Towards Reduced Dielectric Loss and

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C: Energy Conversion and Storage; Energy and Charge Transport

Exploring the Carrier Dynamics in Zinc Oxide-Metal Halide Based Perovskites Nanostructures: Towards Reduced Dielectric Loss and Improved Photocurrent Radhamanohar Aepuru, Somen Mondal, Nandan Ghorai, Viresh Kumar, Himanshu Sekhar Panda, and Hirendra N. Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09403 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018

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The Journal of Physical Chemistry

Exploring the Carrier Dynamics in Zinc Oxide-Metal Halide Based Perovskites Nanostructures: Towards Reduced Dielectric Loss and Improved Photocurrent Radhamanohar Aepuru,§ Somen Mondal,§ Nandan Ghorai,§ Viresh Kumar,‡ H.S. Panda,‡ Hirendra N. Ghosh§,†* §

Institute of Nano Science and Technology, Mohali, Punjab 160064, India.

†

Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai -400085,

India. ‡

Defence Institute of Advanced Technology, Pune-411025, India.

Corresponding Author *E-mail: [email protected], [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT: Metal-halide based perovskites have emerged as a potential candidate for optoelectronic applications due to their impressive performance achieved by tuning the optical/electrical properties through tailoring the perovskite nanostructures. Herein, we report the synthesis of composite nanostructures by incorporation of ZnO (~6 nm) into CsPbBr 3 (CPB) perovskite framework, which has significant enhancement of photocurrent, due to efficient interfacial charge separation and reduced dielectric loss. Detailed steady state and time resolved PL studies have been carried out to understand charge transfer dynamics in CsPbBr 3/ZnO nanostructure composite system. Femtosecond transient absorption and broadband dielectric spectroscopy studies were carried out to determine the charge carrier relaxation and transfer mechanism. Redox energy level diagram suggests photo-excited electron from conduction band (CB) CPB can be transferred to the CB of ZnO NP due to thermodynamic viability. Ultrafast studies reveal the electron transfer take place from the perovskite nanostructures to ZnO NP within ∼500 fs and limits of the recombination process by efficient charge separation and charge accumulation at the interfaces. Dielectric studies also reveal reduced charge leakage in composite nanostructures with efficient charge separation by facilitating the charge accumulation at the interfaces. Overall, the efficient charge transfer and slow carrier recombination with reduced dielectric losses significantly improved the photocurrent behavior CsPbBr3/ZnO nanostructure composite system as desired for optoelectronic devices.


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INTRODUCTION: Exploring novel optoelectronic materials for various energy conversion/storage devices such as photovoltaic, light-emitting diodes and photodetectors have been witnessed in the research community over the past years.1-4 Amongst, perovskites a family of semiconductors have captivated potential interest in the field of optoelectronics due to their superior optical and electronic properties.4 Inorganic/organic perovskites have been studied extensively due to their low cost processing, tunable band gap, high optical absorption and high power conversion efficiency (> 20 %).5-7 Besides the advantageous, the major concerns in organic-inorganic perovskites e.g. CH3NH3PbI3 (MAPbI3) have poor thermal stability and are very sensitive to moisture.6-8 As a result using these materials for device fabrication is always a challenging task. On the other hand, a new class of perovskite materials which are all-inorganic in nature are found to be more stable as compared to organic-inorganic perovskite in ambient condition have been investigated extensively for several applications such as photovoltaic, light-emitting diodes and photodetectors.9-11 This class of inorganic perovskites eg. colloidal cesium lead trihalide was ﬁrst reported by Kovalenko et al.12 are found to have high thermal stability, higher order absorption crosssection, narrow bandwidth photoluminescence (PL) with tunable behavior, and high emission quantum yield (∼90%).12-17 With these prominent properties, the lead-based cesium trihalide nanocrystals have been consequently started in research for various applications.13, 18-24 Due to their exciting optical/electrical properties like adjustable spectral absorption ranges, high light absorption coefficients, intense photoluminescence, low rates of non-radiative charge recombination, long charge diffusion lengths and high carrier mobility25-30 all-inorganic perovskite materials are found to be excellent photoactive materials for photodetectors 3 ACS Paragon Plus Environment


