Ju Won Lim

Enhanced Light Scattering and Trapping Effect of Ag Nanowire Mesh Electrode for High Efficient Flexible Organic Solar Cell

Ag nanowire (NW) mesh is used as transparent conducting electrode for high efficient flexible organic solar cells (OSCs). The Ag NW mesh electrode facilitates light scattering and trapping, allowing enhancement of light absorption in the active layer. OSCs incorporating Ag NW mesh electrode exhibit maximum power conversion efficiency (PCE) of 4.47%, 25%, higher than that of OSCs with a conventional ITO electrode (3.63%).

Effect of geometric lattice design on optical/electrical properties of transparent silver grid for organic solar cells

Silver (Ag) grid transparent electrode is one of the most promising transparent conducting electrodes (TCEs) to replace conventional indium tin oxide (ITO). We systematically investigate an effect of geometric lattice modifications on optical and electrical properties of Ag grid electrode. The reference Ag grid with 5 μm width and 100 μm pitch (duty of 0.05) prepared by conventional photo-lithography and lift-off processes shows the sheet resistance of 13.27 Ω/sq, transmittance of 81.1%, and resultant figure of merit (FOM) of 129.05. Three different modified Ag grid electrodes with stripe added-mesh (SAM), triangle-added mesh (TAM), and diagonal-added mesh (DAM) are suggested to improve optical and electrical properties. Although all three of SAM, TAM, and DAM Ag grid electrodes exhibit the lower transmittance values of about 72 - 77%, they showed much decreased sheet resistance of 6 - 8 Ω/sq. As a result, all of the lattice-modified Ag grid electrodes display significant improvement of FOM and the highest value of 171.14 is obtained from DAM Ag grid, which is comparable to that of conventional ITO electrode (175.46). Also, the feasibility of DAM Ag gird electrode for use in organic solar cell is confirmed by finite difference time domain (FDTD) simulations. Unlike a conventional ITO electrode, DAM Ag grid electrode can induce light scattering and trapping due to the diffuse transmission that compensates for the loss in optical transparency, resulting in comparable light absorption in the photo active layer of poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₀BM). P3HT:PC₆₀BM based OSCs with the DAM Ag grid electrode were fabricated, which also showed the potential for ITO-free transparent electrode.

Comprehensive Study on the Controlled Plasmon-Enhanced Photocatalytic Activity of Hybrid Au/ZnO Systems Mediated by Thermoresponsive Polymer Linkers

Hybrid semiconductor/noble metal nanostructures coupled with responsive polymers were used to probe unique plasmon-mediated photocatalytic properties associated with swelling-shrinking transitions in polymer chains triggered by specific external stimuli. Poly(N-isopropylacrylamide) (PNIPAM) brushes were anchored on Au films by atom transfer radical polymerization and ZnO nanoparticles were immobilized on the PNIPAM layer to explore controlled photocatalytic activity. The plasmon-enhanced photocatalytic activity was dictated by two critical parameters, that is, grafting density and molecular weight of PNIPAM involved in Au film-PNIPAM-ZnO. The effect of the areal density of PNIPAM chains on the temperature-responsive UV light photocatalytic activities showed mutually antagonistic trends at two different temperatures. The performance at high density was higher above a lower critical solution temperature (LCST), that is, under contracted configuration, while the sample with low density showed higher activity below LCST, that is, extended configuration. Among all the cases explored, the UV light activity was highest for the sample with thin PNIPAM layer and high density above LCST. The visible light activity was induced only for thin PNIPAM layer and high density, and it was higher above LCST. The efficiency of photocatalytic decomposition of phenol pollutant was dramatically enhanced from 10% to 55% upon the increase in temperature under visible light illumination.

Plasmon-mediated wavelength-selective enhanced photoresponse in polymer photodetectors

Photodetectors have gained increasing interest in both academic and industrial domains in a ranging number of applications, including image sensing, communications, environmental monitoring and chemical/biological detection. Noble metal nanoparticles exhibit surface plasmons (SPs) associated with the enhancement of a local electromagnetic field surrounding the metal surface, leading to enhanced light absorption. Herein, size-controlled silver nanoparticles (AgNPs), gold nanoparticles (AuNPs) and gold nanorods (AuNRs) have been introduced into polymer photodiodes. The evaluated devices exhibit remarkable photocurrent enhancement at corresponding plasmon resonant wavelengths, directly resulting in a photosensitivity increase. When compared with a pristine photodiode, AgNPs-, AuNPs- and AuNRs-mediated devices reveal maximum enhancements of 46, 49 and 65% for responsivity and 39, 30 and 54% for detectivity for blue (450 nm), green (525 nm) and red (620 nm) light detection, respectively. The linear dynamic range of plasmon-mediated devices and their photoresponse stability have been further explored. The mechanism of plasmon-enhanced photocurrent has been analyzed using steady-state and time-resolved fluorescence measurements. The results indicate that plasmonic structures incorporated into polymer photodetectors can significantly improve the photoresponse in their plasmon resonant regions. This contribution is believed to showcase the effectiveness and attainable wavelength-selectable photodetection by tailoring the size and shape of coupled plasmonic nanostructures.

