Self-Assembling Thiophene Dendrimers with a Hexa-peri-hexabenzocoronene Core-Synthesis, Characterization and Performance in Bulk Heterojunction Solar Cells

The solidstate organization of molecules is an important factor in determining the performance of organic electronic devices. In bulk heterojunction (BHJ) solar cells, the arrangement of electron donor and acceptor materials into distinct crystalline phases of ideal size and distribution can lead to better power conversion efficiencies. The use of fluorenyl hexa-peri-hexabenzocoronene (FHBC) 2 in this study has highlighted the importance of molecular organization to device performance. FHBC compounds 6, 8, and 10, functionalized with a series of thiophene dendrons, were synthesized using Suzuki-Miyaura coupling in high yields. In UV-vis and H-1 NMR spectroscopic studies, all FHBC derivatives showed self-association in solution. Hexagonal packing of columnar structures was observed for solid state samples of FHBC 2 and 8 in two-dimensional wide-angle X-ray scattering experiments. In thin film X-ray experiments, ordered structures were observed in blends of FHBC2 and fullerene acceptor materials indicating that there is phase separation between the donor and acceptor materials and that the self-organization of the FHBC material is unaffected. While the large thiophene dendritic substituent attached to compound 10 broadened its UV-vis absorption profile, the solid state morphology is altered by the bulky thiophene dendrons. These molecular structure variations arc reflected in the performance characteristics of BHJ solar cell devices fabricated using these FHBC compounds as electron donor materials. Power conversion deficiency of 2.5% was achieved for a device containing cornpound 10 with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor material. This compares favorably with devices fabricated with pure dendritic thiophene materials and illustrates the positive effect of molecular self-organization on device performance.