Wan Jae Dong

Three-Dimensional Nanobranched Indium–Tin-Oxide Anode for Organic Solar Cells

A nanostructured three-dimensional (3D) electrode using transparent conducting oxide (TCO) is an effective approach for increasing the efficiency of optoelectronic devices used in daily life. Tin-doped indium oxide (ITO) is a representative TCO with high conductivity and a high work function for anode applications. This paper reports the fabrication of a large-area ITO nanostructure with a branch shape using an electron beam evaporation process at temperatures as low as 80 °C, which was free of any carrier gas and catalyst. The large surface to volume ratio in the anode by the ITO nanobranches increases both the hole mobility by a 3D pathway and light absorbance by scattering, resulting in organic solar cells with a 12% increase in photocurrent and 20% photoconversion efficiency based on the bulk heterojunction of P3HT [region-regular poly(3-hexylthiophene)] and PCBM [phenyl-C61-butyric acid methyl ester].

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.

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.

Flexible a-Si:H Solar Cells with Spontaneously Formed Parabolic Nanostructures on a Hexagonal-Pyramid Reflector

Flexible amorphous silicon (a-Si:H) solar cells with high photoconversion efficiency (PCE) are demonstrated by embedding hexagonal pyramid nanostructures below a Ag/indium tin oxide (ITO) reflector. The nanostructures constructed by nanoimprint lithography using soft materials allow the top ITO electrode to spontaneously form parabolic nanostructures. Nanoimprint lithography using soft materials is simple, and is conducted at low temperature. The resulting structure has excellent durability under repeated bending, and thus, flexible nanostructures are successfully constructed on flexible a-Si:H solar cells on plastic film. The nanoimprinted pyramid back reflector provides a high angular light scattering with haze reflectance >98% throughout the visible spectrum. The spontaneously formed parabolic nanostructure on the top surface of the a-Si:H solar cells both reduces reflection and scatters incident light into the absorber layer, thereby elongating the optical path length. As a result, the nanopatterned a-Si:H solar cells, fabricated on polyethersulfone (PES) film, exhibit excellent mechanical flexibility and PCE increased by 48% compared with devices on a flat substrate.

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.

Monolithic Nanoporous In–Sn Alloy for Electrochemical Reduction of Carbon Dioxide

Nanostructured metal catalysts to convert CO2 to formate, which have been extensively studied over decades, have many problems such as durability, lifetime, high process temperature, and difficulty in controlling the morphology of nanostructures. Here, we report a facile method to fabricate monolithic nanoporous In-Sn alloy, a network of nanopores, induced by electroreduction of indium tin oxide nanobranches (ITO BRs). The electroreduction process concentrated a local electric field at the tip of the nanostructure, leading to current-assisted joule-heating to form a nanoporous In-Sn alloy. Scanning electron microscopy images showed that the nanopore size of In-Sn alloy could be controlled from 1176 to 65 nm by tuning the electroreduction condition: the applied potential and the time. As a result, formate Faradaic efficiency could be improved from 42.4% to 78.6%. Also, current density was increased from -6.6 to -9.6 mA/cm2 at -1.2 VRHE, thereby resulting in the highest HCOO- production rate of 75.9 μmol/(h cm2). Detachment of catalysts from the substrate was not observed even after a long-term (12 h) electrochemical measurement at high potential (-1.2 VRHE). This work provides a design rule to fabricate highly efficient and stable oxide-derived electrocatalysts.

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.

Efficiency enhancement and angle-dependent color change in see-through organic photovoltaics using distributed Bragg reflectors

A distributed Bragg reflector (DBR) is conducted as a bottom reflector in see-through organic photovoltaics (OPVs) with an active layer of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The DBR consists of alternative layers of the high- and low-refractive index materials of Ta2O5 (n = 2.16) and SiO2 (n = 1.46). The DBR selectively reflects the light within a specific wavelength region (490 nm–630 nm) where the absorbance of P3HT:PCBM is maximum. The see-through OPVs fabricated on DBR exhibit efficiency enhancement by 31% compared to the device without DBR. Additionally, the angle-dependent transmittance of DBR is analysed using optical simulation and verified by experimental results. As the incident angle of light increases, peak of reflectance shifts to shorter wavelength and the bandwidth gets narrower. This unique angle-dependent optical properties of DBR allows the facile color change of see-through OPVs.