes the interfacial hole transport energy barrier at the interface of the DMBSeD thin-film and the bottom GZO electrode. The memory device GZO/DMBSeD/Al based on nanofibrous switching layers shows switching characteristics at compliance current of 10 mA with V_set =0.79 V and V_reset =-0.55 V. This work will be beneficial for the rational design of advanced next-generation organic memory devices by controlling the nanostructured morphology of active organic switching layer for enhanced charge-transfer phenomenon.

Nucleation Control-Triggering Cocrystal Polymorphism of Charge-Transfer Complexes Differing in Physical and Electronic Properties

Binary charge-transfer complex polymorphs composed of perylene and 4,8-bis(dicyanomethylene)-4,8-dihydrobenzo-[1,2-b:4,5-b']-dithiophene (DTTCNQ) were synthesized separately via a simple artificial nucleation-tailoring method, in both macroscopic and microscopic cocrystal engineering manners. The two polymorphs were testified to be independently thermosalient in the solid state, and the specific self-assembly derived from homogeneous or heterogeneous nucleation by assistance of governable thermodynamic/kinetic drive, leading to a change in the ordered p-n stacking structure. The as-prepared polymorphic microcrystals afforded a significantly varied (opto)electronic property: high n-type transporting and good photoresponsivity for β-complex, and ambipolar transporting with ignorable photoresponsivity for α-complex, attributing to the different charge-transfer and supramolecular alignment. This work provides us a new route to the exploitation of donor-acceptor complex family, making it possible to develop functional materials and devices based on variable supramolecular binary structures.

High-performance optical memory transistors based on a novel organic semiconductor with nanosprouts

Here, we developed a novel organic semiconductor of 2,7-di(anthracen-2-yl)naphthalene (2,7-DAN), which not only exhibits outstanding hole-transport with a mobility of 3.3 cm2 V-1 s-1 but also shows a high photoresponsivity of 8000 A W-1 with detectivity as high as 1.2 × 1014 Jones. Most importantly, optical memory transistors (OMTs) based on it display an excellent memory effect due to the inhomogeneity (nanosprouts) of the 2,7-DAN film only, different from traditional strategies towards memory devices like the application of a floating gate layer, an electret layer or a photochromic molecule. The findings distinctly make 2,7-DAN an excellent candidate for high performance nonvolatile OMTs with a simpler structure.

π-Extended hexadeca-substituted cobalt phthalocyanine as an active layer for organic field-effect transistors

The structural modification of the phthalocyanine skeleton with a hexadeca substitution pattern is a promising approach for the fabrication of active layers for OFETs.