Hybrid Polyrhizal CdS Nanobelts/All‐Inorganic Perovskite Nanoflake Heterojunction toward Ultrahigh‐Performance Optoelectronic Devices

Both all‐inorganic lead‐free perovskite and CdS nanobelts possess outstanding photo‐detective properties. Herein, a hybrid photodetector based on four CdS nanobelts (NBs)/Cs3Sb2Br9 nanoflake (NF) heterojunction fabricated by a dry‐transfer tactic is designed. In this structure, four parallel CdS nanobelts almost fully cover the Cs3Sb2Br9 NF so that their photoelectronic advantage is demonstrated. The Cs3Sb2Br9 is exploited as an efficient light absorber and component for the construction of type‐II energy band with CdS. Compared with a single CdS NB/Cs3Sb2Br9 NF device, the hybrid four CdS NBs/Cs3Sb2Br9 NF device increases the hybrid area ratio of the channel from 36% to 86%, and demonstrates an appealing performance on an ultrahigh ON/OFF current ratio of 1.54 × 105, remarkable responsivity of 4.13 × 103 A W−1, large detectivity of 1.47 × 1015 Jones, and tremendous external quantum efficiency of 1.14 × 106 %, which are 9.5, 3.8, 7.2, and 3.8 times greater than the single CdS NB/Cs3Sb2Br9 NF device, respectively. Moreover, the photoresponse wavelength of the hybrid four CdS NBs/Cs3Sb2Br9 NF device is broadened to 400–520 nm. This work offers a strategy to enhance the photoelectric performance of hybrid devices, along with the illustration of vital insight into advanced device designs for violet‐green photodetectors.


Introduction
Contemporary integrated photoelectric detectors are crucial to many fields, such as remote-sensing, missile warning, machine DOI: 10.1002/aelm.202300383vision, intelligent health detection, and hand gesture detector, etc. [1][2][3] For the past few years, there is a growing demand for ultrahigh performance photodetectors with high responsivity, excellent detectivity, fast photoresponse speed.For this purpose, cadmium sulfide (CdS) displays a direct band gap of 2.43 eV, ntype conductive characteristic, high electron mobility, superior thermal stability, and low work function as one of the most researched II-VI semiconductors.The appealing properties make it the most potential candidate for optoelectronic devices.The low dimensional CdS nanostructures exhibit excellent photosensitivity to visible light owing to the large surface volume ratio. [4,5]evertheless, the low dimensional CdS nanostructures have large surface states density, and the absorption of surface oxygen is closely related to the photoconductivity behavior.The photocurrent will be reduced due to the presence of many of sulfur vacancies. [6]The formation of heterojunction can effectively passivate the trap or defects, and isolate CdS from