• Energy Research

AbstractIn the context of developing transparent near‐IR absorbing dye‐sensitized solar cells, diketopyrrolopyrrole (DPP) cyanine dyes have recently emerged as an alternative to strongly aggregating linear cyanines. In our efforts to increase both the power conversion efficiency (PCE) and the average visible transmittance (AVT), a thienylated version, called TB202, that shows a red‐shifted absorption with respect to our champion dye TB207 was designed. However, the lower energy LUMO level of TB202 brings along a lower driving force (−ΔG) for carrier injection, which we recently identified as the main parameter limiting the PCE to 1.5 % in the best device conditions. In the present paper, we publish a detailed account of the effect of the de‐aggregating cheno‐deoxycholic acid (CDCA) for both TB207 and TB202. Both transient absorption (TAS) and fluorescence up‐conversion (FLUPS) data are presented, which allow to quantitively compare the effect of −ΔG and the CDCA concentration, in terms of the kinetic competition of ensemble averaged carrier injection and monomer‐to‐aggregate energy transfer (ET) rates. A comprehensive picture emerges on how ET is reduced by higher CDCA concentrations, leading in the best device conditions to injection efficiencies in the range of 65 % for TB207 and only 35 % for TB202.

Most photovoltaic (PV) technologies are opaque to maximize visible light absorption. However, see-through solar cells open additional perspectives for PV integration. Looking beyond maximizing visible light harvesting, this work considers the human eye photopic response to optimize a selective near-infrared sensitizer based on a polymethine cyanine structure (VG20-C x ) to render dye-sensitized solar cells (DSSCs) fully transparent and colorless. This peculiarity was achieved by conferring to the dye the ability to strongly and sharply absorb beyond 800 nm (S0-S1 transition) while rejecting the upper S0-S n contributions far in the blue where the human retina is poorly sensitive. When associated with an aggregation-free anatase TiO2 photoanode, the selective NIR-DSSC can display 3.1% power conversion efficiency, up to 76% average visible transmittance (AVT), a value approaching the 78% AVT value of a standard double glazing window while reaching a color rendering index (CRI) of 92.1%. The ultrafast and fast charge transfer processes are herein discussed, clarifying the different relaxation channels from the dye monomer excited states and highlighting the limiting steps to provide future directions to enhance the performances of this nonintrusive NIR-DSSC technology.

AbstractThe development of transparent solar cells extends the applications of photovoltaics by offering the opportunity to substitute the gigantic surface coverage of windows by solar panels to produce electricity. Herein, we report a new family of NIR‐sensitizers based on pyrrolopyrrole cyanine dyes, particularly efficient for the development of fully transparent and colorless dye‐sensitized solar cells since a record efficiency of 2.5 % was achieved with an average visible transmittance (AVT) of 76 % and a color rending index (CRI) of 93.