How hybrid solar-cell materials may capture more solar energy

July 27, 2015

Innovative techniques for reducing solar-cell installation costs by capturing more solar energy per unit area by using hybrid materials have recently been announced by two universities.

Capturing more of the spectrum

Chemists at the University of California, Riverside have found an ingenious way to lower solar cell installation costs by reducing the size of solar collectors (credit: David Monniaux)

The University of California, Riverside strategy for making solar cells more efficient is to use the near-infrared region of the sun’s spectrum, which is not absorbed by current solar cells.

The researchers report in Nano Letters that a hybrid material that combines inorganic materials (cadmium selenide and lead selenide semiconductor nanocrystals) with organic molecules (diphenylanthracene and rubrene) could allow for an increase of solar photovoltaic efficiency by 30 percent or more, according to Christopher Bardeen, a UC Riverside professor of chemistry.

The new material also has wide-ranging applications such as in biological imaging, data storage and organic light-emitting diodes. “The ability to move light energy from one wavelength to [a] more useful region — for example, from red to blue — can impact any technology that involves photons as inputs or outputs,” he said.

The research was supported by grants from the National Science Foundation and the U.S. Army.

Plasmonic nanostructures and metal oxides

Rice researchers selectively filtered high-energy hot electrons from their less-energetic counterparts using a Schottky barrier (left) created with a gold nanowire on a titanium dioxide semiconductor. A second setup (right), which included a thin layer of titanium between the gold and the titanium dioxide, did not filter electrons based on energy level. (credit: B. Zheng/Rice University)

Meanwhile, new research from Rice’s Laboratory for Nanophotonics (LANP) has found a way to boost the efficiency and also reduce the cost of photovoltaic solar cells by using high-efficiency light-gathering plasmonic nanostructures combined with low-cost semiconductors, such as metal oxides.

“We can tune plasmonic structures to capture light across the entire solar spectrum,” claims Rice’s Naomi Halas, co-author of an open-access paper in Nature Communications. “The efficiency of [conventional] semiconductor-based solar cells can never be extended in this way because of the inherent optical properties of the semiconductors.”

The researchers found in an experiment that a solar cell using a “Schottky barrier” device allowed only “hot electrons” (electrons in the metal that have a much higher energy level) to pass from a gold nanowire to the semiconductor, unlike an “Ohmic device,” which let all electrons pass.

Today’s most efficient photovoltaic cells use a combination of semiconductors that are made from rare and expensive elements like gallium and indium, so this finding promises to further reduce the cost of solar cells.

Abstract of Hybrid Molecule–Nanocrystal Photon Upconversion Across the Visible and Near-Infrared

The ability to upconvert two low energy photons into one high energy photon has potential applications in solar energy, biological imaging, and data storage. In this Letter, CdSe and PbSe semiconductor nanocrystals are combined with molecular emitters (diphenylanthracene and rubrene) to upconvert photons in both the visible and the near-infrared spectral regions. Absorption of low energy photons by the nanocrystals is followed by energy transfer to the molecular triplet states, which then undergo triplet–triplet annihilation to create high energy singlet states that emit upconverted light. By using conjugated organic ligands on the CdSe nanocrystals to form an energy cascade, the upconversion process could be enhanced by up to 3 orders of magnitude. The use of different combinations of nanocrystals and emitters shows that this platform has great flexibility in the choice of both excitation and emission wavelengths.

Abstract of Distinguishing between plasmon-induced and photoexcited carriers in a device geometry

The use of surface plasmons, charge density oscillations of conduction electrons of metallic nanostructures, to boost the efficiency of light-harvesting devices through increased light-matter interactions could drastically alter how sunlight is converted into electricity or fuels. These excitations can decay directly into energetic electron–hole pairs, useful for photocurrent generation or photocatalysis. However, the mechanisms behind plasmonic carrier generation remain poorly understood. Here we use nanowire-based hot-carrier devices on a wide-bandgap semiconductor to show that plasmonic carrier generation is proportional to internal field-intensity enhancement and occurs independently of bulk absorption. We also show that plasmon-induced hot electrons have higher energies than carriers generated by direct excitation and that reducing the barrier height allows for the collection of carriers from plasmons and direct photoexcitation. Our results provide a route to increasing the efficiency of plasmonic hot-carrier devices, which could lead to more efficient devices for converting sunlight into usable energy.