2D Ruddlesden–Popper perovskite sensitized SnP₂S₆ ultraviolet photodetector enabling high responsivity and fast speed

Abstract

As the newly developed wide-bandgap semiconductors, two-dimensional layered metal phosphorus chalcogenides (2D LMPCs) exhibit enormous potential applications in ultraviolet (UV) photodetection due to their superior optoelectronic performance. However, 2D LMPC-based UV photodetectors generally suffer from low responsivity and slow response speed, which hinder their practical applications. Here, we present an effective strategy of sensitizing 2D LMPC UV photodetectors with a 2D Ruddlesden–Popper (RP) perovskite to enable high responsivity and fast response speed. As a demonstration, a hybrid heterojunction composed of RP perovskite (PEA)₂PbI₄ and a 2D SnP₂S₆ flake is fabricated by spin-coating method. Benefitting from the strong optical absorption of (PEA)₂PbI₄ and the efficient interfacial charge transfer caused by the favorable type-II energy band alignment, the as-fabricated 2D SnP₂S₆/(PEA)₂PbI₄ hybrid heterojunction photodetectors show high responsivity (67.1 A W⁻¹), large detectivity (2.8 × 10¹¹ Jones), fast rise/delay time (30/120 μs) and excellent external quantum efficiency (22825%) at 365 nm. Under field-effect modulation, the responsivity of the heterojunction photodetector can reach up to 239.4 A W⁻¹, which is attributed to the photogating mechanism and reduced Schottky barriers. Owing to the excellent photodetection performance, the heterojunction device further shows superior imaging capability. This work provides an effective strategy for designing high-performance UV photodetectors toward future applications.

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New concepts

Two-dimensional layered metal phosphorus chalcogenides (2D LMPCs) are highly desired wide-bandgap semiconductors for ultraviolet (UV) photodetection. However, the 2D LMPC-based UV photodetectors commonly suffer from low responsivity and slow response speed, which seriously hinders their practical applications. In this work, taking advantage of the enhanced optical absorption as well as the favorable energy band alignment, a high-performance UV photodetector based on the 2D Ruddlesden–Popper (RP) perovskite (PEA)₂PbI₄ and SnP₂S₆ flake hybrid heterojunction has been demonstrated. In comparison to most 2D LMPC-based photodetectors, the heterojunction photodetector exhibits a high responsivity of 239.4 A W⁻¹ and a fast response time of 30 μs at 365 nm, with great potential in photoelectric imaging applications. This work provides an efficient strategy for the design and fabrication of high-performance UV photodetectors toward multiple applications.

1. Introduction

Ultraviolet (UV) photodetectors have drawn extensive attention for their wide application in military surveillance, optical communications, flame detection, environmental monitoring, and biological analysis.^1–4 Two-dimensional layered metal phosphorus chalcogenides (2D LMPCs) with wide bandgaps of 1.3–4.5 eV are prospective candidates for next-generation UV photodetectors due to their excellent optoelectronic properties and ease of integration.^5–7 Currently, tremendous efforts have been devoted to investigate the UV photodetection performance of 2D LMPCs with various compositions and structures.^7–9 Despite the significant progress made in 2D LMPC-based UV photodetectors, most of them exhibit low responsivity and slow response speed, due to the weak optical absorption and inherent defect trapping caused by their atomically thin nature.^8,10

Constructing hybrid heterojunctions by combining 2D materials and photosensitizers with strong optical absorption has been demonstrated as an effective strategy to overcome the drawbacks mentioned above and achieve high-performance photodetectors.^11–13 On the one hand, the hybrid heterojunctions combine the advantageous properties of two different materials, providing opportunities to break through the performance limitations of the single material.^13,14 On the other hand, the built-in electric field formed at the hetero-interface facilitates the separation and transfer of photocarriers.¹⁵ The emerging 2D Ruddlesden–Popper (RP) perovskites are considered as suitable UV photosensitizers due to their wide bandgaps, large absorption coefficients, solution-processability and good environmental stability.^16–18 For example, the 2D (BA)₂MA₃Pb₄I₁₃/MoS₂ hybrid heterojunction fabricated by Xie et al. exhibits a high responsivity of 238 A W⁻¹ and a fast response time at the microsecond level.¹⁹ Shen et al. demonstrated that the 2D (PEA)₂PbI₄ sensitized MoS₂ photodetector shows a 100-fold improvement in the response speed (several milliseconds) relative to the pure MoS₂ device.²⁰ Therefore, hybridizing 2D RP perovskites together with 2D LMPCs is highly promising for realizing high-sensitivity and fast-speed UV photodetectors.

