Highly stable and Pb-free bismuth-based perovskites for photodetector applications

Article abstract of DOI:10.1039/d0nj01806f

Herein, we report the synthesis of highly stable, Pb-free bismuth iodide (BiI3 or BI), stoichiometric methylammonium bismuth iodide [(CH3NH3)3Bi2I9 or MA3Bi2I9 or s-MBI] and non-stoichiometric methylammonium bismuth iodide [(CH3NH3)2BiI5 or MA2BiI5 or Ns-MBI] perovskite thin films for photodetector applications. These films are synthesized at room temperature by a single step solution process spin coating method. The structural, optical, and morphological properties of these films were investigated using different characterization techniques such as XRD, Raman spectroscopy, FE-SEM, UV-Visible spectroscopy, etc. Formation of BI, s-MBI and Ns-MBI thin films is confirmed by XRD and Raman spectroscopy measurements. XRD analysis reveals that BI has a hexagonal crystal structure and a P63/mmc hexagonal space group for s-MBI and Ns-MBI. The optical properties of BI thin films show a high absorption coefficient (～104 cm-1) and a band gap of ～1.74 eV. Similarly, s-MBI films have a high absorption coefficient (～103 cm-1) and an indirect band gap of ～1.8 eV. Moving from s-MBI to Ns-MBI, the value of absorption coefficient is ～103 cm-1 and the band gap corresponds to ～2 eV. Finally, photodetectors based on the synthesized BI, s-MBI and Ns-MBI perovskites have been directly fabricated on FTO substrates. All photodetectors exhibited highly stable photo-switching behaviour along with excellent photoresponsivity and detectivity, with a fast response and recovery time. Our work demonstrates that Pb-free BI, s-MBI and Ns-MBI perovskites have great potential in the future for realizing stable photodetectors.

Full text of DOI:10.1039/d0nj01806f

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Jadkar, Y. Hase, S. Rondiya, N. Patil, M. Prasad and S. R. Jadkar, New J. Chem., 2020, DOI:
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PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
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Highly stable and Pb-free Bismuth-based perovskites for photodetector appDliOcIa: 1t0io.10n3s9/D0NJ01806F
Ajinkya Bhorde, Shruthi Nair, Haribhau Borate, Subhash Pandharkar, Rahul Aher, Ashvini Punde,
Ashish Waghmare, Pratibha Shinde, Priti Vairale, Ravindra Waykar, Vidya Doiphode, Vijaya
Jadkar, Yogesh Hase, Sachin Rondiya, Nilesh Patil
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School of Energy Studies, Savitribai Phule Pune University, Pune 411 007 (India)
Mohit Prasad, Sandesh Jadkar*
Department of Physics, Savitribai Phule Pune University, Pune 411 007 (India)
*Corresponding author
E-mail: sandesh@physics.unipune.ac.in
Tele/Fax No.:+ 91 20 2569 5201
ABSTRACT
Herein, we report synthesis of highly stable, Pb-free bismuth iodide (BiI3 or BI), stoichiometric
methylammonium bismuth iodide [(CH3NH3)3Bi2I9 or MA3Bi2I9 or s-MBI] and non-stoichiometric
methylammonium bismuth iodide [(CH3NH3)2BiI5 or MA2BiI5 or Ns-MBI] perovskites thin films for
photodetector application. These films are synthesized at room temperature by single step solution
process spin coating method. The structural, optical, and morphological properties of these films were
investigated using different characterization techniques such as XRD, Raman spectroscopy, FE-SEM,
and UV-Visible spectroscopy etc. Formation of BI, s-MBI and Ns-MBI thin films is confirmed by XRD and
Raman spectroscopy measurements. XRD analysis reveals that BI has hexagonal crystal structure and
P63/mmc hexagonal space group for s-MBI and Ns-MBI. Optical properties of BI thin films show high
absorption coefficient ( 104 cm-1) and band gap of  1.74 eV. Similarly, s-MBI films have high absorption
coefficient ( 103 cm-1) and indirect band gap of  1.8 eV. Moving from s-MBI to Ns-MBI the value of
absorption coefficient  103 cm-1 and band gap corresponds to  2 eV. Finally, photodetectors based on
synthesized BI, s-MBI and Ns-MBI perovskites have been directly fabricated on FTO substrates. All
photodetectors exhibited highly stable photo-switching behaviour along with excellent photoresponsivity
and detectivity, with fast response and recovery time. Our work demonstrates that Pb-free BI, s-MBI and
Ns-MBI perovskites have great potential in future for realizing stable photodetectors.
KEYWORDS: Pb-free and Bi-based perovskites; Photo-detector; Responsivity; Detectivity
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DOI: 10.1039/D0NJ01806F
1: INRODUCTION
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Recently, halide-based perovskite materials have been identified as strong potential candidate for
various opto-electronic applications. Organic-inorganic and hybrid organic-inorganic halide-based
perovskite materials are effective due to their unique properties such as long charge carrier diffusion
length, high absorption coefficient (104-105 cm-1), low synthesis cost [1], high charge carrier mobility [2]
and tunable band gap (1.5 eV-2.3 eV) [3] etc. Metal halide perovskites have emerged as a promising
class of semiconductor materials for wide range of applications, such as solar cells [4-6], photodetectors
[7-9] light-emitting diodes (LEDs) [10-12] and lasers [13, 14] etc. Among these applications,
photodetectors are of primary importance as they bridge the gap between photonics and electronics.
