Photoinduced Electron Injection from Zinc Phthalocyanines into Zinc Oxide Nanorods: Aggregation Effects

Abstract

Phthalocyanines (Pc) are well-known light-harvesting compounds. However, despite the tremendous efforts on phthalocyanine synthesis, the achieved energy conversion efficiencies for Pc-based dyesensitized solar cells are moderate. To cast light on the factors reducing the conversion efficiency, we have undertaken a time-resolved spectroscopy study of the primary photoinduced reactions at a semiconductor-Pc interface. ZnO nanorods were chosen as a model semiconductor substrate with enhanced specific surface area. The use of a nanostructured oxide surface allows to extend the semiconductor-dye interface with a hole transporting layer (spiro-MeOTAD) in a controlled way, making the studied system closer to a solid-state dye-sensitized solar cell. Four zinc phthalocyanines are compared in this study. The compounds are equipped with bulky peripheral groups designed to reduce the self-aggregation of the Pcs. Almost no signs of aggregation can be observed from the absorption spectra of the Pcs assembled on a ZnO surface. Nevertheless, the time-resolved spectroscopy indicates that there are inter-Pc charge separation−recombination processes in the time frame of 1−100 ps. This may reduce the electron injection efficiency into the ZnO by more than 50%, pointing out to a remaining aggregation effect. Surprisingly, the electron injection time does not correlate with the length of the linker connecting the Pc to ZnO. A correlation between the electron injection time and the "bulkiness" of the peripheral groups was observed. This correlation is further discussed with the use of computational modeling of the Pc arrangements on the ZnO surface.

Photoreactions of Macrocyclic Dyes on (1010) Wurtzite Surface – Interplay Between Conformation and Electronic Effects

“…2. Results of DFT modeling are in good agreement with the ones obtained by molecular dynamics simulations performed in the previous study [7]. Out of the four phthalocyanines presented in Tabl.…”

“…Out of compounds 1 -4, compound 3 is the least protected from intermolecular interactions and, therefore, is the most aggregated one. This is corroborated by the time resolved spectroscopy measurements [7], which exhibit the appearance of a sharp transient absorption band at 995 nm for ZnO samples modified with compound 3. This band corresponds to the Pc anion and is not observed for other samples.…”

“…Since modeled molecules with shortest edgeto-surface distances exhibit the fastest electron injection times observed experimentally, we can deduce that the electron transfer reactions are largely controlled by the distance between the dye and the semiconductor surface plane. As noted previously [7], the electron injection time does not correlate with the length of the linker connecting the Pc to ZnO. Indeed, the electron injection time constants for 1 and 2 are practically the same, even though 2 has a longer linker, and one would expect a longer distance be- tween the Pc core and the ZnO surface, leading to a slower injection.…”

“…1. The synthetic details of the compounds differing by the bulky peripheral groups and the length of a linker together with the routine of their spectroscopic measurements were described elsewhere [7]. The role of the linker on the electron transfer occurring in hybrid systems has been the subject of intense investigations for the past few decades [28,29].…”

“…Ones of the most developed and studied hybrids based on these nanostructures are their conjugation with macrocyclic dyes such as porphyrin and phthalocyanine (Pc) derivatives [7][8][9]. It should be noted, however, that hybrids with phthalocyanines are relatively unexplored due to the moderate energy conversion efficiencies achieved so far for Pcbased dye sensitized solar cells.…”