Two-dimensional (2D) Dion–Jacobson (DJ) phase hybrid perovskites with diammonium spacers eliminate the van der Waals gap between adjacent inorganic layers, thus compensating for the disadvantage of Ruddlesden–Popper (RP) phase hybrid perovskites [1–18]. Compared with three-dimensional (3D) hybrid perovskites, the carrier transport performance of 2D hybrid perovskites in the out-of-plane direction is significantly limited due to the limitations of quantum wells and dielectric effects [19,20]. However, it is gratifying that DJ hybrid perovskites have superior out-of-plane carrier transport capabilities than RP hybrid perovskites. For example, Huang et al. directly observed fast carriers transport along out-of-plane direction in (4AMP)(MA)Pb₂I₇ (4AMP is 4-(aminomethyl)piperidinium, MA is methylammonium) [21]. This important feature is very beneficial for constructing high-performance optoelectronic devices. Due to the late start of research on DJ hybrid perovskite and the high difficulty in synthesis, the research on the structure–activity relationship of the optoelectronic devices constructed by them is far inferior to that of RP hybrid perovskites. According to the research reports, we know that the configuration of interlayer organic ammonium cations directly affects the structure and properties of hybrid perovskites [22–26]. Therefore, aromatic diammonium spacers have a higher dielectric constant than aliphatic diammonium spacers with the similar size, thereby increasing the degree of dielectric matching between the organic layer and inorganic layer, which is of particular concern in DJ hybrid perovskites [23,27–30]. In addition, due to the strong rigidity of aromatic diammonium, it is not easy to form 2D structure when constructing hybrid perovskites, resulting in a scarcity of kinds of DJ hybrid perovskites with aromatic spacers [31].

The absorption spectrum range and intensity reflect the responsiveness of hybrid perovskite to different wavelengths of light, and also affect the photodetection band range of such materials. Currently, most of the reported DJ hybrid perovskites photodetectors based on aromatic diammonium are lead-bromide systems, for example, (HIS)(DMA)Pb₂Br₇ (HIS²⁺ is histammonium and DMA⁺ is dimethylammonium) [12], (2AMPY)PbBr₄ (2AMPY is 2-(aminomethyl)pyridinium) [24] and (3AMPY)(EA)Pb₂Br₇ (3AMPY is 3-(aminomethyl)pyridinium, EA is ethylammonium) [32]. Due to their absorption cutoff edge being mostly below 500 nm, the photodetection band range is relatively narrow. In order to broaden the light absorption range of DJ hybrid perovskites, it is urgent to design and construct more lead-iodine hybrid perovskites based on aromatic diammonium cations.

Herein, we have successfully constructed a 2D bilayered DJ hybrid perovskite (3AMPY)(EA)Pb₂I₇ using aromatic diammonium as spacer and EA as perovskitizer. Under the synergistic effect of 3AMPY²⁺ and EA⁺ cations, (3AMPY)(EA)Pb₂I₇ shows a shorter interlayer spacing of 3.456 Å, which is superior to most lead-iodine DJ hybrid perovskites. Benefiting from the shorter interlayer spacing and wider light absorption range, single crystal devices based on (3AMPY)(EA)Pb₂I₇ exhibit excellent photo-response under 520 and 637 nm light irradiation, with a current on/off ratios of up to 10⁴.

The single crystal of (3AMPY)(EA)Pb₂I₇ was synthesized by regulating the stoichiometric ratio of Pb(CH₃COO)₂·3H₂O, EA, and 3AMPY in HI (55%–58%) solution. The single crystal structure of (3AMPY)(EA)Pb₂I₇ was shown in Fig. 1a, which contains double-layered [Pb₂I₇]_n inorganic framework with the EA⁺ cations enclosed in the perovskite cages. Unlike in RP hybrid perovskites, there is only monolayered organic 3AMPY²⁺ cations lying between the adjacent inorganic layer in (3AMPY)(EA)Pb₂I₇, which results in a shorter interlayer spacing of 3.456 Å. In addition, we also compared the interlayer spacing of other lead-iodine DJ hybrid perovskites constructed with aromatic diammonium (Fig. 1b and Table S1 in Supporting information), and the results showed that (3AMPY)(EA)Pb₂I₇ we constructed has the shortest interlayer spacing (Table S1). The shorter interlayer spacing reduces the difficulty of electron transfer in the out-of-plane direction, which is crucial for the performance of the devices. Moreover, the shorter interlayer spacing results in an I—I distance of only 4.112 Å, greatly increasing the interaction between adjacent inorganic sheets. The single-crystal X-ray diffraction (SCXRD) results indicate that (3AMPY)(EA)Pb₂I₇ belongs to the centrosymmetric space group of P2₁/c with unit cell parameters of a = 16.3167(7) Å, b = 18.2945(7) Å, c = 8.8253(4) Å and β = 98.490(2) at 200 K. And the selected Pb—Ⅰ bond lengths, Pb—Ⅰ—Pb bond angels, I—Pb—Ⅰ bond angles were provided in Tables S2–S4 (Supporting information). Moreover, the phase purity of (3AMPY)(EA)Pb₂I₇ has been verified by powder X-ray diffraction (PXRD) patterns in Fig. S1 (Supporting information). The thermal stability of (3AMPY)(EA)Pb₂I₇ was shown in Fig. S2 (Supporting information), there was no significant decomposition below 535 K, indicating its high thermal stability.

