The paper discusses the design of efficient single-photon emitters based on defects or impurities in semiconductors, which are crucial for quantum networks. The authors present a model that captures the essential physics of electron-phonon coupling, which governs the behavior of real single-photon emitters. They critically evaluate common approximations and find that nonradiative processes, particularly multiphonon emission, become dominant at low photon energies, leading to reduced efficiency. The model suggests that reducing the phonon frequency is a promising approach to enhance efficiency. The paper also explores the impact of cavity coupling on efficiency and discusses the feasibility of various defect-based emitters, including those in diamond and two-dimensional materials. The authors conclude by suggesting that the "Goldilocks" single-photon emitter, with a transmission energy around 1.5 eV, is likely to be optimal for both telecom wavelengths and high efficiency.The paper discusses the design of efficient single-photon emitters based on defects or impurities in semiconductors, which are crucial for quantum networks. The authors present a model that captures the essential physics of electron-phonon coupling, which governs the behavior of real single-photon emitters. They critically evaluate common approximations and find that nonradiative processes, particularly multiphonon emission, become dominant at low photon energies, leading to reduced efficiency. The model suggests that reducing the phonon frequency is a promising approach to enhance efficiency. The paper also explores the impact of cavity coupling on efficiency and discusses the feasibility of various defect-based emitters, including those in diamond and two-dimensional materials. The authors conclude by suggesting that the "Goldilocks" single-photon emitter, with a transmission energy around 1.5 eV, is likely to be optimal for both telecom wavelengths and high efficiency.