Replacing Alkoxy Groups with Alkylthienyl Groups: A Feasible Approach To Improve the Properties of Photovoltaic Polymers

Abstract

Polymer solar cells (PSCs) have attracted much attention because of their potential application in flexible, light-weight, and low-cost large-area devices through roll-to-roll printing. The bulk heterojunction PSCs showed advanced features in realizing high efficiencies and solution-processible devices. The active layer in this kind of device consists of an interpenetrating network formed by an electron-donor material blended with an electron-acceptor material. 3] Typically, conjugated polymers are used as electron donors and fullerene derivatives are used as the electron acceptors in the PSCs. Recently, power conversion efficiencies (PCEs) of 6–7% have been realized by using new conjugated polymer donors or new fullerene-derived acceptors. Short circuit current density (Jsc), open circuit voltage (Voc), and fill factors (FF) are key parameters for a PSC device, because the PCE of the device is proportional to the values of the three parameters. To broaden the response wavelength range of a PSC device by using conjugated side chains or narrowband-gap conjugated polymers is an effective way to realize high Jsc values. Conjugated polymers with lower HOMO levels are helpful in realizing high Voc and PCE values, as the Voc value of PSCs is directly proportional to the offset between the HOMO level of electron donor and the LUMO level of electron acceptor. PSiFDTBT, PFDTBT, and PCDTBT are three excellent examples for this concept. Consequently, by using conjugated polymers with lower HOMO levels and also narrow band gaps, high PCEs were realized in different families of conjugated polymers. Conjugated polymers based on benzo[1,2-b :4,5-b’]dithiophene (BDT) units have attracted interest as electron donors in the PSC field in recent years, since the report of Hou and Yang et al. on the synthesis and photovoltaic properties of a series of copolymers based on BDT. Many copolymers of BDT with different conjugated units, such as thieno[3,4b]thiophene (TT), 4,7-dithiophene-2-yl-2,1,3-benzothiadiazole (DTBT), N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD), and bithiazole, etc. were synthesized, and the copolymers showed promising photovoltaic properties. In these BDT-based polymers, the alternative copolymers of BDT and TT, namely PBDTTTs, are an important family of photovoltaic materials. For additional improvements in the photovoltaic performance of the PBDTTTs, structural modifications brought about by using different substituents on BDT, or the copolymerized moieties is of great importance. For example, Liang et al. introduced a fluorine atom into the TT unit of the PBDTTTs, and the HOMO level of the resulting polymer was successfully lowered by approximately 0.12 eV, and thus a higher Voc value was achieved, resulting in a great improvement of PCE. Hou et al. optimized PBDTTTs further by replacing the alkoxycarbonyl group on the TT unit with the alkylcarbonyl groups. The structural modification can also be carried out on the BDT units. In this work, we designed an 5-alkylthiophene-2-yl-substituted BDT monomer and synthesized two new PBDTTT-based polymers having either the thienylsubstituted BDT with alkoxycarbonyl-substituted thieno[3,4b]thiophene (TT-E) or the alkylcarbonyl-substituted thieno[3,4-b]thiophene (TT-C); that is PBDTTT-E-Tand PBDTTTC-T, respectively (Scheme 1). To fully investigate the effect of the thienyl-substituted BDTon the photovoltaic properties of the polymers, two corresponding PBDTTT polymers based on the alkoxy-substituted BDT (BDT-O), PBDTTT-E and PBDTTT-C (Scheme 1), were also prepared. The synthetic route of the thienyl-substituted BDT monomer (BDT-T) is shown in Scheme 1. The branched alkyl group 2-ethylhexyl was employed as the side chain on the thiophene to guarantee high solubility of the target polymers. The TT-E and TT-C monomers are commercially available. The polymers were prepared through a Stille coupling reaction between the bis(trimethyltin) BDT monomers (BDT-T and BDT-O) and the bromides (TT-E and TTC) as shown in Scheme 1. All the polymers are soluble in chloroform (CHCl3), chlorobenzene, and dichlorobenzene. Thermogravimetric analysis (TGA) measurements were employed to evaluate the thermal stability of the polymers. We found that the two-dimentional (2D) conjugated polymers based on alkylthienyl-substituted BDTs are much more stable than their analogues, the alkoxy-substituted BDTs. The TGA plots of these four polymers are shown in Figure 1. It can be seen that the decomposition temperatures [*] Dr. L. Huo, S. Zhang, F. Xu, Prof. J. Hou State Key Laboratory of Polymer Physics and Chemistry Beijing National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of Sciences Beijing 100190 (China) E-mail: hjhzlz@iccas.ac.cn