Next, we also analyzed the influence of perovskitizers on the structure in DJ hybrid perovskites. As shown in Fig. 2, compared with smaller size of MA (2.17 Å), the introduction of EA (2.74 Å) significantly leads to more distortion of the inorganic [PbI₆] octahedra. For quantitative analysis, we introduced the octahedra asymmetry parameter Δd, which is calculated through the following formula [33]:

where d is the mean Pb—Ⅰ bond length and d_n are the six individual Pb—Ⅰ bond length in the octahedra. Through calculation, we found that the degree of distortion of the octahedron in (3AMPY)(EA)Pb₂I₇ (5.09 × 10⁻⁴) is significantly greater than that in (3AMPY)(MA)Pb₂I₇ (1.73 × 10⁻⁴) [4], indicating that the influence of perovskitizers on the structure is very obvious. This result is similar to the reported hybrid perovskites (BA)₂(EA)₂Pb₃I₁₀ and (BA)₂(MA)₂Pb₃I₁₀ (BA is n-butylammonium), which belong to the RP type hybrid perovskites [34]. The large octahedral distortion has a highly impact on the luminescent performance of materials, thus laying the foundation for the future use of such perovskites in the field of luminescence [34]. In addition, the introduction of large size of EA perovskitizer result in an average Pb-I bond length in (3AMPY)(EA)Pb₂I₇ (3.20 Å) greater than that in (3AMPY)(MA)Pb₂I₇ (3.164 Å). The Pb—Ⅰ bond length also has a certain impact on the band gap of hybrid perovskites. Moreover, the distortion levels of [PbI₆] octahedra can also be quantified by defining the distortion index (D) (bond length), quadratic elongation (λ) and bond angle variance (σ²) based on Pb—Ⅰ bond lengths and Pb—Ⅰ—Pb bond angles through VESTA software [4]. The values of D, λ and σ² are listed in Table S5 (Supporting information). From these data, we can see that the distortion of [PbI₆] octahedra in (3AMPY)(EA)Pb₂I₇ is more seriously than (3AMPY)(MA)Pb₂I₇.

For growing bulk single crystals of (3AMPY)(EA)Pb₂I₇, it is obtained by slow-cooling method in a saturated solution containing raw materials (Fig. 3a). Subsequently, we studied the optical absorption ability of (3AMPY)(EA)Pb₂I₇ in Fig. 3b. The (3AMPY)(EA)Pb₂I₇ shows a sharp absorption edge at about 656 nm, affording the optical band gap (E_g) is ~1.89 eV according to the Tauc equation (Fig. 3b inset), which also can be seen it is a direct band gap characteristics. Due to the elongation of Pb—Ⅰ bond lengths, the absorption cut off edge (λ_cutoff) of (3AMPY)(EA)Pb₂I₇ is obvious smaller than that of (3AMPY)(MA)Pb₂I₇ (705 nm) and (3AMPY)(FA)Pb₂I₇ (850 nm) [35], which fully demonstrates that the optical absorption range of the materials can also be controlled by changing the perovskitizers. In order to further investigate the semiconducting properties of (3AMPY)(EA)Pb₂I₇, we calculated its energy band structure using density functional theory (DFT) in Fig. S3 (Supporting information). Both the valence band maximum and conduction band minimum of (3AMPY)(EA)Pb₂I₇ are located at the Z point, once again confirmed its direct band gap semiconductor characteristics. Different from most reported hybrid perovskites whose semiconducting properties are mainly contributed by inorganic frameworks, in (3AMPY)(EA)Pb₂I₇, the interlayer aromatic diammonium 3AMPY²⁺ also contribute to the conduction band, which can be clearly observed in Fig. 3c. The semiconducting performance of (3AMPY)(EA)Pb₂I₇ has been verified from the above aspects, thus providing assurance for its use in photodetection.

