Jyh-Lih Wu

Surface Plasmonic Effects of Metallic Nanoparticles on the Performance of Polymer Bulk Heterojunction Solar Cells

We have systematically explored how plasmonic effects influence the characteristics of polymer photovoltaic devices (OPVs) incorporating a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). We blended gold nanoparticles (Au NPs) into the anodic buffer layer to trigger localized surface plasmon resonance (LSPR), which enhanced the performance of the OPVs without dramatically sacrificing their electrical properties. Steady state photoluminescence (PL) measurements revealed a significant increase in fluorescence intensity, which we attribute to the increased light absorption in P3HT induced by the LSPR. As a result, the rate of generation of excitons was enhanced significantly. Furthermore, dynamic PL measurements revealed that the LSPR notably reduced the lifetime of photogenerated excitons in the active blend, suggesting that interplay between the surface plasmons and excitons facilitated the charge transfer process. This phenomenon reduced the recombination level of geminate excitons and, thereby, increased the probability of exciton dissociation. Accordingly, both the photocurrents and fill factors of the OPV devices were enhanced significantly. The primary origin of this improved performance was local enhancement of the electromagnetic field surrounding the Au NPs. The power conversion efficiency of the OPV device incorporating the Au NPs improved to 4.24% from a value of 3.57% for the device fabricated without Au NPs.

Plasmonic-enhanced polymer photovoltaic devices incorporating solution-processable metal nanoparticles

We have explored the effect of gold nanoparticle (Au NP)-induced surface plasmons on the performance of organic photovoltaic devices (OPVs). The power conversion efficiency of these OPVs was improved after blending the Au NPs into the anodic buffer layer. The addition of Au NPs increased the rate of exciton generation and the probability of exciton dissociation, thereby enhancing the short-circuit current density and the fill factor. We attribute the improvement in device performance to the local enhancement in the electromagnetic field originating from the excitation of the localized surface plasmon resonance.

Cesium carbonate as a functional interlayer for polymer photovoltaic devices

The device characteristics of polymer solar cells with cesium carbonate (Cs2CO3) as an electron-injection interlayer have been investigated. It is found that the insertion of Cs2CO3 at the cathode interface improves the device power conversion efficiency from 2.3% to 3.1%. In order to further understand the mechanism, the interfacial interaction between the active organic layer and the cathode was studied by x-ray photoemission spectroscopy (XPS). The results of XPS measurement indicate the fact that a portion of electrons transfer from the interlayer into the organic layer, resulting in n-type doping. The n-doping effect enhances the efficiency of electron injection and collection. Further, the maximum open-circuit voltage (Voc) was determined from its temperature dependence. For the device with Cs2CO3, the maximum Voc is extremely close to the corresponding value of the energy difference between the highest occupied molecular orbital of the electron donor and the lowest unoccupied molecular orbital of t...

Spatial redistribution of the optical field intensity in inverted polymer solar cells

We have used indium tin oxide (ITO), a transparent conducting oxide, as an optical spacer to improve the performance of inverted polymer solar cells. The optical interference effect resulted in spatial redistribution of the optical field in the devices. Although the degree of light absorption in inverted cells was not increased, the resulting favorable distribution of photogenerated excitons probably decreased the level of exciton quenching near the electrodes. As a result, the introduction of the ITO optical spacer at an appropriate thickness increased the short-circuit current density and the overall power conversion efficiency.

Flexible Polymer Photovoltaic Devices Prepared With Inverted Structures on Metal Foils

We have prepared top-illuminated organic photovoltaic devices on stainless-steel substrates, using molybdenum trioxide/indium tin oxide as the transparent top electrode. Incorporating an Al counter electrode grid having a shadow fraction of 10% effectively reduced the device resistance, thereby enhancing the power conversion efficiency to ca. 3%. This device exhibited superior mechanical flexibility. The inverted device architecture eliminated the need to use low-work-function metals, which are air sensitive and easily oxidized, thereby improving the air stability of the flexible devices.

Delta-doped quantum well structures grown by molecular beam epitaxy

Delta doping in quantum well structures has been studied. The quantum wells consist of a strained InGaAs layer sandwiched between two GaAs layers. The layers were undoped except for a sheet of Si dopants deposited in the middle of the quantum well. Structures with various doses and quantum well thicknesses were studied and compared. Capacitance voltage measurements were carried out to determine the carrier distribution. A very narrow carrier profile with a full width at half maximum of only 12 A has been achieved. This is the narrowest carrier profile ever reported for any growth technique.

Near-infrared laser-driven polymer photovoltaic devices and their biomedical applications

We demonstrate near-infrared laser-driven (NIRLD) organic photovoltaic devices (OPVs), which can directly convert 980 nm light into electrical power. We attribute the NIR photovoltaic response to the long-wavelength absorption of charge transfer (CT) states. Direct excitation through CT states might open up new avenues for harvesting the long-wavelength spectrum of solar irradiation. Further, because of the high transparency of biological tissue toward 980 nm light, these NIRLD OPVs might be a promising wireless electrical source for biological nanodevices.

Resonant tunneling of electrons from quantized levels in the accumulation layer of double-barrier heterostructures

We report the first observation of the resonant tunneling features associated with the quantized levels in the accumulation layer of the double‐barrier resonant tunneling structure (DBRTS) with undoped electrodes. This quantum effect causes additional kinks in the current‐voltage (I‐V) characteristic and an increasingly enhanced oscillation behavior in the differential conductance‐voltage (G‐V) curve. Three discrete quantum levels have been observed based on the room‐temperature G‐V curve. Our measurements are made without the presence of magnetic field and thus the experimental results are totally different from the magneto‐oscillation.

Precise determination of aluminum content in AlGaAs

The Al composition of AlGaAs has been determined by four methods: high‐resolution transmission electron microscopy (HRTEM), reflection high‐energy electron diffraction (RHEED), photoluminescence (PL), and double‐crystal x‐ray diffraction (DCXRD). HRTEM is direct and the most accurate method because it does not involve any formula or extrapolation. Using the result obtained from HRTEM as a standard, we have calibrated the results from other methods. RHEED intensity oscillation is found to be accurate and reliable, if the growth conditions are correctly chosen. Comparing the PL results with those determined from HRTEM and RHEED, we suggest three formulas to determine the Al contents at different temperatures. We also proposed a polynomial to determine the Al concentration using the DCXRD measurement.

Origin of the enhancement of negative differential resistance at low temperatures in double-barrier resonant tunneling structures

An explanation of the increased peak-to-valley current ratio for double-barrier resonant tunneling structures (DBRTSs) operated at low temperatures is proposed. It was found that this phenomenon is an inherent property of DBRTSs not caused by the suppression of thermionic current over barriers. The energy distributions of electrons at different temperatures result in variations of peak and valley currents.<<ETX>>