Wei-Fang Su

Transparent and conducting electrodes for organic electronics from reduced graphene oxide

The deposition and optoelectronic properties of reduced graphene oxide thin films are described. Thin films with thicknesses ranging from 1–10nm have been deposited by the vacuum filtration method. The conductivity of the thin films can be varied over six orders of magnitude by varying the filtration volume of the graphene oxide aqueous suspension while maintaining the transmittance between 60%–95%. In addition, enhancement in the conductance through Cl doping is demonstrated. The combination of the reduction and Cl treatments make the reduced graphene oxide thin films sufficiently conducting to incorporate them as the hole collecting electrode in proof of concept organic photovoltaic devices.

Interfacial Nanostructuring on the Performance of Polymer/ TiO2 Nanorod Bulk Heterojunction Solar Cells

This work presents polymer photovoltaic devices based on poly(3-hexylthiophene) (P3HT) and TiO2 nanorod hybrid bulk heterojunctions. Interface modification of a TiO2 nanorod surface is conducted to yield a very promising device performance of 2.20% with a short circuit current density (J(sc)) of 4.33 mA/cm2, an open circuit voltage (V(oc)) of 0.78 V, and a fill factor (FF) of 0.65 under simulated A.M. 1.5 illumination (100 mW/cm2). The suppression of recombination at P3HT/TiO2 nanorod interfaces by the attachment of effective ligand molecules substantially improves device performance. The correlation between surface photovoltage and hybrid morphology is revealed by scanning Kelvin probe microscopy. The proposed method provides a new route for fabricating low-cost, environmentally friendly polymer/inorganic hybrid bulk heterojunction photovoltaic devices.

Investigation of nanoscale morphological changes in organic photovoltaics during solvent vapor annealing

Improvement of the photovoltaic efficiency via exposure of organic poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) devices to solvent vapor at room temperature is reported. In situ photoluminescence (PL) and Raman spectroscopy, in conjunction with ex situ optical absorption and atomic force microscopy, have been used to provide insight into the nanoscale morphological changes occurring during solvent vapor annealing. We found that in 1 : 1 composites of P3HT : PCBM, suppression of PL, narrowing in line-width of the 1442 cm−1 P3HT Raman peak, and strong modifications in the optical absorption spectra were observed during solvent vapor annealing, while minimal changes occurred in pure P3HT films. We attribute these spectral modifications to de-mixing of PCBM and subsequent stacking of P3HT in coplanar conjugated segments, similar to what is observed during thermal annealing.

Ring-Opening Polymerization

Polymers can be synthesized by polymerizing monomers with ring structure as shown in Table 11.1. The polymerization of these compounds has some aspects of both chain and step polymerizations as far as kinetics and mechanisms are concerned. Table 11.1 Examples of polymers prepared by ring-opening polymerization [1] Polymer type Polymer repeating group Monomer structure Monomer type Polyalkene Open image in new window Open image in new window Cyclic alkene Polyether Open image in new window Open image in new window Cyclic ethera Polyesterb Open image in new window Open image in new window Lactone Polyamide Open image in new window Open image in new window Lactam Polysiloxane Open image in new window Open image in new window Cyclic siloxane Polyphosphazene Open image in new window Open image in new window Hexachloro-cyclotriphosphazenec Polyamine Open image in new window Open image in new window Aziridened aEpoxide (x = 2); oxetane (x = 3) bRing opening of cyclic oligomers has also been developed cPhosphonitrilic chloride trimer dAlso called alkyleneimine

Enhancing photoluminescence quenching and photoelectric properties of CdSe quantum dots with hole accepting ligands

CdSe quantum dots have been encapped with aromatic ligands: α-toluenethiol, thiophenol, and p-hydroxythiophenol to enhance the photoluminescence (PL) quenching and photoelectric properties of the quantum dots. The aromatic ligand capped CdSe quantum dots are prepared through ligand exchange with trioctylphosphine oxide (TOPO) capped CdSe quantum dots. The XPS surface chemistry analysis and elemental analysis has confirmed the success of ligand exchange from TOPO to aromatic ligands. Both XRD and HRTEM-SAED studies indicate the crystalline structure of CdSe quantum dots not only remains but is also improved by the ligand exchange of TOPO with thiol molecules. Time resolved PL decay measurements indicate thiophenol and p-hydroxythiophenol ligands effectively quench the emission and have much shorter PL lifetimes than that of TOPO and that of α-toluenethiol. Thus, both thiophenol and p-hydroxythiophenol can act as an effective acceptor for photogenerated holes through aromatic π-electrons. Thiophenol also exhibits good charge transport behavior showing a 10-fold increase in short circuit current density (Isc) as compared with TOPO in the photocurrent study of fabricated photovoltaic devices.

Tuning perovskite morphology by polymer additive for high efficiency solar cell.