oxygen, promoting the photoelectric characteristics and promoting light capture capability. [7]ecently, low dimensional CdS hybrids with 2D or perovskite materials (WS 2 , [8] MoS 2 , [9] PbS, [10] ZnO, [11] MWCNT, [12] CH 3 NH 3 PbI 3 , [13] and CsPbBr 3 [14,15] ) have been explored and exhibited excellent photoelectric performance.A self-powered photodetector based on the integrated gradient O-doped CdS nanorod array and CH 3 NH 3 PbI 3 demonstrated a remarkable detectivity of 2.1 × 10 13 Jones. [13]The 2D CsPbBr 3 /CdS heterostructures exhibited a brand-new excitonic photovoltaic effect along with high performances including a high switch ratio (I light /I dark ) of 10 5 and a fast response rate of 23 μs are obtained. [14]The CdS@CsPbBr 3 core-shell MW heterojunction photodetector possessed relatively high ratio (10 4 ) of photocurrent to dark current, much higher responsivity (319.79A W −1 ) and faster response time (6.6 ms). [15][27][28][29] However, the high toxicity of lead ion is a major obstacle to the practical application of inorganic cesium-based perovskite in large scale.The crystal structure of the layered modification of Cs 3 Sb 2 Br 9 is derived from the hypothetical perovskite compound CsSbBr 3 (i.e., Cs 3 Sb 3 Br 9 ) by removing every third Sb layer to achieve charge balance.Therefore, Cs 3 Sb 2 Br 9 belongs to the perovskite family. [30]By way for resolving the toxic problem, a few studies are reported on Sbbased Cs 3 Sb 2 Br 9 perovskites with different morphology such as nanoflake as lateral-structured device or photodetector, [31,32] nanocrystals for photocatalytic reduction of CO 2 , [33] quantum dots (QDs) with photoluminescence quantum yield (PLQY), [34,35] single crystals or microplate devices for photodetectors. [36,37]Photodetectors based on Cs 3 Sb 2 Br 9 nanoflake showed a fast respond speed (24/48 ms), large responsivity (3.8A W −1 ), high photodetect sensitivity (2.6 × 10 12 Jones). [32]Cs 3 Sb 2 Br 9 QDs achieved a high PLQY of 46% at 410 nm and exhibited a bright violet emission with a high PLQY of 51.2%, respectively. [34,35]Cs 3 Sb 2 Br 9 single crystals photodetectors have an excellent performance of a responsivity of 2.29 A W −1 , a sharp detectivity of 3.77 × 10 12 Jones. [36]The Cs 3 Sb 2 Br 9 microplate detector yielded an on/off ratio of 2.36 × 10 2 , responsivity of 36.9 mA W −1 , and detectivity of 1.0 × 10 10 Jones. [37]Meanwhile, the hybrid Cs 3 Sb 2 Br 9 nanoflake/CdSe nanobelt device has also been prepared to promote the optoelectronic detective properties due to the large absorption coefficient of Cs 3 Sb 2 Br 9 to light. [38]But the small contact area between them limits their greatest advantage of hybrid device components. In

