Illhwan Lee

A Challenge Beyond Bottom Cells: Top‐Illuminated Flexible Organic Solar Cells with Nanostructured Dielectric/Metal/Polymer (DMP) Films

Top-illuminated flexible organic solar cells with a high power conversion efficiency (≈6.75%) are fabricated using a dielectric/metal/polymer (DMP) electrode. Employing a polymer layer (n = 1.49) makes it possible to show the high transmittance, which is insensitive to film thickness, and the excellent haze induced by well-ordered nanopatterns on the DMP electrode, leading to a 28% of enhancement in efficiency compared to bottom cells.

Spontaneously Formed Nanopatterns on Polymer Films for Flexible Organic Light‐Emitting Diodes

or electron beam lithography [ 18 ] involve multiple processes, low throughput, and high costs, [ 19 ] so these methods are not applicable to roll-to-roll or large-area processes for commercial applications. And these various kinds of nanostructural patterns involved harsh processing conditions to produce a number of defects. So far, this is why such process could not be applied to polymer materials to produce nanostructure patterns. Such problems could be solved by direct treatment using plasma onto soft materials of polymer. Polymer fi lms such as PET are composed of semicrystalline and amorphous domains, resulting in the selectively etch of the amorphous domains under plasma condition. [ 20–24 ] The difference in etch rates in both semicrystalline and amorphous domains causes texturing to spontaneously form a nanostructure on the surface of PET, leading to controlling antirefl ection properties and maintaining its excellent fl exibility. The nanostructure could be directly formed on the surface of PET fi lm without additional mask for etching. Thus it makes the process applicable to roll-to-roll processes for commercial applications. And the nanostructures are formed on several kinds of polymer substrates, such as PI (polyimide), PES (polyethersulfone), and PC (polycarbonate), by plasma treatment (Figure 6S, Supporting Information). Here, we report a novel concept to increase the light outcoupling effi ciency of fl exible OLEDs based on O 2 plasma treatment (Figure 1 b). The sub-micrometer-patterns were produced on the back side of PET substrate for enhancing light out-coupling of OLED. The sub-micrometer-sized pattern (>500 nm) can be obtained using condensed energy of the accelerated ions without additional mask for etching, controlled by plasma condition. According to fi nite-difference time-domain (FDTD) simulation, waveguide modes in PET substrate (>500 nm size) can be remarkably extracted because of reducing the total internal refl ection. Consequently, the luminance of OLED could be improved by 70% (from 17340 to 29420 cd m −2 at 25 mA cm −2 ) using the submicrometer-sized pattern produced by the plasma treatment. Figure 2 shows the atomic force microscopy (AFM) data of plasma-treated PET fi lms to examine the nanostructure size, produced by the selectively etching of amorphous domains. Average surface roughness ( R a ) is defi ned as the average of the individual heights and depths from the arithmetic mean elevation of the profi le. Thus, the height of nanostructure may be considered to be equal to twice of R a . The maximum roughness was obtained at a process pressure of 100 mTorr (Figure S1a, Supporting Information). The DOI: 10.1002/smll.201500821 Nanopatterns

Phase-controllable copper oxides for an efficient anode interfacial layer in organic light-emitting diodes

We investigated phase-controllable copper oxide (CuOx) as a hole injection layer for improving the electrical and optical properties of organic light emitting diodes (OLEDs). The phase of CuOx is changed from CuO to Cu2O by elevating the deposition rate during thermal evaporation, and a non-stoichiometric film mixing with two phases occurs in the intermediate condition. In non-stoichiometric films, unbonded oxygen atoms with a large density of gap states are induced near the Fermi level, acting as a hole transport path. Thus, holes can be more easily injected into the highest occupied molecular orbital states of α-NPD, lowering the hole injection barrier significantly. Films of non-stoichiometric CuOx are inserted at the interface of α-NPD with an Ag electrode, serving as the hole injection layer. Significantly improved current densities and luminance were achieved, reducing the operation voltage at 1 mA cm−2 from 6.1 V to 3.2 V.

Optical Enhancement in Optoelectronic Devices Using Refractive Index Grading Layers

We enhanced the optical transmittance of a multilayer barrier film by inserting a refractive index grading layer (RIGL). The result indicates that the Fresnel reflection, induced by the difference of refractive indices between Si(x)N(y) and SiO2, is reduced by the RIGL. To eliminate the Fresnel reflection while maintaining high transmittance, the optimized design of grading structures with the RIGL was conducted using an optical simulator. With the RIGL, we achieved averaged transmittance in the visible wavelength region by 89.6%. It is found that the optimized grading structure inserting the multilayer barrier film has a higher optical transmittance (89.6%) in the visible region than that of a no grading sample (82.6%). Furthermore, luminance is enhanced by 14.5% (from 10,190 to 11,670 cd m(-2) at 30 mA cm(-2)) when the grading structure is applied to organic light-emitting diodes. Finally, the results offer new opportunities in development of multilayer barrier films, which assist industrialization of very cost-effective flexible organic electronic devices.

