Won-Yong Jin

Highly flexible and transparent conducting silver nanowire/ZnO composite film for organic solar cells

High efficiency and flexible inverted organic solar cells have been fabricated using solution-processed silver nanowire/zinc oxide composite transparent electrodes. The transparent electrodes showed a low sheet resistance of ∼13 Ω·sq−1 and high transmittance of ∼93% as well as superior mechanical flexibility. Power conversion efficiencies of ∼7.57% and ∼7.21% were achieved for devices fabricated on glass and plastic substrate, respectively. Moreover, the flexible devices did not show any degradation in their performance even after being folded with a radius of ∼480 μm.

Silver Nanowire Transparent Conductive Electrodes for High-Efficiency III-Nitride Light-Emitting Diodes.

Silver nanowires (AgNWs) have been successfully demonstrated to function as next-generation transparent conductive electrodes (TCEs) in organic semiconductor devices owing to their figures of merit, including high optical transmittance, low sheet resistance, flexibility, and low-cost processing. In this article, high-quality, solution-processed AgNWs with an excellent optical transmittance of 96.5% at 450 nm and a low sheet resistance of 11.7 Ω/sq were demonstrated as TCEs in inorganic III-nitride LEDs. The transmission line model applied to the AgNW contact to p-GaN showed that near ohmic contact with a specific contact resistance of ~10−3 Ωcm2 was obtained. The contact resistance had a strong bias-voltage (or current-density) dependence: namely, field-enhanced ohmic contact. LEDs fabricated with AgNW electrodes exhibited a 56% reduction in series resistance, 56.5% brighter output power, a 67.5% reduction in efficiency droop, and a approximately 30% longer current spreading length compared to LEDs fabricated with reference TCEs. In addition to the cost reduction, the observed improvements in device performance suggest that the AgNWs are promising for application as next-generation TCEs, to realise brighter, larger-area, cost-competitive inorganic III-nitride light emitters.

Highly stable and efficient inverted organic solar cells based on low-temperature solution-processed PEIE and ZnO bilayers

Highly efficient and air-stable inverted organic solar cells (IOSCs) were fabricated from solution-processed non-conjugated polyethylenimine ethoxylated (PEIE) as the polyelectrolyte, a zinc oxide (ZnO) bilayer as the electron transport layer, and an active layer of thieno[3,4-b]thiophene/benzodithiophene (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). When compared to conventional ZnO thin film devices, the incorporation of ZnO with nano-ridge structures (ZnO-R) and large interfacial areas, in addition to low leakage currents, led to an enhancement in power conversion efficiency from 7.41% to 8.38%. Furthermore, the presence of a thin PEIE layer between ITO and ZnO-R not only suppressed the formation of an oxygen deficient state at the ZnO-R surface, but also improved charge carrier mobilities and prevented leakage currents. Consequently, a maximum (average) efficiency of 8.91% (8.86%) and superior air stability with approximately 65% of the initial efficiency being retained after 326 days of storage under ambient atmosphere were achieved.

Silver nanowires for transparent conductive electrode to GaN-based light-emitting diodes

Transparent, conductive, and uniform Ag nanowires (NWs) were introduced to improve the optical performance of GaN-based light-emitting diodes (LEDs) by a spin-coating technique. The Ag NWs acted as a current spreading layer, exhibiting high transmittance and low sheet resistance, and ultimately leading to high performance GaN-based LEDs with an ultra large size of 5 × 5 mm2. Compared to the transmittance of conventional LEDs without Ag NWs, the relative transmittance of LEDs with Ag NWs was approximately 90% of the overall wavelength region. However, the electroluminescence (EL) intensity of LED with Ag NWs was much higher than that of conventional LEDs without Ag NWs for injection current above 45 mA. In addition, the EL full width at half maximum of LEDs with Ag NWs was much lower than that of conventional LEDs without Ag NWs. Based on these results, we believe that the enhanced optical performance of ultra large LEDs was due to an increase in the current spreading effect.

Thermal pressing of a metal-grid transparent electrode into a plastic substrate for flexible electronic devices

A flexible transparent electrode (TE) is fabricated by thermal pressing of a metal-grid into a plastic film. The metal-grid is prepared by electrohydrodynamic continuous jet printing, which easily provides a high aspect ratio for the printed lines. Embedding the high-aspect-ratio metal-grid results in a smooth surface morphology that promotes the uniform deposition of functional materials over the metal-grid TE. The thermal-pressed metal-grid TEs show excellent electrical and optical performance: a sheet resistance of 0.5 Ω sq−1 and an optical transmittance above 80% lead to a figure of merit of 2000. The flexibility of the thermal-pressed metal-grid TE is investigated under both compressive and tensile bending stresses. Invariant electrical performance is observed for a bending radius of up to 3 mm. Less than 30% degradation of the original electrical performance occurs after 1000 compressive–bending cycles with a radius of 10 mm. Organic solar cells fabricated on the thermal-pressed metal-grid TEs demonstrate acceptable device performance equivalent to devices fabricated on commercial indium tin oxide glass. These properties confirm the feasibility of thermal-pressed metal-grid TEs for use in flexible electronics.

