High-Performance Transistors and Photo Detectors based on Solution-Processed Organic Semiconductors

Abstract

Over the past two decades, organic semiconductors have recently received great attention as components in organic field-effect transistors (OFETs) and organic photo detectors (OPDs) for potential use in flexible, inexpensive, large-area electronics applications. To achieve high-performance OFETs, a variety of research efforts have been applied toward design strategies with the goal of enhancing molecular ordering and electrical properties from developed organic semiconductors may be further enhanced through the optimization of device preparation, such as choosing suitable solvents with high solubility and dielectric treatments by inducing favorable crystalline structures. The most important challenge to realize high-performance OPDs is the need for a high photoconductive gain, which is desirable for low-intensity photodetection. The photoconductive gain is directly proportional to the charge carrier mobility of the majority carriers and the lifetime of the trapped minority carriers. Toachieve high photoconductive gains, it is necessary to introduce strategically designed long-lifetime trap states that effectively confine electrons (holes) so that the holes (electron) can travel many times along the circuits.In chapter 2, I report a novel synthetic strategy designed to uncover the relationship between molecular aggregation in a film state and OFET performance. Four new quaterthiophene derivatives with end-groups composed of dicyclohexyl ethyl (DCE4T), dicyclohexyl butyl (DCB4T), cyclohexyl ethyl (CE4T), and cyclohexyl butyl (CB4T) were designed. The quaterthiophene derivatives with asymmetrically substituted cyclohexyl end-groups (CE4T and CB4T) preferred H-type aggregation whereas those with symmetrically substituted cyclohexyl end-groups (DCE4T and DCB4T) preferred J-type aggregation. The field-effect mobilities of devices that incorporated the asymmetrical molecules, CE4T and CB4T, were quite high, above 10–2 cm2/(Vs), due to H-aggregation, whereas the field-effect mobilities of devices that incorporated symmetrical molecules, DCE4T and DCB4T, were poor, below10–4 cm2/(Vs), due to J-aggregation. In chapter 3, the relationship between molecular aggregation and OFET device performance using designed two end-capped cyclohexylethynylquaterthiophene derivatives(CHE4T and BCHE4T) for use in solution-processed organic transistors was studied. UV–vis absorptionand grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis revealed that CHE4T tended toundergo H-aggregation whereas BCHE4T tended to undergo J-aggregation. The average field-effect mobilities of the devices based on CHE4T and BCHE4T were 2.51×10–2 cm2/(Vs) and 4.76×10–4 cm2/(Vs) in the saturation regime. In chapter 4, we designed and synthesized three novel asymmetric alkylated thiophene-naphthalene oligomers, 5-decyl-2,2':5',2'':5'',2'''-quterthiophene (DtT), 5-decyl-5''-(naphthalen-2-yl)-2,2':5',2''-terthiophene (D3TN), and 5-(4-decylphenyl)-5'-(naphthalen-2-yl)-2,2'-thiophene (DP2TN), in an effort to improve the intermolecular ordering in the thin film state. UV-Vis absorption and GIWAXS analyses indicated that DtT, D3TN, and DP2TN, tended to form H-aggregates with a vertical orientation on the substrates. The ratio of H- to J-aggregates and the distinct differences amongthe film morphologies in the three oligomers were correlated with high field-effect mobilities, up to 3.7 × 10–2 cm2/Vs, in OFETs based on DP2TN.In chapter 5, we used a mixed solvent system composedof chloroform and tetrahydrofuran (THF) to control the solvent properties and optimize the thin film morphology of the newly synthesized 2-(6-((2-ethylhexyl)oxy)naphthalen-2-yl)anthracene (EH-NA) foruse as an active layer in OFETs. To overcome the limitations on solvent properties, mixed solvent systems have been tested in the spin-coating step. By mixing solvents, we were able to optimize the morphology and improve the degree of crystallinity in the thin film, leading to enhanced OFET performance. The mixed solvent system suggested here will enable us to control the film morphology for use in organic thin film transistors.In chapter 6, with the aim of enhancing the performances of OFETs,the present study describes a strategy for slowing down the evaporation rate and increasing the time available for polymer chain alignment by introducing 1-chloronaphthalene (CN) as an additive toa chloroform solution prior to spin-coating an active layer onto a self-assembled monolayers-treated surface during OFET fabrication. PDPPDBTE, PDPPDTSE and PTVTF were used as the semiconducting layer. We also found that the effects of the CN additive strategy depended on the crystalline characteristics of the polymers. In chapter 7, we prepared polymer/nanocrystals hybrid photo detectors with both planar heterojunction (PHJ) and bulk heterojunction (BHJ) geometries. The planar geometry was strategically introduced to avoid compromising the charge carrier mobility of the polymers. The PHJ-OPD could yield a higher hole mobility than could be achieved in a BHJ-OPD without compromising on theselective electron trapping effects. The optimized PHJ-OPD led to a photoconductive detectivity of 1.3 ×1010 cm Hz1/2/W.