Self-biased photodetector using 2D layered bismuth triiodide (BiI₃) prepared using the spin coating method

Abstract

Layered bismuth triiodide (BiI₃) is a 2D material that has emerged as an ideal choice for optical sensors. Although BiI₃ has been prepared using vacuum-based deposition techniques, there is a dearth of research studies on synthesizing this material using chemical route. The present work uses a facile spin coating method with varying rotation speeds (rpm) to fabricate BiI₃ material thin films for photodetection applications. The structural, optical, and morphological study of BiI₃ thin films prepared at 3000–6000 rpm were investigated. XRD analysis indicates formation of BiI₃ films and revealed that BiI₃ has a rhombohedral crystal structure. FESEM analysis showed that BiI₃ films prepared at different rpm are homogeneous, dense, and free from cracks, flaws, and protrusions. In addition, films show an island-like morphology with grain boundaries having different grain sizes, micro gaps, and the evolution of the granular morphology of BiI₃ particles. The UV spectroscopy and photoluminescence analysis revealed that BiI₃ films strongly absorb light in the visible region of spectra with a high absorption coefficient of ∼10⁴ cm⁻¹, have an optical band gap of ∼1.51 eV. A photodetector was realised using fabricated BiI₃ film obtained at an optimum spin speed of 4000 rpm. It showed rapid rise and decay times of 0.4 s and 0.5 s, a responsivity of ∼100 μA W⁻¹, external quantum efficiency of 2.1 × 10⁻⁴%, and detectivity of ∼3.69 × 10⁶ Jones at a bias voltage of 0 V. Our results point towards a new direction for layered 2D BiI₃ materials for the application in self-biased photodetectors.

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1. Introduction

Photodetector is an optoelectronic device that converts optical signals into electrical signals. It works on the principle of photoelectric effect i.e. when photons strike the material, it absorbs energy and generates electron–hole pairs (EHPs), thereby forming excitons. In this information era, photodetectors have become indispensable devices due to their application in optical communication and optoelectronics. In the last two decades, many new materials have been discovered with remarkable optical and electronic properties to realize photodetectors having application in optical communication, image sensing, night surveillance, environmental monitoring, and biological/chemical detection.^1–5 The most popular materials employed for photodetection are crystalline inorganic semiconductors like Si, InGaAs, GaN, etc. These materials have outstanding charge carrier mobility, high stability, and small exciton binding energy.^6,7 These material-based photodetectors have limitations in practical applications due to their high operating voltage, mechanical inflexibility, complex and expensive manufacturing process, and low driving temperature. Therefore, researchers are looking for several other materials to overcome these limitations, such as 2D layered graphene, transition metal dichalcogenides (for example, WS₂ and MoS₂), metal chalcogenides (such as InS₂ and InSe), and metal halides (such as PbI₂ and BiI₃). Photodetectors based on these materials have shown extraordinary detection capability over a broad spectrum ranging from UV to IR.^8–12 Apart from these 2D materials, flexible devices based on organic, inorganic Pb hybrid perovskite materials such as MAPbBr₃,¹³ α-CsPbI₃,^14,15 have shown extraordinary rectifying performance. Li et al.¹⁶ have reported excellent stability of over 1 year in the synthesized material. Despite such significant advancement, there are still challenges that needs to be addressed such as Pb toxicity and stability, which are critical issues in practical applications.¹⁷ To overcome these issues, trials are being undertaken in which organic cation is replaced with inorganic one, replacing divalent lead with other materials such as Ge, Sn, Cu, Fe, Pd, Mn, Sb, and Bi. Among these, Sn and Ge have stability issues, as they easily get oxidized due to low binding energy. Bi₃⁺ and Sb₃⁺ have features similar to Pb²⁺, like same electronegativity and ionic radius.¹⁷ Among all replacements, Bi-based compounds offer great promising properties and they have been scarcely explored.¹⁸ Therefore, to advance perovskite materials for their reliable, practical applications, the present work aims to analyze the substitutional Bi-based material, prominently bismuth triiodide (BiI₃) for self biased photodetector application.

