Trap-limited hole mobility in semiconducting poly(3-hexylthiophene)

Abstract

Bulk transport properties of poly(3-hexylthiophene) (P3HT) were studied by analyzing temperature dependent current-voltage characteristics of the polymer thin films sandwiched between indium tin oxide/polystyrene sulfonate doped polyethylene dioxy-thiophene (ITO/PEDOT) and aluminium electrodes. It was found that the contacts limit charge injection under reverse bias, but under forward bias the current is limited by space charge that accumulates near the hole injecting electrode (ITO/PEDOT) resulting in a rectification of ${10}^5$. The forward current density obeys a power law of the form $J\sim V^m$, with $m>2$, described by space charge limited current in the presence of exponentially distributed traps within the band gap. In this paper we describe the deduction, and discuss the limits, of an expression for the calculation of the total trap density, based on the exponential trap distribution model, which yielded reasonable agreement with our experimental $J\left(V\right)$ data. The total deep hole trap density was estimated to be $3.5\times {10}^{16}{\mathrm{cm}}^{-3}$, and the activation energy extrapolated to zero Kelvin was obtained to be 0.054 eV. Temperature dependent hole mobility in P3HT was also estimated under trap-free space charge conditions, yielding a value of $3\times {10}^{-5}{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$, at 304 K.