Devices such as solar cells, transistors, detectors, sensors, and LEDs are made with semiconductor materials that have charge carriers that are released when the carriers are hit with light. Determining the transport properties of a semiconductor’s charge carriers can help researchers predict how effective the material will be for an optoelectronics application. Until now, the parameters of the transport properties in minority and majority charge carriers in semiconductors have been determined by using different measurement methods for each type of charge. To enable efficient, complete characterization of semiconductors, scientists at Helmholtz-Zentrum Berlin (HZB) developed a method that records 14 different parameters of transport properties in negative and positive charge carriers in a single measurement. The new method, which the researchers call Constant Light-Induced Magneto-Transport (CLIMAT), is based on the Hall effect. According to the researchers, CLIMAT could help scientists accurately assess new materials for optoelectronic devices in far less time than existing methods. The bright spheres represent the bound charge carriers (negative and positive) in the material. A light beam separates these charges, which are then deflected in different ways in the applied magnetic field. With the Constant Light-Induced Magneto-Transport (CLIMAT) method, 14 different parameters of the transport properties in semiconductors can be measured with a single measurement, including density, lifetime, diffusion lengths, and mobility. Courtesy of Laura Canil/HZB. CLIMAT combines light, electrical current, and a magnetic field to assess the transport properties of holes — areas in which electrons are missing, which behave like positive charge carriers — and electrons, which are negative charge carriers, and provide insight into the individual behavior of these carriers. It uses a magnetic field, applied vertically through the sample, and a constant light source for charge separation. The charge carriers move along an electric field and are deflected by the magnetic field according to their mass, mobility, and other properties. CLIMAT resolves the electron and hole concentration by using conductivity and Hall coefficient values with analytical equations. It resolves electron and hole signals to allow a total of 14 properties to be identified from the signals. CLIMAT can resolve electron and hole mobility, lifetime, diffusion coefficient and length, and other transport properties. The use of light extends the range of materials that can be probed by CLIMAT (and by Hall measurements in general). The free carrier injection and sample conductivity are controlled by the light in CLIMAT; thus, Hall effect measurements are not limited by high resistivity, bandgap energy, or thin-film thickness. The researchers also developed correction methods for using CLIMAT with materials affected by grain boundaries and parasitic conductivity. The researchers used CLIMAT to characterize silicon and metal halide perovskites — two materials with substantial differences in charge transport properties. They used experimental data to fit a simulation model that incorporated charge generation and recombination processes. This enabled the researchers to extract crucial parameters associated with traps responsible for charge losses in the material systems. Insight into charge dynamics also helped the researchers identify strategies for reducing charge recombination and enhancing the efficiency of semiconductor devices. To demonstrate the broad applicability of CLIMAT, research teams at HZB, the University of Potsdam, and other institutions in the U.S. and Europe used it to characterize 12 different semiconductor materials, including silicon, halide perovskite films, organic semiconductors such as Y6, semi-insulators, self-assembled monolayers, and nanoparticles. Through these experiments, the researchers demonstrated the versatility of CLIMAT and its potential to contribute to the effective use of semiconductors in applications ranging from solar cells and LEDs to memory devices, sensors, and transistors. “CLIMAT thus provides a comprehensive insight into the complicated mechanisms of charge transport, both positive and negative charge carriers, with a single measurement,” researcher Artem Musiienko said. “This enables us to evaluate new types of semiconductor materials much more quickly, for example, for their suitability as solar cells or for other applications.” The researchers reported that the charge transport properties obtained from CLIMAT in the experiments agreed with the results obtained from photoluminescence quantum yield (PLQY), time-resolved photoluminescence, and time-of-flight techniques. The ability to access 14 different material parameters with one measurement method could open the way for an in-depth exploration of the mechanisms that limit the efficiency and functionality of material structures. The CLIMAT method was approved for patenting by the European Patent Office in 2024. “Negotiations are currently underway with companies about licensing our method,” Musiienko said. The researchers plan to further develop the technology into a compact measuring device about the size of a notebook. The research was published in Nature Communications (www.doi.org/10.1038/s41467-023-44418-1).
