Red luminescent water stable lead-free 2D tin halide perovskite nanocrystals for photodetectors

Abstract

In this report, we have synthesized an environmentally friendly hybrid organic–inorganic layered two-dimensional (2D) lead-free perovskite nanomaterial. The synthesized perovskites, namely (OleylAm)₂SnI₄ (MHP1), exhibit outstanding water stability and emit luminous red light. The photodetector constructed using our material showcases superior characteristics, including a faster response than comparable devices and improved rise and fall times compared to other 2D perovskite nanomaterials.

---

Wells was the first scientist to research metal halide perovskites.¹ However, in the early 1900s, scientist Weber conducted considerable research into their structural properties.^2a,b Perovskite materials perform exceptionally well in optoelectronic devices, with low exciton binding energies, a large optical absorption cross-section, long charge carrier diffusion lengths, and an easily tunable energy band gap.^2c–5a Furthermore, due to their outstanding optoelectronic properties, hybrid organic–inorganic lead halide perovskites have been the subject of substantial investigation. Hybrid lead halide perovskites have unique features that make them useful in a variety of optoelectronic applications such as solar cells, photodetectors, light-emitting diodes (LEDs), and field-effect transistors.^5b,c However, the considerable barrier posed by lead toxicity has driven scientists to investigate alternatives, with a particular emphasis on substituting lead metal with tin (Sn) from the (IV) group. Tin has similar electrical and structural characteristics to lead and additionally is harmless to the environment. As a result, substantial study has begun to determine whether tin halide perovskites are a feasible replacement.^5c It is worth noting that, unlike lead halide perovskites, 2D forms of Sn(II)-based hybrid perovskites have not received extensive attention in nanomaterial research. Mitzi et al. first described two-dimensional (2D) layered perovskite materials with a large spacer organic cation sandwiched between two perovskite layers.^6a,b The chemical formula for these 2D perovskites is A₂BX₄, where “A” represents a long-chain spacer cation, “B” is a divalent cation, and “X” is a halide (for example, Cl, Br, and I). As a result, 2D perovskites have a hydrophobic chain on their surface, which protects them from moisture while also assuring nanomaterial stability. These characteristics have motivated researchers to further explore the optoelectronic properties of 2D Ruddlesden–Popper (RP) halide perovskites.^7a,b Two phases of 2D layered Sn halide perovskites are distinguished by the type of organic spacer cation “A.” The Ruddlesden-phase (A₂BX₄) is the state in which “A” is a monovalent organic spacer cation. On the other hand, the Dion–Jacobson phase (ASnI₄) is the term used if “A” is a divalent organic spacer cation. The type of organic spacer cation at the “A” sites determines how these phases can be distinguished from one another.^7c,d Extensive studies have primarily concentrated on lead analogs, attributed to the increased stability of Pb²⁺ over Sn²⁺ halide perovskites in the presence of moisture. However, the constrained stability of the reduced state (Sn²⁺) presents a limitation for the practical utilization of tin halide perovskites in device fabrication.^7e Furthermore, a significant challenge lies in controlling oxidation in the presence of moisture, which promotes the transition of Sn²⁺ to Sn⁴⁺ states. However, partial oxidation results in self-doping, creating defects and trap states that hinder the performance of devices. On the other hand, the complete oxidation of Sn²⁺ to Sn⁴⁺ leads to the breakdown of the perovskite nanomaterial's structure itself.^{8a,b,9a,b,10a,b} To address this issue, Wang et al. have reported a method to prevent the oxidation of Sn²⁺ in the presence of moisture by using hypophosphorous acid and aimed to produce 2D Sn-based low-dimensional layered perovskite nanomaterials. Ascorbic acid additives were also used to prevent Sn²⁺ oxidation.^9b

