Solution processed CuInS2/SnO2 heterojunction based self-powered photodetector for UV encrypted visible light communication

A photodetector (PD) featuring dual-band detection capability and self-powering attributes is crucial for various applications in sensing, communication, and imaging. Here, we present a self-powered PD based on a solution-processed CuInS2/SnO2 heterojunction capable of detecting ultraviolet (UV) and visible light spectra. The CuInS2 layer was composed of ∼2 nm-sized quantum dots (QDs) synthesized using the hot injection method, while the SnO2 layer was fabricated using a straightforward sol-gel technique. This self-powered PD displayed a significant spectral response across both UV (355 nm) and visible light (532 nm) ranges, all accomplished without the need for external bias. The PD demonstrates rapid detection, with rise and decay times of 125 ms and 156ms for visible light and 85 ms and 200 ms for UV light, respectively, at a power level of 15 mW. The PD achieved responsivity values of 10.66 μA/W and 34.56 μA/W for visible and UV light, respectively. The impressive capability for dual-band detection in both ultraviolet (UV) and visible light showcases the practical feasibility and utility of this device for self-powered photodetection and deciphering UV-encrypted visible light communication. Moreover, its straightforward solution-based processing attribute renders it valuable for the mass production of devices and technology.


Introduction
Photodetectors (PDs) are devices designed to identify or sense light and electromagnetic radiation by converting the light signal into an electrical counterpart.They have diverse applications in measurement and detection, remote sensing, optical communication systems, and environmental monitoring, among others [1,2].PDs that can detect light without the need for external bias are commonly known as self-powered PDs [3].The nonreliance on external power, such as batteries, results in the development of compact, lightweight, portable, and energy-efficient devices.The operational principles of self-powered PDs hinge on the photovoltaic effect exhibited by semiconductors.In essence, semiconductors generate electron-hole pairs (excitons) in response to photon illumination.The photocurrent arises as a result of the separation of these photogenerated electrons and holes, accompanied by the directed movement of the photogenerated electrons [4].In p-n junction PDs, the separation of photogenerated electron-hole pairs is facilitated by the built-in electric field (E b ) [4,5].E b can be established in heterogenous junctions due to the Fermi level difference between the semiconductors.This differential drives the separation and transmission of photogenerated carriers within the junction leading to the generation of photocurrent in the external circuit.
Conventional broadband PDs predominantly rely on single-crystalline semiconductors such as Si, Ge, InGaAs, and GaN [6,7].These bulk materials face challenges such as limited band gap tunability, intricate manufacturing processes, low operational temperature, and elevated driving voltage.Quantum dot-based PDs offer a potential solution to address these issues.QDs stand out from traditional bulk materials due to their ability to absorb light across a wide spectrum, extending from ultraviolet (UV) to mid-infrared wavelengths possessing a high absorption coefficient along with tunable optoelectronic properties and compatibility with flexible substrates [8][9][10].Furthermore, QDs-based devices can be mass-produced through solutionprocessable methods at lower temperatures, leading to significant cost reductions.Nevertheless, current QDbased PDs typically involve materials like CdS, PbS, PbSe, CdSe, and CsPbBr 3 which incorporate toxic heavy metals like Pb and Cd, leading to significant safety and toxicity issues [11][12][13][14].Extensive endeavors have been made to engineer heavy-metal-free QDs possessing exceptional optoelectronic properties to enhance the efficiency of PDs [15].Among various QDs, copper indium sulfide (CuInS 2 ) QDs hold notable attraction for their utilization in optoelectronic devices such as light-emitting diodes, solar cells, and photodetectors [16][17][18][19].
