Inline quality control of passivation stacks in high-efficiency silicon solar cell concepts: Thickness determination from spectro-photometry

Stacks of dielectric thin films are widely used for passivated emitter and rear solar cells based on crystalline silicon. The film thicknesses influence both the electrical and optical properties of these devices. The standard offline method for the optical characterization of these films is ellipsometry. Since the spectral reflectance of solar cells and precursors can be measured inline using spectro-photometry, we evaluate a method for the determination of the passivation layer thicknesses from spectral reflectance data in the UV-Vis and compare it to ellipsometric offline-measurements. The crucial point is to exploit the whole reflectance spectrum instead of utilizing only a single wavelength where a reflectance minimum occurs. It is shown that comparable film thicknesses can be determined from both spectro-photometry and ellipsometry. Furthermore, spectro-photometry is more robust on rough samples. Even film thicknesses below 30 nm can be determined if the reflectance data extends to the ultraviolet part of the spectrum