Then, we assembled a single crystal device (Fig. S4 in Supporting information) based on (3AMPY)(EA)Pb₂I₇ and studied its photodetection performance at different wavelengths. The current–voltage (I—V) curves of (3AMPY)(EA)Pb₂I₇ were measured under 405 nm (Fig. 4a), 520 nm (Fig. 4b) and 637 nm (Fig. 4c) illumination. The (3AMPY)(EA)Pb₂I₇ device exhibits an ultralow dark current (I_dark) of 5.32 × 10⁻¹¹ A at 10 V bias, which is comparable to other DJ hybrid perovskite, such as (HIS)(DMA)Pb₂Br₇ (1.97 × 10⁻¹¹ A) [12]. The maximum photocurrent (I_ph) under 405 nm, 520 nm and 637 nm can reach to 1.45 × 10⁻⁷ A, 8.35 × 10⁻⁷ A, 2.91 × 10⁻⁶ A, respectively, corresponding to on/off ratios ((I_ph-I_dark)/I_dark) of up to ~10³ (405 nm) and ~10⁴ (520 nm and 637 nm) for the devices. This value exceeds the vast majority of hybrid perovskites optoelectronic devices, such as (4-AMP)Cs₂Pb₃Br₁₀ (~10³) [36], (2meptH₂)CsPb₂Br₇ (~10³) [37]. Photoresponsivity (R) and photo-detectivity (D^*) are two important parameters for evaluating the performance of photodetectors. The value R is defined by equation R = I_ph/(PS) [12], where the P is the incident light power density and S is the valid area of devices. The value D^* is estimated by the equation D^* = I_ph/[PS(2qI_dark)^1/2] [12], where q is the elementary electronic charge. The high crystal quality and low dark current make the devices based on (3AMPY)(EA)Pb₂I₇ single crystal exhibit excellent R (887.8 mA/W) and D^* (7.4 × 10¹² Jones) under 637 nm laser irradiation. We can see that with the incident light power density increasing, the R and D^* value show a gradual decreasing variation in Fig. 4d. The D^* value of the (3AMPY)(EA)Pb₂I₇ single crystal device is comparable to commercial silicon photodetector with the D^* of 4 × 10¹² Jones. The response time of the device includes rise time (τ_r) and decay time (τ_f) in Fig. 4e. The definition of response time for optoelectronic devices is that the time interval required for the photocurrent to increase from 10% of the peak to 90% (τ_r) and the time interval required for the photocurrent to decrease from 90% of its peak to 10% (τ_f). The τ_r and τ_f of the (3AMPY)(EA)Pb₂I₇ device are 265 µs and 367 µs, respectively. Finally, we tested the stability of the device under multiple cycles in Fig. 4f, the result shows that there was no obvious attenuation in the photocurrent of the device during the long time running. From structure to detection performance, it is sufficient to demonstrate that (3AMPY)(EA)Pb₂I₇ single crystal device have full potential in photodetection and are also an optional materials for constructing high-performance optoelectronic devices.

In summary, we constructed a bilayered DJ hybrid perovskite (3AMPY)(EA)Pb₂I₇ using aromatic diammonium as spacer and studied the photodetection performance of the single crystal devices. The introduction of EA as a larger perovskitizer into the system increases the stretching of the Pb—Ⅰ bond length and the degree of distortion of the octahedron, confirming that different sizes of perovskitizer have a significant impact on the structure of hybrid perovskites. Moreover, the unique crystal structure of (3AMPY)(EA)Pb₂I₇ gives it a shorter interlayer spacing, and the single crystal device based on it show higher detectivity under 637 nm illumination. Our work not only enriches the kinds of muti-layer DJ hybrid perovskites, but also confirms that DJ hybrid perovskites constructed with aromatic diammonium have great potential as photodetection materials.

The authors declare no competing financial interest.

This work was financially supported by the National Natural Science Foundation of China (Nos. 22005183 and 22275117), the Program of State Key Laboratory of Quantum Optics and Quantum Optics Devices (No. KF202204).