N-acyl-dithieno[3,2-b:2’,3’-d]pyrrole-based low bandgap copolymers affording improved open-circuit voltages and efficiencies in polymer solar cells

• Synthesis of N-acyl-dithieno[3,2-b:2′,3′-d]pyrrole-alt-quinoxaline (DTPQx) copolymers.
• Application in bulk heterojunction polymer solar cells.
• Increased open-circuit voltages by DTP N-acylation and quinoxaline fluorination.
• Power conversion efficiencies up to 4.81%.

Three distinct low bandgap copolymers are synthesized by the combination of N-(2′-propylpentanoyl)dithieno[3,2-b:2′,3′-d]pyrrole (DTP) and (fluorinated) 2,3-bis[5′-(2”-ethylhexyl)thiophen-2′-yl]quinoxaline (Qx) and these PDTPQx derivatives are investigated as electron donor materials in bulk heterojunction polymer solar cells. Due to the DTP N-acylation and the introduction of the Qx units, both the open-circuit voltage (Voc) and the short-circuit current density (Jsc) increase compared to previous devices based on DTP-type donor polymers. Organic solar cells with an average Voc of 0.67 V, a Jsc of 12.57 mA/cm² and a fill factor of 0.54 are obtained, affording a power conversion efficiency of 4.53% (4.81% for the top-performing device), a record value for (N-acyl-)DTP-based polymer solar cells devoid of special interlayer materials. Despite further enhancement of the Voc, the solar cell efficiency declines for the fluorinated PDTPQx copolymers because of the inability to achieve a finely intermixed bulk heterojunction blend nanomorphology.

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