While 3D perovskite nanomaterials exhibit superior potential for light-emitting diodes due to their broad tunability of light emission, high color saturation, low fabrication cost, and ease of formation through solution processability,^11,12a,b they encounter a drawback. The lower exciton binding energy of 3D perovskites leads to the easy dissociation of excitons at room temperature, thereby diminishing the efficiency of radiative recombination.^12c,d The optoelectronic properties of 2D perovskites can be easily modified by employing different spacer cations and incorporating various inorganic layers for photodetector applications.^13a Herein, a woven fibrous photodetector, Cs₃Bi₂I₉, with quantum well modulation of pure phase 2D type perovskites, shows better photodetector applications.^13b–f Moreover, if spacer cations have conjugated, bulky, and rigid structures, they function as encapsulating layers, thereby enhancing the potential and stability of perovskite nanomaterials.¹⁴ In the present investigation, we demonstrate the synthesis of MHP1 perovskite nanomaterials and their remarkable photo-responsivity and water stability. The 2D tin-based crystalline halide perovskite nanomaterials, MHP1 were initially synthesized by us employing an aqueous acid–base strategy without any moisture protection. Scheme 1 shows the synthesis procedure for the luminous MHP1 (Fig. S1, ESI†) perovskite crystalline materials. The synthesis protocol for MHP1 is identical to that reported for the formation of the (C₁₈H₃₅NH₃)₂SnBr₄ nanomaterial.^15a The MHP1 perovskite nanomaterial exhibits robust fluorescence and stability under UV light. Furthermore, we conducted a comprehensive study of the photo-response and optical properties of the synthesized nanomaterial. The combination of outstanding photo-response and conductive properties renders this material particularly intriguing for diverse applications. The optical properties of MHP1 perovskite nanomaterials are investigated using UV-Vis absorption and photoluminescence (PL) spectroscopy. The MHP1 nanomaterial exhibits (Fig. 1a) three distinct absorption peaks; the first peak is observed at 427 nm, which corresponds to the high-energy transition of excitons, and another absorption peak is present at 504 nm, attributed to the charge transfer transition between the organic spacer cations and inorganic layers. Additionally, a sharp absorption peak at 580 nm is observed, generated due to intrinsic exciton absorption.^15b,16a In PL analysis, the MHP1 nanomaterial exhibits dual excitonic emission peaks at 649 and 700 nm (Fig. 1b) due to transfer transition between the organic spacer cations and inorganic layers. These similar emissions are also observed from (PEA)₂SnI₄ single crystals and layered lead halide perovskites.^6b The two peaks of dual emission do not correspond to defect or trap-related states because these states exhibit an absorption cross-section that is quite weak.^16b,c When firmly analyzing PL data, the shorter wavelength is generated from the interaction of 2D (Sn–I) tin iodide isolated layers. In contrast, at the edges of this crystal, a longer wavelength PL peak is observed. Therefore, resolved PL data suggest that the energy levels differ between the edges and isolated 2D (Pb–I) layers. Our synthesized perovskite nanomaterials have similarities to the materials used to create Pb–I layers, according to PL analysis. Thus, we assume that longer emission peaks of wavelength are caused by the crystal edges, whereas shorter wavelength emission is a result of isolated Sn–I layers.