Degradation mechanism of planar-perovskite solar cells: correlating evolution of iodine distribution and photocurrent hysteresis

In this report, we demonstrate that moisture/O2 in ambient air is the major issue for the photovoltaic performance degradation and severe photocurrent hysteresis of non-encapsulated planar-perovskite solar cells. Consequently, this leads to difficulty in determining the real power conversion efficiency (PCE). Upon longer storage time, the evidence of a small amount of iodine in the hole transport layer (HTL) led to hindering the charge transport from the HTL to the anode, thus resulting in the decrease of short-circuit current density and fill factor. Meanwhile, the transient chronoamperometry result suggests that the increase of hysteresis with storage time is ascribed to the changes of activation energy. It is further supported by X-ray photoelectron spectroscopy depth profile analysis, which revealed that penetration of moisture/O2 caused the shifts of iodine distribution within the perovskite layer after aging time of >72 h. Remarkably, effective moisture/O2 passivation can be achieved by combination of polyimide and UV-cured polymer as a novel encapsulation process, which exhibited an impressive stabilized PCE of above 14% (retained 97% of its initial efficiency) and simultaneously maintained the hysteresis up to ∼1000 h.

Ultra-Smooth, Fully Solution-Processed Large-Area Transparent Conducting Electrodes for Organic Devices

A novel approach for the fabrication of ultra-smooth and highly bendable substrates consisting of metal grid-conducting polymers that are fully embedded into transparent substrates (ME-TCEs) was successfully demonstrated. The fully printed ME-TCEs exhibited ultra-smooth surfaces (surface roughness ~1.0 nm), were highly transparent (~90% transmittance at a wavelength of 550 nm), highly conductive (sheet resistance ~4 Ω ◻−1), and relatively stable under ambient air (retaining ~96% initial resistance up to 30 days). The ME-TCE substrates were used to fabricate flexible organic solar cells and organic light-emitting diodes exhibiting devices efficiencies comparable to devices fabricated on ITO/glass substrates. Additionally, the flexibility of the organic devices did not degrade their performance even after being bent to a bending radius of ~1 mm. Our findings suggest that ME-TCEs are a promising alternative to indium tin oxide and show potential for application toward large-area optoelectronic devices via fully printing processes.

Improving light extraction of flexible OLEDs using a mechanically robust Ag mesh/ITO composite electrode and microlens array

In this study, we designed a highly flexible, mechanically robust Ag mesh/ITO composite transparent conducting electrode (TCE) integrated with a microlens array (MLA) to improve the light extraction of organic light-emitting diodes (OLEDs). The mechanical flexibility and durability of a flexible OLED based on a Ag mesh/ITO composite TCE were superior to those of a device with a conventional ITO electrode, retaining ∼100% initial luminance for up to 1000 cycles at a bending radius of ∼2 mm. Additionally, both experimental and theoretical characterizations suggest that the performance of a flexible OLED, such as its external quantum efficiency and angular-dependent emission, vastly improves after the incorporation of MLA due to the additional light scattering induced by the MLA. The combined use of Ag mesh and MLA is a practical way to overcome the brittleness of ITO and improve the out-coupling efficiency of flexible OLEDs (enhancement ratio of ∼1.51).

Dual Light Trapping and Water-Repellent Effects of a Flexible-Based Inverse Micro-Cone Array for Organic and Perovskite Solar Cells

A simple and cost-effective fabrication process of a flexible-based inverse micro-cone array (i-MCA) structure textured on flexible transparent conductive electrodes (TCEs) was successfully demonstrated via a micro-imprinting process. The flexible i-MCA films exhibited an extremely high total transmittance of ∼93% and a haze of ∼95% with reduced reflectance while simultaneously demonstrating water-repellent properties. Introducing i-MCA on the illuminating side of organic solar cells (OSCs)- and perovskite solar cells-rigid glass substrate showed improved power conversion efficiencies (PCEs) due to the light trapping effect by multiple light bounces between cone array structures (forward scattering). This results in an increase of the optical path length in the photoactive layer. Similarly, flexible TCEs embedded with textured i-MCA increased the PCE by 14% for flexible OSCs. More importantly, i-MCA-TCE-based OSCs were highly flexible with 98% retention from the initial PCE at both 0° and at 60° even after 2000 bending cycles at a radius of 2 mm. This finding demonstrates that textured i-MCA is promising for improving: (a) the light harvesting efficiency of solar cells when installed in low-/high-latitude locations and (b) the wearable technology where a flexible device attached on curved objects could retain the PCE, even at an oblique angle, with respect to the normal incidence angle.

The optoelectronic properties of tungsten-doped indium oxide thin films prepared by polymer-assisted solution processing for use in organic solar cells

Tungsten-doped indium oxide (WIO) transparent conducting thin films, to be used in inverted organic solar cells (IOSCs), were prepared by a polymer-assisted solution (PAS) process. Tungsten has high Lewis acid strength and it is, therefore, a promising candidate dopant for formulating high-mobility transparent conducting oxides with high transmittance over a wide range of regions from visible to near-infrared. WIO–PAS was formulated by coordinating W- and In-anionic complexes with a water-soluble polymer, and subsequently spin-coated on glass substrates and heat-treated at elevated temperatures. The final WIO–PAS coating solution was prepared by adding W-PAS and In-PAS with W concentrations in solution ranging from 1 at% to 5 at%. The optimum W concentration resulting in the lowest resistivity of 7.38 × 10−4 Ω cm was 3 at%. The optimum PAS-processed WIO (PAS–WIO) film, having a thickness of 230 nm, had a sheet resistance of 38 Ω sq−1 and an optical transmittance greater than 85%. The potential of the optimum PAS–WIO films in IOSCs was also assessed. The IOSCs prepared with PAS–WIO films had a power conversion efficiency (PCE) of 5.6%, which is comparable to the PCE values of IOSCs with commercial tin-doped indium oxide (ITO) films. This suggests that the PAS–WIO films are a cost-effective alternative to the vacuum-based ITO films used for the fabrication of optoelectronic devices.