Bismuth sulfide quantum dots-CsPbBr3 perovskite nanocrystals heterojunction for enhanced broadband photodetection

Colloidal semiconductor nanocrystals (NCs) or quantum dots (QDs) have shown great potential for solution-processable photodetector due to their exceptional optical and electronic properties. However, broadband and sensitive photodetection from single QDs- based devices is quite challenging. Nano-heterojunction with proper band alignment based on two different materials offers significant advantages for developing broadband photodetector. Herein, we report ultraviolet–visible (UV–Vis) to near-infrared (NIR) light-responsive photodetector based on solution-processed nano-heterojunction of visible light absorber CsPbBr3 perovskite NCs and wide absorption range, environment-friendly Bi2S3 QDs. Our results demonstrate that the CsPbBr3–Bi2S3 nano-heterojunction-based photodetector has higher responsivity (380 μA/W at a wavelength of 532 nm) and higher specific detectivity (1.02 × 105 Jones), as compared to the individual CsPbBr3 or Bi2S3 QDs based devices. Interestingly, the detection wavelength range of our heterojunction device is further extended to the near-infrared region (1064 nm) due to the broadband absorption range of Bi2S3 QDs, which is not observed in the visible light absorber CsPbBr3 devices. Remarkably, the responsivity of the heterojunction device is 90 μA W−1. The enhanced specific detectivity and the broadband response of hybrid devices are attributed to the improved charge carrier generation, efficient charge separation and transfer at the interface between CsPbBr3 and Bi2S3 QDs.


Introduction
Photodetectors, known for their ability to convert optical light into electrical signals, find diverse applications in both industrial and scientific domains, encompassing environmental pollution monitoring, sensing, and water sterilization [1][2][3].Some commercially available photodetector based on Si, GaAs, GaN etc, offers advantages such as high sensitivity, low noise, and compatibility with existing semiconductor manufacturing processes.They are optimized for specific wavelength ranges, allowing for various applications, including communication, sensing, imaging and night vision devices [4][5][6][7].In three-dimensional (3D) materials, the transport of photogenerated carriers (electrons and holes) can be relatively slow due to the longer distance they need to travel, this can result in longer response time and lower sensitivity in photodetection applications [8].To mitigate these limitations, low-dimensional materials have been focussed on as the extraordinary properties of these materials render them highly promising for various applications in electronics, energy, catalysis and medicine [9,10].In the past decade, quantum dots (QDs) have received enormous research interest due to their unique size, shape, and compositional dependent electronic and optical properties, which open the avenue for the next generation of optoelectronic devices.Owing to their unique solution processibility, tunable optical and electronic

Synthesis of CsPbBr 3 QDs
The synthesis of CsPbBr 3 QDs is carried out in a complete argon environment using a Schlenk line system reported previously.At a temperature of 90 °C, the mixture containing 10 ml of 1-octadecene (ODE) mixed with cesium carbonate and lead acetate was degassed for 30 min in a round-bottom flask with a three-neck and a water condensation system.Then, 0.6 ml of oleic acid (OA) and 2 ml of oleylamine (OAm) were added while the system was degassed continuously under argon at 130 °C for 60 min.Under the argon environment, the temperature was set to 170 °C.Upon reaching a temperature of 170 °C, the mixture received an instantaneous addition of 0.4 ml of benzoyl bromide.Subsequently, the blend was rapidly cooled within 10 s by immersing it in an ice bath and maintaining constant stirring.This was done until the colour changed to a yellowish green [22,30,31].

Synthesis of Bi 2 S 3 QDs
Bismuth and sulphur precursors are bismuth neodecanoate and thioacetamide, respectively.The bismuth neodecanoate was dissolved in OA and ODE by stirring at 165 °C with a continuous flow of argon gas through the standard Schlenk line technique, giving a colourless, transparent solution.When thioacetamide was injected into the reaction mixture, it produced a dark brown solution of the hot reaction mixture.This reaction mixture is left at 105 °C for the growth of quantum dots for 60 s.A continuous argon purging process is employed within the reaction flask to guarantee the utmost purity of QDs.Once the reaction mixture was cooled down to room temperature, the solution was washed two times with ethanol, and then the QDs were dispersed in toluene [17,32].