Synthesis of metal halide perovskites for photodetector applications has been reported by several
groups. For example, Hu et al. [15] demonstrated high performance flexible photodetector applications
based on pinhole-free CH3NH3PbI3 perovskite films deposited by vapour solution method. Yang et al.
[16-17] reported synthesis and fabrication of all-inorganic CsPbX3 perovskites for photo-detector
application. However, most of the metal halide perovskites for photodetector are based on toxic lead
(Pb). Furthermore, lifetime of these metal halide perovskites Pb-based photodetectors is low due to
chemical instability of the material. Therefore, it is necessary to replace Pb in these perovskite materials
by non-toxic element. To achieve Pb-free perovskite material, several attempts have been made to
replace Pb by group 14 elements such as tin (Sn) and germanium (Ge) [18, 19]. However, replacement
of Sn and Ge in perovskites results in poor stability due to oxidation effect. Bismuth (Bi) is the adjacent
element of Pb and has properties which are similar and comparable to Pb.
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Thus, to overcome the above problems researchers replaced lead (Pb) by Bismuth (Bi) and have
succeeded in fabricating Pb-free Bi-based single perovskites such as MA3Bi2I9 [20], CsBi3I10 [21],
Cs3Bi2I9 [22], and also double perovskite such as MA2AgBiI6 [23], Cs2AgBiX6 (X = Br, Cl) [24]. Such Pb-
free perovskite materials have been successfully employed in opto-electronic devices. For example,
Zhaoyang Qi et al. [25] have prepared highly stable, Pb-free Cs3Bi2I9 perovskite nanoplates for visible
light photodetection application. They have fabricated photodetector which exhibited fast rise and decay
time of 10.2 and 37.2 ms, respectively with specific detectivity  1010 Jones. Bai et al. [26] have reported
preparation of Cs3Bi2I9 perovskite films for solar cell applications. Cuhadar et al. [27] synthesized
Cs3Bi2I9 perovskite film and have shown its application in low-voltage switching resistive memory.
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Variety of methods are reported for the synthesis of device quality perovskite materials which include two
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step spin-coating and vacuum flash-assisted solution process (VASP) [28], vapor deposition [29, 30],
Gas-induced method [31, 32] etc. However, these methods necessitate costly equipment, and require
high processing temperature and time. One step solution-based room temperature synthesis method
have many merits over the above-mentioned synthesis methods such as easy synthesis, large scale
deposition, less process time and no need of high temperature. The present study is focused on simple-
one step solution processed synthesis method (without any solvent treatment) for device fabrication. We
herein report synthesis of highly stable and Pb-free bismuth iodide (III) (BiI3 or called BI), stoichiometric
methylammonium bismuth iodide [(CH3NH3)3Bi2I9 or MA3Bi2I9 or called s-MBI] and non-stoichiometric
methylammonium bismuth iodide [(CH3NH3)2BiI5 or MA2BiI5 or called Ns-MBI] perovskites thin films at
room temperature by single step solution process spin coating method for photodetector application. The
x-ray diffraction (XRD) and UV-Visible spectroscopy measurements show that BI, s-MBI and Ns-MBI are
highly stable in atmospheric conditions for four months. Finally, photodetectors based on the synthesized
BI, s-MBI and Ns-MBI perovskites have been directly fabricated on FTO substrates. These
photodetectors have highly stable photo-switching behaviour along with excellent photoresponsivity and
detectivity, thereby fast response and recovery time. Our work presents that Pb-free BI, s-MBI and Ns-
MBI perovskites has great potential in future for photodetector application.
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2: EXPERIMENTAL
2.1: Synthesis of MAI crystals
Methylammonium iodide (CH3NH3I, MAI) crystals were synthesized by drop-wise addition of aqueous
hydro-iodic acid (HI, Alfa Aesar, 57 %) into methylamine (CH3NH2, Sigma Aldrich, 40 %) under ice-cold
condition and then stirred for 2 hrs. Solvent from the resultant solution was removed using rotary
evaporator at 60 °C. The yellow-white crystals were obtained after complete evaporation of solvent and
then the remnants were washed several times by diethyl ether [(C2H5)2O]. By filtering the precipitate and
subsequent drying at 70 °C for 12 hrs under vacuum yields white MAI crystals.
2.2: Synthesis of BI, s-MBI and Ns-MBI thin films
Pb-free perovskites BI, s-MBI and Ns-MBI thin films were prepared at room temperature by single step
solution process spin coating method. In the experiment, bismuth triiodide (BiI3, 99.9 %, Sigma Aldrich)
and N, N-Dimethylformamide (DMF) and as-synthesized methylammonium iodide (MAI) crystals were
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used as chemical precursors. All solutions were prepared by adding BiI3 and MAI in varDioOuI:s10p.10ro39p/Do0rtNioJ0n18s06F
(molarity variation of BiI3:MAI) in DMF. For the synthesis of BI solution, 1 M BiI3 was added in 0.5 ml
DMF. Similarly for the synthesis of s-MBI 3:2 M proportion of BiI3:MAI were added in 0.5 ml of DMF and
Ns-MBI solution was obtained by adding 2:1 M proportion of BiI3:MAI in 0.5 ml of DMF. The resultant
solution was transferred for sonication till the solution became crystal clear. These perovskite solutions
were deposited on glass substrate (corning glass # 7059) by simple spin coating method (at 6000 rpm
for 30 s). Spin deposited perovskite thin films were annealed at 90 oC on hot plate for 10 min. Films were
allowed to cool to room temperature and then taken for characterization.
