Electron injection in "electron-only" devices based on a symmetric metal/silole/metal structure

We report on electron injection from two different metal electrodes into three silolie derivatives, namely 2,5-di-(3-biphenyl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PPSPP), 1,2-bis(1-methyl-2,3,4,5,-tetraphenyisilacyclopentadienyl) ethane (2PSP) and 2,5-bis-(2', 2"-bipyridin-6-yl)-1, 1-dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy), previously employed as emissive and electron transport materials in molecular organic light-emitting diodes (MOLEDs). Silole films were sandwiched between symmetric Mg:Ag or bilayer CsF-Al electrodes. The steady-state current density-voltage characteristics were measured as a function of the silole layer thickness for the two cathodes. The trap-free space-charge-limited current based on time-of-flight measurements compared with the injected electron current for PyPySPyPy indicated that Mg:Ag contacts limit the injected current, while CsF-Al contacts behave as quasi-ohmic contacts. Similar findings were obtained for 2PSP and PPSPP allowing steady-state derived electron mobility parameters to be extracted. Based on space-charge-limited conduction analysis of the measured current-voltage characteristics, PyPySPyPy is found to be a superior electron transporting silole with approximately an order, of magnitude higher electron mobility (2.0 x 10(-4) cm(2)/Vs) compared with those of 2PSP (2.4 x 10(-5) cm(2)/Vs) and PPSPP (5.2 x 10(-5) cm(2)/Vs), which is significantly higher than that of the prototype electron transport material tris (8-hydroxyquinolinolato) aluminum (III) (Alq(3)) (6.5 x 10(-7) cm(2)/Vs) at 0.6 MV/cm.