The transparent conducting oxide (TCO) films with low‐refractive‐index (low‐n) are fabricated by the oblique‐angle sputtering method. By using the experimentally measured physical data of the fabricated low‐n TCO films as the simulation parameters, the effect of low‐n TCOs on the performance of a‐Si:H/µc‐Si:H tandem thin film solar cells is investigated using Silvaco ATLAS. The Al‐doped zinc oxide, indium tin oxide (ITO), and Sb‐doped tin oxide films are deposited at the flux incidence angles of θi = 0° (normal sputtering) and θi = 80° from the sputtering target during the sputtering process. The oblique‐angle sputtered films at θi = 80° show the inclined columnar nanostructures compared to those at θi = 0°, modifying the optical properties of the films. This is caused mainly by the increase of porosity within the film which leads to its low‐n characteristics. The a‐Si:H/µc‐Si:H tandem thin film solar cell incorporated with the low‐n ITO film exhibits an improvement in the conversion efficiency of ∼1% under AM1.5g illumination because of its higher transmittance and lower absorption compared to that with the ITO film at θi = 0°, indicating a conversion efficiency of 13.75%.