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Achieving higher brightness of a single-photon emitter (SPE) is central for advanced applications from quantum information processing to quantum sensing. However, most approaches of integrating quantum emitters and photonic resonators require accurately localizing the emitter into a photonic structure, which is a challenge. Here, we report using dielectric microspheres for laser focusing to create SPEs in hexagonal boron nitride and in situ enhance the emission via photonic coupling between the SPE and the dielectric microspheres. The photoluminescence intensity is increased by 2.4-fold, achieving a high brightness SPE with a saturation intensity of up to 19.6 Mcounts s−1. This approach provides a feasible way to generate a high-performance SPE while simultaneously enabling precise coupling of the quantum light source and optical resonators. [ABSTRACT FROM AUTHOR]

Metal halide perovskites exhibit intriguing properties for optoelectronic and electronic applications. Understanding their intrinsic photoexcitation and band‐tail dynamics is essential for exploring novel functionality and improving device performance. This report addresses the band‐tail luminescence of lead bromide perovskite (APbBr3) single crystals, where A is methylammonium, formamidinium, and cesium. Using time‐resolved photoluminescence (PL) spectroscopy, the dynamics of well‐defined dual emission peaks are unambiguously resolved from the picosecond to the microsecond scale. Whereas the long‐lived states exhibit similar recombination dynamics at high temperatures, a clear, fast transition occurs between the high and low energy states at low temperatures, indicating direct communication between these two states. These time‐resolved PL spectra shed light on the intrinsic optical properties of metal halide perovskites. [ABSTRACT FROM AUTHOR]

Low‐dimensional halide perovskites with broad emission are a hot topic for their promising application as white light sources. However, the physical origin of this broadband emission in the sub‐bandgap region is still controversial. This work investigates the broad Stokes‐shifted emission bands in mixed lead‐tin 2D perovskite films prepared by mixing precursor solutions of phenethylammonium lead iodide (PEA2PbI4, PEA = phenethylammonium) and phenethylammonium tin iodide (PEA2SnI4). The bandgap can be tuned by the lead‐tin ratio, whereas the photoluminescence is broad and significantly Stokes‐shifted and appears to be fairly insensitive to the relative amount of Pb and Sn. It is experimentally observed that these low‐dimensional systems show substantially less bandgap bowing than their 3D counterpart. Theoretically, this can be attributed to the smaller spin–orbit coupling effect on the 2D perovskites compared to that of 3D ones. The time‐resolved photoluminescence shows an ultrafast decay in the high‐energy range of the spectra that coincides with the emission range of PEA2SnI4, while the broadband emission decay is slower, up to the microsecond range. Sub‐gap photoexcitation experiments exclude exciton self‐trapping as the origin of the broadband emission, pointing to defects as the origin of the broadband emission in 2D Sn/Pb perovskite alloys. [ABSTRACT FROM AUTHOR]

Low-dimensional hybrid organic–inorganic perovskites (HOIPs) have attracted significant attention for applications such as solar cells, light-emitting diodes, and photodetectors. Compared with their three-dimensional counterparts, low-dimensional perovskites (LDPs) exhibit prominent characteristics such as stronger quantum confinement, remarkable exciton effects, and structural diversity, which are crucial for their nonlinear optical (NLO) properties. However, it remains hard to obtain ideal perovskite crystals with high optical transparency, large NLO responses, and excellent stability simultaneously, which are highly desirable for next-generation integrated photonics. In this work, a wide bandgap (4.2 eV) quasi-1D perovskite single crystal, PEA3PbBr5·H2O, with a considerable second-order NLO coefficient (0.1 pm V−1) that is 1/4 times that of the commercial crystal KH2PO4 (KDP), is reported. Power-, wavelength-, and polarization-dependent experiments are implemented, unfolding a high polarization ratio of up to 94%. Moreover, an unexpectedly anomalous green PL phenomenon from the single crystal is discovered. The upconversion emission in the crystal, as well as the time-resolved photoluminescence dynamics, is also investigated. Such a quasi-1D perovskite may provide a new platform for advancing the application of nonlinear optics. [ABSTRACT FROM AUTHOR]