Possessing attributes such as a customizable direct band gap (1.5 to 2 eV) across the visible and near-infrared spectra, along with low toxicity, cost-effectiveness, a high absorption coefficient, and superb emission tunability, they represent environmental friendly, secure, and promising candidate [20][21][22][23].Various methodologies and device configurations have been employed to investigate the application of CuInS 2 QDs in PDs [24].To fabricate the nanostructured CuInS 2 film, different methodologies including the solvothermal method combined with sputtering or solid-gas reaction, chemical bath deposition, and spin-coating of colloidal QDs have been reported [25][26][27][28].Among these, spin-coating colloidal QDs emerge as the most straightforward, cost-effective, and lowtemperature processable technique, with the added advantage of facilitating mass production of devices.Furthermore, synthesizing colloidal CuInS 2 QDs via the hot injection method enables easy tuning of the material's optical and electronic characteristics [29].In this study, we focused on implementing a self-powered PD comprising solution-processed CuInS 2 QDs film.To attain the self-powered characteristic in CuInS 2 -based PDs, it is essential to incorporate a material capable of forming a type II heterojunction with CuInS 2 .In a type II heterojunction, one semiconductor exhibits relatively higher conduction and valence band energies compared to the other semiconductor [30].This enables efficient carrier separation and directed transmission, resulting in self-powered characteristics in type II heterojunction-based PDs [31][32][33][34][35]. Additionally, customizing the semiconductor properties within this heterojunction enables the broadband detection [32,36].When considering CuInS 2 , options such as TiO 2 , ZnO, and SnO 2 present themselves as viable materials for constructing a type II heterojunction [33,37,38].These materials not only facilitate the fabrication of such heterojunctions but also aid in electron extraction and transport.Among these materials, SnO 2 stands out as the sole candidate possessing a combination of advantageous traits including low-temperature processability, high electron mobility (200 cm 2 /V-sec), and a surface free from -OH groups resulting in increased stability for the adjacent materials [39][40][41].SnO 2 is an n-type wide band-gap (∼3.6 eV) semiconductor and capable of forming a type II heterojunction when combined with the p-type CuInS 2 [42,43].Additionally, SnO 2 is utilized as an electron-transport layer in photovoltaic applications due to its high electron mobility, superior transmittance (90%), and favorable energy level alignments [39,40].The SnO 2 /CuInS 2 heterojunction PD can achieve photoresponse across the UV to visible spectrum.The built-in electric field can effectively facilitate the separation and transmission of photogenerated carriers at the interface, allowing for broad-spectrum photo-response without requiring external bias [44].
In this study, we fabricated FTO/SnO 2 /CuInS 2 /Au structure via solution-processed methods for PD application.For this, SnO 2 film was fabricated via a sol-gel process.CuInS 2 QDs were synthesized via the hot injection method and were used as a precursor to employ the CuInS 2 film.CuInS 2 QDs were characterized using the HRTEM, and the SnO 2 and CuInS 2 films were characterized using the XRD for structural properties.The SnO 2 /CuInS 2 heterojunction was characterized with atomic force microscopy (AFM) for morphology and with UV-visible absorbance and photoluminescence for the optical properties.The fabricated FTO/SnO 2 /CuInS 2 /Au structure shows the self-powered photodetection properties for the visible (532nm) and UV (355nm) spectrum of light with different powers (1 mW, 7 mW, and 15 mW).The different photodetection parameters like the rise time, decay time, sensitivity, responsivity, detectivity, and noise equivalent power (NEP) were calculated.Further, the device has been studied for its utility in decrypting UVencrypted visible light communications.

