We present a quantitative theory of the suppression of the optical linewidth due to charge fluctuation noise in a - diode, recently observed by Anderson et al. [Science 366, 1225 (2019)]. We connect the local electric field with the voltage across the diode, allowing the identification of the defect depth from the experimental threshold voltage. Furthermore, we show that an accurate description of the decoherence of such spin centers requires a complete spin-1 formalism that yields a biexponential decoherence process, and predict how reduced charge fluctuation noise suppresses the spin center’s decoherence rate.