Various embodiments of a power source and method of forming such power source are disclosed. The power source can include a substrate and a cavity disposed in a first major surface of the substrate. The power source can also include radioactive material disposed within the cavity, where the radioactive material emits radiation particles; and particle converting material disposed within the cavity, where the particle converting material converts one or more radiation particles emitted by the radioactive material into light. The power source further includes a sealing layer disposed such that the particle converting material and the radioactive material are hermetically sealed within the cavity, and a photovoltaic device disposed adjacent the substrate. The photovoltaic device can convert at least a portion of the light emitted by the particle converting material that is incident upon an input surface of the photovoltaic device into electrical energy.