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Frontiers in
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Spring 2016

Giving Sunlight a Boost

Converting sunlight to higher energy to better absorb the solar spectrum

Nate Thomas

Theoretical upconversion method. Two photons are absorbed in the thin metal layer, creating two electrons, e1 and e2, and two holes, h1 and h2. A hole is simply the absence of an electron, but this absence behaves much like a particle. Electron e2 and hole h1 then move into the region called the semiconductor quantum well, which is a sandwich of two materials that help keep the electron and hole in the same place. In the final step, e2 and h1 recombine to release an upconverted photon. Figure modified with permision of Guru Naik.

All it takes is standing outside on a hot summer day to feel how powerful the light from the sun is. The sun is our most abundant source of energy, and over the past 60 years, researchers have been pushing the limits on how well solar cells convert sunlight into electricity. However, solar cells have a fundamental limitation. They can only absorb light, or photons, with energy above a particular energy threshold, called the band gap energy. Any light with energy below this band gap energy is lost.

Recently, researchers in the Light-Materials Interactions in Energy Conversion (LMI) and the Center for Excitonics (CE), two Energy Frontier Research Centers (EFRCs), have been developing new technologies to overcome this problem and to push solar cell efficiencies even further. One method to avoid losing the light with energy below the solar cell band gap is called “upconversion.” Intuitively, it is the process of converting two or more low-energy photons, which don’t have enough energy to be absorbed in a solar cell, into a single high-energy photon, which subsequently can be absorbed. Light, however, does not naturally interact with itself. Two photons traveling through the air won’t accidentally combine to create a single photon of higher energy without help. Researchers must instead come up with creative theoretical ways to upconvert sunlight, even before realizing it experimentally.

From two photons into one. All upconversion methods require a primary medium to absorb incident sunlight and emit the resulting upconverted light. Both light absorption and emission processes involve “electrons” and “holes.” We are familiar with electrons as the negatively charged particles that whiz around the nucleus of an atom. When a material, such as a piece of metal, absorbs an incident photon, the energy of one of these electrons goes up. When this electron jumps up in energy, it leaves behind a vacancy aptly named a “hole.” This hole also moves just like an electron! When holes travel around, what actually happens is some other electron at a different position moves to fill the hole, leaving behind a new one. But it is just like the hole has moved. Thus, light absorption results in the creation of mobile electrons and holes. When the light absorption process happens in reverse, light is emitted. A high-energy electron falls back down and fills a hole, or recombines with it, to re-release a photon.

The concept of electron-hole recombination is critical to the upconversion method outlined by Gururaj Naik and others from the LMI. In their work, they propose a theoretical model to upconvert photons (see schematic of their system design).

In their theory, for the first step, a photon is absorbed in the metal layer, causing electron e1 to jump up in energy, leaving behind hole h1. This hole has sufficient energy to sneak across into the adjacent “semiconductor quantum well,” indicated by the curved brown arrow. The electron, however, does not. It stays put. In the second step, this process is repeated, except this time, it is electron e2 and not hole hthat has sufficient energy to jump across the metal/quantum-well interface.

The “semiconductor quantum well” is actually just a sandwich of different materials that helps trap electron e2 and hole h1 in the same place, allowing them to recombine. As shown in the diagram, though, the energy gap between e2 and h1 is greater than it was for either e1 and h1, or e2 and h2. In other words, when electron e2 and hole h1 recombine, the energy of the re-emitted photon will be greater than the energy of either of the initially absorbed photons. We will have successfully upconverted! In their paper, Naik and others conclude that this theoretical scheme can double upconversion efficiency from current techniques while also providing tunability.

Unique to this system is that by changing the size of the metal or by changing the materials in the semiconductor quantum well, researchers can change the energy of the upconverted photon and tune for a particular solar cell.

Letting electrons and holes grow old. The reason for this complicated process is because high-energy, “excited” electrons and holes that directly arise from light absorption do not like to stay in their excited states for very long. It is no easy feat to prevent the original electron-hole pairs, e1-h1 and e2-h2, from losing their energy through heating the metal or from recombining with themselves, both of which would prevent upconversion. Critical to their process is the separation of these pairs such that e2 and h1 can instead recombine with each other.

Mengfei Wu, Daniel Congreve, Mark Wilson, and others from the CE have also come up with a way to make for longer-lived electron-hole pairs necessary for photon upconversion. In their method, they use tiny nanocrystals of lead sulfide (PbS) only ten or so atoms across instead of a layer of metal as the primary light absorber. A layer of these nanocrystals is coated with a layer of organic material, into which electrons and holes can diffuse. A portion of the electron-hole pairs actually change their state and upon entering the organic material become longer lived. These new long-lifetime electron-hole pairs are called “triplet excitons.” We can't make triplet excitons directly from absorbing light, which is why we need both the PbS nanocrystals and the layer of organic material.

These new electrons and holes can actually avoid recombination long enough for interesting reactions to happen. For example, as shown in the figure, two electrons and two holes, or net two triplet excitons, can react to create a single higher energy electron-hole pair. That is just like the result in the previous work when electron e2 and hole h1 ended up with a larger energy gap between them. When this final electron and final hole merge, they release an upconverted photon.

Step-by-step reaction of two electron-hole pairs, called triplet excitons, to create an upconverted photon. Image courtesy of Nathan Thomas.

Wu, Congreve, Wilson, and others succeeded in realizing this technique experimentally. In the image below, they show an infrared laser, which we can't see, shining into these layers of nanocrystals and organic material. The result is a distinct red beam of light, indicating that some of the light from the laser has jumped up in energy to a wavelength we can see.

Experimental realization of upconversion. The red beam of light consists of upconverted light that originated from an infrared laser. Reprinted by permission from Macmillan Publishers Ltd: Nature Photonics, Wu et al., copyright 2016.

These new ideas to upconvert light result from theoretically and experimentally probing the fundamentals of how light interacts with different materials. As the need for high-performance solar cells grows, the challenge is to use this fundamental understanding to find the next methods for controlling and manipulating sunlight. The sun is our most prevalent source of energy. Researchers in EFRCs have developed new, creative ways for photon upconversion necessary to better harness that energy for a more sustainable energy future.

More Information

Naik GV and JA Dionne. 2015. “Photon Upconversion with Hot Carriers in Plasmonic Systems.” Applied Physics Letters 107:133902. DOI: 10.1063/1.4932127

Wu M, DN Congreve, MWB Wilson, J Jean, N Geva, M Welborn, T Van Voorhis, V Bulović, MG Bawendi, and MA Baldo. 2016. "Solid-State Infrared-to-Visible Upconversion Sensitized by Colloidal Nanocrystals." Nature Photonics 10(1):31-34. DOI: 10.1038/nphoton.2015.226

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More Information

Naik GV and JA Dionne. 2015. “Photon Upconversion with Hot Carriers in Plasmonic Systems.” Applied Physics Letters 107:133902. DOI: 10.1063/1.4932127

Wu M, DN Congreve, MWB Wilson, J Jean, N Geva, M Welborn, T Van Voorhis, V Bulović, MG Bawendi, and MA Baldo. 2016. "Solid-State Infrared-to-Visible Upconversion Sensitized by Colloidal Nanocrystals." Nature Photonics 10(1):31-34. DOI: 10.1038/nphoton.2015.226

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.