In photoswitches that undergo fluorescence switching upon ultraviolet irradiation, photoluminescence and photoisomerization often occur simultaneously, leading to unstable fluorescence properties. Here, we successfully demonstrated reversible solid‐state triple fluorescence switching through "Pump–Trigger" multiphoton manipulation. A novel fluorescence photoswitch, BOSA‐SP, achieved green, yellow, and red fluorescence under excitation by pump light and isomerization induced by trigger light. The energy ranges of photoexcitation and photoisomerization did not overlap, enabling appropriate selection of the multiphoton light for "pump" and "trigger" photoswitching, respectively. Additionally, the large free volume of the spiropyran (SP) moiety in the solid state promoted reversible photoisomerization. Switching between "pump" and "trigger" light is useful for three‐color tunable switching cell imaging, which can be exploited in programmable fluorescence switching. Furthermore, we exploited reversible dual‐fluorescence switching in a single molecular system to successfully achieve two‐color super‐resolution imaging. [ABSTRACT FROM AUTHOR]

Color centers, optically active defects within solids, are vital for leading quantum information technologies such as quantum computing and quantum sensing. An essential prerequisite for realizing scalable quantum architectures is the ability to create quantum emitters deterministically. In the last decades, significant efforts have been devoted to selectively generating color centers. One of the most used methods is high‐energy ion implantation. However, this method usually causes extended lattice damage along the entire trajectory because of ions bombardment. Moreover, the position depth of color centers is also limited by the ions penetrate length in crystals. The direct laser‐writing (DLW) technique has recently emerged as a powerful tool to create color centers in solid‐state materials. It can define color centers at arbitrary depths inside the substrate and operate at the ambient environment, therefore, providing a feasible 3D fabrication method for integrated quantum photonics. Here, recent advancements of laser writing of color centers in solid‐state materials are reviewed, from bulk crystals, such as diamond and silicon carbide, to nanostructures, involving single‐walled carbon nanotubes, 2D layered materials, and quantum dots. [ABSTRACT FROM AUTHOR]

2D perovskites offers a rich playing field to explore exciton physics and they possess a great potential for a variety of opto‐electronic applications. Whilst their photophysics shows intricate interactions of excitons with the lattice, most reports have so far relied on single compound studies. With the exception of variations of the organic spacer cations, the effect of constituent substitution on the photophysics and the nature of emitting species, in particular, have remained largely under‐explored. Here PEA2PbBr4, PEA2PbI4, and PEA2SnI4 (where PEA stands for phenylethylammonoium) are studied through a variety of optical spectroscopy techniques to reveal a complex set of excitonic transitions at low temperature. Weak high‐energy features are attributed to vibronic transitions breaking Kasha's, for which the responsible phonons cannot be accessed through simple Raman spectroscopy. Bright peaks at lower energy are due to two distinct electronic states, of which the upper is a convolution of the free exciton and a localized dark state and the lower is attributed to recombination involving shallow defects. This study offers deeper insights into the photophysics of 2D perovskites through compositional substitution and highlights critical limits to the communities' current understanding of processes in these compounds. [ABSTRACT FROM AUTHOR]

The stability of the active layer is an underinvestigated aspect of metal halide perovskite solar cells. Furthermore, the few articles on the subject are typically focused on thin films, which are complicated by the presence of defects and grain boundaries. Herein, a different approach is taken: a perovskite composition that is known to be stable in single crystal form is used, and its (photo‐)physical properties are studied in the form of spin‐coated thin films. The perovskites are lead‐based with cesium and formamidinium as the A‐site cations and iodide and bromide as the halide anions, with the formula Cs0.1FA0.9PbI3−xBrx. These compounds show high potential in terms of stability in single crystal form and closely resemble the compounds that have successfully been used in highly efficient perovskite–silicon tandem solar cells. It is found that a small difference in bromine content (x = 0.45 vs 0.6) has a significant impact in terms of the phase purity and charge carrier lifetimes, and conclude that the thin films of Cs0.1FA0.9PbI2.55Br0.45 have good potential for the use in optoelectronic devices. [ABSTRACT FROM AUTHOR]

2D perovskite materials have recently reattracted intense research interest for applications in photovoltaics and optoelectronics. As a consequence of the dielectric and quantum confinement effect, they show strongly bound and stable excitons at room temperature. Here, the band‐edge exciton fine structure and in particular its exciton and biexciton dynamics in high quality crystals of (PEA)2PbI4 are investigated. A comparison of bulk and surface exciton lifetimes yields a room temperature surface recombination velocity of 2 × 103 cm s−1 and an intrinsic lifetime of 185 ns. Biexciton emission is evidenced at room temperature, with a binding energy of ≈45 meV and a lifetime of 80 ps. At low temperature, exciton state splitting is observed, which is caused by the electron–hole exchange interaction. Transient photoluminescence resolves the low‐lying dark exciton state, with a bright/dark splitting energy estimated to be 10 meV. This work contributes to the understanding of the complex scenario of the elementary photoexcitations in 2D perovskites. [ABSTRACT FROM AUTHOR]

