1,1,4,4-Tetracyanobuta-1,3-diene Substituted Diketopyrrolopyrroles: An Acceptor for Solution Processable Organic Bulk Heterojunction Solar Cells Yuvraj Patil, † Rajneesh Misra,* ,† M. L. Keshtov, ‡ and Ganesh D. Sharma* ,§ † Department of Chemistry, Indian Institute of Technology, Indore, Madhya Pradesh 452020, India ‡ A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova Street, 28, Moscow 119991, Russia § Molecular Electronics and Optoelectronics Device Research Laboratory Department of Physics, The LNM Institute of Information Technology, Jamdoli, Rajasthan 302031, India * S Supporting Information ABSTRACT: Two small molecules composed of 1,1,4,4-tetracyanobuta- 1,3-diene substituted diketopyrrolopyrroles (DPPs) denoted as DPP5 and DPP6 were synthesized and their photophysical and electrochemical properties were investigated. The frontier molecular orbitals based on empirical relation between cyclic voltammetry redox potentials, experimental IP, and EA energies indicate that these two small molecules can be used as an electron acceptor for the polymer bulk heterojunction solar cells. The BHJ solar cells combined with a low band gap D-A copolymer P as an electron donor exhibits promising power conversion efficiency of 3.90% and 4.95%, with DPP5 and DPP6, respectively, after the optimization of active layers, indicating that these small molecules based on DPPs can be the alternative as an electron acceptor to replace fullerene, leading to the low-cost solution-processed polymer solar cells. ■ INTRODUCTION The solution processable derivatives of fullerene such as phenyl-C 60 / 70 butyric acid methyl ester (PC 60 / 70 BM) have been widely used as an electron acceptor in bulk heterojunction (BHJ) solar cell due to their excellent electron acceptor properties, 1-8 such as (i) high n-channel mobility, (ii) lowest unoccupied molecular orbital (LUMO), which is delocalized over the entire molecule and facilitates better electron transport, (iii) exhibiting reversible electrochemical reduction that results in stable reduced charge species, and (iv) formation of domains from solution processed film that are on the appropriate length scale for exciton dissociation with mixed phase with electron donor created. Despite these excellent properties, fullerene acceptors exhibit some drawbacks, such as weak absorption with poor tunability over the intense regions of solar spectrum, which limits the contribution to the photocurrent, morphological instability in thin film blends over time, leading to macroscopic crystallite formation and device degradation, and high synthetic costs. 9 To address these problems, new acceptor materials from simple synthetic steps, high yield, and inexpensive synthetic processes for BHJ active layer used in organic solar cells (OSCs) are needed. 10-12 Recently, substantial progress has been made in the design and synthesis of solution processed nonfullerene electron accept- ors. 13-18 The optical and electrochemical properties of nonfullerene acceptors can be easily tuned and tailored, which may enhance the V oc and J sc of BHJ OSCs. 19-27 When compared to the fullerene and its derivatives, nonfullerene small-molecule acceptors have wider spectral absorption that allows absorbing more sunlight to generate electricity. Recently, Yao et al. reported a power conversion efficiency (PCE) of 6.1% for solution processed BHJ organic solar cells using perylene diimide acceptor and conjugated polymer. 28 More recently, Zhan et al. have reported a PCE of 6.8% for a device consisting of an electron acceptor based on a fused-ring core end-capped with 2-(3-oxo-2,3-dihydroinden-1-ylidene) malo- nonitrile (INCN) units and conjugated polymer without any treatment. 29 Zhao et al. have reported PCE of 6.3% for the device with nonfullerene acceptor and low band gap copolymer. 30 Many other promising nonfullerene acceptors have been used for solution processed organic solar cells with overall PCE in the range 4-8%. 31-38 A variety of nonfullerene acceptors have been designed that have shown improved device efficiency. The dicyano (-CN) acceptors have been extensively employed in optoelectronic devices, 15,39-43 which offer an alternative for electron accept- or. 15,44-56 The -CN substituent can increase the electron affinity and promote the formation of crystallite architecture by Received: December 16, 2015 Revised: March 3, 2016 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.5b12307 J. Phys. Chem. C XXXX, XXX, XXX-XXX