Here we demonstrate a simple and effective method of fabricating polymeric scattering substrate for flexible organic light-emitting diodes (OLEDs) that require no costly patterning, etching, or molding processes, aspects that are desirable for the commercialization of large-scale lighting panels. Systematic study of the influences of relative index of refraction, particle size, and doping concentration on transmittance and haze of transparent colorless polyimide (cPI) films was carried out. It was found that the reduction of transmittance and haze of the doped films decreases along with the decrease of the difference of refractive index between the particles and polymer matrix, and it could be compensated by the increase of particle size or doping concentration.