Hybrid organic-inorganic perovskites are semiconductors that have great potential for optoelectronic applications such as light-emitting diodes, photodetectors, and solar cells. In such devices, the surface plays a crucial role in the performance and stability, as it strongly influences the recombination rate of excited charge carriers. It is reported that molecular ligands such as benzylamine are capable of reducing the surface trap state density in thin films. In this work, we aim to clarify the mechanisms that govern the surface passivation of hybrid perovskites by benzylamine. We developed a versatile approach to investigate the influence of benzylamine passivation on the well-defined surface of freshly cleaved hybrid perovskite crystals. We show that benzylamine permanently passivates surface trap states in these single crystals, resulting in enhanced photoluminescence and charge carrier lifetimes. Additionally, we show that exposure to benzylamine leads to the replacement of the methylammonium cations by benzylammonium, thereby creating a thermodynamically more stable two-dimensional (2D) perovskite (BA)2PbBr4 on the surface of the three-dimensional crystal. This conversion to a 2D perovskite drives an anisotropic etching of the crystal surface, with the {100} planes being most prone to etching. Initially, square etching pits appear spread over the surface. As time elapses, these etching pits broaden and merge to yield large flat terraces that are oriented normally to the cleaving plane when they form. A thorough understanding of the mechanisms governing the surface passivation is crucially important to optimize and design novel passivation schemes, with the ultimate goal of further advancing the efficiency of optoelectronic devices based on hybrid perovskites. [ABSTRACT FROM AUTHOR]

Abstract: Wide bandgap (WB) organic–inorganic hybrid perovskites (OIHPs) with a bandgap ranging between 1.7 and 2.0 eV have shown great potential to improve the efficiency of single‐junction silicon or thin‐film solar cells by forming a tandem structure with one of these cells or with a narrow bandgap perovskite cell. However, WB‐OIHPs suffer from a large open‐circuit voltage (Voc) deficit in photovoltaic devices, which is associated with the phase segregation of the materials under light illumination. In this work the photoinstability is demonstrated and Voc loss can be addressed by combining grain crystallization and grain boundary passivation, achieved simultaneously through tuning of perovskite precursor composition. Using FA0.17Cs0.83PbI3–xBrx (x = 0.8, 1.2 1.5, and 1.8), with a varied bandgap from 1.72 to 1.93 eV, as the model system it is illustrated how precursor additive Pb(SCN)2 should be matched with a proper ratio of FAX (I and Br) to realize large grains with defect‐healed grain boundaries. The optimized WB‐OIHPs show good photostability at both room‐temperature and elevated temperature. Moreover, the corresponding solar cells exhibit excellent photovoltaic performances with the champion Voc/stabilized power output efficiency reaching 1.244 V/18.60%, 1.284 V/16.51%, 1.296 V/15.01%, and 1.312 V/14.35% for WB‐OIHPs with x = 0.8, 1.2, 1.5, and 1.8, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Layered halide perovskites have recently shown extraordinary potential for low‐cost solution‐processable optoelectronic applications because of their superior moisture stability over their 3D counterparts. However, few studies have investigated the effect of light on layered hybrid perovskites. Here, the mechanically exfoliated nanoflakes of the 2D perovskite (PEA)2PbI4 (PEA, 2‐phenylethylammonium) are used as a model to investigate their intrinsic photostability. The light‐induced degradation of the flakes is investigated by using in situ techniques including confocal laser scanning microscopy, wide‐field fluorescence microscopy, and atomic force microscopy. Under resonant photoexcitation, (PEA)2PbI4 degrades to PbI2. It is clearly shown that this process is initiated at the crystal edges and from the surface. As a consequence, the photoluminescence of (PEA)2PbI4 is progressively quenched by surface traps. Importantly, the light‐induced degradation can be suppressed by encapsulation using hexagonal boron nitride (hBN) flakes and/or polycarbonates. This report sheds light on a specific mechanism of light‐induced degradation in layered perovskites and proposes a new encapsulation method to improve their photostability. [ABSTRACT FROM AUTHOR]

