Studies on Interface Science and Engineering in Organic Field Effect Transistors


Student thesis: Doctoral Thesis

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  • Yan YAN


Awarding Institution
Award date19 Oct 2015


Over past two decades, organic field effect transistors (OFETs) have attracted widely scientific and technological interest because of their specific advantages of inexpensive, light-weight and compatibility with flexible substrates, which can be applied in organic electronics such as price-tags, "smart cards", radio-frequency identification tags (RFID), backplane circuitry for active matrix displays and sensors. Therefore, intensive research efforts are focused on the device performance. One of the vigorous investigations is through the interface science and engineering. There are two critically important interfaces in OFETs. i) dielectric/semiconductor interface and ii) semiconductor/metal interface. For the interface between the semiconductor and gate dielectric, it has a significant impact on the morphology of semiconductor which eventually determines the overall electrical performance of the devices. For semiconductor/metal interface, this interface controls the contact resistance and charge injection in OFETs.
This thesis is mainly concerning studies of the interface science and engineering in OFETs. OFETs with self-assembly monolayers (SAMs) modification exhibit a significant improvement of device performance in terms of reducing the off current, minimizing interface trap state densities, enhancing the charge mobility and improving the current stability. Here, we have utilized the ultraviolet/ozone (UVO) treatment to change surface properties of polymeric dielectrics. Then, the chemical vapor deposition (CVD) is used to form SAMs on polymeric insulators. With hexamethyldisilazane on polymeric dielectrics, an ordered pentacene molecular orientation is formed with larger grains, resulting in improved carrier mobilities, and low threshold voltages (VT). Moreover, the UVO treatment is used to enhance the alignment of HMDS on polymeric insulator surfaces and a time dependent effect is observed for the UVO treatment. Poly-4-vinylphenol (PVP), cross-linked PVP and poly(methyl methacrylate) (PMMA) are employed as polymeric insulators. For PVP and cross-linked PVP substrates, a short UVO exposure enhances the HMDS reaction on the polymer surface, and a long UVO exposure shows an adverse effect. On the other hand, PMMA is found to be more sensitive to the UVO treatment and displays performance degradation. These findings will be of value to solution processed insulators for printable electronic applications on flexible substrates.
We have also utilized the confined photocatalytic oxidation (CPO) treatment and the hydrolysis to efficiently convert C-H bonds into C-OH groups on polymeric material surfaces, followed by SAMs decoration on polymeric dielectrics via chemical bonding for the OFETs application. This method is a low temperature process and has negligible etching effect on polymeric dielectric layers. Various types of SAMs have been tested and successfully attached onto the hydroxylated polymeric dielectric surfaces through chemical bonding, ensuring the stability of decorated functional films during the subsequent device fabrication consisting of solution-processingof the polymer active layer. With the surface decoration of functional groups, both n-type and p-type polymers exhibit enhanced carrier mobilities in the unipolar OFETs. In addition, enhanced and balanced mobilities are obtained in the ambipolar OFETs with the blend of polymer semiconductors. The anchored SAMs on the dielectric surfaces dramatically preclude the solvent effect, thus enable an improvement of carrier mobility up to two orders of magnitude. Our study opens a way of targeted modifications of polymeric surfaces and related applications in organic electronics.
We have fabricated self-aligned, full solution process polymer field-effect transistors on flexible substrates through surface selective surface energy regions pattern. Conventional techniques to form selective surface energy regions on rigid inorganic substrates are not suitable for polymer substrates due to the sensitive and soft limitation of intrinsic polymer properties. Therefore, there is a strong demand for finding a novel and compatible method for the polymer surface energy modification. Here, by employing the CPO method, we successfully demonstrate full polymer filed-effect transistors fabricated through four-step spin-coating process on flexible polymer substrates. Electrodes are successfully patterned on polyethylene terephthalate and PMMA surfaces. Moreover, the pattern technique is low-cost, short time, simple operation and suitable for large-area solution-processing techniques with less etching effect. In addition, the insulating property of the polymeric dielectric is not affected. The CPO method can be applied for complicated circuits to form different patterns without destroying bulk material properties. Finally, the self-aligned full polymer field-effect transistors on flexible polymer substrates are fabricated, showing good electrical properties and mechanical flexibility under bending tests.