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Technique Approach

Organic semiconductor based devices have attracted considerable attention for a wide range of applications including organic light-emitting diodes (OLED), organic field effect transistors (OFET), and photovoltaic devices (PV). Realization of desirable molecular architectures having well defined dimensions and controlled functionality for organic semiconducting materials is a key challenge for research in the field of active organic materials. It is critically important to precisely build molecular architectures to afford necessary performance attributes such as good solubility, long-term thermal and oxidative stability, and enhanced π-stacking for superior electrical conductivity.

Previously, we demonstrated the synthesis of a series of oligothiophene grafted polystyrenes where the pendant conjugation system lies perpendicular to the polymer backbone. We have expanded our scope to incorporate [5,5']-diphenyl-[2,2']-bithiophene (PTTP) based semiconducting moieties. The PTTP family of co-oligomers affords good electrical performance because of its molecular geometry and energy levels. To date, studies have focused on PTTP small molecules. Given the advantages of polymers versus small molecules, particularly in terms of their ability to form uniform films, mechanical flexibility, and excellent rheological properties, it is of interest to incorporate PTTP semiconducting blocks into polymeric systems to facilitate solution based processing for flexible electronic device applications. As shown in Scheme 1, we have synthesized a class of polymethacrylate derivatives containing conjugated PTTP cores as pendant side chains via direct free radical polymerization of PTTP-modified methacrylic monomers. The effect of fundamental changes in chemical structure of the PTTP substituted polymers (e.g., alkyl side chain and aliphatic spacer) on the physical properties of the resultant polymers was systematically studied. Properties such as morphology, solubility and electrical performance were evaluated.

Results

The solubility of the PTTP graft polymers prepared in this study was significantly affected by the length of both the alkyl side chain and spacer. For instance, poly(4-(5'-(4-hexylphenyl)-[2,2'-bithiophen]-5-yl)phenethyl methacrylate), p(HPTTPEM) displayed very limited solubility in many common organic solvents at room temperature. However, poly(4-(5'-(4-dodecylphenyl)-[2,2'-bithiophen]-5-yl)phenethyl methacrylate), p(DPTTPEM), was soluble in hot 1,2,4-trichlorobenzene (TCB) and chlorobenzene, and was partially soluble in chloroform. Using a butyl rather than ethyl spacer enhanced polymer solubility. While the length of the alkyl side chain appeared to have no significant effect on monomer reactivity, reactivity did appear to decrease upon introduction of a butyl spacer. Molecular weight (M_w, and M_n), polydispersity (PDI), degree of polymerization (DP), and thermal stability of polymers is given in Table 1.

While the PTTP monomers displayed two distinct transitions in either the heating or cooling process upon DSC characterization, crystalline phase transitions at low temperature, similar to those found for the monomers, were not found for the respective PTTP polymers; a result which is consistent with a previous report and thought to result from the lack of ordered alignment of the PTTP segments along a given direction. Extension of the PTTP pendant side chain to a longer aliphatic segment led to the appearance of a glass transition temperature for p(DPTTPEM) at 181 ^oC. This temperature decreased to 169 ^oC for p(DPTTPBM), the corresponding polymer with a butyl spacer.

Scheme 1. Synthetic route to PTTP graft polymers.

Table 1. Molecular weight and thermal stability of PTTP graft polymers.

The morphology of the PTTP compounds was studied by micro X-ray diffraction (XRD). All monomers showed strong reflection signals up to multiple orders, consistent with the crystalline nature of these compounds revealed by both DSC and polarized optical microscopy. As shown in Table 2, the first order d-spacing varied from ca. 28 Å in HPTTPEM to ca. 35 Å as the alkyl side chain length increased. When a butyl unit was used as the spacer, the d-spacing decreased slightly from ca. 35 Å in DPTTPEM to ca. 31 Å in DPTTPBM. Conceivably, the enhanced flexibility of DPTTPBM induced by the longer butyl spacer allows for folding of the alkyl moieties as well as possible interdigitation of the alkyl chains leading to the decreased interlayer d-spacing. XRD evaluation of the polymers confirmed the DSC results suggesting that they are amorphous. Of interest however, the three major peaks in the wide angle regions (19 – 27^o) remained essentially unchanged (Figure 1). According to the folded chain structure for a PTTP block copolymer proposed by Okamoto and coworkers, this result may be indicative of a face-to-face alignment of the PTTP pendant side chains along the polymer backbone.

Table 2. X-ray diffraction θ - 2θ data for sublimed PTTP films.

Figure 1. Micro X-ray diffraction θ - 2θ scans of sublimed DPTTPEM film and p(DPTTPEM) powder at room temperature.

A bottom-gate, bottom-contact TFT device configuration was utilized in the characterization of FET properties of the monomers and graft polymers. The semiconductors were deposited by spin-coating from chloroform solution, and except for the two polymers with shorter alkyl side chains which were in sufficiently soluble in chloroform, the materials exhibited characteristic p-channel FET behavior. The extracted mobilities in the saturated regime were on the order of 10^-5 cm²V^-1s^-1.

Summary and Future Plans

A “graft” approach to the design and synthesis of polymer based semiconductors has been explored. Through the course of this program, a series of oligo-thiopene substituted polystyrenes and PTTP-modified polymethacrylates were synthesized and evaluated. Significantly, it was found that for the methacrylate system, the pendant PTTP sides chains appeared to be aligned in a face-to-face manner as evidenced by a distinct blue shift in their UV-Vis spectra. Compared to alternative well-developed organic semiconductor materials, there is still substantial space to improve the FET performance characteristics through optimization of the structure of the PTTP materials investigated here. The introduction of the concept of a graft polymer approach could also contribute to the development of new chemical architectures and/or building blocks for applications related to organic electronics.