Despite being riddled with impurities and defects, solution-processed lead-halide perovskites are surprisingly efficient at converting solar energy into electricity. Their efficiency is approaching that of silicon-based solar cells, the industry standard. In a new study published in Nature Communications, physicists at the Institute of Science and Technology Austria (ISTA) present a comprehensive explanation of the mechanism behind perovskite efficiency that has long perplexed researchers.
How can a device assembled with minimal sophistication rival state-of-the-art technology perfected over decades? Over the past 15 years, materials research has witnessed the rise of lead-halide-based perovskites as prospective next-generation solar-cell materials. The puzzle is that despite similar performance, perovskite solar cells are fabricated using inexpensive solution-based techniques, while the industry-standard silicon cells require ultra-pure single-crystal wafers.
Now, postdoc Dmytro Rak and assistant professor Zhanybek Alpichshev at the Institute of Science and Technology Austria (ISTA) have uncovered the mechanism behind the unique photovoltaic properties of perovskites. Their key finding is that while silicon-based technology relies on the absence of impurities, the opposite is true in perovskites: It is the natural network of structural defects in these materials that enables the long-range charge transport necessary for efficient photovoltaic energy harvesting. “Our work provides the first physical explanation of these materials while accounting for most–if not all–of their documented properties,” says Rak. The results could accelerate the transition of next-generation perovskite-based solar cells from the lab to real-world applications.
Perovskites: From obscurity to the limelight
“Lead-halide perovskites” is an umbrella term for a class of compounds discovered in the 1970s. They were named for their superficial structural similarity to perovskites, a broad family of oxide compounds that play a prominent role in materials science. However, apart from their curious ability to form stable hybrid organic–inorganic crystalline structures, lead-halide perovskites did not initially attract broad interest. After standard characterization, they were catalogued and largely forgotten.
However, in the early 2010s, researchers realized that these materials exhibit exceptional photovoltaic performance. Perovskites also proved to be excellent materials for LEDs as well as X-ray detection and imaging. “In addition, these materials exhibit astounding quantum properties, such as quantum coherence at room temperature,” explains Alpichshev, whose group at ISTA investigates complex condensed matter physics phenomena in complex materials.
Fundamentally different solar cell technologies
An efficient solar cell must absorb incident light and convert it effectively into charges—a negatively charged electron and a positively charged “hole.” These charges must then be collected at the solar cell electrodes to produce usable current. This is where it becomes challenging: charges must travel hundreds of microns—equivalent to hundreds of kilometers on a human scale—without getting trapped along the way.
In silicon-based technology, this problem is solved by making the solar-harvesting medium almost free of defects that could trap charges before they reach the collection electrodes. What is unusual about perovskite devices is that, being solution-grown, they are filled with defects. How can charges in such an environment travel long distances to be extracted as useful current? Moreover, why do they even persist long enough to do so?
From hypothesis to an image: Silver ‘angiography’
There is solid evidence that once electrons and holes in perovskites form a bound state—an exciton—they recombine very quickly. In light of this, the observation that electrons and holes remain separated for extended periods inside the materials becomes even more puzzling. To rationalize this apparent paradox, the ISTA researchers conjectured that unaccounted-for internal forces within perovskites must rip the nascent electron-hole pairs apart and prevent them from recombining.
To test this hypothesis, the team introduced electrons and holes deep inside the bulk of a perovskite sample using nonlinear optical methods. This allowed them to detect a finite current flowing in the exact same direction in the material each time a new portion of electrons and holes was introduced—even in the absence of any applied voltage. “This observation clearly indicated that even deep inside single crystals of unmodified, as-grown perovskites, there are internal forces that separate opposite charges,” says Alpichshev.
However, previous characterizations of perovskites had determined that such behavior is incompatible with their intrinsic crystal structure. To resolve this contradiction, the ISTA researchers further proposed that charge separation does not occur uniformly across the sample, but is instead localized at so-called “domain walls”—sites of modified structure that can form microscopic networks spanning the entire sample.
But how could this conjecture be confirmed? How can such a domain-wall network be visualized deep inside the bulk, given that most local probes are sensitive only to the surface, where properties can differ significantly?
To overcome this challenge, Rak drew on his training as a chemist. Noting that perovskites are also good ionic conductors, he asked whether introducing some “marker” ions could be used to highlight domain walls non-destructively. To find out, he developed a new electrochemical staining technique to visualize the material’s domain-wall structure: he made silver ions diffuse into the bulk of the perovskite crystal, where they would preferentially accumulate at domain walls. The ions were then electrochemically transformed into metallic silver, allowing scientists to directly visualize the network running through the entire depth of the materials under a microscope. “This qualitative technique, invented and implemented at ISTA, is much like angiography in living tissues—except that we are examining the micro-structure of a crystal,” says Alpichshev.
Highways for electrons
According to Rak, realizing that a natural network of charge-separating domain walls densely spans the entire bulk of perovskites was a game-changer. As he explains, “If an electron-hole pair is created near a domain wall, the local electric field pulls the electron and the hole apart, placing them on opposite sides of the wall. Unable to recombine immediately, they can drift along the domain walls for what seems like eons on a charge carrier’s timescale and travel long distances.” Thus, the team demonstrated the existence of what they call “highways for charge carriers” inside perovskites. These explain the remarkable charge-transport properties that make perovskites so effective for energy harvesting.
The authors emphasize that the present work provides the first comprehensive and coherent physical explanation of perovskites. “With this comprehensive picture, we are finally able to reconcile many previously conflicting observations about lead-halide perovskites, resolving a long-standing debate about the source of their superior energy-harvesting efficiency,” says Rak.
To date, research has mostly focused on tuning the chemical composition of perovskites, with limited success. Now, the ISTA team’s findings could help researchers engineer perovskites to boost their efficiency without compromising their low-cost production process—heralding the next generation of solar cells.
END