A fast technique offers improved studies of semiconductor charge dynamics

The use of new semiconducting materials is spawning high-performance solar cells at lower costs. Understanding the potential performance of these new semiconductors means understanding the dynamics of their charge carriers.

Existing techniques to screen solar cell materials and study semiconductor charge dynamics typically use high-fluence pulsed optical sources. Labram et al. demonstrated a technique that instead uses steady-state illumination, which they call steady-state microwave conductivity.

Steady-state microwave conductivity is contactless, which makes it faster than standard techniques because it doesn’t require constructing a device or optimizing the microstructure of a semiconductor. Instead, the authors simply illuminate their sample using a green light-emitting diode in a microwave cavity.

While the optimization of solar cells using new materials may take several months of experimental work, the authors’ technique provides immediate and unambiguous information about a semiconductor. Electron mobility traditionally requires electrical contacts to measure. This contactless technique gives a proxy for the mobility-lifetime product of charge carriers, where the lifetime of carriers is the average time it takes for a specific type of charge carrier to recombine.

The authors were surprised to find that the mobility-lifetime product’s proxy in the sample they studied, a hybrid halide perovskite called methylammonium lead iodide, was strongly dependent on optical power density.

Labram said that their technique is important to the field of solar cell research not only because of its speed, simplicity and versatility, but also because of its ability to measure charge dynamics under conditions closer to real operating conditions of solar cells, unlike pulsed-laser-based techniques, which require charge carrier densities significantly higher than in operating solar cells.

Source: “Steady-state microwave conductivity reveals mobility-lifetime product in methylammonium lead iodide,” by John G. Labram, Erin E. Perry, Naveen R. Venkatesan, and Michael L. Chabinyc, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5041959 .