“Purity” is one of the many myths of perfection, suggesting that absolute conformity is the only way to produce supreme value. Just as that claim is false for human populations, new research in physics at the Institute of Science and Technology Austria (ISTA) shows it’s also false for solar cells.
That’s because while silicon photovoltaic systems demand a smoothly uniform structure, perovskite cells contain natural networks of structural “defects” which are clearly assets for efficient solar-electric collection through long-term charge transport. In a paper that was recently published in the journal Nature Communications, physicists Dmytro Rak, Zhanybek Alpichshev, and co-authors at ISTA explain the mechanism that is accelerating perovskite cells in the efficiency race against the pack-leader for decades – silicon cells.
Lead-halide perovskite cells processed in solutions are remarkably good at harvesting electricity from sunlight. Unlike diva-princess silicon cells that won’t produce electricity unless manufactured as ultra-pure single-crystal wafers, rough-and-tough Rosie-the-Riveter perovskite cells just exit their simple, inexpensive solution-based birth-chamber before powering whatever we need.
According to Rak, the ISTA team has revealed “the first physical explanation of these materials while accounting for most – if not all – of their documented properties,” which could soon move perovskite-based solar cells from laboratories into industry and beyond.
This new frontier is a dramatic change for lead-halide perovskites, a range of compounds which were a curiosity researchers discovered back in the 1970s and then promptly ignored until the early 2010s. Similar in structure to regular oxide-compound perovskites, lead-halide perovskites can exist as stable crystalline structures that are both organic and inorganic.
But around 15 years ago, scientists finally learned the many previously unguessed “talents” of lead-halide perovskites that had existed all along, including superb photovoltaic efficiency; flexible application for imaging, X-ray detection, and LEDs; and as Alpichshev notes, “astounding quantum properties, such as quantum coherence at room temperature.” His ISTA team researches complex materials and their complex condensed matter physics phenomena.
So, what makes the ISTA discovery so significant? Solar cells create electrical current by first converting light into electrons (negative charges) and “holes” (positive charges), and then collecting them with electrodes. Given the size of electrons, the distance for their travel to the electrodes is enormous – in relative terms, like that of Frodo’s path from Hobbiton to Mount Doom. And just as Middle Earth offers no smooth, straight path for dropping rings in volcanoes, the micron-scale trail in a solar cell offers plenty of places for wee electrons to get trapped and lost forever.
In a silicon-based cell, the path to an electrode is “paved” so uniformly that electrons can swiftly reach their target. But because solution-manufactured lead-halide perovskite cells are full of “pot-holes,” should they be they disastrous for electron journeys?
Alpichshev’s team conjectured that inside perovskites, electrons and holes can “marry” as excitons that perovskites immediately “divorce” using previously unknown internal forces. So, using nonlinear optical methods, they repeatedly placed electrons and holes inside a perovskite sample and observed finite current flowing in the same direction without applying voltage, meaning that “even deep inside single crystals of unmodified, as-grown perovskites,” says Alpichshev, “there are internal forces that separate opposite charges” at microscopic “domain wall” networks.
As Rak explains, when an exciton exists “near a domain wall, the local electric field pulls the electron and the hole apart, placing them on opposite sides of the wall.” But despite that “divorce,” the perovskite network of “highways for charge carriers” allow excellent energy harvesting.
As a chemist, Rak understood how to verify that theory: by introducing silver ions as “markers” for electrochemically staining domain walls without destroying them, and turning the ionic silver into metallic silver. As Alpichshev explains, this ISTA invention “is much like angiography in living tissues – except that we are examining the micro-structure of a crystal.”
Rak says the discovery is a significant breakthrough, allowing scientists “to reconcile many previously conflicting observations about lead-halide perovskites, resolving a long-standing debate about the source of their superior energy-harvesting efficiency.”
If ISTA’s results allow engineers to boost perovskite solar-cell efficiency while keep costs low, every community on earth needing electricity – especially those with the least access – could experience extraordinary benefits for production and quality of life.
Source: ISTA
