Near-IR dyes for transparent dye-sensitized solar cells: The effects of small driving forces (−ΔG) and excited state quenching by energy transfer to aggregates studied by femtosecond differential absorption and fluorescence spectroscopy for two DPP cyanine dyes.

Abstract

In 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.