Molecular Dynamics and Conductivity of a PTB7:PC71BM Photovoltaic Polymer Blend: A Dielectric Spectroscopy Study

Molecular dynamics of a low-band gap polymer, poly{[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]} (PTB7), and its 1:1.5 wt blend with a fullerene derivative, [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), accessible for organic photovoltaics was studied with the broadband dielectric spectroscopy (BDS) technique in a wide range of frequency (from 10-1 to 106 Hz) and temperature (from -130 to 150 °C). In the BDS spectra, three types of molecular mobility of the polymer chains designated as γ-, β1-, and β2-relaxations, along with conductivity and electrode polarization processes, were recognized, separated, and analyzed. The relaxation processes in PTB7 and PTB7:PC71BM were described with Arrhenius-type equations and related parameters were found. Their analysis allowed us to reveal the influence of the PC71BM molecules on the dynamics of the polymer chains. Based on dc conductivity data, the Cole-Cole diagrams for PTB7 and PTB7:PC71BM were plotted. They allowed for estimating the hole lifetime and its comparison with the hole extraction time.