No abstract

A series of metal‐free acene‐modified triphenylamine dyes (benzene to pentacene, denoted as TPA‐AC1 to TPA‐AC5) are investigated as organic sensitizers for application in dye‐sensitized solar cells (DSSCs). A combination of density functional theory (DFT), density functional tight‐binding (DFTB), and time‐dependent DFT (TDDFT) approaches is employed. The effects of acene units on the spectra and electrochemical properties of the acene‐modified TPA organic dyes are demonstrated. The dye/(TiO2)46 anatase nanoparticle systems are also simulated to show the electronic structures at the interface. The results show that from TPA‐AC1 to TPA‐AC5 with increasing sizes of the acenes, the absorption and fluorescence spectra are systematically broadened and red‐shifted, but the oscillator strength and electron injection properties are reduced. The molecular orbital contributions show increasing localization on the bridging acene units from TPA‐AC1 to TPA‐AC5. From the theoretical examination of some key parameters including free enthalpy related to the electron injection, light‐harvesting efficiency, and the shift of semiconductor conduction band, TPA‐AC3 with an anthracene moiety demonstrates a balance of the above crucial factors. TPA‐AC3 is expected to be a promising dye with desirable energetic and spectroscopic parameters in the DSSC field, which is consistent with recent experimental work. This study is expected to deepen our understanding of TPA‐based organic dyes and assist the molecular design of new metal‐free dyes for the further optimization of DSSCs.

The interlayer pi-pi interaction between finite-size models of graphene sheets was investigated by using a density functional theory method, augmented with an empirical R(-6) term for the description of long-range dispersive interaction; these were calibrated by studying the pi-pi interaction between various benzene dimer configurations and comparing the results with previous calculations. For stacked bilayers (dimers) and multilayers of polyaromatic hydrocarbons, which serve as molecular models of graphene sheets, we found that binding energies and energy gaps are strongly dependent on their sizes, while the stacking order and the number of stacked layers have a minor influence. The remarkably broad variation of the energy gap, ranging from 1.0 to 2.5 eV, due mainly to variation of the model size, suggests the potential of broadband luminescence in the visible range for carbon-based nanomaterials that have pi-pi interacting.