## 1 Introduction  
Development of novel semiconducting conjugated polymers and investigation ofstructure-property relationships always play a critical role in the organic electronic devices. Through rational molecular design, they will be tuned specifically to achieve desired optical and electronic properties for application in electronics and optoelectronics, such as field effect transistors (FETs) [1, 2], photo-detectors [3], light-emitting diodes (LEDs) [4], and photovoltaics (PVs) [5, 6]. Over the past several decades, many well-designed conjugated polymers have been used as photoactive materials in organic photovoltaic (OPV) devices owing to their excellent optoelectronic properties [7, 8]. As a promising conjugated polymer in the OPV field, it should possess broad absorption in the solar spectrum, high charge carrier mobility for charge transport, suitable energy levels, etc. [7; 9]. Therefore, how the designs of the molecular structures can be used to control their properties is of particular importance in the development of conjugated polymers. Among these high performance conjugated polymers, benzo[1,2-b:4,5-b\({}^{\prime}\)]dithiophene (BDT) has been considered as one of the most effective building-blocks for p-type conjugated polymers because it possesses a planar structure and BDT-based copolymers have high hole mobility and suitable electronic energy levels [10; 11; 12]. The bulk heterojunction (BHJ) polymer solar cell devices (PSCs) based on BDT-based copolymers have achieved high power conversion efficiency (PCE) values over 10% [8; 13; 14; 15]. In addition, in order to enhance the short-circuit current density in PSCs, molecular designs should be conducive to enhancing the absorption ability of the conjugated polymer. As shown in Figure 1, two-dimensional (2-D) conjugation by vinylene linkage, such as thienylene-vinylene [16], bi(thienylene-vinylene) [17], phenothiazazine-vinylene [18], triphenylamine-vinylene [19], and terthiophene-vinylene [20], has proven to be a successful strategy to enhance light-harvesting ability, charge mobility and self-assembly, and adjust molecular energy levels to meet the requirements of PSCs [17; 20; 21]. Moreover, [PERSON] and co-workers reported that a series of 2-D conjugated polymers incorporating different thienylene-vinylene derivatives with BDT building blocks, such as PTG1 [22] and PBDT-TID [23], not only attracted much interest but also demonstrated an effective strategy to extend the absorption range and down-shift the HOMO energy level. On the other hand, [PERSON] and co-workers reported that introduction of strong electro-withdrawing groups, such as pyrrol[3,4-c]pyrrole-1,4-dione (DPP), in the 2-D conjugated polymers broadens the spectral absorption via the random polymerization [24]. Taking a comprehensive view of these strategies, it it is of great interest to discover novel 2-D conjugated polymers to improve their optoelectronic and photovoltaic properties, such as enhancing absorption ability and further improving photon-to-electron conversion efficiency, through incorporating different 2-D conjugation blocks with electron-withdrawing and/or electron-donating groups. In our previous studies, terthiophene-vinylene (TTV)-based conjugated polymers possess a broader absorption region and deeper HOMO levels than rr-P3 HT [20; 25; 26]. Herein, a novel 2-D polymer PTTVBDT, as shown in Figure 2, has been designed and synthesized by cooperating TTV and BDT units via Stille polymerization. Furthermore, two electron-withdrawing groups, TPD and DPP units, were employed as the third copolymerization unit to build the terpolymers, PTTVBDT-TPD and PTTVBDT-DPP, respectively. Thieno[3,4-c]pyrrole-4,6-dione (TPD) and DPP units both have been also widely used as building blocks and electron-withdrawing groups in high performance conjugated  
Figure 1: Molecular structures of the 2-D conjugated polythiophene derivatives (R = alkyl group) [16; 17; 18; 19; 20]. polymers due to their symmetric, rigidly fused, and coplanar structures [11; 27; 28; 29; 30].