In this work, we present the first demonstration of a 2D RP perovskite-LMPC hybrid heterojunction for high-performance UV photodetectors. Herein, a typical RP perovskite (PEA)₂PbI₄ and an emerging LMPC SnP₂S₆ are selected as the photosensitizer layer and the carrier transport layer, respectively. Ultraviolet photoelectron spectroscopy (UPS) and photoluminescence (PL) spectra as well as fluorescence lifetime measurements reveal the type-II band alignment of the 2D SnP₂S₆/(PEA)₂PbI₄ hybrid heterojunction, which promotes efficient interfacial charge transfer from (PEA)₂PbI₄ to SnP₂S₆. Owing to the enhanced optical absorption and the efficient interfacial charge transfer, the 2D SnP₂S₆/(PEA)₂PbI₄ hybrid heterojunction photodetector has demonstrated 20-fold enhancement for responsivity (67.1 A W⁻¹) and 50-fold improvement for response time (30 μs) than the pure SnP₂S₆ device at 365 nm. And the responsivity can be further increased to 239.4 A W⁻¹ by the synergistic effect of the photogating mechanism and Schottky barrier reduction under field-effect modulation. In addition, the hybrid heterojunction shows excellent imaging capability, which is evidenced by a single-pixel imaging system. Our demonstration provides an effective strategy towards high-sensitivity and fast-speed UV photodetectors based on 2D RP perovskite–LMPC hybrid heterojunctions.

2. Results and discussion

Fig. 1a presents the specific fabrication process of the 2D SnP₂S₆/(PEA)₂PbI₄ hybrid heterojunction. First, the multilayer SnP₂S₆ flakes are mechanically exfoliated onto a 300 nm SiO₂/Si substrate, as shown in Fig. 1b. The transmission electron microscopy (TEM) image of the SnP₂S₆ flake clearly shows two clear lattice spacing of 0.51 nm with an included angle of 60°, corresponding to (010) and (100) planes of rhombohedral SnP₂S₆ (Fig. 1c),²¹ which indicates that the as-exfoliated SnP₂S₆ flake possesses high crystallinity and cleaves along the (001) plane. Then the 2D perovskite (PEA)₂PbI₄ layer is deposited on the top of an as-exfoliated SnP₂S₆ sample using a facile solution spin-coating method at a spin speed of 3000 rpm for 60 s, followed by an annealing process on a hot plate at 80 °C for 10 min. It should be noted that the spin-coating process and the annealing process are carried out in a glove box. Finally, the 2D SnP₂S₆/(PEA)₂PbI₄ hybrid heterojunction is successfully obtained, as shown in Fig. 1d. The scanning electron microscopy (SEM) image and energy dispersive spectroscopy (EDS) mapping have confirmed that the prepared (PEA)₂PbI₄ layer possesses completely dense morphology and uniform elemental distribution of C, N, Pb and I (Fig. S1, ESI†). The atomic force microscopy (AFM) image of the heterojunction surface is shown in Fig. 1e. The polycrystalline (PEA)₂PbI₄ layer deposited on the SnP₂S₆ flake shows an average grain size of 20–50 nm and continuous surface coverage. To further characterize the structure of the 2D SnP₂S₆/(PEA)₂PbI₄ heterojunction, X-ray diffraction (XRD) measurement was conducted. As shown in Fig. 1f, the peaks at 13.7°, 27.6° and 56.8° can be assigned to the (003), (006) and (0012) crystallographic planes of 2D SnP₂S₆, and the peaks at 5.5°, 10.9°, 16.3°, 21.8°, 27.4° and 32.9° correspond to the (002), (004), (006), (008), (0010) and (0012) crystallographic planes of 2D perovskite (PEA)₂PbI₄.^22,23 Besides, the characteristic peaks of SnP₂S₆ and (PEA)₂PbI₄ can be simultaneously detected in their heterojunction and no additional peaks have appeared, which indicates the successful combination of the (PEA)₂PbI₄ layer and the SnP₂S₆ flake. The optical characteristics of the heterojunction are investigated using ultraviolet-visible absorption spectra. As shown in Fig. 1g, the SnP₂S₆ flake exhibits obvious absorption at 300–500 nm, which results from the optical transition between the valence and the conduction band.²⁴ For (PEA)₂PbI₄, strong absorption at 300–550 nm has been observed, while the sharp absorption peak at about 524 nm can be attributed to the excitonic transition.²⁵ In comparison, the SnP₂S₆/(PEA)₂PbI₄ heterojunction retains the absorption characteristics of (PEA)₂PbI₄ and presents enhanced absorption at 300–500 nm, which is caused by the co-absorption of SnP₂S₆ and (PEA)₂PbI₄.²⁶

