Bulk heterojunction polymer photovoltaic cells (PPVCs) were fabricated by using low energy band-gap conjugated polymers (PFR3-S and PFR4-S) as electron donors and C-60 as an electron acceptor material. The characterization of the structures of the PFR3-S:C-60 and PFR4-S:C-60 mixtures were carried out with atomic force microscopy (AFM), and the PPVCs were characterized with current density-voltage (J-V) and spectral photocurrent measurements. The PFR3-S:C-60 (1:1) and PFR4-S:C-60 (1:1) devices were constructed in ITO/PEDOT:PSS (100 nm)/polymer:C-60 (100 nm)/Ca (50 nm)/Al (100 nm) configurations. In the PFR4-S:C-60 PPVC, the spectral response was found to be extended to 700 nm, and exhibited an improved spectral match with solar radiation. The PFR3-S:C-60 (1:1) and PFR4-S:C60 (1:1) devices were found to have higher energy conversion efficiencies and better short circuit current densities (J(sc)), 3.18 mA/cm(2) and 3.09 mA/cm(2), respectively, than a MEHPPV:C60 (1:1) device. The open-circuit voltages (V-oc) of the PFR3-S and PFR4-S devices were found to be 0.85 and 0.81 V, respectively. The energy conversion efficiencies (eta(e)) of the PFR3-S:C-60 (1:1) and PFR4-S:C-60 (1:1) devices under AM 1.5 solar illumination (100 mW/cm(2)) were found to be as high as 0.98% and 1.02%, respectively. (c) 2005 Elsevier B.V. All rights reserved.