Highly efficient panchromatic dye-sensitized solar cells: Synergistic interaction of ruthenium sensitizer with novel co-sensitizers carrying different acceptor units Dickson D. Babu a , Dalia Elsherbiny b , Hammad Cheema b , Ahmed El-Shafei b, ** , Airody Vasudeva Adhikari a, * a Organic Materials Laboratory, Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Mangalore 575025, India b Polymer and Color Chemistry Program, North Carolina State University, Raleigh, NC 27695, USA article info Article history: Received 29 March 2016 Received in revised form 9 May 2016 Accepted 11 May 2016 Available online 12 May 2016 Keywords: Dye sensitized solar cells Indole Co-sensitizers Acceptors Density functional theory abstract Herein, we report the molecular design, synthesis and photovoltaic performance studies of three new organic co-sensitizers, N 1-3 carrying indole and thiophene units linked to different acceptors/anchoring groups, as co-adsorbents for dye sensitized solar cells. We present the role of anchoring/acceptor units on co-sensitization properties N 1-3 . Their photo-physical and electrochemical results along with molecular geometry, obtained from Density Functional Theory are utilized to rationalize the influence of co- sensitizer structures on photovoltaic properties for DSSCs. We have shown that, the co-sensitization effect is profoundly dependent upon the anchoring/acceptor unit in the co-adsorbents. Among them, N 3 containing 4-aminobenzoic acid shows promising co-sensitization results and exhibits an enhanced efficiency of 9.26%, when co-sensitized with a ruthenium dye, HD-14. Further, the study highlights the importance of molecular matching between the sensitizer and co-sensitizer in enhancing the efficiency. Furthermore, vertical electronic excitations are calculated using time dependent density functional theory studies. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction The need to generate clean yet viable energy is one of the most important scientific and technological challenges in the 21st cen- tury. Due to the possibility of low-cost conversion of photovoltaic energy into electricity, dye-sensitized solar cells (DSSCs) have engrossed great interest in material science throughout the past two decades [1]. Although, sensitizers based on ruthenium com- plexes have delivered high efficiency (~11%) [2,3], they are hammered by their high cost, low molar extinction coefficient, limited availability of the noble metal and environmental side- effects etc. Due to the aforementioned limitations, scientists across the globe have been forced to look for an alternative to the ruthenium complexes and thereby, designing novel and efficient sensitizers for DSSC application has become exceedingly imperative. In this regard, metal-free sensitizers have shown a lot of promise [4,5]. The major drawback with metal-free dyes is their lower overall efficiency as compared to metal-based counterparts. The primary reason for the lower efficiency is the faster charge recombination between the injected electrons in the titanium di- oxide and the oxidized species present in the electrolyte. On the other hand, the merits of metal-free sensitizers include, facile synthesis, high molar extinction coefficients, cost effectiveness and they are appropriate for large scale application of DSSCs. The challenge is to strike a balance between these two aforementioned aspects. Many researchers around the globe have focused on extending the absorption spectrum of the sensitizer and thereby enhancing the overall efficiency [6]. The main dye design philoso- phy implemented by various research groups to broaden the ab- sorption spectra of the sensitizers is donor-p-bridge-acceptor (D- p-A) design strategy [7]. However, the extension of absorption spectrum comes with its share of parallel limitations. Firstly, red- shifting the absorption spectrum by extending the conjugation may result in sensitization problems, i.e. dye aggregation on the semiconductor surface, leading to lower photo-conversion * Corresponding author. ** Corresponding author. E-mail addresses: avachem@gmail.com, avadhikari123@yahoo.co.in, avchem@ nitk.ac.in (A.V. Adhikari). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig http://dx.doi.org/10.1016/j.dyepig.2016.05.016 0143-7208/© 2016 Elsevier Ltd. All rights reserved. Dyes and Pigments 132 (2016) 316e328