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 ∼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.

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