Citation

Geng Yaru, Li Ruiqing, Zhu Tingting, Li Xinling, Guan Qianwen, Ye Huang, Wang Peng, Li Junlin, Luo Junhua. Formamidine intercalation to broaden photoresponsive range in alternating-cations-interlayered hybrid perovskite[J]. Chin. Chem. Lett., 2025, 36(8): 110216  

Yaru Geng^a,b, Ruiqing Li^a, Tingting Zhu^a, Xinling Li^a, Qianwen Guan^a, Huang Ye^a, Peng Wang^a, Junlin Li^a,b, Junhua Luo^a,b,*     

Two-dimensional (2D) organic-inorganic hybrid perovskites (OIHPs) have been regarded as rising-star materials in various optoelectronic applications [1–7] for their excellent semiconductor properties [8,9], especially in the field of photodetection. As one of the important parameters, the photoresponsive range of the materials is crucial for the operating wavelength region of the photodetector. Therefore, it is of great significance to take effective measures to achieve the ideal photoresponsive range in 2D OIHPs. On the one hand, incorporating different metals (Ge, Sn, Pb) [10,11] and halogens (Cl, Br, I) [12–14] elements into OIHPs can modulate the photoresponsive range for their different orbital coupling resulting in different bandgap. On the other hand, compared with metals and halogens, organic components exhibit abundant variety and versatile functions, which provide great potential in modulating the photoresponsive range. Particularly, aromatic amines have been shining in perovskites due to their excellent photo-absorption capacity and charge transport [15,16]. Therefore, it is of great significance to realize the modulation of photoresponsive range in aromatic amine-based perovskites. For example, Fu et al. achieved broadband photoelectric detection in (3AMPY)(FA)Pb₂I₇ by introducing formamidinium (FA) as a perovskitizer into aromatic amine-based perovskites [17]. This is because that FA with conjugated group participates in the bandgap formation thus being a good booster in the photoresponsive range modulation. However, due to the small size, FA normally acts as a perovskitizer locating in the cage determined by the inorganic framework, which impedes their further development in perovskite structure design and next-generation photoelectric materials exploration.

Alternating-cations-interlayered (ACI) hybrid perovskites, an emerging type of 2D OIHPs, possess outstanding structural flexibility which provide the possibility for constructing intercalation FA-based perovskites. In detail, with the feature of two different organic cations (A′ and A″) alternately arranged in the interlayer, ACI hybrid perovskites can accommodate various organic cations with different sizes, such as guanidine, methylamine, ethylamine, aromatic amine, and chiral cation [18–23]. Thereinto, aromatic amine-based ACI (A-ACI) hybrid perovskites can be considered as promising candidates for the next-generation photoelectronic materials because they integrate the advantages of aromatic amines’ excellent charge transport and ACI hybrid perovskites’ shortened interlayer distance. More recently, Dai et al. realized a wider photoresponsive range in A-ACI perovskite (4-AP)(MA)₂Pb₂I₈ reconstructed by introducing intercalation MA into aromatic amine-based OIHP (4-AP)PbI₄ [24]. Although this research provides new insight for broadening the photoresponsive range by constructing ACI hybrid perovskites, the association between intercalation cations and photoelectric performance is still intangible.

Herein, by intercalating FA reconstruction, the photoresponsive range is successfully broadened in A-ACI hybrid perovskites (NMA)₄(FA)₂Pb₃Br₁₂ (2) remolding from (NMA)₄(MA)₂Pb₃Br₁₂ (1, NMA = N-methylbenzylaminium, FA = formamidinium and MA = methylammonium) (Scheme 1). In particular, two compounds adopt a novel configuration that NMA and MA/FA are alternately arranged in the interlayer in a 4:2 manner. Importantly, compared with 1, 2 exhibits a narrower bandgap due to low-lying π^* orbitals of intercalation FA participating in the bandgap formation, accompanied by shortened interlayer distance and reduced skeleton distortion. Furthermore, 2-based photodetectors exhibit a broadening photoresponsive range by 70 nm compared with 1. Meanwhile, 2 displays outstanding photoelectric performance, including considerable switching ratio (7.91 × 10³), excellent detectivity (~10¹¹ Jones), and fast response time (210 µs). These findings not only enrich the structure type of the perovskite family but also provide new insights into structure-property relationships.

Scheme 1

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Scheme 1. The intercalation action reconstruction strategy. Alloying the strategy of substituting FA for MA in new-type ACI hybrid perovskites with organic cations alternately arranged in the interlayer in a 4:2 manner.

