Study On “N-Type Doping” Of Organic Semiconductor Alq

Organic light-emitting diodes（OLEDs）have generated a great deal of interest in both academic and industry communities on account of their wide viewing angles,high resolution,light weight,and fast response time,and have been appiled in flat-panel displays,including in the television and cell phone industries,in recent years.It is well known that both intrinsic carrier concentrations and mobilities of organic semiconductors are very low,and the injection and transport abilities of electrons do not match those of holes,leading to an imbalance between the number of holes and electrons in the emitting layers and limiting the improvement of OLED luminous efficiency.Using electrical doping techniques of n-type doping in the electron transport layer can not only increase the electron concentration,but also reduce the electron injection barrier in some cases.In this thesis,two typical electron injection materials,Li F and Liq（8-hydroxyquinolinatolithium）,are used and investigated as n-type dopants in electron transport layer Alq（tris（8-hydroxyquinolinato）aluminum）,and the results are as follows:First,the effect of Li F as an“n-type dopant”on device performance was investigated,and electron-only devices with Alq:Li F doping and Alq non-doping systems as the electron transporting layers,Li F/Al and Al as different cathode structures,were prepared.The experimental results show that the electrical properties of the devices with the doped layers are inferior to those of non-doped devices with Li F/Al composite cathodes and superior to those of non-doped devices with Al only cathodes,implying that Li F’s doping in Alq exhibits dual effects:Li F molecules near Al cathode interfaces in the doped system display as electron injection layer s,which enhance electron injections and incerase the currents;while the Li F molecules in the bulk of Alq block the electron transports owing to their own insulating properties,consequently decrease the currents.In the absorption tests of the Alq:Li F doped system,no additional absorption peaks corresponding to those in the NPB:Mo O₃ doped system caused by charge transfer are observed.In the fluorescence tests,a slight blue-shift of the photoluminescence spectra is observed which is caused by the solid-state solvation effect.In the electroluminescent devices,the doping of Li F shows a similar behavior,the performance of Li F-doped device is inferior to that of non-doped device with Li F/Al composite cathode and superior to that of non-doped device with Al cathode,the maximum current efficiencies of the three devices are 3.93,5.01 and 2.27 cd/A,respectively.According to the experimental results,Li F cannot achieve a significant“n-doping”effect,which is inconsistent with the results reported in the literature so far.Finally,the effect of the typical organic electron injection material Liq as an“n-type dopant”was tried.Electron-only devices and electroluminescent devices with similarly structures mentioned above were prepared.Similar to Li F,the results show that Liq is also unable to achieve significant“n-type doping”effect.The difference compared to Li F is that the blocking effect of Liq on electron transport in the Alq:Liq system is significantly weaker than that of Li F in the Alq:Li F doped system,which can be attributed to the semiconductor properties of Liq itself.In the electroluminescence tests,the performance of the Liq-doped device is inferior to that of non-doped devices with Liq/Al composite cathode,and superior to that of non-doped devices with Al cathode,with maximum current efficiencies of 3.96,4.27and 2.27 cd/A for the three devices,respectively.