van der Waals Heterostructures for Next-generation Hot Carrier Photovoltaics

与块体半导体相比，空间受限范德华层状材料具有强的库仑相互作用、高激子结合能、减少电荷筛选和低电子-声子耦合，导致热载流子（HC）冷却速度较慢。由于k点动量守恒，在没有声子发射的vdWs异质结构中观察到有效的直接层间HC转移。在石墨烯基vdWs异质结构中，相当高的光吸收导致光载流子密度增强，从而引发热声子瓶颈效应，导致石墨烯中的HC冷却时间延长。适当设计的vdWs异质结构的上述优点无疑有利于高效HC太阳能电池（HCSCs）的制造，限制了HC的超快热化并超过了Shockley-Queisser极限。在这项工作中，将生长具有高光吸收度的低带隙（~1-1.5 eV）不同层厚的过渡金属二硫族化合物（过渡金属：Mo， W；硫族化合物：S， Se, Te），并将其与具有超清洁界面的石墨烯集成在一起，用于制造HCSCs。通过时间分辨和角度分辨光谱学研究HC的动力学，包括HC的类型、温度、HC寿命和载流子乘法，以探测太阳能光驱动HC的光伏特性。优化后的石墨烯/TMD vdWs异质结构将与合适的能量选择触点（ESCs）和具有适当功函数的金属电极相结合，从而在HCSCs中高效收集hc。ESCs的厚度将调整为通过ESCs隧穿到金属电极的最大hc。hc驱动光伏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。大面积HCSCs将通过vdWs材料的晶圆级生长和太阳模拟器（1-SUN AM1.5）下的I-V测量来实现。

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).