Sunghyun Kim

Magnetocaloric materials have the ability to undergo temperature changes when subjected to varying magnetic fields. These materials are of interest due to their potential for innovative cooling applications. This review article summarizes materials that exhibit magnetic ordering within the temperature range required for gas liquefaction and explores their potential applications through the magnetocaloric effect (MCE). The gas liquefaction temperature range is typically assumed to be 20–77 K, however, this study specifically summarizes materials that have a transition temperature near to the hydrogen liquefaction temperature (≈20K). This review article aims to showcase ongoing research on magnetic materials for hydrogen liquefaction. Driven by the depletion of natural resources and environmental concerns, the search for environmentally sustainable fuels has intensified, making hydrogen a promising alternative. However, the liquefaction of hydrogen is highly energy‐intensive. The investigation focuses on identifying and understanding these materials and assessing their suitability for environmentally friendly and sustainable cooling technologies. By harnessing the magnetocaloric effect, these materials exhibit temperature changes in response to an applied magnetic field, offering advantages over traditional cooling methods that are 20–50% more efficient. The review aims to furnish researchers with essential information that can help modify magnetocaloric effect (MCE) materials, enabling them to achieve the desired magnetic ordering temperature conducive to the liquefaction of hydrogen.

The metal halide Cs₃Cu₂I₅ displays anomalous optical properties: an optical absorption onset in the ultraviolet region (∼ 330 nm) with highly efficient luminescence in the blue region (∼ 445 nm). Although self-trapped exciton formation has been proposed as the origin of giant Stokes shift, its connection to the photoluminescence quantum yield exceeding 90% remains unknown. Here, we explore the photochemistry of Cs₃Cu₂I₅ from first-principles and reveal a low energy barrier for exciton self-trapping associated with Cu–Cu dimerization. Kinetic analysis shows that the quantum yield of blue emission in Cs₃Cu₂I₅ is sensitive to the excited carrier density due to the competition between exciton self-trapping and band-to-band radiative recombination.

The thermodynamic limit of photovoltaic efficiency for a single-junction solar cell can be readily predicted using the bandgap of the active light absorbing material. Such an approach overlooks the energy loss due to non-radiative electron-hole processes. We propose a practical ab initio procedure to determine the maximum efficiency of a thin-film solar cell that takes into account both radiative and non-radiative recombination. The required input includes the frequency-dependent optical absorption coefficient, as well as the capture cross sections and equilibrium populations of point defects. For kesterite-structured Cu₂ZnSnS₄, the radiative limit is reached for a film thickness of around 2.6 μm, where the efficiency gain due to light absorption is counterbalanced by losses due to the increase in recombination current.

Improvement in the optoelectronic performance of halide perovskite semiconductors requires the identification and suppression of nonradiative carrier trapping processes. The iodine interstitial has been established as a deep level defect and implicated as an active recombination center. We analyze the quantum mechanics of carrier trapping. Fast and irreversible electron capture by the neutral iodine interstitial is found. The effective Huang–Rhys factor exceeds 300, indicative of the strong electron–phonon coupling that is possible in soft semiconductors. The accepting phonon mode has a frequency of 53 cm^–1 and has an associated electron capture coefficient of 1 × 10^–10 cm³ s^–1. The inverse participation ratio is used to quantify the localization of phonon modes associated with the transition. We infer that suppression of octahedral rotations is an important factor to enhance defect tolerance.

Defects influence the properties and functionality of all crystalline materials. For instance, point defects participate in electronic (e.g. carrier generation and recombination) and optical (e.g. absorption and emission) processes critical to solar energy conversion. Solid-state diffusion, mediated by the transport of charged defects, is used for electrochemical energy storage. First-principles calculations of defects based on density functional theory have been widely used to complement, and even validate, experimental observations. In this ‘quick-start guide’, we discuss the best practice in how to calculate the formation energy of point defects in crystalline materials and analysis techniques appropriate to probe changes in structure and properties relevant across energy technologies.

The Shockley-Queisser (SQ) limit provides a convenient metric for predicting light-to-electricity conversion efficiency of a solar cell based on the band gap of the light-absorbing layer. In reality, few materials approach this radiative limit. We develop a formalism and computational method to predict the maximum photovoltaic efficiency of imperfect crystals from first principles. The trap-limited conversion efficiency includes equilibrium populations of native defects, their carrier-capture coefficients, and the associated recombination rates. When applied to kesterite solar cells, we reveal an intrinsic limit of 20% for Cu₂ZnSnSe₄, which falls far below the SQ limit of 32%. The effects of atomic substitution and extrinsic doping are studied, leading to pathways for an enhanced efficiency of 31%. This approach can be applied to support targeted-materials selection for future solar-energy technologies.

Hematite (α-Fe₂O₃) is the most studied artificial oxygen-evolving photo-anode and yet its efficiency limitations and their origin remain unknown. A sub-picosecond reorganisation of the hematite structure has been proposed as the mechanism which dictates carrier lifetimes, energetics and the ultimate conversion yields. However, the importance of this reorganisation for actual device performance is unclear. Here we report an in situ observation of charge carrier self-localisation in a hematite device, and demonstrate that this process affects recombination losses in photoelectrochemical cells. We apply an ultrafast, device-based optical-control method to resolve the subpicosecond formation of small polarons and estimate their reorganisation energy to be ~0.5 eV. Coherent oscillations in the photocurrent signals indicate that polaron formation may be coupled to specific phonon modes (−1). Our results bring together spectroscopic and device characterisation approaches to reveal new photophysics of broadly-studied hematite devices.

Cs₄PbBr₆ is a member of the extended halide perovskite family that is built from isolated (zero-dimensional) PbBr₆ ⁴⁻ octahedra with Cs⁺ counter ions. The material exhibits anomalous optoelectronic properties: optical absorption and weak emission in the deep ultraviolet (310–375 nm) with efficient luminescence in the green region (∼540 nm). Several hypotheses have been proposed to explain the giant Stokes shift including: (i) phase impurities; (ii) self-trapped exciton; (iii) defect emission. We explore, using first-principles theory and self-consistent Fermi level analysis, the unusual defect chemistry and physics of Cs₄PbBr₆. We find a heavily compensated system where the room-temperature carrier concentrations (9 cm⁻³) are more than one million times lower than the defect concentrations. We show that the low-energy Br-on-Cs antisite results in the formation of a polybromide (Br₃) species that can exist in a range of charge states. We further demonstrate from excited-state calculations that tribromide moieties are photoresponsive and can contribute to the observed green luminescence. Photoactivity of polyhalide molecules is expected to be present in other halide perovskite-related compounds where they can influence light absorption and emission.