Radioisotope Power Systems (RPS) are critical for future space and planetary exploration missions. Small improvements in the RPS performance, weight, size, and/or reliability can have a dramatic effect on the scientific capability of the vehicle and the overall mission costs. Radioisotope thermophotovoltaic (RTPV) energy converters are a particular type of RPS that directly convert the heat produced by a general purpose heat source to electrical power using a specialized photovoltaic (PV) cell. A key element in an RTPV system is the radiative emitter that converts thermal energy to radiative energy that illuminates the PV cell. In this project, Creare and the Massachusetts Institute of Technology (MIT) propose further development of an advanced, 2-D, photonic crystal radiative emitter optimized for RTPV systems that provides high emittance matched to the bandgap of the PV cell with low emittance elsewhere that will provide high system efficiency. In Phase I, we designed, fabricated, and tested prototype emitters. In Phase II, we will improve and scale up the fabrication processes, and fabricate larger, improved test samples, which will be fully characterized for high-temperature emittance and durability. We will also assess the impact of this new emitter on the overall RTPV system design and performance.