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2.3: Material characterization
X-ray diffraction was performed using X-ray diffractometer (Bruker D8 Advance, Germany) having CuKα
line (λ = 1.54056 Å) ranging 2θ values from 5° to 60° with step of 0.1°. The optical band gap of
perovskite thin films were estimated from the absorbance spectra recorded using JASCO, V-670 UV-
Visible-NIR spectrophotometer in the range of 200-800 nm. The HITACHI, S-4800, Japan, field emission
scanning electron microscope (FE-SEM) with operating voltage 10 kV was used to observe the surface
morphology of BI, s-MBI and Ns-MBI thin films. Raman spectra were recorded using Raman
spectroscopy (Renishaw InVia microscope Raman) in the range of 70-250 cm-1. The spectrometer has
backscattering geometry for detection of Raman spectrum with the resolution of 1 cm-1. The excitation
source was 532 nm line of He-Ne laser. The power of the Raman laser and its spot size were kept < 5
mW and  1 μm respectively to avoid possible heating effect on the sample. Thickness of all the
synthesized Bi-based perovskites films was determined by profilometer (KLA-Tencor, P-16+) and it was
found to be in the range of 500-600 nm.
2.4: Device fabrication and photo-response measurements
BI, s-MBI and Ns-MBI photodetectors were fabricated on FTO coated glass substrate. Firstly, the FTO
substrates were etched out from the regions under the anode contact by using 30 % hydrochloric acid
(HCl) and zinc powder. The FTO substrates were then cleaned with ultrasonication in double distilled
water (DDW), acetone and isopropyl alcohol (IPA) for 15 min each. In second step, an electron transport
layer (ETL) of compact TiO2 (c-TiO2) was deposited on pre-cleaned FTO substrate by using RF-
magnetron sputtering. In third step active perovskite layer of BI, s-MBI or Ns-MBI was deposited on
FTO/c-TiO2 by spin coating method to form FTO/c-TiO2/BI or s-MBI or Ns-MBI structure. Prepared
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structure was then annealed on hot plate at 70 for 10 min. After that, for the top electrodeVicewoAnrtticalecOt,nline
DOI: 10.1039/D0NJ01806F
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graphite paste was prepared by mixing graphite powder in isopropyl alcohol (IPA) solution and the same
was deposited over the FTO/c-TiO2/BI or s-MBI or Ns-MBI structure to form FTO/c-TiO2/BI or s-MBI or
Ns-MBI/Graphite structure. The complete device fabrication process is shown in figure 1.
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Figure 1: Schematic of complete device fabrication process used in the present study
All the opto-electronic measurements were carried out using Keithley 2400 system source meter that
was attached to a computer through a GPIB 488A interface. For white light illumination, a PEC-L01
Portable Solar Simulator was used for measuring the photo-response of fabricated device FTO/c-TiO2/BI
or s-MBI or Ns-MBI/Graphite. The photocurrent-time characteristic curves were measured under
standard AM 1.5 sunlight (100 mW/cm2). Figure 2 shows the schematic of opto-electronic measurement
set up used in the present study. The effective area of the photodetector was 0.25 cm2.
Figure 2: Schematic of BI, s-MBI and Ns-MBI photodetector fabricated and experimental set up used to study photo
response properties
3: RESULTS AND DISCUSSION
3.1: Structural properties
X-ray diffraction (XRD) and Raman spectroscopy were used to confirm the formation and to analyse the
structural properties of synthesized bismuth iodide (BI), stoichiometric methylammonium bismuth iodide
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(s-MBI) and non-stoichiometric methylammonium bismuth iodide (Ns-MBI) thin films pDreOpI:a10r.e10d39b/Dy0NsJp01in806F
coating method. Figure 3(a) shows the XRD pattern of as-prepared BI and same BI sample exposed to
normal atmospheric conditions (relative humidity of 55-65 % and temperature of 25-35 °C) for 4 months
without any encapsulation.
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Figure 3: X-ray diffraction pattern of spin coated a) BI, b) s-MBI and c) Ns-MBI perovskite thin films
Presence of multiple peaks in XRD pattern indicates that all films are polycrystalline in nature. Films
show diffraction peaks at 2θ  12.78°,14.22°, 25.72°, 26.97°, 39.09°, 41.56° and 52.94° which
corresponds to (003), (101), (006), (113), (009), (300) and (0012) diffraction planes respectively. All
diffraction peaks can be indexed as hexagonal BiI3 phase with lattice parameters a = 0.7538 nm and c =
2.0805 nm, which are consistent with the values reported in the literature (JCPDS file # 07-0269) [33]
and have space group C2 [34]. It is interesting to note that XRD pattern recorded after 4 months
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exposure of BI films to normal atmospheric conditions didn’t show any significant change in the
diffraction peak intensity and position. Furthermore, no additional diffraction peaks were observed even
after exposure of BI films to humid air for 4 months. These results suggest that BI thin films are
environmentally stable. Figure 3(b) and figure 3(c) shows the XRD pattern of s-MBI and Ns-MBI thin
films along with XRD patterns of same s-MBI and Ns-MBI thin films recorded after 4 months exposed to
normal atmospheric conditions that too without any encapsulation. As seen both films have diffraction
peaks at 2θ  8.30°, 12.56°, 16.30°, 24.60°, 25.40°, 41.56° and 50.30° corresponding to diffraction
planes (002), (101), (004) (006), (202), (208) and (308) respectively. These diffraction peaks match well
with the theoretically calculated and experimentally observed XRD patterns of MBI published earlier [35].
