A solution-processable double-cable polymer (PFT-PDI), composed of the backbone poly(fluorene-alt-hiophene) (PFT), the n-butoxyl linker, and the pendants perylenediimides (PDI), was developed. PFT-PDI was almost nonconductive with the hole and electron mobilities in the order of 10(-10). There was no charge transfer but energy transfer from the donor PFT chain to the acceptor PDI units. With a hole-transporting channel from the stacked PFT units and an electron-transporting channel along PDI chain, PFTPDI at the P3HT/PCBM interface facilitated the effective charge generation from P3HT excitons and charge transporting and enhanced the cell photocurrent. The encapsulated cell ITO/MoO/P3HT:PCBM:PFT-PDI/LiF/Al with doping PFT-PDI of 3 wt % demonstrated the maximum power conversion efficiency (PCE) of 4.50%, increasing by 27.5%, relative to PCE of 3.53% from the cell without doping. The PFT-PDI doping much improved the cell's stability with the loss of the initial PCE of 5.8%, in contrast to 29.7% from the reference device after being stored for 7 days.