Synthetic Metals 191 (2014) 168–176 Contents lists available at ScienceDirect Synthetic Metals jo ur nal homep age: www.elsevier.com/locate/synmet Synthesis of fluorene based two acceptor random copolymers for organic solar cell applications Renchu Scaria a,b , S.K. Dhawan a,∗ , Suresh Chand b a Polymeric and Soft Materials Section, National Physical Laboratory, (CSIR), New Delhi 110 012, India b Organic and Hybrid Solar Cell Group, Physics of Energy Harvesting Division, National Physical Laboratory, (CSIR), New Delhi 110 012, India a r t i c l e i n f o Article history: Received 17 November 2013 Received in revised form 17 February 2014 Accepted 21 February 2014 Available online 28 March 2014 Keywords: Polyfluorenes Donor–acceptor copolymers Suzuki polycondensation Organic solar cells a b s t r a c t A series of fluorene based random copolymers incorporating two acceptor units – 4,7-dithienyl-2, 1,3-benzothiadiazole and N-heterocyclic moieties (2,1,3-benzothiadiazole, [2,3-b]- phenanthrenequinoxaline, [2,3-b]-acenaphthenequinoxaline) have been synthesized by Suzuki polycondensation method. The synthesized polymer films exhibit strong absorption in the range from 300 nm to 700 nm with good solubility and thermal stability. The optical band gap of the synthesized copolymers are in the range of 1.86–1.90 eV. The HOMO energy levels of the polymers calculated from cyclic voltammetry measurements have been found to be in the range from -5.45 to -5.70 eV. Using these new electron donor copolymers, solar cells were fabricated and highest efficiency was obtained with PFQx(phen)DTBT as an electron donor and PC 71 BM as an electron acceptor. In fact with PFQx(phen)DTBT:PC 71 BM in the ratio of 1:4, a power conversion efficiency (PCE) ∼0.47% has been achieved with open-circuit voltage (V oc ) 0.59 V, short circuit current density (J sc ) ∼2.49 mA/cm 2 and fill factor (FF) ∼ 32% under the illumination of AM1.5G, 100 mW cm -2 . © 2014 Elsevier B.V. All rights reserved. 1. Introduction Donor–acceptor bulk hetrojunction (BHJ) organic solar cells are fast emerging as a potential, futuristic, flexible, large area cost effective technology. Power conversion efficiency (PCE) more than 10% has already been achieved in these solar cells using tandem structures of the device [1]. The two important steps in polymer solar cells include the efficient light harvesting and efficient trans- port of charge carriers. As regards efficient harvesting of photons the approaches being followed include modification/tailoring the band gap of existing electron donating polymers or developing entirely new polymers. Polyfluorenes are a class of stable conju- gated polymers that have received wide attention for use in solar cells due to their high HOMO energy as using high HOMO energy materials could potentially lead to larger device working voltages and overall higher efficiencies [2–4]. Donor–acceptor (D–A) con- jugated copolymers based on fluorene donor with wide variety of acceptors such as derivatives of quinoline, quinoxaline (Qx), 2,1,3- benzothiadiazole (BT) and 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) have already been studied for various applications because ∗ Corresponding author. Tel.: +91 11 4560 9401; fax: +91 11 2572 6938. E-mail addresses: skdhawan@mail.nplindia.ernet.in, skdhawan@mail.nplindia.org (S.K. Dhawan). the optical and electronic properties of polymers could be tun- able through the intramolecular charge transfer (ICT) from the donor to acceptor [3,5–10]. However, several reported D–A copoly- mers showed poor power conversion efficiency in solar cells due to higher electronic band gap, which limited their photon absorp- tion within the solar spectrum. For example, 9,9-dialkylfluorene and 2,1,3- benzothiadiazole and quinoxaline based copolymers have shown power conversion efficiency of <0.02% with PCBM as electron acceptor [11,12]. However, the alternating copolymer of fluorene with DTBT unit (APFO3, PF10DTBT) has shown better effi- ciency than their BT or Qx counterparts [13–18]. This shows that apart from limited photon absorption, the mismatch of the energy level between electron-donating polymer and electron acceptor significantly contribute to lower efficiency of the solar cells. Thus, well-chosen donor and acceptor groups are particularly desirable for low band gap polymers which can provide a significant enhance- ment of the ICT intensity and conjugated length that leads to a better extended absorption and higher absorption coefficient. Semi-random copolymers containing multiple distinct acceptor monomers have been synthesized to achieve large current densities and high efficiencies due to favourable properties such as semicrys- tallinity, high hole mobility, and importantly broad, uniform, and strong absorption of the solar spectrum [19–21]. Apart from a few limited reports on random tercopolymers with two acceptor units, there have been no systematic study into the synthesis of http://dx.doi.org/10.1016/j.synthmet.2014.02.027 0379-6779/© 2014 Elsevier B.V. All rights reserved.