Kyung-Koo Lee

Ether-based electrolytes are widely investigated for lithium metal batteries (LMBs) owing to their favorable compatibility with metallic lithium. However, conventional linear glymes strongly solvate Li+, leading to sluggish desolvation, while their limited oxidative stability restricts high-voltage operation. Here, we report an asymmetric, partially fluorinated ether, diethylene glycol 2,2,2-trifluoroethyl ethyl ether (F3DEGDEE), as a single solvent. Raman spectroscopy, molecular dynamics simulations, and density functional theory calculations reveal that asymmetric fluorination moderately weakens Li+–solvent coordination and promotes FSI− participation in the primary solvation sheath, while avoiding excessive aggregation and viscosity observed in the symmetrically fluorinated analogue. This balanced solvation environment preserves sufficient ionic conductivity, facilitates Li+ desolvation, and enhances oxidative stability. Preferential anion decomposition forms an inorganic-rich SEI with low and stable transport barriers, enabling uniform and reversible Li plating/stripping. The optimized 2 M LiFSI/F3DEGDEE electrolyte achieves 99.02% Coulombic efficiency over 500 cycles in Li||Cu cells and stable Li||Li cycling for >2000 h. Li||LFP cells retain 81.8% capacity after 2000 cycles at 5 C, while Li||NMC811 cells operated at 4.4 V retain 74% after 200 cycles at 1 C. These results highlight asymmetric partial fluorination as an effective strategy to synergize ion transport, desolvation kinetics, and oxidative stability for high-performance LMBs.

Research on electron transporting layers (ETLs) for organic solar cells (OSCs) has advanced rapidly in recent years; however, most studies have focused primarily on improving device efficiency rather than stability. Here, we report new ETL polymers, PPV and PPA, incorporating PDINN moieties through vinyl and ethynyl linkers, respectively. Among these materials, PPV‐M delivers the best photovoltaic performance, achieving power conversion efficiencies of 18.45% and 16.97% in D18:Y6‐BO‐ and PM6:BTP‐eC9‐based devices, respectively, surpassing those of PDINN‐based counterparts (17.79% and 16.85%). Under thermal aging at 65°C, PPV‐M‐based devices exhibited improved operational retention compared with PDINN‐based devices, with T ₈₀ values of 637 h and 8 h, respectively. This improved efficiency and stability originate from the polymeric nature of PPV‐M, which enables smooth and uniform coverage of the photoactive layer and provides a higher glass transition temperature than the small‐molecule PDINN, thereby suppressing thermally induced molecular diffusion. These results demonstrate that polymerizing electron‐transporting units into well‐defined ETLs is an effective molecular design strategy for achieving both high efficiency and enhanced thermal stability in OSCs.

Direct photopatterning with photoactivated cross-linkers provides a powerful route to highly integrated, solution-processed electronics. We introduce Diazo-6Bx, a six-branched diazo-based cross-linker that combines high miscibility with organic semiconductors and efficient C–H insertion reactivity. Comparative studies with azide-functionalized cross-linkers (phenyl- and tetrafluorophenyl-based structural analogues, namely, Ph-6Bx and 6Bx, respectively) reveal that the fluorine-free structure of Diazo-6Bx and its superior compatibility with host materials enable highly uniform cross-linking. This results in high-fidelity photopatterning with steep taper angles, excellent pattern retention, and minimal line-width variation. In solution-processed organic thin-film transistors (OTFTs), Diazo-6Bx enables the fabrication of unit devices that outperform those based on 6Bx, as well as large-area, high-resolution transistor arrays and logic circuits with excellent operational stability and scalability. These results demonstrate that a careful cross-linker design will play a crucial role in patterning next-generation electronic materials in extremely high fidelity.

