The development of organic light emitting diodes (OLEDs) and the use of emitting molecules have strongly stimulated scientific research of emitting compounds. In particular, for OLEDs it is required to harvest all singlet and triplet excitons that are generated in the emission layer. This can be achieved using the so‐called triplet harvesting mechanism. However, the materials to be applied are based on high‐cost rare metals and therefore, it has been proposed already more than one decade ago by our group to use the effect of thermally activated delayed fluorescence (TADF) to harvest all generated excitons in the lowest excited singlet state S₁. In this situation, the resulting emission is an S₁→S₀fluorescence, though a delayed one. Hence, this mechanism represents the singlet harvesting mechanism. Using this effect, high‐cost and strong SOC‐carrying rare metals are not required. This mechanism can very effectively be realized by use of Cu^Ior Ag^Icomplexes and even by purely organic molecules. In this investigation, we focus on photoluminescence properties and on crucial requirements for designing Cu^Iand Ag^Imaterials that exhibit short TADF decay times at high emission quantum yields. The decay times should be as short as possible to minimize non‐radiative quenching and, in particular, chemical reactions that frequently occur in the excited state. Thus, a short TADF decay time can strongly increase the material's long‐term stability. Here, we study crucial parameters and analyze their impact on the TADF decay time. For example, the energy separation ΔE(S₁–T₁) between the lowest excited singlet state S₁and the triplet state T₁should be small. Accordingly, we present detailed photophysical properties of two case‐study materials designed to exhibit a large ΔE(S₁–T₁) value of 1000 cm⁻¹(120 meV) and, for comparison, a small one of 370 cm⁻¹(46 meV). From these studies—extended by investigations of many other Cu^ITADF compounds—we can conclude that just small ΔE(S₁–T₁) is not a sufficient requirement for short TADF decay times. High allowedness of the transition from the emitting S₁state to the electronic ground state S₀, expressed by the radiative ratek^r(S₁→S₀) or the oscillator strengthf(S₁→S₀), is also very important. However, mostly small ΔE(S₁–T₁) is related to smallk^r(S₁→S₀). This relation results from an experimental investigation of a large number of Cu^Icomplexes and basic quantum mechanical considerations. As a consequence, a reduction ofτ(TADF) to below a few μs might be problematic. However, new materials can be designed for which this disadvantage is not prevailing. A new TADF compound, Ag(dbp)(P₂‐nCB) (with dbp=2,9‐di‐n‐butyl‐1,10‐phenanthroline and P₂‐nCB=bis‐(diphenylphosphine)‐nido‐carborane) seems to represent such an example. Accordingly, this material shows TADF record properties, such as short TADF decay time at high emission quantum yield. These properties are based (i) on geometry optimizations of the Ag^Icomplex for a fast radiative S₁→S₀rate and (ii) on restricting the extent of geometry reorganizations after excitation for reducing non‐radiative relaxation and emission quenching. Indeed, we could design a TADF material with breakthrough properties showingτ(TADF)=1.4 μs at 100 % emission quantum yield.

A design strategy for the development of Ag(I)-based materials for thermally activated delayed fluorescence (TADF) is presented. Although Ag(I) complexes usually do not show TADF, the designed material, Ag(dbp)(P₂-nCB) [dbp = 2,9-di-n-butyl-1,10-phenanthroline, and P₂-nCB = nido-carborane-bis(diphenylphosphine)], shows a TADF efficiency breakthrough exhibiting an emission decay time of τ(TADF) = 1.4 μs at a quantum yield of Φ_PL = 100%. This is a consequence of three optimized parameters. (i) The strongly electron-donating negatively charged P₂-nCB ligand destabilizes the 4d orbitals and leads to low-lying charge (CT) states of MLL′CT character, with L and L′ being the two different ligands, thus giving a small energy separation between the lowest singlet S₁ and triplet T₁ state of ΔE(S₁–T₁) = 650 cm^–1 (80 meV). (ii) The allowedness of the S₁ → S₀ transition is more than 1 order of magnitude higher than those found for other TADF metal complexes, as shown experimentally and by time-dependent density functional theory calculations. Both parameters favor a short TADF decay time. (iii) The high quantum efficiency is dominantly related to the rigid molecular structure of Ag(dbp)(P₂-nCB), resulting from the design strategy of introducing n-butyl substitutions at positions 2 and 9 of phenanthroline that sterically interact with the phenyl groups of the P₂-nCB ligand. In particular, the shortest TADF decay time of τ(TADF) = 1.4 μs at a Φ_PL value of 100%, reported so far, suggests the use of this outstanding material for organic light-emitting diodes (OLEDs). Importantly, the emission of Ag(dbp)(P₂-nCB) is not subject to concentration quenching. Therefore, it may be applied even as a 100% emission layer.