1. Thermodynamic limits of photovoltaic conversion
Experimental solar cells have reached efficiencies over 40%, but still higher conversion efficiencies above 90% are physically possible, and allowed by the 1st and 2nd laws of thermodynamics. As solar cells approach theoretical efficiency limits, the reciprocal processes of radiative recombination and electron-hole pair photogeneration create an intense photon gas partially confined in the semiconductor. When photons emitted by electron-hole recombination are recycled to form new electron-hole pairs, the excess charge carrier concentration climbs dramatically, increasing voltage above that expected from conventional semiconductor physics. The escape of photons from the semiconductor ultimately results in the minimum energy loss of a solar cell operating at its theoretical efficiency limit – solar cell operation at the photonic limit.
We are exploring the physics of single-junction solar cells as they approach this photonic limit to learn how to best approach the theoretical efficiency of PV cells. For III-V cells we look at maximizing back surface reflectance and other light management methods, and are interested in how these concepts affect less ideal materials, as when significant Shockley-Read-Hall recombination is present in polycrystalline CdTe, CIGS, and perovskite solar cells, in indirect gap materials like silicon, and at high injection levels. At a fundamental level, we are interested in how the description of thermodynamics can be generalized to include hot-carrier populations that are not in equilibrium with themselves, and thermodynamics of nanoscale and picoscale systems that may be governed by particle entanglement and other quantum-mechanical effects, and by systems with small numbers of particles. We are also interested in how PV cells can be integrated in devices and systems to exceed conventional efficiency limits, as in multijunction cells – the only solar cell technology yet demonstrated to exceed single-junction Shockley-Queisser efficiency limits – and hybrid thermal-PV systems.