Energy resolved-electrochemical impedance spectroscopy investigation of the role of Al-doped ZnO nanoparticles in electronic structure modification of polymer nanocomposite LEDs

Abstract The work is focused on power efficiency and luminance of polymer light-emitting diodes (PLED) based on polymer matrix and nanoparticulate filler. As polymer matrices poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) were used. Nanofillers, aluminium-doped ZnO nanoparticles, were prepared by microwave-assisted polyol method and characterized by XRD, SEM, and TEM. Obtained nanoparticle colloids were mixed with dissolved polymer. Then, nanocomposite active layers of prepared PLEDs were spin-cast in one processing step. Specific optoelectronic and electric properties of nanocomposite materials and performance of PLED devices were investigated using UV–Vis absorption, photoluminescence, energy-resolved electrochemical impedance spectroscopy (ER-EIS), and I-V and luminance measurements. The addition of Al-doped ZnO nanoparticles improved power efficiency of diodes exhibiting an order of magnitude enhancement in electroluminescence intensity, luminance (from 1 900 to 20 700 cd/m2 for MEH-PPV and from 4 600 to 38 000 cd/m2 for F8BT) and had pronounced effect on opening bias voltage of final devices. In addition, two effects of nanoparticulate doping were revealed. The first, specific one, related with electronic structure of used nanoparticles. The second, non-specific, which indirectly contributes to structural ordering of the polymer matrix. Observed phenomena are related to electronic band structure affected by the addition of nanoparticles as revealed by ER-EIS.