Although there are ambiguities regarding crystal structure of MBI. For example, Jakubas and Sobczyk
[36] and Park et al. [37] reported that MBI has hexagonal crystalline structure with P63/mm as space
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group whereas Szklarz et al. [38] had reported that MBI has an orthorhombic structure with Cmcm(63) as
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space group. The XRD peaks of MBI perovskite thin films obtained in this study are close to the results
reported by Jakubas and Sobczyk [36] and Park et al. [37] suggesting that MBI perovskite has
hexagonal crystalline phase with P63/mm as space group. Thus, the calculated lattice parameters are
found to be a = 0.8612 nm, b = 0.8531 nm and c = 2.1971 nm. These lattice parameters match well with
the DFT calculated lattice parameters reported by Chen et al. [39] and experimentally verified reported
values of Kamminga et al. [40]. No change in diffraction peak intensity and position has been observed
in the XRD pattern recorded after 4 months exposure of both s-MBI and Ns-MBI thin films to normal
atmosphere suggesting that synthesized films are environmentally stable. On the other hand, the x-ray
diffraction intensity of non-encapsulated organic-inorganic hybrid perovskite CH3NH3PbI3 was found to
decrease even with exposure to humid air for 5 min and almost vanishes when the exposure duration
was of 30 mins [41].
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Formation of BI, s-MBI and Ns-MBI thin films is also confirmed by Raman spectroscopy. Figure 4 shows
the Raman spectra of BI, s-MBI and Ns-MBI thin films in the range 75-250 cm-1 prepared by spin coating
method.
Figure 4: Raman spectra of spin coated a) BI, b) s-MBI and c) Ns-MBI perovskite thin films
As seen the Raman spectra of BI film show a strong active shoulder at  112 cm-1 and two weaker peaks
at  85 cm-1 and  159 cm-1. These peaks match well with the previously reported Raman spectra of BI
in the literature [34, 42]. The line at 154 cm-1 may be assigned to 2nd order Raman line which is only
observed in the bulk materials at lower temperature [43] or may be a new vibrational mode. Raman
spectra for s-MBI and Ns-MBI show strong Raman active peak at  108 cm-1 and  145 cm-1 which are
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characteristics bands observed for MBI perovskite films [35]. The density functional theory (DFT)
calculated vibrational modes by Öz et al. [35] at  23, 35, 47 and 59 cm-1 and the Bi-I-Bi scissoring
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modes at  73 cm-1 and  87 cm-1 are not observed in Raman spectra of these films. A possible reason
for it may be coupling to the Bi-I stretch mode (108 cm-1). The Bi-I stretching vibrations are observed at
108 cm-1 and 145 cm-1 which are reproduced well in the recorded Raman spectrum. Our results are also
in good agreement with the similar compounds containing (Bi2I9)3- [44]. Thus, Raman spectroscopy
analysis further confirm the formation of BI, s-MBI and Ns-MBI thin films synthesized using spin coating
method.
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3.2: Morphological properties
Surface morphology of spin coated Bi-based perovskites was analysed using FE-SEM technique. Figure
5 shows the FE-SEM micrographs of BI, s-MBI and Ns-MBI thin films. The inset shows corresponding
magnified FE-SEM images. As seen from figure 5(a) BI film show dense and uniform distribution of
micron size crystals having chain like structure which cover the entire substrate area.
Figure 5: FE-SEM micrographs of a) BI, b) s-MBI and c) Ns-MBI perovskite thin films deposited by spin coating method.
The inset shows corresponding magnified images.
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The morphology of s-MBI films shows hexagonal shaped MBI crystals with smooth curvaturesVaiewt AertvicelerOynline
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corner. Micron sized MBI hexagons are vertically as well as horizontally aligned with micron size void
space [Figure 5(b)]. These voids can be removed by chemical engineering such as solvent treatment
[45]. Figure 5(c) depicts the FE-SEM micrographs of Ns-MBI thin films. As seen the Ns-MBI film has
hexagonal shaped crystals with porous part at the centre of hexagons. These hexagons are aligned
horizontally over entire substrate surface with micro-voids present in between them. The origin of
formation of micro-voids in the film is not clear, however we think that evaporation of solvent during the
film formation may have created the voids. These voids may hamper the energy conversion efficiency of
perovskite solar cells.
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3.3: Optical properties analysis
Optical properties such as optical absorption and band gap of as synthesized BI, s-MBI and Ns-MBI thin
films were evaluated using UV-Visible spectroscopy. Figure 6 shows the optical absorption of as
deposited BI, s-MBI and Ns-MBI thin films and the insets of each of figure 6 show the typical Tauc plot,
opted for band gap estimation.
