Hydrogen is an important element in crystalline silicon used for solar cells because it can passivate and neutralize a wide variety of structural defects and metal impurities that lower a solar cell’s efficiency. Recently, however, hydrogen has been connected to negative effects, the most prominent ones being so-called light induced degradation (LID) mechanisms. Multicrystalline silicon solar cells in particular suffer from light and elevated temperature-induced degradation (LeTID), which can cause a drop in the efficiency by up to 10%. Although the origin of LeTID is unknown, the effect is observed only in cells that have been intentionally hydrogenated – an observation that suggests hydrogen, either by itself or in a complex with another defect, is responsible. However, hydrogen is a small and quite elusive atom, and therefore quite difficult to observe directly. It is also often present in low concentrations (≤10¹⁴ atoms/cm^-3) and exists in configurations that are difficult to observe with many characterization techniques.

Researchers at the University of Oslo and the Institute for Energy Technology have worked together to develop a method to detect hydrogen-related defects in multicrystalline silicon wafers using Fourier Transform-Infrared (FT-IR) spectroscopy. FT-IR spectroscopy is a non-destructive technique that characterizes impurity atoms by their vibrations. It is already used by the solar-cell industry for routine quantification of oxygen and carbon impurities in silicon. Unlike these standard measurements, which are performed at room temperature, detection of hydrogen impurities requires more sensitive measurements at cryogenic temperatures, typically 5.0 K. Furthermore, previous FT-IR studies of hydrogen-related defects were limited to slabs 1-2 cm thick, which are difficult to compare with commercial silicon wafers, which are only ~200 µm thick.

In their recent publication in the Journal of Applied Physics, the researchers demonstrated two key findings. First, they showed that they could prepare a stack of thin (180 µm thick) commercially-available multicrystalline silicon wafers suitable for IR measurements.Since the new approach can be used on wafers, it opens the possibility to study hydrogen complexes and their concentrations throughout the solar cell process, and even after illumination and heating. Thus, new insights in the behavior of hydrogen during both production of, and operation of solar cells can be gained. Second, they combined IR spectroscopy with carrier lifetime measurements in wafers subjected to a degradation-regeneration cycle to gain insight into the hydrogen-related point defects that could be connected to the LeTID process. In fact, they showed that the concentrations of complexes formed between hydrogen and either boron or gallium decreased throughout the entire LeTID process. They also showed that a significant amount of hydrogen is bound to carbon impurities, which should be considered for LeTID. Although their results did not allow them to pinpoint a specific defect as the cause of LeTID, the method they demonstrated can be used to detect and explore the behavior of hydrogen-related point defects for other types of silicon wafers (monocrystalline) and under conditions corresponding to other light-induced degradation processes.

The article can be found here: https://doi.org/10.1063/1.5142476 

Text written by Phillip Michael Weiser, Picture by Robert Karsthof