Soo-Ghang Ihn

Control of naturally coupled piezoelectric and photovoltaic properties for multi-type energy scavengers†

In this paper, we present a simple, low-cost and flexible hybrid cell that converts individually or simultaneously low-frequency mechanical energy and photon energy into electricity using piezoelectric zinc oxide (ZnO) in conjunction with organic solar cell design. Since the hybrid cell is designed by coupled piezoelectric and photoconductive properties of ZnO, this is a naturally hybrid architecture without crosstalk and an additional assembling process to create multi-type energy scavengers, thus differing from a simple integration of two different energy generators. It is demonstrated that the behavior of a piezoelectric output is controlled from alternating current (AC) type to direct current (DC)-like type by tailoring mechanical straining processes both in the dark and under light illumination. Based on such controllability of output modes, it is shown that the performance of the hybrid cell is synergistically enhanced by integrating the contribution made by a piezoelectric generator with a solar cell under a normal indoor level of illumination. Our approach clearly demonstrates the potential of the hybrid approach for scavenging multi-type energies whenever and wherever they are available. Furthermore, this work establishes the methodology to harvest solar energy and low-frequency mechanical energies such as body movements, making it possible to produce a promising multi-functional power generator that could be embedded in flexible architectures.

Low-temperature growth and characterization of ZnO thin films for flexible inverted organic solar cells

Flexible inverted organic solar cells (IOSCs) with a zinc oxide (ZnO) thin film layer acting as an effective electron transport layer and a low reflective light absorber were fabricated in this study. The high quality ZnO thin films on flexible indium tin oxide-coated polyethersulfone (PES) substrates were grown in a non-vacuum process using mist pyrolysis chemical vapor deposition at a low temperature of 180 °C without any thermal damage to the PES substrate. The power conversion efficiency of the flexible IOSCs with ZnO thin films averaged 3.1% at a simulated air-mass of 1.5 global full-sun (100 mW cm−2) illumination, which was greater than that of flexible IOSCs with a ZnO thin film formed using a sol–gel method (approximately 2.3%).

An Alternative Host Material for Long‐Lifespan Blue Organic Light‐Emitting Diodes Using Thermally Activated Delayed Fluorescence

It has been challenging to find stable blue organic light emitting diodes (OLEDs) that rely on thermally activated delayed fluorescence (TADF). Lack of stable host materials well‐fitted to the TADF emitters is one of the critical reasons. The most popular host for blue TADF, bis[2‐(diphenylphosphino)phenyl] ether oxide (DPEPO), leads to unrealistically high maximum external quantum efficiency. DPEPO is however an unstable material and has a poor charge transporting ability, which in turn induces an intrinsic short OLED operating lifespan. Here, an alternative host material is introduced which educes the potential efficiency and device lifespan of given TADF emitters with the appropriateness of replacing the most popular host material, DPEPO, in developing blue TADF emitters. It simultaneously provides much longer device lifespan and higher external quantum efficiency at a practical brightness due to its high material stability and electron‐transport‐type character well‐fitted for hole‐transport‐type TADF emitters.

Autocatalytic effect of amine-terminated precursors in mixed self-assembled monolayers

We investigated the formation of mixed self-assembled monolayers (SAMs), comprising two silanes terminated with an amine and a non-reactive functional group, to demonstrate the autocatalysis of the mixed SAMs by the amine-terminated precursors. Measurements of surface energy and angle-resolved X-ray photoelectron spectroscopy on the mixed SAMs revealed that the final composition and the surface coverage of the mixed SAMs after deposition strongly depended on the presence and the concentration of the aminesilane precursor in solution. To explain the observed dependence, we further suggested a mechanism based on the autocatalytic effect of the aminesilane precursor on the mixed SAM. These results highlight the complex role of the aminesilane, which behaves simultaneously as a component of the mixed SAM and a catalyst. An important implication of this is that the compositions of mixed SAMs can be significantly influenced by the autocatalytic effect of the component carrying a reactive functional group such as amine.

Degradation of blue-phosphorescent organic light-emitting devices involves exciton-induced generation of polaron pair within emitting layers

Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established. Here we report a new mechanism for degradation of emitting layers in blue-phosphorescent devices. We investigate binary mixtures of a wide bandgap host and a series of novel Ir(III) complex dopants having N-heterocyclocarbenic ligands. Our mechanistic study reveals the charge-neutral generation of polaron pairs (radical ion pairs) by electron transfer from the dopant to host excitons. Annihilation of the radical ion pair occurs by charge recombination, with such annihilation competing with bond scission. Device lifetime correlates linearly with the rate constant for the annihilation of the radical ion pair. Our findings demonstrate the importance of controlling exciton-induced electron transfer, and provide novel strategies to design materials for long-lifetime blue electrophosphorescence devices.The short lifetime of blue-phosphorescent organic light-emitting devices owing to material degradation impedes their practical potential. Here, Kim et al. study the molecular mechanism of the degradation that involves exciton-mediated electron transfer as a key step for the generation of radical ion pairs.

Auger electron nanoscale mapping and x-ray photoelectron spectroscopy combined with gas cluster ion beam sputtering to study an organic bulk heterojunction

The lateral and vertical distributions of organic p/n bulk heterojunctions for an organic solar cell device are, respectively, investigated using nanometer-scale Auger electron mapping and using X-ray photoelectron spectroscopy (XPS) with Ar gas cluster ion beam (GCIB) sputtering. The concentration of sulfur, present only in the p-type material, is traced to verify the distribution of p-type (donor) and n-type (acceptor) materials in the blended structure. In the vertical direction, a considerable change in atomic sulfur concentration is observed using XPS depth profiling with Ar GCIB sputtering. In addition, Auger electron mapping of sulfur reveals the lateral 2-dimensional distribution of p- and n-type materials. The combination of Auger electron mapping with Ar GCIB sputtering should thereby allow the construction of 3-dimensional distributions of p- and n-type materials in organic photovoltaic cells.

Enhancement of the power conversion efficiency in a polymer solar cell using a work-function-controlled TimSinOx interlayer

Work-function-adjustable TimSinOx provides an opportunity to optimize the energy-level alignment at the photoactive/cathode interface in the polymer bulk heterojunction solar cell. The work function of TimSinOx is engineered by adjusting the Si mol% during the sol–gel reaction. The controlled work function provides an energetically downhill cascade pathway for electrons from the electron acceptor to the cathode, which contributes to improvement in electron collection at the cathode. The valence band maxima of TimSinOx also become deeper as the Si mol% increases and the hole-blocking ability of TimSinOx is enhanced as a result. Accordingly, polymer solar cells fitted with the optimized TimSinOx exhibit enhanced performance.