Figure 6: UV-Visible absorption spectra of spin coated a) BI, b) s-MBI and c) Ns-MBI perovskite thin films. Inset in each
figure shows typical Tauc plot used to estimate band gap of BI, s-MBI and Ns-MBI thin films respectively.
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In the direct transition semiconductor, the optical energy band gap (Eopt) and the optical absorption
5
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coefficient () are related by [46],
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8
(αE)1/ 2  B1/ 2(E-Εopt
)
….(1)
9
Where  is the absorption coefficient, B is the optical density of state and E is the photon energy. The
absorption coefficient () can be calculated from the transmittance of the films with the formula,
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1
1
T




α  ln
d


…..(2)
Where d is the thickness of the films and T is the transmittance. Therefore, the optical band gap is
obtained by extrapolating the tangential line to the photon energy (E = h) axis in the plot of (h)2 as a
function of h (Tauc plot).
Figure 6(a) shows that absorption of BI start from  710 nm and have broad absorption spectra up to 
220 nm with highest value of absorption coefficient ( 104 cm-1). Inset of figure 6(a) shows the typical
Tauc plot for BI having band gap 1.74 eV, which matches with the literature values [34, 35]. Figure 6(b)
shows the absorption of s-MBI which starts from  660 nm and has broad absorption spectra up to  220
nm with highest value of absorption coefficient ( 103 cm-1). Inset of figure 6(b) shows the typical Tauc
plot for s-MBI with indirect band gap of  1.8 eV [47, 48]. Moving from stoichiometric MBI to non-
stoichiometric MBI the optical properties varies little bit. The optical absorption and optical band gap of
Ns-MBI is shown in figure 6(c). Increase in concentration of MAI in MBI (i. e. Ns-MBI) the absorption
edge shift towards lower wavelength. For Ns-MBI thin film optical absorption starts from  620 nm and
have broad absorption spectra up to  220 nm with highest value of absorption coefficient ( 103 cm-1).
Inset of figure 6(c) shows typical Tauc plot for Ns-MBI having indirect band gap of  2 eV. We have also
recorded the optical absorption spectra and calculated the optical band gap of all three BI, s-MBI and
Ns-MBI thin films after four months from the date of synthesis. All the synthesized samples even after 4
months exhibited similar optical properties such as absorption and band gap values. These results
further confirm environmental stability of Bi-based perovskites.
3.4: Photodetector properties
A photodetector is a light sensor that can convert incident radiation into electrical signal. The working
mechanism of a photodetector is well-known and many reviews have explained its basic principle in
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detail [49-51]. In a photo-detector, absorbed photon energy is utilized for the generationDoOfI:e10le.10c3t9ro/Dn0NhJo01le806F
pairs called excitons. The stability and recyclability of a photodetector under light illumination is critical for
its long-term operation. Thus, the time-resolved photo-response measurement has been conducted
under alternative dark and white light illumination (100 mW/cm2) at zero bias voltage. Figure 7(a1), 7(b1)
and 7(c1) represents time-resolved photo-response [(I-t) curves] of BI, s-MBI and Ns-MBI perovskite
photo-detectors. Almost no deterioration of photocurrent is observed, revealing that the photo-response
of all the fabricated BI, s-MBI and Ns-MBI devices is stable and repeatable. Furthermore, sharp rise and
decay of photocurrent is observed, when the solar simulator is turned ON and OFF immediately,
signifying the fast response and recovery characteristic of BI, s-MBI and Ns-MBI perovskite-based
photodetectors under white light illumination. As seen from the XRD and FE-SEM analysis (Figure 3 and
Figure 5) the synthesized BI, s-MBI and Ns-MBI materials are highly crystallized. As a result, the photo-
induced free carriers recombine very fast instead of slowly recombining with the trapped carriers. This
gives the fast photo-response characteristics. Such fast photo-response has been reported by Luo et al.
[52] for InSe-based photodetectors and Cao et al. [53] for GaSe photodetectors. In order to evaluate the
response and recovery time, each photodetector’s enlarged view of a single ON-OFF cycle need to be
considered.
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Figure 7: Time-resolved photo-response characteristics of a1) BI, b1) s-MBI and c1) Ns-MBI perovskite thin film
photodetector under alternating dark and illumination conditions and a2), b2) and c2) are single cycle photo-response of
BI, s-MBI and Ns-MBI perovskite photodetectors respectively for estimation of response and recovery time
Figure 7(a2), 7(b2) and 7(c2) shows the dynamic response and recovery data for one cycle of BI, s-MBI
and Ns-MBI perovskite-based photodetectors respectively. The time taken by the photodetector to reach
90 % of the maximum photocurrent value from its dark current value is called the response time (Rise) of
that photodetector. In the same analogy, the time required for photodetector to reach 10 % of maximum
photocurrent response is called the decay time (Decay) of that photo-detector [54]. The response and
decay time for BI photodetector is found 2 s and 1 s respectively, for s-MBI photodetector 2 s and 2 s
respectively and for Ns-MBI photodetector 3 s and 2 s respectively.
