An Amorphous Spirobifluorene-Phosphine-Oxide Compound as the Balanced n-Type Host in Bright and Efficient Light-Emitting Electrochemical Cells with Improved Stability

Abstract

A rational host–guest concept design for the attainment of high efficiency at strong luminance from light-emitting electrochemical cells (LECs) by suppression of exciton-polaron quenching [Tang et al., Nature Communications 2017, 8, 1190] has been reported. However, a practical drawback with the presented host–guest LEC devices was that the operational stability is insufficient for many applications. Here, a systematic study is performed, revealing that a major culprit for the limited operational stability is that the employed n-type host, 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7), has a strong propensity for crystallization and that this crystallization results in a detrimental phase separation of the constituents in the active material during device operation. The authors, therefore, identify an alternative class of concept-functional n-type hosts in the form of spirobifluorene-phosphine-oxide compounds, and report that the replacement of OXD-7 with amorphous 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene results in a much improved operational lifetime of 700 h at >100 cd m−2 during constant-bias driving at an essentially retained high current efficacy of 37.9 cd A−1 and a strong luminance of 2940 cd m−2.