Electrospun Nanofibers for Optoelectronic Device Applications

One-dimensional (1D) nanostructures coupled with its possible technological applications have fueled the exponential growth of optoelectronics because of the anisotropic transportation of charge-carrier. The geometry confinement provided by 1D nanostructures also alter the optoelectrical characteristic compared to their bulkcounterpart. Among of the various fabrication of 1D nanostructure, electrospinning is a versatile assembly method for fabricating uniform and ultrafine nanofibers with different patterning via various geometric collectors. Also, electronspun nanofibers can serve as matrix for functional materials such as metallic nanoparticle, conjugated polymer and quantum dot for diverse application in optoelectronic device. However, the applications of such nanofibers in optoelectronic devices have not been fully explored yet. Furthermore, the unique process of electrospinning enable the distinctive characteristic of electrospun nanofiber is also not investigated. In this thesis, we produce diverse functional ES nanofibers and explore their morphology, opto-electrical properties and applications on organic field-effect transistor (OFET) and organic photovoltaic(OPV). The details of each topic are summarized as below: In chapter 2, an extra electric field below the spinneret in electrospinning setup was introduced for producing poly(3-hexylthiophene) (P3HT) nanofibers. The liquid jet is greatly prolonged by the additional extensional force and thinner fibers can thusbe obtained. The chain conformation and orientation in fibers are probed by differential scanning calorimetry (DSC) and X-ray diffraction techniques. Under the influence of the secondary electric field, P3HT chains are extensively stretched and aligned along the fiber axis. The electrospun P3HT nanofibers are fabricated into field-effect transistors and the charge carrier mobilities of the nanofibers with and without secondary electric field are found to be 1.54×10-4 and 1.62×10-1 cm2 V-1 s-1, respectively. The dramatic enhancement of mobility by more than 1000 times is due to the effective charge transport through the delocalization of electrons along the highly extended and oriented P3HT backbones rather than the ordinary π-π stacking. In addition to P3HT, we find this simple method also works for other poly(3-alkylthiophene). In chapter 3, the significant enhancement of the P3HT:PC61BM ([6,6]-phenyl C61-butyric acid methyl ester) photovoltaic devices using different patterns of the electrospun Ag/PVP composite nanofibers, including non-woven, aligned-, and crossed-patterns were reported. The composite electrospun nanofibers were prepared through the in-situ reduction of silver (Ag) nanoparticles in Ag/poly(vinyl pyrrolidone) (PVP) through two-fluid coaxial electrospinning technique. The composition, crystalline orientation, and particle size of Ag were successfully manipulated by controlling the core/shell solution concentration (AgF-1, AgF-2 and AgF-3), as evidenced by FE-SEM, TEM and SAED analyses. The smallest diameter of thecomposite nanofibers led to the highest orientation of the Ag nanoparticles and resulted in the largest conductivity comparable to that of ITO, due to the geometrical confinement. Such composite nanofibers exhibited the surface plasmon resonance (SPR) effect, evidenced by the absorption peak around 425 nm, which provide near field enhancement of electromagnetic field around active layer. In addition, the composite nanofibers with the crossed- or nonwoven patterns further enhanced high carrier mobility, compared to those of aligned-pattern. It led to the 18.7% enhancement on the power conversion efficiency of photovoltaic cell (ITO/composite nanofibers/PEDOT:PSS/P3HT:PC61BM/Ca/Al) compared to the parent device. The above results indicated the high conductivity and SPR effect of the Ag/PVP electrospun nanofibers could significantly improve the photocurrent as well as PCE, leading to promising organic solar cell applications. In chapter 4, plasmonic-enhanced luminescent solar concentrator (LSC) electrospun nanofiber by coaxial electrospinning technique with poly[2,7-(9,9-dihexylfluorene)-alt-4,7-(2,1,3-benzothiadiazole)](PFBT) nanoparticle as the LSC and Ag nanoparticle as the SPR center was explored. The ES nanofiber, crosslinked poly(methacrylic acid) (PMAA),provides a solvent-proof matrix for LSC without changing the conventional OPVs configuration. Besides, the in-situ reduction of Ag nanoparticle simultaneously enhanced the exciton generation of PFBT and active material with the SPR effect. The dualfunctional ES nanofibers allow significant light harvesting through down conversion and enhanced exciton generation, leading to remarkably 18 % enhancement on the PCE for both P3HT:PC61BM and PTB7 (polythieno[3,4-b]-thiophene-cobenzodithiophene): PC71BM([6,6]-phenyl C71-butyric acid methyl ester) [6,6]-phenyl) photovoltaic cells. This configuration provides a novel method to integrate the plasmonicenhanced LSC electrospun nanofiber into OPV device with the combination of two light trapping methods (SPR and LSC) and the maintenance of active area coverage. The above studies address the characteristics of electrospun nanofibers can be feasibly manipulated the composition and process condition during electrospinning. The orientation property in micro-scope and macro-scope of electrospun nanofibers also indicate the unique application in optoelectronic devices.