Fig. 1 Preparation and characterization of the SnP₂S₆/(PEA)₂PbI₄ heterojunction. (a) Schematic diagram of the SnP₂S₆/(PEA)₂PbI₄ heterojunction prepared by a spin-coating method. (b) Optical image of SnP₂S₆ flakes before spin-coating (PEA)₂PbI₄. (c) TEM image of the SnP₂S₆ flake. (d) Optical image of SnP₂S₆ flakes after spin-coating (PEA)₂PbI₄. (e) AFM image of the (PEA)₂PbI₄ layer deposited on the SnP₂S₆ flakes. (f) XRD patterns and (g) absorption spectra of the SnP₂S₆ flake, the (PEA)₂PbI₄ layer and the heterojunction. (h) Band alignment of the SnP₂S₆/(PEA)₂PbI₄ heterojunction.

To better understand the energy band structure of the heterojunction, we have performed ultraviolet photoelectron spectroscopy (UPS) measurements. The UPS results of SnP₂S₆ and (PEA)₂PbI₄ are shown in Fig. S2 (ESI†). The work function (W_F) of SnP₂S₆ and (PEA)₂PbI₄ can be calculated to be 5.22 eV and 4.47 eV according to W_F = hν − E_s, where hν and E_s refer to the photon energy (21.22 eV for the He I light source) and the second electron cutoff energy, respectively. Therefore, the Fermi level (E_F) of SnP₂S₆ and (PEA)₂PbI₄ is determined to be −5.22 eV and −4.47 eV when the vacuum level is taken as the reference. As can be seen from the valence band spectra, the difference between the valence band maximum (E_v) and the E_F of SnP₂S₆ and (PEA)₂PbI₄ is 1.51 eV and 1.1 eV. The E_v of SnP₂S₆ and (PEA)₂PbI₄ can be estimated to be −6.62 eV and −5.26 eV, respectively. Subsequently, the optical band gap of SnP₂S₆ and (PEA)₂PbI₄ is determined to be 2.5 eV and 2.3 eV,^21,22 and thus a type-II band alignment of the SnP₂S₆/(PEA)₂PbI₄ heterojunction is proposed in Fig. 1.