BiI₃ is a favorable 2-D layered material from the metal halides family. It exhibits a repeating unit of sandwiched I–Bi–I layers with strong Bi–I ionic bonds in mono layers and weak van der Waals interactions between them.¹⁹ The unique properties include a wide bandgap of 1.67 eV,²⁰ high dielectric constant,²¹ high electron mobility (260 ± 50 cm² V⁻¹ s⁻¹)²² or 1000 ± 200 with Sb doping,²³ make it an attractive material for next-generation photonic and optoelectronic devices. The material has been successfully employed for solar photovoltaics,²⁴ laser CUT-OFF applications,²⁵ nuclear radiation detectors,²⁶ X-ray detection,^27,28 nanoscale pressure sensors,²⁹ and photocatalytic applications,³⁰ etc.

Various methods have been used to prepare BiI₃, which includes physical vapor deposition (PVD),³¹ hydrothermal method,³² hot wall technique³³ and thermal evaporation.³⁴ Different morphologies such as 1-D, 2-D, and 3-D nanoparticles/nanosheets of BiI₃ can be obtained using above listed methods.^35,36 However, these synthesis techniques require high processing temperature, costly equipment, and coherent processing is time-consuming. Furthermore, their high processing temperature limits their practical applications in flexible, large-area, low-cost portable devices. On the other hand, a one-step solution-based method such as spin coating is widely adopted for synthesizing high-quality films due to its easy and time-saving process.³⁷ Apart from this, spin coating has various advantages, such as low-temperature processes, requirement of low-cost precursors, and it is environmental benign and safe. Due to closed reaction processes, better stoichiometry is achieved and by controlling process parameters one can control morphology and particle size. Furthermore, it does not require any seed-catalyst and expensive surfactant.

Herein the study focuses on photodetector fabrication using a simple and inexpensive one-step solution-processed spin coating method. Bismuth iodide(III) (BiI₃) thin films were used in afore-mentioned photodetector, which were annealed at 160 °C for 45 min. Furthermore, the effect of spin speed on the structural, morphological, and optical properties of BiI₃ thin films was investigated. Finally, at an optimized spin speed (4000 rpm), BiI₃-based photodetectors were directly fabricated on FTO substrates. The fabricated photodetector shows stable photoswitching behavior, remarkable detectivity and photoresponsivity, and rapid response with fast recovery times. The present work demonstrates that BiI₃ has great potential for photodetector applications.

2. Experiment

2.1. Preparation of the electron transport layer (ETL)

An electron transport layer (ETL) compact TiO₂ was deposited on FTO substrate by RF sputtering technique. A four-inch target of TiO₂ (99.99%, VIN Karola) was used for TiO₂ film deposition. First, the substrates were sequentially cleaned with soap water, isopropyl alcohol, ethanol, and distilled water in an ultrasonicated bath for 10 min each. Then, substrates were loaded onto a substrate holder, and the deposition system was evacuated to a back pressure of 1 × 10⁻⁸ mbar. Compact TiO₂ films were deposited by maintaining the deposition parameters at 4 × 10⁻² mbar with constant 150 W RF power for 2 h. The prepared TiO₂ films were subsequently annealed at a temperature of 400 °C for 1 h.

2.2. Preparation of BiI₃ thin films

Metal halide BiI₃ thin films were prepared at room temperature in an ambient atmosphere by the spin coating deposition technique. In this experiment, we have used (BiI₃) and N,N-dimethylformamide (DMF) as chemical precursor and solvent, respectively. To synthesize the solution of BiI₃, 1 molar BiI₃ was added to 1 ml DMF and stirred for 2.5 h without heat treatment. Before deposition, the mixed solution was filtered with a PTAA-0.45 μm filter. Then, this filtered solution was used in the spin coating method to prepare BiI₃ thin films on the RF-sputtered c-TiO₂ films. The spin speed was varied from 3000 rpm to 6000 rpm in the steps of 1000 rpm. Other parameters, such as spin time and concentration of the solution, were kept constant. Afterwards, the deposited films were heated at 160 °C for 45 min on a hot plate in an ambient atmosphere. Finally, the films were allowed to cool to room temperature. Fig. 1 represents the synthesis protocol adapted for realization of BiI₃ thin films.

Fig. 1 Synthesis protocol for BiI₃ thin films.

