Thermal noise of hot carriers under natural sunlight

•Noise thermometry can measure hot carrier temperature under natural sunlight.•Calibration of equipment is possible using heated metal film resistors.•Graphite and PbTe thin films demonstrate hot carriers under concentrated sunlight.•We see no carrier heating above lattice temperature in bismuth under sunlight.•New candidate materials for hot carrier solar cell absorbers can be assessed. The efficiency of hot carrier solar cells strongly depends on the attainable carrier temperature in the absorber material. Measurements of hot carrier temperature under natural concentrated sunlight were performed at thin films of graphite, bismuth and PbTe, cooled through AlN ceramic substrate. The carrier temperature was assessed by magnitude of thermal noise voltage of charge carriers in the frequency range between 300 and 20,000 Hz according to Nyquist formula. Noise measurement equipment was calibrated in a range of sample impedances by using heated metal film resistors. We found that the carrier temperature in graphite films can exceed 600 °C under up to 15,000× suns and is much higher than the estimated lattice temperature in the film under the given cooling conditions. Secondary electron micrographs illustrate a good mechanical contact between the graphite film and the AlN substrate. We found also distinct heating of charge carriers in PbTe up to about 100 °C. Contrary to that, no measurable carrier heating above the lattice temperature was detected in bismuth films. Johnson noise thermometry demonstrates potential in identification of attractive candidate materials for absorbers in hot carrier solar cells.