Inchan Hwang

Investigating the origin of S-shaped photocurrent-voltage characteristics of polymer:fullerene bulk-heterojunction organic solar cells

We herein investigated the origin of the S-shape behaviour exhibited near the compensation voltage in the photocurrent density-voltage (Jph-V) characteristics of organic solar cells. P3HT:PCBM bulk-heterojunction solar cells annealed at a too high temperature show the S-shaped Jph-V characteristics. Optical microscopy images revealed the interface degradation. Utilizing a drift-diffusion model, we found that the S-shape behavior cannot be seen by a sole effect of charge accumulation or imbalance of charge carrier mobilities. Rather, the effect of both combined resulting in strong bimolecular recombination causes a reduction in photocurrent near the compensation voltage, producing the S-shape feature.

Superior bipolar resistive switching behaviors of solution-processed HfAlOx thin film for nonvolatile memory applications

We report on the bipolar resistive switching behavior and uniform cumulative distribution of set/reset voltage observed from the resistive random access memory (ReRAM) device based on a solution-processed HfAlOx thin film. The HfAlOx thin film was spin-coated from a mixture of HfOx and AlOx solutions at ratio. The dominant conduction mechanism of a solution-processed HfAlOx thin film is found to be the ohmic behavior in the low-resistance state (LRS) and high-resistance state (HRS) in the low-voltage region, whereas the Poole–Frenkel emission is dominant in the HRS in the high-voltage region. Outstanding reliability in terms of endurance and retention characteristics was achieved. The solution-processed HfAlOx thin film can be a promising candidate for the application in ReRAM devices with glass or flexible substrates.

Enhanced Polarization Ratio of Electrospun Nanofibers with Increased Intrachain Order by Postsolvent Treatments

Polarized emission that is beneficial to lighting and display applications can be demonstrated by aligning emissive chromophores, which can be achieved using an electrospinning technique. We investigate the photophysical properties of nanofibers based on poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]/poly(ethylene oxide) blends both with and without postsolvent treatments. Two different solvents were sequentially used in an attempt to extract the insulating electrospinnable polymer and increase the polarization ratio of the nanofiber meshes by molecular reorganization. The polarization ratio of emission from the nanofiber meshes treated with N,N-dimethylformamide (DMF) following treatment with acetonitrile solvents was found to be increased. An increase in the 0-0 emission vibronic intensity relative to that of the 0-1 peak and a reduction in the photoluminescence (PL) bandwidth were found. In addition, the PL decays faster and the parallel component along the nanofiber axis increases after the DMF treatment, indicating that the radiative recombination process becomes faster. Our results consistently show that postsolvent treatment promotes stronger J-aggregate character, with longer coherence lengths of the exciton along the long axis of the nanofibers, due to enhanced intrachain order.

Toward High Conductivity of Electrospun Indium Tin Oxide Nanofibers with Fiber Morphology Dependent Surface Coverage: Postannealing and Polymer Ratio Effects

High electrical conductivity of metal oxide thin films needs uniform surface coverage, which has been the issue for the thin films based on electrospun nanofibers (NFs) that have advantage over the sputtered/spin-coated films with respect to large surface area and mechanical flexibility. Herein, we investigated a reduction in the sheet resistance of electrospun indium tin oxide (ITO) NF films with improved surface coverage. We found that the surface coverage depends significantly on the electrospinnable polymer concentration in the precursor solutions, especially after post-hot-plate annealing following the infrared radiation furnace treatment. The postannealing process increases crystallinity and oxygen vacancies. However, with a higher PVP content, it makes the surface of ITO NFs more prominently rough as a result of the formation of larger sphere-shaped ITO particles on the NF surface, which gives rise to poor surface coverage. A less poly(vinylpyrrolidone) (PVP) content in ITO NF films by electrospinning for short deposition times was found to improve surface coverage even after postannealing. The sheet resistance notably decreases, down to as low as 350 Ω/sq, with a high transmittance of over 90%. Our study provides an understanding on how to achieve high electrical conductivity of ITO NF films with high surface coverage, which can be utilized for the optoelectronic and sensing applications.

Probing molecular orientation of P3HT nanofibers in fiber-based organic solar cells

Molecular orientation of conjugated polymers plays a key role in exciton generation/separation and charge transport, and thus significantly influence photovoltaic devices. Herein, we fabricated fiber-based organic solar cells and investigated the photovoltaic parameters with different diameters of fibers and PCBM diffusion. The open-circuit voltage that varies with molecular orientation whether it is face-on or edge-on was observed to differ. The investigation of the open-circuit voltage dependence reveals that thick fibers have core/shell like structures with different orientations. Thick fibers have face-on in the core and edge-on orientations in the shell. The face-on orientations are not preferentially formed in thin fibers, but the PCBM diffusion can induce face-on orientations that exist within the intermixed phase. Our results may shed a light on better understanding on fiber-based solar cells and suggest a way toward improving photovoltaic efficiency.

Organic light-emitting devices based on solution-processable small molecular emissive layers doped with interface-engineering additives

In this study, we investigate small molecular organic light-emitting diodes (SM-OLEDs) consisting of emission layers (EMLs) fabricated using a solution-coating process of self-metered horizontal dip- (H-dip-) coating. The EML used was composed of a co-mixed small molecular host matrix of hole-transporting 4,4′,4′′-tris(9-carbazolyl)-triphenylamine (TcTa) and electron-transporting 2,7-bis (diphenyl phosphoryl)-9,9′-spirobifluorene (SPPO13) doped with blue-, green-, and/or red-emitting phosphorescent iridium complexes. To improve the electron-injecting and hole-blocking properties at the cathode interface and to enhance the film-forming capabilities, an interface-engineering additive of poly(oxyethylene tridecyl ether) (PTE) was mixed with the small molecular EMLs. Using PTE additives was shown to reduce dramatically the formation of film defects such as nano-pinholes in the EMLs, resulting in thin and homogeneous PTE-mixed EMLs with smooth surface morphologies, even when using a single H-dip-coating process. The use of simple H-dip-coated EMLs mixed with PTEs in SM-OLEDs resulted in good device performance, with maximum luminance levels of 29 200 cd m−2, 115 000 cd m−2, and 16 400 cd m−2, with corresponding peak current efficiencies of 18.8 cd A−1, 31.2 cd A−1, and 10.0 cd A−1, for blue, green, and red SM-OLEDs, respectively. Furthermore, we demonstrated the feasibility of fabricating large-area and high-performance solution-processable SM-OLEDs using H-dip-coated EMLs doped with PTEs. These results clearly indicate that H-dip-coated small molecular EMLs mixed with PTE can be used to yield simple, bright, and efficient solution-processable SM-OLEDs.