2.3. Device fabrication and photoresponse measurements

The device fabrication approach is divided into two steps. In the first step, we deposited BiI₃ layer on FTO/compact-TiO₂ by spin coating. Then in the second step, we deposited graphite on FTO/compact-TiO₂/BiI₃ as a bottom contact of the device for charge extraction using simple doctor blade method. The graphite paste was prepared by adding isopropyl alcohol (IPA) solution to graphite powder. Fig. 2 shows the device schematics for the BiI₃-based photodetector and photoresponse measurement setup. The photoresponse measurements of prepared BiI₃ films were conducted using a Keithley 2450 source-meter connected to the computer. White light illumination was performed via a solar simulator (ORIEL SOL 2A 94022A Class ABA). Time-dependent photoswitching studies were measured under standard AM 1.5 sunlight (100 mW cm⁻²) with zero bias applied voltage at room temperature. The effective area of the photodetector used was 2 cm².

Fig. 2 Schematic of the photodetector and its properties measurement setup.

2.4. Characterization

The X-ray diffraction analysis was recorded with Cu-Kα radiation (λ ∼ 15.4 nm) using a Bruker D8 Advance diffractometer (Germany) in the 2θ = 10° to 60°. The UV-Visible-NIR spectrophotometer (JASCO, V-670) was used for the optical properties study. The absorption spectra were recorded between the wavelength of 200 nm to 800 nm. The photoluminescence spectra were recorded on a Fluorolog HORIBA Jobin Yvon spectrophotometer. Morphological micrographs were taken with different magnifications by an FEI Nova NanoSEM 450 microscope. The thickness of all the fabricated BiI₃ thin films was measured by the Dektak XT Bruker profilometer.

3. Results and discussion

3.1. Variation in thickness

Fig. 3 shows the variation of BiI₃ film thickness as a function of the spin speed. As expected, the thickness of the BiI₃ film decreases with an increase in spin speed. It drops from 520 nm to 384 nm as the spin speed is increased from 3000 to 6000 rpm. In this spin coating technique, the thickness (t) of the film is defined by equation,³⁸where t_o is the film thickness at the initial liquid, τ is the spin duration, and K is the system constant represented by K = ρω²/3η, where ρ indicates the liquid density, ω represents the spin speed, and η denotes the liquid viscosity. Thus, if the spin duration is constant for a specified precursor, then the thickness of the film decreases as the spin speed increases.

Fig. 3 Variation in film thickness as a function of the spin speed of BiI₃ films.

3.2. Structural analysis

The formation of BiI₃ thin film was confirmed through XRD analysis. Fig. 4 shows the XRD pattern of BiI₃ thin films deposited on FTO by spin coating at different spin speeds. The XRD pattern of FTO is also incorporated in Fig. 4, and the peaks labeled with asterisks (*) correspond to the FTO substrate. The appearance of multiple peaks in the XRD pattern indicates polycrystalline nature of the films. The major diffraction peaks are observed at 2θ ∼ 12.6°, 13.5°, 25.5°, 26.8°, 35.1°, 41.5°, 43.6°, 46.1°, 54.5°, and 58.1°, which correspond to the (003), (101), (006), (113), (211), (300), (208), (119), (226), and (309) crystal planes, respectively, of the rhombohedral crystal structure of BiI₃ with space group R3̄. It is observed that as spin speed increases, the preferred orientation of BiI₃ crystallites changes from (003) to (113). All the peaks are well-matched with the standard diffraction pattern of BiI₃ (JCPDS data card # 48-1795). The inter-planar spacing between atoms (d-spacing) for BiI₃ films at different spin speeds is determined by Bragg's law,

2d sin θ = nλ; (2)

Fig. 4 XRD pattern of BiI₃ films deposited spin coating at different spin speeds.

The observed interplanar spacing value of the BiI₃ film is 3.32 Å, which is in agreement with the previously reported d-value (3.38 Å) of bulk BiI₃.³⁹ These results confirmed the formation of BiI₃ thin films by a cost-effective and single-step spin coating method. The lattice parameters (a, b, and c) and the unit-cell volume (V) of BiI₃ films were calculated by using,⁴⁰

V = a²c; (4)

The obtained values of lattice constants are a ∼ 5.327 Å, c ∼ 21.051 Å and V = 597.36 ų. These values match well with previously reported data,^41–43 further confirming the formation of BiI₃ films. The average grain size (d_X-ray) of BiI₃ films was determined from the (003)/(113) plane located at 2θ ∼ 12.06°/26.8° using Debye–Scherrer's equation,⁴⁴

where λ indicates the wavelength, β represents full width at half maximum (FWHM), and θ represents Bragg's diffraction angle. As seen in Table 1, the average grain size does not show any particular trend with increased spin speed. However, the BiI₃ thin film prepared at 4000 rpm has the largest grain size.

