The nature of electronic and carrier transport properties of M-(quinolate)₂ complexes (M = Be, Mg, Ca, and Sr): A computational study

A series of metal-quinolate (MQ₂) materials (M = Be²⁺, Mg²⁺, Ca²⁺, and Sr²⁺) were theoretically investigated to understand their electronic and charge transport properties as an application of electron injection layer (EIL) and additive material to electron transporting layer (ETL). It was found that the Be²⁺, Mg²⁺, and Ca²⁺ are tetragonally coordinated to the quinolate ligands whereas the Sr²⁺ is planarly coordinated. The analyses of electronic properties indicated that the stable lowest unoccupied molecular orbital (LUMO) energy level of these materials relative to LiQ is expected to improve the electron injection from cathode and enhance the electron transport in ETL. In perspective of electrochemical reactivity, the chemical hardness values of MQ₂ materials showed that hole and electron carriers can be easily facilitated in order of BeQ₂ < MgQ₂ < CaQ₂ < SrQ₂. In particular, the results of charge carrier transport properties clearly showed that k_e of SrQ₂ is 64% higher than that of LiQ, which is an advantage for both EIL and ETL additive. Comprehensive analyses showed that SrQ2 is superior to LiQ as a replacement in perspective of electron injection and transport properties.