(NMA) ₄(MA)₂Pb₃Br₁₂ (1) can be obtained from the N-methylbenzylamine (NMA), Pb(AC)₂·3H₂O, and MA with the stoichiometric ratio in aqueous HBr solutions. For (NMA)₄(FA)₂Pb₃Br₁₂ (2), FA is substituted for MA with the same mole and the proportions of other components remain unchanged. As shown in Fig. S1 (Supporting information), the phase purity of 1 and 2 can be certified by the powder X-ray diffraction (PXRD) measurements which match well with the simulated results, respectively. Single crystal X-ray diffraction discloses that both 1 and 2 crystallize in the same orthorhombic system space group Pbcn with crystal and refinement data and more detailed crystallographic data as shown in Tables S1−S5 (Supporting information). As depicted in Fig. 1, the inorganic layer is composed of [PbBr₆]^4- basic structural units, which are connected via the corner-sharing and further extended along the ab-plane. The adjacent inorganic layers along the c-axis direction are separated by the organic layers, which are alternately arranged by NMA and MA/FA in a 4:2 manner. 1 and 2 adopt a novel ACI structure type, differing from previously reported ACI hybrid perovskites that intercalation cations alternately arranged in a 1:1 manner [18,19,25].

Figure 1

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Fig. 1. The crystal structures of (a) 1 and (b) 2. Equatorial Pb-Br-Pb bond angles in (c) 1 and (d) 2. For clarity, H atoms are omitted.

The two intercalation cations, MA and FA, possess different configurations and sizes, thereby generating different effects on crystal structure and semiconductor properties. On the one hand, the two cations both perform in +1 form and the difference is that FA has a coplanar structure and contains a delocalized bond of π₄³. The conjugated groups may lead to a reduced bandgap, which will be described later. On the other hand, FA (253 pm) possesses a larger size than MA (217 pm) [26]. In the unit cell, FA exhibits more sterically matched with the large hindrance cation NMA, therefore resulting in different interlayer distances and inorganic skeleton distortions. Analyses based on single structures (Figs. 1a and b) indicate that the interlayer distance of 2 (7.36 Å) is smaller than 1 (7.56 Å), which can be ascribed to the different penetration depths of NMA (Fig. S2 in Supporting information) [13]. Meanwhile, Pb-Br-Pb bond angles (β) are measured to evaluate the distortions of PbBr₆ octahedra by the deviation of the maximal. value (β") and minimal value (β’) of Pb-Br-Pb bond angles, ∆β = β"– β’ [27]. A larger deviation implies a greater distortion degree of the inorganic skeleton, which will hinder carrier transport. The calculated results indicate that 2 has a flatter skeleton with a value of 2.84° compared with 1 (8.59°) (Figs. 1c and d). In a word, the shortened interlayer distance and more regular inorganic skeleton in 2 jointly contribute to enhancing the interlayer orbital electron coupling, thereby reducing the bandgap and improving photoelectric performance [28,29].

To explore their semiconducting properties, the absorption spectra are performed in Figs. 2a and b. The absorption cut-off of 1 is located at about 455 nm, and the bandgap of 1 is estimated to be 2.82 eV according to the Tauc equation (Fig. S3 in Supporting information). Differently, 2 exhibits an absorption edge at 550 nm with a narrow bandgap of 2.22 eV. The bandgap of 2 is narrower than most 2D monolayered lead-bromine OIHPs (Table S6 in Supporting information) and even comparable to some 2D multilayered perovskites, for example, (IA)₂(MA)₂Pb₃Br₁₀ (2.3 eV) and (TRA)₂CsPb₂Br₇ (2.4 eV) [30,31]. To further study their electronic properties, the band structures and partial density of states (DOS) analyses of 1 and 2 are implemented using the first-principles density functional theory (DFT) calculations. As shown in Fig. 2c, the conduction band minimum (CBM) and the valence band maximum (VBM) of 1 are both located at the G point demonstrating a direct bandgap feature, and the calculated bandgap is 2.57 eV. By contrast, the calculated bandgap of 2 exhibits a narrow bandgap of 2.15 eV with the indirect bandgap feature (Fig. 2d). Ulteriorly, the analysis based on partial DOS is performed to explore the source of this narrower bandgap (Figs. 2e and f, Fig. S4 in Supporting information). The VBM and CBM of 1 mainly arise from Br-p orbitals and Pb-p orbitals, respectively, which coincide with most lead halide hybrid perovskites where the bandgap is mainly attributed to the contribution of inorganic components. In 2, the VBM derives from Br-p orbitals, and interestingly, the CBM mainly arises from C-2p and N-2p orbitals of FA cation. Therefore, it can be concluded that the low-lying π^* orbitals of FA participate in the conduction band formation and substantially decrease the compound’s bandgap. In other words, the FA-derived conduction band in 2 lies below the Pb-6p-orbitals band, causing the bandgap transition between Br-4p orbitals and C-2p and N-2p orbitals of FA cation and thereby reducing the bandgap.

