Electronic structures and charge redistribution between P3HT and (10,2)/(6,5) carbon nanotubes (CNT) are investigated by density functional theory calculations. The simulations show that electron is transferred from flat P3HT to (10,2) CNT while hole is transferred from (6,5) CNT to wrapped P3HT due to the different work functions of these materials. The large built-in potential can compete to exciton binding energy, leading to efficient charge separation across the type-II photovoltaic heterojunctions. Electron transfer faster than hole is expected because the electron donor state is much more delocalization, creating larger donor-acceptor coupling, which provides critical insights of organic photovoltaic solar cells. (C) 2014 Elsevier B. V. All rights reserved.