Chwenhaw Liao

Heat and light induce distinct but interacting degradations in perovskite solar cells. We quantify heat–light interactions using a synergy metric. Nip cells show high Pb & I diffusions, Pb ⁰ defects, and outgassing. Findings guide stable cell designs.

Nanomaterials with a strong room-temperature optical response to magnetic fields are highly desirable for applications in sensing and photonics. Therefore, the ability to tune this response presents an opportunity to develop nanoscale magneto-optical devices. 2D metal halide perovskites are promising materials for optoelectronics, but typically exhibit very weak magneto-optical effects at room temperature. In this article, a 15× enhancement of the magnetic field effect on photoluminescence (magneto-photoluminescence) is demonstrated in colloidal (PEA)₂PbI₄ 2D perovskite nanosheets at room temperature. The results show that an external species can influence the exchange interaction energy and consequently the splitting between bright and dark exciton states in 2D perovskite nanosheets, which ultimately governs the magneto-photoluminescence. Additionally, the average photoluminescence quantum yield (PLQY) is increased from 15.2% in bulk single crystals to 23.1% in nanosheets (with a maximum recorded PLQY of 39.61%) produced using liquid-phase exfoliation, which also exhibited reduced trap emission compared with bulk or tape-exfoliated crystals in this study. This work demonstrates a simple method of engineering exciton states in 2D perovskites, assisting the development of optoelectronic technology and representing a crucial step toward producing nanoscale room-temperature magneto-optical devices.

Triple‐junction solar cells theoretically outperform their double-junction and single‐junction counterparts in power conversion efficiency, yet practical perovskite–perovskite–silicon devices have fallen short of both theoretical limits and commercial targets. To address surface defects in the top perovskite junction, we introduce a piperazine-1,4-diium chloride treatment, which replaces less stable lithium fluoride. For interfacing the top and middle perovskite junctions, we optimize the size of gold nanoparticles deposited on atomic layer-deposited tin oxide for best ohmic contacting with minimal optical losses. Applying these strategies, our champion 1-cm ² triple‐junction cell achieved a third party-verified reverse‐scan power conversion efficiency of 27.06% with an open circuit voltage of 3.16 V. Scaling up to 16 cm ² , the device produced a certified steady‐state power conversion efficiency of 23.3%. Device longevity also improved by eliminating methylammonium and incorporating rubidium into the perovskite bulk alongside the piperazine-1,4-diium chloride surface layer. An encapsulated 1-cm ² cell retained 95% of its initial efficiency after 407 h at maximum power point and passed the IEC 61215 thermal cycling test. These results represent advancements towards efficient and stable perovskite–perovskite–silicon triple-junction solar cells.

The successful development of optoelectronic devices is contingent on a detailed understanding of interactions between light and excited energy states in photoactive materials. In 2D perovskites, excitons are the dominant photogenerated species and their energetic structure plays a pivotal role, governing photon absorption and emission processes. In these materials, dark exciton states can undergo photoluminescence due to relaxation of selection rules and this process can be modulated by an external magnetic field, enabling unambiguous identification of the exciton fine structure. Previous reports of magneto‐optical spectroscopy on 2D perovskites are restricted to the macroscopic response, where key information is lost regarding the microscopic heterogeneity of the photoluminescence. Here, magneto‐optical microscopy is used for the first time on perovskite materials to elucidate the spatial variation of exciton emission processes. In 2D perovskite thin films, regions of localized bright and dark exciton populations are distinguished, correlated to the film morphology. In single crystals, dark excitons become localised at the edges, where excitons can be trapped in two distinct types of sub‐gap states. This work represents significant progress in understanding the properties of exciton emission in 2D perovskites, which is crucial for the development and optimization of optoelectronic technology.

Two-dimensional (2D) organic-inorganic metal halide perovskites have gained immense attention as alternatives to three-dimensional (3D) perovskites in recent years. The hydrophobic spacers in the layered structure of 2D perovskites make them more moisture-resistant than 3D perovskites. Moreover, they exhibit unique anisotropic electrical transport properties due to a structural confinement effect. In this study, four lead-free Dion-Jacobson (DJ) Sn-based phase perovskite single crystals, 3AMPSnI₄, 4AMPSnI₄, 3AMPYSnI₄, and 4AMPYSnI₄ [AMP = (aminomethyl)-piperidinium, AMPY = (aminomethyl)pyridinium] are reported. Results reveal structural differences between them impacting the resulting optical properties. Namely, higher octahedron distortion results in a higher absorption edge. Density functional theory (DFT) is also performed to determine the trends in energy band diagrams, exciton binding energies, and formation energies due to structural differences among the four single crystals. Finally, a field-effect transistor (FET) based on 4AMPSnI₄ is demonstrated with a respectable hole mobility of 0.57 cm² V^-1 s^-1 requiring a low threshold voltage of only -2.5 V at a drain voltage of -40 V. To the best of our knowledge, this is the third DJ-phase perovskite FET reported to date.

Most of the reported 2D Ruddlesden–Popper (RP) lead halide perovskites with the general formula of A_n+1B_nX_3n+1 (n = 1, 2, …) comprise layered perovskites separated by A‐site‐substituted organic spacers. To date, only a small number of X‐site‐substituted RP perovskites have been reported. Herein, the first inorganic‐cation pseudohalide 2D phase perovskite single crystal, Cs₂Pb(SCN)₂Br₂, is reported. It is synthesized by the antisolvent vapor‐assisted crystallization (AVC) method at room temperature. It exhibits a standard single‐layer (n = 1) Ruddlesden–Popper structure described in space group of Pmmn (#59) and has a small separation (d = 1.69 Å) between the perovskite layers. The SCN⁻ anions are found to bend the 2D Pb(SCN)₂Br₂ framework slightly into a kite‐shaped octahedron, limiting the formation of a quasi‐2D perovskite structure (n > 1). This 2D single crystal exhibits a reversible first‐order phase transformation to 3D CsPbBr₃ (Pm3m #221) at 450 K. It has a low exciton binding energy of 160 meV—one of the lowest for 2D perovskites (n = 1). A Cs₂Pb(SCN)₂Br₂‐single‐crystal photodetector is demonstrated with respectable responsivity of 8.46 mA W⁻¹ and detectivity of ≈1.2 × 10¹⁰ Jones at a low bias voltage of 0.5 V.

Lead halide perovskites (LHPs) have become a promising alternative for a wide range of optoelectronic devices, thanks to their solution‐processability and impressive optical and electrical properties. More recently, LHPs have been investigated in magneto‐optic studies and have exhibited spin‐polarized emission, photoinduced magnetization, and long spin lifetimes. Here, the viability of methylammonium lead bromide (MAPbBr₃) single crystals as solution‐processed Faraday rotators is demonstrated. Compared to terbium gallium garnet, the industry standard in the visible, it is found that MAPbBr₃ exhibits Verdet constants (i.e., strength of Faraday effect) of similar or greater magnitude (up to 2.5x higher), with lower temperature dependence. Due to its low trap absorption, it is calculated that an optical isolator made from MAPbBr₃, with appropriate antireflection coatings, should reach ≈95% transmission and achieve 40 dB isolation for incoming powers of over 2 W. It is also shown that the Verdet constant of MAPbBr₃ can be calculated accurately from its dispersion in refractive index, allowing the possibility to predict similar effects in other perovskite materials.