Hot carrier photovoltaics in van der Waals heterostructures

Successfully designing an ideal solar cell requires an understanding of the fundamental physics of photoexcited hot carriers (HCs) and the underlying mechanism of unique photovoltaic performance. Harnessing photoexcited HCs offers the potential to exceed the thermodynamic limit of power conversion efficiency, although major loss channels employing ultrafast thermalization of HCs severely restrict their utilization in conventional bulk-absorber-based solar cells. Spatially confined semiconductors, especially 2D van der Waals (vdW) materials, offer several advantages, such as strong Coulomb interaction, high exciton binding energy, strong carrier–carrier scattering and weak carrier–phonon coupling, resulting in slow HC cooling and restricted loss channels. This Review provides a detailed mechanistic understanding of the HC cooling dynamics in confined vdW layered materials for efficiently utilizing HCs and discusses the role of carrier multiplication in designing a solar cell with the power conversion efficiency exceeding the Shockley–Queisser limit. Additionally, we analyse the major energy loss channels that limit the efficiency of a conventional solar cell, as well as the promises held by the 2D vdW heterostructures for an efficient HC solar cell. Furthermore, we highlight the challenges and opportunities involved in successfully utilizing HCs in practical solar cells with efficiencies beyond the thermodynamic limit. Conventional solar cells suffer from the thermalization energy loss of hot carriers, which sets the Shockley–Queisser limit. This Review highlights the insightful mechanisms of slow hot carrier cooling dynamics and efficient carrier transport/extraction in 2D van der Waals heterojunctions, following opportunities in practical hot carrier solar cells.