Device fabrication
The substrates, n-doped Si with SiO 2 coating with gold contact, were cleaned by ultrasonication in deionized water, acetone and isopropyl alcohol (IPA).A multi-layered spin-coated thin film was fabricated by solid-state ligand exchange treatment using layer-by-layer deposition [33].The coating was done at 1500 rpm for 30 s using 50 μl of dispersed Bi 2 S 3 QDs.The ligand exchange process was done by applying 400 μl of tetrabutylammonium bromide solution onto each deposited layer, followed by soaking for 10 s and spin coating at 3000 rpm for 30 s.Each treated film was rinsed using IPA, then toluene, and then spin-coated at the same parameters.It is annealed for 15 min at a temperature of 120 °C, which remove the solvent residue, thereby improving the contact between different layers.After that, a second layer of Bi 2 S 3 was formed using the same steps to increase the thickness of the layer.After this layer, CsPbBr 3 was deposited by spin coating at 1000 rpm for 30 s, and annealing was done for 5 min at 100 °C.The observed absorption edge for CsPbBr 3 quantum dots shown in figure 2(a), was 500 nm, with a very sharp absorption, whereas Bi 2 S 3 quantum dots have the absorption coefficient beginning at 1200 nm and continuing onwards until 400 nm.The spectrum becomes broad, ranging from 400-1200 nm, once the Bi 2 S 3 /CsPbBr 3 heterojunction between the two materials is formed, which is due to co-absorption of CsPbBr 3 and Bi 2 S 3 films.It also allowed us to see an enhancement in the overall absorption intensity due to the material's direct physical superposition [21,35].The absorption spectra of heterostructure show the characteristic of Bi 2 S 3 with CsPbBr 3 absorption edge with decreased sharpness, which indicates the quenching arising from a supposed energy transfer, as mentioned in [21], from perovskite to the Bi 2 S 3 QDs, which is an ultrafast and high-efficiency process.As the illumination region shifted to the near-infrared region, the only observed photocurrent was from Bi 2 S 3 due to the charge carriers it generated.It also showed that CsPbBr 3 acted as a hole transport material rather than a charge carrier donor [36].According to the Tauc plot analysis, CsPbBr 3 displays a direct band gap of 2.4 eV, whereas the band gap for the Bi 2 S 3 /CsPbBr 3 heterojunction is measured at 2.3 eV.The reduction in band gap in the case of heterostructure occurs due to the emergence of distinct states within the bandgap, facilitating the migration of electrons from the valence bands to the newly formed states in the gap region [13,37].It is important to note that the individual band gap of Bi 2 S 3 is lower, at 1.5 eV, as shown in supplementary figure S1.The photophysical investigation was done using steady-state as well as time-resolved fluorescence spectra.At 504 nm, the steady-state photoluminescence (PL) spectra show a peak for pristine CsPbBr 3 QDs and heterostructure, but quenching of large magnitude is observed in the heterostructure in comparison to CsPbBr 3 , as shown in figure 2(b).This is attributed to the extremely efficient charge transfer due to the successful superposition in the heterostructure, which shows large amount of quenching in intensity [21].

