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Radioluminescent isotope cells (RLICs) have the advantages of a long lifetime and high stability due to the use of phosphor material with excellent radiation resistance. Current research efforts mainly focus on the improvement of energy conversion efficiency. This study presents a 63Ni-based RLIC with enhanced photon transport interfaces. The ZnS:Cu phosphor layer is spin-coated directly onto the surface of an AlGaInP-based photovoltaic cell (PC) to achieve efficient coupling of photons by optimizing the transmission interface, and a metal film is sputtered onto the ZnS:Cu layer to reflect radioluminescence towards the PC. Theoretical simulations and experiments are used to compare and validate the integration designs of the ZnS:Cu layer and metal reflective films (Ag, Al, and Ni). It is demonstrated that the RLIC based on the spin-coated ZnS:Cu/PC structure with a 100 nm thick Ag film can increase the output power by 52.6%, compared to conventional RLICs based on adhesive ZnS:Cu/BOPP/PC structure. Maximum efficiency of 0.92% is expected under beta radiation of 63Ni. The enhancement of photon transport is attributed to fluorescence backward reflection and refractive index matching at the interfaces.