MgO nano-facet embedded silver-based dielectric/metal/dielectric transparent electrode

We replace Indium Tin Oxide (ITO) with an MgO nano-facet Embedded WO(3)/Ag/WO(3)(WAW) multilayer for electrodes of high efficiency OLEDs. WAW shows higher values for transmittance (93%) and conductivity (1.3×10(5) S/cm) than those of ITO. Moreover, WAW shows higher transmittance (92.5%) than that of ITO (86.4%) in the blue region (<500 nm). However, due to the large difference in refractive indices (n) of glass (n=1.55) and WO(3) (n=1.95), the incident light has a small critical angle (52°). Thus, the generated light is confined by the glass/WAW interface, resulting in low light outcoupling efficiency (~20%). This can be enhanced by using a nano-facet structured MgO (n=1.73) layer and a ZrO(2) (n=1.84) layer as a graded index layer. Using these optimized electrodes, ITO-free, OLEDs with various emission wavelengths have been produced. The luminance of OLEDs using MgO/ZrO(2)/WAW layers is enhanced by 24% compared to that of devices with ITO.

Electron injection in magnesium-doped organic light-emitting diodes

We investigated alkali metal doping mechanism by comparative analysis between an Mg-Alq3 co-deposition (Mg:Alq3) and an Mg deposition on Alq3 films (Mg/Alq3). The operating voltage decreased by 0.4 V and the luminance increased by 60 cd/m2 at 11 mA/cm2 for devices constructed from the Mg:Alq3. However, the device characteristics of Mg/Alq3 samples were degraded. Our experimental results using an in-situ photoemission study showed that alkali metal doping in Alq3 did not induce band bending, but reduce electron injection barrier by charge transfer from alkali metals to Alq3 molecules.

Transparent electrode of nanoscale metal film for optoelectronic devices

Abstract. This paper reviews the principles, impediments, and recent progress in the development of ultrathin flexible Ag electrodes for use in flexible optoelectronic devices. Thin Ag-based electrodes are promising candidates for next-generation flexible transparent electrodes. Thin Ag-based electrodes that have a microcavity structure show the best device performance, but have relatively low optical transmittance (OT) due to reflection and absorption of photons by the thin Ag; this trait causes problems such as spectral narrowing and change of emission color with viewing angle in white organic light-emitting diodes. Thinning the Ag electrode to <−10  nm thickness (ultrathin Ag) is an approach to overcome these problems. This ultrathin Ag electrode has a high OT, while providing comparable sheet resistance similar to indium tin oxide. As the OT of the electrode increases, the cavity is weakened, so the spectral width of the emission and the angular color stability are increased.

Transparency controllable silver-based electrode for flexible optoelectronics

The transmittance of Ag-based electrode increased through suppressing surface plasmons (SPs) coupling. When 10-nm-thick Ag was deposited on small-dielectric-constant (e) film (LiF, SiO), SPs coupling was induced, resulting in low transmittance (<40%) in visible region. At the Ag/large-e oxide interface (WO3 and MoO3), SPs were suppressed, and the film showed increased transmittance (∼80%). Organic light emitting diodes using Ag/WO3 (e: 35) as a transparent electrode showed 1.26 times greater luminance and 32.6% greater power efficiency than using Ag/LiF (e: 5). These results provide us with an important guideline for enhancing the transmittance of Ag/dielectric film by controlling SPs coupling.

Spontaneously Embedded Scattering Structures in a Flexible Substrate for Light Extraction

A flexible hazy substrate (FHS) with embedded air bubbles to increase light extraction efficiency of organic light-emitting diodes (OLEDs) is reported. In order to embed the air bubbles in the flexible substrate, micropatterned substrates are fabricated by plasma treatment, and then coated with a planarization layer. During the planarization layer coating, air bubbles are trapped between the substrate and the planarization layer. The haze of the FHS can be controlled from 1.7% to 68.4% by changing the size of micropatterns by adjusting the plasma treatment time. The FHS shows average haze of 68.4%, average total transmittance of 90.3%, and extremely flat surface with average roughness (R a ) of 1.2 nm. Rigorous coupled-wave analysis and finite-difference time-domain simulations are conducted to demonstrate that the air bubbles in the substrate can effectively extract photons that are trapped in the substrate. The FHS increases the power efficiency of OLEDs by 22% and further increases by 91% combined with an external extraction layer. Moreover, the FHS has excellent mechanical flexibility. No defect has been observed after 10 000 bending cycles at bending radius of 4 mm.

Simple and scalable growth of AgCl nanorods by plasma-assisted strain relaxation on flexible polymer substrates

Implementing nanostructures on plastic film is indispensable for highly efficient flexible optoelectronic devices. However, due to the thermal and chemical fragility of plastic, nanostructuring approaches are limited to indirect transfer with low throughput. Here, we fabricate single-crystal AgCl nanorods by using a Cl2 plasma on Ag-coated polyimide. Cl radicals react with Ag to form AgCl nanorods. The AgCl is subjected to compressive strain at its interface with the Ag film because of the larger lattice constant of AgCl compared to Ag. To minimize strain energy, the AgCl nanorods grow in the [200] direction. The epitaxial relationship between AgCl (200) and Ag (111) induces a strain, which leads to a strain gradient at the periphery of AgCl nanorods. The gradient causes a strain-induced diffusion of Ag atoms to accelerate the nanorod growth. Nanorods grown for 45 s exhibit superior haze up to 100% and luminance of optical device increased by up to 33%.