Result and discussion
Time-resolved PL spectra were used to study the excited state charge transfer process, as shown in figure 2(c).After fitting all the profiles of PL decay by DAS 6 software using the tri-exponential functions, equation (1a), shows (α 1 , α 2 , α 3 ) as the relative amplitude and time constant (τ 1 , τ 2 , τ 3 ) showing the decay times, respectively.
The decay profiles were fitted with three parts of the lifetimes, which were assigned as the fast decay (τ 1 ), intermediate decay (τ 2 ), and slowest decay (τ 3 ).Detailed parameters of time-resolved PL decay from triexponential fitting for pristine and Bi 2 S 3 /CsPbBr 3 heterojunction are shown in supplementary table S7.As per previous reports, fast decay component is linked to radiative recombination from surface states, while the moderate and slowest decay components are associated with donar-acceptor recombination and conduction band to acceptor recombination, respectively [38].The presence of a fast decay component indicates the defects within the materials, which act as non-radiative recombination centres.However, CsPbBr 3 has more surface states or recombination centres that lead to quicker decay than Bi 2 S 3 /CsPbBr 3 , because in case of heterostructure, there will be surface defect passivation due to the Bi 2 S 3 layer.From the supplementary table S7, it can be seen that, values of intermediate decay (τ 2 ), and slowest decay (τ 3 ) decrease in case of heterostructure which represent supress radiative recombination and the average lifetime decreases from 4.76 ns to 3.65 ns, calculated from the formula (1b).Reduction in PL lifetime and PL quenching in the case of heterostructure represents the electron transfer from the excited state of CsPbBr 3 to Bi 2 S 3 [39].
The arrangement of layers in heterostructure is such that photons first fall on the CsPbBr 3 layer, then transfer to the Bi 2 S 3 layer, forming the bipolar carrier transport layer [40].This type of heterostructure arrangement can absorb light most effectively.Under light excitation, the photon falls on the CsPbBr 3 , so there will be an ultrafast energy transfer, and most of the exciton generated in this layer will transfer to the next layer, the Bi As the CBM and VBM of Bi 2 S 3 are respectively lower and higher than those of CsPbBr 3 , the built-in electric field is generated due to the differences in energy band alignment between the two different semiconductor materials that make up the junction, leading to charge redistribution.Overall, the formation of excitons at the junction is driven by the built-in electric field, both electrons and holes will experience a driving force towards Bi 2 S 3 [35,41].Now, as the energy of the photons is higher than the band gap of CsPbBr 3 , these will get absorbed in the CsPbBr 3 layer and create excitons.These excitons will migrate towards the Bi 2 S 3 under the established driving force at the interface.However, when the band is bent downward, the holes in the CsPbBr 3 layer are confined to the valence band due to the formation of a potential barrier shown in figure 3(c).Thus, the heterojunction improves device performance by promoting charge transfer and reducing carrier recombination.Under external bias, the carriers can be effectively collected by the external circuit, thereby enhancing the photoconductivity [35].The phenomenon above is restricted to the UV and Vis spectra, wherein photon energies surpass a threshold that allows absorption by CsPbBr 3 .Both materials can absorb the higher energy spectrum of light simultaneously and can actively increase the photo-generated carriers, leading to greater photoconductivity.
Conversely, for photons having lower energy than the band gap of CsPbBr 3 , CsPbBr 3 film will no longer act as an absorbing layer.The photons will pass through this layer and get absorbed by the Bi 2 S 3 layer.The excited electrons in Bi 2 S 3 cannot enter the CsPbBr 3 layer through photo-thermionic emission due to a barrier at the interface, shown in figure 3(d).But at the same time, the photo-generated holes in the Bi 2 S 3 valence band would transfer to the CsPbBr 3 valence band via thermionic emission, dissociating the excitons in the Bi 2 S 3 layer.The promoted excitons dissociation and charge carriers transfer contribute to reducing carrier recombination and enhanced optical parameters like responsivity and fast response speed [41].
Figure 4 shows the device's current-voltage (I-V) characteristics of the device in the voltage range of −15 to 15 V under dark and light illumination.The fabricated device's ohmic behaviour was revealed by I-V analysis, with a very low dark current resulting from the edge contacts.From the I-V measurement, it can be deduced that this heterostructure will show photodetection capabilities.When photons from a laser source fall on a heterostructure, there will be a generation of charge carriers, which cause an increase in photocurrent in accordance with the intensity of the incident photons [43,44] The key parameters of the fabricated PDs, such as responsivity (R), noise equivalent power (NEP), specific detectivity (D * ), and quantum efficiency (η) were determined using the equations (2)-( 5) respectively [45].
The power of a signal at which its signal-to-noise ratio is the attribution of unity in the output bandwidth of one hertz is given by the noise equivalent power (NEP), The inverse of NEP Signals is defined as specific detectivity, The Quantum efficiency (η) of the fabricated PDs has been calculated by using the following relation, Where I ph , P d , and A o are the photocurrent, power density, and active area of the fabricated device, the value of the active area is 15.71 × 10 −4 cm 2 , I d is the dark current, e is the elementary charge, h is Plank's constant, c is the speed of light, λ is the incident photon wavelength of the illuminating source.
Figures 5(a)-(c) shows the power-dependent photocurrent for the device in the UV-NIR region.For each on/off cycle, the device shows a change in photocurrent in the power range of 0.15-2.35mW.When a large number of photons interact with the material, a large number of charge carriers are excited in the conduction band, so there will be a high current.The highest I ph of 103 nA was observed at 2.35 mW, which reduces to 59 nA at 0.15 mW for the laser at 532 nm, as shown in figure 5  It is clear from the data that, with bias, there is an increase in I ph for different wavelength lasers.The I ph value for the visible region was 38 nA at 5 V, which increased to 107 nA at 15 V.This is because the carrier drift velocity increases at a high bias voltage, and the photoelectric current is amplified quickly by several orders of magnitude [44].Photo-response (I-t) measurements for the pristine device, Bi 2 S 3 and CsPbBr 3 , are shown in supplementary figures S7 and S8 respectively.Photo-response speed determines the capability of a photodetector to follow a fast-switching optical signal.The time that photocurrent takes to reach the maximum value of 90% from 10% and then to return from 90% back to 10% of the maximum value is the rise and fall time, respectively.Thus, the rising and falling time of Bi 2 S 3 /CsPbBr 3 heterostructuresbased PD is 73.82/62.68ms, 132/65 ms, and 380/343 ms for the UV, Vis, and NIR regions, respectively as shown in supplementary figure S6.Response speed increases as we move UV to Vis region, due to heat created by UV light in the device and working of photodetector affected which increase the response time.Further when we move from Vis to NIR region, only Bi 2 S 3 act as an absorbing layer, there will be less amount of photocurrent generated, shows slow response and there is an increase in response time and rise and fall time with pristine devices (CsPbBr 3 and Bi 2 S 3 ) shown in supplementary table S3.
Figure 6 shows devices' specific detectivity and responsivity of devices with varying power and voltage.Specific detectivity and responsivity decrease as incident power increases.A high D * and R value corresponds to the traps of minority carriers, such as holes under weak light illumination at low optical power [46].It makes the holes incompetent to recombine with the electrons and to make them able to recombine, they must be excited again by the valance band.This process is similar to lengthening the carrier's recombination lifetime, corresponding to high D * and R value.The determined values of specific detectivity for the UV, Vis, and NIR regions at lower power are 0.

