The differential impact of light soaking and dark annealing on hydrogen species during a LeTID cycle in boron-doped FZ silicon: A quantitative FT-IR study

Hydrogen is recognized as the key species responsible for light- and elevated- temperature-induced degradation (LeTID) in silicon. The simplest hydrogen species detectable is acceptor-hydrogen, HB in boron-doped material. Many studies track HB trough resistivity measurements and infer the evolution of the hydrogen molecule, H2, often excluding other potentially relevant hydrogen-related species from consideration. This study presents a Fourier transform-infrared analysis of the evolution of the hydrogen species, HB, H2, the hydrogen dimer H*2, and the vacancy-hydrogen complexes VH4 and V2H6, during dark annealing and light soaking. The findings are compared to changes in lifetime-equivalent defect density observed over a LeTID cycle. Before solar cell processing the hydrogen species found are HB, H2, V2H6, and VH4. After processing, H*2 is formed, HB and H2 increase in concentration, while VH4 and V2H6 disappear. Light soaking at 75 ◦C and 1 sun-equivalent and dark annealing at 175 ◦C both reduce H2 and H*2 concentrations, while HB concentration increases with dark annealing but decreases during light soaking. Extended dark annealing results in a re-appearance of VH4. A considerable role of H*2 and VH4 in the hydrogen balance in a LeTID cycle is demonstrated. Finally, several unidentified peaks are observed in the wavenumber range of 3000 cm-1 to 3300 cm-1, showing sensitivity to different treatment conditions. Specifically, peaks at 3029.1 cm-1, 3102.1 cm-1, and 3238.9 cm-1 grow only when light soaking took place as final step. Conversely, a peak at 3025.8 cm-1 increases under dark annealing conditions and decreases with prolonged light soaking.