Solution processable planar heterojunction perovskite solar cell is a very promising new technology for low cost renewable energy. One of the most common cell structures is FTO/TiO2/CH3NH3PbI3-xClx/spiro-OMeTAD/Au. The main issues of this type of solar cell are the poor coverage and morphology control of the perovskite CH3NH3PbI3-xClx film on TiO2. For the first time, we demonstrate that the problems can be easily resolved by using a polymer additive in perovskite precursor solution during the film formation process. A 25% increase in power conversion efficiency at a value of 13.2% is achieved by adding 1 wt % of poly(ethylene glycol) in the perovskite layer using a 150 °C processed TiO2 nanoparticle layer. The morphology of this new perovskite was carefully studied by SEM, XRD, and AFM. The results reveal that the additive controls the size and aggregation of perovskite crystals and helps the formation of smooth film over TiO2 completely. Thus, the Voc and Jsc are greatly increased for a high efficiency solar cell. The amount of additive is optimized at 1 wt % due to its insulating characteristics. This research provides a facile way to fabricate a high efficiency perovskite solar cell by the low temperature solution process (<150 °C), which has the advancement of conserving energy over the traditional high temperature sintering TiO2 compact layer device.

A large interconnecting network within hybrid MEH-PPV/TiO 2 nanorod photovoltaic devices

This is a study of hybrid photovoltaic devices based on TiO2 nanorods and poly[2-methoxy-5-(2 � -ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV). We use TiO 2 nanorods as the electron acceptors and conduction pathways. Here we describe how to develop a large interconnecting network within the photovoltaic device fabricated by inserting a layer of TiO2 nanorods between the MEH-PPV:TiO2 nanorod hybrid active layer and the aluminium electrode. The formation of a large interconnecting network provides better connectivity to the electrode, leading to a 2.5-fold improvement in external quantum efficiency as compared to the reference device without the TiO2 nanorod layer. A power conversion efficiency of 2.2% under illumination at 565 nm and a maximum external quantum efficiency of 24% at 430 nm are achieved. A power conversion efficiency of 0.49% is obtained under Air Mass 1.5 illumination. (Some figures in this article are in colour only in the electronic version)

Near-ultraviolet photodetector based on hybrid polymer/zinc oxide nanorods by low-temperature solution processes

In this article, we have proposed a nanostructured near-ultraviolet photodetector (<400nm) based on the ZnO nanorod/polyfluorene hybrid by solution processes at low temperature. The current-voltage characteristic of the hybrid device demonstrates the typical pn-heterojunction diode behavior, consisting of p-type polymer and n-type ZnO nanorods, respectively. The relative quantum efficiencies of the hybrid device exhibit a nearly three order difference while illuminated under UV and visible light, respectively. The responsivity for the device can reach to 0.18A∕W at 300nm by applying a bias of −2V, which provides a route to fabricate a low-cost near-UV photodetector.

Nitrogen-Doped Anatase Nanofibers Decorated with Noble Metal Nanoparticles for Photocatalytic Production of Hydrogen

We report the synthesis of N-doped TiO(2) nanofibers and high photocatalytic efficiency in generating hydrogen from ethanol-water mixtures under UV-A and UV-B irradiation. Titanate nanofibers synthesized by hydrothermal method are annealed in air and/or ammonia to achieve N-doped anatase fibers. Depending on the synthesis route, either interstitial N atoms or new N-Ti bonds appear in the lattice, resulting in slight lattice expansion as shown by XPS and HR-TEM analysis, respectively. These nanofibers were then used as support for Pd and Pt nanoparticles deposited with wet impregnation followed by calcination and reduction. In the hydrogen generation tests, the N-doped samples were clearly outperforming their undoped counterparts, showing remarkable efficiency not only under UV-B but also with UV-A illumination. When 100 mg of catalyst (N-doped TiO(2) nanofiber decorated with Pt nanoparticles) was applied to 1 L of water-ethanol mixture, the H(2) evolution rates were as high as 700 μmol/h (UV-A) and 2250 μmol/h (UV-B) corresponding to photo energy conversion percentages of ∼3.6 and ∼12.3%, respectively.

Nanostructured metal oxide/conjugated polymer hybrid solar cells by low temperature solution processes

In this article, we have proposed a nanostructured photovoltaic device based on the ZnO nanostructures/poly(3-hexylthiophene)(P3HT):TiO2 nanorod hybrid by solution processes at low temperature. An array of ZnO nanorods with a larger size of ∼50 nm in diameter and ∼180 nm in length are grown to provide direct pathways for efficient charge collection. TiO2 nanorods with a size of ∼5 nm in diameter and ∼20–30 nm in length are incorporated into polymers to facilitate charge separation and transport by providing an increased interfacial area and a more effective transport pathway. The device performance with the inclusion of TiO2 nanorods exhibits a seven times increase in the short circuit current with respect to that without TiO2 nanorods. The device performance can be further enhanced after completely removing the residual surfactant on the TiO2 nanorods using the ligand exchange method, giving a short circuit current density of 2.67 mA cm−2 and a power conversion efficiency of 0.59% under Air Mass 1.5 (100 mW cm−2) illumination.