Conclusion
In summary, the heterojunction of Bi 2 S 3 QDs and CsPbBr 3 perovskite NCs was successfully fabricated and studied for a broad range of spectra.CsPbBr 3 NCs and Bi 2 S 3 QDs were synthesized using the hot injection method and characterized by x-ray diffraction and transmission electron microscopy.Various optical excitation wavelengths (355 nm, 532 nm, 1064 nm) were employed to examine the device's power and voltage-dependent photo-response, responsivity, specific detectivity, and response time of the device.The outcomes of this study showcase that CsPbBr 3 -Bi 2 S 3 nano-heterojunction exhibits enhanced performance characteristics.Notably, it shows elevated responsivity (380 μA W −1 ) and improved specific detectivity 1.02 × 10 5 Jones at the wavelength of 532 nm, surpassing the capabilities of individual CsPbBr 3 and Bi 2 S 3 devices.Remarkably, the responsivity and detectivity of the nano-heterojunction device is 90 μA W −1 and 0.06 × 10 5 Jones at 1064 nm wavelength which is not observed in the visible light absorber CsPbBr 3 devices.We anticipate that this work could provide an efficient approach to constructing heterojunction between NIR absorbing Bi 2 S 3 QDs and visible light absorber CsPbBr 3 perovskite NCs to achieve broadband photodetector, which can be promising for wide spectral range photodetection applications such as image sensing and optical communication.and AcSIR, India, for PhD registration.The author would thank Geetanjali Calley (Technical officer) of CSIR-NPL, New Delhi, for XRD.

Figure 3 (
a) depicts the device fabrication process with steps.The thickness of the Bi 2 S 3 QDs and CsPbBr 3 films is 30 and 40 nm, respectively, shown in supplementary figure S5.

