In this paper, we conducted DFT and TDDFT calculations on three double heteroleptic Cu(I) complexes to understand how different substituents on N^N ligands influence the phosphorescence quantum yield (PLQY). Both radiative and nonradiative decay processes were thoroughly investigated. Factors that determine the rate of radiative process (k_r) were considered, including the lowest triplet excited state E(T₁), the transition dipole moment M_Sm,j of the S_m → S₀ transition, the spin-coupled matrix element SOC, and the singlet–triplet splitting energies ΔE(S_m–T₁). The results indicate that E(T₁), M_Sm,j and SOC increase and ΔE(S_m–T₁) decreases upon introducing –Ph and –CH₂– groups on the N^N ligands. The net results lead to a gradual increase of k_r in the three Cu(I) complexes, from 1 (0.48 × 10⁴ s⁻¹) to 2 (0.64 × 10⁴ s⁻¹) and then to 3 (1.61 × 10⁴ s⁻¹). The rate of nonradiative decay process (k_nr) was computed by a convolution method. We explored how k_nr is determined by SOC between T₁ and S₀ states (〈T₁|SOC|S₀〉²), effective energy gap ΔE′ and the Huang–Rhys factor (S). We found that 〈T₁|SOC|S₀〉² and ΔE′ contribute significantly to k_nr, but S does not determine the order of k_nr. k_nr gradually decreases from complex 1 (2.51 × 10⁶ s⁻¹) to 2 (0.32 × 10⁶ s⁻¹) and then to 3 (0.14 × 10⁶ s⁻¹) after introducing –Ph and –CH₂– groups on the N^N ligands. The computed PLQYs for the three complexes are 1: 0.0019, 2: 0.0198, and 3: 0.1011. These are quantitatively consistent with the experimental observation (1: 0.0028, 2: 0.0061, and 3: 0.1000).