Relaxation processes in a polymer composite for bulk heterojunction: A dielectric spectroscopy study

Poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4ʹ,7ʹ-di-2-thienyl-2ʹ,1ʹ,3ʹ-benzothiadiazole)] (PCDTBT) and its blend with [6,6]-phenyl C71-butyric acid methyl ester (PCBM) available for photovoltaic application were investigated with Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and broadband dielectric spectroscopy (BDS). In the BDS spectra, the molecular dynamics, polarization, and conductivity processes were identified, analyzed, and discussed for both PCDTBT and the PCDTBT/PCBM blend. The dielectric constant of the blend was successfully predicted when the dielectric constants of PCDTBT and PCBM and their weight ratio in the blend were taken into account. Enhanced dc conductivity due to increased charge carrier mobility was detected in the blend. The energetic offset of the photovoltaic blend under study was shown to be directly estimated from the BDS data.