Porphyrin-diindenothieno[2,3-b ]thiophene alternating copolymer-a blue-light harvester in ternary-blend polymer solar cells

Porphyrin, despite chosen by Nature as light har- vesting units, hasn't revealed its full potentials as a structural unit in porphyrin-incorporated polymers (PPors). A novel PPor was synthesized to investigate the origins of the low perform- ances of PPor-based polymer solar cells (PSCs). The polymer features broad absorption in the blue-light region, because the diindenothieno(2,3-b)thiophene (DITT) unit extended the conju- gation in the polymer backbone. PPor-DITT/PC71BM based PSCs have a high Voc (0.79 V). Their low Jsc and fill factor (FF) were attributed to the un-optimized morphology, as indicated by the photoluminescence quenching and atomic force micros- copy (AFM) experiments. Using PPor-DITT as a blue-light har- vesting dopant in an amorphous host leverage the strong 400- 550 nm absorption of PPor-DITT and circumvent the difficulties in reaching optimized morphology in the PPor/PCBM thin films. An addition of 2 wt % of PPor-DITT in ternary-blend PSCs resulted in a 10 % increase of external quantum effi- ciency (EQE) in the blue-light region. However, in a crystalline host, the dopant decreased the crystallinity of the host and led to large drops in FF and power conversion efficiencies (PCEs). The study provides an alternative route and expands the appli- cation of PPors in PSCs as a blue-light harvester in ternary- blend PSCs using amorphous polymers as host. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 5032- 5040, 2012