On the underlying mechanism of light-induced patterning of N719-stained photoanodes for "photovoltaic photographs”

Abstract

Recently, "photovoltaic photographs" were proposed as a creative application of photovoltaic technologies, relevant in fields such as architecture. A high-resolution image is created in solar cells by light-induced patterning of the photoactive layer, causing a local change in the appearance of the solar cell. Here, we investigate the physico-chemical changes induced by this process in the photoactive layer of proof-of-concept N719 photovoltaic photographs, to better understand the underlying mechanisms and further develop the concept. By combining a variety of techniques, we show a previously unreported multi-step degradation of the isothiocyanate ligand of the dye, correlated to visual color changes. Time-resolved UV-VIS spectroscopy revealed the catalytic role played by TiO2, causing a blueshift (35 nm) in the dye's 495 nm metal-to-ligand charge-transfer peaks within 10 h. This is confirmed through infrared spectroscopy showing a 24 cm-1 shift to smaller wavenumbers of the CN-stretching vibration. Finally, time-of-flight secondary ion mass spectrometry (ToF-SIMS) reveals the multi-step nature of the degradation, through the transient increase of an NCO-signal. These insights are of importance for a better understanding of the photo-induced degradation of N719, a more substantiated control of the patterning process, and to design appropriate light-induced patterning techniques for other classes of solar cells.