The performance of a photodetector is further characterized by various parameters such as photo
responsivity, detectivity, photosensitivity, probable quantum efficiency etc. The photo-responsivity (R) of
a photo-detector defined as photo-excited current generated per unit power of incident light on the
effective area of photodetector and is given by the equation [55],
I
P xA
….. (3)
R

Whereas ΔI = Iphoto-Idark is the change in the photocurrent due to irradiation of light, P is the intensity of
incident photons (100 mW/cm2) and A is area of photodetector (0.25 cm2). The photoresponsivity of BI,
s-MBI and Ns-MBI perovskite-based photodetectors is found to be  0.5 mA/W, 0.1 mA/W and 0.2 mA/W
respectively.
The photosensitivity () of a photodetector is defined as the ratio between the difference in photo current
(Iphoto) and dark current (IDark) values to the dark current and is expressed as [56],
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IPhoto IDark
…..(4)


IDark
The estimated value of photosensitivity of BI, s-MBI and Ns-MBI perovskite-based photodetectors is
found to be 114, 118, and 75 respectively.
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To evaluate the efficiency of the electron transport and carrier collection we have estimated the probable
internal quantum efficiency (IQE) of photodetector using following equation [57],
hxc xR
…..(5)
IQE (%) 
x 100 %
ex
Where h is a Plank’s constant, c is a velocity of light, e charge of the electron, λ is a wavelength of the
incident radiations and R represents the photoresponsivity. The value of quantum efficiency of BI, s-MBI
and Ns-MBI perovskite-based photodetectors is found to be 124x10-3 %, 20 x10-3 % and 41 x10-3 %
respectively.
Similarly, Detectivity (D*), is an important parameter of photodetectors which gives a measure of quality
of device. It is measured in the unit of Jones and is given by [58],
R
D*

….. (6)
1 2
(2 xe xJDark
)
Where, Rλ is the photoresponsivity, JDark is the dark current density and e is electron charge. We have
observed average value of detectivity for BI, s-MBI and Ns-MBI perovskite photo-detectors is  1.30x107
Jones,  0.61x107 Jones and  0.70x107 Jones respectively. A comparative analysis of BI, s-MBI and
Ns-MBI based photodetector with reference to their response and decay time, responsivity, detectivity,
photosensitivity, probable quantum efficiency etc. is shown in Table 1.
Table 1: Comparative analysis of BI, s-MBI and Ns-MBI based photodetector
Response time
(s)
Decay time
(s)
Photo-responsivity
(mA/W)
Probable quantum
efficiency x10-3(%)
Detectivity
(Jones)
Photodetector
BI
2
2
3
1
2
2
0.5
0.1
0.2
124
20
1.30x107
0.61x107
0.70x107
s-MBI
Ns-MBI
41
CONCLUSIONS:
We have synthesized highly stable, Pb-free BI, s-MBI and Ns-MBI perovskite thin films at room
temperature by single step solution process spin coating method for photodetector application. The
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structural, optical, and morphological properties of these films were investigatedDOuIs: i1n0.g1039d/iDff0eNrJe0n18t06F
characterization techniques such as XRD, Raman spectroscopy, FE-SEM, UV-Visible spectroscopy etc.
Formation of BI, s-MBI and Ns-MBI thin films are confirmed by XRD and Raman spectroscopy. The XRD
and UV-Visible spectroscopy measurements showed that BI, s-MBI and Ns-MBI are highly stable in
atmospheric conditions for four months. Further, XRD analysis showed that BI has hexagonal crystal
structure and P63/mmc hexagonal space group for s-MBI and Ns-MBI. UV-Visible spectroscopy analysis
showed that BI films has high absorption coefficient of  104 cm-1 and band gap of  1.74 eV. Similarly, s-
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MBI also has high absorption coefficient (103 cm-1) and indirect band gap of  1.8 eV. Moving from s-
MBI to Ns-MBI the value of absorption coefficient  103 cm-1 and band gap becomes  2 eV. Finally,
photodetectors based on the synthesized BI, s-MBI and Ns-MBI perovskites have been directly
fabricated on FTO substrates. All photodetectors showed highly stable photo-switching behaviour along
with excellent photoresponsivity and detectivity, fast response and recovery time. The present results
demonstrate great potential of highly stable and low-toxic BI, s-MBI and Ns-MBI perovskite for future
opto-electronic applications.
ACKNOWLEDGMENT
Ajinkya Bhorde is thankful to Department of Science and Technology (DST), Government of India for
INSPIRE fellowship. Ravindra Waykar, Shruthi Nair, Subhash Pandharkar, Ashish Waghmare, Pratibha
Shinde, Ravindra Waykar and Yogesh Hase are thankful to the Ministry of New and Renewable Energy
(MNRE), Government of India for the financial support under National Renewable Energy Fellowship
(NREF) program. Haribhau Borate is grateful to University Grant Commission (UGC), New Delhi and
Pune District Education Association (PDEA), Pune for providing Teacher Fellowship under faculty
development program. One of the authors Sandesh Jadkar is thankful to University Grants Commission
(UPE program), New Delhi and Indo-French Centre for the Promotion of Advanced Research-CEFIPRA,
Department of Science and Technology, New Delhi for special financial support.
