Electroluminescence of Conjugated Rigid-rod Polymer Tuned by Emission Layer Thickness

\textit{Bi}layered light emitting diodes were fabricated with fully conjugated rigid-rod polymer poly-$p$-phenylenebenzobisoxazole (PBO) on top of hole conducting poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) using indium-tin-oxide on glass as the substrate and also the electron injecting cathode. The hole injecting anode was Al vacuum evaporated onto the PBO layer. Electroluminescence emission was obtained showing a \textit{blue} shift in emission $\lambda _{max.}$ together with a lowered threshold voltage for the \textit{bi}layered polymer diodes having a smaller PBO layer thickness. When the PBO thickness changing from 90 nm to 27 nm with a constant PEDOT:PSS layer thickness of 54 nm, the $\Delta \lambda _{max.}$ and the $\Delta $V$_{threshold}$ were 70 nm and 3.5 V, respectively.