Jun Yong Kim

Efficient perovskite solar cells (PSCs) require precisely engineered internal electric fields to promote charge separation and suppress recombination. Although p–n homojunctions offer an intrinsic route to generate such fields, their implementation has been hindered by interfacial degradation and the processing constraints of conventional multilayer fabrication. In this work, interfacial‐defect‐free MAPbI ₃ p–n homojunctions are constructed by integrating stoichiometry‐controlled self‐doping with solvent‐assisted hot‐pressing transfer (HPT). This approach enables independent modulation of the doping polarity in the top and bottom MAPbI ₃ layers, resulting in structurally coherent bilayers with reinforced built‐in fields. Experimental characterization combined with drift–diffusion simulations shows that device performance is optimized when the top MAPbI ₃ layer is strongly p‐doped and the bottom layer is moderately n‐doped, a doping configuration that strengthens the internal field, enhances carrier separation, and reduces hysteresis. Under these optimized conditions, the p–n homojunction architecture delivers a relative improvement in power conversion efficiency of approximately 22% compared to the intrinsic bilayer device. These findings establish a unified experimental–theoretical framework for HPT‐based homojunction PSCs and highlight controlled doping asymmetry as a central design principle for high‐efficiency perovskite photovoltaics.

Developing spectrally precise, compact electroluminescent (EL) devices is critical for emerging photonic technologies, including advanced displays and integrated photonic systems. Although recent advances in emitter materials have enabled narrowband emission with full width at half maximum (FWHM) values as low as 25 nm, their practical applications are hindered by stability issues, fabrication complexity, and limited environmental compatibility. Optical microcavities improve spectral precision through high‐quality factors but require complex reflector structures and simultaneous optical and electrical optimization. Here, a universal strategy is presented to achieve spectrally precise emission from broadband organic light‐emitting diodes (OLEDs) by enhancing the Purcell effect through dual‐microcavity resonances. A secondary cavity atop the OLED separates optical and electrical design while generating dual‐microcavity resonances. Coupling between excitons and dual‐microcavity enhances the Purcell effect, leading to an increased spontaneous emission rate. Spectrally tunable, ultrapure green emission (FWHM = 21 nm) is demonstrated from phosphorescent OLEDs (intrinsic FWHM = 60 nm), achieving ≈65% spectral narrowing. The devices also exhibit a high luminance of 1.241 × 10 ⁵ cd m ⁻² , strong directionality, and suppressed efficiency roll‐off. Our approach is compatible with state‐of‐the‐art emitters, polaritons, and photonic architecture, offering a promising route toward advanced photonic systems requiring monochromatic emission from compact EL devices.

Microcavity structures are used in inorganic‐, organic‐, quantum‐dot‐, and perovskite‐based electroluminescent (EL) devices to advance next‐generation displays. However, there are difficulties in controlling electrical characteristics and patterning processes for producing different thicknesses for each red, green, and blue (RGB) subpixel, and the issues are more challenging in the high‐resolution display for future realistic media. Here, a novel design method is presented for a dual‐microcavity structure that controls high‐order modes of a second cavity stacked on top of EL devices with the same cavity length for each subpixel to produce multiple peaks at RGB resonant wavelengths. The dual‐microcavity effect demonstrated by top‐emitting organic light‐emitting diodes (OLEDs) can be conveniently fabricated via in situ deposition. By modulating the high‐order modes, the spectral characteristics of each RGB dual‐microcavity top‐emitting OLED (DMTOLED) are manipulated while its electrical properties are maintained. Green DMTOLED exhibits a maximum luminance of 2.075 × 10⁵ cd m⁻², allowing applications not only for commercialized displays but also for outdoor augmented reality and automotive displays. Furthermore, dual‐microcavity structures with narrow spectral bandwidths can be applied to next‐generation EL devices for more realistic media. The method is expected to be applied industrially, promoting the advancement of EL devices for next‐generation displays.