Results and Discussion
Synthesis of Cs 3 Sb 2 Br 9 NF with highly crystal quality is vital to gain high performance photodetector.Compared with the CdS powder raw materials, the Raman peaks of the CdS NBs at 301 and 600 cm −1 have a frequency shift of 7 and 14 cm −1 , respectively, which affected by the phonon limiting effect of the nanomaterials. [39]XPS of single Cd 3d and S 2p is recorded, as illustrated in Figure 2e,f  hybrid CdS NB/Cs 3 Sb 2 Br 9 NFs is higher than that of either CdS NBs or Cs 3 Sb 2 Br 9 NFs and was also broadened to 510 nm, compared to Cs 3 Sb 2 Br 9 NFs. Figure S3a,b (Supporting Information) denotes the optical bandgap of Cs 3 Sb 2 Br 9 NF (2.58 eV) and CdS NB (2.43 eV), which are extracted from the following equation: [40] (h where  is absorption coefficient, h is Plank's constant,  is radiation frequency, A is constant, and E g is optical bandgap.Figure S3c,d (Supporting Information) depict the UPS spectrum of single Cs 3 Sb 2 Br 9 NF and CdS NB, respectively.It is observed that the conduction and valance bands of Cs 3 Sb 2 Br 9 NF are higher than those of CdS NBs.Therefore, the type-II energy band is formed. [41]Figure 3c demonstrates the time-resolved PL decay of Cs 3 Sb 2 Br 9 NF device and hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device.A built-in electric field will be formed in the hybrid CdS NBs/ Cs 3 Sb 2 Br 9 NF heterojunction when CdS is in contact with Cs 3 Sb 2 Br 9 .The photogenerated electron-hole pairs or weak-bonded excitons will be separated by the built-in electri-cal field from the heterojunction, leading to reduced radiative recombination of charge carriers. [42]The carrier separation is accelerated so that life time  of the hybrid device (1.56 ns) is higher than that of the single device (0.84 ns) and has lower non-radiative recombination.As a result, it is more suitable for fabricating the high-performance photodetectors. [43,44]o explore the photoelectric properties of the Cs 3 Sb 2 Br 9 NF device and the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device, the spectral responsivity in the range of 300 -700 nm is investigated at an applied bias voltage of 3 V, as displayed in Figure 3d.It revealed that the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device owns photoresponsivity in the range of 400 -520 nm, demonstrating its well potential in violet-green band light detection.
In order to systematically explore and compare the photoelectric performance of single Cs 3 Sb 2 Br 9 NF device and hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device, I-V curves of two devices are recorded under 448 nm light irradiation with the power density of 1.4 mW cm −2 and in dark.Figure 5a-c present I-V characteristics curves of the Cs 3 Sb 2 Br 9 NF device and the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device under 448 nm light irradiation with different power density at 3 V, respectively.Notably, the photocurrent abruptly increases with increasing light intensity.It describes that the efficiency of the photogenerated carrier is proportional to the absorbed photon flux. [45]Furthermore, Figure 5b-d exhibit the corresponding I-P curves of the two devices under the same conditions, respectively.To expound the relationship between the photocurrent versus light intensity by a power law I = AP  , [46] where I, A, and P are photocurrent, constant, and light intensity, respectively.The  determines the response of photocurrent to light intensity and the fitting of the experimental results show that  are 0.84 and 0.88 for the Cs 3 Sb 2 Br 9 NF device and the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device respectively, conforming to photogenerated electron-hole pairing, capturing, and recombining. [47]The fitted value is approaching the ideal value of 1, suggesting that there exists fewer trap centers, and defects in the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device.
The responsivity (R), [48] external quantum efficiency (EQE), [49] and detectivity (D*) [50] are three key parameters for photodetectors, which can be expressed by the following equations: where I ph , I d , P, A, h, c, e, and  are photocurrent, dark current, light power intensity, effective area of device, Plank's constant, light speed, charge of unit, and wavelength of incident light.Figure 6a-c plot R and EQE of the Cs 3 Sb 2 Br 9 NF device and the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device, respectively.Significantly, R and EQE of the latter were calculated as 4.13 × 10 3 A W −1 and 1.14 × 10 6 %, which are 1.12 × 10 3 times and 1.10 × 10 3 times as large as that of the former (R = 3.74 