Studies of Polymer Field-Effect Transistors

Thesis (Ph.D.)--University of Washington, 2012The era of plastic electronics is rapidly emerging due to the increasing development and application of low-cost, printable, shape-conforming, and large-area devices, such as organic field-effect transistors (OFETs) and circuits, organic solar cells, and organic light-emitting devices. Deepening our understanding of charge-carrier dynamics in polymer semiconductors is critical to the future advances in organic electronics. This dissertation focuses on studies of OFETs and aims to better understand the charge transport properties of polymer semiconductors and factors that influence the performance of OFETs. Case studies of structure-morphology-property relationships in unipolar p- and n-channel polymer OFETs as well as ambipolar OFETs reveal that variations in molecular structure and processing affect electronic energy levels, solid-state morphology and crystallinity, and thus the magnitude and polarity of charge carriers. The studies resulted in achievement of high-performance OFETs with high charge-carrier mobility of up to 0.3 cm2/Vs. The morphology and electronic energy levels are also related to ambient stability and durability of polymer OFETs through kinetics and thermodynamics of interaction between the semiconductor and extrinsic molecules in ambient air. Air-stable ambipolar OFETs were realized by utilizing unipolar p- and n-type polymer semiconductors as the active channel elements. Complementary digital logic circuits such as inverters and NAND- and NOR-gates, were also demonstrated using the unipolar and ambipolar OFETs. Device engineering studies show that electron mobility and electrical stability of n-channel polymer OFETs can be significantly enhanced by inserting a low-dielectric-constant polymer dielectric buffer layer at the semiconductor/dielectric interface. Electron mobility was found to increase exponentially with decreasing dielectric constant of the buffer layer. Finally, poly(3-butylthiophene)-nanowire/polystyrene nanocomposites were also investigated as a means of controlling the solid-state morphology of active thin films in OFETs. High dc conductivity and high hole mobility were obtained throughout a wide range of the nanowire compositions (2-100 wt%) due in part to the very low percolation threshold (0.5 wt%)

Synergistic Effects of Processing Additives and Thermal Annealing on Nanomorphology and Hole Mobility of Poly(3-hexylthiophene) Thin Films

Control of the nanoscale molecular ordering and charge-carrier mobility of poly(3-hexylthiophene-2,5-diyl) (P3HT) was achieved by the combined use of processing additives and thermal annealing. Evaluation of four processing additives (1,8-octanedithiol (ODT), diphenyl ether (DPE), 1-chloronaphthalene (CN), and 1,8-diiodooctane (DIO), which are commonly used for the fabrication of organic solar cells, revealed that the nanoscale molecular ordering and, therefore, the charge-carrier mobility, are largely affected by the additives, as demonstrated by spectral absorption, X-ray diffraction, and atomic force microscopy. Thermal annealing selectively influenced the morphological changes, depending on the solubility of P3HT in the additive at high temperature. In the case of CN, in which P3HT can be dissolved at moderate temperature, significant molecular ordering was observed even without thermal annealing. For DIO, in which P3HT is only soluble at elevated temperature, the mobility reached 1.14 × 10−1 cm2 V−1 s−1 only after annealing. ODT and DPE were not effective as processing additives in a single-component P3HT. This study provides insight for designing the processing conditions to control the morphology and charge-transport properties of polymers

Enhanced carrier mobility and electrical stability of n-channel polymer thin film transistors by use of low-k dielectric buffer layer

© 2011 American Institute of Physics. The electronic version of this article is the complete one and can be found at: http://dx.doi.org/10.1063/1.3655680DOI: 10.1063/1.3655680Insertion of a low-k polymer dielectric layer between the SiO₂ gate dielectric and poly(benzobisimidazobenzophenanthroline) (BBL) semiconductor of n-channel transistors is found to increase the field-effect mobility of electrons from 3.6 × 10⁻⁴ cm²/Vs to as high as 0.028 cm²/Vs. The enhanced carrier mobility was accompanied by improved multicycling stability and durability in ambient air. Studies of a series of eight polymer dielectrics showed that the electron mobility increased exponentially with decreasing dielectric constant, which can be explained to result from the reduced energetic expense of charge-carrier/dipole interaction

Synthesis and side-chain engineering of phenylnaphthalenediimide (PNDI)-based n-type polymers for efficient all-polymer solar cells

We designed and synthesized a series of n-type conjugated polymers by introducing phenylnaphthalenediimide (PNDI) as a novel n-type building block, and investigated the effect of side-chain engineering of the polymer acceptors on the performance of all-polymer solar cells (all-PSCs). The optical, electrochemical, and structural properties of the polymers with three different side chains of 2-ethylhexyl (PPNDI-EH), 2-butyloctyl (PPNDI-BO), and 2-hexyldecyl (PPNDI-HD) groups were examined. Interestingly, the PNDI-based polymer having the longest side chain showed a higher degree of edge-on oriented intermolecular assembly in thin films, thereby resulting in the highest field-effect electron mobility among the three polymers. Also, we examined the performance of PNDI-based polymers as polymer acceptors in all-PSCs. Unlike the trend in the field-effect transistor, the PPNDI-BO-based all-PSCs exhibited the highest power conversion efficiency (PCE) of 4.25% among the three polymer blends. This was attributed to the well-balanced hole/electron transport and higher exciton dissociation probability in the PPNDI-BO-based all-PSCs, benefitted from the well-intermixed blend morphology between the polymer donor and PPNDI-BO

High-Mobility n‑Type Conjugated Polymers Based on Electron-Deficient Tetraazabenzodifluoranthene Diimide for Organic Electronics

High-mobility p-type and ambipolar conjugated polymers have been widely reported. However, high-mobility n-type conjugated polymers are still rare. Herein we present poly­(tetraazabenzodifluoranthene diimide)­s, PBFI-T and PBFI-BT, which exhibit a novel two-dimensional (2D) π-conjugation along the main chain and in the lateral direction, leading to high-mobility unipolar n-channel transport in field-effect transistors. The n-type polymers exhibit electron mobilities of up to 0.30 cm²/(V s), which is among the highest values for unipolar n-type conjugated polymers. Complementary inverters incorporating n-channel PBFI-T transistors produced nearly perfect switching characteristics with a high gain of 107

High Mobility Thiazole–Diketopyrrolopyrrole Copolymer Semiconductors for High Performance Field-Effect Transistors and Photovoltaic Devices

New donor–acceptor copolymers incorporating both a strong electron-accepting diketopyrrolopyrrole unit and a weak electron-deficient thiazolothiazole or benzobisthiazole moiety were synthesized, characterized, and found to exhibit very high charge carrier mobility. Stille coupling copolymerization gave copolymers having moderate number-average molecular weights of 17.0–18.5 kDa with polydispersities of 3.3–4.0 and optical band gaps of 1.22–1.38 eV. High performance p-channel field-effect transistors were obtained using the thiazolothiazole-linked copolymers, PDPTT and PDPTTOx, giving hole mobilities of 0.5 and 1.2 cm²/(V s), respectively, with on/off current ratios of 10⁵ to 10⁶. In contrast, the benzobisthiazole-linked copolymer PDPBT had a substantially lower field-effect mobility of holes (0.005 cm²/(V s)) due to its amorphous solid state morphology. Bulk heterojunction solar cells fabricated by using one of the thiazolothiazole-linked copolymer, PDPTT, as electron donor and PC₇₁BM acceptor show a power conversion efficiency of 3.4% under 100 mW/cm² AM1.5 irradiation in air