On the Underlying Mechanism of Light-Induced Patterning of N719-Stained Photoanodes for "Photovoltaic Photographs"

TL;DR

The paper investigates the mechanism behind light-induced patterning in N719-stained TiO2 photoanodes used for photovoltaic photographs. By combining time-resolved UV-VIS spectroscopy, FTIR, GC-MS, and TOF-SIMS with controlled illumination, the authors demonstrate that TiO2 catalyzes a multi-step degradation of the NCS ligand, beginning with sulfur loss and oxidation to sulfur-oxide species and forming transient NCO- intermediates, while bipyridine remains relatively intact. The spectral blue-shift and protonation/counter-ion changes correlate with visual patterning and performance losses in illuminated regions, and TOF-SIMS maps reveal spatially localized degradation consistent with an oxygen-catalyzed mechanism. These findings provide a mechanistic framework for controlled pv-photo patterning and offer guidance for applying the approach to other classes of solar cells while addressing stability considerations.

Abstract

Recently, "photovoltaic photographs" were proposed as a creative application of photovoltaic technologies, relevant in fields such as architecture or the automotive industry. In this application an image is created by light-induced patterning of the photoactive layer, causing a local change in the appearance of the solar cell. In order to further develop this concept, it is crucial to elucidate the mechanism underlying these local changes. Here, UV-VIS and infrared spectroscopic techniques, as well as Gas Chromatography - Mass Spectrometry and time-of-flight secondary ion mass spectrometry, have been used to investigate the physico-chemical changes induced by this process in the photoactive layer of proof-of-concept photo-patterned dye-sensitized solar cells. We show that, for N719 photovoltaic photographs, TiO2 plays a crucial role and that the dye undergoes a multi-step chemical degradation, related to its isothiocyanate ligand. These insights are of importance for a better understanding of the photo-induced degradation of N719, a more substantiated control of the light-induced patterning process, and to design appropriate light-induced patterning techniques for other classes of solar cells.

Figures (6)

• Figure 1: Left: Photograph of a 100 mm x 100 mm "pv-photo", prepared using N719 and containing an image of a popular japanese cartoon figure. Right: Chemical structure of the N719 dye
• Figure 2: Schematic representation showing the illumination step in the preparation of test anodes for analysis. Light from a Xe short arc lamp was shone onto stained photoanodes (consisting of anatase TiO2 nanoparticles, screenprinted onto fto covered glass) that were partially covered by a mask. The masked area (covered) was minimally exposed to light while the unmasked area (illuminated) showed strong discolouration.
• Figure 3: Normalised time resolved UV-VIS spectra for N719 in ethanol (top) and N719 extracted from a suspension of stained TiO2 nanoparticles (bottom). The colour gradient represents illumination time, with a lighter colour corresponding to longer times. The highlighted blue-shift is perceptible as a change of colour of the solution.
• Figure 4: Left: spectrum of the illuminated (a and b) and covered (c) areas of anodes prepared according to \ref{['sec:methods:ftir']}. The spectrum for the covered area is identical to that of a dark area sample (figure S3). The coloured regions highlight vibrational bands that were identified using the covered area. Right: Chemical structure of the N719 dye, colour coded to link ligands and functional groups to features in the spectrum.
• Figure 5: ToF-SIMS comparison of three areas from N719-dyed TiO_2 anodes: a covered area, an area illuminated for 4 hours, and another illuminated for 22 hours. Spectral intensities are shown for (a) NCS-, (b) SO_4-, and (c) NCO-. The time-dependent nature of the photodegradation is highlighted in (d). The inset shows the absolute peak intensities of Ru(NCS)(NCS)-, Ru(NCS)(NC)-, and Ru(NC)(NC)-, while the main plot presents their relative intensities, calculated as the absolute intensity of each species divided by the absolute intensity of Ru(NCS)(NCS)- in the corresponding area. Error bars indicate 95 % confidence intervals of the means.