^6b,17 Herein, we have additionally performed the time-dependent PL analysis (Fig. S2, ESI†) of our material demonstrating their water stability. Further, we carried out a thin-film X-ray diffraction (XRD) investigation (Fig. 1c) to examine the diffraction pattern and crystalline nature of our synthesized MHP1 material. Three distinct diffraction peaks, corresponding to the (001), (002), and (003) crystal planes, are visible in the thin film XRD pattern at 2θ = 2.40°, 4.83°, and 7.26°, respectively. These results validate the crystalline nature of the produced perovskite material.^18,19a The presence of the oleylammonium cation in the synthesized perovskite nanomaterial is frequently identified through Fourier transform infrared spectroscopy (FTIR) analysis. The characteristic peak associated with the ammonium carbon chain, ranging from 1240–1800 and 2700–3000 cm⁻¹, is evident in the FTIR spectrum. When the NH₃⁺ group binds with the SnI₄ octahedron moiety, the –NH stretching vibration peak appears at 3400 cm⁻¹ that shifts to 3300 cm⁻¹ (Fig. 1d). Additional peaks for =C–H and –C–H extending at 2850–3000 cm⁻¹ are also obtained. At 1400 to 1700 cm⁻¹, the C=C stretching and –CH₃ bending modes are observed. Therefore, it is evident from these observations that the oleylammonium cation is crucial to the synthesis of perovskite nanomaterials.^19b The calculated value of the CIE coordinate is obtained for X = 0.668 and Y = 0.312 for our perovskite nanomaterial. The CIE coordinates (Fig. 1e) confirmed that the color space and quality of the material are visible to the human eye. X-ray photon spectroscopy (XPS) was employed to investigate the oxidation state and surface analysis of the elements C, N, Sn, and I present on the surface of the MHP1 nanomaterial. Furthermore, narrow scans show that every element is present in an appropriate oxidation state (Fig. 1f). In Fig. 1g, a significant peak of C 1s is observed at 284 eV, while the N 1s peak (Fig. 1h) appears at 89 eV. Similarly, Sn exhibits two peaks (Fig. 1i) located at 488.11 eV for Sn 3d_5/2 and 496.26 eV for Sn 3d_3/2 orbitals, respectively. Furthermore, XPS peak data demonstrate two peaks of iodine (Fig. 1j) at 618.61 eV for I 3d_5/2 and another peak at 630.06 eV for I 3d_3/2, respectively. Additionally using photoresponse and XPS data (Fig. S3, ESI†) we have demonstrated enhanced Sn²⁺ stability in our synthesized material. Moreover, the thermal stability of the synthesized perovskite materials was investigated using thermo gravimetric analysis (TGA) presented in Fig. S4 (ESI†), where the temperature was increased by 10 °C per minute. Due to the reaction between hypophosphorous acid and water, the TGA spectrum of MHP1 exhibits a mass loss at 80 to 90 °C, releasing H₂ gas and phosphorous acid. Another hump, observed at 180 °C, corresponds to the decomposition of oleylamine originating from the material. A significant decomposition hump at 380 °C was observed, indicating the complete loss of MHP1 material. Scanning electron microscopy (SEM) was employed to examine (Fig. 2a) the morphology of MHP1, which was found to be quasi-spherical. Furthermore, elemental mapping (Fig. 2b–e) confirms the element distribution according to their composition. Herein, transmission electron microscopy (TEM) also confirms (Fig. 2f) that the morphology of MHP1 is quasi-spherical, and elemental mapping (Fig. S5, ESI†), and energy dispersive X-ray (EDX) spectra from TEM reveal that the elements Sn and I are present in an almost 1 : 4 ratio (Table S1, ESI†). Additionally, selected area electron diffraction patterns (SAED) (Fig. 2g) and HRTEM (Fig. 2h) confirm the crystalline nature of MHP1. Herein we have also demonstrated the outstanding stability of our perovskite material (Fig. 2i) in water over a month at 4 °C.

