Mar 21 2018
This illustration depicts what happens inside a hybrid perovskite material in the first few trillionths of a second after it is hit with simulated sunlight (upper left). The blue and green spheres are atoms, arranged in double pyramids as shown at left. When light hits, electrons start to separate from positively charged “holes,” the first step in creating an electrical current (yellow streaks). Meanwhile, atoms begin to vibrate within the perovskite’s lattice-like structure. Scientists detected these processes by analyzing the terahertz radiation (red streaks) the processes released. (Credit: Greg Stewart/SLAC National Accelerator Laboratory)
Researchers have gained new knowledge of basic properties of hybrid perovskites, which were unknown until now. Hybrid perovskites are inexpensive materials with the ability to improve or even function as a substitute for traditional solar cells developed using silicon.
When viewed under a microscope, a perovskite slice resembles an abstract mosaic of random grains of crystal. The riddle is how such a patchwork of defective, tiny grains has the ability to convert sunlight into electricity with an efficiency comparable to that of a single crystal of pure silicon.
Latest research conducted by researchers from Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory provides intriguing insights. The researchers offer a fresh knowledge of the way electric charges get separated in perovskites a few billionths of a second after the absorption of light, which is the first, critical step in producing electric current. They have reported their findings in the March 15, 2018, issue of the Advanced Materials journal.
The research is the first ever to investigate the inner dynamics of hybrid perovskites at the atomic level by employing laser pulses with an intensity matched with that of solar radiation, thereby mimicking natural sunlight. According to the researchers, their finding could result in enhancements in the performance of perovskite solar cells and an innovative way to investigate their functionality.
Perovskites and Silicon
Majority of the solar cells used at present are developed by using purified silicon synthesized at temperatures of more than 3000 °F, or 1600 °C. These sturdy silicon panels can withstand all types of weather conditions for several decades.
Despite being considerably less durable, perovskite solar cells are more flexible and thinner than silicon cells and can be synthesized at ambient temperature by using a hybrid blend of inexpensive inorganic and organic materials, such as methylammonium, lead, and iodine.
The scientists, including Michael McGehee, a Stanford co-author, have demonstrated that the efficiency of perovskite solar cells in transforming light into electricity is quite similar to that of commercially available silicon cells and can even outperform them. This blend of flexibility, efficiency, and easy production has led to a global quest for developing commercial-grade perovskites that can remain unaffected upon exposure to precipitation and heat for a longer period of time.
Perovskites are very promising materials for photovoltaics. But people wonder how they can achieve such high efficiencies.
Burak Guzelturk, Postdoctoral Scholar, Stanford
Electrons and Holes
The functioning of all solar cells is based on the same principle. Photons in the sunlight ingested by the crystalline material push negatively charged electrons into an excited state. Positively charged spaces or “holes” that separate from one another are formed in the place of the liberated electrons. Such a separation results in the generation of electric current.
The highly ordered atomic structure of pure silicon offers a direct path for holes and electrons to move through the solar cell. However, in the case of perovskites, it is not so easy.
Perovskites are typically filled with defects. They’re not even close to being perfect crystals, yet somehow the electric currents don’t see the defects.
Aaron Lindenberg, Associate Professor, Stanford
Terahertz Emission
For this research, the scientists used laser pulses to activate the waves of sunlight from both ends of the visible light spectrum, that is, the low-energy infrared light and the high-energy violet light. The outcomes were evaluated at the picosecond timescale, where one picosecond is equal to one-trillionth of a second.
“In the first picoseconds after sunlight hits the perovskite, the electrons and holes in the crystalline lattice start to split,” elucidated Lindenberg. “The separation was uncovered by measuring the emission of high-frequency terahertz light pulses oscillating a trillion times per second from the perovskite thin film. This is the first time anyone has observed terahertz emission from hybrid perovskites.”
The terahertz emission also showed that holes and electrons had close interactions with lattice vibrations in the crystalline material. These interactions, which take place on a femtosecond timescale, could assist in describing the way electric currents propagate through the patchwork of crystal grains in hybrid perovskites.
“As the electric charges separate, we observe a sharp spike in the terahertz emission, matching a vibrational mode of the material,” stated Guzelturk. “That gives us clear evidence that the electrons and holes are strongly coupling with the atomic vibrations in the material.”
This discovery increases the probability that coupling to the lattice vibration could protect the holes and electrons from charged defects in the perovskite, thereby shielding the electric current as it propagates through the solar cell. A similar course of events has been hypothesized by other research groups.
This is one of the first observations of how the local atomic structure of a hybrid perovskite material responds in the first trillionths of a second after absorbing sunlight. Our technique could open up a new way of probing a solar cell right when the photon is absorbed, which is really important if you want to understand and build better materials. The conventional way is to put electrodes on the device and measure the current, but that essentially blurs out all of the microscopic processes that are key. Our all-optical, electrode-less approach with femtosecond time resolution avoids that problem.
Aaron Lindenberg, Associate Professor, Stanford
Hot Electrons
The team also discovered that terahertz light fields are considerably stronger when high-energy light waves are used to strike the perovskite.
“We found that radiated terahertz light is orders of magnitudes more intense when you excite the electrons with violet light versus low-energy infrared light,” stated Lindenberg. “That was an unexpected result.”
According to Guzelturk, this finding could offer an innovative understanding of high-energy “hot” electrons.
“Violet light imparts electrons with excess kinetic energy, creating hot electrons that move much faster than other electrons,” he stated. “However, these hot electrons lose their excess energy very rapidly.”
Lindenberg further added that tapping the energy of hot electrons can result in a new era of highly efficient solar cells.
One of the grand challenges is finding a way to capture the excess energy from a hot electron before it relaxes. The idea is that if you could extract the current associated with hot electrons before the energy dissipates, you could increase the efficiency of the solar cell. People have argued that it’s possible to create hot electrons in perovskites that live much longer than they do in silicon. That’s part of the excitement around perovskites.
Burak Guzelturk, Postdoctoral Scholar, Stanford
The research demonstrated that in the case of hybrid perovskites, hot electrons get isolated from holes more efficiently and quickly than electrons activated by infrared light.
“For the first time we can measure how fast this separation occurs,” stated Lindenberg. “This will provide important new information on how to design solar cells that use hot electrons.”
Toxicity and Stability
Guzelturk stated that the capability of evaluating terahertz emissions could also result in innovative studies for producing non-toxic substitutes to traditional lead-based perovskites.
“Most of the alternative materials being considered are not as efficient at generating electricity as lead,” he noted. “Our findings might allow us to understand why lead composition works so well while other materials don’t, and to investigate the degradation of these devices by looking directly at the atomic structure and how it changes.”
Staff scientist Christopher Tassone and postdoctoral scholar Karsten Bruening with SLAC’s SSRL Materials Science Division; Hemamala Karunadsa, assistant professor of chemistry, and graduate students Rebecca Belisle, Rohit Prasanna, and Matthew Smith from Stanford; and Venkatraman Gopalan, professor of materials science and engineering, and graduate student Yakun Yuan from Pennsylvania State University are the other co-authors of the research.
The SLAC, the National Science Foundation, and the DOE Office of Science funded the study. Stanford and SLAC jointly operate SIMES.