Thermomechanical model of solar cells

TL;DR

The paper reframes photovoltaic conversion as an open-cycle thermodynamic process using a thermodynamic photon gas, where the chemical potential $\mu$ embodies extractable work and sets the voltage scale. It derives a maximum chemical potential $\mu_{max} = k_B T_o \ln\left( \frac{\Phi_{E_g}(T_S)}{\Phi_{E_g}(T_o)} \right) = q V_{oc}^{max}$ and shows that isochoric thermalization followed by isothermal expansion accounts for losses, including a current-driven term that reproduces the standard diode relation $J = J_\ell - J_o \left( e^{ qV/(k_B T_o)} - 1 \right)$. The framework recovers the Shockley–Queisser detailed balance and reveals an availability correction to $V_{oc}$ of about $26$ mV, implying a modest but meaningful reduction to the SQ limit. Overall, the work offers a physically transparent thermodynamic interpretation of photovoltaic conversion and the origin of efficiency-limiting losses.

Abstract

The paper considers a model for the solar cell as a mechanical open-cycle thermodynamic engine where the chemical potential is produced in an isochoric process corresponding to the thermalization of electron-hole pairs. Expansion of the beam under one-sun illumination and current generation are described as isothermal lost work. More generally, voltage produced in an open cycle process corresponds to availability, leading to a correction to the Shockley-Queisser detailed balance limit.

Figures (3)

• Figure 1: Chemical work produced by photon cooling (thermalization) from temperature $T_S$ to temperature $T_o$, compared with mechanical work along the isotherm BC.
• Figure 2: Isothermal losses pictures in the $p-v$ plane.
• Figure 3: Open and closed cycle conversion, illustrated on the example of a steam turbine.