Synthesis and characterization of red light-emitting fluorene-based copolymers and their applications in photovoltaic cells

Abstract

We report here the synthesis via Suzuki polymerization of fluorene-based copolymers containing various energy bandgap dyes (R1, R2, R3, R4, CAR, and, PZB) and their application into electronic devices such as EL devices and photovoltaic cells. The UV-vis. absorption and PL emission spectra of the polymers were measured to investigate their optical properties and EL spectra and IVL characteristics were measured to show how they had the possibility for commercializing. Conventional PLED devices were fabricated in the configuration [ITO/PEDOT-PSS/polymer/Ca/Al] using the polymers as emitting layers. Also, we applied some of polymers into photovoltaic cells because they had low energy bandgap below 2.1 eV. Firstly, four novel fluorene-based light-emitting polymers (PFR1, PFR2, PFR3, and PFR4) containing different comonomers have been designed, synthesized, and characterized. The single layer LED devices fabricated from these polymers emitted light from bluish green to pure red. The results show that the color of the light emitted by the homopolymer, poly(9,9-bis(2``-ethylhexyl)fluorene), could be tuned by incorporating comonomers (R1, R2, R3, and R4) with narrower band-gaps. The energy band-gap of the copolymers with those comonomers varied according to the position of the cyano group in the vinylene unit and the type of aromatic group (thiophene or phenylene) incorporated. Notably, PFR4 showed pure red emission (chromaticity values x = 0.67, y = 0.34) that is almost identical to the standard red (0.66, 0.34) demanded by the national television system committee (NTSC) and also exhibited a low turn-on voltage. And then, using new polymerization methods, Suzuki coupling reaction, four fluorene-based alternating polymers (PFR1-S, PFR2-S, PFR3-S, and PFR4-S) containing different comonomers (R1, R2, R3, and R4) have been designed, synthesized, and characterized. Single layer LED devices fabricated from these polymers emitted bluish green to pure red light. The absorptio...