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

长期以来，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令人兴奋的前景，该电池能够将更多的太阳能转化为有用的电力。当用于生产太阳能电池的半导体中能量远高于吸收阈值(或带隙)的光子被吸收时，热载流子就会被光产生。对于大多数材料来说，热载流子随后会迅速与材料相互作用产生热量。这种寄生热能无法转化为有用的电能，因此是商业太阳能电池的主要损耗过程。到目前为止，陆地上使用的太阳能电池的功率转换效率仅为30%。如果在产生热量之前利用“热”光生载流子，太阳能电池的转换效率预计将超过60%。这将大大降低系统成本，增加光伏技术的全球影响，从而大大促进可持续和清洁能源的发展。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，需要进行重大创新和开发新的系统和架构，以有效地分离热产生过程。实现这一目标的一个可能途径是通过山谷光伏，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热载流子，在高迁移率晶体管中，热载流子转移到太阳能电池吸收体结构中的所谓卫星谷。这一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将其移除的机会。尽管取得了这样的成功，但在实现实用的谷型光伏太阳能电池之前，还有几个重要的基本过程需要了解。特别是，虽然已经证明了将热载流子转移和储存到卫星谷，从而减少了热损失，但由于迄今开发的原理验证装置中对载流子提取的寄生障碍，移除载流子并提供有用的电压和电流的能力仍然存在问题。在这个项目中，我们建议对III-V异质结进行全面的研究，包括材料生长、光学光谱和器件物理，例如提供一个实用的热载流子太阳能电池。这项研究将利用PI所展示的重要新结果，系统地优化基于谷间散射或由谷间散射增强的热载流子太阳能电池的器件设计和运行。具体地说，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特性的新型势垒/选择性接触层。此外，需要新的架构，不仅分散高能光生载流子，而且在太阳能电池工作点维持较大的内部电场，因此还可以利用较低能量的光生载流子来进一步提高设备的效率。这项工作的应用性和基础性使参与该项目的研究生和本科生接触到不同的研究活动。这将使他们能够发展相当多的实用技能，同时使他们接触到材料开发和技术转让的过程，这是研究生研究中独一无二的经历，对他们未来的职业生涯具有重要价值。该计划还为少数族裔学生提供研究机会，包括在PI组的暑期研究。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.