Synthesis of CuInS 2 QDs
CuInS 2 QDs were prepared under an inert atmosphere using a hot injection method, following a previously reported procedure [29,45].First, a mixture containing 464 mg of indium acetate, 76 mg of copper iodide, 1 ml of dodecanethiol, and 10 ml of 1-octadecene was prepared in a 50 ml four-neck flask fitted with a water condenser.This mixture was stirred at 120 °C for 30 minutes while argon gas was purged into the flask.Subsequently, 0.5 ml of oleic acid was incorporated into the mixture using an injection, and the process of continuous degassing with argon gas flow was maintained for an additional 30 minutes.Afterward, the temperature of the flask was elevated to 210 °C, and the reaction was allowed to progress for 30 minutes until a deep red colloidal solution formed.The QDs were precipitated by the addition of acetone to the solution.Following centrifugation at 5000 rpm, the precipitate was collected and dispersed in toluene.The QDs were purified through successive cycles of dispersion in toluene followed by precipitation with acetone.Finally, the QDs were dispersed in 4 ml of anhydrous toluene.
2.3.Preparation of SnO 2 precursor 0.15 mmol (i.e.33.75 mg) of SnCl 2 •2H 2 O was dissolved in 1 ml of ethanol.This solution was stirred for 3 h at 60 °C and then aged for 24 h.

Fabrication of device
The device structure of FTO/SnO 2 /CuInS 2 /Au with CuInS 2 QDs an active layer was fabricated according to the following procedure.At first, 2 mm-wide strips of FTO were etched onto the FTO-coated glass.The patterned fluorine tin oxide (FTO) coated glass substrate underwent cleaning with detergent, deionized water, acetone, and isopropanol in an ultrasonic bath, and then dried in a vacuum oven.Subsequently, the SnO 2 precursor was spin-coated onto clean FTO substrates at 3000 rpm for 30 seconds, followed by annealing at 180 °C for 60 minutes on a hot plate to form the SnO 2 layer.For applying the CuInS 2 active layer onto the SnO 2 film, the dispersion solution containing CuInS 2 QDs in toluene was spin-coated at 2000 rpm for 30 seconds.Following this, the active layer was annealed at 120 °C for 10 minutes on a hot plate.Finally, a 100 nm thick and 2 mm wide strip of Au as the top electrode was thermally evaporated under the pressure of 5 × 10 −6 mbar.The Au strip was deposited to align at a right angle with the bottom FTO strip electrode, determining the size of the fabricated device as 2 mm × 2 mm.

Characterization and measurements
Characterization techniques utilized included x-ray diffraction (XRD) using a Rigaku Model ULTIMA-IV, operated at 40 kV/40 mA.UV-Vis spectroscopy was conducted using a SHIMADZU UV-Vis Spectrophotometer Model 2401PC.The AFM analysis was conducted by the NT-MDT Solver pro instrument.Field emission scanning electron microscopy (FESEM) analysis employed a TESCAN MAGNA GMH instrument.I-V characterization was performed using a Keithley source meter 2450, and impedance analysis was conducted with a HIOKI IM 3536 LCR Meter.For photodetection, the monochromatic lasers of 355 nm and 532 nm were utilized.

