Charge transport study of semiconducting polymers and their bulk heterojunction blends by capacitance measurements

We use frequency dependent capacitance measurements to probe carrier mobilities and transport parameters of six representative semiconducting polymers and some of their bulk heterojunction (BHJ) blends. With a suitable choice of a hole injection layer, well-defined signals for hole transport characterization can be obtained for the pristine polymers [J. Appl. Phys. 99, 013706 (2006)]. However, ill-defined signals with negative capacitances, arising from undesirable electron leakages, are obtained for the BHJ blends. The problem of electron leakage can be circumvented by inserting an electron blocking and trapping layer under the cathode. As a result, hole transport properties of BHJ blends can be obtained. For the BHJ of poly(3-hexylthiophene) blended with [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC ₆₁BM), the hole mobilities seem to be insensitive to the composition of the BHJ, indicating the P3HT component in the BHJ is well connected. On the other hand, for poly[N-9"-hepta-decanyl-2,7-carbazole-alt-5,5-(4′, 7′-di-2-thienyl-2′,1′,3′-benzothiadia zole)] doped with [6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC₇₁BM), a clear reduction of the hole mobility is observed as the polymer composition is reduced. Temperature dependent experiments were performed. The data are analyzed by the Gaussian Disorder Model. We found that the energetic disorder is independent of the composition of the BHJ. Organic photovoltaic performances of BHJ blends are also measured in this contribution. The correlation between device performance and energetic disorder of the BHJ will be discussed.