According to previous reports, SnP₂S₆ is an n-type semiconductor while (PEA)₂PbI₄ is commonly considered as a p-type semiconductor.^27,28 When SnP₂S₆ and (PEA)₂PbI₄ are in contact with each other, a depletion region would form, and the E_F gradually tends to align and reaches a new equilibrium as the electrons diffuse from (PEA)₂PbI₄ into SnP₂S₆.^29,30 Benefitting from the charge transfer, the energy band at the interface would bend and a stable built-in electric field with barrier height of 0.7 eV (the E_F difference of two materials) is established on both sides of the interface.^31,32 Therefore, the photoexcited electron–hole pairs in (PEA)₂PbI₄ are separated by the built-in electric field, leading to electrons diffusing into SnP₂S₆ while the holes are retained in (PEA)₂PbI₄, as presented in Fig. 2a.^20,33 To further explore the interfacial carrier dynamics, the steady-state photoluminescence (PL) and fluorescence lifetime measurements have been conducted. Generally, the photo-generated electron–hole pairs in the (PEA)₂PbI₄ layer would recombine rapidly. However, after being combined with the SnP₂S₆ flake, the transfer of electrons into SnP₂S₆ would effectively reduce the rate of photo-generated electrons and holes recombination in (PEA)₂PbI₄, resulting in the quenching of PL intensity and the shortening of the fluorescence lifetime, as shown in Fig. 2b and c.^13,34 In order to gain a clearer understanding of charge transfer at the interface, the PL intensity mapping and fluorescence lifetime imaging are performed on the region shown in Fig. 2d. As shown in Fig. 2e and f, the outline of the dark SnP₂S₆ flake can be clearly resolved, indicating that the PL intensity and the fluorescence lifetime of (PEA)₂PbI₄ are significantly suppressed after being combined with SnP₂S₆. These results confirmed that such type-II band alignment of the SnP₂S₆/(PEA)₂PbI₄ heterojunction could facilitate efficient charge separation and transfer.

Fig. 2 Charge transfer behavior of the SnP₂S₆/(PEA)₂PbI₄ heterojunction. (a) Schematic diagram of the charge transfer process of the SnP₂S₆/(PEA)₂PbI₄ heterojunction under the illumination condition. (b) Steady-state PL spectra and (c) fluorescence lifetime of (PEA)₂PbI₄ and the heterojunction. (d) Optical image of the SnP₂S₆/(PEA)₂PbI₄ heterojunction. (e) PL intensity mapping and (f) fluorescence lifetime mapping of (d).

Fig. 3a presents the schematic diagram of the SnP₂S₆/(PEA)₂PbI₄ heterojunction photodetector with Cr/Au (10/50 nm) as electrodes, and the detailed preparation process can be seen in the experimental section. To evaluate the optoelectronic performance of the heterojunction, the photodetectors based on SnP₂S₆ and (PEA)₂PbI₄ have also been prepared for comparison (Fig. S3, ESI†). Here, the light activated area S of the three devices is about 12 μm², and the thickness of the SnP₂S₆ flake and the (PEA)₂PbI₄ layer is about 18 nm and 60 nm, respectively. The current–voltage (I_ds–V_ds) characteristics of the three devices under dark and illumination conditions are shown in Fig. 3b. The dark current (I_dark) of the heterojunction device is higher than that of both SnP₂S₆ and (PEA)₂PbI₄ devices, which is consistent with the findings in MoS₂/CsPbBr₃ and WSe₂/CH₃NH₃PbI₂.^26,35 Herein, the (PEA)₂PbI₄ layer serves the double role of a light harvester and a hole conductor. Under laser illumination, the electron–hole pairs generated in the (PEA)₂PbI₄ layer would be separated driven by the built-in electric field, and the electrons are transferred into the SnP₂S₆ flake, which would raise the E_F of SnP₂S₆ and lower the E_F of (PEA)₂PbI₄, resulting in the reduction of the Schottky barrier at contacts and the enhancement of the light current (I_light) of the heterojunction device (Fig. 3f).³⁵Fig. 3c and d presents the spectral response range of the three devices measured at 5 V bias, and the responsivity (R) can be calculated by R = I_ph/PS, where I_ph = I_light − I_dark refers to the photocurrent, P refers to the power density of the incident laser and S refers to the device effective area.³¹ Owing to the enhanced absorption and interfacial charge transfer, the photoresponse of the heterojunction device is consistently higher than that of pure SnP₂S₆ and (PEA)₂PbI₄ devices. As shown in Fig. 3e, the responsivity of the SnP₂S₆/(PEA)₂PbI₄ heterojunction device has been improved by about an order of magnitude compared with that of the pure SnP₂S₆ device, and a sudden increase at about 520 nm is caused by the complex exciton states in (PEA)₂PbI₄.³⁶ Besides, the heterojunction device exhibits a high responsivity in the wavelength range of 300–500 nm, followed by a dramatic decrease in the wavelength range of 500–600 nm. Under 400 nm incident laser, the responsivity of the heterojunction device reaches a maximum of 66.4 A W⁻¹, which can be attributed to the stronger absorption at this wavelength.