Inverted perovskite solar cells (PSCs), a promising photovoltaic technology, require effective hole-selective contacts─often provided by self-assembled monolayers (SAMs)─to achieve high performance and stability. We systematically engineered five phenothiazine-based SAMs (MeO- to CN-2EPT) by varying their terminal substituents to tune their molecular dipole moments and highest occupied molecular orbital (HOMO) energy levels, thereby precisely controlling the interfacial energetics. Our study revealed a critical trade-off between charge extraction efficiency and stability. Br-2EPT demonstrated optimal energetic alignment with the perovskite valence band maximum (VBM = -5.68 eV), yielding the highest device power conversion efficiency (PCE of 19.9%) and minimal recombination losses. Conversely, CN-2EPT showed the strongest chemical interaction via its cyano functionality, leading to superior defect passivation and outstanding long-term stability (retaining 97.8% PCE after 100 h of MPP tracking and 95% after 70 days of thermal aging). However, the deeper HOMO energy level of CN-2EPT created a hole-extraction barrier, significantly limiting its fill factor (FF) and ultimate efficiency. These results highlight the importance of interfacial dipole engineering in SAMs and underscore that the critical design principle for next-generation hole-selective layers is the judicious combination of strong passivating groups with a HOMO energy level that closely matches the perovskite valence band to maximize both efficiency and long-term durability.

Surface and interfacial defects represent the primary loss pathways limiting the power conversion efficiency (PCE) and the operational stability of inverted perovskite solar cells (PSCs). Here, we report a sequential bimolecular passivation strategy employing propane-1,3-diammonium diiodide (PDAI) followed by (E)-[(4-trifluoromethyl)styryl]phosphonic acid (4TF) to simultaneously mitigate grain boundary and surface-state traps. PDAI acts as a deep-level defect passivant by penetrating grain boundaries, effectively eliminating interfacial pinholes and inducing a Fermi level (E_F) shift that reduces the work function (W_F) from 4.40 to 4.28 eV. Subsequent 4TF treatment facilitates coordination bonding with undercoordinated Pb²⁺ sites, establishing a positive surface dipole that shifts the W_F to 4.54 eV. This dual-functional approach optimizes band alignment with the C₆₀ electron transport layer, while yielding a compact, pinhole-free morphology. Consequently, PSCs treated with PDAI/4TF deliver a champion PCE of 24.6%, significantly surpassing the PDAI-only (PCE = 23.17%) and control (PCE = 21.21%) devices. These findings underscore the effectiveness of combined defect and dipole control for tailoring the perovskite/ETL interface in high-performance perovskite solar cells.

The commercialization of high-energy-density lithium metal batteries (LMBs) is critically hindered by the uncontrolled growth of lithium dendrites and the unstable solid electrolyte interphase (SEI), which lead to poor cycling stability and safety issues. Herein, a multifunctional separator (LNO@ZIF-67/PP) is proposed to comprehensively address these challenges through a synergistic approach. A porous zeolitic imidazolate framework-67 (ZIF-67) layer is coated onto a commercial polypropylene (PP) separator to physically regulate Li+ flux, while its pores are pre-loaded with lithium nitrate (LiNO3) to chemically engineer a robust SEI. The ZIF-67 framework endows the separator with enhanced thermal stability, electrolyte wettability, and a high Li+ transference number (tLi+ = 0.75). Concurrently, the controlled release of LiNO3 facilitates the in-situ formation of a stable, inorganic-rich SEI, predominantly composed of highly conductive Li3N. This dual-function design transforms the typical dendritic deposition into a smooth, dense morphology, enabling a near-perfect Li plating/stripping Coulombic efficiency of 99.68% and ultra-stable cycling in Li||Li symmetric cells for over 500 h. When evaluated in full-cell configurations, the LNO@ZIF-67/PP separator enables stability in both Li||LiFePO4 and high-voltage Li||NCM811 cells, even under demanding conditions with thin (20 µm) lithium anodes. Specifically, the Li||LiFePO4 cells maintained a high capacity retention of 95.5% over 300 cycles, while the high-voltage Li||NCM811 cells demonstrated stable operation with a capacity retention of 69.7% after 300 cycles under harsh conditions (4.4 V cut-off). This work presents a powerful strategy that integrates physical guidance from porous hosts and chemical functionality from active additives to stabilize the lithium metal anode, offering a scalable solution for high-performance LMBs.

An engineered DEGDEE solvent creates stable Li ⁺ solvation, promoting inorganic-rich interphases for enhanced Li‖LiFePO ₄ cycling stability.