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(PDs).31-34 For optoelectronic applications halide perovskites are simply processed into polycrystalline thin films on various substrates. It is reported in literature that the crystal structures, morphology, and optoelectronic properties of halide perovskites strongly depend on both compositions and synthetic procedure. Numerous efforts have been made to develop allinorganic perovskites (CsPbX3) of varying size, shape and composition to obtain the desired properties.35, 36 In a recent report, a facile anti-solvent precipitation method was employed for the synthesis of various colloidal nanocrystals (quantum dots, nanorods, nanocubes etc.) under ambient conditions by varying the medium, ligand content and reaction time and exhibited morphology dependent

photoluminescence

(PL)

properties.35

To realize superior

optoelectronic behavior like both in solar cell and photo-detector higher charge separation and slower charge recombination is expected. In our previous study colloidal CsPbBr 3 composite system was synthesized by hot injection method and studied the charge transfer and recombination dynamics in ultrafast time scale.6 A colloidal heterovalent (Bi3+) doped CsPbBr3 nanocrystals were prepared by hot injection method and studied interfacial charge transfer process in ultrafast time scale. The results suggested, efficient charge transfer in the nanocrystals is facilitated by metal doping which increases the driving-force between the molecular acceptor and donor moieties upon metal doping. 36 It has been realized to improve the device performance, major requirements are efficient charge separation and transfer at the interfaces, slow recombination process and large carrier mobility. 37,38 Recently, Xin Hu et al. demonstrated CH3NH3PbI3 (MAPbI3) based photodectectors and measured the photocurrent behavior at different wavelength of incident light.39 Similarly, Zhenjun Tan et al. demonstrated photocurrent behavior of 2D hybrid (C4H9NH3)2PbBr4 perovskite based photodetectors which have shown extremely low dark current (∼10-10 A).40 Very recently a new strategy has been


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adopted where hybrid nanostructures were realized by introducing carrier tracks (CNTs, Au and ZnO nanoparticles) in the nanostructures to promote charge transportation, which influences the optoelectronic performance.41-43 Despite extensive research on CsPbBr3 based optoelectronic devices, the detailed mechanism of carrier relaxation and transfer process of the induced carriers at the interfaces and to QDs in the hybrid or composite systems are not fully explored. Very recently, Pavel et al. proposed a kinetic model to explain the photodynamics of MAPbI3 matrix embedded with PbS/CdS QDs using transient absorption and terahertz spectroscopy. The model describes majority of the carriers in perovskite domain diffuse towards QDs, in which electrons are diffused towards the MAPbI3/PbS-CdS interface and holes are migrated towards the valence band of PbS/CdS QDs. 44 Now to improve the device performance of CsPbBr3 based photodetectors, understanding the carrier dynamics in these composite systems are need to be explored. Critical questions such as carrier lifetime, recombination dynamics and their influence on the dielectric properties are needed to be addressed before device fabrication. In the present investigation, we have demonstrated simple and facile technique for the synthesis of CsPbBr3 nanostructures decorated with ZnO quantum dots. The characterization of the CsPbBr3 nanostructures was carried out through steady state optical absorption and luminescence spectroscopy as well as X-ray diffraction and high resolution TEM studies. The detailed carrier dynamics were investigated by using various spectroscopic techniques such as time-resolved absorption and emission techniques and correlated with broadband dielectric impedance spectroscopy. It has been observed that upon photoexitation and applied electric field the charge carriers are transported to the interfaces of CsPbBr3/ZnO composite. The dielectric results suggest reduced charge leakage in the composites which further remarkably 5 ACS Paragon Plus Environment


improved the photocurrent behavior by limiting the recombination of photoexcited electron– hole pairs in the perovskites. To our surprise the photocurrent of the CsPbBr 3/ZnO composite nanostructures is significantly enhanced as compared to the CsPbBr3 nanostructures. Overall, the newly synthesized composite nanostructures exhibit excellent optical and electrical properties which have a wide potential application in various optoelectronic devices such as photodetectors and solar cells etc.

