Gunhee Lee

Superflexible, high-efficiency perovskite solar cells utilizing graphene electrodes: towards future foldable power sources

With rapid and brilliant progress in performance over recent years, perovskite solar cells have drawn increasing attention for portable power source applications. Their advantageous features such as high efficiency, low cost, light weight and flexibility should be maximized if a robust and reliable flexible transparent electrode is offered. Here we demonstrate highly efficient and reliable super flexible perovskite solar cells using graphene as a transparent electrode. The device performance reaches 16.8% with no hysteresis comparable to that of the counterpart fabricated on a flexible indium-tin-oxide electrode showing a maximum efficiency of 17.3%. The flexible devices also demonstrate superb stability against bending deformation, maintaining >90% of its original efficiency after 1000 bending cycles and 85% even after 5000 bending cycles with a bending radius of 2 mm. This overwhelming bending stability highlights that perovskite photovoltaics with graphene electrodes can pave the way for rollable and foldable photovoltaic applications.

Plasmonic Organic Solar Cells Employing Nanobump Assembly via Aerosol-Derived Nanoparticles

We report the effect of a nanobump assembly (NBA) constructed with molybdenum oxide (MoO3) covering Ag nanoparticles (NPs) under the active layer on the efficiency of plasmonic polymer solar cells. Here, the NPs with precisely controlled concentration and size have been generated by an atmospheric evaporation/condensation method and a differential mobility classification and then deposited on an indium tin oxide electrode via room temperature aerosol method. NBA structure is made by enclosing NPs with MoO3 layer via vacuum thermal evaporation to isolate the undulated active layer formed onto the underlying protruded NBA. Simulated scattering cross sections of the NBA structure reveal higher intensities with a strong forward scattering effect than those from the flat buffer cases. Experimental results of the device containing the NBA show 24% enhancement in short-circuit current density and 18% in power conversion efficiency compared to the device with the flat MoO3 without the NPs. The observed improvements are attributed to the enhanced light scattering and multireflection effects arising from the NBA structure combined with the undulated active layer in the visible and near-infrared regions. Moreover, we demonstrate that the NBA adopted devices show better performance with longer exciton lifetime and higher light absorption in comparison with the devices with Ag NPs incorporated flat poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Thus, the suggested approach provides a reliable and efficient light harvesting in a broad range of wavelength, which consequently enhances the performance of various organic solar cells.

Ultra-sensitive Pressure sensor based on guided straight mechanical cracks.

Recently, a mechanical crack-based strain sensor with high sensitivity was proposed by producing free cracks via bending metal coated film with a known curvature. To further enhance sensitivity and controllability, a guided crack formation is needed. Herein, we demonstrate such a ultra-sensitive sensor based on the guided formation of straight mechanical cracks. The sensor has patterned holes on the surface of the device, which concentrate the stress near patterned holes leading to generate uniform cracks connecting the holes throughout the surface. We found that such a guided straight crack formation resulted in an exponential dependence of the resistance against the strain, overriding known linear or power law dependences. Consequently, the sensors are highly sensitive to pressure (with a sensitivity of over 1 × 105 at pressures of 8–9.5 kPa range) as well as strain (with a gauge factor of over 2 × 106 at strains of 0–10% range). A new theoretical model for the guided crack system has been suggested to be in a good agreement with experiments. Durability and reproducibility have been also confirmed.

Transparent ITO mechanical crack-based pressure and strain sensor

Sensors to detect motion with high precision have been extensively studied in diverse engineering research fields. Among them, transparent devices, which have strong adaptability in various fields such as display panels, have not gained much academic interest. In this study, we present a highly sensitive pressure and strain sensor based on a cracked transparent epilayer, indium-tin oxide (ITO), deposited on a transparent PET substrate. This sensor system, with which we demonstrate how to detect pressure and finger motions, exhibits ultra-sensitivity to strain (gauge factor about 4000 at 2% strain), pressure (sensitivity is about 1.91 kPa−1 at pressures from 30 to 70 kPa), and transparency (up to 89% at a wavelength of 560 nm). Also, durability has been validated over 5000 cycles. The sensor thus boasts broad applications including touchscreens and motion detectors.

