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Tin-based halide perovskite materials are of interest for light-emitting applications due to their broad yellow emission, but they are known to exhibit inferior stability compared to the lead- based perovskites due to tendency of Sn2+ to be readily oxidized. Here we investigated the ambient stability of a Dion-Jacobson tin bromide perovskite. We found that the optical properties of the samples were more significantly affected by the moisture rather than the ratio of Sn2+/Sn4+. The exposure to humidity resulted in changes in the ratio of 2D HDASnBr4 and hydrated 1D HDA3SnBr8 phases, with initial increase (up to ~40 h) in luminescence attributed to increasing fraction of hydrated 1D phase which contributes to self- trapped exciton emission. With further exposure to humidity, the material starts to degrade resulting in reduced luminescence after ~40 h. The use of additives for the suppression of oxidation of Sn2+, namely SnCl2 and tetraethyl orthosilicate (TEOS), results in prolonging the lifetime of the samples (>260 h for thin films with additives, compared to 180 h for thin films without additives). However, the persistence of luminescence with increased time of ambient exposure in samples prepared with additives can be attributed to the changes in the evolution of phase composition (rati-os of 2D and hydrated 1D phases) over time rather than suppressing the oxidation of Sn2+.

This work is partially supported by the Hong Kong Research Grants Council (project numbers AoE/P-02/12 and C6023-19G) and the William Mong Institute of Nano Science and Technology (project number WMINTST19/SC04). A.S. also acknowledges the support from King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No: OSR-2019-CARF/CCF-3079.

Summary There is an urgent demand for all-polymer organic solar cells (AP-OSCs) to gain higher efficiency. Here, we successfully improve the performance to 16.09% by introducing a small amount of BN-T, a B←N-type polymer acceptor, into the PM6:PY-IT blend. It has been found that BN-T makes the active layer, based on the PM6:PY-IT:BN-T ternary blend, more crystalline but meanwhile slightly reduces the phase separation, leading to enhancement of both exciton harvesting and charge transport. From a thermodynamic viewpoint, BN-T prefers to reside between PM6 and PY-IT, and the fraction of this fine-tunes the morphology. Besides, a significantly reduced nonradiative energy loss occurs in the ternary blend, along with the coexistence of energy and charge transfer between the two acceptors. The progressive performance facilitated by these improved properties demonstrates that AP-OSCs can possibly comparably efficient with those based on small molecule acceptors, further enhancing the competitiveness of this device type.

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 A

A novel refractive index (RI) sensor based on micro-fiber with micro-arched transition (MMAT) was proposed. The power extinction ratio of interference spectra was enhanced by the micro-arched transition. Although the waist diameter of micro-fiber was less than 6 μm, the power extinction ratio was larger than 10 dB, which was critical to the spectral dip measurement. The relationship between waist diameter of micro-fiber and RI sensitivity was systematically investigated. It turned out that higher RI sensitivity of MMAT could be obtained by decreasing the waist diameter of micro-fiber. While the waist diameter was 5.6 μm, the RI sensitivities of MMATs were up to 2739.8 nm/RIU, 4515.7 nm/RIU and 12,620.0 nm/RIU in the RI range of 1.333–1.3580, 1.3705–1.3905 and 1.3940–1.3980 respectively. The sensor was applied to detecting the concentrations of bovine serum albumin (BSA) solutions and showed the sensitivity of 0.3 nm/(mg/ml). Owing to its small volume and high sensitivity, this kind of senor can be expected to widely apply in fields of biology, chemistry and physics.

We demonstrate a high-sensitivity relative humidity (RH) sensor based on microfiber coated with three-dimensional graphene network (3-DGN). The 3-DGN was fabricated by self-assembly of polystyrene (PS) microspheres which were each covered by reduced graphene oxide (rGO) nanosheets. The transmission electron microscopy (TEM) image and the Raman spectra indicated that the rGO nanosheets effectively covered the surface of each PS microsphere, with the 3D rGO network serving as the active material. Absorption of water molecules by the 3-DGN layer increases the effective refractive index of the 3-DGN cladding, leading to the decreasing of optical transmission power. Due to the high surface-to-volume ratios of the 3-DGN, which enhances its interaction with the water molecules, the sensor achieved a high linear humidity sensitivity of -4.118 dB/%RH over the humidity range 79.5–85.0 %RH and -0.224 dB/%RH over the range 50.5–70.6 %RH. Such high performance shows that the microfiber coated with 3-DGN has great potential for application in widespread fields, such as chemical and food processing and biology.

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.

Transient optical spectroscopy is used to quantify the temperature-dependence of charge separation and recombination dynamics in P3TEA:SF-PDI2 and PM6:Y6, two non-fullerene organic photovoltaic (OPV) systems with a negligible driving force and high photocurrent quantum yields. By tracking the intensity of the transient electroabsorption response that arises upon interfacial charge separation in P3TEA:SF-PDI2, a free charge generation rate constant of ≈2.4 × 1010 s−1 is observed at room temperature, with an average energy of ≈230 meV stored between the interfacial charge pairs. Thermally activated charge separation is also observed in PM6:Y6, and a faster charge separation rate of ≈5.5 × 1010 s−1 is estimated at room temperature, which is consistent with the higher device efficiency. When both blends are cooled down to cryogenic temperature, the reduced charge separation rate leads to increasing charge recombination either directly at the donor-acceptor interface or via the emissive singlet exciton state. A kinetic model is used to rationalize the results, showing that although photogenerated charges have to overcome a significant Coulomb potential to generate free carriers, OPV blends can achieve high photocurrent generation yields given that the thermal dissociation rate of charges outcompetes the recombination rate.

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.

We demonstrated strain sensing of a microfiber with a microarched transition region, which was fabricated by flame heated tapering. Due to multimode interference of different propagation modes of microfiber, two main transmission dips were observed at 1215.0 and 1469.8 nm. Enhanced by the microarched transition region, the depth of the dip was up to 19 dB at 1215.0 nm. The position of the dip red-shifted while the axial strain changed from 0 to 1166.2 μϵ. The axial strain sensitivity was up to 56.6 pm/μϵ, which was one order of magnitude higher than that of the traditional optical strain sensor based on microfiber or fiber Bragg grating. The linear correlation coefficient was 98.21%. This kind of microfiber with a microarched transition region can be widely used in various physical, chemical, and biological sensing and detection fields.