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

We investigated the molecular weight dependence of the conformational phase transition behavior of thin films of poly[bis(p-n-butylphenyl)silane] (PBP S), which is a near-ultraviolet (NUV) electroluminescence (EL) material for polysilane-based polymer light-emitting diodes (LEDs). A low molecular wei ght PBPS with efficient NUV EL exhibited a phase transition from a disorder ed to an ordered backbone conformation when we increased the temperature. B y contrast, the opposite transition behavior was observed for a high molecu lar 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 t he temperature, whereas that of the high molecular weight polymer deteriora ted. The time dependence of the EL investigation suggests that, although th e EL intensity continued to increase for the first few hours, insufficient Joule heat was generated to induce a phase transition in the entire PBPS la yer of an LED made of the low molecular weight PBPS during continuous opera tion . We conclude that the conformational phase transition was not a key f actor 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 eff iciency of the external electrodes. (C) 2000 American Institute of Physics. [S0021-8979(00)01019-7].