Scheme 1 Schematic representation of the synthesis of 2D tin halide perovskite nanocrystals.

Fig. 1 (a) UV-vis spectra, (b) photo luminescence spectra, (c) thin film XRD, (d) FTIR spectra of the perovskite material, (e) CIE coordinate spectra of (oleylamine)₂SnI₄, (f) survey scan: narrow scan of (g) C, (h) N, (i) Sn, and (j) I atoms.

Fig. 2 (a) FE-SEM images of the nanocrystals at 100 nm scale bar, (b) overall elemental mapping of the image, (c) Sn, (d) I and (e) C, atoms, (f) TEM image at 500 nm scale bar, (g) SAED pattern, (h) images of the fringes and (i) image of (oleylamine)₂SnI₄ perovskites under water after one month showing their stability.

To investigate the photo response, temporal response, and detectivity of the photodetector, we have fabricated a device similar to the one shown in Fig. 3a. A UV LED (385 nm) powered by a key sight function generator is utilized to confirm the characteristics of the photodetector. Various intensities of UV light were illuminated on the photodetector to quantify its performance (Fig. 3b). It displays the device's photo response under different illumination intensities of 385 nm wavelength light. This illustrates that increasing the intensity of the incident UV light increases the photocurrent. A long-term photoresponse is also recorded under periodic illumination conditions to ensure the device's stability (Fig. S6, ESI†). Furthermore, to assess the performance of the photodetector, temporal response, responsivity (R), and specific detectivity (D) are considered at various power densities of incident light. The temporal response is explicitly characterized by the device's rise and fall times. The rise time refers to the duration during which the photo response of the device increases from 10% to 90% of its maximum, while the fall time indicates when the photo response decreases from 90% to 10% of its maximum. Rise and fall times are measured using an oscilloscope under UV light with an intensity of 15 mW cm⁻² (Fig. 3c), illustrating that the rise time of the device is 0.91 ms, and the fall time is 1.28 ms, both of which are faster than those reported for previously studied Sn-based perovskite photodetectors, as shown in Table S2 (ESI†). Responsivity is the efficiency at which a photodetector can respond to incident light. It is calculated as the photocurrent ratio to the incident light's power density and expressed as R = (J_light − J_dark)/(P_inc). Here, J_light is the current density in the presence of light, J_dark is the current density in the dark, and P_inc is the power density of the incident light. The responsivity of the detector is 1.1825 mA W⁻¹ at 8 mW cm⁻², which increases with the intensity of the light and reaches to 2.8381 mA W⁻¹ at a power density of 15 mW cm⁻², as shown in Fig. 3d. When the incident power density increases, it leads to a lower recombination rate. Consequently, there is an associated increase in photo-generated current, contributing to the rise of responsivity and specific detectivity. The numerical modeling conducted by Klee et al. elucidates the reciprocal connection between the recombination rate and light intensity.²⁰ Another noteworthy attribute of a photodetector is specific detectivity, defining the minimum light intensity detectable by the device. It is influenced by both the responsivity and noise characteristics of the photodetector. If shot noise is considered the primary contributor to the dark current, the specific detectivity is expressed as D* = R/(2qJ_dark)^0.5, where q represents the elementary charge. The specific detectivity, D*, escalates with a rise in incident light intensity. Specific detectivity achieved for the device increased from 1.98 × 10⁹ to 5 × 10⁹ Jones by increasing the intensity of UV light from 8 mW cm⁻² to 15 mW cm⁻² (1 Jones = cm Hz^0.5 Watt⁻¹). Finally, we have synthesized lead-free layered, two-dimensional (2D) Sn-based perovskite nanomaterial MHP1. Without applying any voltage, we successfully fabricated a device with a high photoresponsive value using these materials. The obtained photoresponsivity value exceeds that of prior research, as indicated (Table S2, ESI†). Additionally, the device exhibits rapid reaction times, with a rise time of 910 μs and a fall time of 1.28 ms. With a specific detectivity of 5 × 10⁹ Jones and a responsivity (R) of 2.83 mA W⁻¹ in 1.28 ms.

Fig. 3 (a) Schematic representation of the I–V studies, (b) photo response of the detector under different intensities of light, (c) temporal response of the photodetector, and (d) responsivity and detectivity with different power densities of the incident light.

P. K. gratefully acknowledges the Science and Engineering Research Board (CRG/2020/000702), New Delhi, India. B. L., P. Kumar and AS acknowledge UGC, and SK acknowledges CSIR, India, for their doctoral fellowship. M. B. acknowledges the Department of Science and Technology, India (DST/INT/SWD/VR/P-13/2019). The authors also acknowledge the Institute Instrumentation Centre (IIC), IITR for providing the instrumentation.

