They are not very efficient. 2000 watts of heat, for 125 watts of electricity. 6.25% efficiency.
This thing better be much more efficient, or you will need a big heat sink to cool your phone.
Thermoelectrics vs betavoltics is apples to Oranges. Thermoelectric is limited by carnot and the Peltier effect. Betavoltics is the direct capture of a high energy beta particles electrostatic and kinetic energy into the crystalline lattice of the diamond matrix. Betavoltic batteries are not new the limiting factors have always been the semiconductors used for particles capture degrade rapidly due to well the radiation flux. That and isotopes that decay with solely beta radiation are rare most have multiple decay modes and nearly all have a gamma ray mode which requires massive shielding with steel, lead , or.concrete. Nickel has a isotope that’s beta decay and a few companies have made betavoltic devices for remote use. Carbon 14 while solid is not always a good choice as it burns to CO2 which would be radioactive CO2 gas. One of the primary radioactive gasses from Chernobyl was C14 as CO2 and CO when the reactors graphite blocks burned in air that one event altered planetary inventory of C14 in the environment to such a degree its still detectable today 30 years later.