Non‐destructive lateral mapping of the thickness of the photoactive layer in polymer‐based solar cells

Abstract

Non‐destructive lateral mapping of the thickness of the photoactive layer in poly(3‐hexyl‐thiophene) : 1‐(3‐methoxy‐carbonyl)propyl‐1‐phenyl‐(6,6)C61 (P3HT : PCBM) solar cells is demonstrated. The method employs a spatially resolved (XY) recording of ultraviolet‐visible spectra in reflection geometry at normal incidence, using a dense raster defined by a circular probe spot of 800‐µm diameter. The evaluation of the thickness of the photoactive layer at each raster point employs an algorithm‐driven comparison of the measured absorption spectrum with spectral features, as compiled from the corresponding simulated spectrum. For the robustness of the applied algorithm toward noise in the recorded absorption data to be increased, a new minimum finder algorithm is described and implemented. The thickness evaluation relies on the correct assignment of extrema in the experimental absorption spectra to the corresponding extrema in the simulated absorption spectra, and a new algorithm for this is also implemented and described. For a level of confidence for the method to be established, first thickness mapping is performed for a set of reference samples consisting of P3HT : PCBM spin‐coated on indium tin oxide‐coated float glass substrates. After this, two application examples for solar cells processed either by spin coating or slot die coating of the P3HT : PCBM layer follow. The spin‐coated solar cells have glass as the substrate with the P3HT : PCBM spun at different spinning speeds. The slot die‐coated solar cells were processed on polyethylene terephthalate foil in a roll‐to‐roll experiment involving a continuously changing P3HT : PCBM concentration along the printing direction. Copyright © 2011 John Wiley & Sons, Ltd.

Photochemical stability of conjugated polymers, electron acceptors and blends for polymer solar cells resolved in terms of film thickness and absorbance

“…An optical fiber-based CCD spectrometer (Avantes AvaSpec 1024 with a 400 mm quartz fiber) and a halogen/deuterium light source (Avantes AvaLight-DHc) were used to record the absorption spectra in a transmission geometry in the range of 300 to 900 nm at set intervals based on the approach described in ref. 23. By using collimating lenses adjusted normal to the sample, a parallel light probe was obtained by which a circular area of Ø 3 mm was probed.…”

Large area organic solar cells

“…By using the optical constants of the PCDTBT:PC 70 BM layer it was possible to calculate the thickness from the reflectivity using optical modeling similar to the method reported by Sylvester-Hvid at al. [205]. The optical constants were derived from measurements with a J.…”