<p>We proposed a superconducting diode as a novel superconducting device for the power receiver of wireless power transmission in the space solar power system (SSPS), which is expected to be a renewable energy source in the future. In order to realize efficient power reception and rectification for wireless power transmission, we investigated the deposition method of MgB₂ film as a suitable material for superconducting diodes, the surface planarization process to improve rectification efficiency, and an evaluation method for rectification in superconducting diodes at high frequency in the microwave range. We present the basic concept of the superconducting diodes after explaining the theory on surface barrier to the flux quantum in superconductors. Earlier studies of various superconducting diodes are also reviewed.</p><p>As results of this project, MgB₂ films were prepared by the pulsed laser deposition using a Nd:YAG laser with in-situ post annealing process. The film shows T_c^onset = 33.6 K and J_c = 0.9×10⁶ A/cm² at 20 K and under self-field. The surface planarization of REBa₂Cu₃O_y films increased the rectification rate up to 145 % compared to the untreated sample due to the enhancement of the surface barrier. Wet polishing with loose abrasives is promising for the surface planarization of the films. We established a test facility of superconducting diode to explore for a wide range of frequencies from 60 Hz to 80 MHz and confirmed the rectification at frequencies up to 80 MHz in the REBa₂Cu₃O_y films. In the future, it is expected that we will fabricate superconducting diodes using the MgB₂ films, develop a polishing technique for mass production, and develop a rectification test method at a microwave frequency of 5.8 GHz. Furthermore, it is desirable to fabricate a rectenna with a superconducting diode built into the antenna, and to develop a device with the functions of high-efficiency power reception, rectification, and storage of microwaves for wireless power transmission including the SSPS.</p>