High-efficiency dye-sensitized solar cells fabricated using D-D-p-A (donor-donor/p-spacer-acceptor) architecture Gachumale Saritha a , Ramalinga Viswanathan Mangalaraja b , Sambandam Anandan a,⇑ a Nanomaterials and Solar Energy Conversion Laboratory, Department of Chemistry, National Institute of Technology, Trichy 620 015, India b Advanced Ceramics and Nanotechnology Laboratory, Department of Materials Engineering, Faculty of Engineering, University of Concepcion, Concepcion 407-0409, Chile article info Article history: Received 24 December 2016 Received in revised form 14 February 2017 Accepted 22 February 2017 Keywords: Dye-sensitized solar cell Metal-free organic dye Co-sensitization Electron injection Electron lifetime abstract Herein, we designed and synthesized two new metal-free organic dyes CCC ((E)-2-cyano-3-(6-((E)-(9-e thyl-9H-carbazol-3-ylimino)methyl)-9-hexyl-9H-carbazol-3-yl)acrylic acid) and CTC ((E)-2-cyano- 3-(4-((4-((E)-(9-ethyl-9H-carbazol-3-ylimino)methyl) phenyl)(phenyl)amino)phenyl)acrylic acid) of D-D-p-A (donor-donor/p-spacer-acceptor) architecture. N-hexyl carbazole and triphenylamine were introduced as a donor as well as p-spacer, N-ethyl carbazole as an additional donor and cyano acrylic acid as acceptor. To evaluate photovoltaic properties of dye-sensitized solar cells (DSSCs), their photophysical, electrochemical and theoretical calculations were performed. The overall conversion efficiencies of the sensitizers CCC and CTC were 3.12% and 2.63%, respectively. Upon co-sensitized with N719 dye, fabri- cated solar cell with CCC dye showed highest short circuit current density of 19.33 mA/cm 2 , open circuit voltage of 0.745 V and fill factor of 0.473 resulting in highest power conversion efficiency (PCE) of 8.01%, which is higher than DSSCs based on individual dyes may be due to greater incident photon current con- version efficiency and broader visible light absorption. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Dye-sensitized solar cells (DSSCs) have been widely investi- gated because of their high performance in converting solar energy to electric energy (Gratzel, 2005), which are alternatives to the conventional solid silicon-based photovoltaic devices (He et al., 2011; Gratzel, 2001) due to the low cost of production. The advan- tages of DSSCs are lightweight, low cost, uncomplicated molecular tuning, intense absorption wavelengths and easy to fabricate with modified colors (Hagfeldt et al., 2010). The sensitizer plays a piv- otal role in the DSSCs process is may be due to the light harvesting efficiency and in turn, facilitate electron injection into the conduc- tion band (CB) of oxide semiconductor from the excited state of the dye upon light absorption (Tiwana et al., 2011). Many researchers put a lot of efforts to develop novel and efficient photosensitizers. Apart from ruthenium polypyridine and zinc porphyrin complexes, more attention has been paid to metal-free organic dyes because of their low cost of production, easy purification, and good flexibility upon modification of the dye structures. Photo conversion effi- ciency (PCE) of DSSC sensitized by Ru complex and Zn porphyrin dyes have been achieved maximum 11.1% (Chiba et al., 2006) and 13% (Mathew et al., 2014), respectively. However, the limited ruthenium resource, difficulty in the purification of ruthenium complexes and lower molar extinction coefficient values limit their applications in DSSCs. Further, the major drawback of porphyrin- based sensitizer exhibits very weak absorption characteristics in the spectral region between the Soret and Q bands (Fan et al., 2015). In this context, many metal-free organic dyes have been developed to harness photons in near infra-red (NIR) region (Numata et al., 2014; Qin et al., 2014, 2012). For designing efficient sensitizers (Mishra et al., 2009), so far various electron donor moi- eties such as, indole (Yang et al., 2014), triarylamine (Liang and Chen, 2013) and squaraine (Shi et al., 2011), have been employed in the D-p-A architecture and this dipolar configuration creates an effective intramolecular charge transfer (ICT) from donor to acceptor during electron excitation. N-hexyl carbazole (planar) and triphenylamine (non-planar) act as primary donor. In addition to this, we introduced N-ethyl substituted carbazole as an additional donor i.e. secondary donor. The reason for choosing N-alkyl substituted carbazole as a secondary donor due to its unique optical, electrical, and chemical properties. Further, the rigid framework of carbazole has been used widely as a functional building block or a substituent in the construction of organic mole- cules for organic light emitting diodes (Chaskar et al., 2011) and liquid crystals (Sienkowska et al., 2007). Further, numerous investigations on p-conjugated molecules with donor–acceptor http://dx.doi.org/10.1016/j.solener.2017.02.046 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: sanand@nitt.edu, sanand99@yahoo.com (S. Anandan). Solar Energy 146 (2017) 150–160 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener