Soohwan Sul

Quantum-Dot-Sensitized Solar Cell with Unprecedentedly High Photocurrent

The reported photocurrent density (JSC) of PbS quantum dot (QD)-sensitized solar cell was less than 19 mA/cm2 despite the capability to generate 38 mA/cm2, which results from inefficient electron injection and fast charge recombination. Here, we report on a PbS:Hg QD-sensitized solar cell with an unprecedentedly high JSC of 30 mA/cm2. By Hg2+ doping into PbS, JSC is almost doubled with improved stability. Femtosecond transient study confirms that the improved JSC is due to enhanced electron injection and suppressed charge recombination. EXAFS reveals that Pb-S bond is reinforced and structural disorder is reduced by interstitially incorporated Hg2+, which is responsible for the enhanced electron injection, suppressed recombination and stability. Thanks to the extremely high JSC, power conversion efficiency of 5.6% is demonstrated at one sun illumination.

Heterogeneous stacking of nanodot monolayers by dry pick-and-place transfer and its applications in quantum dot light-emitting diodes

Layered assembly structures composed of nanomaterials, such as nanocrystals, have attracted considerable attention as promising candidates for new functional devices whose optical, electromagnetic and electronic behaviours are determined by the spatial arrangement of component elements. However, difficulties in handling each constituent layer in a material-specific manner limit the 3D integration of disparate nanomaterials into the appropriate heterogeneous electronics. Here we report a pick-and-place transfer method that enables the transfer of large-area nanodot assemblies. This solvent-free transfer utilizes a lifting layer and allows for the reliable transfer of a quantum dot (QD) monolayer, enabling layer-by-layer design. With the controlled multistacking of different bandgap QD layers, we are able to probe the interlayer energy transfer among different QD monolayers. By controlling the emission spectrum through such designed monolayer stacking, we have achieved white emission with stable optoelectronic properties, the closest to pure white among the QD light-emitting diodes reported so far.

A high performance green-sensitive organic photodiode comprising a bulk heterojunction of dimethyl-quinacridone and dicyanovinyl terthiophene

A fullerene-free bulk-heterojunction (BHJ) organic photodiode (OPD) with high efficiency and green-color selectivity is reported. Using N,N-dimethyl quinacridone (DMQA) as a donor and dibutyl-substituted dicyanovinyl-terthiophene (DCV3T) as an acceptor, a maximum external quantum efficiency (EQE) of over 67% at 540 nm was achieved at −5 V bias. The OPD performance together with their electrical and optical behaviors were investigated by varying the ratio of donor and acceptor components and measuring the absorption coefficient, charge carrier generation, and charge transport. The composition rich in DMQA exhibited a high yield of photogenerated charge carriers and a low absorption intensity, whereas the material rich in DCV3T had a high absorption intensity and low yield of charge carriers. It was found that the 1 : 1 ratio of components showed the best device performance due to its relatively high absorption and efficient photogeneration of charge carriers. Furthermore, electrical characterization of our BHJ OPDs indicated that a balance of electron and hole mobilities is important for enhancing EQE.

Degradation of blue-phosphorescent organic light-emitting devices involves exciton-induced generation of polaron pair within emitting layers

Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established. Here we report a new mechanism for degradation of emitting layers in blue-phosphorescent devices. We investigate binary mixtures of a wide bandgap host and a series of novel Ir(III) complex dopants having N-heterocyclocarbenic ligands. Our mechanistic study reveals the charge-neutral generation of polaron pairs (radical ion pairs) by electron transfer from the dopant to host excitons. Annihilation of the radical ion pair occurs by charge recombination, with such annihilation competing with bond scission. Device lifetime correlates linearly with the rate constant for the annihilation of the radical ion pair. Our findings demonstrate the importance of controlling exciton-induced electron transfer, and provide novel strategies to design materials for long-lifetime blue electrophosphorescence devices.The short lifetime of blue-phosphorescent organic light-emitting devices owing to material degradation impedes their practical potential. Here, Kim et al. study the molecular mechanism of the degradation that involves exciton-mediated electron transfer as a key step for the generation of radical ion pairs.

