Researchers at the Karlsruhe Institute of Technology (KIT) in Germany have detected strips of nanostructures with alternating directions of polarization in the perovskite layers, that might serve as transport paths for charge carriers.
The perovskites used by the KIT team were defined as metal organic compounds with a special crystal structure and excellent photovoltaic properties. An interdisciplinary team of researchers analyzed perovskite solar cells by means of piezoresponse force microscopy and found ferroelectric nanostructures in the light-absorbing layers. Ferroelectric crystals form domains of identical electrical polarization direction. The KIT scientists observed that, during thin-layer development, lead iodide perovskites form about 100 nm wide stripes of ferroelectric domains with alternating electric fields. This alternating electric polarization in the material might play an important role in the transport of photogenerated charges out of the solar cell and, hence, explain the special photovoltaic properties of perovskites.
"These ferroelectric structures of a few 10 nm in size might form nearly perfectly separated transport paths for charge carriers in the solar cell," the team explains. Researchers have been looking for such structures for years in order to improve the efficiency of solar cells. "In perovskite solar cells, these structures seem to develop by themselves under certain conditions". Former theoretical studies of other researchers already predicted these advantageous nanostructures. So far, however, no evidence has been found. The scientists of KIT studied ferroelectricity of lead halogenide perovskites under the "NanoSolar" project funded by the Baden-Württemberg Foundation.