Quantum Chemical Designing of Efficient Sensitizers for DSSC Bull. Korean Chem. Soc. 2013, Vol. 34, No. 7 2093 http://dx.doi.org/10.5012/bkcs.2013.34.7.2093 Quantum Chemical Designing of Efficient Sensitizers for Dye Sensitized Solar Cells Muhammad Imran Abdullah, Muhammad Ramzan Saeed Ashraf Janjua, † Asif Mahmood, †,* Sajid Ali, ‡ and Muhammad Ali § Institute of Chemistry, University of the Punjab, Lahore, Pakistan † Department of Chemistry, University of Sargodha, Sargodha, Pakistan. * E-mail: asifmahmood023@gmail.com ‡ Department of Physics, Agriculture University, Faisalabad, Pakistan § Department of Physics, Umea University, Umea, Sweden Received March 8, 2013, Accepted April 20, 2013 Density functional theory (DFT) was used to determine the ground state geometries of indigo and new design dyes (IM-Dye-1 IM-Dye-2 and IM-Dye-3). The time dependant density functional theory (TDDFT) was used to calculate the excitation energies. All the calculations were performed in both gas and solvent phase. The LUMO energies of all the dyes were above the conduction band of TiO2, while the HOMOs were below the redox couple (except IM-Dye-3). The HOMO-LUMO energy gaps of new design dyes were smaller as compared to indigo. All new design dyes were strongly red shifted as compared to indigo. The improved light harvesting efficiency (LHE) and free energy change of electron injection ΔG inject of new designed sensitizers revealed that these materials would be excellent sensitizers. The broken coplanarity between the benzene near anchoring group having LUMO and the last benzene attached to TPA unit in all new design dyes consequently would hamper the recombination reaction. This theoretical designing will the pave way for experimentalists to synthesize the efficient sensitizers for solar cells. Key Words : Dye-sensitized solar cells, Indigo, Light harvesting efficiency, Electron injection, Density func- tional theory Introduction Dye-sensitized solar cells (DSSC) attain consideration because of their sky-scraping light to electricity conversion efficiencies, simple and low cost manufacturing. 1-3 The sensitizer is a critical element in DSSC, which improves the power conversion efficiency and increases the stability of the devices. The Ruthenium base photosensitizers give a solar energy to electricity conversion efficiency of 10% in aver- age. 2 Metal free organic DSSCs have benefits over metal holding sensitizers, e.g., easy and cheap preparation methods, environment friendly and elevated molar extinction coeffi- cient. 4 Different metal free dyes have been examined which have comparable efficiencies to metal holding sensitizers. 5-7 Designing of dye sensitizer plays an important role in the optimization of DSSC, 8 and it depends on the quantitative information of dye sensitizer. In most of the organic sensitizers presence of donor, bridge and acceptor (DBA) moieties is very important to get better performance of the photoinduced intramolecular charge transfer. During electronic transition charge transfer depends on the conjugation across the donor and anchoring groups. Efficiency of organic sensitizers decreases due to dye aggre- gation and charge recombination. 9 It has been established that the triphenylamine (TPA) 10 derivatives as electron donor and cyanoacetic acid moiety as electron acceptor are better options to improve the efficiency. 9 It is estimated that TPA can restrain the cationic charge from the semiconductor surface therefore block the recombination. TPA also has a characteristics steric hindrance that can put off undesirable dye aggregation at the semiconductor surface. 11 To model and design efficient metal-free sensitizers for DSSC, suitable DBA systems are needed whose properties can be altered by applying the drivable structural modifications. In this research work, we have studied the indigo and new- ly designed indigo base dyes. We have designed compounds by the substitution of TPA moiety in indigo dye as donor and cyanoacetic acid moiety, NO 2 and CN as electron acceptor. To evaluate the effect of auxillary donor on dye efficiency, Dimethyl Vinyl -CH=C(CH 3 ) 2 Methoxy and N,N-Dimethyl Amine groups have introduced on TPA moiety. This study will provide the help to experimentalists to synthesize the more efficient dyes for dye sensitized solar cells. Computational Details Density functional theory (DFT) and time dependant den- sity functional theory (TD-DFT) calculations were perform- ed to determine geometries, electronic structures and elec- tronic absorption spectra of indigo base dyes. All the calcu- lations, both in gas and solvent phase, were performed using Gaussian09 package. 12 All calculations were performed by employing CAM-B3LYP functional and 6-311+G* basis set. Polarizable continuum model (PCM) was used to study solvent effects. The free energy change for electron injection onto a titanium dioxide (TiO 2 ) surface and dye’s excited state oxidation potential were calculated using mathematical equations.