Effects of structural defects on hole drift mobility in aryl-substituted polysilanes

The effects of a silicon-based structural defect on the hole transport properties of poly(methylphenylsilane) were studied by the DC time-of-flight technique. The defect was chemically introduced as silicon branching of the silicon backbone by copolymerizing methylphenyldichlorosilane and p-tolyltrichlorosilane with sodium. A good empirical relation was obtained between the hole drift mobility (μ _E−0) at a zero electric field limit and the defect density. The value of μ _E−0 decreased exponentially from 4 × 10⁻⁴ to 5 × 10⁻⁶cm² V¹ s⁻¹ with an exponential increase in defect density from 0.0024 to 0.18. On the basis of Bässler's disorder formalism, hole transport in the polymers was quantitatively analysed, revealing the different contributions of diagonal and off-diagonal disorder with the defect density. The degree of fluctuation in the density of state mainly determined the hole drift mobility in specimens with fewer defects, while intersite hopping distance controlled the mobility in the polymers with more defects.