REFERENCES:
[1] P. Cheng, T. Wu, Y. Li, L. Jiang, W. Deng, and K. Han, New J. Chem., 2017, 41, 9598
[2] G. Hodes, Science 2013, 342, 317
[3] M. A. Green, A. H. Baillie, and H. J. Snaith, Nat. Photonics, 2014, 8, 506
[4] A. Kojima, K. Teshima, Y. Shirai, and T. Miyasaka, J. Am. Chem. Soc., 2009, 131, 6050
14

Page 15 of 18
New Journal of Chemistry
1
2
3
View Article Online
DOI: 10.1039/D0NJ01806F
[5] L. Etgar, P. Gao, Z. Xue, Q. Peng, A. K, Chandiran, B. Liu, and M. Grätzel, J. Am. Chem. Soc.,
4
5
6
2012, 134, 17396
7
8
9
[6] M. Liu, M. B. Johnston, and H. J. Snaith, Nature, 2013, 501, 395
[7] D. Dong, H. Deng, C. Hu, H. Song, K. Qiao, X. Yang, J. Zhang, F. Cai, J. Tang, and H.
Song, Nanoscale, 2017, 9, 1567
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
[8] H. Deng, X. Yang, D. Dong, B. Li, D. Yang, S. Yuan, K. Qiao, Y. B. Cheng, J. Tang, and H. Song,
Nano Lett., 2015, 15, 7963
[9] S. Tong, H. Wu, C. Zhang, S. Li, C. Wang, J. Shen, S. Xiao, J. He, J. Yang, J. Sun, and Y. Gao,
Org. Electron., 2017, 49, 347
[10] X. Yang, X. Zhang, J. Deng, Z. Chu, Q. Jiang, J. Meng, P. Wang, L. Zhang, Z. Yin, and J. You,
Nat. Commun., 2018, 9, 570
[11] H. Wu, S. Xu, H. Shao, L. Li, Y. Cui, and C. Wang, Nanoscale, 2017, 9, 16858
[12] S. J. Kim, J. Byun, T. Jeon, H. M. Jin, H. R. Hong, and S. O. Kim, ACS Appl. Mater. Interfaces,
2018, 10, 2490
[13] X. Wang, M. Shoaib, X. Wang, X. Zhang, M. He, Z. Luo, W. Zheng, H. Li, T. Yang, X. Zhu, L. Ma,
and A. Pan, ACS Nano 2018, 12, 6170
[14] Y. Zhong, Q. Wei, Z. Liu, Q. Shang, L. Zhao, R. Shao, Z. Zhang, J. Chen, W. Du, C. Shen, Y.
Zhang, P. Zhang, G. Gao, X. Xing, X. Liu, and Q. Zhang, Small 2018, 14, 1801938
[15] W. Hu, W. Huang, S. Yang, X. Wang, Z. Jiang, X. Zhu, H. Zhou, H. Liu, Q. Zhang, X. Zhuang, J.
Yang, D. H. Kim, and A. Pan, Adv. Mater. 2017, 29, 1703256
[16] T. Yang, F. Li, and R. Zheng, ACS Appl. Electron., 2019, 1, 1348
[17] T. Yang, Y. Zheng, Z. Du, W. Liu, Z. Yang, F. Gao, and Yang, W. ACS Nano., 2018, 12, 1611
[18] N. Noel, S. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A. Haghighirad, A. Sadhanala, G.
Eperon, S. Pathak, M. Jhonson, A. Petrozza, L. Herz, H. Snaith, Energy Environ. Sci., 2014, 7,
3061
[19] F. Hao, C. Stoumpos, D. Cao, R. Chang, and M. Kanatzidis, Nat. Photon., 2014, 8, 489
[20] R. Hoye, R. Brandt, A. Osherov, V. Stevanović, S. Stranks, M. Wilson, H. Kim, A. Akey, J.
Perkins, R. Kurchin, J. Poindexter, E. Wang, M. Bawendi, V. BuloviC, and T. Buonassisi, Chem.
Eur. J., 2016, 22, 2605
15

New Journal of Chemistry
Page 16 of 18
1
2
3
4
5
6
7
8
9
View Article Online
[21] M. B. Johansson, H. Zhu, and E. M. J. Johansson, J. Phys. Chem. Lett., 2016, 7, 3D4O6I:710.1039/D0NJ01806F
[22] H. Zhang, Y. Xu, Q. Sun, J. Dong, Y. Lu, B. Zhang, and W. Jie, CrystEngComm, 2018, 20, 4935
[23] P. Cheng, T. Wu, Y. Li, L. Jiang, W. Deng, and K. Han, New J. Chem., 2017, 41, 9598
[24] E. T. McClure, M. R. Ball, W. Windl, and P. M. Woodward, Chem. Mater., 2016, 28, 1348
[25] Z. Qi, X. Fu, T. Yang, D. Li, P. Fan, H. Li, F. Jiang, L. Li, Z. Luo, X. Zhuang, and A. Pan, Nano
Res., 2019, 12, 1894
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
[26] F. Bai, Y. Hu, Y. Hu, T. Qiu, X. Miao, and S. Zhang, Sol. Energy Mater. Sol. Cells, 2018, 184, 15
[27] C. Cuhadar, S. G. Kim, J. M. Yang, J. Y. Seo, D. Lee, and N. G. Park, ACS Appl. Mater.