High-performance Fuel Cell with Stretched Catalyst-Coated Membrane: One-step Formation of Cracked Electrode

We have achieved performance enhancement of polymer electrolyte membrane fuel cell (PEMFC) though crack generation on its electrodes. It is the first attempt to enhance the performance of PEMFC by using cracks which are generally considered as defects. The pre-defined, cracked electrode was generated by stretching a catalyst-coated Nafion membrane. With the strain-stress property of the membrane that is unique in the aspect of plastic deformation, membrane electrolyte assembly (MEA) was successfully incorporated into the fuel cell. Cracked electrodes with the variation of strain were investigated and electrochemically evaluated. Remarkably, mechanical stretching of catalyst-coated Nafion membrane led to a decrease in membrane resistance and an improvement in mass transport, which resulted in enhanced device performance.

Precise Morphology Control and Continuous Fabrication of Perovskite Solar Cells Using Droplet-Controllable Electrospray Coating System

Herein, we developed a novel electrospray coating system for continuous fabrication of perovskite solar cells with high performance. Our system can systemically control the size of CH3NH3PbI3 precursor droplets by modulating the applied electrical potential, shown to be a crucial factor for the formation of perovskite films. As a result, we have obtained pinhole-free and large grain-sized perovskite solar cells, yielding the best PCE of 13.27% with little photocurrent hysteresis. Furthermore, the average PCE through the continuous coating process was 11.56 ± 0.52%. Our system demonstrates not only the high reproducibility but also a new way to commercialize high-quality perovskite solar cells.

Crack-based strain sensor with diverse metal films by inserting an inter-layer

Various sensory systems to detect human motions have been developed for wearable healthcare and artificial electronic skins. Recently, an ultrasensitive mechanical crack-based strain sensor inspired by a spiders slit organ has been proposed. In spite of its high sensitivity, flexibility, and fascinating sensing ability to vibration, the materials that can be used to manufacture the sensor are limited to certain kinds because of the low adhesion between the substrate and a metal film. Therefore, the compatibility of materials with the substrate is a crucial issue in developing a practical sensor system. Here, we present a mechanical crack-based strain sensor with diverse metal (Au, Ag and Pt) films by introducing an inter-layer. Two inter-layers are used; a Cr layer is for generating cracks and MoO3 layer for enhancing the adhesion between the substrate and the metal layer. When cracks are generated on the Cr layer, they are propagated to the conductive metal layers (Au, Ag and Pt). Our crack-based strain sensor exhibited reproducibility and durability with high sensitivity to strain (GF = ∼1600 for Au and Ag layered crack sensors at 2% strain, GF = ∼850 for Pt layered sensor at 2% strain).

Photocurable PUA (Poly Urethaneacrylat) cantilever integrated with ultra-high sensitive crack-based sensor

This paper describes the fabrication and characterization of ultra-high sensitive polymer cantilever to precisely monitor the change in contraction force of cardiomyocytes. The mechanical crack-based sensor inspired from a spider showed enhanced sensitivity towards strain and vibration in nature. In spite of all these interesting characteristics, the crack-based sensor has not yet been used for biomedical applications such as detecting a small force generated from cells. Herein we made successful attempt to develop a novel sensing technique for measuring the contraction force of cardiomyocytes. This idea can be applied for the development of cantilever-based cardiac toxicity screening systems.

Nanobump assembly for plasmonic organic solar cells

We demonstrate novel plasmonic organic solar cells (OSCs) by embedding an easy processible nanobump assembly (NBA) for harnessing more light. The NBA is consisted of precisely size-controlled Ag nanoparticles (NPs) generated by an aerosol process at atmospheric pressure and thermally deposited molybdenum oxide (MoO3) layer which follows the underlying nano structure of NPs. The active layer, spin-casted polymer blend solution, has an undulated structure conformably covering the NBA structure. To find the optimal condition of the NBA structure for enhancing light harvest as well as carrier transfer, we systematically investigate the effect of the size of Ag NPs and the MoO3 coverage on the device performance. It is observed that the photocurrent of device increases as the size of Ag NP increases owing to enhanced plasmonic and scattering effect. In addition, the increased light absorption is effectively transferred to the photocurrent with small carrier losses, when the Ag NPs are fully covered by the MoO3 layer. As a result, the NBA structure consisted of 40 nm Ag NPs enclosed by 20 nm MoO3 layer leads to 18% improvement in the power conversion efficiency compared to the device without the NBA structure. Therefore, the NBA plasmonic structure provides a reliable and efficient light harvesting in a broad range of wavelength, which consequently enhances the performance of organic solar cells.