This study reports the fabrication and electrical characterization of metal-electrolyte-semiconductor (MES) capacitors based on printed ZnO as a semiconducting layer and a cellulose-based eco-friendly electrolyte. The results demonstrate that MES capacitors behave similarly to their solid-state metal-insulator-semiconductor (MIS) counterparts. Capacitance–voltage measurements show well-defined charge saturation and depletion plateaus, and the Mott–Schottky plots exhibit the expected linear behavior, allowing the extraction of the semiconductor free carrier density. Upon UV light exposure with varying irradiance, the flat-band voltage shifts consistently with a light-induced increase in free carrier density, and the capacitance increases due to trap filling. These behaviors are confirmed by measurements using a phototransistor structure. The frequency response of the MES capacitors also reveals low-frequency relaxation peaks in the capacitance spectrum, which align with a continuous distribution of interface state density reaching approximately ∼4.86 × 1013 cm–2·eV–1. These findings confirm that MES capacitors can effectively quantify electronic states at the electrolyte/semiconductor interface. This capability, along with a high capacitance ratio [Clight – Cdark]/Cdark at very low voltages, is particularly valuable for applying MES devices in a wide range of sensing applications, especially in field-effect photodetectors.