With emerging thin-film PIN-based optoelectronics devices, a significant research thrust is focused on the passivation of trap states for performance enhancement. Among various methods, the capacitance frequency technique (CFT) is widely employed to quantify the trap-state parameters; however, the trapped charge-induced electrostatic effect on the same is not yet established for such devices. Herein, we present a theoretical methodology to incorporate such effects in the CF characteristics of well-established, but not limited to, carrier-selective perovskite-based PIN devices. We show that the electrostatic effect of trapped charges leads to non-linear energy bands in the perovskite layer, which results in the underestimation of the trap density from existing CFT models. Consequently, a parabolic band approximation with effective length (PBAEL) model is developed to accurately predict the trap density for shallow or deep states from CFT analysis. In addition, we demonstrate that the attempt-to-escape frequency, which dictates the trapping dynamics, can be well extracted by eliminating the electrostatic effect at a reduced perovskite thickness. We believe that our work provides a unified theoretical platform for CFT to extract trap-state parameters for a broad class of organic, inorganic, and hybrid semiconductor-based thin-film devices for energy-conversion applications such as solar cells, LEDs, artificial photosynthesis, etc.