A W −1 and    Table 1 describes comparison of photoelectric performance of Cs 3 Sb 2 Br 9 NF composite with different quantities of CdS NBs measured under the same condition.The results show that with increasing number of the CdS NBs, the hybrid area on the channel increases from 36% to 86%.At the same time, the photoelectric properties have also significantly improved.Compared to the single CdS NB/Cs 3 Sb 2 Br 9 NF device, the light/dark current ratio of the hybrid four CdS NBs/Cs 3 Sb 2 Br 9 NF device is increased by 9.5 times, both responsivity and EQE are increased 3.8 times, and detectivity is increased by 7.2 times, respectively.In order to explore the contribution of CdS NB to hybrid device, the photoelectric performance of different quantities of CdS NBs are also compared, as shown in Table S1 (Supporting Information).Similarly, as the number of CdS NB increased from one to four, the effective area of CdS in the channel increased from 30 to 323 μm 2 .Compared with a single CdS NB device, the light/dark current ratio of four CdS NBs device is increased by 7.3 times, the response rate and EQE are both increased by 4.2 times, and the detectivity is increased by 4.0 times.
The photoresponsive property and speeds of single Cs 3 Sb 2 Br 9 NF device and hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device are further studied by monitoring fast change light signals.Figure 7a-c show the photoresponse time cyclic curves of single and hybrid devices by regularly controlling ON/OFF of 448 nm light.The results exhibit the stable and rapid response of single and hybrid devices to 448 nm light.The response speed is also a vital indicator for sensibility of photoelectric device.The rise time for one cycle is defined as from 10 -90% of the maximum value, and the fall time is from 90 -10% of the maximum value. [51]The rise/fall time of one cycle of single Cs 3 Sb 2 Br 9 NF device is 10.7/11 ms, as displayed in Figure 7b.Meanwhile, that of the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device is 18.2/18.7 ms, as depicted in Figure 7d.
Table 2 lists the key parameters of some previously reported photoelectric devices, which based on perovskite, CdS, and 2D materials.Table 2 shows that the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device exhibits outstanding photoelectric properties than the single perovskite devices and composite devices based on CdS including higher on/off ratio, R, and D*, and EQE.Therefore, the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device has an advantage over other hybrid devices based on CdS NBs/nanowires in the field of violet-green band spectral detection.
Based on UPS data, the energy band alignment between Cs 3 Sb 2 Br 9 NF and CdS NBs is displayed in Figure 8a.A typical type-II energy band is formed due to the conduction and valance bands of Cs 3 Sb 2 Br 9 NF are higher than CdS NBs. [41]And the Schottky contact is formed between Cs 3 Sb 2 Br 9 NF and CdS NBs, generating a strong built-in potential at Schottky junction interface.Figure 8b shows the schematic energy band of Cs 3 Sb 2 Br 9 NF and CdS NBs without contact.Obviously, the charge transfer is not happening between them.The operating state of the contacted hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device in dark is depicted in Figure 8c.The Schottky barrier decreases in that the charge interface changes the Femi level, resulting in the dark current of hybrid device to increase in dark.Figure 8d illustrates the working state of the contacted hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device under 448 nm light irradiation.Many electron-hole pairs are generated under light irradiation.
The photogenerated holes will be transferred to Cs 3 Sb 2 Br 9 NF and trapped, while the photogenerated electrons will be transferred to CdS NBs and circulate in the circuit.And they can recirculate many times, leading to a higher photocurrent during the lifetime of photogenerated electrons. [54]PL mapping (Figure S7, Supporting Information) of the CdS NBs/Cs 3 Sb 2 Br 9 NF shows PL intensity increasing (PL peak width decreasing) away from the heterojunction boundary, which might be due to the quenching effect of the heterojunction, since the photogenerated electronhole pairs or weak-bonded excitons will be separated by the  built-in electrical field from the heterojunction, leading to reduced radiative recombination of charge carriers. [42]The PL quenching effect indicates that efficient charge carrier separation occurs at the interface of the heterojunction, confirming the strong coupling of the two semiconductors forming the heterojunction. [55]