Table 1 Microstructural parameters of BiI₃ films prepared at different spin speeds

Spin speed (rpm)	dX-ray (nm)	Preferred orientation	Degree of crystallinity (%)	ε (10^−3)	δ × 10^14 (line per m^2)	α (10^−3)	Texture coefficient (TC)
							(003)	(113)
3000	58	(003)	46	2.2	2.97	1.82	1.14	0.76
4000	68	(113)	63	1.3	2.16	1.08	0.54	1.28
5000	54	(113)	59	1.5	3.43	1.36	0.74	1.10
6000	26	(113)	56	1.4	14.8	2.83	0.61	0.96

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To examine the effect of spin speed on the microstructural properties of BiI₃ films, we have calculated the dislocation density, microstrain, stacking fault probability, degree of crystallinity, texture coefficients, etc. These values are listed in Table 1. The crystal structure of BiI₃ is a layered 2-D material built from BiI₆ octahedra with 1/3 of the vacant cation sites. It has been reported that each unit cell of BiI₃ contains three stacked I–Bi–I layers, and in each layer, three closed-pack atomic sheets are stacked in the I–Bi–I sequence.⁴⁵ As a result, faults in stacking are generally found in BiI₃ crystals.^46,47 These crystal imperfections and distortions induce microstrain (ε) in the film, which is given by,⁴⁸

The calculated microstrain values for BiI₃ film are shown in Table 1. The lowest microstrain was observed for the BiI₃ film prepared at 4000 rpm. The microstrain in the film depends on the grain boundaries of the crystallites.⁴⁹ The grain size reduces the grain boundaries, hence the microstrain in the film.

The stacking fault probability (α) is the fraction of layers that undergoes sequential stacking faults in a crystal⁵⁰ and can be measured by,⁵¹

Among all prepared BiI₃ thin films, the lowest stacking fault density was observed for the 4000 rpm sample, implying improved crystallinity. Sometimes, the crystalline peaks overlap with the amorphous hump in the XRD pattern. The crystalline phase can be calculated based on the percentage of crystallinity. Also, it can be calculated by the crystalline XRD peak integrated over the area divided by the film's total integrated area in the XRD pattern.⁵²

where A_C is the crystalline XRD peak integrated area and A_T is the total integrated area of the XRD pattern. The degree of crystallinity critically depends on the spin speed. The highest crystallinity degree (63%) is observed for the BiI₃ film prepared at 4000 rpm. The material properties are strongly influenced by the dislocation/crystallographic defect or irregularity within a crystal structure. The dislocation density (δ) is the length of dislocation lines per unit volume of the crystal and was determined by using the following relation,⁵³where n represents the factor equal to unity for the minimum dislocation density and d_X-ray is the crystallite size. The lowest dislocation density of 1.08 × 10⁻³ was obtained for the BiI₃ thin film prepared at 4000 rpm.

To examine the effect of spin speed on the preferential orientation of crystallites along the (hkl) plane in the film, the texture coefficient [(TC)_hkl] was calculated. It is calculated using,^54,55

where I_i is the relative intensity, I_o indicates the integral intensity of the JCPDS data (powder diffraction pattern) of the corresponding plane i, and N represents the number of reflections in the XRD pattern. If the TC_(hkl) value is equal to 1 that means no preferred orientation,⁵⁶ whereas a TC greater than 1 indicates the preferred orientation of the crystallites in that particular direction.⁵⁷ The calculated values of the texture coefficients are shown in Table 1. The BiI₃ thin films prepared at 3000 and 4000 rpm has texture coefficients 1.14 and 1.28 for the (003) and (113) planes. These results suggest that as the spin speed changes from lower to higher values, the orientation changes from the (003) direction to the (113) direction.

