Constraints imposed by the sparse solar photon flux on upconversion and hot carrier solar cells

Efficiently utilising near-infrared solar radiation remains a challenge in advanced solar energy conversion concepts for photocatalysis and photovoltaics. Here we argue that the particulate nature of the solar photon flux limits the efficiency of schemes that rely on the interaction between optically excited states to exploit low energy photons, such as upconversion and hot carrier systems. The solar photon flux poses stringent constraints on the optical absorption strengths and excited state lifetimes required of such schemes to be efficient. We survey reported device architectures and material properties, and present a quantitative assessment of the extent to which they are limited by the sparse solar photon flux. We illustrate why two photon absorption and other non-linear optical methods are completely unsuitable for augmenting solar energy conversion efficiency, and where existing upconversion and hot carrier solar cell schemes stand against the photon flux criterion. Finally, we describe the opportunities that strategies such as sensitisation and phononic (vibrational) engineering present for bridging the gap between current devices and their efficiency limits.