New low band-gap alternating polyfluorene derivatives for photovoltaic cells

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 C60 as an electron acceptor material. The characterization of the structures of the PFR3-S : C60 and PFR4-S : C60 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 : C60 (1 : 1) and PFR4-S : C60 (1 : 1) devices were constructed in ITO/PEDOT : PSS (100 nm)/polymer : C60 (100 nm)/Ca (50 nm)/Al (100 nm) configurations. In the PFR4-S : C60 PPVC, the spectral response was found to be extended to 700 nm, and exhibited an improved spectral match with solar radiation. The PFR3-S : C60 (1 : 1) and PFR4-S : C60 (1 : 1) devices were found to have higher energy conversion efficiencies and better short circuit current densities (Jsc), 3.18 mA/cm2 and 3.09 mA/cm2, respectively, than a MEH-PPV : C60 (1 : 1) device. The open-circuit voltages (Voc) of the PFR3-S and PFR4-S devices were found to be 0.85 and 0.81 V, respectively. The energy conversion efficiencies (ηe) of the PFR3-S : C60 (1 : 1) and PFR4-S : C60 (1 : 1) devices under AM 1.5 solar illumination (100 mW/cm2) were found to be as high as 0.98% and 1.02%, respectively.