Diketo-pyrrolo-pyrrole-based medium band gap copolymers for efficient plastic solar cells: Morphology, transport, and composition-dependent photovoltaic behavior

Two diketo-pyrrolo-pyrrole-based donor-acceptor copolymers, PCPDT-PDPP and PDTP-PDPP, were synthesized and applied as p-type component for bulk heterojunction solar cells. Using a facile molecular engineering of the main chain linking bridge, we have successfully tuned the band gap of the materials from low to medium level, which results in an improved balance between the device open circuit voltage and short circuit current. The power conversion efficiencies of the polymer/PC 70BM combinations achieve 2.03% (PCPDT-PDPP) and 1.32% (PDTP-PDPP) without any interface engineering or post-treatment. A totally different composition-dependent photovoltaic behavior was also discovered for each of the two materials, although they share similar donor-acceptor (D-A) structure. Morphological and photophysical studies reveal a phase structure dominated, light harvesting subordinated mechanism, in which some nonkernel molecular level factors, such as the branch chain density and the molecular weight, are found to play important roles. © 2011 American Chemical Society.link_to_subscribed_fulltex