2, Texas A&M University, College Station, Texas, United States
Photon upconversion is a process in which two or more low-energy photons are sequentially absorbed in a material and a high-energy photon is emitted. Efficient upconversion has numerous potential applications in energy harvesting. For example, a well-known challenge in photocatalytic water splitting for hydrogen fuel generation is that only a small portion of the incident solar spectrum has photon energies larger than the bandgap of the photocatalyst. Similar limitations apply to photovoltaics, where the inability to harvest low-energy photons is a significant contributor to the Shockley-Quessier limit on the net solar energy harvesting efficiency for a single junction device. Photon upconversion provides an exciting opportunity to "recycle" these wasted low-energy photons by converting them into higher-energy photons that can be harvested by the photovoltaic or photocatalytic material. Such photon upconversion technologies could also be employed to generate visible light from near-infrared solar energy or waste heat (thermal radiation) generated by electronics.
There are a few existing photon upconversion materials, but they do not meet the performance metrics necessary for significant impact on energy harvesting applications. Moreover, it is difficult to engineer these materials for specific applications. Semiconductors have inherently broadband absorption, capturing essentially all photons with energy above their bandgap. The absorption and emission wavelengths in semiconductor nanostructures can be controlled with both composition and size through quantum confinement effects. Moreover, semiconductor materials can be combined in complex heterostructures to guide the transfer of charges. For these reasons, semiconductors provide an appealing platform for photon upconversion.
We have developed a new semiconductor nanoparticle approach to photon upconversion. We will describe the underlying design principles and present numerical simulations that demonstrate the potential impact of such upconversion materials on solar energy harvesting. We will describe the synthesis and characterization of colloidal upconversion nanoparticles that make the creation of an “upconversion paint” a realistic possibility. We will describe how theses structures can be engineered to alter absorption and emission wavelengths and improve efficiency. We will demonstrate the viability of this new approach to photon upconversion and discuss the challenges and prospects for reaching efficiency levels that will make this technology commercially viable.