Photocurrent Enhancement of BODIPY-Based Solution-Processed Small-Molecule Solar Cells by Dimerization via the Meso Position

Three 4,4-difluoro-4-bora-3a,4a-diaza-s-indancene (BODIPY)-based small molecule donors H-T-BO, Br-T-BO, and DIMER were synthesized and fully characterized. Although modification at the meso position has a subtle influence on the light-harvesting ability, energy levels, and phase sizes, it has a striking effect on the packing behavior in solid film as two-dimension grazing incidence X-ray diffraction (2D GIXRD) and X-ray diffraction (XRD) confirm. Br-T-BO exhibits better packing ordering than H-T-BO in pristine film, which is beneficial from reinforced intermolecular interaction from halogen atoms. However, when [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is blended, no diffraction patterns corresponding to the monomeric donor can be seen from the XRD data and both H-T-BO- and Br-T-BO-based blend films give a slightly blue-shifting absorption peak with respect to their neat ones, both of which imply destruction of the crystalline structure. As for DIMER, the enhancement of the intermolecular interaction arises not only from the expansion of the backbone but the "steric pairing effect" brought on by its twisted structure. When blended with PC71BM, the diffraction patterns of DIMER are, however, kept well and the absorption peak position remains unchanged, which indicates the ordered packing of DIMER is held well in blend film. In coincidence with the fact that packing ordering improves from H-T-BO to Br-T-BO and DIMER in pristine films and the ordered packing of DIMER even in blend film, DIMER-based devices show the highest and most balanced hole/electron mobility of 1.16 × 10(-3)/0.90 × 10(-3) cm(2) V(-1) s(-1)with respect to Br-T-BO (4.71 × 10(-4)/2.09 × 10(-4) cm(2) V(-1) s(-1)) and H-T-BO (4.27 × 10(-5)/1.00 × 10(-5) cm(2) V(-1) s(-1)) based ones. The short-circuit current density of the three molecule-based cells follows the same trend from H-T-BO (6.80) to Br-T-BO (7.62) and then to DIMER (11.28 mA cm(-2)). Finally, the H-T-BO-, Br-T-BO-, and DIMER-based optimal device exhibits a power conversion efficiency of 1.56%, 1.96%, and 3.13%, respectively.