We fabricated the parabola-shaped subwavelength grating (SWG) nanostructures on indium tin oxide (ITO) films/Si and glass substrates using laser interference lithography, dry etching, and subsequent re-sputtering processes. The efficiency enhancement of an a-Si:H/μc-Si:H tandem thin film solar cell was demonstrated theoretically by applying the experimentally measured data of the fabricated samples to the simulation parameters. Their wetting behaviors and effective electrical properties as well as optical reflectance properties of ITO SWGs, together with theoretical prediction using a rigorous coupled-wave analysis method, were investigated. For the parabola-shaped ITO SWG/ITO film, the solar weighted reflectance (SWR) value was ~10.2% which was much lower than that (i.e., SWR~20%) of the conventional ITO film, maintaining the SWR values less than 19% up to a high incident angle of 70° over a wide wavelength range of 300-1100 nm. Also, the ITO SWG with a superhydrophilic surface property (i.e., water contact angle of 6.2°) exhibited an effective resistivity of 2.07 × 10(-3) Ω-cm. For the a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the parabola-shaped ITO SWG/ITO film as an antireflective electrode layer, the conversion efficiency (η) of 13.7% was theoretically obtained under AM1.5g illumination, indicating an increased efficiency by 1.4% compared to the device with the conventional ITO film (i.e., η = 12.3%).