Cadmium selenide (CdSe) nanoparticles of 2-7 nm diameter were synthesized by a hot injection method and their TOPO ligands exchanged with pyridine by a liquid-liquid extraction process. The effects of the ligand exchange on the material properties of the nanoparticles and device performance of the bulk hetero-junction (BHJ) solar cell were systematically investigated. The wavelength shift to a shorter value observed in the PL spectra of the pyridine-capped CdSe indicates that the size of nanoparticles decreased due to the exchange of the surface ligands of the CdSe from TOPO to pyridine. Improvements in the power conversion efficiency of BHJ solar cells made with pyridine-capped CdSe are mainly attributed to the enhanced current collection, J(sc)=0.43 mA/cm(2) (TOPO) to 1.4 mA/cm(2) (pyridine). (C) 2011 Elsevier B.V. All rights reserved.