Structural colors have been reported instead of conventional dye- or pigment-based color filters. Color selectivity can degrade as structure-based optical resonances are accompanied by several resonance modes. In this work, we suggest a simple and effective design of the plasmonic color filter (PCF) that integrated the PCF with the one-dimensional (1D) photonic crystal (PhC). The introduced PhC creates an optical band gap and suppresses undesired peaks of the PCF caused by the high-order resonance mode. Finally, the suggested structure provides a high color purity. This study can be a guideline for technology that replaces conventional color filters.

In this paper, we propose an optimized structure of thin Cu(In,Ga)Se₂ (CIGS) solar cells with a grating aluminum oxide (Al₂O₃) passivation layer (GAPL) providing nano-sized contact openings in order to improve power conversion efficiency using optoelectrical simulations. Al₂O₃ is used as a rear surface passivation material to reduce carrier recombination and improve reflectivity at a rear surface for high efficiency in thin CIGS solar cells. To realize high efficiency for thin CIGS solar cells, the optimized structure was designed by manipulating two structural factors: the contact opening width (COW) and the pitch of the GAPL. Compared with an unpassivated thin CIGS solar cell, the efficiency was improved up to 20.38% when the pitch of the GAPL was 7.5–12.5 μm. Furthermore, the efficiency was improved as the COW of the GAPL was decreased. The maximum efficiency value occurred when the COW was 100 nm because of the effective carrier recombination inhibition and high reflectivity of the Al₂O₃ insulator passivation with local contacts. These results indicate that the designed structure has optimized structural points for high-efficiency thin CIGS solar cells. Therefore, the photovoltaic (PV) generator and sensor designers can achieve the higher performance of photosensitive thin CIGS solar cells by considering these results.

CsPbIBr₂, a cesium-based all-inorganic halide perovskite (CsPe), is a very promising alternative material to mainstream organic–inorganic hybrid halide perovskite (HPe) materials owing to its exceptional moisture stability, thermal stability, and light stability. However, because of the wide band gap (2.05 eV) of CsPbIBr₂, it has a low power conversion efficiency (PCE), which hinders its application in highly efficient solar cells. In this study, a facile nanoimprinted one-dimensional grating nanopattern (1D GNP) formation on mesoporous TiO₂ (mp-TiO₂) photoelectrodes was introduced to improve the effective light utilization and enhance the performance of CsPbIBr₂ perovskite solar cells (PSCs). The 1D GNP structure on the mp-TiO₂ layer increases the light absorption efficiency by diffracting the unabsorbed light into the active mp-TiO₂ and CsPbIBr₂ layers as well as increasing the charge separation and collection due to the extended interfacial contact area between the mp-TiO₂ and CsPbIBr₂ layers. Consequently, both the current density (J_SC) and the fill factor (FF) of the fabricated cells improved, leading to over a 20% enhancement in the solar cell’s PCE. Thus, this periodic grating structure, fabricated by simple nanoimprinting, could play an important role in the large-scale production of highly efficient and cost-effective Cs-based PSCs.

An optical encapsulation structure has been proposed to realize high external quantum efficiency (EQE) and color purity in flexible bottom‐emission organic light‐emitting diodes (OLEDs). It was designed based on the Fabry‐Perot Interferometer (FPI) using optical simulation. The structure showed three sharp peaks in red, green and blue wavelength bands, respectively.

We propose an optimized structure for a humidity-sensing colorimetric sensor that utilizes a metal-organic framework to form a one-dimensional photonic crystal. The structure changes spectral reflectivity by switching its photonic band gap (PBG). We found that, although photonic crystals on a highly-reflective substrate increased the peak more than a transparent substrate, the resulting ripples in the wavelength band near the PBG destroy our ability to detect the color conversion. We also show that, despite the blue-shift of the peak PBG wavelength for higher angles of incidence, our sensor can still create significant color conversion at nonzero angles when the environmental humidity changes. Therefore, photonic crystals on a transparent substrate that utilize PBG-switching can operate as moisture-sensitive colorimetric sensors. Our study will help optimize sensors and industrial nano-structures.