Transformed Double-Capped Gold Nanorods in Dye Co-Sensitized Solar Cells for Semitransparent Windows

Abstract: Background: Dye sensitized solar cells (DSSCs) containing two different dyes were recently used forapplications to windows. To enhance the efficiency of this type of solar cells by means of the effect of localizedsurface plasmon resonance (LSPR), we produced gold nanorods (GNRs) with an aspect ratio (a.r.) equal to 3:1and tos 4:1. With an actual window application in mind, and mainly to prevent corrosion by the redox mediator inthe cell, we considered the capping of GNRs before introducing them into the titanium oxide (TiO2) layer of theanode. In particular, we made a double-capping with silica and titania layers for a limited total thickness (i.e.,about 6 nm), while still allowing a significant localized LSPR effect despite the increased distance between goldand dye molecules. We documented the different transformations in dimensions of the two types of capped goldnanorods (c-GNRs) due to the effect of sintering. Our aim was to evaluate the influence that these transformationswould have on the photovoltaic performances of DSSCs.Methods: We added c-GNRs with a ratio of 2% in w/w to a transparent semiconductor paste, which was doctorbladed on the photoanodes of the co-sensitized solar cells made with commercially available organic sensitizers(L1 or L0) and the squaraine SQ2, which acted as a co-sensitizer. The films had a thickness of about 6 ?m andwere sintered at 450°C. We used transmission electron microscopy (TEM) analysis to document the transformations,absorbance and absorptance spectra in order to control the effects of these modifications, and transmittancespectra for evaluating the see-through effects. We performed current-voltage, external quantum efficiency(EQE%) and electrochemical impedance spectroscopy (EIS) characterizations of the DSSCs.Results: The semiconductor films with c-GNRs that had GNRs with an a.r. equal to 4:1 (c-GNRs 4:1) had lowerabsorption and higher transmission as compared to those with GNRs a.r equal to 3:1 (c-GNRs 3:1). Only the c-GNRs 3:1, which retained a similar shape and an a.r. equal to 1.5 after sintering, produced an enhancement in thepower conversion efficiency ?% (23%), current Jsc (8%), and voltage Voc (2.5%) when used in combination withthe dye cocktail containing the organic dye L1. On the contrary, the presence of c-GNRs 4:1 negatively influencedthe photovoltaic performances of the cells containing this dye cocktail. The same occurred for both types ofc-GNRs with the dye cocktail containing L0.Conclusion: The use of c-GNRs 3:1 could actually improve the efficiency of co-sensitized DSSCs. On the otherhand, the transformed dimensions of the c-GNRs 4:1 negatively influenced the photovoltaic characteristics whenwe used the same concentration of nanoparticles, and a semiconductor paste in small grains (i.e., about 20 nm).We attributed this fact both to a reduced penetration of the dyes in the films and to an inferior plasmonic effect.