Abstract
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.
Key points
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Photogenerated hot carriers can be harnessed in spatially confined photovoltaic materials (2D van der Waals heterostructures), owing to slow hot carrier cooling and restricted loss channels, resulting in power conversion efficiency beyond the Shockley–Queisser limit.
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Weak optical absorbance of graphene can be compensated by integrating with 2D van der Waals layered semiconductors yielding relatively high absorbance. Subsequently, the enhanced photocarrier density invokes the hot-phonon bottleneck effect, leading to prolonged hot carrier cooling in graphene, which results in the synergistic hot-carrier-driven photovoltaic performance.
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Unlike conventional bulk heterostructures, direct interlayer hot carrier transfer on the ultrafast timescale can be efficient in van der Waals heterostructures without phonon emission due to momentum conservation at the K-point.
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In graphene-based 2D van der Waals heterostructures, graphene can serve as both the injector and the collector of quasi-thermalized hot carriers with high quantum efficiency.
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Both 2D van der Waals layered materials and perovskite nanostructures demonstrate high carrier multiplication conversion efficiency. Moreover, 2D van der Waals heterostructures also demonstrate highly efficient interlayer carrier multiplication near room temperature.
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van der Waals heterostructure stacking with a hot carrier absorber material, perovskite nanostructures, may offer synergistic hot carrier and carrier multiplication effects, which lead to the enhanced solar cell performance (maximum open-circuit voltage (\({V}_{{\rm{OC}}}^{{\rm{HC}}}\)) and short-circuit current (\({I}_{{\rm{SC}}}^{{\rm{CM}}}\))).
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References
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Glossary
- Tandem cells
-
High-performance multijunction photovoltaic devices that sequentially absorb wide-range solar energy.
- Carrier multiplication
-
(CM). Interband carrier–carrier scattering increasing the overall carrier density in the conduction band, also referred to as inverse Auger recombination.
- Decay cascade
-
When a LO phonon decays its energy by successive emission of multiple optical and acoustic phonons.
- Auger recombination
-
(AR). Interband carrier–carrier scattering reducing the overall carrier density in the conduction band.
- Internal quantum efficiency
-
Ratio between the number of photocarriers collected and the number of photons absorbed by the graphene layer.
- A-exciton, B-exciton and C-exciton states
-
A and B excitonic states arise due to spin–orbit-coupling-induced valence band splitting at the K-point. The C-exciton originates from the band nesting region on the higher-energy side.
- Auger heating
-
Auger recombination increases the total energy of carriers, therefore, heating the electronic system.
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Paul, K.K., Kim, JH. & Lee, Y.H. Hot carrier photovoltaics in van der Waals heterostructures. Nat Rev Phys 3, 178–192 (2021). https://doi.org/10.1038/s42254-020-00272-4
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DOI: https://doi.org/10.1038/s42254-020-00272-4
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