Donor-acceptor block copolymers (BCP), incorporating poly(3-hexylthiophene) (P3HT), and a polystyrene copolymer with pendant fullerenes (PPCBM) provide desired stable nanostructures, but mostly do not exhibit balanced charge carrier mobilities. This work presents an elegant approach to match hole and electron transport in BCP by blending with molecular PCBM without causing any macrophase separation. An insufficient electron mobility of PPCBM can be widely compensated by adding PCBM which is monitored by the space-charge limited current method. Using X-ray diffraction, atomic force microscopy, and differential scanning calorimetry, we verify the large miscibility of the PPCBM: PCBM blend up to 60 wt % PCBM load forming an amorphous, molecularly mixed fullerene phase without crystallization. Thus, blending BCP with PCBM substantially enhances charge transport achieving an electron mobility of mu(e) = (3.2 +/- 1.7) x 10(-4) cm(2) V-1 s(-1) and hole mobility of mu(h) = (1.8 +/- 0.6) x 10(-3) cm(2) V-1 s(-1) in organic field-effect transistors (OFET). The BCP: PCBM blend provides a similarly high ambipolar charge transport compared to the established P3HT: PCBM system, but with the advantage of an exceptionally stable morphology even for prolonged thermal annealing. This work demonstrates the feasibility of high charge transport and stable morphology simultaneously in a donor-acceptor BCP by a blend approach. (C) 2016 Wiley Periodicals, Inc.