The transport properties of two photovoltaic polymers, poly(3- hexylthiophene) (P3HT) and poly(2,7-carbazole) derivative (PCDTBT), and their polymer:fullerene bulk heterojunction (BHJ) are studied by space-charge-limited current (SCLC), dark-injection space-charge-limited current (DI-SCLC), and admittance spectroscopy(AS). For a nominally hole-only device, electrons leakage occurs. This results in a current larger than the theoretical SCLC and ill-defined DI-SCLC and AS signals. In order to prevent electron leakage, a hole-transporting but electron blocking/trapping thin layer is added between active layer and Au. The layer composed of copper phthalocyanine (CuPc) doped into an amine-based small molecule. Using this interlayer, well-defined carrier transit time can be obtained for mobility extraction. With a suitable interlayer to suppress undesirable carrier injection and transport, these techniques should find broad applications in the transport characterization of narrow gap photovoltaic polymers and BHJ blends.