Abstract: The low power conversion efficiency (PCE) of tin‐based hybrid perovskite solar cells (HPSCs) is mainly attributed to the high background carrier density due to a high density of intrinsic defects such as Sn vacancies and oxidized species (Sn4+) that characterize Sn‐based HPSCs. Herein, this study reports on the successful reduction of the background carrier density by more than one order of magnitude by depositing near‐single‐crystalline formamidinium tin iodide (FASnI3) films with the orthorhombic a‐axis in the out‐of‐plane direction. Using these highly crystalline films, obtained by mixing a very small amount (0.08 m) of layered (2D) Sn perovskite with 0.92 m (3D) FASnI3, for the first time a PCE as high as 9.0% in a planar p–i–n device structure is achieved. These devices display negligible hysteresis and light soaking, as they benefit from very low trap‐assisted recombination, low shunt losses, and more efficient charge collection. This represents a 50% improvement in PCE compared to the best reference cell based on a pure FASnI3 film using SnF2 as a reducing agent. Moreover, the 2D/3D‐based HPSCs show considerable improved stability due to the enhanced robustness of the perovskite film compared to the reference cell. [ABSTRACT FROM AUTHOR]

Mixed iodide-bromide organolead perovskites with a bandgap of 1.70-1.80 eV have great potential to boost the efficiency of current silicon solar cells by forming a perovskite-silicon tandem structure. Yet, the stability of the perovskites under various application conditions, and in particular combined light and heat stress, is not well studied. Here, FA0.15Cs0.85Pb(I0.73Br0.27)3, with an optical bandgap of ≈1.72 eV, is used as a model system to investigate the thermal-photostability of wide-bandgap mixed halide perovskites. It is found that the concerted effect of heat and light can induce both phase segregation and decomposition in a pristine perovskite film. On the other hand, through a postdeposition film treatment with benzylamine (BA) molecules, the highly defective regions (e.g., film surface and grain boundaries) of the film can be well passivated, thus preventing the progression of decomposition or phase segregation in the film. Besides the stability improvement, the BA-modified perovskite solar cells also exhibit excellent photovoltaic performance, with the champion device reaching a power conversion efficiency of 18.1%, a stabilized power output efficiency of 17.1% and an open-circuit voltage ( Voc) of 1.24 V. [ABSTRACT FROM AUTHOR]

The mechanism behind the temperature dependence of the device performance in hybrid perovskite solar cells (HPSCs) is investigated systematically. The power conversion efficiency (PCE) of the reference cell using [60]PCBM as electron extraction layer (EEL) drops significantly from 11.9% at 295 K to 7% at 180 K. The deteriorated charge carrier extraction is found as the dominant factor causing this degradation. Temperature dependent spectroscopy and charge transport studies demonstrate that the poor electron transport in the [60]PCBM EEL at low temperature leads to inefficient charge carrier extraction. It is further demonstrated that the n-type doping of [60]PCBM EEL or the use of an EEL (fulleropyrrolidine with a triethylene glycol monoethyl ether side chain) with higher electron transport capability is an effective strategy to achieve HPSCs working efficiently over a broad temperature range. The devices fabricated with these highly performing EELs have PCEs at 180 K of 16.7% and 18.2%, respectively. These results support the idea that the temperature dependence of the electron transport in the EELs limits the device performance in HPSCs, especially at lower temperatures and they also give directions toward further improvement of the PCE of HPSCs at realistic operating temperatures. [ABSTRACT FROM AUTHOR]

Organic single crystals have a great potential in the field of organic optoelectronics because of their advantages of high carrier mobility and high thermal stability. However, the application of the organic single crystals in light-emitting devices (OLEDs) has been limited by single-layered structure with unbalanced carrier injection and transport. Here, fabrication of a multilayered-structure crystal-based OLED constitutes a major step toward balanced carrier injection and transport by introducing an anodic buffer layer and electron transport layer into the device structure. Three primary color single-crystal-based OLEDs based on the multilayered structure and molecular doping exhibit a maximum luminance and current efficiency of 820 cd cm−2 and 0.9 cd A−1, respectively, which are the highest performance to date for organic single-crystal-based OLEDs. This work paves the way toward high-performance organic optoelectronic devices based on the organic single crystals. [ABSTRACT FROM AUTHOR]

