Seungjun Oh

Photoelectron spectroscopic study of band alignment of polymer/ZnO photovoltaic device structure

Using x-ray photoelectron spectroscopy, we investigated the band alignment of a Ag/poly(3-hexylthiophene-2,5-diyl) (P3HT)/ZnO photovoltaic structure. At the P3HT/ZnO interface, a band bending of P3HT and a short surface depletion layer of ZnO were observed. The offset between the highest occupied molecular orbital of P3HT and the conduction band minimum of ZnO at the interface contributed to the open circuit voltage (Voc) was estimated to be approximately 1.5 ± 0.1 eV, which was bigger than that of the electrically measured effective Voc of P3HT/ZnO photovoltaic devices, meaning that the P3HT/ZnO photovoltaic structure has the potential to provide improved photovoltaic properties.

Hard x-ray photoelectron spectroscopy study on band alignment at poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/ZnO interface

We used hard x-ray photoelectron spectroscopy to investigate the interfacial electronic states of a poly(styrenesulfonate) doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) contact on a ZnO single crystal. An understanding of the interfacial band structure is useful for putting the organic contact to practical use. We observed upward band bending of the ZnO layer a few nanometers from the interface. The detected ZnO bulk region exhibited a flat band structure, meaning that the PEDOT:PSS does not greatly deplete the ZnO layer. The band bending caused the charge injection barrier formation with the result that the contact exhibited the Schottky property.

Nanoimprint for Fabrication of Highly Ordered Epitaxial ZnO Nanorods on Transparent Conductive Oxide Films

We established a process of growing large-scale highly ordered ZnO nanorod arrays on a transparent conductive oxide layer. Nanoimprint lithography was employed specifically to produce large-scale ordered ZnO nanorod arrays. The growth conditions were examined by comparing nanorod growth on different substrates, namely, aluminum-doped zinc oxide (AZO) and indium tin oxide (ITO). Structural characterization revealed that the crystallinity and flatness of the ZnO seed layer were key factors in improving uniformity as regards the diameter, height, and orientation of the ZnO nanorods.

Improving the performance of inorganic-organic hybrid photovoltaic devices by uniform ordering of ZnO nanorods and near-atmospheric pressure nitrogen plasma treatment

We investigated the performance of hybrid photovoltaic devices composed of ZnO and poly(3-hexylthiophene) (P3HT). The uniform ordering of ZnO nanorods (NRs) and nitrogen plasma treatment at near-atmospheric pressure offer advantages in modifying the ZnO NR surface. Uniform ordering of the ZnO NRs promoted the effective infiltration of P3HT, increasing the donor–acceptor interface area, which is directly related to short-circuit current density (JSC). Near-atmospheric pressure treatment compensated carriers to form a highly resistant interlayer at the ZnO surface, which reduced carrier recombination and, as a result, increased the open circuit voltage (VOC). Combining these two approaches achieved five-fold increase in JSC compared to that of the planar heterojunction, while the VOC was increased up to 0.71 V.

Arbitrary cross-section SEM-cathodoluminescence imaging of growth sectors and local carrier concentrations within micro-sampled semiconductor nanorods.

Future one-dimensional electronics require single-crystalline semiconductor free-standing nanorods grown with uniform electrical properties. However, this is currently unrealistic as each crystallographic plane of a nanorod grows at unique incorporation rates of environmental dopants, which forms axial and lateral growth sectors with different carrier concentrations. Here we propose a series of techniques that micro-sample a free-standing nanorod of interest, fabricate its arbitrary cross-sections by controlling focused ion beam incidence orientation, and visualize its internal carrier concentration map. ZnO nanorods are grown by selective area homoepitaxy in precursor aqueous solution, each of which has a (0001):+c top-plane and six {1–100}:m side-planes. Near-band-edge cathodoluminescence nanospectroscopy evaluates carrier concentration map within a nanorod at high spatial resolution (60 nm) and high sensitivity. It also visualizes +c and m growth sectors at arbitrary nanorod cross-section and history of local transient growth events within each growth sector. Our technique paves the way for well-defined bottom-up nanoelectronics.

Nanochannel effect in polymer nanowire transistor with highly aligned polymer chains

We established a process to develop well-defined polymer nanowire transistors made of liquid-crystalline semiconducting poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2). Nano-scaled channels have been fabricated in gate insulator layers of SiO2 using a lithography technique. The nanochannels offer templates for forming polymer nanowires in which polymer chains are uniaxially aligned along the nanochannels through a nano-confinement effect. In addition, this process prevents the occurrence of serious damage during the inevitable etching process that is used to separate the nanowires from each other. We examined the electrical properties and polymeric chain alignment of F8T2 nanowires. Nanowire transistors exhibited carrier mobilities of 3.5 and 2.8 × 10−3 cm2/Vs for 54 and 130 nm wide nanowires, respectively. The carrier mobilities were about three times larger than that of a thin film transistor. Polarized UV-vis absorption analysis clarified that the improved carrier mobility can be attributed to the enh...

