Synthesis And Characterization Of Multihydroxylated Small-molecule Hole-transport Materials

Organic light-emitting diodes （OLEDs） based on solution processing have beenreceiving increasing attention due to the possibility to afford low-cost, large-area and flexibledevices. Nevertheless, solution processing presents demanding challenges such as interlayermixing of adjacent functional layers. In this context, we report the synthesis of a new class ofmultihydroxylated small-molecule hole-transport compounds in an attempt to obtainalcohol-processable hole-transport materials that are compatible to the emitting layer, whichgenerally consists of hydrophobic material（s）. The molecular structure propertiesrelationships such as on solubility in alcohol and weakly-polar solvents and hole-transportmobility are discussed. It is noteworthy that a compound with high solubility in alcoholsolvents whilst insoluble in weakly polar solvents was used as a hole-transport layer inbottom-anode OLEDs that consisted of a solution-processed thin polymer emitting layer,leading to promising device performance. The details as follows:1. Three multihydroxylated hole-transport molecules,2OH-1,2OH-2and6OH, havebeen synthesized.2OH-1is soluble in both alcohol and weakly polar solvents such as tolueneand xylenes, with a solubility of at least20mg/mL. Further attaching four triphenylaminogroups to the endgroups of2OH-1,2OH-2is obtained. However, the resulting solubility inalcohol and weakly polar solvents is severely reduced, less than ca.1mg/mL. The synthesisof6OH fails to improve solubility, especially in alcohol solvents. 2. On the basis of the above work,10OH has been designed and synthesized with10hydroxyl groups in a molecule.10OH shows a high solubility in polar solvents whilst barelysoluble in weakly polar solvents. For instance, more than50mg of10OH facilely dissolves in1mL methanol, ethanol or2-propanol at room temperature, while0.1mg of the compound isinsoluble in1mL toluene, p-xylene, and chlorobenzene. Furthermore, this multihydroxylatedcompound is capable of affording uniform amorphous, smooth and transparent films,spin-cast from2-propanol with a high Tgof190°C. The resistance to weakly polar solventsuch as toluene of the resulting film is confirmed by AFM and UV-Vis absorption experiments.Hole mobility of10OH is4.6×10-6cm2V-1s^-1measured by fabricating single carrierhole-only device ITO/MoO₃（10nm）/10OH （130nm）/MoO₃（10nm）/Al （100nm）.10OH isutilized as a hole-transport layer in OLED [ITO/10OH （30nm）/P-PPV （80nm）/CsF （1.5nm）/Al （200nm）] exhibited a maximal luminous efficiency （LEmax） as high as19.8cd A-1anda low turn-on voltage of2.6V.3. Subsequently, in order to reduce visible light absorption of the above10OHhole-transport compound,4OH and5OH have been synthesized. While4OH has a highsolubility in alcohol solvents,5OH is almost insoluble. Hole mobility of4OH, revealed by SCLC at low voltages, is1.5×10^-7cm²V-1s^-1.