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Materials

Colored Solar Cells Span The Spectrum

Materials: Adding a photonic crystal to a perovskite creates solar cells in a range of colors

by Neil Savage
February 17, 2015

CORRECTION: This story was updated on March 16, 2015, to correct the method the researchers used to control the color of the solar cells.

The facades of office buildings standing tall in the sunlight present tempting targets for photovoltaics researchers, who see them as the perfect place to put solar cells. Some researchers have pursued this idea, designing cells made of perovskites, a class of inexpensive crystalline minerals that could convert sunlight to electricity with an efficiency similar to silicon. Unfortunately, such solar cells are typically a dull brown, limiting their attractiveness to architects. Now researchers have found a way to imbue these cells with a choice of colors across the spectrum (Nano Lett. 2015, DOI: 10.1021/nl504349z).

Upon Reflection
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Credit: Nano. Lett.
Perovskite solar cells (2 cm wide on each side) get their colors from alternating layers of silicon dioxide nanoparticles and titanium dioxide embedded inside, which reflect specific wavelengths of light. Varying the thickness of the layers gives the different colors shown.
20150217lnp1-colorsolarcells.jpg
Credit: Nano. Lett.
Perovskite solar cells (2 cm wide on each side) get their colors from alternating layers of silicon dioxide nanoparticles and titanium dioxide embedded inside, which reflect specific wavelengths of light. Varying the thickness of the layers gives the different colors shown.
Colored Cell
[+]Enlarge
Credit: Nano. Lett.
A new perovskite solar cell gets its colors from an integrated photonic crystal. A glass substrate (bottom, light gray) is covered with a layer of conducting fluorine-doped tin oxide (purple). Above that, alternating layers of titanium dioxide (brown) and silicon dioxide nanoparticles (tan spheres) form the photonic crystal. On top, a layer of perovskite (dark gray), a hole-transport material (light purple), and a gold electrical contact complete the cell.
20150217lnp1-layereddiagram.jpg
Credit: Nano. Lett.
A new perovskite solar cell gets its colors from an integrated photonic crystal. A glass substrate (bottom, light gray) is covered with a layer of conducting fluorine-doped tin oxide (purple). Above that, alternating layers of titanium dioxide (brown) and silicon dioxide nanoparticles (tan spheres) form the photonic crystal. On top, a layer of perovskite (dark gray), a hole-transport material (light purple), and a gold electrical contact complete the cell.

Henry J. Snaith, a physicist at Oxford University, and colleagues expanded this color palette by incorporating a layered photonic crystal into the solar cell, which tunes the wavelength of reflected light. They started with a sheet of glass coated with fluorine-doped tin oxide to make it conductive and then deposited alternating layers of titanium dioxide and silicon dioxide nanoparticles on that surface. Within each nanoparticle layer, they added a polymer to fill in the spaces between the particles. After producing several layers, they heated the device to decompose the polymer and then added perovskite precursors. During this annealing process, the perovskite, CH3NH3PbI3-xClx, penetrated the pores in the photonic crystal all the way down to the glass.

By adjusting the thickness of the layers in the photonic crystal, the researchers can control the wavelength of reflected light, and thus the color of the solar cell. But the reflection comes with a trade-off: The perovskite absorbs less light, which can reduce the cell’s efficiency. A blue solar cell, for instance, is about 30% less efficient than an ordinary perovskite cell. But Snaith says the high efficiency of perovskites—20% for the best reported cells, similar to silicon—makes the loss acceptable. The team created solar cells in six colors ranging from blue to red, and almost all had efficiencies ranging between 6.0 and 8.8%.

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