Juyoung Ham

Design of broadband transparent electrodes for flexible organic solar cells

Broadband transparent electrodes, Ta2O5/Ag/WO3−x, are successfully designed to enhance light absorption and carrier transport properties in organic solar cells (OSCs) as an alternative to ITO/PEDOT:PSS. Employing the optical constant matching layer, zero reflection conditions could be achieved over a broad range of wavelengths. Moreover, the non-stoichiometric WO3−x could induce a large density of gap states near the Fermi level via quick thermal deposition, acting as the transport path of carriers. Significantly improved current densities were achieved, increasing the power conversion efficiency from 2.1% to 2.9%, values which are comparable to conventionally fabricated devices on ITO/PEDOT:PSS.

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

Ultrafast laser-assisted synthesis of hydrogenated molybdenum oxides for flexible organic solar cells

A novel method to synthesize a hydrogenated molybdenum oxide (HyMoO3−x) thin film by irradiation of photons using a KrF laser (λ = 248 nm) on an ammonium heptamolybdate ((NH4)6Mo7O24·4H2O) precursor layer is demonstrated. The laser-assisted synthesis is simple, and can be conducted in an ambient atmosphere without damaging the underlying bottom electrode and plastic substrate. The exposure time (30 ns) is extremely short compared to thermal annealing (>3 min). Because the high-energy photons are absorbed by the MoO3 layer and provide the activation energy for the reaction, the hydrogen atoms that dissociate from the ammonium molecules bond to the MoO3; this process yields a HyMoO3−x thin-film. By controlling the laser energy, the stoichiometry of the HyMoO3−x layer can be manipulated to simultaneously obtain advantageous electrical properties of both high work function (5.6 eV) and electrical conductivity (9.9 μS cm−1). The HyMoO3−x hole transport layer (HTL) is successfully demonstrated on flexible top-illuminated PTB7:PCBM organic solar cells (OSCs). This OSC has good mechanical flexibility, and 75% higher short-circuit current than the device with a PEDOT:PSS HTL. Finite-domain time-difference simulations were conducted to verify the enhancement of the photocurrent. The thin layer of HyMoO3−x was proven to be suitable for the microcavity condition which allows a resonant wavelength match to the PTB7:PCBM active layer.

Wavelength-Scale Structures as Extremely High Haze Films for Efficient Polymer Solar Cells

Wavelength-scale inverted pyramid structures with low reflectance and excellent haze have been designed for application to polymer solar cells (PSCs). The wavelength-scale structured haze films are fabricated on the back surface of glass without damages to organic active layer by using a soft lithographic technique with etched GaN molds. With a rigorous coupled-wave analysis of optical modeling, we find the shift of resonance peaks with the increase of patterns diameter. Wavelength-scale structures could provide the number of resonances at the long wavelength spectrum (λ = 650-800 nm), yielding enhancement of power conversion efficiency (PCE) in the PSCs. Compared with a flat device (PCE = 7.12%, Jsc = 15.6 mA/cm(2)), improved PCE of 8.41% is achieved in a haze film, which is mainly due to the increased short circuit current density (Jsc) of 17.5 mA/cm(2). Hence, it opens up exciting opportunities for a variety of PSCs with wavelength-scale structures to further improve performance, simplify complicated process, and reduce costs.

Eco-friendly graphene synthesis on Cu foil electroplated by reusing Cu etchants

Graphene film grown by chemical vapor deposition using Cu substrate is promising for industrial applications. After etching the Cu substrate, which is essential step in graphene transfer process, the etchant solution must be chemically treated to prevent water pollution. Here we investigated that a method of reusing Cu etchant used to synthesize graphene, the synthesis of graphene on the resulting reused Cu films (R-G), and the application of R-G to organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). The turn-on voltage of OLEDs based on the R-G electrode was 4.2 V, and the efficiencies of OPVs based on the R-G electrode were 5.9–5.95%, that are similar to or better than those of the indium-tin-oxide-based devices. These results suggest that the reusing of Cu foil by the electroplating method could reduce the cost of graphene synthesis, thus opening a wide range of applications in graphene electronics.

Continuous 1D-Metallic Microfibers Web for Flexible Organic Solar Cells

We report the use of a continuous 1D-metallic microfibers web (MFW) as transparent electrode for organic solar cells (OSCs). The MFW electrode can be produced with a process that involves simple electrospinning and wet etching of metal thin film. Au MFW exhibits a maximum optical transmittance of 90.8% (at 15 Ω/sq of the sheet resistance) and excellent mechanical flexibility. The MFW structure has an average width in the range from 4 to 6 μm and a junction-free structure, resulting in very smooth surface roughness. The OSCs with Au MFW electrode exhibited a higher power conversion efficiency (PCE) of 3.50% than the device with an indium tin oxide electrode (PCE = 3.20%). The optical modeling calculation showed that the Au MFW electrode induced light scattering and improved the light absorption in the active layer, resulting in an improved PCE in the OSCs.

Antireflective indium-tin-oxide nanobranches for efficient organic solar cells

Indium tin oxide (ITO) nanobranched structures with low reflectance have been designed for application to organic solar cells. The ITO nanobranches are deposited on the front surface of glass without damages to organic active layer by using an electron beam deposition. Through a finite-difference time-domain, we find that the field intensity in the glass region is enhanced with ITO nanobranches. Consequently, the number of incident photons induces strong absorption within the PTB7-PC70BM active layer, leading to the enhanced short circuit current density (Jsc). Compared with a flat device (power conversion efficiency (PCE) = 6.53%, Jsc = 13.3 mA/cm2), improved PCE of 7.09% is achieved in an antireflection coating, which is mainly due to the increased Jsc of 14.2 mA/cm2.

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%.

Extremely flat metal films implemented by surface roughness transfer for flexible electronics

We present an innovative approach to fabricate an extremely flat (EF) metal film which was done by depositing metal on an extremely flat mother substrate, then detaching the metal from the substrate. The detached flexible metal films had a roughness that was within 2% of the roughness of the mother substrate, so EFs with Ra < 1 nm could be fabricated using the surface roughness transfer method. With quantitative analysis using in situ synchrotron XPS, it was concluded that the chemical reaction of oxygen atoms with the metal film played a critical role in designing a peel-off system to get extremely flat metal films from the mother substrate. The OLED was successfully implemented on the metal film. The OLEDs luminance could be increased from 15 142 to 17 100 cd m−2 at 25 mA m−2 by replacing the glass substrate with an EF copper (Cu) substrate, due to the enhanced heat dissipation during the operation. This novel method can be very useful for mass production of large scale, low-cost and high quality metal films using roll-to-roll process.