Conclusions
In summary, we have prepared the hybrid CdS NBs/Cs

Experimental Section
Preparation of Cs 3 Sb 2 Br 9 NFs: Briefly, 0.2 mmol of SbBr 3 (99%, Shanghai Macklin Biochemical Ltd. Corp.) and 0.3 mmol of CsBr (99.9%,Xi'an Plymer Light Technology Corp.) were first dissolved in 5 mL of hydrobromic acid (HBr) (48%, Shanghai Macklin Biochemical Ltd. Corp.).Then, the mixture solution was kept in an oil bath heated to 135 °C for 8 min to obtain clear yellow perovskite precursor solution.SiO 2 /Si substrates were ultrasonically cleaned in acetone, ethanol, and ultrapure water.For hydrophilic treatment, the substrates were cleaned in a high vacuum plasma processor for 5 min.Afterward, the yellow precursor solution was dropped onto a SiO 2 /Si substrate heated to 140 °C and evaporated for 15 min to synthesize Cs 3 Sb 2 Br 9 NFs.
Synthesis of CdS NBs by CVD: First, the quartz tube with a diameter of 2.5 cm was cleaned, and a layer of gold film with a thickness of ≈10 nm was plated on the cleaned SiO 2 /Si substrates with a small ion sputtering instrument.After that, the ceramic boat with appropriate CdS powders (99.999%,Shanghai Macklin Biochemical Ltd. Corp.) was placed in the middle of the quartz tube, and the gold-coated substrates were put downstream of the ceramic boat.Finally, the quartz tube was placed in a three-temperature tubular furnace, and the internal vacuum reached 150 torr.The temperature rose to 300 °C at a rate of 15 °C /minute, and then rose to 820 °C for 120 min.Ar gas (with 5% H 2 ) with a flow rate of 15 sccm was injected throughout the process.
Fabrication of Transferrable CdS NBs/Cs 3 Sb 2 Br 9 NF PD: The CdS NBs/Cs 3 Sb 2 Br 9 NF photodector (PD) was fabricated by a dry-transfer tactic, as shown in Figure S5 (Supporting Information).First, the electrodes (10 nm Ti/60 nm Au) with 10 μm channel were deposited onto the Cs 3 Sb 2 Br 9 NF by electron beam evaporation.Afterwards, a pipette gun was used to extract 50 μL CdS dispersing solution and drop it on a clean substrate.The CdS NBs were gently glued by a piece of PDMS thin film (0.5 cm × 0.5 cm) after the solution was completely evaporated, and then a single CdS NB was transferred onto the top of Cs 3 Sb 2 Br 9 NF due to the heat release property of the PDMS thin film.Finally, the above transfer steps were repeated until four parallel CdS NBs completely cover the Cs 3 Sb 2 Br 9 NF in the channel.CdS NBs/Cs 3 Sb 2 Br 9 NF heterojunction was consequently formed, and the CdS NBs/Cs 3 Sb 2 Br 9 NF heterojunction was annealed at 150 °C for 30 min under the protection of Ar to gain better contact.
this work, CdS nanobelts (NBs) are obtained by chemical vapor deposition (CVD) and Cs 3 Sb 2 Br 9 nanoflakes (NFs) are obtained by inverse temperature crystallization (ITC).The hybrid four CdS NBs/Cs 3 Sb 2 Br 9 NF device (hereafter denoted as the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device) are fabricated via transferring four CdS NBs onto the single Cs 3 Sb 2 Br 9 NF by dry-transfer method.The contact area between four CdS NBs and single Cs 3 Sb 2 Br 9 NF almost covers the entire channel in the Au electrodes.The coverage of the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device nearly approaches 90%.Cs 3 Sb 2 Br 9 NF not only enhances light absorption but also takes part in the establishment of type-II band alignment with CdS NBs.In addition, compared with the single Cs 3 Sb 2 Br 9 NF device, the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device obtains outstanding photoelectric performance with large I on /I off ratio, superior responsivity, detectivity, and EQE.The work revealed that the designed hybrid device provides an available strategy to achieving ultrahigh performance photodetectors.
