Benzothiadiazole-based polymer for single and double junction solar cells with high open circuit voltage† Swaminathan Venkatesan, Evan C. Ngo, Qiliang Chen, Ashish Dubey, Lal Mohammad, Nirmal Adhikari, Abu Farzan Mitul and Qiquan Qiao * Single and double junction solar cells with high open circuit voltage were fabricated using poly{thiophene- 2,5-diyl-alt-[5,6-bis(dodecyloxy)benzo[c][1,2,5]thiadiazole]-4,7-diyl} (PBT-T1) blended with fullerene derivatives in different weight ratios. The role of fullerene loading on structural and morphological changes was investigated using atomic force microscopy (AFM) and X-ray diffraction (XRD). The XRD and AFM measurements showed that a higher fullerene mixing ratio led to breaking of inter-chain packing and hence resulted in smaller disordered polymer domains. When the PBT-T1:PC 60 BM weight ratio was 1 : 1, the polymer retained its structural order; however, large aggregated domains formed, leading to poor device performance due to low fill factor and short circuit current density. When the ratio was increased to 1:2 and then 1 : 3, smaller amorphous domains were observed, which improved photovoltaic performance. The 1 : 2 blending ratio was optimal due to adequate charge transport pathways giving rise to moderate short circuit current density and fill factor. Adding 1,8-diiodooctane (DIO) additive into the 1 : 2 blend films further improved both the short circuit current density and fill factor, leading to an increased efficiency to 4.5% with PC 60 BM and 5.65% with PC 70 BM. These single junction solar cells exhibited a high open circuit voltage at 0.9 V. Photo-charge extraction by linearly increasing voltage (Photo-CELIV) measurements showed the highest charge carrier mobility in the 1 : 2 film among the three ratios, which was further enhanced by introducing the DIO. The Photo-CELIV measurements with varying delay times showed significantly higher extracted charge carrier density for cells processed with DIO. Tandem devices using P3HT:IC 60 BA as bottom cell and PBT-T1:PC 60 BM as top cell exhibited a high open circuit voltage of 1.62 V with 5.2% power conversion efficiency. 1. Introduction Polymer solar cells (PSCs) have received prominent attention in the last two decades as an alternative to inorganic solar cells because of their low production cost and compatibility with exible substrates. 1–9 Such solar cells can be processed from solution using a variety of low cost techniques such as spray coating, 10–12 dip casting, 13 screen printing, 13 spin coating, 14 and inkjet printing. 15 This makes them a candidate for large-scale production via roll-to-roll processing. 16 Despite those advan- tages, the efficiency and lifetime of polymer solar cells are still too low for commercialization. Poly(3-hexylthiophene) (P3HT) is one of the most widely studied conjugated polymers for photovoltaic applications, and when blended with fullerene, it results in efficiencies up to 5%. One of the performance- limiting factors in P3HT is the photocurrent generation owing to its higher band gap; therefore, only a small part of the solar spectrum is absorbed. Hence there is a need for a low band gap conjugated polymer for broader solar spectrum utilization. Recently, a variety of donor–acceptor (D–A) or push–pull type polymer structures 17 have been proposed and synthesized to obtain polymers with lower band gap and higher charge carrier mobility. These D–A polymers consist of alternating electron- rich and electron-decient units. Based on the selection of these units, the energy levels and charge transport properties of the donor polymers can be nely tuned. Among the various acceptor units, 2,1,3-benzothiadiazole (BT) has been used in optoelec- tronic devices 18–22 with wide success owing to the resulting low- lying HOMO level of the polymer, which leads to not only larger open circuit voltage (V oc ) but also to higher stability against oxidation. Helgesen et al. 22 reported a series of 2,1,3-benzothia- diazole (BT)-based co-polymers with different substituted thio- phene groups as donor units; however, their study focused primarily on synthesis and optical property characterization. One of the polymers in the literature reported moderate photo- voltaic efficiency of 2.22% and a V oc of 0.93 V. However, to attain high power conversion efficiency using low band gap polymers Center for Advanced Photovoltaics, Department of Electrical Engineering, South Dakota State University, Brookings, SD, USA. E-mail: qiquan.qiao@sdstate.edu † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr01040j Cite this: DOI: 10.1039/c4nr01040j Received 25th February 2014 Accepted 11th April 2014 DOI: 10.1039/c4nr01040j www.rsc.org/nanoscale This journal is © The Royal Society of Chemistry 2014 Nanoscale Nanoscale PAPER Published on 16 April 2014. Downloaded by University of Arizona on 21/05/2014 17:32:17. View Article Online View Journal