Chun-Yang Lu

Porphyrins for efficient dye-sensitized solar cells covering the near-IR region

A series of new porphyrins (LWP1–4) for dye-sensitized solar cells (DSSCs) were prepared by attaching pyrene or a 4-dimethylaminophenyl group in combination with anthracene to modify the porphyrin core. Fundamental studies showed that incorporation of these moieties renders feasible tuning of spectral and redox properties of the porphyrins. Significantly, DSSCs adopting the LWP1 dye exhibit energy conversion up to 800 nm without compromising the overall efficiency. This achievement is attributed to the collective effects of the broadened and red-shifted IPCE spectra, elevated energy levels at the excited states of the dyes, suitable dye soaking processes, and suitable electron-donating substituents.

Molecule-modulated photoconductivity and gain-amplified selective gas sensing in polar GaN nanowires

We report the strong molecular effects on the surface-dominant photoconductivity with high-gain transport in the polar GaN nanowires. Both the transient and steady-state photocurrents are sensitive and selective to the adsorptions of oxygen and hydrogen. The surface band bending of GaN nanowires is proposed to be effectively reduced or enhanced by oxygen or hydrogen, respectively, as a donorlike or acceptorlike surface state. The molecular effect, corroborated with the high-gain photoresponse nature of GaN nanowires is found to amplify the molecule-selective photocurrent signal by near three orders of magnitude higher than its counterpart in dark current. The molecule-tunable photoconductivity, as demonstrated here, would benefit a variety of applications, ranging from the high-gain optoelectronic devices, photoelectric energy transducer, as well as gas and chemical sensors.

Enhancing Optical Out-Coupling of Organic Light-Emitting Devices with Nanostructured Composite Electrodes Consisting of Indium Tin Oxide Nanomesh and Conducting Polymer.

A nanostructured composite electrode consisting of a high-index indium-tin-oxide nanomesh and low-index high-conductivity conducting polymer effectively enhances coupling of internal radiation of organic light-emitting devices into their substrates. When combining this internal extraction structure and the external extraction scheme, a very high external quantum efficiency of nearly 62% is achieved with a green phosphorescent device.

Enhancing light out-coupling of organic light-emitting devices using indium tin oxide-free low-index transparent electrodes

With its increasing and sufficient conductivity, the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been capable of replacing the widely used but less cost-effective indium tin oxides (ITOs) as alternative transparent electrodes for organic light-emitting devices (OLEDs). Intriguingly, PEDOT:PSS also possesses an optical refractive index significantly lower than those of ITO and typical organic layers in OLEDs and well matching those of typical OLED substrates. Optical simulation reveals that by replacing ITO with such a low-index transparent electrode, the guided modes trapped within the organic/ITO layers in conventional OLEDs can be substantially suppressed, leading to more light coupled into the substrate than the conventional ITO device. By applying light out-coupling structures onto outer surfaces of substrates to effectively extract radiation into substrates, OLEDs using such low-index transparent electrodes achieve enhanced optical out-coupling and external quantum efficiencies in comparison with conventional OLEDs using ITO.

Unlocking the Full Potential of Conducting Polymers for High‐Efficiency Organic Light‐Emitting Devices

By carefully tuning the thicknesses of low-optical index PEDOT:PSS and high-index ITO layers in organic light-emitting devices (OLEDs), very high optical coupling efficiencies can be obtained through the generation of appropriate microcavity effects. These experiments result in an external quantum efficiency (EQE) of 33.7% for green phosphorescent OLEDs and even higher EQEs of 54.3% can be obtained by adopting an external out-coupling lens.

Gallium nitride microcavities formed by photoenhanced wet oxidation

We report the formation of gallium nitride (GaN) microcavities by manipulating a photoenhanced oxidation rate difference between the polar and nonpolar crystallographic planes of GaN. When immersed in a buffered acetic (CH3COOH) electrolyte of pH∼6.2 at room temperature, it is shown that the photo-oxidation can proceed at a rate that is one order of magnitude slower on the nonpolar plane of {11¯00}GaN than on the polar plane of {0001¯}GaN due to the reduced surface field action. Gallium nitride microcavities bounded by optically smooth {11¯00} and {11¯03} facets can thus be preferentially formed on the c-plane sapphire substrate after dissolving the oxide layer. The optical properties of these GaN hexagonal cavities reveal characteristic peaks of whispering gallery modes in resonance with the GaN band edge emission spectrum. A typical cavity Q factor of 103 is observed in these GaN microcavities due to a reduced optical scattering loss in the wet chemical reaction process.

Plasmonic ITO-free polymer solar cell

The aluminum and sliver multilayered nano-grating structure is fabricated by laser interference lithography and the intervals between nanoslits is filled with modified PEDOT:PSS. The grating structured transparent electrode functions as the anti-reflection layer which not only decreases the reflected light but also increases the absorption of the active layer. The performances of P3HT:PC₆₁BM solar cells are studied experimentally and theoretically in detail. The field intensities of the transverse magnetic (TM) and transverse electrical (TE) waves distributed in the active layer are simulated by rigorous coupled wave analysis (RCWA). The power conversion efficiency of the plasmonic ITO-free polymer solar cell can reach 3.64% which is higher than ITO based polymer solar cell with efficiency of 3.45%.

Analyses of optical out-coupling of organic light-emitting devices having micromesh indium tin oxide and conducting polymer as composite transparent electrode.

UNLABELLED We report the characterization and analyses of organic light-emitting devices (OLEDs) using microstructured composite transparent electrodes consisting of the high-index ITO (indium tin oxide) micromesh and the low-index conducting polymer PEDOT PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)], that are fabricated by the facile and convenient microsphere lithography and are useful for enhancing light extraction. The rigorous electromagnetic simulation based on the three-dimensional finite-difference time-domain (FDTD) method was conducted to study optical properties and mechanisms in such devices. It provides a different but consistent viewpoint/insight of how this microstructured electrode enhances optical out-coupling of OLEDs, compared to that provided by ray optics simulation in previous works. Both experimental and simulation studies indicate such a microstructured electrode effectively enhances coupling of internal radiation into the substrate, compared to devices with the typical planar ITO electrode. By combining this internal extraction structure and the external extraction scheme (e.g. by attaching extraction lens) to further extract radiation into the substrate, a rather high external quantum efficiency of 46.8% was achieved with green phosphorescent OLEDs, clearly manifesting its high potential.

Enhance Light Out-Coupling of OLEDs: Low-Index Active Materials and Horizontal Dipole Emitters

We report that judicious use of low-index active organic materials and transparent electrodes in OLEDs, together with preferentially horizontal dipole emitter, can effectively enhance optical coupling both into substrate and directly into air.

High-Efficiency Transparent Small-Molecule Organic Light-Emitting Devices Adopting Laminated Transparent Top Electrodes

We demonstrate a highly efficient transparent OLED with laminated transparent electrode. With use of small-molecule materials, rather high EQE and current efficiency of up to (11.4%, 43.1 cd/A), comparable to the reference device, were obtained.