Organic-inorganic perovskite materials are key components of the new generation of solar cells. Understanding the properties of these materials is important for improving the lifetime and quality of solar cells. Led by Professor Yabing Qi, researchers from the Energy Materials and Surface Sciences (EMSS) Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), in collaboration with Professor Youyong Li’s group from Soochow University (China) and Professor Nam-Gyu Park’s group from Sungkyunkwan University (Korea), recently reported in theJournal of the American Chemical Society the first atomic resolution study of organic-inorganic perovskite.

Perovskites are a class of materials with the general chemical formula ABX3. A and B are positive ions bound by negative X ions. Organic-inorganic perovskites used in solar cells are usually methylammonium lead halides (CH₃NH₃PbX₃, where X is bromine, iodine or chlorine). The OIST scientists used a scanning tunneling microscope to create topographic images of the surface of a single crystal of methylammonium lead bromide (CH₃NH₃PbBr₃).

Quantum Tunneling

The scanning tunneling microscope uses a conducting tip that moves across the crystal’s surface in a manner very similar to a finger moving across a Braille sign. While the bumps in Braille signs are a few millimeters apart, the microscope detects bumps that are more than a million times smaller (atoms and molecules). This is achieved by the quantum tunneling effect, which is the ability of an electron to pass through a barrier. The probability of an electron passing between the material surface and the tip depends on the distance between the two. The resulting atomic-resolution topographic images reveal positions and orientations of atoms and molecules, and also provide a detailed look at structural defects in the surface.

“At room temperature, atoms and molecules are quite mobile, so we decided to freeze the crystal to almost absolute zero (-269ºC) to get a good picture of its atomic structure,” says Robin Ohmann, Ph.D., a member of the EMSS Unit and the first author of the paper. The crystal was cut and studied in a vacuum to avoid surface contamination. Ohmann’s colleagues from Soochow University calculated atomic structures using principles of quantum physics and then compared them with scanning tunneling microscopy data.

The researchers discovered that methylammonium molecules can rotate and that they favor specific orientations that lead to two types of surface structures with distinctly different properties. Apart from rotation, these molecules affect positions of neighboring bromine ions, further altering the atomic structure. Since the structure dictates the electronic properties of the material, the geometric positions of atoms are essential for the understanding of solar cells.

Scanning tunneling microscope images also reveal local imperfections caused by dislocations of molecules and ions and, probably, missing atoms. These imperfections may affect device performance by changing electrical properties such as conductivity, for example.

The structure of perovskite materials is temperature-sensitive, and the observed structure of the frozen crystal might not be fully identical to the structure at room temperature. However, the comprehensive description of perovskite crystals at the atomic level paves the way to better understanding of their behavior under real-life conditions. The current findings shed light on molecule-ion interplay on the surface of an organic-inorganic crystal and should help to improve designs of future solar cells. The next goal of the researchers is to examine interactions between perovskites and other molecules (e.g., water molecules that are known to interfere with the performance of solar cells).