Metastable Zone Engineering With Excess Halide for Low‐Defect MAPbBr 3 Single Crystals with Ultra‐Low X‐Ray Detection Limit

The lead halide perovskite single crystals (SCs) have already emerged as a series of star materials for solar cells, lasers and detectors due to the complete crystal structure, fewer defects and grain boundaries. Previous studies have demonstrated that MASCN gas can convert δ‐FAPbI 3 films into α‐FAPbI 3 below the phase transition temperature. However, applying this strategy to the exploration of FAPbI 3 SCs remains a challenge. Herein, we designed a constant‐pressure gas diffusion setup to treat δ‐FAPbI 3 SCs with MASCN vapor, successfully obtaining photoactive α‐FAPbI 3 SCs with enhanced optoelectronic properties. X‐ray detectors based on the processed SCs exhibited significantly improved performance, achieving a high sensitivity of 4.46 × 10 4 µC Gy air −1 cm −2 , a ratio of sensitivity to dark current density as high as 1.06 × 10 12 µs Gy air −1 , and a detection limit as low as 150 nGy air s −1 . Furthermore, due to the strong polarizing nature of sulfur, some SCN − ions formed stable bonds with Pb 2+ in the crystal lattice, thereby enhancing the environmental stability of FAPbI 3 . The processed SCs maintained their phase purity without degradation after one month of storage in ambient conditions. This work offers a new strategy for repurposing δ‐FAPbI 3 SCs and advances the development of perovskite SC X‐ray detectors.

High-quality perovskite single crystals with large size are highly desirable for the fundamental research and high energy detection application. Here, a simple and convenient solution method, featuring continuous-mass transport process (CMTP) by a steady self-supply way, is shown to keep the growth of semiconductor single crystals continuously stable at a constant growth rate until an expected crystal size is achieved. A significantly reduced full width at half-maximum (36 arcsec) of the (400) plane from the X-ray rocking curve indicates a low angular dislocation of 6.8 × 106 cm-2 and hence a higher crystalline quality for the CH3 NH3 PbI3 (MAPbI3 ) single crystals grown by CMTP as compared to the conventional inverse temperature crystallization (ITC) method. Furthermore, the CMTP-based single crystals have lower trap density, reduced by nearly 200% to 4.5 × 109 cm-3 , higher mobility increased by 187% to 150.2 cm2 V-1 s-1 , and higher mobility-lifetime product increased by around 450% to 1.6 × 10-3 cm2 V-1 , as compared with the ITC-grown reference sample. The high performance of the CMTP-based MAPbI3 X-ray detector is comparable to that of a traditional high-quality CdZnTe device, indicating the CMTP method as being a cost-efficient strategy for high-quality electronic-grade semiconductor single crystals.

Explaining the time-dependent evolution of photoluminescence spectra of halide perovskite single crystals after pulsed excitation requires the consideration of a range of physical mechanisms, including electronic transport, recombination and reabsorption. The latter process of reabsorption and re-generation of electron-hole pairs from a photon created by radiative recombination in the single crystal itself is termed photon recycling and has been a highly controversial topic. We use photoluminescence experiments performed under different illumination conditions combined with numerical simulations that consider photon recycling to show which parameters affect temporal decays, spectral shifts and differences in the illumination direction. In addition, we use numerical simulations with and without photon recycling to understand the relative importance of charge-carrier transport and photon recycling. We conclude that under most relevant illumination conditions and times after the pulse, electronic transport is more important than photon recycling for the spectral behavior of the transients. However, inclusion of photon recycling is imperative for the understanding of the absolute density of electrons and holes present in the crystal during a certain time after the pulse.

