Novel NIR-absorbing conjugated polymers for efficient polymer solar cells: effect of alkyl chain length on device performance

Three low bandgap conjugated polymers, i.e., PDTPBT-C8, PDTPBT-C6 and PDTPBT-C5, which consist of alternating N-alkyl dithieno[3,2-b: 2',3'-d] pyrrole and 2,1,3-benzothiadiazole units and carry 1-octylnonyl, 1-hexylheptyl and 1-pentylhexyl as side chains, respectively, were synthesized. These polymers show strong absorption in the wavelength range of 600-900 nm with enhanced absorption coefficient as the length of alkyl chain decreases. The film morphology of the polymers and 1-(3-methoxycarbonyl) propyl-1-phenyl-[6,6]-C-61 (PCBM) blends is also dependent on the alkyl chain length. As the length decreases, the film becomes more uniform and the domian size decreases from 400-900 nm for PDTPBT-C8 to similar to 50 nm for PDTPBT-C5. Bulk heterojunction photovoltaic solar cells (PSCs) were fabricated based on the blend of the polymers and PCBM with a weight ratio of 1: 3. The device performance is dramatically improved as the length of the side chain decreases, due to enhanced film absorption coefficient and improved film morphology. With the polymer PDTPBT-C5, which carries the shortest alkyl chain, power conversion efficiency (PCE) up to 2.80% has been achieved. This result indicates that optimizing the structure of the solublizing alkyl chain is also crucial for the design and synthesis of high performance PSC polymeric materials.