Metal-free perovskites are of interest for their chemical diversity and eco-friendly properties, and recently have been used for X-ray detection with superior carrier behavior. However, the size and shape complexity of the organic components results in difficulties in evaluating their stability in high-energy radiation. Herein, we introduce multiple hydrogen-bond metalfree PAZE-NH 4 X 3 •H 2 O perovskite, where H 2 O leads to more hydrogen bonds appearing between organic molecules and the perovskite host. As suggested by the theoretical calculations, multiple hydrogen bonds promote stiffness of the lattice, and increase the diffusion barrier to inhibit ionic migration. Then, low trap density, high μτ products and structural flexibility of PAZE-NH 4 Br 3 •H 2 O give a flexible X-ray detector with the highest sensitivity of 3708 μC Gy air À 1 cm À 2 , ultra-low detection limit of 0.19 μGy air À 1 s À 1 and superior spatial resolution of 5.0 lp mm À 1 .

Two-dimensional (2D) Ruddlesden-Popper (RP) CsPbI 3 perovskite possesses superior phase stability by introducing steric hindrance. However, due to the quantum and dielectric confinement effect, 2D structures usually exhibit large exciton binding energy, and the charge tunneling barrier across the organic interlayer is difficult to eliminate, resulting in poor charge transport and performance. Here, a multiple-ring aromatic ammonium, 1-naphthylamine (1-NA) spacer is developed for 2D RP CsPbI 3 perovskite solar cell (PSC). Theoretical simulations and experimental characterizations demonstrate that the 2D RP CsPbI 3 perovskite using 1-NA spacer with extended π-conjugation lengths reduces the exciton binding energy and facilitates the efficient separation of excitons. In addition, its cations have a significant contribution to the conduction band, which can reduce the bandgap, promote electronic coupling between organic and inorganic layers, and improve interlayer charge transport. Importantly, the strong π-π conjugation of 1-NA spacer can enhance intermolecular interactions and hydrogen bonding, and prepare high-quality films with preferred vertical orientation, resulting in lower defect density, and directional charge transport. As a result, the (1-NA) 2 (Cs) 3 Pb 4 I 13 PSC exhibits a record 16.62% performance with enhanced stability. This work provides an efficient approach to improve charge transport and device performance by developing multiple-ring aromatic spacers.

Metal-free perovskites (MFPs) possess excellent photophysical properties of perovskites while avoiding the introduction of toxic metal ions and organic solvents, and have been expanded to X-ray detection. However, iodine-based high-performance MFPs are tended to oxidation, corrosion, and uncontrolled ion migration, resulting in poor material stability and device performance. Herein, the strongly electronegative PF 6 − pseudohalide is used to fabricate the large-size MDABCO-NH 4 (PF 6 ) 3 (MDBACO = methyl-N′diazabicyclo[2.2.2]octonium) single crystals (SCs) for solving the problems of iodine ions. After the introduction of PF 6 − pseudohalides, the Coulomb interaction and hydrogen bonding strength are enhanced to alleviate the ion-migration and stability problems. Moreover, combined with theoretical calculations, PF 6 − pseudohalides increase the ion-migration barrier, and affect the contribution of its components to the energy band for a broadening bandgap. Meanwhile, the improved physical properties, such as large activation energy of ionic migration, high resistivity, and low current drift, further expand its application in low-dose and sensitive X-ray detection. Finally, the X-ray detector based on MDABCO-NH 4 (PF 6 ) 3 SCs achieves a sensitivity of 2078 μC Gy air −1 cm −2 (highest among metal-free SCs-based detectors) and the lowest detectable dose rate (16.3 nGy air s −1 ). This work has expanded the selection of MFPs for X-ray detectors and somewhat advanced the development of high-performance devices.

Since the first CsPbX 3 perovskite solar cells (PSCs) made a power conversion efficiency (PCE) of 2.9% in 2015, their performance skyrocketed to an unprecedented value beyond 20%. [1,2] However, the black-phase CsPbX 3 tends to transfer into the nonperovskite phase at room temperature resulting in deterioration of photovoltaic performance. [3][4][5] To overcome these defects, massive efforts have been made by researchers, including developing new counterparts such as Ruddlesden-Popper (RP) CsPbI 3 and Dion-Jacobson (DJ) CsPbI 3 . [6,7] These types of 2D CsPbI 3 show improved stability for its hydrophobicity of the bulk aliphatic or alkylammonium cations (e.g., phenylethylammonium)), which would induce steric hindrance effect against the phase-transition process. [6][7][8] However, every coin has two sides. The organic cations also induce poor migration of photogenerated carriers in the perovskite film, which results in the low PCE of corresponding devices. [9][10][11][12] Recently, bulky-cation engineering is usually used to improve the carrier transition process of 2D perovskites (especially 2D RP type) by introducing organic cations with different sizes, charges, and shapes. Their carrier behaviors are improved in terms of their different suitabilities of hydrogen-bonding capacity, stereochemical configuration, and space-filling capability. [13][14][15] Li et al. demonstrated a novel bulky organic cation of 3-aminopropionitrile (3-APN) to construct pure 2D (3-APN) 2 PbI 4 with small I⋯I distance and favorable growth direction. 3-APN cation also induces intramolecular hydrogen bonding to align the energy level of the device for effective interfacial charge transfer. [16] Recently, Ren et al. presented a new bulky alkylammonium of 2-(methylthio) ethylamine hydrochloride (MTEACl) to realize sulfur-sulfur interaction, which enhanced charge transport and stabilized its framework. [17] Also, Xu et al. developed a series of multiplering aromatic ammoniums of 1-naphthalenemethylammonium (NpMA) and 9-anthracenemethylammonium (AnMA) to explore their difference in carrier behavior. It benefited from the hydrogen bonding formation between organic cations and inorganic layers, which ensured ultrafast exciton migration process for exciton separation, charge transition, and collection. [18] However, the exploration of organic cations in CsPbX 3 is limited to the common cations of PEA þ and BA þ . Some of the novel functional groups designed in organic cations could induce fantastic photoelectronic properties and device improvement as well.The chemical tuning of the spacer organic cation with fluoro (F) substitution has been widely explored for improving the basic properties of A-site cations. [19] Pan et al. demonstrated that F-substitution PEA þ is beneficial to improving the visible

Metal-free perovskites are of interest for their chemical diversity and eco-friendly properties, and recently have been used for X-ray detection with superior carrier behavior. However, the size and shape complexity of the organic components results in difficulties in evaluating their stability in high-energy radiation. Herein, we introduce multiple hydrogen-bond metalfree PAZE-NH 4 X 3 •H 2 O perovskite, where H 2 O leads to more hydrogen bonds appearing between organic molecules and the perovskite host. As suggested by the theoretical calculations, multiple hydrogen bonds promote stiffness of the lattice, and increase the diffusion barrier to inhibit ionic migration. Then, low trap density, high μτ products and structural flexibility of PAZE-NH 4 Br 3 •H 2 O give a flexible X-ray detector with the highest sensitivity of 3708 μC Gy air À 1 cm À 2 , ultra-low detection limit of 0.19 μGy air À 1 s À 1 and superior spatial resolution of 5.0 lp mm À 1 .