Thickness Determination of Dielectric Layer Stacks for In-Line Applications

Duration: October 2012 - May 2015
Contracting Authority / Sponsors German Federal Ministry of Economic Affairs and Energy (BMWi)
© Fraunhofer ISE
Fig. 1: Back surface of a PERC solar cell precursor with a double-layer coating. Its reflectance properties can be used to determine the thickness of the individual layers in-line during production.
© Fraunhofer ISE
Fig. 2: The layer thicknesses determined from reflectance spectra for double-coated PERC precursors agree significantly better with the reference values (from spectral ellipsometry of flat samples with single coatings) than those determined by laser ellipsometry.

The PERC concept (Passivated Emitter and Rear Cell) is a promising approach to increase the efficiency of solar cells and involves coating the back surface of a solar cell, typically with two thin dielectric layers. The layer thickness affects the electrical and optical properties of these solar cells. The usual method to characterize the layers is ellipsometry. At Fraunhofer ISE, a procedure based on spectral reflectance was developed which, in contrast to ellipsometry, allows the thickness of doublelayer thin-film stacks to be determined reliably also on slightly micro-rough surfaces and during a running solar cell production process.

The spectral reflectance of the back surface of PERC solar cell precursors depends essentially on the refractive indices and thicknesses of the deposited dielectric films and can be calculated from these data. This relationship can be used to determine the film thicknesses by fitting the simulated reflectance Rsim to the reflectance Rm measured after film deposition. In the calculation, the refractive indices are assumed to be known and the thickness of both dielectrics is varied until the best fit of Rsim to Rm is obtained.

The method was tested with PERC solar cell precursors. These feature double layers of differing thickness consisting of silicon-rich silicon oxynitride (SiRiON) and silicon nitride (SiNx) (Fig. 2). The film thicknesses were determined with a laser ellipsometer and with the curve-fitting procedure from the spectral reflectance. Reference measurements of single coatings deposited on flat samples were made with a spectral ellipsometer. The thickness values determined with the laser ellipsometer deviate systematically to lower (SiNx) or higher (SiRiON) values, whereas the reflectance-based procedure displays higher accuracy without systematic deviations (Fig. 2). It is thus well suited for determining the thickness of thin coating stacks on PERC solar cells in-line during a running production process.