Some implementations include a downhole power source to be positioned in a wellbore to supply electrical power to an electrical device in the wellbore, the downhole power source comprising a radioisotope source to be positioned downhole in the wellbore to emit radiation. One or more semiconductor layers to be positioned downhole in the wellbore may be configured to capture the emitted radiation from the radioisotope source and to may be configured to generate the electrical power based on the captured radiation.