Title: A unified approach to the thermodynamics of a photovoltaic system

Abstract: Thermodynamics is accepted as a universal truth, encompassing all macroscopic objects. Therefore, it is surprising to find that, within our current understanding, the photovoltaic effect has so far eluded the first and second laws of thermodynamics. The inconsistency emerges from the fact that photovoltaics obey a distinct law of detailed balance1. Since radiative processes depend on only two independent variables that are the chemical potential and the temperature, the detailed balance, and the two laws of thermodynamics cannot be mutually solved. In this work, we resolve this incompatibility by proposing that the system is controlled by yet a third independent variable, which is related to the emissivity. This unification not only advances our fundamental understanding of light-matter interactions but, perhaps more importantly, allows us to assess the limiting factors of advanced photovoltaic concepts designed for elevated temperatures. These include thermophotovoltaics2, thermoradiative and thermophotonic solar power conversion, and radiative cooling, which are instrumental in our ability to develop advanced renewable energy technologies.