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).
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%.