Hybrid bismuth halides perovskites have emerged as promising candidates for X-ray detection, due to the strong absorptivity of high-energy X-ray photons, high resistivity, large carrier diffusion length and low toxicity. However, the mostly investigated hybrid bismuth iodides single crystals are usually opaque and require a harsh synthesis process. Herein, novel one-dimensional (1D) pentamethylenediamine bismuth bromide (PDA)BiBr5 single crystals were synthesized via an antisolvent-assisted crystallization method at room temperature. Bulk (PDA)BiBr5 single crystals have sizes of 10×1.3×1.5 mm3 and high transparency. They are shown to have low density of defects of 2.0×1010 cm-3 and obvious photoconductivity. Moreover, they exhibit large bulk resistivity of 2.13×1011 Ω cm and good X-ray attenuation coefficient. Consequently, the vertical structured (PDA)BiBr5 single crystal X-ray photoconductor produces a sensitivity of 3.8 μC Gyair -1 cm-2 . This study provides a facile strategy for synthesizing bulk hybrid bismuth bromides single crystals with potential X-ray detection application.

The spontaneous polarization in ferroelectric perovskites offers a promising route toward self‐powered X‐ray detection, yet the microscopic link between ferroelectric phase transition and detector‐relevant carrier dynamics remains largely unexplored. Here, using 1D [3‐(aminomethyl)piperidinium]BiI 5 single crystals (3AMP SCs) as a model system, we uncover how a specific first‐order ferroelectric–paraelectric transition mechanism directly governs polarization‐driven carrier transport and photovoltaic behavior. Structural analyses reveal that room‐temperature ferroelectricity originates from the non‐centrosymmetric ordering of 3AMP 2 + cations and strong organic–inorganic interfacial coupling, while the transition at 364 K is driven by highly anharmonic hydrogen‐bond dynamics and stabilized by the rigidity of the Bi–I framework. This cooperative mechanism generates a robust polarization field along the c‐axis, which reduces exciton binding energy, suppresses trap‐assisted recombination, and enables efficient carrier separation through the bulk and anomalous photovoltaic effects. As a direct consequence of this phase‐transition‐controlled transport behavior, the 3AMP SC‐based detector achieves record‐performance self‐powered X‐ray detection, with a sensitivity of 566.79 µC Gy air −1 cm −2 , an ultra‐low detection limit of 5.24 nGy air s −1 , and excellent long‐term stability. This work establishes a clear structure–phase transition–polarization–transport–performance relationship in Bi‐based halide perovskites, providing a new framework for designing ferroelectric materials for self‐powered optoelectronic devices.

Self‐powered perovskite X‐ray detectors have drawn increasing attention due to the merits of low noise, low power consumption as well as high portability and adaptability. However, the active layer thickness is usually compromised by the small carrier diffusion length, which leads to inefficient X‐ray attenuation and hence low sensitivity of the detectors. Herein, self‐powered and highly sensitive single‐crystal perovskite X‐ray detectors are achieved by finely controlling the crystal thickness and optimizing their carrier transport properties. Perovskite single crystals with thickness of around 800 µm are grown by a two‐step crystal growth process to realize the full attenuation of hard X‐ray with the energy of 80 keV. And the incorporation of formamidinium (FA) (FA = CH(NH2)2+) cation into methylammonium lead triiodide (MAPbI3) (MA = CH3NH3+) increases the mobility‐lifetime (µτ) product of the single crystals by nearly one order of magnitude, leading to a record X‐ray detection sensitivity of 8.7 × 104 µC Gyair−1 cm−2 under zero bias. Moreover, the eliminated external bias and reduced trap density weaken the field‐driven ion migration effect, and therefore result in a low detection limit of 27.7 nGy s−1. This work represents an effective way to achieve self‐powered perovskite X‐ray detectors with both high sensitivity and low detection limit.

