This review focuses on the state‐of‐the‐art of solid‐state lighting devices (SSLDs)—that is, organic light‐emitting diodes (OLEDs) and light‐emitting electrochemical cells (LECs)—prepared with transition metal complexes featuring thermally activated delayed fluorescence (TADF) mechanism. First, the TADF mechanism is briefly introduced, as well as the experimental and theoretical methods applied to study TADF in transition metal complexes. Second, the review presents an exhaustive overview of OLED and LEC devices incorporating organometallic TADF emitters. For each type of device, the description of TADF is organized by respective elements focusing on each emission color, that is, blue, green/yellow, orange/red and, if existing, white. Finally, insights and future potential development of organometallic TADF emitters for lighting devices are comprehensively discussed. Overall, this review complements recent ones focused on TADF small molecules applied to the SSLD field.

Recently, the use of a new family of electroluminescent copper(I) complexes—i.e., the archetypal [Cu(IPr)(3‐Medpa)][PF6] complex; IPr: 1,3‐bis‐(2,6‐di‐iso‐propylphenyl)imidazole‐2‐ylidene; 3‐Medpa: 2,2′‐bis‐(3‐methylpyridyl)amine—has led to blue light‐emitting electrochemical cells (LECs) featuring luminances of 20 cd m−2, stabilities of 4 mJ, and efficiencies of 0.17 cd A−1. Herein, this study rationalizes how to enhance these figures‐of‐merit optimizing both device fabrication and design. On one hand, a comprehensive spectroscopic and electrochemical study reveals the degradation of this novel emitter in common solvents used for LEC fabrication, as well as the impact on the photoluminescence features of thin‐films. On the other hand, spectro‐electrochemical and electrochemical impedance spectroscopy assays suggest that the device performance is strongly limited by the irreversible formation of oxidized species that mainly act as carrier trappers and luminance quenchers. Based on all of the aforementioned, device optimization was realized using ionic additives and a hole transporter either as a host–guest or as a multilayered architecture approach to decouple hole/electron injection. The latter significantly enhances the LEC performance, reaching luminances of 160 cd m−2, stabilities of 32.7 mJ, and efficiencies of 1.2 cd A−1. Overall, this work highlights the need of optimizing both device fabrication and design toward highly efficient and stable LECs based on cationic copper(I) complexes.

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