The Fabrication And Application Of Conjugated Polymer Nanostructure Arrays With Highly Oriented Internal Structure

Due to the charge transport in conjugated polymer being anisotropic, resulting fromelectrons delocalized along the polymer backbone and the overlap of orbits, the controlover the molecular and crystal orientation of conjugated polymer is a key element indetermining its property and application, and becomes one of the most significant researchtopics in the fields of polymer and condensed matter physics. In this thesis, the conjugatedpolymers poly(3-hexylthiophene)(P3HT) and thieno (3,4-b) thiophene-alt-benzodithi-ophene copolymer (PTB7) are selected as the study objects. We focus on the preferentialmolecule or crystal orientation of conjugated polymer nanostructure array, including thefabrication of polymer nanostructure array with the aid of the solvent-assistant room-temperature nanoimprinting method (SART-NIL), the rules of molecular and crystalalignment in polymer nanostructure arrays, the relationship between the molecular orcrystal orientation and property, and the effect of molecular or crystal orientation in theconjugated polymer nanostructure on the performance of ordered heterojunction polymersolar cell.The uniform arrays of hexagonally packed P3HT and PTB7nanopillars are fabricatedby the SART-NIL method, due to the conjugated polymer being easily oxidized anddecomposed at elevated temperature. The nanopillars, bearing a diameter of35~130nmand a center-to-center distance (period) of55~150nm, precisely replicate the dimensionsof anodic aluminum oxide (AAO) molds. AAO templates are prepared by a two-stepanodization method.The molecular and crystal orientation of conjugated polymer nanostructure arrayshave been discussed in detail with the aid of the synchrotron radiation grazing incidencewide angle X-rays diffraction (GIWAXD) measurements. The SART-NIL process inducesface-on chain and crystal alignment in the P3HT nanopillars, and the preferentialorientation can be rationalized based on the confinement of the rod-like P3HT crystalswithin the nanostructure arrays. Furthermore, a critical diameter which is equal or smallerthan the length of P3HT rod-like crystals is needed to induce the favorable alignment within nanopillars. The face-on chain and crystal alignment solely depends on thediameters of nanopores, rather than on the solvents, temperatures, substrates and nanopillarheights. The face-on molecular and crystal orientation is also obtained within theconjugated polymer PTB7films and nanostructure arrays.The conducting atomic force microscopy (C-AFM) is used to investigate the influenceof the molecular and crystal orientation on the conducting ability. The average currentflowing through the P3HT nanopillars with face-on alignment is enhanced obviouslycompared to the film with edge-on orientation. The enhancement of conducting capabilityis due to the favorable electron delocalization along the-stacking direction compared tothe side chain direction. In addition, with the decrease of confinement dimension, thedegree of face-on molecular and crystal orientation is improved and the conductingcapability of corresponding nanostructure arrays is also enhanced. Compared to theunprocessed film, no alteration of conducting capacity within PTB7nanopillars is due tothe unchanged molecular and crystal orientation.The conjugated polymer nanostructure array with preferential molecule orientation isemployed to effectively fabricate ordered polymer/fullerene bulk heterojunction solar cells,and we focus on the effect of molecular and crystal orientation on the performance ofordered bulk heterojunction solar cell. The face-on molecular and crystal alignmentimproves exciton dissociation and charge transportation in the ordered bulk heterojunctionsolar cells, as a result, the device performance is enhanced, especially in the short circuitcurrent, fill factor and power conversion efficiency.According to the study of conjugated polymer nanostructure array with preferentialmolecular and crystal orientation, it develops a new method to control the molecularorientation of conjugated polymer, and also provides new insight and guidance forunderstanding the structure-property relationships of conjugated polymer. In addition, thisstudy may also lay a foundation for the application of conjugated polymer nanostructurearray to the semiconducting device.