Structure difference of sorbitol derivatives influences the crystallization and performance of P3OT/PCBM organic photovoltaic solar cells

In this work, we compare the effects of sorbitol derivatives (1,3:2,4-dibenzylidene sorbitol (DBS), 1,3:2,4-di(p-methylbenzylidene) sorbitol (MDBS) and 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol (DMDBS)) on the performances of poly(3-octyl thiophene)/[6,6]-phenyl C₆₁-butyric acid methyl ester (P3OT/PCBM) bulk heterojunction (BHJ) organic photovoltaic (OPV) devices and explore the mechanism. Differential scanning calorimetry (DSC) and atomic force microscopy (AFM) measurements indicate that DBS, MDBS and DMDBS are nucleating agents of P3OT. DMDBS has the strongest molecular polarizability and exhibits the best propensity for self-assembly in 1,2-dichlorobenzene (ODCB). The strong π-π stacking of aromatic benzylidene group and the high density of the fibrillary aggregates supply more nucleation surfaces for P3OT, leading DMDBS has the highest nucleation efficiency (NE). Sorbitol derivatives accelerate the crystallization rate (G) of P3OT with the order as G_P3OT/DMDBS > G_P3OT/MDBS > G_P3OT/DBS > G_P3OT. The acceleration of the crystallization increases the number of tie molecules, causing the improvement of the connectivity between ordered regions, resulting dramatically increasing the carrier transport of P3OT. G_P3OT/DMDBS is highest, the connectivity between ordered regions is best in P3OT with DMDBS. UV–vis measurement indicates that the intra-chain order of P3OT reduces with the addition of sorbitol derivative, and the intra-chain order of P3OT with DMDBS is lowest. The P3OT/PCBM/sorbitol derivative BHJ OPV devices were fabricated and show that the short circuit current J_{SC P3OT/DMDBS} > J_{SC P3OT/MDBS} > J_{SC P3OT/DBS} > J_{SC P3OT}. It hints that the connectivity of tie molecules plays a significant role in defining semiconducting polymer transport characteristics, and is perhaps more important than molecular level interactions (inter- and intra-chain order) for efficient macroscopic charge carrier transport. Finally, it shows that adding sorbitol derivatives can improve the power conversion efficiency (PCE) of P3OT/PCBM BHJ OPV device, the best PCE as 1.77% is obtained in the P3OT/PCBM/DMDBS device.