Yun-Yue Lin

Blue photoluminescence from chemically derived graphene oxide.

Blue photoluminescence from chemically derived graphene oxide Goki Eda, Yun-Yue Lin, Cecilia Mattevi, Hisato Yamaguchi, Hsin-An Chen, I-Sheng Chen, Chun-Wei Chen, and Manish Chhowalla 1 Department of Materials, Imperial College, Exhibition Road, London SW7 2AZ, UK. 2 Department of Materials Science and Engineering, Rutgers University 607 Taylor Road, Piscataway, NJ 08854, USA. 3 Department of Materials Science and Engineering, National Taiwan University No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan.

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.

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.

The influence of interface modifier on the performance of nanostructured ZnO/polymer hybrid solar cells

We have demonstrated an improvement of photovoltaic performance based on the nanostructured ZnO/poly(3-hexylthiophene) (P3HT) hybrid through interface molecular modification on ZnO nanorod surface. By probing the carrier dynamics at ZnO/P3HT interfaces, we have found that the interfacial molecules can play the role of assisting charge separation and suppression of back recombination at interfaces, which accounts for the observed enhanced short circuit current (Jsc) and open circuit voltage (Voc) in photovoltaic performance.

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.

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.

Improved performance of polymer/TiO2 nanorod bulk heterojunction photovoltaic devices by interface modification

In this article, the polymer photovoltaic devices based on the poly(3-hexylthiophene)/TiO2 nanorods hybrid material is present. An enhancement in the device performance can be achieved by removing or replacing the insulating surfactant on the TiO2 nanorod surface with a more conductive ligand, which can play the role to assist charge separation efficiency or also to prevent from back recombination, giving a large improvement in the short circuit current and fill factor. The relatively high power conversion efficiency of 1.7% under simulated AM 1.5 illumination (100mW∕cm2) can be achieved, providing a route for fabricating low-cost, environmentally friendly polymer photovoltaic devices by all-solution processes.