Exploratory synthesis and luminescence study of the first pi-conjugated tin-based alternating copolymers for blue light-emitting diodes at the very low operating voltage

Abstract

Tin-based alternating copolymers with a uniform pi-conjugated segment were synthesized using the Heck reaction between distyrylstannane monomer and various difunctionalized monomers. The UV-vis absorption maximum peaks of the resulting polymers in chloroform solution and in thin films appeared in the wavelength range of 347-394 nm. Upon photoexcitation with light of wavelength 350 nm, their photoluminescence spectra exhibited an emissive maximum peak around 470-502 nm, corresponding to blue light emission. Multilayered light-emitting diodes with ITO/PEDOT (50 nm)/polymer (80 nm)/Ca (50 nm)/Al (200 nm) composition were fabricated. These LED devices exhibited an emissive maximum peak in the range of 464-472 nm. All of the materials exhibited a very low turn-on voltage of less than 4 V. From cyclic voltammetiic studies and optical data, the LUMO level is estimated to be 3.20 eV for SnPhFPV and 2.90 eV for SnPhPVK, and the ionization potentials (HOMO level) were estimated to be 5.90 eV for SnPhFPV and 5.60 eV for SnPhPVK. This lowered LUMO level, in comparison to those of poly(p-phenylene) derivatives (such as PPP, ladderlike PPP, and PAF), reduces the energy barrier to electron injection, resulting in a lowering of the operating voltage in the polymeric LED. Compared to devices based on the tin-based polymers alone, devices based on blends between the tin-based polymers and PVK showed improved efficiency of power and luminescence by at least 2-4 times and 3-6 times, respectively. Also for these polymer:PVK blends, the electroluminescence efficiency enhances up to the range of 0.1-0.3 lm/W, and the purity of the emitted blue color improves. These results may be attributed to the intramolecular confinement by diluting the EL polymers with PVK. One of the devices based on a blend system between a tin-based polymer and PVK has a brightness of 2047 cd/m(2) at 11 V with a power efficiency of 0.3 cd/A.