Results and discussion
The structural characteristics of the deposited films of SnO 2 and CuInS 2 were studied via x-ray diffraction (XRD).Figures 1(a The CuInS 2 QDs obtained were analyzed using high-resolution transmission electron microscopy (HRTEM).The resulting micrographs are presented in figures 2(a) and (c), depicting lower and higher resolutions, respectively.Additionally, the particle size distribution is illustrated in figure 2(b), revealing that the QDs exhibit sizes ranging from 1.2 to 3.7 nm, with an average size of 2.1 ± 0.5 nm.The high-resolution micrograph displays the lattice fringes pattern as depicted in the inset of figure 2(c).The spacing between the lattice fringes is found to be 0.32 nm which corresponds to the (112) plane of crystalline CuInS 2 QDs. Figure 2(d The SnO 2 and the CuInS 2 /SnO 2 structured films were also characterized through atomic force microscopy (AFM) for the surface morphology.Figures 3(a   This can be attributed to the non-radiative relaxation of excitons via an alternative decay pathway induced by the interface of CuInS 2 with SnO 2 .Time-resolved photoluminescence (TRPL) analysis was also performed for both CuInS 2 and CuInS 2 /SnO 2 heterojunctions.The results, illustrated in figure S3 (supplementary information), reveal lifetimes of 80.7 ns for CuInS 2 and 77 ns for CuInS 2 /SnO 2 at the 465 nm peak.This decrease in lifetime aligns with the non-radiative relaxation of carriers.
Figure 5 shows the schematic of the photodetection devices with an FTO/SnO 2 / CuInS 2 /Au structure.Figure 6(a) shows the I-V behavior of the FTO/SnO 2 /CuInS 2 /Au device in the dark and under the illumination of visible light (532 nm) with a power of 1 mW, 7 mW, and 15 mW.notable leakage current within the device.This leakage can be ascribed to the uneven thickness of both the SnO 2 and CuInS 2 layers, as evidenced in the AFM micrographs illustrated in figure 3, potentially creating pathways for current leakage.Additionally, defects within the material also provide avenues for leakage current, further contributing to the deviation of the device's IV curve from ideal rectification behavior.The IV behavior of the device also shows that the current in the device is not coinciding with zero at zero bias under the illumination of the light as shown in the insets of figures 6(a) and (b).The current also increases with the increment of the power of the incident light.This shows the self-powered behaviour of the device.The device's spectral response at zero bias was also measured in the UV-visible range (300-800 nm) and is depicted in figure S5 (supplementary information).The test conducted within the wavelength range of 300-800 nm demonstrates that the device can detect both UV and the entire visible spectrum of light.Moreover, the photocurrent response shows an increasing trend as the spectrum transitions from the visible to the UV side.
To study the self-powered photoresponse behavior, the device has been tested for photocurrent at no bias voltage under the illumination of the wavelengths of 532 nm (visible light) and 355 nm (UV light) with different powers of 1 mW, 7 mW, and 15 mW.The photocurrent has been measured for the on/off time of 10 seconds for each light source.Figure 7(a) shows the photocurrent characteristic of the device at no bias voltage for visible light (532 nm) with different incident powers.As the power is increased the photocurrent is also increasing. Figure 7(b) shows the photocurrent characteristics with UV light (355 nm) illumination of different incident powers and under no bias voltage conditions.In this too, the photocurrent is increasing in the same pattern i.e. the photocurrent increased as the incident power increased.
The photodetector's various parameters were determined based on the photocurrent behavior.For incident powers of 1 mW, 7 mW, and 15 mW, the rise times for visible light are 130 ms, 130 ms, and 125 ms, while the corresponding decay times are 170 ms, 152 ms, and 156 ms, respectively.Additionally, for the same incident powers, the rise times for UV light are 128 ms, 97 ms, and 85 ms, with decay times of 210 ms, 237 ms, and 200 ms, respectively.Figures 8(a) and (b) illustrate the rise and decay times corresponding to a 15 mW incident power for visible and UV light, respectively.Different parameters of PD like the sensitivity (S), Responsivity (R), NEP, detectivity (D) and quantum efficiency (η) have also been calculated for the different powers of the visible and UV light and are summarized in table S1 (supplementary information).These parameters are defined as [47]: where I L and I D are current under the dark and light conditions respectively.I ph = (I L -I D ) is the photocurrent, A 0 is the active area of the device, e is the charge of the electron, h is the plank's constant, c is speed of light, λ is the wavelength of the incident light and P inc is the incident power.The sensitivity, NEP, responsivity, and detectivity for the different powers of the visible and UV light are depicted in figure 9.The sensitivity of the device is depicted in figure 9(a).At different incident power levels of visible light (1 mW, 7 mW, and 15mW), the sensitivity is measured as 24%, 79%, and 104% respectively.For the same level of power in UV light, the sensitivity is measured as 84%, 