We have studied hybrid solar cells based on the polymer poly(3-hexylthiophene) (P3HT) and colloidal CdSe nanocrystals. Using CdSe nanospheres with varying size, we have found that the power conversion efficiency (eta(P)) of these devices increases monotonically with the CdSe nanocrystal size, from eta(P)=(0.39 +/- 0.04)% under AM1.5G solar illumination for 4.0 +/- 0.2 nm size nanospheres to eta(P)=(1.9 +/- 0.2)% for 6.8 +/- 0.5 nm size nanospheres. The efficiency increase with nanocrystal size is mostly due to a significant increase in the short-circuit current, whereas the open-circuit voltage and fill factor of the solar cells are less affected. The devices also exhibit abnormal initial aging behavior when exposed to air, as an increase in both the short-circuit current and open-circuit voltage during the first 30 min leads to a significant increase in eta(P). (C) 2010 Elsevier B.V. All rights reserved.