Figure 2

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Fig. 2. The absorption spectra of (a) 1 and (b) 2. The calculated band structures of (c) 1 and (d) 2 (the value of 2 after correction of scissor operator (+0.6 eV)). The partial DOS of (e) 1 and (f) 2.

Photo-absorption capacity plays a conclusive role in operating the wavelength range of photoelectric devices. Theoretically, photoelectric devices can directly convert light signals to electric signals within the absorption range. Consequently, wavelength-dependent photoresponse is important evidence to verify the photo-absorption range of the two compounds (Fig. S5 in Supporting information). As shown in Fig. 3a, the photocurrent of 1 has shown a dramatic decrease at 450 nm, and there is no response at the wider band, which is closely associated with its photo-absorption properties. Differently, considerable photocurrent can be achieved under green light (520 nm) irradiation in 2 (Fig. 3b and Fig. S6 in Supporting information). In addition, the wavelength-dependent photoresponse of the 2-based photodetector was provided using the continuous spectrum from 300 nm to 700 nm wavelength. As shown in Fig. S7 (Supporting information), the spectral photoresponsivity reached the highest value at around 410 nm, then decreased until it disappeared near 550 nm. This result is in good agreement with the absorption spectrum of 2, which also validates the feasibility and effectiveness of the intercalation cation modification strategy.

Figure 3

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Fig. 3. The photocurrent of (a) 1 and (b) 2 under 404, 520, 637, 660, and 785 nm light illumination with the same light power density. (c) The I-V curves of 2 under dark and 404 nm laser irradiation with different intensities. (d) Responsivity (R) and detectivity (D*) of 2 under 404 nm light illumination and 10 V. (e) The linear dynamic range of 2 under 404 nm light illumination. (f) Photoresponse time of 2 under 404 nm light illumination.

We systematically investigate the photoelectric performance of the 2-based photodetectors under 404 nm. As demonstrated in Fig. 3c, the I-V characteristics of 2 show that the photocurrent (I_ph) increases with the increase of bias voltage and light density. Under the incident light power density of 56.66 mW/cm² (V_bias = 10 V), 2-based photodetectors exhibit an excellent photocurrent of 88.6 nA. With a pretty low dark current (0.0112 nA), a considerable switching ratio of 7.91 × 10³ can be obtained, which is comparable to most reported perovskite-based detectors, such as (s-BA)₂(MA)Pb₂I₇ (5.14 × 10²) [32], (BA)₂(DMA)Pb₂Br₇ (1.14 × 10³) [33] and so on. Next, responsivity (R) and detectivity (D^*) can be used to evaluate the performance of photodetectors. As illustrated in Fig. 3d, R and D^* increase sharply with reducing light power intensity, and the maximum values of R and D^* are 3.99 mA/W and 3.27 × 10¹¹ Jones. The linear dynamic range (LDR) of the 2-based photodetector has been performed (Fig. 3e), the photodetectors exhibit linear response with the light power density from 0.24 µW/cm² to 56.66 mW/cm² and the calculation for LDR is 107.46 dB. Moreover, the rise and fall time of 2 are measured to be 210/340 µs respectively (Fig. 3f). All these results indicate that it is feasible to achieve high-performance photoelectric detection in intercalation FA-based ACI perovskites.

In summary, by intercalating FA reconstruction, the photoresponsive range is successfully broadened in A-ACI hybrid perovskites (NMA)₄(FA)₂Pb₃Br₁₂ (2) reconstructing from (NMA)₄(MA)₂Pb₃Br₁₂ (1). Two compounds adopt an unprecedented configuration that NMA and MA/FA are alternately arranged in the interlayer in a 4:2 manner. In particular, compared with 1, 2 exhibits a narrower bandgap due to low-lying π^* orbitals of intercalation FA participating in the conduction band, accompanied by shortened interlayer distance and reduced skeleton distortion. Further, 2-based photodetectors exhibit a broadening photoresponsive range by 70 nm compared with 1. Additionally, 2 displays fascinating photoelectric performance including a considerable switching ratio of 7.91 × 10³, an excellent detectivity of 3.27 × 10¹¹ Jones, and a fast response time of 210 µs. This research not only opens up an effective way to broaden the photoresponsive range but also provides new insights into structure-property relationships.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Yaru Geng: Writing – review & editing, Writing – original draft, Validation, Formal analysis, Data curation. Ruiqing Li: Writing – review & editing, Validation, Formal analysis. Tingting Zhu: Writing – review & editing. Xinling Li: Writing – review & editing. Qianwen Guan: Writing – review & editing. Huang Ye: Writing – review & editing. Peng Wang: Writing – review & editing. Junlin Li: Writing – review & editing. Junhua Luo: Writing – review & editing, Supervision, Conceptualization.

This work was financially supported by the National Natural Science Foundation of China (Nos. 22435005, 22193042, 21921001, 52202194, 22305105, 22201284), and the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (No. ZDBSLY-SLH024).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110216.