| dc.description.abstract | A new series of liner organic compounds containing 1-hexyl-4,5-dicyanoimidazole (DCI) unit have been synthesized. These compounds can be of asymmetric type with a D-S-A motif, or of symmetric type with a A-S-D-S-A motif, where D is the electron donor such as diphenylamino or anthracenyl moiety, A was the electron acceptor (DCI), and S is theπ-conjugated spacer of different length. The compounds have high molar absorption coefficients (23,200−53,100 M-1cm-1) in the region 342−504 nm, and the absorption coefficient and wavelength increase as the molecules have better co-planarity. Among the compounds, the anthracene-based molecules with rather planar π−conjugated system have superior light-harvesting behavior. The longer wavelength absorptions of these molecules are π−π* transitions with charge-transfer character due to the presence of the electron-withdrawing DCI group. Theoretical calculations indicate that these compounds possess fairly small dihedral angles of the conjugation spacer (3.1−20.4o) except that between DCI and the neighboring phenyl groups (42.3−46.3o). The photovoltaic devices of BHJ configuration using these DCI-containing compounds and PCBM (phenyl-C61-butyric acid methyl ester) as the donor and the acceptor, respectively, were also studied. The photovoltaic cell fabricated with the blend of TVPY−DCI and PCBM in a 1:2 ratio exhibits the highest power conversion efficiency (?) reaching 0.33%. Except for T−DCI, all the devices fabricated with the DCI molecules exhibited high open-circuit voltage (VOC) in the range of 0.89−1.09 V, which can be attributed to the more stabilized HOMO levels of the DCI compounds. In spite of good light-harvesting behavior, the low solubility of anthracene-based molecules in common organic solvents prevent formation of films with good quality, which led to low performance of the devices based on them. This is further evidenced by the atomic force microscopy (AFM) image, a plethora of aggregation was found for these molecules which led to low current density (JSC). On the contrary, the AFM image of compound TVPY−DCI showed rather flattened active layer, which resulted high current density and power conversion efficiency.
| en_US |