Temperature dependent charge transport studies across thermodynamic glass transition in P3HT:PCBM bulk heterojunction: insight from J-V and impedance spectroscopy

Temperature dependent charge transport properties of P3HT:PCBM bulk heterojunction are analysed by dc and ac measurements under dark conditions across a wide temperature range of 110-473 K, which includes the thermodynamic glass transition temperature (T-g similar to 320 K) of the system. A change from Ohmic conduction to space charge limited current conduction at higher (>= 1.2V) applied bias voltages above >= 200 K is observed from J-V characteristics. From capacitance-voltage (C-V) measurement at room temperature, the occurrence of a peak near the built-in voltage is observed below the dielectric relaxation frequency, originating from the competition between drift and diffusion driven motions of charges. Carrier concentration (N) is calculated from C-V measurements taken at different temperatures. Room temperature mobility values at various applied bias voltages are in accordance with that obtained from transient charge extraction by linearly increasing voltage measurement. Sample impedance is measured over five decades of frequency across temperature range by using lock-in detection. This data is used to extract temperature dependence of carrier mobility (mu), and dc conductivity (sigma(dc)) which is low frequency extrapolation of ac conductivity. An activation energy of similar to 126 meV for the carrier hopping process at the metal-semiconductor interface is estimated from temperature dependence of sigma(dc). Above T-g, mu levels off to a constant value, whereas sigma(dc) starts to decrease after a transition knee at T-g that can be seen as a combined effect of changes in mu and N. All these observed changes across T-g can be correlated to enhanced polymer motion above the glass transition.