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Technique Approach

As an alternative to silicon based cells, π-conjugated organic materials have been shown to be attractive candidates in electronic devices because of their compatibility with high through-put, low cost processing techniques, and their capability to be precisely functionalized through the techniques of organic synthesis to afford desired performance attributes. Most π-conjugated linear polymers possess rigid aromatic backbones that serve as the charge transport medium.  Potential issues surrounding the use of such conjugated polymers arise such as twisting when those materials are used in PV cells comprised of ordered rigid electron acceptors. An alternative approach to overcome these problems is to synthesize polymers with semiconducting units grafted onto a flexible backbone allowing for good solubility and necessary π-stacking among the grafted side chains. 

Herein we try to synthesize oligothiophenes grafted polystyrenes with the pendant conjugation systems perpendicular to the polymer backbone. To avoid steric hindrance that prevents a direct free radical polymerization of the oligothiophene-modified styrene monomer to form high molecular weight graft polystyrene, 2-(4-vinylphenyl)thiophene (5) was first synthesized and polymerized to form poly(thiophenyl styrene) (PTS).  The PTS precursor can be easily modified to produce a stannylated derivative (Sn-PTS).  The oligothiophene semiconducting units can be then grafted onto the polystyrene backbone to form the graft polymers PH3TS and PH5TS via Stille coupling reaction as shown in Scheme 1.

Scheme 1. Synthetic route of PH3TS and PH5TS.^a

Preliminary Results

Molecular weights of the resulting polymers were determined by gel permeation chromatography (GPC) as listed in Table 1.  The dry PTS is a light yellow powder (M_n = 13000, PDI = 2.01) and very soluble in common organic solvents such as DCM and THF.  The resulting polymer PH3TS obtained from the grafting reaction is a yellow powder (M_n = 19700 and DPI = 2.16), which is soluble in many common organic solvents such as diethyl ether, THF, DCM, and chloroform.  The polymer PH5TS is a reddish powder, partially dissolved in THF, CHCl₃ or tri-chlorobenzene to form a reddish suspension.

Table 1.  Summary of properties of the polymers PTS, PH3TS and PH5TS.

The thermal curves of PH3TS and PTS seem very similar, with the same decomposition temperature (5 wt % loss) at 325 ^oC by TGA.  Compared to them, PH5TS demonstrates lower thermal stability with T_d at 298 ^oC.  There was no clear evidence of any usual glass and crystallization behavior for both polymers as detected by DSC and hence the polymers were assumed to involve highly hindered segmental motions and appeared to be amorphous. 

The normalized UV-Vis absorption spectra of PTS, PH3TS and PH5TS in chloroform are reported in Figure 1.  As it was expected, PH5TS with the longest conjugated side chain in this study exhibits a more red-shifted peak absorption maximum of 407 nm while PH3TS and PTS exhibit the absorption maximum at 383 and 293 nm, respectively. 

Figure 1. UV-Vis absorption spectra of Polymers PTS, PH3TS and PH5TS in CHCl₃ (~0.1 mg/mL).

To study the intra- and inter- molecular interactions, the polymer solution was studied by fluorescence spectroscopy.  As shown in Figure 2, these polymers show fluorescence emissions in the visible region upon excitation at a certain wavelength (275 nm for the PTS, 375 nm for the PH3TS and PH5TS).  A progressive red-shifted wavelength was found as the chain length of the grafted oligothiophenes increased.  Emission peaks were observed at 411, 485 and 514 nm for the polymers PTS, PH3TS, and PH5TS, respectively.  But an apparent difference in the shape of the emission peak of PTS was observed compared to those of PH3TS and PH5TS.  The peak of PTS was split into two sub-peaks at 387 and 409 nm with a shoulder at 431 nm, indicating strong inter-chain interactions.

Figure 2.  Fluorescence spectra of polymers in CHCl₃.

As shown in Figure 3, only oxidation peaks were revealed for both polymers by cyclic voltammetry.  The onset of the oxidation occurred at approximately 0.46 V (vs. FeCp₂^+/0) and reached maximum at 0.64 V (vs. FeCp₂^+/0).  As for PH5TS, due to its limited solubility in DCM, a uniform film could hardly be deposited onto the platinum electrode.  It resulted in an irreversible oxidation process with decreased anodic current in subsequent scan cycles. The HOMO energy levels were calculated using the equation

, where E_ox is the half-wave of oxidation potential vs. FeCp₂^+/0.  Because both polymers could hardly be reduced, their LUMO energy levels were roughly estimated from the HOMO energy levels and optical bandgaps (E_g^opt).  As shown in Table 2, relatively low HOMO levels of -5.34 and -5.22 eV were obtained for PH3TS and PH5TS. 

Figure 3. Cyclic Voltammograms for a) PH3TS and b) Ph5TS.

Table 2.  Optical and electrochemical properties of polymers.

Summary and Future Plan

We have shown a rational molecular strategy to design and synthesize semiconductor grafted polymers by grafting π-conjugated oligothiophenes onto a polystyrene backbone.  The thermal stability, optical and electrochemical properties have been studied. However, due to its limited solubility of PH5TS in common organic solvents, it is very difficult to fabricate organic thin-film transistor devices out of this polymer in a solution process for a consistent performance. The solubility of this material needs to be enhanced in the future.  This can be realized by attaching longer alkyl side chain onto the pendant thiophene groups.  Meanwhile, a spacer will be inserted between polymer backbone and each grafted semiconducting core.  It will provide a way to study how flexibility of pendant semiconducting cores impacts packing among the grafted side chains and future facilitate charge carrier mobilizing. Additionally, including the oligothiophene, new semiconducting structure will be explored in the future. For examples, diphenylene dithiophene is a well studied organic semiconductor with comparable mobility and relatively low HOMO level. It will be an attractive building block for synthesizing new class of air stable semiconducting polymers.