A method of manufacturing an organic-inorganic composite thin film may include: forming a thin film from a paste that includes an inorganic powder and an organic compound binder by using a screen printing process; and/or performing a pressing process and a heating process with respect to the thin film. The heating process may be performed at a glass transition temperature of the organic compound binder or in a temperature range higher than the glass transition temperature of the organic compound binder. An X-ray detector configured to detect X-rays irradiated from an outside of the X-ray detector may include: a photoconductive material layer in which electron-hole pairs are formed due to absorption of the X-rays. The photoconductive material layer may be formed of an organic-inorganic composite thin film that includes an inorganic powder and an organic compound binder.