Heavy-Atom-Free Sensitizers for NIR-to-Visible Solar Photon Upconversion

University Of Washington, Seattle WA

Investigators

Abstract

Non-technical Description Harvesting solar energy efficiently and inexpensively is a grand challenge to producing clean, sustainable energy. The spectrum of light emitted by the sun spans a vast range, from the far infrared to ultraviolet. However only a fraction of this light can be absorbed by the active layer in a solar cell. If one can broaden the spectral range of sunlight that a solar panel can capture, the overall power generated increases. One strategy to increase the fraction of light absorbed is to combine two low-energy photons and fuse their energy into one higher-energy photon, a process called upconversion. The upconverted photon can then be absorbed and used to generate electricity from light that would otherwise be wasted. The goal of this project is to identify and design new molecules that can be used for upconversion in more efficient solar cells. Researchers will combine synthetic chemistry and optical characterization with theory and machine learning to achieve this goal. Beyond its scientific and technological impacts, participants will engage local and regional community college students with authentic and sustained exposure to original research experiences. One specific avenue the team is pursuing is to attempt to infuse photon upconversion research concepts into course-based academic laboratory experiments at a local community college. This approach to engage community college students with exposure to original research is intended to help overcome the geographic constraints that may hinder these students from taking part in more traditional research experience for undergraduate (REU) programs that require students to travel long distances to the research site. Technical Description Photon upconversion could significantly enhance solar cell efficiencies to routinely meet or exceed the Shockley-Queisser limit. One promising strategy for deploying photon upconversion to enhance the efficiency of solar photovoltaics is triplet-triplet annihilation upconversion because it can occur under low-intensity, non-coherent illumination from solar photons. Historically, the materials that have been explored for photon upconversion have included sensitizers based on precious metal complexes, arylhalides, and quantum dots. The first class is not economically viable due to the high cost of the metal, the second class is unstable under illumination, and the third class suffers from parasitic absorption of the upconverted light by the sensitizer. Given these materials constraints, the PI’s group identifies heavy-atom-free upconversion sensitizers based on thionated squaraine-based materials, which they explore as potential candidates for achieving NIR-to-visible photon upconversion for solar photovoltaic applications. The goal is to clarify elusive structure-function relationships that have historically made it challenging to identify NIR-absorbing triplet sensitizers in general, and more specifically, hitherto intractable to achieve NIR-to-Visible photon UC without using expensive precious metal centers or arylhalides that readily photodegrade, which are both unattractive for solar applications. One key goal of this work is to address specific questions such as, “What molecular descriptors are most prescriptive for simultaneously achieving NIR absorption and high intersystem crossing yields?” The team combines experimental and theoretical approaches based on ultrafast pump-probe spectroscopy, ab initio predictions, and reinforcement learning to identify and evaluate new molecular design strategies for materials that they synthesize and test in this project. The project also supports new Course-based Undergraduate Research Experiences (CUREs) for community college students to conduct advanced photonic materials research relevant for next-generation solar energy devices. 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.