Optical properties of a series of monosilylene-oligothienylene copolymers and the application to light-emitting diodes

UV-vis absorption, photoluminescence (PL), and electroluminescence (EL) properties of a series of monosilylene-oligothienylene copolymer films have been studied. Both the absorption and PL spectra of these films exhibited a red shift with increasing the number of thienylene rings in the repeat unit of a polymer, indicating that the optical properties of these polymers are controlled mainly by the aromatic units, i.e. oligothienylene in the main chain. Polymer light-emitting diodes with a simple structure of Al/electrolyte-doped copolymer/ITO were fabricated using these copolymers as an emitting layer. Clear and stable EL spectra were observed only when the EL devices were in the reverse bias, i.e. Al as anode and ITO as cathode. The EL spectra resembled the corresponding PL spectra and also exhibited a red shift with the increase in the number of thienylene rings. It was also found that the turn-on voltage of the EL devices decreased with the increase in the number of thienylene rings, which is due, most likely, to the increase of charge carrier mobility and decrease of the bandgap as the pi-conjugation length increases.