Molecular weight dependent electroluminescence of silicon polymer near-ultraviolet light-emitting diodes

We investigated the electroluminescence (EL) of single-layer near-ultraviol et (NUV) light-emitting diodes (LEDs) made from poly[bis(p-n-butylphenyl)si lane] (PBPS) with three different molecular weights (MWs). Although the NUV EL spectra of the three LEDs exhibited no noticeable differences, we obser ved a marked MW dependence on such aspects of the operating performance as the EL external quantum efficiency, EL threshold current density and electr ic field, which were improved as the MW of PBPS decreased. The MW dependenc e of the hole transport behavior suggested that the MW decrease promoted po sitive space charge formation in the PBPS layer during LED operation. We at tributed the origin of the MW dependence of the LED performance to this pos itive space charge formation, which played an important role in improving t he electron-hole supply balance from the external electrodes of the LED. (C ) 2000 American Institute of Physics. [S0021- 8979(00)06717-7].