On-line Monitoring of Photovoltaics Production

Abstract

Two approaches are reviewed for the application of spectroscopic ellipsometry (SE) to on-line monitoring of thin film photovoltaics (PV) production. In the first approach, through-the-glass SE is applied for serial point-by-point measurements spanning the area of a thin film PV panel 60 cm × 120 cm in size. An ellipsometer detection system is used that incorporates two one-dimensional detector arrays for spectroscopy over a wide photon energy range (0.75–3.5 eV, limited by glass absorption at high energies). The PV panel in this review is fabricated starting from soda-lime glass with four oxide layers deposited on its surface, including the transparent top contact. A CdS/CdTe semiconductor bilayer is deposited subsequently on the top contact, functioning as the PV heterojunction. In the on-line analysis configuration, the coated glass panel moves along a roller conveyer with the film side facing up and passes a station designed for on-line mapping by SE. The polarization generation and detection arms of the ellipsometer located beneath the panel scan from side to side and acquire SE data in a through-the-glass measurement mode. In this approach, a maximum of ~30 locations can be measured in the one minute time period required for the 120 cm long panel to travel by the SE station; the largest fraction of the time is consumed by ellipsometer translation. The effective thickness of CdS (or CdS material volume/area), which includes bulk and interface layer components, is deduced in SE data analysis. This thickness is found to be a robust parameter that can be used in modeling to predict photo-generated charge carrier collection for the CdTe PV modules. The second approach for on-line monitoring reviewed here employs an instrument with an expanded beam for line imaging across a PV substrate/film-stack structure with a maximum image width of 15 cm. In this approach, a detection system is used incorporating a two-dimensional detector array; the two array indices are exploited for spectroscopy (1.3–3.3 eV) and line imaging in parallel. Thus, imaging width-wise and mapping length-wise is performed without ellipsometer translation, enabling high speed multilayer uniformity evaluations in flexible roll-to-roll PV production. The application reviewed here involves film-side analysis of multilayer fabrication on a moving length of 12.7 cm wide flexible polyimide foil substrate mounted within a cassette for roll-to-roll deposition. Maps are acquired in situ after deposition of individual Ag and ZnO layers, functioning together as the back reflector and back contact, as well as after deposition of n-type doped hydrogenated amorphous silicon (a-Si:H n-layer) as a component of a thin film a-Si:H n-i-p solar cell structure. Areas of the flexible coated PV panels up to 12 cm × 45 cm in size were characterized to determine layer thicknesses and optical properties. Parametric expressions incorporating Drude, critical point oscillator, and modified Lorentz oscillator terms were employed to describe the complex dielectric functions of thin film Ag and ZnO, and the a-Si:H n-layer, respectively. Currently, ~30 point line images can be collected every 20 cm of length when using an average 120 cm/min substrate speed. Prospects exist for increasing length-wise resolution significantly to ~0.5 cm, using high speed detection schemes demonstrated previously.