Multilayer Dye Aggregation at Dye/TiO2 Interface via π…π Stacking and Hydrogen Bond and Its Impact on Solar Cell Performance: A DFT Analysis

Lei Zhang,Weifeng Rao,Xiaogang Liu,Jingfa Li

doi:10.1038/srep35893

Abstract

Multilayer dye aggregation at the dye/TiO2 interface of dye-sensitized solar cells is probed via first principles calculations, using p-methyl red azo dye as an example. Our calculations suggest that the multilayer dye aggregates at the TiO2 surface can be stabilized by π…π stacking and hydrogen bond interactions. Compared with previous two-dimensional monolayer dye/TiO2 model, the multilayer dye aggregation model proposed in this study constructs a three-dimensional multilayer dye/TiO2 interfacial structure, and provides a better agreement between experimental and computational results in dye coverage and dye adsorption energy. In particular, a dimer forms by π…π stacking interactions between two neighboring azo molecules, while one of them chemisorbs on the TiO2 surface; a trimer may form by introducing one additional azo molecule on the dimer through a hydrogen bond between two carboxylic acid groups. Different forms of multilayer dye aggregates, either stabilized by π…π stacking or hydrogen bond, exhibit varied optical absorption spectra and electronic properties. Such variations could have a critical impact on the performance of dye sensitized solar cells.

Highlights

Dye sensitized solar cells (DSSCs) utilize molecular dye absorbers to convert photo energy into electrical power[1,2,3,4]
The aggregation of p-methyl red azo dyes has been evaluated in previous research in the lateral direction relative to the TiO2 surface[24], and the study on the p-methyl red azo dyes serves as a good platform to further analyze their intermolecular interactions and thereby construct the multilayer aggregation model
We show that multilayer aggregates may form along the c-axis of the dye/TiO2 unit cell via intermolecular interactions, such as π...πstacking and hydrogen bond interactions

Summary

Introduction

Dye sensitized solar cells (DSSCs) utilize molecular dye absorbers to convert photo energy into electrical power[1,2,3,4]. It has been identified that molecular dye aggregates are responsible for novel optoelectronic properties for functional molecular dyes in DSSCs and related devices, and numerous researches have been performed to manipulate molecular aggregates and fine-tune their materials properties[17,18,19,20,21,22] To this end, dimeric dye aggregates at different anchoring positions on the TiO2 surface have been analyzed by Filippo et al.[23], offering nanoscopic insights on experimental phenomena that is inconsistent with the monomeric adsorption model. To more closely reassemble experimental scenarios, higher degrees of dye aggregates, such as trimer, tetramer and pentamer, have been constructed via first principles calculations[24] These aggregation models qualitatively explain the “H-aggregation” phenomena of dyes at the dye/TiO2 interface, where blue-shifted light www.nature.com/scientificreports/ Figure 1. The aggregation of p-methyl red azo dyes has been evaluated in previous research in the lateral direction relative to the TiO2 surface[24], and the study on the p-methyl red azo dyes serves as a good platform to further analyze their intermolecular interactions and thereby construct the multilayer aggregation model

Methods

Results

Conclusion