Ya-Ze Li

The synthesis, structure, and properties of 5,6,11,12-tetraarylindeno[1,2-b]fluorenes and their applications as donors for organic photovoltaic devices

The synthesis, structure, and properties of three new 5,6,11,12-tetraarylindeno[1,2-b]fluorenes are reported. The highly twisted conformations between an indeno[1,2-b]fluorene core and peripheral aryl substitutions endow these indeno[1,2-b]fluorene derivatives with good photostability for use as electron donors for vacuum-deposited photovoltaic devices. The optimized device based on a TAInF2 donor blended with C70 as an electron acceptor produces a high open-circuit voltage (>0.9 V) and a power conversion efficiency of 2.91%. This work demonstrates the first application of an indenofluorene derivative as an electron donor in organic solar cells.

Cathodic-controlled and near-infrared organic upconverter for local blood vessels mapping.

Organic materials are used in novel optoelectronic devices because of the ease and high compatibility of their fabrication processes. Here, we demonstrate a low-driving-voltage cathodic-controlled organic upconverter with a mapping application that converts near-infrared images to produce images of visible blood vessels. The proposed upconverter has a multilayer structure consisting of a photosensitive charge-generation layer (CGL) and a phosphorescent organic light-emitting diode (OLED) for producing clear images with a high resolution of 600 dots per inch. In this study, temperature-dependent electrical characterization was performed to analyze the interfacial modification of the cathodic-controlled upconverter. The result shows that the upconverter demonstrated a high conversion efficiency of 3.46% because of reduction in the injection barrier height at the interface between the CGL and the OLED.

Influence of Singlet and Charge-Transfer Excitons on the Open-Circuit Voltage of Rubrene/Fullerene Organic Photovoltaic Device

We demonstrated that the open-circuit voltage (VOC) of rubrene/C60 organic photovoltaic (OPV) devices can be substantially improved by changing the rubrene thickness. A shoulder exhibited in a range of 500-550 nm was observed. This result indicated that the singlet excitons of rubrene were increased when the thickness of the rubrene layer was increased. Capacitance-voltage measurements were conducted for estimating the built-in potential of the devices. The calculated VOC was higher than that of the experiment, thus indicating that energetic losses occurred in the devices. We reused the reciprocity and revised Marcus theory for determining the charge-transfer (CT) properties of the devices. The CT properties of the CT states at the rubrene/C60 interface remained similar. The nonradiative energetic losses become smaller when the rubrene layer was increased, thus indicating the bimolecular recombination was increased. The increased recombination thermally activated the electrons in C60 into rubrene for forming the singlet excitons in rubrene. The reduction in reorganization energy indicated that the electroluminescence of rubrene was enhanced, thereby improving VOC. These results proved that the two-step thermal activation of C60 electrons and the improved VOC of rubrene were caused by the increased singlet excitons of rubrene.

Downscaling the Sample Thickness to Sub-Micrometers by Employing Organic Photovoltaic Materials as a Charge-Generation Layer in the Time-of-Flight Measurement

Time-of-flight (TOF) measurements typically require a sample thickness of several micrometers for determining the carrier mobility, thus rendering the applicability inefficient and unreliable because the sample thicknesses are orders of magnitude higher than those in real optoelectronic devices. Here, we use subphthalocyanine (SubPc):C70 as a charge-generation layer (CGL) in the TOF measurement and a commonly hole-transporting layer, N,N’-diphenyl-N,N’-bis(1,1’-biphenyl)-4,4’-diamine (NPB), as a standard material under test. When the NPB thickness is reduced from 2 to 0.3 μm and with a thin 10-nm CGL, the hole transient signal still shows non-dispersive properties under various applied fields, and thus the hole mobility is determined accordingly. Only 1-μm NPB is required for determining the electron mobility by using the proposed CGL. Both the thicknesses are the thinnest value reported to data. In addition, the flexibility of fabrication process of small molecules can deposit the proposed CGL underneath and atop the material under test. Therefore, this technique is applicable to small-molecule and polymeric materials. We also propose a new approach to design the TOF sample using an optical simulation. These results strongly demonstrate that the proposed technique is valuable tool in determining the carrier mobility and may spur additional research in this field.

Low resistance and high work-function WO3/Ag/MoO2 multilayer as transparent anode for bright organic light-emitting diodes

The authors report efficient organic light-emitting diodes (OLEDs) using a high conductive transparent WAM multilayer as anode electrode [WAM=WO3 (30nm)/Ag (10 nm)/MoO2 (5 nm)], which was prepared by thermal evaporation under room temperature condition to form the smooth morphology on anode surface, leading to reduce the injection barrier between metal/organic interface. The WAM anode shows a low sheet resistance of 8.27 Ω/sq, suitable injection workfunction of ~5.68 e V, and high optical transmittance of ~80% at range of visible light from 400 to 550 nm. In addition, a hole only WAM-device by atomic-force microscopy and electrical characterizations has demonstrates that the efficient hole injection property is existing at WAM/NPB interface as compared with the standard transparent conductive oxide of indium tin oxide. From the device characterization, the device performance of green fluorescent OLED based on WAM anode exhibiting a high brightness of 150 000 cd/m2 and current efficiency of 20 cd/A at driving voltage of 8 V has been achieved.

A wide absorption donor-acceptor active layer for vacuum-deposited organic photovoltaic devices with a 6.8 % power conversion efficiency

In this letter, the authors present a highly efficient small-molecule organic photovoltaic (OPV) with a donor-acceptor bulk-heterojunction of a novel electron donor material and C70. As a result, the OPV exhibited a highest power conversion efficiency of 6.8%, an open circuit voltage of 0.79 V, a short-circuit current density of 16.13 mA/cm2 and a fill factor (FF) of 53.9 % under 1 sun solar illumination (AM 1.5G).

Low resistance and high work-function WO 3 /Ag/MoO 2 multilayer as transparent anode for bright organic light-emitting diodes

The authors report efficient organic light-emitting diodes (OLEDs) using a high conductive transparent WAM multilayer as anode electrode [WAM=WO3 (30nm)/Ag (10 nm)/MoO2 (5 nm)], which was prepared by thermal evaporation under room temperature condition to form the smooth morphology on anode surface, leading to reduce the injection barrier between metal/organic interface. The WAM anode shows a low sheet resistance of 8.27 Ω/sq, suitable injection workfunction of ~5.68 e V, and high optical transmittance of ~80% at range of visible light from 400 to 550 nm. In addition, a hole only WAM-device by atomic-force microscopy and electrical characterizations has demonstrates that the efficient hole injection property is existing at WAM/NPB interface as compared with the standard transparent conductive oxide of indium tin oxide. From the device characterization, the device performance of green fluorescent OLED based on WAM anode exhibiting a high brightness of 150 000 cd/m 2 and current efficiency of 20 cd/A at driving voltage of 8 V has been achieved.