A new conjugated polymer based on 5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione, named as PBDTBDD, was designed, synthesized, and applied in polymer solar cells (PSCs). A power conversion efficiency (PCE) of 6.67% was obtained from the PBDTBDD/PC61BM-based PSC, which is a remarkable result for the PSCs using PC61BM as electron acceptor. The PBDTBDD/PC61BM-based device exhibits a narrow absorption band and excellent quantum efficiency in the range from 500 to 700 nm. Furthermore, PBDTBDD shows a strong aggregation effect in solution state, and the study indicates that although the temperature used in solution preparation has little influence on molecular orientation as well as crystallinity of the D/A blend, it plays an important role in forming proper domain size in the blend. This work provides a good example to reveal the correlation between the morphology of the blend films and the processing temperature of the solution preparation. Furthermore, the study in this work suggests an interesting and feasible approach to modulate domain size without changing crystallinity of the blend films in PSCs.

This paper investigates the effect of mesophase separation on the crystallization behavior of olefin block copolymers (OBCs) with different octene contents, which were synthesized by chain shuttling technology. Crystallization always occurs simultaneously but competitively with mesophase separation in OBCs. Because of the reason that the crystallization temperature is lower than the mesophase separation temperature for the OBCs, the mesophase separation can start first; large portions of the crystallizable hard blocks are confined in the mesophase-separated domains and could not contribute to the formation of radial lamellar stacks. In addition, the mesophase separation creates a stereo-hindrance effect; crystal lamellae could only grow through the interstitial space between the dispersed domains. As a result, large and compact crystals could not be formed. As the octene content increased in the sample, mesophase separation becomes more and more dominant, and the crystal morphology degrades sharply from spherulites to fragmentary lamella structures. It is found that increasing the annealing time during development of the mesophase-separated structure has a similar effect to increasing the octene content in the sample. However, all of the OBCs can form nearly the same crystalline morphology if the mesophase separation is suppressed, from which we can postulate that the nature of crystallization due to the crystallizable hard blocks in OBCs should be similar.