Ultrafast photocarrier dynamics in nanocrystalline ZnO x N y thin films

We examined the ultrafast dynamics of photocarriers in nanocrystalline ZnOxNy thin films as a function of compositional variation using femtosecond differential transmittance spectroscopy. The relaxation dynamics of photogenerated carriers and electronic structures are strongly dependent on nitrogen concentration. Photocarriers of ZnOxNy films relax on two different time scales. Ultrafast relaxation over several picoseconds is observed for all chemical compositions. However, ZnO and oxygen-rich phases show slow relaxation (longer than several nanoseconds), whereas photocarriers of films with high nitrogen concentrations relax completely on subnanosecond time scales. These relaxation features may provide a persistent photocurrent-free and prompt photoresponsivity for ZnOxNy with high nitrogen concentrations, as opposed to ZnO for display applications.

Understanding the structural, electrical, and optical properties of monolayer h-phase RuO2 nanosheets: A combined experimental and computational study

The structural, electrical, and optical properties of monolayer ruthenium oxide (RuO2) nanosheets (NSs) fabricated by chemical exfoliation of a layered three-dimensional form of K-intercalated RuO2 are studied systematically via experimental and computational methods. Monolayer RuO2 NS is identified as having a distorted h-MX2 structure. This is the first observation of a RuO2 NS structure that is unlike the t-MX2 structure of the RuO2 layers in the parent material and does not have hexagonal symmetry. The distorted h-MX2 RuO2 NSs are shown to have optical transparency superior to that of graphene, thereby predicting the feasibility of applying RuO2 NSs to flexible transparent electrodes. In addition, it is demonstrated that the semiconducting band structures of RuO2 NSs can be manipulated to be semi-metallic by adjusting the crystal structure, which is related to band-gap engineering. This finding indicates that RuO2 NSs can be used in a variety of applications, such as flexible transparent electrodes, atomic-layer devices, and optoelectronic devices.Two-dimensional materials: A more transparent way to get in contactNanosheets of ruthenium oxide could make excellent transparent electrical contacts, show researchers from Korea. Graphene is the wonder material of the last decade owing to its amazing electrical, mechanical and thermal properties. Scientists are thus keen to fabricate single layers of atoms other than carbon. Now, Dong-Su Ko, Jung-Hwa Kim and Jong Wook Roh from the Samsung Advanced Institute of Technology and co-workers have combined experiments and theory to fully characterize this unusual two-dimensional material. They created their nanosheets by exfoliating a three-dimensional block of ruthenium oxide. Transmission electron microscopy, X-ray diffraction experiments and first-principles calculations showed that the two-dimensional material has a distorted atomic arrangement, which makes it a semiconductor rather than a metal like its parent material. Furthermore, ruthenium oxide nanosheets are more transparent than graphene, making them useful for flexible transparent electrodes.As a new two-dimensional (2D) material, monolayer ruthenium oxide (RuO2) nanosheets (NSs) have distorted h-MX2 type crystal structures that lead to semiconducting properties and good optical transmittance. This study suggests that monolayer RuO2 can be useful in applications of flexible optoelectronics.

All dielectric metasurface nano-fabrication based on TiO2 phase shifters

All dielectric metasurface of low loss TiO2 in visible wavelengths was devised forming subwavelength-scale nanostructures. DC-magnetron sputtering of oxygen-reduced TiOx (x<;2) target with reactive oxygen gas made dense amorphous TiO2 layers of 1.35nm Ra. A 380nm-thick TiO2 has low extinction coefficient (k) under 1x10-5, transparency of 98.33% and high refractive index (n) of 2.55 at 485nm wavelength. Highly precise TiO2 meta-atoms were successfully defined with 100nm-300nm feature size. Phase shift properties of TiO2 metasurface were measured. Finally, we constructed dielectric metaphotonic platform for various optical devices such as band pass filters, flat lens and beam deflectors in visible ranges.