Data availability

The data supporting this article have been included in the ESI.†

Conflicts of interest

There are no conflicts of interest to declare for this work.

Notes and references

1. H. L. Wells, Z. Anorg. Chem., 1893, 3, 195–210.
2. (a) D. Weber, J. Chem. Sci., 1979, 34, 939–941; (b) D. Weber, J. Chem. Sci., 1978, 33, 1443–1445; (c) F. Cao and L. Li, Adv. Funct. Mater., 2021, 31, 2008275.
3. (a) P. Bansal and P. Kar, Chem. Commun., 2019, 55, 6543–6546; (b) A. Jha, H. Shankar, S. Kumar, M. Shankar and P. Kar, Nanoscale Adv., 2022, 4, 1779–1785.
4. H. Shankar, S. Ghosh and P. Kar, J. Alloys Compd., 2020, 844, 156148.
5. (a) M. A. Green, A. Ho-Baillie and H. J. Snaith, Nat. Photonics, 2014, 8, 506–514; (b) L. C. Schmidt, A. Pertegas, S. Gonzalez-Carrero, O. Malinkiewicz, S. Agouram, G. M. Espallargas, H. J. Bolink, R. E. Galian and J. Perez-Prieto, J. Am. Chem. Soc., 2014, 136, 850–853; (c) H. Huang, L. Polavarapu, J. A. Sichert, A. S. Susha, A. S. Urban and A. L. Rogach, NPG Asia Mater., 2016, 8, e328.
6. (a) D. B. Mitzi, C. Feild, W. Harrison and A. M. Guloy, Nature, 1994, 369, 467–469; (b) V. V. Nawale, T. Sheikh and A. Nag, J. Phys. Chem. C, 2020, 124(38), 21129–21136.
7. (a) L. Lanzetta, M. J. Marin-Beloqui, I. Sanchez-Molina, D. Ding and S. A. Haque, ACS Energy Lett., 2017, 2, 1662–1668; (b) L. Hou, Y. Zhu, J. Zhu and C. Li, J. Phys. Chem. C, 2019, 123, 31279–31285; (c) C. C. Stoumpos, D. H. Cao, D. J. Clark, J. Young, J. M. Rondinelli, J. I. Jang, J. T. Hupp and M. G. Kanatzidis, Chem. Mater., 2016, 28, 2852–2867; (d) L. Mao, W. Ke, L. Pedesseau, Y. Wu, C. Katan, J. Even, M. R. Wasielewski, C. C. Stoumpos and M. G. Kanatzidis, J. Am. Chem. Soc., 2018, 140, 3775–3783; (e) F. Wang, J. Ma, F. Xie, L. Li, J. Chen, J. Fan and N. Zhao, Adv. Funct. Mater., 2016, 26, 3417–3423.
8. (a) T. M. Koh, T. Krishnamoorthy, N. Yantara, C. Shi, W. L. Leong, P. P. Biox, A. C. Grimsdale, S. G. Mhaisalkar and N. Mathews, J. Mater. Chem. A, 2015, 3, 14996–15000; (b) S. Gupta, D. Cahen and G. Hodes, J. Phys. Chem. C, 2018, 122, 13926–13936.
9. (a) Z. Chang, Z. Lu, W. Deng, Y. Shi, Y. Sun, X. Zhang and J. Jie, Nanoscale, 2023, 15, 5053–5062; (b) F. Cao, W. Tian, M. Wang, M. Wang and L. Li, InfoMat, 2020, 2, 577–584.
10. (a) L. Liu, F. Cao, L. Bian, M. Wang, H. Sun and L. Li, Sci. Chin. Mater., 2023, 66, 4696–4703; (b) J. Yang, H. Zhang, H. Cui, S. Xu, G. Pan, H. Gao, Z. Zhang and Y. Mao, ACS Photonics, 2024, 11, 1181–1188.