Reduced graphene oxide wrapped core–shell metal nanowires as promising flexible transparent conductive electrodes with enhanced stability

Transparent conductive electrodes (TCEs) are widely used in a wide range of optical-electronic devices. Recently, metal nanowires (NWs), e.g. Ag and Cu, have drawn attention as promising flexible materials for TCEs. Although the study of core-shell metal NWs, and the encapsulation/overcoating of the surface of single-metal NWs have separately been an object of focus in the literature, herein for the first time we simultaneously applied both strategies in the fabrication of highly stable Ag-Cu NW-based TCEs by the utilization of Ag nanoparticles covered with reduced graphene oxide (rGO). The incorporation of Ag nanoparticles by galvanic displacement reaction was shown to significantly increase the long term stability of the electrode. Upon comparison with a CuNW reference, our novel rGO/Cu-AgNW-based TCEs unveiled remarkable opto-electrical properties, with a 3-fold sheet resistance decrease (from 29.8 Ω sq-1 to 10.0 Ω sq-1) and an impressive FOM value (139.4). No detrimental effect was noticed in the relatively high transmittance value (T = 77.6% at 550 nm) characteristic of CuNWs. In addition, our rGO/Cu-AgNW-based TCEs exhibited outstanding thermal stability up to 20 days at 80 °C in air, as well as improved mechanical flexibility. The superior performance herein reported compared with both CuNWs and AgNWs, and with a current conventional ITO reference, is believed to highlight the great potential of these novel materials as promising alternatives in optical-electronic devices.

Optoelectronic hybrid perovskite materials and devices (Conference Presentation)

