Photon-Transport Interface Optimization of GAGG:Ce Scintillating Waveguides for Radio-Photovoltaic Cells

Radio-photovoltaic (RPV) cells can provide self-powered and long-lifetime electricity for complex and harsh environments, but their energy conversion efficiency is limited by interfacial losses during radiation-to-light and light-to-electricity conversion. Here, we propose a photon-transport interface engineering strategy for RPV cells driven by radioisotope radiation sources, using a GAGG:Ce scintillating waveguide as the radiation-to-light conversion layer. The original scintillator layer suffers from radioluminescence (RL) absorption at the radiation-source loading surface and low edge light extraction efficiency (27.6%). Because the refractive index of GAGG:Ce is high, only a limited fraction of isotropically generated RL photons can directly escape from a smooth GAGG:Ce/air interface within one pass. An annealed 100 nm Ag reflective film reduces photon leakage from the main surface, improves RL photon transport, and maintains low particle-energy absorption at sufficiently high incident energies, producing a 20.8% enhancement in emission intensity. In parallel, a disordered rough edge surface improves photon extraction by changing local emission angles; Under the present open-edge configuration, the edge with an Sa value of 434.2 nm produces the highest measured output, corresponding to an edge-emission enhancement of at least 77% relative to the smooth edge. Together, these interfacial modifications improve edge emission and electrical output, yielding a maximum electrical power of 0.648 μW at an X-ray tube voltage of 30 kV and current of 1.0 mA. An absolute ECE is not reported because the X-ray power incident on and absorbed by the scintillator was not independently calibrated.