Effect of thermodynamic glass transition on charge transport properties in a benzodithieno-imidazole π-conjugated polymer: fullerene blend

In this work, the charge transport properties of bulk heterojunction formed by p-type benzodithieno-imidazole (BDTI-Br) based π-conjugated polymer and PCBM acceptor has been investigated in photovoltaic configuration under dark conditions. Temperature dependent current-voltage (J − V) characteristics show a transition from ohomic () to trap dominated space charge limited current with a power law J ∼Vm, m ≥ 2 for higher applied voltages. Capacitance-voltage measurement shows that capacitance value at reverse bias is drift controlled and the accumulation of diffused charge carriers contribute at forward bias. An equivalent impedance model gives charge carrier lifetime values in milliseconds and electron mobility values in the range of ∼10−5 cm2 V−1 s−1; in line with that obtained by charge extraction by linearly increasing voltage method. The extrapolated dc conductivity values in 113 K–473 K show Arrhenius like behavior with an activation energy of 145 meV. Notable changes in temperature dependent conductivity, carrier concentration and carrier mobility values around 320 K indicate a strong correlation of charge transport process with structural changes that may occur in bulk heterojunction around its thermodynamic glass transition temperature.