Fig. 3 Photodetection performance comparison of the SnP₂S₆/(PEA)₂PbI₄ heterojunction with the pristine SnP₂S₆ flake and the pure perovskite. (a) Schematic diagram of the SnP₂S₆/(PEA)₂PbI₄ heterojunction device. (b) I_ds–V_ds characteristics of SnP₂S₆, (PEA)₂PbI₄ and SnP₂S₆/(PEA)₂PbI₄ heterojunction devices under dark and illumination conditions. (c and d) Photocurrent and responsivity of SnP₂S₆, (PEA)₂PbI₄ and SnP₂S₆/(PEA)₂PbI₄ heterojunction devices in the wavelength range of 300–700 nm. (e) Enhanced responsivity of the SnP₂S₆/(PEA)₂PbI₄ heterojunction relative to the pristine SnP₂S₆ flake. (f) Working mechanism of the SnP₂S₆/(PEA)₂PbI₄ heterojunction device.

Since the SnP₂S₆/(PEA)₂PbI₄ heterojunction device exhibits excellent photoresponse in the UV band, we further systematically evaluated its optoelectronic performance at 365 nm. Fig. 4a and b present the I_ds–V_ds characteristics and time-resolved photocurrent of the heterojunction device under laser illumination with various densities from 0.76 mW cm⁻² to 7.15 mW cm⁻². It has been observed that the photocurrent shows a gradually increasing trend as more photo-generated carriers would be excited at stronger laser in the heterojunction.³⁷ Besides, the heterojunction device exhibits excellent on/off switching stability and reversibility. The dependence between photocurrent and laser power density can be fitted by I_ph ≈ P^θ.³⁸ As shown in Fig. 4c, the power-law index (θ) of the heterojunction device is 0.61, which deviates from the ideal value (θ = 1). This might be caused by the defect states within the two materials or at their hetero-interface.³⁷ Apart from responsivity, the specific detectivity (D*) is another key parameter to measure the sensitivity of the photodetector, which can be calculated by the equation (where e represents the elementary charge).³⁹ As shown in Fig. 4d, the responsivity and detectivity of the heterojunction device increase as the laser power density decreases, and the maximum values are up to 67.1 A W⁻¹ and 2.8 × 10¹¹ Jones with a laser power intensity of 0.76 mW cm⁻², demonstrating the excellent detection capability for weak light. In addition, the photoelectric performance of pure SnP₂S₆ and (PEA)₂PbI₄ devices under 365 nm laser illumination is presented in Fig. S4 (ESI†). Compared with the pure SnP₂S₆ device (R = 3.1 A W⁻¹, D* = 2.7 × 10¹⁰ Jones, P = 0.76 mW cm⁻²) and (PEA)₂PbI₄ device (R = 18.6 A W⁻¹, D* = 8.7 × 10¹⁰ Jones, P = 0.76 mW cm⁻²), the heterojunction device exhibits higher responsivity and detectivity. The external quantum efficiency (EQE) of the heterojunction device can be calculated by EQE = hcR/e (h, c and λ refer to the Planck constant, speed of light and incident laser wavelength).³⁹ As shown in Fig. 4e, the EQE is as high as 22825% under a laser power density of 0.76 mW cm⁻², which confirms the outstanding photoelectric conversion efficiency of the heterojunction device.⁴⁰ The on/off ratio (I_light/I_dark) of the heterojunction device is calculated to be ∼10⁴, indicating its antinoise ability. Besides, the stability and long-term operability of the heterojunction device confirmed that the photocurrent and dark current of the heterojunction device hardly changed even after about 200 cycles of on/off switching operation (Fig. 4f). To further evaluate the performance stability of the heterojunction device, we also measured its photoresponse curve after 2 weeks storage in the glove box (Fig. S5, ESI†). It can be seen that the heterojunction device still maintains good switching stability but the curve becomes less smooth, which may be attributed to the fact that (PEA)₂PbI₄ is vulnerable to UV illumination.