Interfaces, 2018, 10, 29741
[28] X. Li, D. Bi, C. Yi, J. Décoppet, J. Luo, S. M. Zakeeruddin, A. Hagfeldt, M. Grätzel, Science, 2016,
353, 58
[29] X. Chen, H. Cao, H. Yu, H. Zhu, H. Zhou, L. Yang, and S. Yin, J. Mater. Chem., 2016, 4, 9124
[30] M. K. Assadi, S. Bakhoda, R. Saidur, and H. Hanaei, Renew. Sust. Energ. Rev, 2018, 81, 2812
[31] Y. Zhou, and N. P. Padture, ACS Energy Letters, 2017, 2, 2166
[32] Z. Zhou, Z. Wang, Y. Zhou, S. Pang, D. Wang, H. Xu, Z. Liu, N. Padture, and G. Cui, Angew.
Chem. Int. Ed., 2015, 54, 9705
[33] H. Wang, J. Tian, K. Jiang, Y. Zhang, H. Fan, J. Huang, and Y. Song, RSC Adv., 2017, 7, 43826
[34] Y. Yang, C. Wang, J. Hou, and J. Dai, Mater. Lett. 2003, 57, 2185
[35] S. Öz, J. C. Hebig, E. Jung, T. Singh, A. Lepcha, S. Olthof, and S. Mathur, Sol. Energy Mater. Sol.
Cells, 2016, 158, 195
[36] R. Jakubas, L. Sobczyk , Phase Trans. 1990, 20, 163
[37] B. W. Park, B. Philippe, X. Zhang, H. Rensmo, G. Boschloo, E. Johansson, Adv. Mater. 2015, 27,
6806-6813
[38] P. Szklarz, A. Pietraszko, R. Jakubas, G. Bator, P. Zielinski, M. Galazka, J. Phys. 2008, 20,
255221
[39] X. Chen, Y. Myung, A. Thind, Z. Gao, B. Yin, M. Shen, S. Cho, P. Cheng, B. Sadtler, R. Mishra,
and P. Banerjee, J. Mater. Chem., A, 2017, 5, 24728
[40] M. Kamminga, A. Stroppa, S. Picozzi, M. Chislov, I. Zvereva, J. Baas, A. Meetsma, G. Blake, and
T. Palstra, Inorg. Chem., 2017, 56, 33
16

Page 17 of 18
New Journal of Chemistry
1
2
3
View Article Online
[41] N. Rajamanickam, S. Kumari, V. K. Vendra, B. W. Lavery, J. Spurgeon, T. DrDuOffIe: 1l0,.1a0n39d/DM0N.J0K18.06F
4
5
6
Sunkara, Nanotechnology, 2016, 27, 235404
7
8
9
[42] H. Hsueh, R. Chen, H. Vass, S. Clark, G. Ackland, W. Poon, and J. Crain, Phys. Rev. B 1998, 58,
14812
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
[43] U. Bogner, and G. Roska, J. Lumin. 1981, 24-25, 683
[44] M. Wojtas, R. Jakubas, and J. Baran, Vib. Spectrosc., 2005, 39, 23
[45] D. Gedamu, I. M. Asuo, D. Benetti, M. Basti, I. Ka, S. G. Cloutier, and R. Nechache, Sci. Rep.,
2018, 8, 12885
[46] J. Tauc, Mater. Res. Bull. 1970, 5, 721
[47] C. Lan, J. Luo, S Zhao, C. Zhang, W. Liu, T. Ma, and S. Hayase, J. Alloy. Compd., 2017, 7, 834
[48] Z. Zhang, X. Li, X. Xia, Z. Wang, Z. Huang, B. Lei, and Y. Gao, J. Phys. Chem. Lett., 2017, 8,
4300
[49] T. Zhai, L. Li, X. Wang, X. Fang, Y. Bando and D. Golberg, Adv. Funct. Mater., 2010, 20, 4233
[50] F. H. L. Koppens, T. Mueller, P. Avouris, A. C. Ferrari, M. S. Vitiello and M. Polini, Nat.
Nanotechnol., 2014, 9, 780
[51] Z. Alaie, S. M. Nejad and M. H. Yousefi, Mater. Sci. Semicond. Process, 2015, 29, 16
[52] W. G. Luo, Y. F. Cao, P. A. Hu, K. M. Cai, Q. Feng, F. G. Yan, T. F. Yan, X. H. Zhang, and K. Y.
Wang, Adv. Opt. Mater., 2015, 3, 1418
[53] Y. F. Cao, K. M. Cai, P. A. Hu, L. X. Zhao, T. F. Yan, W. G. Luo, X. H. Zhang, X. G. Wu, K. Y.
Wang, and H. Z. Zheng, Sci. Rep., 2015, 5, 8130
[54] P. K. Kannan, D. J. Late, H. Morgan, and C. S. Rout, Nanoscale, 2015, 7, 13293
[55] M. Z. Hossain, S. L. Rumyantsev, K. M. F. Shahil, D. Teweldebrhan, M. Shur, and A. A. Balandin,
ACS Nano, 2011, 5, 2657
[56] A. A. Hussain, A. R. Pal, D. S. Patil, Org. Electron., 2014, 15, 2107
[57] W. W. Xiong, J. Q. Chen, X. C. Wu, and J. J. Zhum, J. Mater. Chem. C, 2015, 3, 1929
[58] S. Liu, Z. Wei, Y. Cao, L. Gan, Z. Wang, W. iXu, X. Guo and D. Zhu, Chem. Sci., 2011, 2, 796
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DOI: 10.1039/D0NJ01806F
TABLE OF CONTENT
SEM IMAGE
PHOTODETECTOR
RESPONSE
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