Radiovoltaic devices can directly convert high-energy β-electron or a-particle emissions from radioisotopes to electrical power through the generation and separation of electron hole pairs in semiconductors. By carefully matching length scales involved in the physics of absorbing high-energy electrons in semiconductors and the diffusion lengths of electrons and holes, it is possible to increase the power conversion efficiency. This paper demonstrates a distinct increase in conversion efficiency in p-i-n diodes compared to pn diodes.