Fig. 4 UV detection performance of the SnP₂S₆/(PEA)₂PbI₄ heterojunction device. (a) I_ds–V_ds curves of the device in the dark and under 365 nm laser illumination with various power densities. (b) Time-resolved photoresponse under 365 nm laser illumination with various power densities. (c) The dependence of photocurrent on laser power density at V_ds = 5 V. (d and e) Responsivity, detectivity, on/off ratio and EQE as a function of laser power density at V_ds = 5 V. (f) Photostability measurement of the device under 365 nm laser illumination with P = 0.76 mW cm⁻² and V_ds = 5 V. (g) The dependence of current decay on pulsed laser frequency at V_ds = 5 V. (h) Photoresponse of the SnP₂S₆/(PEA)₂PbI₄ heterojunction device under a 365 nm pulsed laser with various frequencies. (i) Response time of the device under a 365 nm pulsed laser with a frequency of 3000 Hz.

Generally, the response speed determines the capability of the photodetector to detect the rapid change of the optical signals. Therefore, we further investigated the response characteristics of the heterojunction device to a pulsed laser with various frequencies. It can be seen from Fig. 4h that the heterojunction device exhibits stable and reversible response to a pulsed laser with a frequency of 100 Hz, 1000 Hz and 2000 Hz, and its photocurrent remains above 50% of the initial value even under a 2000 Hz pulsed laser. The relationship between the current delay [(I_max − I_min)/I_max] and the pulsed laser frequency is further extracted in Fig. 4g, and the 3 dB cut-off frequency (the frequency-dependent photocurrent drops to 0.707 of the peak value) of the heterojunction device is estimated to be about 1500 Hz.⁴¹ As shown in Fig. 4i, the rise time (τ_rise) and delay time (τ_delay) of the heterojunction device in a complete switching cycle are 30 μs and 120 μs at a 3000 Hz pulsed laser, indicating that it is much faster than the pure SnP₂S₆ device (Fig. S6, ESI†). The fast photoresponse of the heterojunction device can be attributed to the charge separation at the hetero-interface as well as the efficient charge collection in the bottom SnP₂S₆ flake.^12,13

The gate-modulated photoresponse has also been investigated in order to reveal the underlying mechanism of photodetection in the heterojunction device. Fig. 5a presents the transfer characteristic curves of the heterojunction device under 365 nm laser illumination with various power densities, and its transfer characteristic curve under dark conditions is shown in Fig. S7 (ESI†). According to previous reports, the back-gate mainly modulates the Fermi level of n-type SnP₂S₆ due to the charge screening effect of the SnP₂S₆ flake underneath, while the top (PEA)₂PbI₄ layer has not been affected.^42,43 Therefore, the heterojunction device exhibits a typical n-type behavior under dark conditions. Under 365 nm laser illumination, the current of the heterojunction device dramatically increases with increasing power density within the gate voltage range from −60 V to 60 V. The dependence between the photocurrent and the laser power density at various gate voltages is shown in Fig. 5b, and the θ values fitted by the power law I_ph ≈ P^θ are displayed in Fig. 5c. When the gate voltage varies from −60 V to 60 V, the θ value of the heterojunction device gradually changes from 0.98 to 0.41, indicating the transition of the photocurrent generation mechanism from photoconductive effect (θ = 1) to photogating effect (0 < θ < 1). Such gate-modulated photoresponse behavior has also been reported in hybrid heterojunction devices with a similar structure.^44,45 As shown in Fig. 5d, the gate-tunable responsivity and detectivity of the heterojunction device have been demonstrated, which rapidly increase as the gate voltage changes from −60 V to 60 V. This behavior can be explained by the gate-modulated band structure at the SnP₂S₆–electrode contact interface.^33,46 As presented in Fig. 5e, the Fermi level of the SnP₂S₆ flake underneath moves down and the Schottky barrier at the contact interface significantly increases when V_gs < V_th (V_gs and V_th refer to the gate voltage and threshold voltage, respectively). Right now, the heterojunction device is in the off state, and the channel current is contributed by photo-generated carriers.^33,37 In addition, when the heterojunction device is in the on state (V_g > V_th), the Fermi level of the SnP₂S₆ flake underneath moves towards the conduction band and the Schottky barrier (Φ_SB) at the contact interface decreases.³⁷ Therefore, the thermionic and tunneling current gradually increases, resulting in the enhanced channel current (Fig. 5f).⁴⁷ The synergistic effect of the tunable Schottky barrier and the photogating mechanism can drastically increase the channel current, achieving excellent photodetection performance. Obviously, the responsivity and detectivity of the heterojunction device can reach up to 239.4 A W⁻¹ and 1.9 × 10¹¹ Jones at V_gs = 60 V, respectively.

