Photoconductivity in fullerene-doped polysilane thin films

Abstract

Flash photolysis time-resolved microwave conductivity (FP-TRMC) measurements have been performed to study the transport properties of charge carriers in the polysilane films. The effect of C 60 concentrations on photoconductivity of poly(methylphenylsilane) (PMPS) and poly( n-hexylphenylsilane) (PHPS) films has been studied using a variety of excitation light sources. The value of ϕ ∑ μ , which is the product of quantum efficiency of photocarrier generation ( ϕ) and sum of moibilities of the carriers ( ∑ μ ), depends strongly on C 60 concentration upon exposure of 532 nm. The quantum efficiency reaches ∼0.06 with C 60 concentration at 5.7 mol%, and saturates at the higher concentrations. The ϕ ∑ μ value increases considerably upon exposure of 193 nm up to 1.22 × 10 −3 cm 2/Vs without significant dependence on C 60 concentration, suggesting the direct formation of electron-hole pair by 6.39 eV photon absorption with the higher ϕ value at ∼0.12. The present technique gave the estimates of intra-chain mobility of charge carriers in PMPS and PHPS, and one-order magnitude higher intra-chain mobility was observed in PHPS than that in PMPS. This is suggestive of a highly ordered Si backbone conformation in PHPS. The intra-chain mobility in PMPS, however, is at least two orders of magnitude higher than the mobility values estimated by conventional time-of-flight techniques. Thus we conclude that the higher mobility than ∼10 −3 cm 2/Vs ( E ∼ 0) is realizable even with PMPS by precise synthesis and device fabrication free from disordering, defects, impurities, etc.