Despite the rich experience gained in controlling the microstructure of perovskite films over the past several years, little is known about how the microstructure affects the device properties of perovskite solar cells (HPSCs). In this work, the effects of the perovskite film microstructure on the charge recombination and light-soaking phenomenon in mixed halide HPSCs are investigated. Devices with noncompact perovskite morphology show a severe light soaking effect, with the power conversion efficiency (PCE) improved from 3.7% to 11.6% after light soaking. Devices with compact perovskite morphology show a negligible light soaking effect, with PCE slightly increased from 11.4% to 11.9% after light soaking. From device investigations, photoluminescence, and impedance spectroscopy measurements, it is demonstrated that interface electron traps at the grain boundaries as well as at the crystal surface dominate the light soaking effect. Severe trap-assisted recombination takes place in HPSCs using noncompact films, while it is effectively eliminated in devices with compact films. Moreover, how the grain size of the perovskite film affects the light soaking phenomenon is investigated. In the case of compact perovskite films, the size of the grains has a limited effect on the light soaking. In these compact films, grains are fused and trap states are effectively reduced. [ABSTRACT FROM AUTHOR]

Mixed iodide–bromide (I–Br) organolead perovskites are of great interest for both single junction and tandem solar cells since the optical bandgap of the materials can be tuned by varying the bromine to iodine ratio. Yet, it remains unclear how bromine incorporation modifies the properties of the perovskite solar cells. Here we use methylammonium lead iodide–bromide [MAPb(I1−xBrx)3 (x = 0–0.11)] as a model system to study the question. Through elemental analysis we found that bromine exhibits an increased concentration towards the interface between perovskite and TiO2, and that such interface bromine accumulation is more pronounced when a HI acid additive is used to produce the perovskite films. Opto-electrical characterization results further suggest that the carrier lifetime in the perovskite and the charge extraction at the perovskite/TiO2 interface are improved in the mixed halide perovskite samples. Accordingly, the optimal thickness of the perovskite active layer is increased from 330 nm for MAPbI3 to 510 nm for MAPb(I0.89Br0.11)3, resulting in an average efficiency increase of 1.6%. The study highlights the role of bromine in passivating defect states at grain boundaries and interfaces in mixed halide perovskite solar cells and demonstrates the use of solvent additives in tuning the electronic properties of the mixed halide perovskite materials. [ABSTRACT FROM AUTHOR]

Formamidinium lead iodide (FAPbI3) has a broader absorption spectrum and better thermal stability than the most famous methylammonium lead iodide, thus exhibiting great potential for photovoltaic applications. In this report, the light-induced photoluminescence (PL) evolution in FAPbI3 thin films is investigated. The PL intensity evolution is found to be strongly dependent on the atmosphere surrounding the samples. When the film is exposed to air, its photoluminescence intensity is enhanced more than 140 times after continuous ultraviolet laser illumination for 2 h, and the average lifetime is prolonged from 17 to 389 ns. The enhanced photoluminescence implies that the trap density is significantly reduced. The comparative study of the photoluminescence properties in air, nitrogen, and oxygen/helium environment suggests that moisture is important for the PL enhancement. This is explained in terms of moisture-assisted light-healing effect in FAPbI3 thin films. With this study, a new method is demonstrated to increase and control the quality of hybrid perovskite thin films. [ABSTRACT FROM AUTHOR]

A metal-coated hemisphere microcavity is realized by a simple self-assembled process. Plasmon-photon hybrid modes are produced by the metallic microcavity, and the lasing wavelength exhibits a blue shift when compared with no-metal cavities. Simulation shows that the blue shift originates from the redistribution of the electric-field intensity in the microcavity. The metal-coated microcavity exhibits excellent lasing performance at room temperature with a quality factor of $\sim 2500$ , owing to high-smooth surface of the hemisphere. Our findings may provide a promising candidate for fundamental investigation of plasmon and photon modes interaction and cavity electrodynamics. [ABSTRACT FROM PUBLISHER]

