"Our findings suggest that compression can allow us to tailor the wavelength of absorbed light. This compression may be attained through either mechanical or chemical means."Wendy Mao
Researchers at Stanford SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory have demonstrated a way to increase the voltages of perovskite solar-cell absorbers just by applying external pressure. Squeezing the solar cells made from artificial crystalline structures called perovskites, a potentially useful and cheap photovoltaic material, helped to improve their performance.
Perovskites, which are also benefitting research into new lasers and LEDs, come in many crystalline structures, such as hybrid perovskites that are made of lead, iodine or bromine, and organic compounds. Solar cells fabricated out of perovskites have already revealed some promise, but in this study, published in the journal ACS Central Science [Jaffe et al. ACS Cent. Sci. (2016) DOI: 10.1021/acscentsci.6b00055], it was shown that applying pressure can alter the properties of these materials, as well as how they respond to light. A dramatic increase in the electronic conductivity of the materials at high pressures was also observed.
To explore how pressure affects the way hybrid perovskites react to light, samples of the material were positioned in a diamond-anvil cell, a high-pressure device with two opposing diamonds. Each sample was positioned between the diamonds before being squeezed at very high pressures. It was found that, under compression, a sample that is usually orange would become lighter in color, indicating the perovskite was absorbing higher-energy light waves. However, when the pressure was increased, the sample darkened, indicating that lower-energy light was also being absorbed.
They tracked the positions of atoms upon compression with X-ray diffraction, which helped to identify how the structure of the materials reacts to pressure. As co-leader Wendy Mao said, “Our findings suggest that compression can allow us to tailor the wavelength of absorbed light. This compression may be attained through either mechanical or chemical means.”
Other studies have pinpointed that hybrid perovskites can efficiently absorb sunlight before converting it to electricity, with some managing to achieve efficiencies of over 20%, which is similar to that of commercially available silicon solar cells. As co-leader Hemamala Karundasa also states, “this work shows that pressure is a tuning knob for improving the properties of perovskite absorbers in a predictable way”.
Some research groups have already produced cheap tandem solar cells fabricated from perovskite that is placed on top of silicon, although achieving the necessary high voltages for high-efficiency tandem cells has not been straightforward. The findings from this new study suggest that pressure could increase the voltages of perovskite solar cells, something that requires further research.