Solar Cell Performance of Phenanthrodithiophene–Isoindigo Copolymers Depends on Their Thin-Film Structure and Molecular Weight

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Phenanthro[1,2-b:8,7-b′]dithiophene (PDT)–isoindigo (IID)-based polymers 12OD (L) and 12OD (H) with a different molecular weight were synthesized and characterized. By using further purified PDT and IID monomers, the high-molecular-weight polymer 12OD (H) with number-average molecular weight (Mn) over 50 kDa was obtained. Both 12OD (L) and 12OD (H) polymers have the same energy gap and highest occupied molecular orbital (HOMO) energy levels, indicating that the influence of molecular weight on their electronic structure is negligible, although 12OD (H) has stronger aggregation behavior than 12OD (L). 12OD (H)-based solar cells fabricated by using optimal solvent and additives showed an increased short-circuit current density (Jsc) with same open-circuit voltage (Voc) and fill factor (FF), resulting in a significantly improved power conversion efficiency (PCE) of up to 6.1%, which is approximately 70% higher than that of the 12OD (L)-based cell (3.5%). This result is due to the different molecular orientation caused by the higher molecular weight. Grazing incidence wide-angle X-ray scattering (GIWAXS) analyses revealed that the blended film of 12OD (H)/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) formed a face-on orientation with a long-range ordered structure, while a low crystalline edge-on structure was observed in the blended film of 12OD (L)/PC61BM. Such high crystalline and favorable molecular orientation could promote light harvesting efficiency and hole transporting ability, resulting in high Jsc and thus an excellent PCE. From the detailed GIWAXS analysis, the orientation change of 12OD (H) was induced by the addition of PC61BM. This result suggests that the strong aggregation ability of 12OD (H) can drive crystallization and favors active interaction with PC61BM to form a face-on orientation. In this study, the increase in molecular weight can improve not only the thin-film structure such as crystallinity and phase separation structure but also the molecular orientation in the PDT-based polymer system.