/11th July 2017, Sandra Henderson/, On their quest to better understand and thus be able to optimize perovskite solar cells, researchers from the Karlsruhe Institute of Technology (KIT) in Germany have discovered in the perovskite layers strips of nanostructures with alternating directions of polarization that could serve as transport paths for charge carriers. “Perovskite solar cells are widely considered the shooting stars of modern photovoltaics,” says Dr Alexander Colsmann, head of KIT’s Organic Photovoltaics Group, adding that their underlying working principles are not yet well understood, however. Thus, the KIT team set out to investigate the material properties and working principles of organometal halide — or more specifically, methylammonium lead iodide (MAPbI3) — perovskite thin-films.
“Initially, we were looking for ways to determine the orientation and structure of small crystal grains in the polycrystalline perovskite layers in the solar cells,” he says, noting that previous theoretic works have predicted ferroelectric domains in methyl ammonium lead iodide thin-films.
Recording ferroelectric nanostripes with atomic force microscopy
Colsmann and his team studied the perovskite thin-films using piezoresponse force microscopy, an atomic force microscopy measurement technique suited to probe ferroelectric properties. “After extensive studies and parameter variations, we recorded images of distinct ferroelectric domains which are nanostructures of the same electrical polarization direction,” he reports.
A significant discovery on the quest to better perovskite solar cells
Researchers have been looking for such nanostructures in an effort to hopefully make perovskite solar cells’ light-absorbing layers more robust to environmental impacts and to be able to substitute the lead content with environmentally friendlier elements. “It has been an extensive discussion within the community, whether or not MAPbI3 is ferroelectric,” Colsmann shares. “We think that we have provided unambiguous proof that MAPbI3 is indeed ferroelectric,” he reveals. “We further demonstrated alternating ferroelectric orientation in sufficiently large crystals, forming stripes all over perovskite crystals.”
The organic photovoltaics expert explains that according to previous simulations of other research groups, the alternating electrical polarization may play an important role for the transport of photogenerated charge carriers, enabling their effective extraction from the solar cells. This ferroelectric pattern of alternating electrical fields may enable positive and negative charge carriers to propagate along separated pathways. “Hence,” Colsmann concludes, “recombination losses would be very low which in turn would explain the remarkable photovoltaic properties of MAPbI3.”
Designing improved perovskite solar cells — the study’s impact
The research advance achieved by Colsmann and his colleagues at KIT could help scientists design the next and improved generation of perovskite solar cells. “This discovery helps the photovoltaic community to better understand the working mechanism of perovskite solar cells,” he agrees. “Our findings may furthermore define an important criterion for future non-toxic photovoltaic materials: Ferroelectricity may be a material property to enable high-performance power conversion.”
What is more, Colsman argues if alternating ferroelectric domains are pivotal to high power conversion efficiencies, scientist can now target optimized layer deposition techniques to yield such striped nano-domains.
When asked what excites him personally the most about the discovery of these nanostripes in perovskite solar cells, Colsmann replies, “Nature has a lot of fascinating mechanisms in its toolbox to make things work very efficiently. The nanostripes look as if they were designed on purpose in the lab.”
Colsmann says next steps for his team will include manipulation of the ferroelectric domains to further enhance the device performance and reliability.
The research is detailed in the article “Ferroelectric domains in methylammonium lead iodide perovskite thin-films,” published in Energy & Environmental Science Journal.
Written by Sandra Henderson, Research Editor, Solar Novus Today
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