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Molecular Assembly and Device Physics of High-Mobility Organic Field-Effect Transistor Fabricated from Donor-Acceptor Copolymers

  • Author(s): Phan, Hung
  • Advisor(s): Nguyen, Thuc-Quyen
  • et al.

The Field Effect Transistor (FET) is inarguably the most important circuit element in modern electronics. Metal-Oxide FETs (MOSFETs), the most common type of FET, are integrated in microprocessors in almost all electronic devices: mobile phones, computers, digital cameras, and digital printers, to name a few. MOSFETs are normally fabricated on top of rigid single crystalline silicon, which requires high temperature processing (~1000 oC). Recently, a need has emerged for flexible electronics in a variety of applications. Examples include form-fitting healthcare-monitoring devices, flexible displays, and flexible radio frequency identification tags (RFID). Organic FETs (OFETs) are viable candidates for flexible electronics because they are based on semiconducting π-conjugated materials, including small molecules and polymers, which can be solution-processed at low temperature on flexible substrates. Solution-processing may enable the use of high throughput methods such as roll-to-roll coating and inkjet printing for low-cost manufacturing. In the molecular perspective, the limitless ability to tune the properties of these materials just by a small modification of the conjugated backbone or sidechains makes them attractive to both academic research and industrial manufacturing. Between the two materials, semiconducting polymers offer better potential for the formation and mechanical properties of thin films, compared to their small molecule counterparts.

For OFETs to be industrially viable, however, they must first have high charge carrier mobility. Recent advances in molecular designs and device engineering have seen significant increases in the mobility of OFETs fabricated with conjugated polymer (or PFETs). In this dissertation, it is shown that obtaining polymer-chain alignment is critical to improve the mobility of PFETs. In addition, the charge transport mechanism is investigated to explain the high mobility in PFETs with aligned polymers. Most importantly, the mechanism of electrical instability and non-ideality (i.e. the double-slope) of high mobility PFETs with a certain degree of ambipolarity is unraveled.

Firstly, the alignment of polymer chains inside polymer fiber bundles is revealed by high-resolution atomic force microscopy (AFM). This alignment is enabled by nano-grooves of ca. 50 nm wide and 2-5 nm deep on SiO2 substrates used for fabricating the PFETs. Mobility of charge transport along the direction of the polymer fiber is an order of magnitude higher than that of charge transport perpendicular to the fiber direction. It indicates that aligned polymer chains facilitate fast intrachain charge transport.

Secondly, the charge transport mechanism is determined to be the thermally-activated hopping of charge carriers. This is an important finding because it has been speculated that band transport is possible in OFETs fabricated from well-aligned polymer fibers. With a normal range of molecular weight (30 kDa to 100 kDa), which is feasible for industrial scale-up, the stretched length of the corresponding polymer chain, ranging from 50 – 150 nm, is not enough to cross the full channel length of the OFETs. In addition, polymer chains in a solution-processed thin film are likely to have kinks and twists that disrupt the perfect electronic coupling necessary for band transport. It implies that polymers should be designed to facilitate not only chain alignment but also the strong electronic coupling in the π-π stacking direction between chains for efficient hoppings between chains with low energetic barriers.

Finally, the electrical instability and its associated non-ideal device characteristics is thoroughly investigated. Electrical stability is as important as high charge carrier mobility for OFETs to be commercialized in large-scale production. The use of low bandgap donor-acceptor (D-A) polymers for obtaining high mobility introduces undesirable electron current in p-type PFETs. The effect of electron transport and trapping on hole conduction in p-type OFETs has not been addressed. In this work, p-type PFETs fabricated with SiO2 dielectrics and with a certain degree of ambipolarity exhibit electrical instability (a change in current upon bias stressing) and non-ideality (double-slope in transfer curves) of hole current. It is determined that electron trapping and the subsequent formation of -SiO- charges on SiO2/polymer interface are the principle origins of the instability and the double-slope. Suppressing these undesired properties is essential to make PFETs industrial viable. It should be noted that the double-slope has been a long debate without a solid explanation in the OFET research community.

In sum, this thesis shows a comprehensive understanding of the structure-processing-property relationship of PFETs fabricated with well-aligned polymers. New findings in the thesis provide important guidelines for molecular design and device engineering of high-mobility and practical PFETs. These guidelines have been successfully demonstrated by us and our collaborators at UCSB.

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