11. A. Wang, Y. Guo, Z. Zhou, X. Niu, Y. Wang, F. Muhammad, H. Li, T. Zhang, J. Wang, S. Nie and Z. Deng, Chem. Sci., 2019, 10, 4573–4579.
12. (a) Y.-H. Kim, S. Kim, S. H. Jo and T.-W. Lee, Small Methods, 2018, 2, 1800093; (b) D. Han, M. Imran, M. Zhang, S. Chang, X.-G. Wu, X. Zhang, J. Tang, M. Wang, S. Ali, X. Li, G. Yu, J. Han, L. Wang, B. Zou and H.-Z. Zhong, ACS Nano, 2018, 12, 8808–8816; (c) F. Zhang, H. Zhong, C. Chen, X.-G. Wu, X. Hu, H. Huang, J. Han, B. Zou and Y. Dong, ACS Nano, 2015, 9, 4533–4542; (d) X.-K. Liu and F. Gao, J. Phys. Chem. Lett., 2018, 9, 2251–2258.
13. (a) A. Wang, Y. Guo, Z. Zhou, X. Niu, Y. Wang, F. Muhammad, H. Li, T. Zhang, J. Wang, S. Nie and Z. Deng, Chem. Sci., 2019, 10, 4573–4579; (b) X. Deng, Z. Li, F. Cao, E. Hong and X. Fang, Adv. Funct. Mater., 2023, 33, 2213334; (c) X. Zhang, Z. Li, E. Hong, M. Deng, L. Su and X. Fang, Adv. Funct. Mater., 2023, 2312293; (d) Z. Li, X. Liu, C. Zuo, W. Yang and X. Fang, Adv. Mater., 2021, 33, 2103010; (e) E. Hong, Z. Li, X. Zhang, X. Fan and X. Fang, Adv. Mater., 2024, 2400365; (f) E. Hong, Z. Li, T. Yan and X. Fang, Nano Lett., 2022, 22(21), 8662–8669.
14. J. V. Passarelli, D. J. Fairfield, N. A. Sather, M. P. Hendricks, H. Sai, C. L. Stern and S. I. Stupp, J. Am. Chem. Soc., 2018, 140(23), 7313–7323.
15. (a) S. Ghosh, J. Kumar, G. K. Nim, M. Bag and P. Kar, Chem. Commun., 2023, 59, 2110; (b) T. Ishihara, J. Takahashi and T. Goto, Phys. Rev. B: Condens. Matter Mater. Phys., 1990, 42, 11099–11107.
16. (a) S. Kumar, S. Ghosh and P. Kar, J. Phys. Chem. B, 2023, 127(10), 2138–2145; (b) T. Sheikh, A. Shinde, S. Mahamuni and A. Nag, ACS Energy Lett., 2018, 3, 2940–2946; (c) M. Bruzzi, F. Gabelloni, N. Calisi, S. Caporali and A. Vinattieri, Nanomaterials, 2019, 9(2), 177.
17. T. Sheikh, V. Nawale, N. Pathoor, C. Phadnis, A. Chowdhury and A. Nag, Angew. Chem., Int. Ed., 2020, 59, 11653–11659.
18. Y. Gao, Z. Wei, P. Yoo, E. Shi, M. Zeller, C. Zhu, P. Liao and L. Dou, J. Am. Chem. Soc., 2019, 141, 15577–15585.
19. (a) X. Zhang, C. Wang, Y. Zhang, X. Zhang, S. Wang, M. Lu, H. Cui, S. V. Kershaw, W. W. Yu and A. L. Rogach, ACS Energy Lett., 2019, 4(1), 242–248; (b) A. Suhail, A. Saini, S. Beniwal and M. Bag, J. Phys. Chem. C, 2023, 127(34), 17298–17306.
20. J. He, D. He, Y. Wang and H. Zhao, Opt. Exp., 2015, 23(26), 33370–33377.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc02404d

---