Charge transport in conjugated polymers

Conjugated polymers can be viewed as one-dimensional semiconductors, in which the semi­conducting properties are attributed to the extended n electron systems formed by the nz elec­trons of the c-conjugated backbone. Although microwave conductivity techniques [54, 55], showed that the on-chain mobility of conjugated polymers can reach very high values, the macroscopic transport properties are controlled by orders of magnitude slower interchain hop­ping processes. The solubility and processability of conjugated polymers are achieved by attaching side chains to the conjugated backbone. Naturally, the length of the insulating side chain affects the interchain electronic coupling, thereby influencing mobility, as is evidenced by the decreasing charge carrier mobility in a series of regioregular polyalkylthiophenes with increasing side chain length [56]. Furthermore, the regularity of the side chain attachment plays an important role in the solid state morphology and ordering. For example, regioregular poly(3-hexylthiophene), in which the solubilizing side chains are attached in a regular pattern, forms crystalline phases [57], while regiorandom poly(3-hexylthiophene-2,5-diyl (P3HT) is typically amorphous. The electronic properties of these two materials are also quite different. Regioregular P3HT exhibits one of the highest mobilities among conjugated polymers in the FET structure, meanwhile mobility in regiorandom P3HT is three to four orders of magnitude lower [58, 59].

The effect of side chain substitution on charge carrier mobility was investigated for a series of alkoxy-PPVs in the work of Martens et al. [60]. The charge carrier mobility measured by space charge limited current measurements of the symmetrically substituted OC10C10-PPV is one order of magnitude higher than the asymmetrically substituted, regiorandom OC1C10- PPV (also known as MDMO-PPV). From the temperature and electric field dependence of the mobility studies it was concluded that increased on-chain and interchain disorder is mainly responsible for the lower mobility of regiorandom MDMO-PPV.

Based on the above arguments, it is expected that increasing the regioregularity of conju­gated polymers is viable way to improve their charge carrier mobility. Lutsen et al. [61] have recently synthesized a soluble regioregular MDMO-PPV polymer based on the sulfinyl pre­cursor route [62]. The charge transport properties of this regioregular polymer were compared to the commercially available regiorandom MDMO-PPV using the ToF technique.

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