Room temperature synthesis of perovskite (MAPbI3) single crystal by anti-solvent assisted inverse temperature crystallization method

Large‐area perovskite (PVK) X‐ray detectors exhibit significant potential for commercial applications. However, the production of uniform and dense thick films as well as achieving efficient carrier transport over tens of microns in thick PVK films, remain major challenges for highly sensitive X‐ray detection. Herein, an innovative vacuum‐assisted blade coating strategy is proposed using PVK inks that strictly controls the nucleation and growth of PVKs to prepare large‐area, high‐quality MAPbI 3 homojunction thick films. Arising from the formation of a type II homojunction between the top and bottom PVK layers, and effective reduction of the density of defect states, the resultant homojunction film exhibits impressive performances, including an increase in carrier lifetime from 1272 to 9335 ps, an increase in surface photovoltage change from 93 to 386 mV, and nearly three times higher carrier mobility‐lifetime product compared with pristine NMP‐PVK film. Consequently, the X‐ray detector based on homojunction film demonstrates a high sensitivity of 1.3 × 10 5 µC Gy air −1 cm −2 , surpassing the most previously reported values for X‐ray detector using blade coating method. This research provides a convenient approach for preparing large‐area PVK thick films and establishes a solid foundation for the development of X‐ray detection.

We report the growth and characterization of different bulk single crystals of organo lead mixed halide perovskites CH3NH3PbI3−xBrx by two different crystal growth approaches: (i)anti-solvent diffusion, and (ii) inverse temperature crystallization. In order to control the size and the shape of crystals, we have investigated different experimental growth parameters such as temperature and precursor concentration. The morphology of obtained crystals was observed by optical microscope, whereas their intrinsic crystalline properties were characterized by single crystal as well as powder X-ray diffraction. The results illustrated that the growth and crystalline structure of mixed halide perovskites CH3NH3PbI3−xBrx could be easily tuned.

In this work, we report two kinds of detectors: one is a perovskite-like (CH3NH3)3Sb2Br9 (MA3Sb2Br9) MSM-type photodetectors; and, the other one is quasi 2-dimensional (PEA)2(MA)3Sb2Br9 transistor-type thin film alcohol detectors. The MA3Sb2Br9 bulk visible photodetector is prepared by inverse temperature crystallization method and the quasi 2-dimensional (PEA)2(MA)3Sb2Br9 transistor-type thin film alcohol detector is prepared by spin coating method. Firstly, we have fabricated a photodetector based on MA3Sb2Br9 perovskite-like single crystal due to the Sb-based perovskite is a material that are more stable in air and moisture than Pb-based perovskites. Here, MA3Sb2Br9 single crystals were synthesized by inverse temperature crystallization process with precursor solution at three different growth temperatures. MA3Sb2Br9 single crystal with an optimum growth temperature of 60 °C presents the best owing to excellent crystal structure and optical absorption properties. On the other hand, recently, we also have fabricated an alcohol detector based on the quasi 2-dimensional (Q2D) (PEA)2(MA)3Sb2Br9 transistor-type thin films. Here, MA3Sb2Br9 films were spin-coated on the glass substrates with ITO pattern. X-ray diffraction (XRD) patterns, absorbance, and current-voltage were employed to examine the characterizations of the Q2D (PEA)2(MA)3Sb2Br9 films and devices. One diffraction peak at 30.2o corresponding to the cubic crystal (022) phase was observed. The position of absorption edges of MA3Sb2Br9 and Q2D (PEA)2(MA)3Sb2Br9 film were around 518 and 500 nm, respectively. It is corresponding to the band gap of MA3Sb2Br9 and Q2D (PEA)2(MA)3Sb2Br9. The MA3Sb2Br9 and Q2D (PEA)2(MA)3Sb2Br9 perovskite-like alcohol detector exhibits high responsivity of 74 and 83 for 5 % of alcohol concentration, respectively. Besides, the rise time and fall time were 1.85 and 0.77 sec for alcohol detection, respectively.