Fig. 5 Gate-modulated UV detection performance of the SnP₂S₆/(PEA)₂PbI₄ heterojunction device. (a) Transfer characteristic curves of the device under 365 nm laser illumination with various power densities. (b) The dependence of photocurrent on laser power density at different gate voltages. (c) The extracted θ as a function of gate voltage. (d) Gate-tunable responsivity and detectivity of the device. (e and f) The carrier transport mechanism of the SnP₂S₆/(PEA)₂PbI₄ heterojunction device at V_gs < V_th and V_gs > V_th under 365 nm laser illumination.

Due to the fascinating photodetection performance of the heterojunction device, we further investigated its imaging applications.^48,49 The schematic diagram of a single-pixel imaging system is shown in Fig. 6a. A black acrylic mask with the imaging target (hollow pattern) is located between the laser and the 2D SnP₂S₆/(PEA)₂PbI₄ heterojunction photodetector, which is controlled by two stepping motors (along the X and Y direction respectively) and can be moved continuously line by line. During the imaging process, the photodetector can accurately detect the laser signal passing through the target and generate photocurrent, which can be collected by the computer. Meanwhile, the location information of the imaging target is also recorded by the computer. Finally, the imaging system converts the current signals into a set of gray values and outputs an image. The size of the imaging target is 200 × 100 pixels, and the scan step along the X direction is 10 pixels. The whole imaging process is conducted in the ambient environment. Fig. 6b and c present the imaging results of the heterojunction device under 405 and 520 nm laser illumination, respectively. Benefiting from the high on/off ratio and the fast response speed, the patterns can be clearly distinguished and possess obvious boundaries, indicating the great potential of the 2D SnP₂S₆/(PEA)₂PbI₄ heterojunction device in the field of photoelectric imaging.

Fig. 6 Photocurrent imaging application of the SnP₂S₆/(PEA)₂PbI₄ heterojunction device. (a) Schematic diagram of the imaging system. (b and c) Output imaging patterns under 405 and 520 nm laser illumination, respectively.

3. Conclusions

In summary, we have successfully fabricated a 2D SnP₂S₆/(PEA)₂PbI₄ hybrid heterojunction for high-performance UV photodetectors by the facile solution spin-coating process. The hybrid heterojunction exhibits significantly enhanced optical absorption in the wavelength range of 300–500 nm, which is attributed to the co-absorption of SnP₂S₆ and (PEA)₂PbI₄. Besides, through PL spectra, fluorescence lifetime and UPS measurements, the type-II energy band alignment leading to photocarrier separation and transfer at the SnP₂S₆ and (PEA)₂PbI₄ heterojunction interface has been demonstrated. Based on the enhanced absorption and efficient charge transfer, the heterojunction photodetector exhibits a high responsivity of 67.1 A W⁻¹, a large detectivity of 2.8 × 10¹¹ Jones, a fast rise/delay time of 30/120 μs and an excellent external quantum efficiency of 22825% under 365 nm laser illumination. Compared with the pure SnP₂S₆ device, the responsivity of the hybrid heterojunction device is improved by about 20 times and the response time is improved by 50 times. Due to the synergistic modulation of the photogating effect and Schottky barriers by gate voltage, the responsivity of the heterojunction device can be optimized to 239.4 A W⁻¹. Further, the excellent imaging capability of the heterojunction device has also been demonstrated. We envision that the 2D SnP₂S₆/(PEA)₂PbI₄ hybrid heterojunction photodetector presented in this work lays the foundation for the design and implementation of high-performance UV photodetectors.

