Hot carrier solar cells were first proposed many decades ago. Over the intervening years, there has been a continuing quest to create these cells, which hold promise to shatter the Shockley–Queisser efficiency limit on single-junction solar cells. One approach considered is to use satellite valleys of the conduction band as metastable states for storing hot electrons until they can be extracted. Experimental efforts, however, have shown the presence of a barrier between the two materials, likely at the heterostructure interface between the absorber and extraction layer. Transfer across the interface is a real-space event rather than a momentum-space process. If the two bands from, and to, which the electron moves are not perfectly aligned, then tunneling must occur. The determination of the evanescent wave numbers that appear in tunneling coefficients are not the simple ones in textbooks but must be found from the full complex band structure of the two materials. Here, the nature of these evanescent states and their role in the tunneling of carriers across typical interfaces is examined using empirical pseudopotential methods.

The down-conversion or down-shifting of high-energy photons into “cold” photons—photons with reduced energy—presents an innovative strategy for enhancing the performance of solar photovoltaic (PV) systems. We explore the multiple benefits of cold photon generation, including reduced thermal load, optimized light management, and the potential to surpass traditional limits of PV systems. We examine the core thermodynamic principles governing cold photon generation and its effects on PV efficiency, highlighting opportunities for collimated cold photon emission. By reducing the entropy of photons through collimation, we can trade energy that would typically be lost as heat for increased efficiency, allowing for gains rather than losses. In addition, we discuss the practical challenges of current down-conversion and shifting materials and processes, while offering insights into the technological advancements required for the seamless integration of cold photon technologies into existing PV infrastructure.