Li2CO3 as an n-type dopant on Alq3-based organic light emitting devices

Lithium carbonate (Li(2)CO(3)) is used as an n-type dopant to improve the electro-optical properties of the Alq(3)-based organic light-emitting diodes (OLEDs). Li(2)CO(3)-doped tris(8-hydroxyquinolinato) aluminum (Alq(3)) layers with various concentrations were prepared by the co-evaporating method. When 10% Li(2)CO(3) was doped into an Alq(3) electron-transporting layer, device properties such as the turn-on voltage, the maximum luminance, and the device efficiency were improved, becoming better than LiF and comparable to devices with Cs(2)CO(3) dopants. Li(2)CO(3)-doped Alq(3) layers possess an enhanced electron injection, transporting and hole-blocking ability. The higher mobility of the Li(2)CO(3)-doped samples has been derived using space-charge-limited current (SCLC) measurements. X-ray and ultraviolet photoelectron spectroscopy results show that some electrons transfer from Li(2)CO(3) into Alq(3), which increases the electron concentration in Alq(3) films and moves the Fermi level close to the lowest unoccupied molecular orbital (LUMO) of Alq(3). The charge transfer was further confirmed by absorption and photoluminescence (PL) spectra. Therefore, the electron injection and transport efficiency are both enhanced due to a lower electron injection barrier and a higher electron mobility, which improve the charge carrier balance in OLEDs and lead to better device efficiency. (C) 2011 American Institute of Physics. [doi:10.1063/1.3585767]