106%, and 134% respectively.The responsivity of the device is depicted in figure 9(b).The device's responsivity to visible light, measured at power levels of 1 mW, 7 mW, and 15 mW, is determined to be 10.66 μA/W, 5.36 μA/W, and 3.17 μA/W, respectively.Similarly, for UV light at equivalent power levels, the responsivity is observed to be 34.56 μA/W, 6.62 μA/W, and 4.15 μA/W.The NEP of the detector lies in 10 −9 W/ Hz as illustrated in figure 9(c).The detectivity of the device has also been calculated and illustrated in figure 9(d).The quantum efficiency of the device has also been calculated and depicted in    maxima (VBM) energies of both materials results in the formation of a type II heterojunction, as depicted in figure 10(a).Due to the Fermi level energy (E F ) being higher in the n-type SnO 2 compared to the p-type CuInS 2 , electron migration from SnO 2 to CuInS 2 and hole migration from CuInS 2 to SnO 2 occur at the interface to maintain equilibrium.This creates a band bending at the interface, forming a built-in electric field (E b ) from the n-type to the p-type semiconductor, as illustrated in figure 10(b).This facilitates the movement of photogenerated electrons from CuInS 2 to SnO 2 and photo-generated holes from SnO 2 to CuInS 2 .When photons enter the CuInS 2 /SnO 2 heterojunction after passing through the FTO, these create excitons (hole-electron pairs) within the material.The separation of these excitons at the junction and their migration to the respective electrodes generate photocurrent in the device.In the case of visible light, photogenerated excitons in CuInS 2 contribute to the photocurrent, as the absorbance of SnO 2 is limited to the UV region.However, for UV light, photogenerated excitons in both materials contribute to the photocurrent.
The device was also exposed to visible light(532 nm, 3 mW) UV light (355 nm, 15 mW), and mixed UVvisible light during the transient measurement at no biasing voltage as shown in figure 11.The results indicate a reduced photocurrent in response to visible light, an elevated photocurrent for UV light, and a surpassing of both photocurrent levels when exposed to mixed UV-visible light.These findings underscore the device's ability to differentiate and respond to each condition through changes in photocurrent.This behavior can be utilized in decoding the visible light communication encrypted with UV light [33,50,51].In this encrypted communication, distinct states such as 00, 01, 10, and 11 can align with conditions where no light is present, visible light is active, UV light is active, and both lights are concurrently active, respectively, as depicted in figure 11.As the device effectively identifies each condition through precise changes in photocurrent, it proves to be well-suited for serving as a decryption device in communications of this nature.
The device underwent impedance spectroscopy, wherein impedance measurements were conducted across a frequency range of 10 Hz to 1 MHz at 0V level.The frequency versus impedance plot is illustrated in figure 12(a).Additionally, the real impedance (Z') versus imaginary impedance (Z") plot, commonly known as the Cole-Cole plot, is presented in figure 12(b).To comprehend the device's behavior, the Cole-Cole curve was fitted using Z-simpwin software.The curve fitting revealed a circuit behavior resembling three RC components in series, accompanied by a contact resistance R 0 (depicted in figure 12(b)).The contact resistance R 0 measures 112Ω.The resistances R 1 , R 2 and R 2 are on the scale of 3.87 × 10 6 Ω, 7.28 × 10 6 Ω and 6.95 × 10 5 Ω.The capacitances C 1 , C 2 and C 3 are in the range of 6.68 × 10 −11 F, 6.3 × 10 −10 F and 5.64 × 10 −11 F. As SnO 2 behaves as an n-type semiconductor and CuInS 2 QDs act as a p-type semiconductor, a depletion region emerges at their junction.Therefore, the three RC components correspond to the SnO 2 film, CuInS 2 film, and the SnO 2 /CuInS 2 junction as depicted in figure 12(c).This elevated resistance can be attributed to the low crystallinity of SnO 2 and the capping of organic ligands on the QDs, which imparts insulating behavior.The low crystallinity of the material provides the higher grain boundaries which contribute to the lower conductivity of the film and as the epicenter of the recombination of the excitons leading to the lower photocurrent in the material [52,53].Longchain organic ligands like oleic acid and dodecanethiol, which facilitate the nucleation, encapsulate the CuInS 2 QDs during the synthesis.The insulating nature of these organic ligands adversely affects the conductivity of the QDs-based films leading to lower performance of the devices [54,55].Additionally, lattice defects within the materials and at the SnO 2 /CuInS 2 interface serve as centers for non-radiative exciton decay [56].Consequently, due to these limitations, the observed photocurrent in the device remains relatively low, typically within the nano-ampere range.Although the device shows reduced performance due to these drawbacks, the device shows dual-band (UV and visible) detection with a fast response in the millisecond range.This makes it suitable for encrypted visible light communication.The drawbacks can be addressed by using various surface engineering methods to further enhance the performance of the device.

