Abstract
Lithium carbonate (Li2CO3) is used as an n-type dopant to improve the electro-optical properties of the Alq3-based organic light-emitting diodes (OLEDs). Li2CO3-doped tris(8-hydroxyquinolinato) aluminum (Alq3) layers with various concentrations were prepared by the co-evaporating method. When 10% Li2CO3 was doped into an Alq3 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 Cs2CO3 dopants. Li2CO3-doped Alq3 layers possess an enhanced electron injection, transporting and hole-blocking ability. The higher mobility of the Li2CO3-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 Li2CO3 into Alq3, which increases the electron concentration in Alq3 films and moves the Fermi level close to the lowest unoccupied molecular orbital (LUMO) of Alq3. 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.
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