Total energy efficiency would entail extracting electron-volt from the photon. There isn’t any real cap here except how much energy you can stuff into a vaguely

Total energy efficiency would entail extracting electron-volt from the photon. There isn’t any real cap here except how much energy you can stuff into a vaguely stable photon and how much energy your PV cell can withstand. PV cells usually excite at one one energy level; any photon with more energy just gives the electron extra speed, which is wasted. This paper is claiming to be able to extract the excess kinetic energy from the electron through collision by colliding with another electron and knocking it loose.