Symmetry and coplanarity of organic molecules affect their packing and photovoltaic properties in solution-processed solar cells.

In this study we synthesized three acceptor-donor-acceptor (A-D-A) organic molecules, TB3t-BT, TB3t-BTT, and TB3t-BDT, comprising 2,2'-bithiophene (BT), benzo[1,2-b:3,4-b':5,6-d″]trithiophene (BTT), and benzo[1,2-b;4,5-b']dithiophene (BDT) units, respectively, as central cores (donors), terthiophene (3t) as π-conjugated spacers, and thiobarbituric acid (TB) units as acceptors. These molecules display different degrees of coplanarity as evidenced by the differences in dihedral angles calculated from density functional theory. By using differential scanning calorimetry and X-ray diffractions for probing their crystallization characteristics and molecular packing in active layers, we found that the symmetry and coplanarity of molecules would significantly affect the melting/crystallization behavior and the formation of crystalline domains in the blend film with fullerene, PC61BM.

Distribution of crystalline polymer and fullerene clusters in both horizontal and vertical directions of high-efficiency bulk heterojunction solar cells.

In this study, we used (i) synchrotron grazing-incidence small-/wide-angle X-ray scattering to elucidate the crystallinity of the polymer PBTC12TPD and the sizes of the clusters of the fullerenes PC61BM and ThC61BM and (ii) transmission electron microscopy/electron energy loss spectroscopy to decipher both horizontal and vertical distributions of fullerenes in PBTC12TPD/fullerene films processed with chloroform, chlorobenzene and dichlorobezene. We found that the crystallinity of the polymer and the sizes along with the distributions of the fullerene clusters were critically dependent on the solubility of the polymer in the processing solvent when the solubility of fullerenes is much higher than that of the polymer in the solvent. In particular, with chloroform (CF) as the processing solvent, the polymer and fullerene units in the PBTC12TPD/ThC61BM layer not only give rise to higher crystallinity and a more uniform and finer fullerene cluster dispersion but also formed nanometer scale interpenetrating network structures and presented a gradient in the distribution of the fullerene clusters and polymer, with a higher polymer density near the anode and a higher fullerene density near the cathode.

Synthesis, electrochemistry, absorption and electro-polymerization of aniline-ethynyl metalloporphyrins.

Free-base, nickel(II), zinc(II) and iron(III) mono(4-aniline-ethynyl)biphenylporphines (AEBPPs) were synthesized and their electro-polymerization reactions were studied. Electrochemical and UV-Visible experiments showed that AEBPPs were electro-polymerized onto Pt and indium tin oxide (ITO) surfaces without significantly changing their redox and spectral properties. This suggests that the pi-conjugations of these porphyrins were not greatly affected upon electro-polymerization.