Bright, stable, and efficient red light-emitting electrochemical cells using contorted nanographenes†
This work rationalizes, for the first time, the electroluminescent behavior of a representative red-emitting contorted nanographene – i.e., hexabenzoovalene derivative – in small molecule light-emitting electrochemical cells (SM-LECs). This new emitter provides devices with irradiances of ca. 220 μW cm−2 (242 cd m−2), external quantum efficiencies (EQE) of 0.78% (<25% loss of the maximum theoretical EQE), and stabilities over 200 h. Upon optimizing the device architecture, the stability increased up to 3600 h (measured) and 13 000 h (extrapolated) at a high brightness of ca. 30 μW cm−2 (34 cd m−2). This represents a record stability at a high brightness level compared to the state-of-the-art SM-LECs (1000 h at 0.3 μW cm−2). In addition, we rationalized one of the very rare LEC examples in which the changes of the electroluminescence band shape relates to the dependence of the relative intensity of the vibrational peaks with electric field, as corroborated by dynamic electrochemical impedance spectroscopy assays. Nevertheless, this exclusive electroluminescence behavior does not affect the device color, realizing one of the most stable, bright, and efficient red-emitting SM-LECs up to date.