## 1 Introduction  
Solution-processed organic solar cells have been studied due to the social demand of low-cost, lightweight, flexible, and printable devices in large scale [1; 2; 3; 4; 5]. Significant progress of organic solar cells has been demonstrated by utilizing a bulk heterojunction (BHJ) strategy consisting of narrow-bandgap conjugated polymers as p-type semiconductors and fullerene derivatives as n-type semiconductors, [6; 7; 8; 9; 10; 11; 12; 13] leading to desirable, nanoscale phase separation of the two organic components and the achievement of a high power conversion efficiency (PCE) [14; 15; 16; 17]. In order to optimize such a BHJ solar cell system, many efforts have been mainly focused on developing novel, p-type conjugated polymers because of their unique light-harvesting properties, good film formation, and extended electron delocalization through their backbone structures synthesized by alternative copolymerization with electron donor (D) and acceptor (A) units, which can lead to good charge carrier mobilities and narrow energy band gaps [18; 19; 20; 21; 22; 23; 24]. However, the intrinsically broad polydispersity of their molecular weights often limit their purification, reproducibility of performance, and cause significant batch-to-batch variation of the performance in functioning devices [25]. Conversely, solution-processed conjugated small molecules have attracted significant attention as candidates for active materials in BHJ solar cells as p-type semiconductor components due to their well-defined molecular structures and molecular weights and their easy purification methods [26; 27; 28; 29; 30]. Thus, various types of small molecules such as organic dyes, [31; 32; 33; 34; 35] oligothiophenes, [36; 37; 38; 39; 40] and D-A type molecules have been designed and synthesized to optimize the BHJ solar cell systems [41; 42; 43; 44; 45]. However, most of them showed low PCEs because it is still difficult to obtain the desirable phase separation morphologies of those two organic components. Therefore, there is a high demand for the discovery of more applicable molecular structures to the BHJ systems and thus, the design and synthesis of appropriate molecules for efficient performance still merits extensive research [25; 26]. Recently, extended D-A structures such as D-A-D-A-D-type conjugated small molecules have been considered as the most successful alternatives to polymers for BHJ solar cells, comprising nearly 10% of PCEs since the choice of D and A units and conjugated length of the framework can tailor thermal properties, optical properties, and ordered self-assemblies in their solid states [46; 47; 48; 49; 50; 51]. Currently, electron-deficient thieno[3,4-c]pyrrole-4,6-dione (TPD) units have been developed as very promising building blocks for conjugated polymers for effective BHJ solar cells [9; 10; 13; 17; 51; 52; 53; 54; 55; 56]. Each TPD unit has a relatively strong electron-withdrawing property which can lower the highest occupied molecular orbital (HOMO) level of the polymeric materials against the vacuum level and then, increase open circuit voltage (\(V_{\rm{oc}}\)) in BHJ solar cells when it is fabricated with phenyl-C\({}_{61}\) or C\({}_{70}\)-butyric acid methyl ester (PCBM) derivatives [57; 58; 59]. Recent reports of BHJ solar cells employing TPD-based polymers have achieved PCEs up to 8.5% [17]. However, TPD-based molecules have yet to be extensively investigated as materials for small molecule(SM)-BHJ solar cells. In 2012, [PERSON] et al. reported a series of TPD(A)-based p-type organic semiconductors conjugated with triphenylamine (D) showing D-A-D structures that exhibited a PCE of 3.3% from functioning BHJ solar cell systems [60]. Even though the D-A-D structures with TPD units showed quite successful performances, we believe that a D-A-D-A-D strategy can provide more structural diversity than a D-A-D strategy.