van der Waals Heterostructures for Next-generation Hot Carrier Photovoltaics

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In contrast to the bulk semiconductors, spatially confined van der Waals (vdWs) layered materials possess strong Coulomb interaction, high exciton binding energy, reduced charge screening and low electron-phonon coupling, leading to a slower hot carrier (HC) cooling. Efficient direct interlayer HC transfer has been observed in vdWs heterostructures without phonon emission due to momentum conservation at K-point. In a graphene-based vdWs heterostructure, considerably high optical absorbance leads to the enhanced photocarrier density, which invokes the hot-phonon bottleneck effect, leading to prolonged HC cooling in graphene. The aforementioned advantages of suitably designed vdWs heterostructures are certainly advantageous for fabrication of efficient HC solar cells (HCSCs), restricting the ultrafast thermalization of HCs and exceeding the Shockley-Queisser limit. In this work, low bandgap (~1-1.5 eV) transition metal dichalcogenides (TMDs) of various layer thicknesses (transition metal: Mo, W; chalcogenide: S, Se, Te) with high optical absorbance will be grown and integrated with graphene having ultraclean interface for the fabrication of HCSCs. HC dynamics including the type of HC, temperature, HC lifetime, and carrier multiplication will be investigated by time- and angle-resolved photoemission spectroscopy to probe the solar light driven HC photovoltaic characteristics. Optimized graphene/TMD vdWs heterostructures will be integrated with proper energy selective contacts (ESCs) and metal electrodes with appropriate work functions for the efficient HCs collection in HCSCs. The thickness of the ESCs will be tuned for the maximum HCs tunneling to the metal electrodes through the ESCs. Demonstration of HC-driven photovoltaics will be carried out by current-voltage (I-V) measurements with various energetic laser illuminations. Large area HCSCs will be realized with wafer-scale growth of vdWs materials and I-V measurements under solar simulator (1-SUN AM1.5).