Heat Flow through Nonideal Contacts in Hot-Carrier Solar Cells

Hot-carrier solar cells (HCSCs) are optoelectronic heat engines that could, in principle, achieve efficient solar energy conversion with a high detailed-balance limit of 85% at full solar concentration. Approaching this limit requires negligible carrier energy loss to the lattice and reversible carrier extraction via narrow, highly conductive energy windows. Practical implementations of HCSCs with nonideal contacts such as two-dimensional (2D) contacts and thermionic barriers have excess heat flows resulting in irreversible extraction. We examine the electronic heat flows in such schemes to explain where power losses originate and show that they determine carrier temperatures and device performance, providing insight into optimal characteristics of an HCSC. A compromise between maximizing current extraction and minimizing the associated heat flow determines the optimal extraction energy. This optimization for a thermionic barrier contact shows that remarkably low carrier temperatures (similar to 500 K) are required for optimal operation at 45% efficiency for a 1-eV-band-gap absorber under the particle conservation model, compared to similar to 1000 K at 60% efficiency for a black-body absorber under the impact ionization model, even in the absence of carrier-phonon interactions. Furthermore, we quantify the irreversible component of the heat flow and show that it is minimal for highly conductive 2D contacts that approach the Carnot limit. The results suggest that heat flows in HCSCs can be probed by measuring carrier temperature trends by varying applied bias and contact parameters, which may present clearer evidence of HCSC operation.