Crystal Morphology and Dimensionality Determine the Electronic Response in Novel Semiconducting Materials

2016Crystal morphology and dimensionality of single-crystal semiconducting materials can have dramatic effects on the optical and electrical properties. Organic photovoltaic (OPV) materials and fluorinated graphene folds are two materials that show these dependencies. Anthradithiophene (ADT), is an OPV that can be functionalized with different side groups to form different crystal stacking. Photoconductance measurements were taken on three different morphologies of ADT: ADT-TES-F, which exhibits 2D π-stacking, ADT-TSBS-F, which has a 1D sandwichherringbone stacking, and amorphous ADT. Polarization dependence of the spatially resolved light absorption and photoluminescence were used to identify single crystal domains in the TES and TSBS samples. Then confocal scanning was used to spatially resolve the photoconductance of the three different morphologies. In the amorphous sample, the photoconductance was uniform over the sample, but experienced quenching over the gold substrate. However, the crystalline samples showed enhanced photoconductance at the interface between the gold and glass substrates. This suggests that the electron-hole pair separation is primarily entropically driven, rather than by the classic photovoltaic charge separation. The interface breaks the symmetry of the crystal samples aiding the separation of the electron hole pairs, while, in the amorphous sample, the disordered molecules already provide the entropy needed for separation, so no enhancement is seen at the interface.\ud \ud Fluorinated graphene folds are a new material not previously published. The folds have a width on the order of tens of nanometers making them a quasi-one dimensional material. Using preliminary images from Cornell University, including pump-probe and SEM images, a list was made of 15 devices that may be functional. By comparison of resistances between electrodes on the devices, two connected devices were identified. Electrodes connected by a fold showed resistances on the order of 10⁵ Ω, where as the unconnected devices had resistances on the order of 10⁷ Ω and higher. Attempts were made to spatially resolve the photocurrent using confocal photocurrent scanning. However, noise in the setup drowned out the response of the fold and prevented the photocurrent from being resolved