Pressure Tunes Absorption Properties of Perovskite Solar Cells

The voltage and electronic conductivity of perovskite solar cells have been shown to increase under high pressure. The findings suggest that either chemical or mechanical compression could enable tunable perovskite solar cells capable of absorbing different wavelengths of light.

Perovskites — metallic crystalline structures — come in several structures, including hybrids made of lead, iodine or bromine, and organic compounds. The inexpensive materials have potential applications in LEDs and lasers, but one of the most popular areas of research involves solar cells. Hybrid perovskites absorb sunlight and convert it to electricity, with reported efficiencies over 20 percent, rivaling commercial silicon solar cells.

Top: A normal perovskite crystal (left) bends and twists after being squeezed between two diamonds. Bottom: The orange perovskite crystal changes color as pressure is applied, indicating that different wavelengths of light are being absorbed. Courtesy of Adam Jaffe and Yu Lin/Stanford & SLAC.

Now researchers at Stanford University have assessed how pressure affects the way hybrid perovskites respond to light. To do so, they loaded perovskite samples ((MA)PbX₃, where MA = CH₃NH₃⁺ and X = Br⁻ or I⁻) in a diamond-anvil cell, a high-pressure device consisting of two opposing diamonds. Each sample was placed between the diamonds and squeezed at very high pressures.

The results were visible. One sample, normally orange, turned lighter in color under compression, an indication that the perovskite was absorbing higher-energy light waves. As pressure increased, the sample darkened, indicating that lower-energy light was also being absorbed.

"By tracking the positions of atoms upon compression using x-ray diffraction, we can explain exactly how the materials' structure responds to pressure," said professor Hemamala Karunadasa said. "Overall this work shows that pressure is a tuning knob for improving the properties of perovskite absorbers in a predictable way."

Several research groups have been developing low-cost tandem solar cells made of perovskite stacked on top of silicon, but obtaining the high voltages required for high-efficiency tandem cells has proven difficult. Results of the Stanford study suggest that pressure can increase the voltages of perovskite solar cells and should be investigated further.

The research was published in ACS Central Science (doi: 10.1021/acscentsci.6b00055).