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

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 this program 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 to systematically 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 and technology 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.