Hybrid perovskite single crystal with extended absorption edge and environmental stability: Towards a simple and easy synthesis procedure

Due to their exceptional properties, the study of hybrid perovskite (HyP) structures and applications dominate current photovoltaic prospects. Methylammonium lead tri-iodide perovskite (MAPI) is the model compound of the HyP class of materials that, in a few years, achieved, in photovoltaics, a power conversion efficiency of 25%. The attention on HyP has recently moved to large single crystals as emerging candidates for photovoltaic application because of their improved stability and optoelectronic properties compared to polycrystalline films. To control the quality and symmetry of the large MAPI single crystals, we proposed an original method that consisted of adding short-chain carboxylic acids to the inverse temperature crystallization (ICT) of MAPI in γ-butyrolactone (GBL). The crystals were characterized by single-crystal X-ray diffraction (SC-XRD), X-ray powder diffraction (XRPD) and Raman spectroscopy. Based on SC-XRD analysis, MAPI crystals grown using acetic and trifluoroacetic acids adopt a tetragonal symmetry “I4cm”. MAPI grown in the presence of formic acid turned out to crystallize in the orthorhombic “Fmmm” space group demonstrating the acid’s effect on the crystallization of MAPI.

The emerging 2D layered perovskites have promising optoelectronic properties, good intrinsic stability and reduced ion migration, making them effective for detecting X‐ray radiation. However, their application is constrained by poor out‐of‐plane carrier transport. In this study, inch‐sized high‐quality CsPb2Br5 layered single crystals (SCs) are developed using an organic ligand‐assisted solution process. By modifying the surface energy, the anisotropy of crystal growth is conquered, resulting in CsPb2Br5 SCs with sufficient thickness for X‐ray detection. Importantly, this modification significantly enhanced the crystal quality as the grown CsPb2Br5 SCs exhibited longer photoluminescence lifetime and smaller trap density. Notably, the CsPb2Br5 SCs demonstrate unprecedented out‐of‐plane carrier transport, achieving a high carrier mobility‐lifetime product of 2.53 × 10−2 cm2V−1. This can be attributed to the small interlayer distance and the strong interlayer force of Cs─Br bonding. Furthermore, CsPb2Br5 SCs possess other intriguing attributes for X‐ray detection, including high bulk resistivity and outstanding thermal stability. These advantageous properties enable high‐performance vertical‐structure X‐ray detection with a superior sensitivity of up to 8865.6 µC Gyair−1cm−2 and a low detectable dose rate of 12.7 nGyairs−1. Additionally, CsPb2Br5 SCs exhibit high spatial resolution in X‐ray imaging and exceptional thermal stability, making them promising candidates for nondestructive determination.

Ultrasensitive and stable X-ray detection using zero-dimensional lead-free perovskites

Polycrystalline perovskites have attracted extensive interest in the field of X-ray detection/imaging due to their excellent compatibility with scalable processing. However, despite recent advances, polycrystalline perovskite X-ray direct detectors are plagued by the relatively high dark currents in imaging applications. Herein, we experimentally demonstrate that the dark current of polycrystalline perovskite X-ray detectors can be controlled by tuning the composition of perovskites. In particular, the incorporation of bromine (Br) into methylammonium lead iodide (MAPbI3) modifies the bandgap, increases electrical resistance, reduces the defects, improves the crystallinity, and suppresses the nonradiative recombination, thereby comprehensively causing the reduction of dark currents. At a doping concentration of 15%, the bulk resistivity of MAPb(I1-xBrx)3 layer increases to 1.3 × 108 Ω·cm, leading to a low dark current density down to 0.801 nA·cm-2. Consequently, the X-ray detector shows a low detection limit of 118.4 nGyair s-1 and a sensitivity to noise-current ratio of 7.27 × 1011 μC Gyair-1 A-1. By integrating the Br-doped MAPbI3 X-ray detector into a readout integrated circuit, the X-ray imager exhibits high-quality imaging capability. The results reveal a simple and effective strategy to minimize dark currents in perovskite X-ray detectors, and the detector has high potential in high-performance X-ray flat-panel imagers for industrial and medical applications.