3.3. Morphological analysis

The film's surface morphology and surface chemistry are crucial for improving the device's performance.⁵⁸ Thus, we investigated the effect of spin speed on the morphological properties, such as surface coverage and grain orientations, of BiI₃ thin films. Fig. 5 shows FE-SEM images of BiI₃ films deposited at different spin speeds. Before imaging, films were coated with Pt by the sputtering method. The FESEM images for all BiI₃ films are homogeneous, dense, and free from flaws, cracks, and protrusions. It was observed that the change in spin speed invokes a distinct difference in grain growth. As a result, the films show an island-like morphology with grain boundaries having different grain sizes. Some micro gaps can also be seen in grains. These gaps are created in the growing films due to uneven evaporation of the solvent. The average size of island-like grains is 400–1200 nm. The evolution of the granular morphology of BiI₃ particles can also be seen on these grains. The average particle size is 20–150 nm. The BiI₃ film prepared at 4000 rpm indicates that the surface is rough with grain boundaries of 100 nm and larger.

Fig. 5 FESEM micrographs of BiI₃ films prepared at different spin speeds (a) 3000 rpm, (b) 4000 rpm, (c) 5000 rpm, and (d) 6000 rpm.

3.4. Optical analysis

To understand the prominence of BiI₃ for optoelectronic applications, optical properties are studied using UV Visible and photoluminescence spectroscopies. Fig. 6(a) shows the absorption profile of BiI₃ films deposited at different spin speeds in the 200–800 nm range. For all BiI₃ films, the absorption onset is observed at 700 nm. It was observed that an absorption spectrum shows a wide area of absorbance in the region of UV visible from 350 nm to 650 nm. The highest absorbance is observed for the BiI₃ film due to the largest average grain size. The XRD pattern supports this conjecture. It was found that the optical bandgaps calculated from different methods are quite different. However, the most reliable method for determining the bandgap of BiI₃ is UV-Visible spectroscopy.⁵⁹ Thus, the optical bandgap (E_g) is evaluated from the absorption coefficient as a part of wavelength. By Tauc's relation, the absorption coefficient (α) and photon energy (hν) are related to each other as,⁶⁰

(αhν)ⁿ = A(hν − E_g) (11)

where α indicates an absorption coefficient, A is the constant, hν indicates photon energy, and n is either 2 or 1/2 for a direct or an indirect bandgap semiconductor. The absorption coefficient (α) is calculated using,⁶¹where t represents the thickness of the films. Tauc's plot of prepared BiI₃ films at different spin speeds is shown in Fig. 6(b). The calculated band gap values are 1.59, 1.51, 1.54, and 1.58 eV for BiI₃ films prepared at 3000, 4000, 5000, and 6000 rpm, respectively, and match with the previously reported values.⁶² These results indicate that the spin speed does not influence the bandgap of BiI₃ films. As seen in Fig. 6(c), BiI₃ films prepared at different spin speeds have a much higher absorption coefficient of ∼10⁴ cm⁻¹ in the visible spectra region. The BiI₃ film prepared at 4000 rpm has a remarkable absorption coefficient compared to 5000 and 6000 rpm, resulting in the potential candidature for photodetector application as the penetration depth of the radiation into the device material is determined by the absorption coefficient.

Fig. 6 (a) Optical absorption spectra of BiI₃ films (b) Tauc's plot used to estimate the bandgap and (c) absorption coefficient (d) photoluminescence spectra of BiI₃ films at different rotation speeds.

The photoluminescence (PL) study explains the material's bandgap, states of defect, and recombination processes. The PL plots for BiI₃ thin films prepared at different spin speeds at a wavelength of 620 nm are shown in Fig. 6(d). No significant shift was observed in the PL peak position for change in spin speed. Instead, the resultant PL peaks were observed at wavelengths ∼683–690 nm, corresponding to 1.82–1.78 eV.

3.5. Photodetector analysis

The photodetector working mechanism is based on converting absorbed photons into an electric signal. The basic functioning of the photodetector and various associated mechanisms are well explained in the literature.^63–65 As seen from the structural properties, BiI₃ films are highly crystalline, and the optical properties show that BiI₃ films have optical absorption below 720 nm. Thus, photodetectors were fabricated by depositing BiI₃ at different spin speeds (or thicknesses) on an FTO substrate in FTO/c-TiO₂/BiI₃/graphite configuration. Fig. 7 shows the typical photodetector properties of the BiI₃-based photodetector fabricated using thin film prepared at 4000 rpm. Herein, the photoresponse of the BiI₃-based system is evaluated under dark and white-light illumination (100 mW cm⁻²) at room temperature at 0 V (self-bias). Fig. 7(a) shows the current–voltage (I–V) characteristics of the BiI₃-based photodetector under both dark and white light illumination conditions. The semiconducting behavior of the prepared BiI₃ films is confirmed as the current increases under white light illumination. Fig. 7(b) shows the current versus time (I–t) plot of the fabricated BiI₃-based photodetector device at no bias voltage. The device exhibited a constant current over the number of cycles, good repeatability, and stability.

