A high-efficiency solid-state nuclear voltaic converter employs microcrystalline wide bandgap semiconductor materials including diamond, silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga2O3), boron nitride (BN), and aluminum nitride (AlN) to convert ionizing radiation from radioisotopes into electrical power. The microcrystalline grains, having controlled dimensions of approximately 1 to 100 nanometers, exploit quantum mechanical phenomena including quantum confinement, quantum-confined Stark effect (QCSE), Lamb shift, Casimir effect, and Purcell effect to optimize charge generation, separation, and collection. The converter is adaptable to layered planar configurations with alternating radioisotope and semiconductor layers, and spherical configurations with concentric semiconductor shells interspersed with radioisotope material. The spherical geometry forms a resonant cavity enhancing the Purcell effect. The invention provides enhanced energy conversion efficiency, robust radiation tolerance, and design flexibility for applications including space exploration, medical devices, and remote sensors.