Solvent-Induced Bright Emission in Lead-Free Organic-Inorganic Antimony Bromides with Reversible Transformation.

Lead-free low-dimensional organic-inorganic metal halides have gained increasing attention in a wide range of applications due to their low toxicity, outstanding optical performance, and structural tunability. In this work, a general method of incorporating organic molecule into sodium antimony bromides is introduced. The 1D Na3SbBr6(C2H6OS)6 and Na3SbBr6(C4H8OS)6 single crystals exhibit bright yellow and orange emission with PL peaks at 610 and 664 nm, and high photoluminescence quantum yields (PLQYs) of 85% and 60%, respectively. These two compounds can be reversibly converted into each other by the removal and addition of the organic components. Their exceptional luminescent performance enables them to be used as solid-state phosphors for the fabrication of yellow and orange down-conversion LEDs. A white LED with a high color rendering index (CRI) of 95 is also fabricated by using Na3SbBr6(C2H6OS)6 as the yellow phosphor. The universality of this method is demonstrated by synthesizing other members of this family with diverse A-groups, including methylammonium (MA) and formamidinium (FA). This work provides an effective strategy for the development of diverse lead-free and high-performance organic-inorganic hybrid materials and indicates these organic-inorganic hybrid compounds are promising luminescent materials for lighting or displays.

Isomerization of Benzothiadiazole Yields a Promising Polymer Donor and Organic Solar Cells with Efficiency of 19.0.

The exploration of high-performance and low-cost wide-bandgap polymer donors remains critical to achieve high-efficiency nonfullerene organic solar cells (OSCs) beyond current thresholds. Herein, the 1,2,3-benzothiadiazole (iBT), which is an isomer of 2,1,3-benzothiadiazole (BT), is used to design wide-bandgap polymer donor PiBT. The PiBT-based solar cells reach efficiency of 19.0%, which is one of the highest efficiencies in binary OSCs. Systemic studies show that isomerization of BT to iBT can finely regulate the polymers' photoelectric properties including i) increasing the extinction coefficient and photon harvest, ii) downshifting the highest occupied molecular orbital energy levels, iii) improving the coplanarity of polymer backbones, iv) offering good thermodynamic miscibility with acceptors. Consequently, the PiBT:Y6 bulk heterojunction (BHJ) device simultaneously reaches advantageous nanoscale morphology, efficient exciton generation and dissociation, fast charge transportation, and suppressed charge recombination, leading to larger VOC of 0.87 V, higher JSC of 28.2 mA cm-2, greater fill factor of 77.3%, and thus higher efficiency of 19.0%, while the analog-PBT-based OSCs reach efficiency of only 12.9%. Moreover, the key intermediate iBT can be easily afforded from industry chemicals via two-step procedure. Overall, this contribution highlights that iBT is a promising motif for designing high-performance polymer donors.

Improved Structural Order and Exciton Delocalization in High-Member Quasi-Two-Dimensional Tin Halide Perovskite Revealed by Electroabsorption Spectroscopy.

Engineering of quasi-two-dimensional (quasi-2D) tin halide perovskite structures is a promising pathway to achieve high-performance lead-free perovskite solar cells, with recently developed devices demonstrating over 14% efficiency. Despite the significant efficiency improvement over the bulk three-dimensional (3D) tin perovskite solar cells, the precise relationship between structural engineering and electron-hole (exciton) properties is not fully understood. Here, we study exciton properties in high-member quasi-2D tin perovskite (which is dominated by large n phases) and bulk 3D tin perovskite using electroabsorption (EA) spectroscopy. By numerically extracting the changes in polarizability and dipole moment between the excited and ground states, we show that more ordered and delocalized excitons are formed in the high-member quasi-2D film. This result indicates that the high-member quasi-2D tin perovskite film consists of more ordered crystal orientations and reduced defect density, which is in agreement with the over 5-fold increase in exciton lifetime and much improved solar cell efficiency in devices. Our results provide insights on the structure-property relationship of high-performance quasi-2D tin perovskite optoelectronic devices.

Highly Stable Tetrahydrothiophene 1-Oxide Caged Copper Bromide and Chloride Clusters with Deep-Red to Near-IR Emission.

All-inorganic copper(I)-based metal halides have emerged as promising candidates for the replacement of lead perovskites because of their outstanding optical properties. However, the limited structure tunability prohibits their further exploration of properties including red photoluminescence (PL). Here, we report a series of red-emissive lead-free hybrid organic-inorganic copper halides A6(C4H8OS)12[Cu8X13][Cu4X4(OH)(H2O)] (ACX-THTO, A = K, Rb, and Cs; X = Cl, Br; THTO = C4H8OS) with the highest photoluminescence quantum yield (PLQY) of 42%. These compounds possess similar crystal structures, and their emission can be tuned in the spectral range of 676-732 nm by controlling their compositions. Additionally, by removing and adding THTO, the reversible transformation between CsCu2Br3 featuring one-dimensional (1D) chains and Cs6(C4H8OS)12[Cu8Br13][Cu4Br4(OH)(H2O)] (CCB-THTO) with zero-dimensional (0D) clusters can be realized. We also demonstrate that the incorporation of THTO in the crystal structures instead of dimethyl sulfoxide (DMSO) can significantly enhance the stability and PL of compounds with the same inorganic components.

Ultrafast freestanding microfiber humidity sensor based on three-dimensional graphene network cladding.

An all-fiber humidity sensor is proposed and fabricated by depositing three-dimensional graphene network (3DGN) around the surface of a freestanding microfiber (MF). The high specific surface area and porosity of 3DGN enhances its interaction with water molecules, allowing high performance of the humidity sensor. The sensor can operate in a wide relative humidity (RH) range of 11.6%RH-90.9%RH with a high sensitivity of -2.841 dB/%RH in the RH range (80.3%RH - 90.9%RH). The response and recovery times of this type of microfiber sensor are measured respectively to be 57 ms and 55 ms, which are one order magnitude faster than those of other fiber RH sensors activated by two-dimensional materials coating. Such an all-fiber RH sensor with high sensitivity and fast response property possesses great potential of application in widespread fields, such as biology, chemical processing and food processing.

High-sensitivity humidity sensing of side-polished optical fiber with polymer nanostructure cladding.

This paper presented a high-sensitivity humidity sensor comprised of a side-polished fiber with polymer nanostructure cladding, which was constructed by dehydrating dichromate gelatin film on the polished surface. Due to the intermodal interference of the core mode and cladding mode, two main transmission dips were observed at 1184.4 nm and 1325.6 nm. These two transmission dips showed significant sensitivity to humidity. The position of transmission dip at 1325.6 nm shifted 22 nm while the relative humidity (RH) changed from 30% RH to 50% RH. The humidity sensitivity was up to 1.12 nm/% RH, and its linear correlation was 98.45%. The humidity sensing characteristic of nanostructure cladding fiber was successfully demonstrated. This novel sensor is compatible with optical fiber systems and has high potential in optical sensing applications.