Fig. 7 BiI₃-based photodetector device characteristics for spin speed 4000 rpm (a) typical current–voltage (I–V) characteristics for dark and under white light illumination, (b) current versus time plot for dark and under white light illumination conditions, and (c) current versus time plot for rise and decay time measurement.

The photodetector's rise time (τ_Rise) is the time required to reach 90% of its maximum photocurrent value from its dark current value. Similarly, the photodetector's decay time (τ_Decay) is the time required to reach 10% of its minimum dark current value from its photocurrent value.⁶⁶ The photodetector's rise and decay times were calculated by enlarging a single photoresponse cycle. Fig. 7(c) shows a single cycle I–t curve of the BiI₃-based photodetector for calculating the rise and decay time for a spin speed of 4000 rpm. The rise and decay time values are ∼0.4 s and ∼0.5 s, respectively. The presence of defects and the recombination of charge carriers strongly affect the response of a photodetector. The surface morphology of the films also affects the carrier transportation and, thus, the device's performance. It has been reported that void and crack-free films are required for ideal optoelectronic devices.⁶⁷ The fast rise and decay times observed for the BiI₃-based photodetector are due to the film's void-free nature and compact surface morphology, enhancing the charge transport of photoinduced charge carriers within the material. XRD (Fig. 4) and FESEM (Fig. 5) analyses further support this conjecture.

Fig. 8 shows the schematic of the band diagram and photogenerated electron transport mechanism of the BiI₃-based photodetector at 0 V bias condition. The energy level positions are obtained from the reported data.⁶⁸ The rectifying behavior can be attributed to formation of a barrier between the TiO₂–BiI₃ interface due to difference in energy levels. In the present case, the energy difference between the conduction band maxima of BiI₃ and TiO₂ is ∼0.1 eV. Therefore, upon illumination, the photogenerated electrons can be pulled quickly towards TiO₂ and the external contacts at zero external bias.

Fig. 8 (a) Band diagram and (b) photogenerated electron transport mechanism of the BiI₃-based photodetector at 0 V bias condition.

Further, the performance of the BiI₃-based photodetector fabricated at 4000 rpm was examined by determining the photosensitivity (ξ), photoresponsivity (R), external quantum efficiency (EQE), and detectivity (D*). The photoresponsivity parameter regulates the briskness of the response to an optical signal of a photodetector. It is represented by the photocurrent produced per unit incidence of light power density on the device,⁶⁹

where I_photo is the generated photocurrent, S is the active area of the photodetector, and P_in is the power of incident radiation or light. The calculated value of photoresponsivity for the BiI₃-based photodetector fabricated at 4000 rpm is listed in Table 2. High crystallinity and crystallite size are responsible for the smooth transport of photogenerated free electrons, significantly improving photodetectors' overall performance.⁷⁰

Table 2 Figure of merit for BiI₃-based photodetector fabricated at 4000 rpm under white light illumination

R (μA W^−1)	ξ	EQE × 10^−4 (%)	D* × 10^6 (Jones)	τRise (s)	τDecay (s)
100	51	2.2	3.69	0.4	0.5

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Another critical parameter of a photodetector is its external quantum efficiency (EQE). It gives an idea about the efficiency of photodetectors in converting photons into separated charge carriers.⁷¹ It is defined as the ratio of the total number of holes or electrons converted via the stimulation of photons for an applied energy source. It is evaluated by using following equation,⁷²

where q is the absolute value of electron charge, h indicates Planck's constant, c represents the speed of light, and λ is the wavelength of illuminated light.

Another crucial parameter of the photodetector is photosensitivity, which represents the change in current with respect to the dark current. The photosensitivity (ξ) is the difference in current (ΔI) normalized to the dark current given by,⁷³

The sensitivity of a device can be recognized by parameter detectivity. It is the ability of a photodetector to sense weak intensity signals. The detectivity of the device is given by^74,75

where I_dark denotes the dark current. The noise limiting detectivity was 10⁶ Jones at 0 V bias voltage, comparable to the previously reported detectivity for the BiI₃ photodetector.

