Charge transport mechanism in bis(double-decker lutetium(III) phthalocyanine) (Lu₂Pc₄) thin film

The dc and ac dark conductivity measurements have been performed on spin-coated Lu₂Pc₄ thin film in the frequency range 40-10⁵ Hz and in the temperature range 50-290 K. Measurements revealed that dc behaviour of the film can be described by Mott's three-dimensional variable range hopping (VRH) model, T^-1/4 law, in the whole temperature range. The ac conductivity σ(ω) varied as ω^s, where the exponent s is found temperature dependent variable and always less than unity. The measured ac conductivity data are discussed in terms of classical models based on pair approximation and a unified theory. Although the ac behaviour can be approximately explained by the correlated barrier hopping model below the 190 K, the temperature dependencies are stronger than any classical model predicts. The log-log plot of the normalized conductivity σ(ω)/σ(0) with reduced frequency ω̃/σ̃(0) showed that, the ac conductivity data agrees fairly well with the predictions of the extended pair approximation (EPA) model due to Summerfield and Butcher.