Figure 1a,b depict the unit cell structure and 2D layered structure of Cs 3 Sb 2 Br 9 NF from crystallographic data.Obviously, 10 Cs cations surround the bioctahedral (Sb 2 Br 9 ) 3− clusters in single Cs 3 Sb 2 Br 9 crystallographic cell to get charge balance.Figure 1c shows optical microscopy image of Cs 3 Sb 2 Br 9 NFs.Each Cs 3 Sb 2 Br 9 NF owns smooth surface and hexagonal structure.The X-ray powder diffraction (XRD) pattern of the Cs 3 Sb 2 Br 9 NFs is displayed in Figure 1d.The main diffraction peaks (diffraction crystal plane) are located at 18.26°(002), 27.48°(003), 32.05°(222), and 56.97°(006), which are consistent with the hexagonal structure [p3m1 (164), Joint Committee on Powder Diffraction Standards (JCPDS) card No. 77-1055].The electronic band alignment with the metal is confirmed before device fabrication.Figure 1e portrays the atomic force microscopy (AFM) of the Cs 3 Sb 2 Br 9 NF with a thickness of ≈110 nm. Figure 1f denotes the position of the valence band maximum (VBM) of Cs 3 Sb 2 Br 9 to be 5.51 eV, which was determined by ultraviolet photoelectron spectroscopy (UPS).The X-ray photoelectron spectroscopy (XPS) is employed to investigate the chemical component and valence state of the Cs 3 Sb 2 Br 9 NF.Cs 3d, Sb 3d, and Br 3d are collected, as shown in Figure 1g-i.For Cs 3d 5/2 and Cs 3d 3/2 , the peaks are located at 724.2 and 738.09 eV, respectively.There are two strong peaks at around 530.1 and 539.46 eV, which are assigned to Sb 3d 5/2 and Sb 3d 3/2 .The marked peaks Br 3d 5/2 and Br 3d 3/2 are located at 68.36 and 69.4 eV, respectively.FigureS1a(Supporting Information) shows EDX spectrum of Cs 3 Sb 2 Br 9 NF.It is obvious that there are no other impurities except Cs, Sb, and Br elements.The scanning electron microscope (SEM) image of the single Cs 3 Sb 2 Br 9 NF with a side length of 50 μm, as visualized in FigureS1b(Supporting Information).Its corresponding elemental mapping is presented in FigureS1b(Supporting Information).The nanoflake has uniform distribution of elements with clear and sharp edges, indicating good crystallinity.Figure2ais SEM image of CdS NBs.It is found that the obtained CdS NBs are hundreds of microns in length.A magnification image of single CdS NB with a thickness of ≈40 nm is highlighted in the inset of Figure2a.Figure 2b envisages XRD pattern of CdS NBs.All diffraction peaks are in accordance with a hexagonal wurtzite structure [p63mc (186), JCPDS card No. 41-1049].Figure 2c denotes the atomic model of single CdS NB with hexagonal wurtzite structure.Raman spectrum of the CdS NBs is elucidated in Figure 2d.The results are detected at an excitation wavelength of 532 nm, with the five main Raman peaks located at 89, 212, 252, 301, and 600 cm −1 , respectively.294 and 586 cm −1 are the main peaks of the CdS powder raw materials.
, respectively.The couple peaks at 405.15 and 411.89 eV are attributed to Cd 3d 5/2 and Cd 3d 3/2 .While the strong peaks located at 161.79 and 162.97 eV correspond to S 2p 3/2 and S 2p 1/2 , respectively.The EDX spectrum and mapping of CdS NB are depicted in Figure S2 (Supporting Information).The peaks of both Cd and S elements are sharp, no other impurities were detected.Furthermore, the EDX mapping shows that the evenly elemental distribution of Cd and S in the dispersed CdS NB, suggesting that the prepared CdS NB is perfectly transferred without damage to surface and structure.It's also laying a foundation for the subsequent formation of hybrid photoelectric device with Cs 3 Sb 2 Br 9 NF.To investigate and compare optical properties of Cs 3 Sb 2 Br 9 NFs, CdS NBs, and hybrid CdS NBs/Cs 3 Sb 2 Br 9 NFs, the photoluminescence (PL) and UV-vis absorption spectrum are shown in Figure 3a,b respectively.In contrast to single Cs 3 Sb 2 Br 9 NFs and CdS NBs, the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NFs has two significant peaks, which agree with the PL peak of Cs 3 Sb 2 Br 9 NFs and CdS NBs, respectively.In the same way, the UV-Vis absorption of

