Solar cells based on perovskites can convert more than 20% of the incident light directly into usable power. However, the reasons behind this high efficiency potential are so far mainly unknown.
In their research into the underlying physical mechanisms, scientists at the Karlsruhe Institute of Technology (KIT) have for the first time detected strips of nanostructures with alternating directions of polarization in the perovskite layers. The structures could serve as transport paths for charge carriers, as the KIT researchers reported in the last Energy & Environmental Science Journal.
The perovskites used in the project “NanoSolar”, financed by the Baden-Württemberg Foundation, are metal-organic compounds with a crystalline structure and outstanding photovoltaic properties. The aim of the multidisciplinary KIT team headed by Dr. Alexander Colsmann is to make the light-absorbing layers of the solar cells more robust to environmental impacts, and to replace the lead contained therein with more environmentally friendly elements. For this purpose, the researchers analyzed perovskite solar cells using a special type of scanning force microscopy, and found ferroelectric nanostructures within the light-absorbing layers.
According to KIT, ferroelectricity means that crystals have an electrical polarization and form regions with the same polarization direction, known as domains. The scientists have now observed that during the thin layer development, the lead iodide perovskite forms approximately 100 nanometer-wide strip-shaped ferroelectric domains with alternating electric fields. These might play a decisive role in the transport of the photogenerated charges out of the solar cell and, thus, explain the special properties of perovskites in photovoltaics. Previous theoretical work had predicted the existence of these nanostructures, but until now no evidence had been found.
By Carl Johannes Muth