Conclusion
In summary, the CuInS 2 QDs of ∼2 nm size were synthesized via the hot injection method.These QDs were employed to form a thin film of CuInS 2 to construct an FTO/SnO 2 /CuInS 2 /Au structured photodetection device.The device shows a self-powered photodetection with a fast response in UV and visible range of light.The photogenerated excitons in CuInS 2 are contributing to the photocurrent.The device exhibits a self-powered photoresponse in dual-band light (UV and visible), rendering it suitable for deciphering visible light encrypted with UV light communication.Moreover, the impedance characteristics reveal a significant resistance in the device, potentially stemming from the low crystallinity of SnO 2 and the insulating ligands capping CuInS 2 QDs.Addressing these limitations in the future could lead to an improvement in the device's performance.
photoluminiscence (PL) spectra of the QDs as shown in figure S1(b) (supplementary information) shows a broad peak centered at 716 nm.The SnO 2 and the CuInS 2 /SnO 2 structured films were also characterized through atomic force microscopy (AFM) for the surface morphology.Figures3(a) and (b) show 5 μm × 5 μm AFM micrographs of SnO 2 film and CuInS 2 film over the SnO 2 films respectively.SnO 2 film shows uniformity over the substrate with a root mean square (RMS) roughness of 1.4 nm.While the CuInS 2 film shows a lower RMS roughness of 0.5 nm.The SnO 2 film and CuInS 2 /SnO 2 heterojunction were also characterized via FESEM.The FESEM micrograph of the SnO 2 photoluminiscence (PL) spectra of the QDs as shown in figure S1(b) (supplementary information) shows a broad peak centered at 716 nm.The SnO 2 and the CuInS 2 /SnO 2 structured films were also characterized through atomic force microscopy (AFM) for the surface morphology.Figures3(a) and (b) show 5 μm × 5 μm AFM micrographs of SnO 2 film and CuInS 2 film over the SnO 2 films respectively.SnO 2 film shows uniformity over the substrate with a root mean square (RMS) roughness of 1.4 nm.While the CuInS 2 film shows a lower RMS roughness of 0.5 nm.The SnO 2 film and CuInS 2 /SnO 2 heterojunction were also characterized via FESEM.The FESEM micrograph of the SnO 2
Figure 6(b) shows the I-V behavior of the FTO/SnO 2 /CuInS 2 /Au device in the dark and under the illumination of UV light (355 nm) with a power of 1 mW, 7 mW, and 15 mW.The IV characteristics do not demonstrate the anticipated perfectly rectifying behavior, as typically seen in a type II heterojunction.The deviation of the IV curve from rectification indicates a

Figure 5 .
Figure 5. Schematic of the device.

Figure 6 .
Figure 6.I-V characteristics of the device in (a) visible light and (b) UV light exposure.

Figure 7 .
Figure 7. Photocurrent characteristic of the device in (a) visible light and (b) UV light exposure.

Figure 8 .
Figure 8. Rise time and decay time for (a) visible light and (b) UV light exposure.

Figure 11 .
Figure 11.Photocurrent of the device in mixed UV and visible light; and the corresponding digital communication for each condition.