| dc.description.abstract | Previous research in our lab found that mixed monolayer formed from the mixture of dipolar CF3Bn2CH2SH and F5BnCH2SH molecules on Au resulted in higher binding density than that formed from single component solution and the work function obtained from mixed layer modification was higher than that from single compound monolayer. In this thesis the mixed monolayers of related phosphonic acids with different molecular length or aromatic rings of different electrostatic characters on aluminium and ITO surface were studied by reflection absorption IR, XPS, electrochemistry, and work function, in the hope to achieve higher work function by mixed monolayer modification. The results showed that the work function was not raised by mixed monolayers of CF3Bn2PA and F5BnPA. Possibly because the phosphonic acid group is bulkier so that the inetermolecular interaction is not sensitive to the charge character. Mixed monolayers of more polar 4FPyPA and CF3Bn2PA can indeed increase the binding density but not the work function.
Monolayer of more polar NO2BnPA molecule gave similar work function as F5BnPA and 4FPyPA, probably due to the tilt angle and lower surface density. When mixed SAM was formed with NO2BnPA, the work function was slightly improved in certain ratio, however, the surface density was not. Based on the thickness measurement, it is proposed that NO2BnPA monolayer becomes more perpendicular due to the attraction of electron-poor aromatic ring.
Standard organic light-emitting diodes and hole-only devices with mixed monolayer-modified ITO as the anode were prepared. Correlations of current density with work function and luminous efficiency were observed. The devices with higher ratio NO2BnPA gave higher luminous efficiency with lower work function and current density. It is suggested that this device structure has excessive hole charges over the electron charges. With increasingly higher work function, more holes are injected, and more holes are wasted without recombination, resulting decreasing efficiency. However, the device with mixed SAMs of CF3Bn2PA and F5BnPA, a reversed trend was observed. Higher current density led to higher luminous efficiency. The cause is not clear. | en_US |