EXPERIMENTAL SECTION: Synthesis of zinc oxide quantum dots (ZnO QDs): ZnO QDs were prepared by using solvothermal process reported earlier. 45 In brief, Zinc acetate dihydrate [Zn(CH3COO)2.2H2O] (0.46 g) was dissolved in 50 ml of DMF. The solution was gradually heated to 95 oC and maintained for 5 hours. The resultant ZnO QDs solution was whitish color and subsequently washed with ethanol several times by centrifugation, and finally redispersed in toluene. The as-prepared ZnO QDs were further utilized to synthesized CsPbBr3-ZnO composite nanostructures. Synthesis of CsPbBr3-ZnO composite nanostructures: Pristine CsPbBr3 nanostructures were prepared by using a facile, cost effective and room temperature chemical route as reported earlier.42 In similar manner, CsPbBr3-ZnO nanostructures were prepared by in-situ addition of ZnO QDs. Briefly, 0.212 g of CsBr and 0.183 g of PbBr2 were dissolved in a solution of 14 mL DMSO and 1 mL HAc. After complete dissolution, ligand (0.2 g of octadecaylamine dissolved in 10 mL HAc) was mixed to the above metal halide solution in continuously stirring condition. Thereafter, CsPbBr 3 nanostructures were formed instantly with addition of well dispersed ZnO QDs (3 wt %) in toluene. The 6 ACS Paragon Plus Environment

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resultant precipitate was centrifuged and washed in toluene to remove residual ligands and was re-dispersed in toluene.

CHARACTERIZATION: The crystal structure of CsPbBr3 nanostructure, ZnO QDs and CsPbBr 3-ZnO composite nanostructures were determined through powder X-ray diffractometer (BRUKER D8 ADVANCE) with Cu-Kα radiation (λ = 1.542 Å) with scanning angle (2𝜃) range of 10-80o. The morphology of the synthesized nanostructures were obtained by using Transmission Electron Microscope (JEOL JEM 2100) at an accelerating voltage of 200 kV. The absorption spectra were recorded using a Shimadzu UV-2600 UV-vis spectrophotometer. Fluorescence measurements were made using Edinburge FS5 spectrofluorometer. The fluorescence lifetimes were measured by the method of time-correlated single-photon counting using a Deltaflex Modular Fluorescence Lifetime System (HORIBA Scientific) with nano-LED pulsed diode light source of 402 nm. The instrument response function (IRF) of the setup was ~200 ps. Femtosecond pump–probe transient absorption studies were performed using Ultrafast Systems HELIOS transient absorption spectrometer. The laser system is a Coherent regenerative amplifier (Astrella Ultrafast Ti:Sapphire Amplifier) (800 nm with repetition rate of 1 kHz and pulse duration of 1ns (45%) 503 ps (-35%) 149 ps (50%)

>1ns (-2%)

533 nm CsPbBr3ZnO

510 nm 533 nm

1.35 ps (66.5%) 590 fs (-100%)

>1ns (-15%) >1 ns (15%)

We have also monitored the bleach recovery kinetics at 533 nm for CsPbBr 3 nanostructures, which can be fitted with single exponential growth with time constant of g = 600 fs (100%) and bi-exponential decay with time constants of τ1 = 1 ps (55%), and τ2 > 2 ns (45%) (Table 1). The single exponential growth component (600 fs) can be attributed to carrier cooling dynamics from upper excitonic states to the conduction band edge. The bleach recovery components of the kinetics trace can be attributed to the recombination reaction between photoexcited electron and hole in CsPbBr3 nanostructures. To determine the electron transfer dynamics correctly we have carried out TA studies in CsPbBr3-ZnO composite nanostructures 18 ACS Paragon Plus Environment

