(a) Power conversion efficiency of the two cells (reference in red and highly doped in green) under standard test conditions (dotted lines) and under real operating conditions (solid lines). (b) Spectral absorptivity under normal incidence of the two solar cells (reference cell in red and heavily doped cell in green), which is the ratio of the absorbed to the incident solar power. According to Kirchhoff's law of radiation, the absorptivity is equal to the spectral emissivity, which is the ratio of the radiation emitted to the radiation emitted by a blackbody at a given wavelength. The emission spectrum of the blackbody at 25 ∘C is shown in the light blue. (c) Normalised efficiency (η/η _stc ) as a function of solar cell temperature, from which the temperature coefficient β can be extracted by linear fitting. (d) Temperature-voltage characteristic. (e) Heat sources and sinks of the two cells at three specific operating points: the short-circuit (J _sc ), the maximum power (MPP), and the open-circuit (V _oc ).