Gwan Ho Jung

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].

Role of Ultrathin Metal Fluoride Layer in Organic Photovoltaic Cells: Mechanism of Efficiency and Lifetime Enhancement

Although rapid progress has been made recently in bulk heterojunction organic solar cells, systematic studies on an ultrathin interfacial layer at the electron extraction contact have not been conducted in detail, which is important to improve both the device efficiency and the lifetime. We find that an ultrathin BaF2 layer at the electron extraction contact strongly influences the open-circuit voltage (Voc ) as the nanomorphology evolves with increasing BaF2 thickness. A vacuum-deposited ultrathin BaF2 layer grows by island growth, so BaF2 layers with a nominal thickness less than that of single-coverage layer (≈3 nm) partially cover the polymeric photoactive layer. As the nominal thickness of the BaF2 layer increased to that of a single-coverage layer, the Voc and power conversion efficiency (PCE) of the organic photovoltaic cells (OPVs) increased but the short-circuit current remained almost constant. The fill factor and the PCE decreased abruptly as the thickness of the BaF2 layer exceeded that of a single-coverage layer, which was ascribed to the insulating nature of BaF2 . We find the major cause of the increased Voc observed in these devices is the lowered work function of the cathode caused by the reaction and release of Ba from thin BaF2 films upon deposition of Al. The OPV device with the BaF2 layer showed a slightly improved maximum PCE (4.0 %) and a greatly (approximately nine times) increased device half-life under continuous simulated solar irradiation at 100 mW cm(-2) as compared with the OPV without an interfacial layer (PCE=2.1 %). We found that the photodegradation of the photoactive layer was not a major cause of the OPV degradation. The hugely improved lifetime with cathode interface modification suggests a significant role of the cathode interfacial layer that can help to prolong device lifetimes.

Enhancement of light reflectance and thermal stability in Ag–Cu alloy contacts on p-type GaN

The mechanism for thermally stable Ag–Cu alloy Ohmic contact on p-type GaN was investigated. Ag–Cu contact showed lower contact resistivity as low as 8.6×10−6Ωcm2, higher reflectance of 84% at 460nm, and better thermal stability than Ag contact after annealing in air ambient. The formation of Ag–Ga solid solution lowered the contact resistivity. Additionally the formation of Cu oxide suppresses the Ag oxidation and increases the work function of the Ag–Cu contact via decreasing the Schottky barrier height for hole injection. Precipitation of Cu oxide at grain boundaries suppresses the Ag agglomeration, leading to enhanced light reflectance as well as thermal stability.

Effects of Mg Additive on Inhibition of Ag Agglomeration in Ag-Based Ohmic Contacts on p-GaN

We investigate the effect of Mg additive on the inhibition of Ag agglomeration in Ag contacts on p-GaN. The Mg-containing Ag contact shows low contact resistivity of 6.3 × 10- 5 Ω cm 2 , high reflectance of 85.5% at 460 nm wavelength, and better thermal stability than the Ag contact after annealing in air ambient. Synchrotron radiation photoemission spectroscopy revealed that Mg atoms dissolving in the Ag contact reacted with oxygen atoms to form Mg oxides and increases the work function via decreasing the Schottky barrier height for hole injection. This leads to the inhibition of Ag atoms from lattice diffusion in the contact and to the suppression of the formation of Ag agglomeration, leading to enhanced light reflectance and thermal stability.

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.

Strain induced suppression of silver agglomeration of indium-containing silver contact

The mechanism for thermally stable indium-containing silver [Ag(In)] Ohmic contact on p-type GaN has been investigated. The specific contact resistivity as low as 3.8×10−5 Ω cm2 and a high reflectance of 88.4% at a 460 nm wavelength were obtained by annealing Ag(0.5 wt % In) alloy contact at 450 °C in air ambient. The In atoms in Ag matrix made In–O chemical bonds, producing a tensile stress in the film. This compensated thermal compressive stress built in the Ag film. As a result, In atoms in Ag film play a role in preventing Ag contact from agglomeration, leading to high reflectance and good thermal stability.

Highly reflective Ag-Cu alloy-based ohmic contact on p-type GaN using Ru overlayer

We report on a metallization scheme of high reflectance, low resistance, and smooth surface morphology ohmic contact on p-type GaN. Ag-Cu alloy/Ru contact showed low contact resistivity as low as 6.2 x 10(-6) Ohms cm(2) and high reflectance of 91% at 460 nm after annealing at 400 degrees C in air ambient. The oxidation annealing promoted the out-diffusion of Ga atoms to dissolve in an Ag-Cu layer with the formation of an Ag-Ga solid solution, lowering the contact resistivity. The Ru overlayer acts as a diffusion barrier for excessive oxygen incorporation during oxidation annealing, resulting in high reflectance, good thermal stability, and smooth surface quality of the contact.

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.

Origin of gap states in the electron transport layer of organic solar cells

The mechanism of the formation of gap states in bathocuproine (BCP) used as an electron transport layer (ETL) in poly(3-hexylthiophene) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) based solar cell is investigated. The photo-generated electrons in the active layer are extracted to a cathode electrode through the gap states of BCP located near the lowest unoccupied molecular orbital (LUMO) level of the electron acceptors. In the most extensively used BCP, the origin of the formation of gap states is not well understood. We show that gap states in BCP are induced by out-diffusion of P3HT molecules into the BCP layer. A Secondary Ion Mass Spectrometry (SIMS) depth profile reveals that the out-diffusion of P3HT occurs during the deposition of BCP on the P3HT:PCBM active layer. The mixing of P3HT molecules with the BCP layer induces a chemical reaction between BCP and P3HT to produce the N–S bonds (between the nitrogen from BCP and the sulfur from P3HT), which acts as gap states in BCP. The power conversion efficiency increased from 0.03% to 3.0% as the deposition rate is reduced from 5.0 A s−1 to 0.1 A s−1. This increase originates from the change in the diffusion length of P3HT with the deposition rate of BCP, leading to an increase in conductivity as well as the alignment of the Fermi level through the gap states.