Crystallographic polarity effect of ZnO on thin film growth of pentacene

The spontaneous polarization effect of ZnO on the thin film growth of pentacene, which is a typical π conjunction organic semiconductor, was investigated. Pentacene thin films were grown on polar ZnO surfaces by ultraslow organic film physical vapor deposition to obtain layer-by-layer growth. X-ray diffraction measurements revealed that pentacene molecules stand upright on polar ZnO surfaces, and that the films consist of two polymorphs, namely, the thin-film and bulk phases. The thin-film phases of pentacene were observed regardless of the polarity of the ZnO substrate at a thickness of less than six molecular layers. However, pentacene on a Zn-polar ZnO substrate gradually changed to the bulk phase unlike that on an O-polar ZnO substrate. Kelvin probe force microscopy measurements revealed that the surface potential of pentacene becomes more positive with increasing pentacene thickness at less than two molecular layers. The variation in the potential of pentacene on the Zn-polar ZnO substrate was larger than that of pentacene on the O-polar ZnO substrate. These findings indicate that the polarity of the semiconductor can control the growth and electronic state of the inorganic/organic semiconductor interface.

Polymer chain alignment and transistor properties of nanochannel-templated poly(3-hexylthiophene) nanowires

Nanowires of semiconducting poly(3-hexylthiophene) (P3HT) were produced by a nanochannel-template technique. Polymer chain alignment in P3HT nanowires was investigated as a function of nanochannel widths (W) and polymer chain lengths (L). We found that the ratio between chain length and channel width (L/W) was a key parameter as regards promoting polymer chain alignment. Clear dichroism was observed in polarized ultraviolet-visible (UV-Vis) absorption spectra only at a ratio of approximately L/W = 2, indicating that the L/W ratio must be optimized to achieve uniaxial chain alignment in the nanochannel direction. We speculate that an appropriate L/W ratio is effective in confining the geometries and conformations of polymer chains. This discussion was supported by theoretical simulations based on molecular dynamics. That is, the geometry of the polymer chains, including the distance and tilting angles of the chains in relation to the nanochannel surface, was dominant in determining the longitudinal alignme...

Bottom-electrode effect on switching behavior and interface reaction in nanoionic-based resistive changing memory

The bottom-electrode effect on a Cu/HfO2 stack structure, which is an oxide-based resistive random access memory (ReRAM) structure, and the resistance switching behavior of the structures were investigated by hard X-ray photoelectron spectroscopy and by comparing the Pt and TiN bottom electrodes. In the Pt bottom electrode, a forward bias voltage induced the reduction of the unintentionally oxidized Cu top electrode and the Cu ion migration in the HfO2 layer, resulting in the switching from the high resistivity to the low resistivity at approximately ±1 V. In contrast, the TiN bottom electrode induced the formation of oxygen vacancies in the HfO2 layer and the thick Cu2O layer at the Cu/HfO2 interface, namely, it induced oxygen migration rather than Cu migration. The switching voltage of the Cu/HfO2/TiN structure was twice that of the Cu/HfO2/Pt structure. The switching mechanism in a nanoionic-type ReRAM structure can be controlled by changing the bottom electrode.

Bias induced Cu ion migration behavior in resistive change memory structure observed by hard X-ray photoelectron spectroscopy

The Cu ion migration behavior of a Pt/Cu/HfO2/Pt structure, which is an oxide-based resistive random access memory (ReRAM) and exhibits resistance switching behavior at voltages of ±0.8 V, was investigated by hard X-ray photoelectron spectroscopy under a bias operation. A forward bias application, during switching from a high resistive state (HRS) to a low resistive state, reduced the Cu2O bonding state at the interface and the intensity ratio of Cu 2p3/2/Hf 3d5/2 (Cu/Hf) by 23 ± 5%, providing evidence of reductions in unintentionally formed Cu2O and Cu diffusion into the HfO2 layer. After switching to HRS again, Cu/Hf increased by 15 ± 5%, indicating that the Cu ion moved back to the top electrode side, though oxygen showed no bias voltage dependence. Consequently, the Cu ion has a key role in the switching. We directly observed the Cu migration behavior related to the resistive change at the Cu/HfO2 interface under bias operation.