Change in the quantum efficiency due to relative thickness variation of hole transport and emitting layers in a self-assembled device

Abstract

Using (Poly(phenylene vinylene) (PPV)/Poly(styrene sulfonate) (PSS)) and (PPV/Poly(methacrylic acid) (PMA)) self-assembled bilayers exhibiting different hole transport properties, we investigated the competing effect on quantum efficiency of the variation of relative thicknesses of (PPV/PSS) and (PPV/PMA) bilayers in indium tin oxide (ITO)/(PPV/PSS)n/(PPV/PMA)m/Al devices with fixed 20 bilayers (i.e. n+m=20). The (PPV/PSS) bilayer is known to be better as a hole transport layer due to higher leakage current and lower quantum efficiency in comparison with the (PPV/PMA) emitting bilayers. With increasing the number of (PPV/PSS) bilayers from 0 to 20 bilayers, the quantum efficiency of the multilayer self-assembled device was at maximum with 3 bilayers of (PPV/PSS) in contact with the ITO anode and the efficiency montonically decreased upon further increase of the (PPV/PSS) bilayer above 3. In the case of the increase of (PPV/PSS) bilayer thickness by the addition of NaCl salt to the PSS solution during the layer deposition process, the quantum efficiency of the device was significantly improved and also maximized with the 3 (PPV/PSS) bilayers. The significant change on the quantum efficiency by a simple control of relative bilayer numbers is mainly attributed to the competing effects caused by the increase of hole transport (PPV/PSS) bilayer at the expense of reduced emitting (PPV/PMA) bilayer. It was also found that the enhancement of the quantum efficiency by the addition of NaCl salt to the PSS solution is mainly caused by sufficient surface coverage of the (PPV/PSS) hole transport bilayers on the ITO electrode.