DFT and TD-DFT study on geometries, electronic structures and electronic absorption of some metal free dye sensitizers for dye sensitized solar cells T. Mohr a,b , V. Aroulmoji a,⇑ , R. Samson Ravindran a , M. Müller b , S. Ranjitha c , G. Rajarajan a , P.M. Anbarasan d a Mahendra Educational Institutions, Mahendhirapuri, Mallasamudram, Namakkal District, Tamil Nadu, India b Department of Energy and Drive Systems, University of Applied Sciences, Ulm, Germany c Velalar College of Engineering and Technology, Thindal, Erode, Tamil Nadu, India d Department of Physics, Periyar University, Salem, Tamil Nadu, India highlights  Connecting a thiophene bridge with henna enhances the efficiency.  In the absorption band n to p * transitions of the electrons are observed.  The natural bond orbital (NBO) of henna and anthocyanin proposes a (D–p–A) system. graphical abstract article info Article history: Received 2 May 2014 Received in revised form 11 July 2014 Accepted 29 July 2014 Available online 12 August 2014 Keywords: DFT and TD-DFT Geometric structure HOMO and LUMO orbitals DSSC Dye sensitizer Absorption spectrum abstract The geometries, electronic structures, polarizabilities and hyperpolarizabilities of 2-hydroxynaphthalene- 1,4-dione (henna1), 3-(5-((1E)-2-(1,4-dihydro-1,4-dioxonaphthalen-3-yloxy) vinyl) thiophen-2-yl)-2-iso- cyanoacrylic acid (henna2) and anthocyanin dye sensitizers were studied based on density functional theory (DFT) using the hybrid functional B3LYP. The Ultraviolet–Visible (UV–Vis) spectrum was investigated by using a hybrid method which combines the properties and dynamics of many-body in the presence of time-dependent (TD) potentials, i.e. TDSCF-DFT (B3LYP). Features of the electronic absorption spectrum in the visible and near-UV regions were plotted and assigned based on TD-DFT calculations. Due to the absorption, bands of the metal–organic compound are n ? p * present. The calculated results suggest that the three lowest energy excited states of the investigated dye sensitizers are due to photoinduced electron transfer processes. The interfacial electron transfer between semiconductor TiO 2 electrode and dye sensitizer is owing to an electron injection process from excited dye to the semiconductor’s conduction band. The role of linking the henna1 dye with a carboxylic acid via a thiophene bridge was analyzed. The results are that using a stronger p-conjugate bridge as well as a strong donator and acceptor group enhances the efficiency. Ó 2014 Elsevier B.V. All rights reserved. Introduction Nowadays it is quite usual to focus in the generation of electric- ity on renewable energies. The consequences of Fukushima tragedy http://dx.doi.org/10.1016/j.saa.2014.07.094 1386-1425/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 1066–1073 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa