Chun-Feng Lin

Transparent Organic Upconversion Devices for Near‐Infrared Sensing

Transparent organic upconversion devices are shown in a night-vision demonstration of a real object under near-infrared (NIR) illumination in the dark. An extraordinarily high current gain - reflecting the on-off switching effect - greater than 15 000 at a driving voltage of 3 V is demonstrated, indicating the high sensitivity to NIR light and potential of using the proposed upconverter in practical applications. A maximum luminance exceeding 1500 cd m(-2) at 7 V is achieved. Unlike previous studies, where 2D aperture projection is reported, the current study shows 3D images of real objects under NIR illumination in the dark.

Characterization of intrinsic hysteresis of pentacene-based organic thin-film transistor through in-situ real-time electrical measurement

The intrinsic hysteresis of a pentacene-based organic thin-film transistor was characterized through home-designed in-situ real-time electrical measurement. The device exhibited intrinsic hysteresis after the device fabrication without breaking the vacuum, which has not been observed previously. Similar behavior was observed when introducing the nitrogen gas. Compared with the measurement condition of vacuum or nitrogen gas, exposure to the ambient air resulted in a severe hysteresis. It was attributed to both the acceptor-like traps at the organic/dielectric interface and the donor-like traps in the transport channel. When the chamber was vacuumed out again, a significantly reduced hysteresis was obtained almost the same as that measured just after device fabrication, indicating the reversibility of the extrinsic hysteresis. We also related the hysteresis to the morphological change under different deposition rates of pentacene. The smoother surface at higher deposition rate caused reduced hysteresis because of the elimination of vacancies near the pentacene/dielectric interface.

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.

Improvement in the Power Conversion Efficiency of Bulk Heterojunction Photovoltaic Device via Thermal Postannealing of Subphthalocyanine:C70 Active Layer

The authors report an efficient organic photovoltaic device based on subphthalocyanine (SubPc):C70 bulk heterojunction (BHJ) via the postannealing treatment. The power conversion efficiency is improved from 4.5% to 5.5% due to the increase in short-circuit current density () from 8.8 to 12.7 mA/cm2 with the expense of decreased fill factor from 52% to 42%. From external quantum efficiency measurements, the spectral shape-independent enhancement over the entire spectrum suggests that the increased mainly originates from improved charge collection efficiency. To confirm this inference, the hole and electron mobilities in the BHJ are estimated from the space-charge limited current, showing improved transport properties at the optimum temperature. Moreover, the morphologic change is also studied as a function of annealing temperature. A larger grain size is observed with increasing temperature due to the phase separation of SubPc and C70. However, at higher temperatures the strong aggregation of C70 molecules may interrupt the pathway of SubPc, resulting in hindered charge transport and, hence, reduced .

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.

Improving the performance of subphthalocyanine/C60 planar heterojunction organic photovoltaic device through the insertion of molybdenum oxide anodic buffer

Here, an efficient subphthalocyanine (SubPc)/C60 heterojunction organic photovoltaic device is demonstrated by using MoO3 as the anodic buffer. In comparison with the device without any treatments, the insertion of MoO3 leads to a significant increase in open-circuit voltage due to a better energy level alignment of the SubPc, which is similar to the use of oxygen-plasma. In addition, MoO3 serves as an optical spacer to tune the SubPc/C60 interface at the optimum optical field distribution. As a result, the short-circuit current density is considerably improved as predicted using the simulation model based on the transfer matrix. A slightly increased fill factor implies the efficient hole extraction after the insertion of MoO3. Moreover, the device with MoO3 as anodic buffer shows an elongated lifetime as compared with the device with oxygen-plasma treatment.

Origin of the Improved Power Conversion Efficiency of Pentacene/C60 Heterojunction Photovoltaic Devices through the Purification of Donor Material

The authors report the impact of the crystallinity property of the electron donor on the performance of a pentacene/C60 organic photovoltaic device. After subjecting pentacene to sublimation twice, all the photovoltaic parameters showed significant improvements leading to enhancement of the power conversion efficiency from 0.9 to 2.2% under air mass 1.5G solar illumination. This is attributed to the well-packed molecular structure in the pentacene thin film, as observed by X-ray diffraction, which leads to high carrier mobility and hence high photocurrent. Moreover, the elimination of microscopic pinholes or defect sites due to the improvement in the degree of the pentacene thin film reduces the dark current and therefore increases the photovoltage. The external quantum efficiency and space-charge limited current are used to analyze the relationship between the quality of thin film electron donor and device performance.

Origin of the open-circuit voltage in small-molecular organic photovoltaic devices: Effects of deposition rate control of donor material

Small-molecular organic photovoltaic devices with different open-circuit voltage (VOC) were fabricated by simply controlling the deposition rate of donor material. Higher VOC was obtained when the donor deposited at a higher rate and vice versa. The origin of the improved VOC was studied by means of morphological change and temperature-dependent current density–voltage characteristics. The presence of pinholes as shown in atomic force microscopic images indicated the strong molecular interaction at the lower donor deposition rate, resulting in the severe leakage current from acceptor to anode as observed in the dark current. Equivalent circuit model and temperature-dependent dark currents were utilized to realize the effect of reverse saturation current on VOC. The higher barrier height at the donor–acceptor interface was attributed to the improved VOC for the device with higher donor deposition rate.

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.