Figure 1 .
Figure 1.XRD patterns of purified (a) CsPbBr 3 QDs (b) Bi 2 S 3 QDs with corresponding JCPDS file.High-magnification TEM image of (c) CsPbBr 3 QDs with HRTEM image having lattice spacing of 0.29 nm (d) Bi 2 S 3 QDs with HRTEM image with a lattice spacing of 0.35 nm.
Figure 1(c) shows, a high-magnification TEM image of CsPbBr 3 QDs, without any sign of aggregation being clearly visible, along with an HRTEM image in the inset, which exhibits lattice fringes of exceptionally crystalline nanocrystals featuring a lattice spacing of 0.29 nm, corresponding to the (200) plane of the monoclinic CsPbBr 3 QDs.

Figure 1 (
d) shows a high-magnification TEM image of spherical Bi 2 S 3 QDs, inset showing lattice fringes with a lattice spacing of 0.35 nm associated with the (310) plane of the Bi 2 S 3 QDs.Low-magnification image with a size distribution histogram, revealing a remarkably narrow size distribution with an average size of 8.5 ± 0.5 nm for CsPbBr 3 and average size 3.9 ± 0.5 nm for Bi 2 S 3 QDs respectively shown in supplementary figure S2.
2 S 3 layer.The device's configuration and electrode structure are shown in supplementary figure S4, and the channel area is 15.71 × 10 −4 cm 2 [21].The working mechanism of the device can be understood by the energy band diagram of the Bi 2 S 3 /CsPbBr 3 heterostructure, as shown in figures 3(b)-(d).For CsPbBr 3 , the energies of the conduction band minimum (CBM) and valence band maximum (VBM) are −3.6 and −5.9 eV [21, 41] respectively, while the CBM and VBM energies for Bi 2 S 3 are −4.2 and −5.7 eV [42] shown in figure 3(b).
. I ph values of Bi 2 S 3 /CsPbBr 3 heterostructurebased broadband PDs under different optical excitation sources with different power are shown in supplementary table S1.I ph values of pristine and Bi 2 S 3 /CsPbBr 3 heterostructure-based broadband PDs at the highest optical power are shown in supplementary table S2.Photo-response (I-t) measurements were performed for repeated on/off cycles (10 s each) with the Keithley instrument 2450 under 355, 532, and 1064 nm laser illuminations for the UV, Vis, and NIR regions, respectively.Power and voltage-dependent characteristics of the fabricated device are shown in figure 5.The value of I ph increases with increasing power (0.15-2.35 mW) and voltage (5-15 V).

Figure 5 (
a), shows that the I ph value for UV light was 80 nA at 2.35 mW and 39 nA at 0.15 mW.With an NIR light illumination device showing the same pattern as the UV and Vis regions with low photocurrent, an I ph value of 30 nA was obtained at 2.35 mW, while 14 nA was obtained at 0.15 mW, as shown in figure 5(c).Figures 5(d)-(f) shows voltage-dependent photo-response for a Bi 2 S 3 /CsPbBr 3 device in the UV-NIR region, where voltage-dependent photocurrent is observed, which possesses good optical switching behaviour.
43 × 10 5 , 1.02 × 10 5 and 0.06 × 10 5 Jones, respectively, and the determined values of responsivity for the UV, Vis, and NIR regions at the lowest power region are 260 μA W −1 , 380 μA W −1 , and 90 μA W −1 , respectively.A comparison table of R and D * of Bi 2 S 3 /CsPbBr 3 heterostructure-based broadband PDs with pristine materials under different optical excitation sources at the lowest power is shown in supplementary tables S4 and S5.Quantum efficiency (η) of Bi 2 S 3 /CsPbBr 3 heterostructure-based broadband PDs with pristine materials under different optical excitation sources at the lowest power is shown in

Figure 5 .
Figure 5. Time-dependent photo-response of the Bi 2 S 3 /CsPbBr 3 PD at different power with the illumination source (a) 355 nm, (b) 532 nm, and (c) 1064 nm.Time-dependent photo-response of the Bi 2 S 3 /CsPbBr 3 PD at different voltage with the illumination source (d) 355 nm, (e) 532 nm, and (f) 1064 nm.

Figure 6 .
Figure 6.Power dependent parameter of the device (a) Specific detectivity (b) Responsivity.voltage-dependent parameter of the device (c) specific detectivity (d) responsivity.