The four new Ag(I) complexes Ag(phen)(P-2-nCB) (1), Ag(idmp)(P-2-nCB) (2), Ag(dmp)(P-2-nCB) (3), and Ag(dbp)(P-2-nCB) (4) with P-2-nCB = bis(diphenylphosphine)-nido-carborane, phen = 1,10-phenanthroline, idmp = 4,7-dimethyl-1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, and dbp = 2,9-di-n-butyl-1,10-phenanthroline were designed to demonstrate how to develop Ag(I) complexes that exhibit highly efficient thermally activated delayed fluorescence (TADF). The substituents on the 1,10-phenanthroline ligand affect the photophysical properties strongly (i) electronically via influencing the radiative rate of the S-1 -> S-0 transition and (ii) structurally by rigidifying the molecular geometry with respect to geometry changes occurring in the lowest excited S-1 and T-1 states. The oscillator strength of the S-1 -> S-0 transition f(S-1 <-> S-0)- an important parameter for the TADF efficiency being proportional to the radiative rate-can be increased from f(S-1 <-> S-0) = 0.0258 for Ag(phen)(P-2-nCB) (1) to f(S-1 <-> S-0) = 0.0536 for Ag(dbp)(P-2-nCB) (4), as calculated for the T-1 state optimized geometries. This parameter governs the radiative TADF decay time (tau(r)) at ambient temperature, found to be tau(r) = 5.6 mu s for Ag(phen)(P-2-nCB) (1) but only tau(r) = 1.4 mu s for Ag(dbp)(P-2-nCB) (4)-a record TADF value. In parallel, the photoluminescence quantum yield (Phi(PL)) measured for powder samples at ambient temperature is boosted up from Phi(PL), = 36% for Ag(phen)(P-2-nCB) (1) to Phi(PL) = 100% for Ag(dbp)(P-2-nCB) (4). This is a consequence of a cooperative effect of both decreasing the nonradiative decay rate and increasing the radiative decay rate in the series from Ag(phen)(P-2-nCB) (1), Ag(idmp)(P-2-nCB) (2), and Ag(dmp)(P-2-nCB) (3) to Ag(dbp)(P-2-nCB) (4). Another parameter important for the TADF behavior is the activation energy of the S-1 state from the state T-1, Delta E(S-1-T-1). Experimentally it is determined for the complexes Ag(dmp)(P-2-nCB) (3) and Ag(dbp)(P-2-nCB) (4) to be of moderate size of Delta E(S-1-T-1) = 650 cm(-1).