Preparation Of White Organic Light Emitting Diodes Based On Localized Surface Plasmon Resonance Of Au,Ag Nanoparticles

The study investigates the localized surface plasmon resonance(LSPR)of Au NPs,Ag NPs and their mixed NPs on the electroluminescence(EL)of white organic light emitting diodes(WOLED).Firstly,the effect of different concentration and position of Au and Ag NPs on the EL performance has been discussed.After studying the OLEDs with Au and Ag NPs which are inserted between indium tin oxide(ITO)and PEDOT:PSS hole injection layer(HIL)or mixed with PEDOT:PSS as HIL,we find out that the OLEDs with Au and Ag NPs inserted between ITO and PEDOT:PSS is much better in EL performance,and the most proper concentration of NPs is determined.The effect of LSPR of Au NPs,Ag NPs and Au,Ag mixed NPs in different ratio(4:1,3:1,1:1,1:3,1:4)on the EL performance of blue,green and white OLEDs has been discussed.The results indicate that either the neat Au,Ag NPs or the mixed NPs have positive effect on the EL performance of OLEDs.Especially the OLEDs with mixed NPs in ratio of 3:1 are the most impressive,namely,the external quatumn efficiency(EQE)and power efficiency(PE)of B3-1(blue OLED with Au and Ag mixed NPs in volume ratio of Au NPs solution to Ag NPs solution of 3:1,the following G3-1 and W3-1 represent for green and white OLEDs with NPs of the same ratio)are 21.88%and 22.48 lm/W respectively,which are improved by 33.41%and 45.31%compared to those of B0(blue OLED without NPs,the following G0 and W0 represent green and white OLEDs without NPs,respectively).For G3-1 and W3-1,the EQE of them are 12.79%and 15.19%,which are improved by 32.40%and 29.06%compared to those of G0 and W0,and the PE of them are 45.21 lm/W and 15.03 lm/W,which are improved by 64.52%and 23.00%compared to those of G0 and W0,respectively.The improvement of EL performance can be attributed to the more obvious LSPR of Au,Ag mixed NPs which have much more overlap area in widened absorption spectra and emission spectra of emission layers(EML)than those of neat Au,Ag NPs.Though the distance of metal nanoparticles(MNPs)and emission layer will be shortened by the Ag NPs in larger size,which will affect the LSPR negatively and lead to the deficit of OLED's EL performance by quenching of exciton as well.Thus the mechanism of improvement of OLED with Au,Ag mixed NPs can be described as a competition of the enhancement of LSPR by mixed NPs which has widened absorption spectra and the quenching of exciton,so that ratio of Au NPs to Ag NPs should be adjusted,and the most appropriate ratio in this study is 3:1.Moreover,the photophysical properties such as exciton's decaying time of different white emission layers has been investigated using transient photoluminescence(PL)spectra.The results demonstrate the mechanism of the LSPR improving OLED's EL properties,which is the decaying time of exciton has been decreased extremely because the radiative transition rate is raised by Au,Ag mixed NPs.The energy transfer rate KET in white emission layers shows that the KET of blue EML to red and green EML and green EML to red EML has been increased significantly.Consequently red,green and blue emission in WOLED is enhanced by Au,Ag mixed NPs in appropriate ratio,which causes the general improvement of EL performance.