The calculated values of photosensitivity (ξ), photoresponsivity (R), external quantum efficiency (EQE), and detectivity (D*), along with the rise time (τ_Rise) and decay time (τ_Decay) of the BiI₃-based photodetector fabricated using thin film prepared at 4000 rpm, are listed in Table 2.

The photosensitivity (ξ), detectivity (D*), rise time (τ_Rise), and decay time (τ_Decay) of some recently reported BiI₃-based photodetectors fabricated using different methods are compared with the present work in Table 3.

Table 3 Comparative analysis of some recently reported BiI₃-based photodetectors

Fabrication method	ξ	D* (Jones)	τRise/τDecay	Ref.
Spin-coating	0.5 mA W^−1 @ 0 V	1.3 × 10^7	2 s/1 s	76
PVD	2.2 A W^−1 @ 5 V	1.6 × 10^12	2 s/5.3 ms	77
PVD	2.8 A W^−1 @ 5 V	1.2 × 10^12	3 s/9 ms	78
Spin-coating	100 μA W^−1 @ 0 V	3.9 × 10^6	0.4 s/0.5 s	Present work

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Although the rise time and decay time observed for our BiI₃-based photodetector are 0.4 s and 0.5 s, respectively, the material still has a lot of scope for improvement and needs to be explored further. Nevertheless, after comparing, we found that the photodetector has appreciable performance at 0 V bias (self-biased) than the BiI₃-based photodetectors tested at applied biases. Thus, we believe that BiI₃ can be a promising material for photodetector application.

4. Conclusions

In conclusion, we have prepared BiI₃ thin films via simple and one-step spin coating technique at different spin speeds. The morphological, structural, and optical properties of BiI₃ films prepared at 3000–6000 rpm were investigated. The formation of BiI₃ films was confirmed through the XRD pattern and revealed that BiI₃ has a rhombohedral crystal structure. FESEM analysis showed that BiI₃ thin films prepared at different rpm are homogeneous, dense, and free from flaws, cracks, and protrusions. In addition, films show an island-like morphology with grain boundaries having different grain sizes, micro gaps, and the evolution of the granular morphology of BiI₃ particles. Furthermore, the UV-Visible and photoluminescence spectroscopy study indicate that the BiI₃ films absorb light strongly in the visible spectra region with a higher absorption coefficient (∼10⁴ cm⁻¹) and have an optical band gap of ∼1.51 eV. Finally, a photodetector was fabricated using a BiI₃ thin film prepared at an optimum spin speed of 4000 rpm and showed the fast rise and decay times of 0.4 s and 0.5 s, a responsivity of ∼100 μA W⁻¹, the external quantum efficiency of 2.1 × 10⁻⁴%, and a detectivity ∼3.69 × 10⁶ Jones at a 0 V bias. The present results demonstrated that BiI₃ could be a promising candidate for other optoelectronic applications.

Author contributions

Ashvini Punde: conceptualization, methodology, investigation and writing-original draft. Shruti Shah: investigation and methodology. Yogesh Hase: visualization and formal analysis. Ashish Waghmare: visualization and formal analysis. Pratibha Shinde: visualization and formal analysis. Bharat Bade: visualization and formal analysis. Habib Pathan: formal analysis and validation. Mohit Prasad: formal analysis and validation. Shashikant Patole: writing-review and editing. Sandesh Jadkar: supervision, writing-review, and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Ashvini Punde is grateful to the Mahatma Jyotiba Phule Research and Training Institute (MAHAJYOTI), Government of Maharashtra, for the Mahatma Jyotiba Phule Research Fellowship (MJPRF). Shruti Shah, Yogesh Hase, Ashish Waghmare, Pratibha Shinde, and Bharat Bade are thankful to the Ministry of New and Renewable Energy (MNRE), Government of India, for the financial support under the National Renewable Energy Fellowship (NREF) program. Mohit Prasad and Sandesh Jadkar thank the Indo-French Centre for the Promotion of Advanced Research-CEFIPRA, Department of Science and Technology, New Delhi, for special financial support. Finally, Shashikant P. Patole would like to thank Khalifa University for its financial support through the internal fund for high-quality publications.

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