Facile Synthesis of Key Building Blocks of D18 Series Conjugated Polymers for High-Performance Polymer Solar Cells

D18 and its derivatives (such as D18-Cl) newly emerged as high-performance conjugated polymers for bulk heterojunction (BHJ) solar cells, with efficiency breaching 17% when paired with small molecule acceptors such as Y6 and its derivatives. However, the complex synthesis of D18 series has limited their accessibility to the research community. We redesigned the original synthetic scheme of D18, particularly the key monomer, 5,8-bis(5-bromo-4-(2-butyloctyl)thiophen-2-yl)-dithieno[3 ',2 ':3,4;2 '',3 '':5,6]benzo[1,2-c][1,2,5]thiadiazole (Br2-T2-DTBT), and replaced a few key steps in the original scheme that were hard to reproduce and/or low yielding with simple to operate and high yielding reactions. Carother's equation was successfully applied to control/tune the molar mass of D18-Cl, and three representative molar masses (high, medium, and low) of D18-Cl were subject to device testing. It is discovered that a medium molar mass (56 kg/mol) D18-Cl offered the highest performance (average 16.6%) when paired with Y6 as the acceptor in a typical BHJ device configuration, consistent with the literature report. This optimized synthesis of D18 series offers great synthetic tunability, and the modular nature/optimized key reactions will facilitate the design and synthesis of conjugated polymers for optoelectronics.