Side-Chain Effect on Cyclopentadithiophene/ Fluorobenzothiadiazole-Based Low Band Gap Polymers and Their Applications for Polymer Solar Cells Yongxi Li, †,‡ Jingyu Zou, † Hin-Lap Yip, † Chang-Zhi Li, † Yong Zhang, † Chu-Chen Chueh, † Jeremy Intemann, † Yunxiang Xu, † Po-Wei Liang, † Yu Chen, ‡, * and Alex K.-Y. Jen †, * † Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, United States ‡ Key Lab for Advanced Materials, Institute of Applied Chemistry, East China University of Science and Technology, 130 Mei Long Road, Shanghai 200237, China * S Supporting Information ABSTRACT: A series of cyclopentadithiophene-based low band gap conjugated polymers with varied side-chain patterns and F-substituents were synthesized. By replacing the shorter 2-ethylhexyl (EH) side-chain with the longer 3,7-dimethyloctyl (DMO) side-chain, it resulted in significant changes to the optical, electrochemical, and morphological properties of the polymers, as well as the subsequent performance of devices made from these materials. Solar cells fabricated from polymer with 2-ethylhexyl (EH) side-chain and monofluoro substituent exhibits increased open circuit voltage, short circuit current and fill factor, resulting in the highest power conversion efficiency (5.5%) in this series of polymers. This finding provides valuable insight for making more efficient low band gap polymers. R ecent progress on developing p-type conjugated polymers has led to significant advances in polymer solar cells (PSCs). Power conversion efficiency (PCE) of single junction PSCs have already exceeded 9% in several recently reported devices. 1 However, there are still challenges that need to be addressed in order to further improve the performance of these devices. 2-6 The main limitations lie on that the polymers used cannot absorb light efficiently to generate enough charge carriers and their intrinsically low charge mobility. By stacking both large and small band gap polymers into a tandem solar cell, it can enhance light absorption and PCE. However, one of the problems that strongly impedes the progress of tandem solar cell development is the lacking of high-performance low band gap polymers (E g < 1.5 eV). 7-18 Therefore, it is important to develop suitable small band gap polymers that can be used to fabricate PSCs with reduced energy loss and increased photocurrent density. Recently, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1- b;3,4-b′]dithiophene)-alt-4,7-(monofluoro-2,1,3-benzothiadia- zole)] (PCPDTFBT) has been reported by both Jen 19 and Neher 20 as an efficient small band gap (1.44 eV) polymer. A relatively high PCE of 5.51% could be obtained without using additional solvent additives during device fabrication. The resulting devices could reach an open-circuit voltage (V oc ) of 0.75 V and a short-circuit current density (J sc ) of 15.0 mA cm -2 . However, these devices only showed a relatively low fill factor (FF) of 0.49. Although this polymer can be considered for use in tandem solar cells, its high lying highest occupied molecular orbital (HOMO) energy level limits the V oc of the resultant cell. Owing to the electron-withdrawing character of fluorine (F) atom, conjugated polymers with F groups functionalized on their backbones usually exhibit lower HOMO energy levels, thus it can increase the V oc of the corresponding device. 21-26 We have previously introduced difluorobenzothiadiazole to further deepen the HOMO level of the PCPDTFBT polymer. As expected, the new polymer poly[2,6-(4,4-bis(2-ethylhexyl)- 4H-cyclopenta[2,1- b;3,4- b′ ]dithiophene)- alt -4,7-(di fluoro- 2,1,3-benzothiadiazole)] (PCPDTDFBT) showed an increased V oc of 0.84 eV, which is about ∼0.1 eV higher than that of PCPDTFBT. This V oc is one of the highest among currently available low band gap polymers. Nonetheless, the solubility of PCPDTDFB is quite poor due to enhanced F-H, F-F interactions and strong stacking of polymer. It strongly limits its processability, which can only be processed through hot o- DCB. This not only hampers the formation of good quality film, but also creates significant problem for device fabrication. In order to generate high V oc while maintaining good solution processability of the polymer, we have devoted significant efforts in engineering its alkyl side-chains. This has been proven to be an effective way to modulate the solubility of polymers. Recently, Mü llen 22 has demonstrated that longer linear alkyl chains C 16 could be used to replace shorter branched C 2,8 to enhance packing of polymer chains to achieve very high mobility (3.3 cm 2 V -1 s -1 ). Pei 27 has also reported Received: May 3, 2013 Revised: June 21, 2013 Published: June 28, 2013 Article pubs.acs.org/Macromolecules © 2013 American Chemical Society 5497 dx.doi.org/10.1021/ma4009302 | Macromolecules 2013, 46, 5497-5503