Dual‐Phosphorescent Heteroleptic Silver(I) Complex in Long‐Lasting Red Light‐Emitting Electrochemical Cells

Abstract The design of red‐emitting silver(I) complexes and their implementation in thin‐film lighting are still challenging as (i) their high ligand‐field splitting energy leads to high‐energy emissions with a controversial mechanism (thermally activated delayed fluorescence vs fluorescence/phosphorescence), and (ii) their low electrochemical stability leads to the formation of silver nanoclusters, limiting device stability to a few seconds. Herein, a thoughtful complex design [Ag(xantphos)(deebq)]PF₆ combining a large‐bite angle diphosphine ligand (xantphos), a rigid, sterically hindered, π‐extended biquinolin (deebq) is reported. In contrast to prior‐art, this complex possesses (i) efficient red‐emission (λ_em = 660 nm; photoluminescence quantum yield of 42%) assigned to a thermally equilibrated dual‐phosphorescent emission based on spectroscopic/theoretical studies and (ii) stable reduction behavior without forming silver nanoclusters. This results in the first red light‐emitting electrochemical cells featuring (i) improved stability of two orders of magnitude compared to prior‐art (from seconds to hours) at irradiances of 20 µW cm⁻², and (ii) a new degradation mechanism exclusively related to p‐doping as confirmed by electrochemical impedance spectroscopy analysis. Indeed, a multi‐layered architecture to decouple hole injection/transport and exciton formation enables a further 2‐fold enhanced irradiance/stability. Overall, this work illustrates that deciphering the rules for silver(I) complex design for lighting is tricky, but worthwhile.