Department of Materials Science and Engineering, University of Utah , 36 S. Wasatch Dr., Salt Lake City, Utah 84112, United States.
J Am Chem Soc. 2013 Nov 6;135(44):16490-6. doi: 10.1021/ja407024u. Epub 2013 Oct 23.
Donor-acceptor nanofibril composites have been fabricated, and the dependence of their photocurrent response on the structure and morphology of the donor part has been systematically investigated. The nanofibril composites were composed of template nanofibers, assembled from an electron acceptor molecule, perylene tetracarboxylic diimide (PTCDI), onto which (through drop-casting) various electron donor molecules (D1-D4) were coated. The donor molecules have the same π-conjugated core, but different side groups. Due to the different side groups, the four donor molecules showed distinctly different propensity for intermolecular aggregation, with D1-D3 forming segregated phases, while D4 prefers homogeneous molecular distribution within the film. It was found that the nanofibril composites with D4 exhibit the highest photocurrent, whereas those with aggregation-prone D1-D3 exhibited much lower photocurrent under the same illumination condition. Solvent annealing is found to further enhance the aggregation of D1-D3 but facilitate more uniform molecular distribution of D4 molecules. As a result, the photocurrent response of PTCDI fibers coated with D1-D3 decreased after vapor annealing, whereas those coated with D4 further increased. The detrimental effect of the aggregation of donor molecules on the PTCDI fiber is likely due to the enhanced local electrical field built up by the high charge density around the aggregate-nanofiber interface, which hinders the charge separation of the photogenerated electron-hole pair. The results reported in this study give further insight into the molecular structural effect on photoconductivity of hybrid materials, particularly those based on donor-acceptor composites or interfaces, and provide new molecular design rules and material processing guidelines to achieve high photoconductivity.
供体-受体纳米纤维复合材料已经制备，并且系统地研究了供体部分的结构和形态对其光电流响应的依赖性。纳米纤维复合材料由模板纳米纤维组成，这些模板纳米纤维由电子受体分子——苝四羧酸二酰亚胺（PTCDI）组装而成，通过滴铸法在其上涂覆了各种电子给体分子（D1-D4）。这些给体分子具有相同的π共轭核心，但具有不同的侧基。由于不同的侧基，这四个给体分子表现出明显不同的分子间聚集倾向，D1-D3 形成分离相，而 D4 则更喜欢在薄膜内均匀的分子分布。结果发现，具有 D4 的纳米纤维复合材料表现出最高的光电流，而在相同的照明条件下，易于聚集的 D1-D3 的纳米纤维复合材料表现出的光电流要低得多。溶剂退火被发现进一步增强了 D1-D3 的聚集，但有利于 D4 分子更均匀的分子分布。结果，用 D1-D3 涂覆的 PTCDI 纤维在蒸汽退火后光电流响应降低，而用 D4 涂覆的光电流响应进一步增加。给体分子聚集对 PTCDI 纤维的不利影响可能是由于在聚集-纳米纤维界面周围的高电荷密度引起的局部电场增强，这阻碍了光生电子-空穴对的电荷分离。本研究的结果进一步深入了解了分子结构对混合材料光电导率的影响，特别是那些基于供体-受体复合材料或界面的材料，为实现高光电导率提供了新的分子设计规则和材料处理指南。