Hybrid organometal halide perovskites have been demonstrated to have outstanding performance as semiconductors for solar energy conversion. Further improvement of the efficiency and stability of these devices requires a deeper understanding of their intrinsic photophysical properties. Here, the structural and optical properties of high-quality single crystals of CH3NH3PbI3 from room temperature to 5 K are investigated. X-ray diffraction reveals an extremely sharp transition at 163 K from a twinned tetragonal I4/mcm phase to a low-temperature phase characterized by complex twinning and possible frozen disorder. Above the transition temperature, the photoluminescence is in agreement with a band-edge transition, explaining the outstanding performances of the solar cells. Whereas below the transition temperature, three different excitonic features arise, one of which is attributed to a free-exciton and the other two to bound excitons (BEs). The BEs are characterized by a decay dynamics of about 5 μs and by a saturation phenomenon at high power excitation. The long lifetime and the saturation effect make us attribute these low temperature features to bound triplet excitons. This results in a description of the room temperature recombination as being due to spontaneous band-to-band radiative transitions, whereas a diffusion-limited behavior is expected for the low-temperature range. [ABSTRACT FROM AUTHOR]

Organic single crystals have attracted great attention because of their advantages of high charge-carrier mobility, high chemical purity, and potential for flexible optoelectronic devices. However, their intrinsic properties of sensitive to organic solvent and fragile result in a difficulty in the fabrication of the organic crystal-based devices. In this work, a simple and non-destructive technique of template stripping is employed to fabricate single-crystal-based organic light-emitting devices (OLEDs). Efficient and uniform carrier injection induced by an improved contact between crystals and both top and bottom electrodes is realized, so that a homogeneous and bright electroluminescence (EL) are obtained. Highly polarized EL and even white emission is also observed. Moreover, the crystal-based OLEDs exhibit good flexibility, and keep stable EL under a small bending radius and after repeated bending. It is expectable that this technique would support broad applications of the organic single crystals in the crystal-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

Organic single-crystalline materials have attracted great attention for laser applications. However, the fabrication of laser resonators and pattern of crystals are still intractable problems. Organic single crystals have been limited to fundamental property studies despite their superior photonic characteristics. In this work, whispering-gallery mode (WGM) resonators of BP1T and BP2T crystalline materials have been fabricated through a combination method with improved lithography and dry etching. Crystalline microresonators with different geometries over a large area are top-down fabricated with submicrometer spatial resolution. WGM lasing oscillation from circular, hexagonal, pentagonal and square resonators is definitively observed. The BP1T and BP2T crystals are characterized with high refractive index, and stable lasing in aqueous solution is demonstrated besides in the air environment. It is expected that organic crystalline materials would be used for the practical applications in a variety of organic electronic and optical devices. [ABSTRACT FROM AUTHOR]

This paper reports the two-photon absorption (TPA) and the spectral-narrowed light emission (SNLE) from the crystals of 9, 10-distyrylanthracene (DSA) derivatives. The results obtained by the Z-scan method show that the tested molecules possess high TPA cross sections. Upon near-IR excitation, strong two-photon-excited fluorescence could be observed in the crystals of four types of materials. Furthermore, SNLE with low threshold occurs in three of the four crystals under pumping with the second harmonic generated in the amplifier. Single crystals of the DSA derivatives exhibit unique photonic properties, including strong solid-state fluorescence, large TPA cross section, and stimulated emission. The results demonstrate that these are potential candidates for compact and practical solid-state laser applications. [ABSTRACT FROM PUBLISHER]

Organic fluorophores with tunable π‐conjugated paths have attracted considerable attention owing to their diverse properties and promising applications. Herein, we present a tailored butterfly‐like molecule, 2,2’‐(2,5‐bis (2,2‐diphenylvinyl)‐1,4‐phenylene)dinaphthalene (BDVPN), which exhibits diverse photophysical features in its two polymorphs. The BP phase crystal, with its “aligned wings” conformation, possesses emissive characteristics that are nearly identical to those in dilute solutions. In contrast, the BN phase crystal, which adopts an “orthogonal wings” conformation, exhibits an unusual hypsochromic‐shifted emission compared to its dilute solution counterparts. This intriguing hypsochromic‐shifted emission originates from the reduction in the effective conjugated length of the molecular skeleton. Notably, BN phase crystals also exhibit exceptional optical performance, featuring high‐efficiency emission (76.6 %), low‐loss optical waveguides (0.571 dB mm−1), deep‐blue amplified spontaneous emission (ASE) with narrow full width at half maximum (FWHM: 6.4 nm), and a unique 200 nm bathochromic shift of piezochromic luminescence. [ABSTRACT FROM AUTHOR]