Photoiduced Electron Transfer Across Phthalocyanine Layers on ZnO Semiconductor
Authors: Tkachenko Nikolai V. 
Published: February 2020

Photo-reactions in hybrid organic-semiconductor structures have significant importance for developing applications with probably most visible being solar cells [1]. The progress with the applications depends crucially on our knowledge and ability to control electronic interactions between the semiconductor and organic molecules attached to it. The properties of the latter, the organic dye layer on semiconductor surface, are heavily affected by inter-molecular interactions in the layer, which complicate engineering of well functioning devices.
One promising class of organic dyes in photonic applications is phthalocyanine (Pc). Pcs are generally photo and chemically stable compounds. Pcs have extended π-system resulting in intense absorption bands in the red part of the spectrum, which is beneficial for solar cell applications. There was a number of attempts to use Pcs in dye sensitized solar cell (DSSC), but the obtained efficiencies were essentially lower than that for the best dyes sensitisers [1]. The most probable reason for this is strong aggregation tendency of Pcs which are essentially flat molecules with very well known tendency of stacking due to π-interaction between the macrocycles. The aggregation enables fast deactivation channels for the excited state [2], which competes with the electron injection to semiconductor in DSSC, or charge separation between two organic counterparts in bulk heterojunctions. To reduce the aggregation effect, synthetic efforts were undertaken to decorate Pcs with bulky groups preventing aggregation but having no or little effect on Pc electronic properties.
This presentation focuses on photo-physics of monolayers of a series of aggregation protected Pcs on surface of ZnO nanorods [3]. The main tool for the study was femtosecond transient absorption spectroscopy. It revealed dynamics of electron injection form photoexcited Pc to ZnO substrate. The average time constant of the electron injection was found to be in the range 1-5 ps, but the dependence of the time constant on the bulky substituents and type of linker connecting Pc to ZnO surface was not apparently obvious. A reasonable explanation of the results was found after molecular modelling of Pcs on the ZnO surface. The modelling suggests that Pc has tendency to tilt toward the surface and the final configuration and the distance from Pc to ZnO surface is determined by both the linker and peripheral groups.
An advantage of using array of vertically aligned ZnO nanorods is that it has essentially higher specific surface area compared to that of a flat surface, but is relatively ease to access. The outer side of Pcs layer was covered by a layer of secondary electron donor or hole transporting material, Spiro-OMeTAD (Spiro), by simple spin coating [4]. The coating changes the order of charge transfer reactions drastically. The charge separation at the Pc-Spiro interface becomes dominating channel of the Pc excited state relaxation, resulting in Pc anion - Spiro cation pair. The Pc anion injects the electron to ZnO but with average time constant much longer (roughly 100 ps) than the electron injection form the Pc singlet excited state in the absence of Spiro.
References
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