van der Waals Heterostructures for Next-generation Hot Carrier Photovoltaics

与块状半导体相比，空间受限的货车德瓦尔斯（vdWs）层状材料具有强库仑相互作用、高激子结合能、降低的电荷屏蔽和低的电子-声子耦合，导致较慢的热载流子（HC）冷却。在vdWs异质结构中，由于K点处的动量守恒，已经观察到有效的直接层间HC转移，而没有声子发射。在基于石墨烯的vdWs异质结构中，相当高的光吸收导致增强的光生载流子密度，这引起热声子瓶颈效应，导致石墨烯中延长的HC冷却。适当设计的vdWs异质结构的上述优点对于高效HC太阳能电池（HCSC）的制造当然是有利的，限制HC的超快热化并超过肖克利-奎塞尔极限。在这项工作中，将生长具有高吸光度的各种层厚度（过渡金属：Mo，W;硫属化物：S，Se，Te）的低带隙（~1- 1.5eV）过渡金属二硫属化物（TMD），并将其与具有超净界面的石墨烯集成以用于制造HCSC。HC动力学，包括HC的类型，温度，HC寿命，和载流子倍增将通过时间和角度分辨光电子能谱探测太阳光驱动的HC光伏特性进行研究。优化的石墨烯/TMD vdW异质结构将与适当的能量选择性接触（ESC）和金属电极集成，具有适当的功函数，用于在HCSC中有效收集HC。ESC的厚度将针对通过ESC隧穿到金属电极的最大HC进行调整。HC驱动的光致发光的演示将通过各种高能激光照明的电流-电压（I-V）测量来进行。在太阳模拟器（1-SUN AM1.5）下，通过vdWs材料的圆片级生长和I-V测量，将实现大面积HCSC。

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).