While the field of perovskite-based optoelectronics has mostly been dominated by photovoltaics, light-emitting diodes and transistors, semiconducting properties peculiar to perovskites make them interesting candidates for innovative and disruptive applications in light signal detection. Perovskites combine effective light absorption in the broadband range with good photo-generation yield and high charge carrier mobility, which combination provides promising potential for exploiting sensitive and fast photodetectors that are targeted for image sensing, optical communication, environmental monitoring, or chemical/biological detection. Currently, organic-inorganic hybrid and all-inorganic halide perovskites with controlled morphologies of polycrystalline thin films, nano-particles/wires/sheets, and bulk single crystals have shown key figure-of-merit features in terms of their responsivity, detectivity, noise equivalent power, linear dynamic range, and response speed. The sensing region has been covered from ultraviolet–visible–near infrared (UV–Vis–NIR) to gamma photons, based on two- or three-terminal device architectures. Diverse photoactive materials and devices with superior optoelectronic performances have stimulated attention from researchers in multidisciplinary areas. We offer a comprehensive overview of the recent progress of perovskite-based photodetectors, focusing on versatile compositions, structures, and morphologies of constituent materials, and diverse device architectures toward the superior performance metrics. Combining the advantages of both organic semiconductors (facile solution processability) and inorganic semiconductors (high charge carrier mobility), perovskites are expected to replace commercial silicon for future photodetection applications. The optical and electronic properties of noble metallic nanoparticles can be exploited to enhance the performance of inorganic/organic photodetectors. We integrated a uniformly-distributed layer of Au nanorods (AuNRs) into vertically-structured perovskite photoconductive photodetectors and report, as a result, perovskite-AuNR hybrid photodetectors that exhibit significant photocurrent enhancements. Ultimately it achieves a responsivity of ~320 A/W at a low driving voltage of -1 V. This is an improvement of 60% compared to the responsivity of pristine devices (~200 A/W). The high responsivity and low driving voltage place this device among the highest-performing perovskite-based thin-film photoconductive photodetectors reported. We characterized the stability and linearity of the photoresponse following repeated light/dark cycles. The hybrid device also shows a fast response (with the decay time of ~95 ns) compared to pristine devices (~230 ns). The improvements in photodetection performance are attributed to plasmon-enhanced optical absorption, as well as advances in charge extraction and transport. Metal halide perovskites have rapidly advanced thin film photovoltaic performance; as a result, the materials’ ob-served instabilities urgently require a solution. Using density functional theory (DFT), we show that a low energy of formation, exacerbated in the presence of humidity, explains the propensity of perovskites to decompose back into their precursors. We find, also using DFT, that intercalation of phenylethylammonium between perovskite layers in-troduces quantitatively appreciable van der Waals interactions; and these drive an increased formation energy and should therefore improve material stability. Here we report the reduced-dimensionality (quasi-2D) perovskite films that exhibit improved stability while retaining the high performance of conventional three-dimensional perovskites. Continuous tuning of the dimensionality, as assessed using photophysical studies, is achieved by the choice of stoi-chiometry in materials synthesis. We achieve the first certified hysteresis-free solar power conversion in a planar per-ovskite solar cell, obtaining a 15.3% certified PCE, and observe greatly improved performance longevity. The same protocol was applied to develop highly stable and efficient photodectors in diverse device configurations. Organometal halide perovskites exhibit large bulk crystal domain sizes, rare traps, excellent mobilities, and carriers that are free at room temperature – properties that support their excellent performance in charge-separating devices. In devices that rely on the forward injection of electrons and holes, such as light-emitting diodes (LEDs), excellent mobilities contribute to the efficient capture of nonequilibrium charge carriers to rare nonradiative centres. Moreover, the lack of bound excitons weakens the competition of desired radiative over undesired nonradiative recombination. Here we also report a perovskite mixed material, one comprised of a series of differently quantum-size-tuned grains, that funnels photoexcitations to the lowest-bandgap light-emitter in the mixture. The materials function as charge carrier concentrators, ensuring that radiative recombination successfully outcompetes trapping and hence nonradiative recombination. We use the new material to build devices that exhibit an external quantum efficiency (EQE) of 8.8% and a radiance of 80 Wsr-1m-2. These represent the brightest and most efficient solution processed near-infrared LEDs to date. Here we show that, by concentrating photoexcited states into a small subpopulation of radiative domains, one can achieve a high quantum yield even at low excitation intensities. We tailor the composition of quasi-2D perovskites to direct the energy transfer into the lowest-bandgap minority phase, and to do so faster than it is lost to non-radiative centres. The new material exhibits 60% photoluminescence quantum yield at excitation intensities as low as 1.8 mW/cm2, yielding a ratio of quantum yield to excitation intensity of 0.3 cm2/mW; this represents a two-orders of magnitude decrease in the excitation power required to reach high efficiency compared to the best prior reports. Using this strategy, we report LEDs with EQEs of 7.4% and a high luminescence of 8400 cd/m2.

Optimization of coupled plasmonic effects for viable phosphorescence of metal-free purely organic phosphor

Metal-free purely organic phosphorescent molecules are attractive alternatives to organometallic and inorganic counterparts because of their low cost and readily tunable optical properties through a wide chemical design window. However, their weak phosphorescent intensity due to inefficient spin-orbit coupling and, consequently, prevailing non-radiative decay processes limit their practical applicability. Here, we systematically studied phosphorescence emission enhancement of a purely organic phosphor system via plasmon resonance energy transfer. By precisely tuning the distance between purely organic phosphor crystals and plasmonic nanostructures using layer-by-layer assembled polyelectrolyte multilayers as a dielectric spacer, maximum 2.8 and 2.5 times enhancement in photoluminescence intensity was observed when the phosphor crystals were coupled with ∼55 nm AuNPs and ∼7 nm AgNPs, respectively, at the distance of 9.6 nm. When the distance is within the range of 3 nm, a dramatic decrease in phosphorescen...

Electrical and Optical Properties of Asymmetric Dielectric/Metal/Dielectric (D/M/D) Multilayer Electrode Prepared by Radio-Frequency Sputtering for Solar Cells

【Transparent and conductive multilayer thin films consisting of three alternating layers FZTO/Ag/

High-Performance UV–Vis–NIR Phototransistors Based on Single-Crystalline Organic Semiconductor–Gold Hybrid Nanomaterials

WO_3

Fluorine doped zinc tin oxide multilayer transparent conducting Oxides for organic photovoltaic׳s Cells

have been fabricated by radio-frequency (RF) sputtering for the applications as transparent conducting oxides and the structural and optical properties of the resulting films were carefully studied. The single layer fluorine doped zinc tin oxide (FZTO) and tungsten oxide (