Molecular weight dependence of the conformational phase transition and electroluminescence of diarylpolysilane diodes

Abstract

We investigated the molecular weight dependence of the conformational phase transition behavior of thin films of poly[bis(p-n-butylphenyl)silane] (PBPS), which is a near-ultraviolet (NUV) electroluminescence (EL) material for polysilane-based polymer light-emitting diodes (LEDs). A low molecular weight PBPS with efficient NUV EL exhibited a phase transition from a disordered to an ordered backbone conformation when we increased the temperature. By contrast, the opposite transition behavior was observed for a high molecular weight PBPS. The photoluminescence (PL) quantum yield of the PBPS films changed in association with the thermal phase transition behavior; the PL quantum yield of the low molecular weight PBPS was improved by increasing the temperature, whereas that of the high molecular weight polymer deteriorated. The time dependence of the EL investigation suggests that, although the EL intensity continued to increase for the first few hours, insufficient Joule heat was generated to induce a phase transition in the entire PBPS layer of an LED made of the low molecular weight PBPS during continuous operation . We conclude that the conformational phase transition was not a key factor as regards efficient NUV EL generation, but may have occurred at the molecules in contact with the external electrodes of the LED, and may have been partially responsible for the improvement in the carrier injection efficiency of the external electrodes.