Figure 1 .
Figure 1.Optical and AFM images, XRD, UPS, and XPS curves of 2D layered Cs 3 Sb 2 Br 9 NFs.a) Crystal structure of a Cs 3 Sb 2 Br 9 unit cell; b) 2D layered diagram; c) Optical image; d) XRD pattern; e) AFM thickness; f) UPS spectrum; g-i) XPS of Cs, Sb, and Br, respectively.
Figure 4a-d visualized schematic diagrams of single Cs 3 Sb 2 Br 9 NF device and hybrid

Figure 2 .
Figure 2. SEM, XRD, atomic model, Raman, and XPS spectra of CdS NBs.a) SEM image, inset: its thickness of single CdS NB; b) XRD pattern; c) Atomic model of wurtzite structure CdS NB; d) Raman spectrum; e,f) XPS of Cd, and S, respectively.

Figure 3 .
Figure 3. a) PL spectrum of Cs 3 Sb 2 Br 9 NF device, CdS NBs device and the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device; b) UV-vis absorption of Cs 3 Sb 2 Br 9 NFs, CdS NBs and the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF; c) Time-resolved PL decay and d) Wavelength-dependent responsivity of Cs 3 Sb 2 Br 9 NF device and the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device.

Figure 4 .
Figure 4. I-V curves of the Cs 3 Sb 2 Br 9 NF device and the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device.a) Schematic illustration; b) Cs 3 Sb 2 Br 9 NF device; c) Logarithmic curves of (b); d) Schematic illustration and optical image of four CdS NBs covered on the channel; e) the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device; f) Logarithmic curves of (e).

Figure 5 .
Figure 5. a) I-V characteristics, and b) Corresponding I-P curve of Cs 3 Sb 2 Br 9 NF device under 448 nm light irradiation.c) I-V characteristics, and d) Corresponding I-P curve of the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device under 448 nm light irradiation.

EQE = 1 .
04 × 10 3 %), respectively.Therefore, D* for the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device was estimated to be 1.47 × 10 15 Jones, as shown in Figure 6d, being 5.53 × 10 2 times as large as that of D* (2.53 × 10 12 Jones) of the single device in Figure 6b.It is found that the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device owns better photoelectric performance than single Cs 3 Sb 2 Br 9 NF device.Furthermore, the photoelectric properties of the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device under 510 nm light irradiation are also measured according to the spectral response of the hybrid device, as displayed in Figure S4 (Supporting Information).The results show that the photocurrent increases with increasing optical power density and the I-P fitting value () is 0.89.The corresponding R, EQE, and D* are 3.5 × 10 3 A W −1 , 8.5 × 10 5 %, and 1.25 × 10 15 Jones.Figure S6 (Supporting Information) plots the photocurrent of the original hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device and after storage in the air for one month, respectively.After one month, the photocurrent of the device drops to 88.23% of the original device, indicating well stability of the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device in the atmosphere.

Figure 7 .
Figure 7. a) Photoresponse property and b) Response speeds of Cs 3 Sb 2 Br 9 NF device under 448 nm light irradiation.c) Photorespontive property and d) Response speeds of the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device under 448 nm light irradiation.

Figure 8 .
Figure 8. a) Diagram of the energy band and the charge transfer between Cs 3 Sb 2 Br 9 NF and CdS NBs.b) Circumstance when the Cs 3 Sb 2 Br 9 NF and CdS NBs are not in contact.Operating circumstance of the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device under c) in dark and d) 448 nm light irradiation.

Table 1 .
Comparison of photoelectric properties of Cs 3 Sb 2 Br 9 NF combined with different amounts of CdS NBs.

Table 2 .
Comparison of key parameters for photodetectors based on perovskite, CdS, and 2D materials.
3 Sb 2 Br 9 NF device for violet-green band light detection by dry-transfer method.This method guaranteed the ≈90% coverage of CdS NBs to Cs 3 Sb 2 Br 9 NF so that the hybrid device possessed synergistic photoelectronic advantage.It is found that the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device displayed an outstanding photoelectric performance with an ultrahigh light/dark current ratio of 1.54 × 10 5 , responsivity of 4.13 × 10 3 A W −1 , large detectivity of 1.47 × 10 15 Jones, EQE of 1.14 × 10 6 %, and fast response speed (18.2/18.7 ms).More importantly, compared with single Cs 3 Sb 2 Br 9 NF device, the spectral response range of the hybrid CdS NBs/Cs 3 Sb 2 Br 9 NF device is noticeably broadened to 400-520 nm.These values are superior to those of CdS based hybrid photoelectric devices.Above all, the maximum contact area of the two materials by dry-transfer tactic made the hybrid CdS NBs/Cs 3 Sb 2 Br 9NF device exhibit an excellent photoelectric performance.The work provides a novel strategy for constructing high-quality CdS/perovskite heterostructures for highperformance violet-green band spectral detection, secure violetgreen optical communication, and imaging applications.