Towards the Realization of the Hot Carrier Solar Cell using Valley Photovoltaics

热载流子太阳能电池的概念一直被认为是实现超高效率太阳能电池的一个令人兴奋的前景，它能够将更多的太阳能量转化为有用的能量。当光子的能量远高于用于生产太阳能电池的半导体中的吸收阈值（或带隙）时，就会产生热载流子。对于大多数材料，热载流子随后迅速与材料相互作用以产生热量。这种寄生热能不能转化为有用的能量，因此在商用太阳能电池中是一个主要的损失过程。迄今为止，陆地上使用的太阳能电池的功率转换效率被限制在~ 30%。如果在产生热量之前利用“热”光产生的电荷载流子，预计太阳能电池的转换效率将超过60%。这将大大降低系统成本，增加光伏技术的全球影响，从而对可持续和清洁能源作出重大贡献。这项研究还为新一代太阳能电池技术创造了潜力，产生了新的设备和消费产品，并对未来的能源产生了重大影响。然而，提取热载流子是极具挑战性的，需要大量的创新和开发新的系统和架构，以有效地解耦热量产生过程。实现这一目标的一个可能途径是通过谷光电，这是PI最近开发的一种协议，通过在高迁移率晶体管中观察到的自然过程来存储高能量热载子，其中热载子转移到太阳能电池吸收器结构中的所谓卫星谷。这个过程减缓了热量的产生，并提供了在这些载体失去能量和产生热量之前去除它们的机会。尽管取得了这样的成功，但在实际谷光电太阳能电池能够实现之前，还有几个重要的基本过程需要了解。特别是，尽管热载流子转移和储存到卫星谷，从而减少了热损失，但由于迄今为止开发的原理验证设备中载流子提取的寄生障碍，去除载流子并提供有用电压和电流的能力仍然存在问题。在这个项目中，我们将对III-V异质结构进行全面的研究，包括材料生长、光谱学和器件物理学，以实现一个实用的热载流子太阳能电池。所提出的研究将采用PI所证明的重要新结果，系统地优化基于谷间散射或通过谷间散射增强的热载流子太阳能电池的器件设计和运行。具体来说，增强热载流子提取的材料系统和太阳能电池结构将需要具有特定性能的新型屏障/选择接触层。此外，新的架构将是必要的，不仅散射高能量的光载流子，而且在太阳能电池的工作点维持大的内部电场，因此也可以利用低能量的光生载流子进一步提高设备的效率。这项工作的应用和基础性质使参与该计划的研究生和本科生接触到多样化的研究活动。这将使他们在接触材料开发和技术转移过程的同时，发展相当多的实用技能，这是研究生研究中独特的经历，对他们未来的职业生涯具有重要价值。该项目还为少数民族学生提供了研究机会，包括在PI小组中进行暑期研究。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The concept of a hot carrier solar cell has long been considered an exciting prospect for the realization of ultra-high efficiency solar cells with the ability to convert more of the sun’s energy to useful power. Hot carriers are photogenerated when photons with energy well above the threshold (or band gap) for absorption in the semiconductor used to produce the solar cell are absorbed. For most materials, the hot carriers then rapidly interact with the material to generate heat. This parasitic thermal energy cannot be converted to useful power and is therefore a major loss process in commercial solar cells. To date, the solar cells used terrestrially are limited to power conversion efficiencies of ~ 30%. If “hot” photogenerated charge carriers were harnessed prior to generating heat, the conversation efficiency of a solar cell has been predicted to exceed 60%. This would significantly reduce system costs and increase the global impact of photovoltaic technology,therefore contributing significantly to sustainable and clean energy sources. This research also creates the potential for a new generation of solar cell technologies, generating new devices and consumer products, as well as having significant implications for future sources of energy. Extracting hot carriers is, however, extremely challenging, and requires significant innovation and the development of novel systems and architectures that effectively decouple heat generation processes. One possible avenue to achieve this goal is via valley photovoltaics, a protocol recently developed by the PI to store the high energy hot carriers through a natural process observed in high mobility transistors in which hot carriers transfer to so-called satellite valleys in the structure of the solar cell absorber. This process slows heat generation and provides the opportunity to remove these carriers before they lose energy and create heat. Despite this success, there are several important fundamental processes to understand before a practical valley photovoltaic solar cell can be realized. In particular although the transfer and storage of hot carriers to the satellite valleys and therefore reduced heat loss has been demonstrated– the ability to remove the carriers and provide useful voltage and current remains problematic due to parasitic barriers to carrier extraction in the proof-of-principle devices developed to date. In thisprogram a comprehensive investigation of III-V heterostructures is proposed encompassing material growth, optical spectroscopy, and device physics such as to deliver a practical hot carrier solar cell. The proposed research will take the important new results demonstrated by the PI tosystematically optimize the device design and operation of a hot carrier solar cell based on, or enhanced by, intervalley scattering. Specifically, material systems and solar cell structures that enhance hot carrier extraction will require novel barrier/selective-contact layers with specific properties. Furthermore, novel architectures will be necessary that not only scatter high energy photocarriers but also sustain large internal electric fields at the operating point of the solar cell,so photogenerated carriers at lower energies can also be harnessed further improving the efficiency of the device.The applied and fundamental nature of this work exposes the graduate and undergraduate students involved in this program to a diverse research activity. This will enable them to develop considerable practical skills while exposing them to the process of materials development andtechnology transfer, an experience unique in graduate research and of significant value to their future careers. The program also provides research opportunities for minority students, including summer research in the group of the PI.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.