Deep-Red Electroluminescent Polymers:  Synthesis and Characterization of New Low-Band-Gap Conjugated Copolymers for Light-Emitting Diodes and Photovoltaic Devices

A novel series of semiconducting conjugated copolymers, derived from alkyl-substituted fluorene, 4,7-diselenophen-2'-yl-2,1,3-benzothiadiazole (SeBT), and 4,7-diselenophen-2'-yl-2,1,3-benzoselenadiazole (SeBSe), was synthesized by a palladium-catalyzed Suzuki coupling reaction with various feed ratios. The optical band gap of copolymers is very low, 1.87 eV for SeBT and 1.77 eV for SeBSe. The efficient fast energy transfer from fluorene segments to narrow-band-gap sites was observed. The emission of photoluminescence and electroluminescence is dominated by narrow-band-gap species and peaked at 670−790 nm, in the range from deep-red to near-infrared (NIR). The external electroluminescent (EL) quantum efficiencies reached 1.1% and 0.3% for devices from these two types of copolymers, respectively. Bulk−heterojunction polymer photovoltaic cells (PPVCs) made from composite thin film of the copolymer 9,9-dioctylfluorene and SeBT (PFO−SeBT) in blend with fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as an active layer show promising performances. The energy conversion efficiency (ECE) is up to 1% under AM1.5 solar simulator (78.2 mW/cm2). The spectral response is extended up to 675 and 750 nm for PPVCs from PFO−SeBT and PFO−SeBSe, respectively.