4. Experimental section

4.1 Preparation of 2D SnP₂S₆ flakes

At first, SnP₂S₆ single crystals were synthesized by a chemical vapor transport (CVT) method. About a 1 g mixture of Sn, P and S powders with a molar ratio of 1 : 2 : 6 was placed in a vacuum-sealed quartz tube. Then, the quartz tube was put in a dual-temperature zone tubular furnace, and the two ends of the quartz tube were heated to 450 °C (source end) and 400 °C (growth end) within 400 min, respectively. Later, they were kept at the temperature for 7 days, and then naturally cooled to room temperature. After breaking the quartz tube, the SnP₂S₆ single crystals with high crystallinity can be obtained. Finally, a mechanical exfoliation method was used to prepare the 2D SnP₂S₆ flakes.

4.2 Preparation of (PEA)₂PbI₄ solution

In our study, 2 mmol of PEAI powder and 1 mmol of PbI₂ powder were mixed and dissolved in 4 mL of N,N-dimethylformamide (DMF) solution. Then the solution was heated and stirred at 70 °C for at least 120 min to ensure the complete dissolution of the two powders. It should be noted that the prepared (PEA)₂PbI₄ solution is stored in a glove box to prevent deterioration.

4.3 Fabrication of the SnP₂S₆/(PEA)₂PbI₄ hybrid heterojunction device

At first, the 300 nm SiO₂/Si substrate used in this study needs to be treated by O₂ plasma (10 sccm, 20 min, 50% power) to ensure the smooth process of the subsequent perovskite spin-coating. The 2D SnP₂S₆ flakes were exfoliated onto the treated substrate, followed by standard electron beam lithography (EBL) and thermal evaporation of 10/50 nm Cr/Au electrodes. Then, 20 μL of (PEA)₂PbI₄ solution was spin-coated on the as-fabricated SnP₂S₆ device with 3000 rpm spin speed for 60 s. After spin-coating the (PEA)₂PbI₄ layer, the device was annealed at 80 °C for 10 min to obtain the SnP₂S₆/(PEA)₂PbI₄ hybrid heterojunction. All the fabrication processes involving the perovskite were conducted in a glove box.

4.4 Characterization

The morphology and structure of the as-prepared SnP₂S₆ flake, (PEA)₂PbI₄ perovskite and SnP₂S₆/(PEA)₂PbI₄ heterojunction were characterized by atomic force microscopy (AFM, Dimension Icon, Bruker), scanning electron microscopy (SEM, Quanta 650, FEI), transmission electron microscopy (TEM, Tecnai G2 F30, FEI) and X-ray diffractometry (XRD, D2 Phaser, Bruker). The energy band alignment of the SnP₂S₆ flake, (PEA)₂PbI₄ perovskite and SnP₂S₆/(PEA)₂PbI₄ heterojunction was measured by ultraviolet photoelectron spectroscopy (UPS, Axis Ultra DLD, Kratos). Besides, the photoluminescence spectra were collected by using a confocal microscope (Alpha 300R, WITec) equipped with a 532 nm laser. And the optical absorption spectra were investigated by using an ultraviolet visible near-infrared spectrophotometer (SolidSpec-3700, SHIMADZU). The photoelectronic measurements of the SnP₂S₆ flake, (PEA)₂PbI₄ perovskite and SnP₂S₆/(PEA)₂PbI₄ heterojunction were performed in a probe station (TTPX, Lakeshore), which was connected with a semiconductor analyzer (B1500A, Agilent). The power-tunable 365 and 532 nm lasers were used as illumination sources to explore the photoresponse of the devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21825103, U21A2069). We also acknowledge the technical support from the Analytical and Testing Center in Huazhong University of Science and Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nh00466f

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