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in ultrafast time scale after exciting the samples at 420 nm and shown in Figure 4B. The transient spectrum of CsPbBr3-ZnO composite nanostructures looks very similar to that of pure CsPbBr3 nanostructures. However, over a close look it’s clearly suggest that intensity of both positive absorption band and bleach band reduces. In particular, bleach intensity reduces drastically and positive absorption band shows a hump at 532 nm. Now to understand charge carrier and charge transfer dynamics we have monitored the kinetics at 510 nm (Figure 5b) and 533 nm (Figure 5d) and compared with pure CsPbBr3 nanostructures. In the composite nanostructures, the decay kinetics of excited CsPbBr3/ZnO are significantly affected by the presence of ZnO. The TA kinetic trace Figure 5b at 510 nm suggests the signal of excited loosing ~76% within 200 ps. The residual signal ~26% of the initial intensity is left after 200 ps. The fast decay is mainly caused by electron transfer from the perovskite CB to the CB of ZnO minimizing the intraband transitions in CsPbBr3 which supported by the decrease in positive TA intensity of CsPbBr3-ZnO composite nanostructures compared to CsPbBr3 nanostructures. The kinetics at 510 nm for CsPbBr3-ZnO composite nanostructures is fitted with bi-exponential growth with time constants of τ1g= 200 fs (33.5%), and τ2g= 1.35 ps (66.5%) and multi-exponential recovery with time constants τ1= 61 ps (50%), τ2= 503 ps (35%), and τ3= >1ns (15%) respectively (Table 1). The growth and decay kinetics of positive absorption band at 510 nm in CsPbBr3-ZnO composite nanostructures is slower as compared to pure CsPbBr3 nanostructures, which might be due to charge separation between CsPbBr 3 and ZnO in the composite system. On the other the kinetics at 533 nm can be fitted multiexponentially with single exponential bleach with time constant τ1

growth

= 390 fs (-32%)

followed by single exponential growth of positive absorption of 590 fs (100%) and followed by tri-exponential decay of positive absorption with time constants τ1 = 25 ps (-3%), τ2= 149



ps (-50%) and τ3= >1 ns (-15%). The kinetics at the excitonic bleach looks completely different for CsPbBr3 nanostructures as compared to that of CsPbBr3-ZnO composite nanostructures (Figure 5B). The intensity of the original bleach drastically reduced which eventually appears as positive absorption with much longer lifetime. In general, the exciton bleach in semiconductor /quantum dot materials appears due to presence of electron in the conduction band. In the present system, the decrement of bleach intensity suggests disappearance of electron from the CsPbBr3 nanostructures by the electron transfer process from CsPbBr3 nanostructures to ZnO. The growth time 590 fs of positive absorption at 533 nm can be attributed to the electron transfer time from the CB of CsPbBr 3 to ZnO in the composite nanostructures. Overall, the multi-exponential decay components suggest recombination process is minimized due to the charge transfer in the composite nanostructures. Femto-second transient absorption studies clearly suggests that due to photo-excitation in the CsPbBr 3-ZnO composite nanostructures we can observe efficient and higher charge separation in the composite nanostructures as compared to pure CsPbBr 3 nanostructures, which eventually can improve photo-conductivity performance. Dielectric studies: Broadband dielectric/impedance spectroscopy is one of the most promising, and powerful technique to investigate the overall electrical properties of materials over a wide range of temperature and frequency. Previously, dielectric and impedance spectroscopic studies were carried out on various nanostructures/nanocomposites materials. 55-58 The significant contribution of dielectric behavior such as dielectric loss, charge separation, carrier relaxation and carrier mobility influencing the device performance can be demonstrated by using these techniques. Till date in the literature, dielectric properties of CsPbBr 3 based composite 20 ACS Paragon Plus Environment

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nanostructures were not demonstrated and also direct correlation between dielectric properties and ultrafast charge transfer dynamics were never discussed in any of the hetero-structure materials. After following the charge carrier dynamics through time-resolved luminescence and Femto-second transient absorption studies it is expected that CsPbBr 3-ZnO composite nanostructures can be better materials for photo-detectors as compared to pure CsPbBr 3 nanostructures. In the present investigation frequency dependent dielectric permittivity and dielectric loss have been measured in both the materials. Figure 6A shows the frequency dependent permittivity (ԑ') and Figure S4 shows imaginary part of permittivity of newly synthesized CsPbBr3 nanostructures and CsPbBr3-ZnO composite nanostructures. Frequency dependent real (ԑ') and imaginary (ԑ'') parts of complex permittivity (ԑ*) expressed as: 55,59,60 ԑ∗ = ԑ′ − 𝑗 ԑ′′ =

ԑ′ = −

ԑ′′ = −

Where Co =

ԑ

′

′′

′′

(2a)

′′

′

′ ′

(2)

(2b)

′′

, which is the capacitance of the empty cell, ԑ o is the permittivity of the vacuum

(8.854 × 10-12 Fm-1), A is the area of the sample and d is the thickness of the sample. 𝑍 and 𝑍′′ are the real and imaginary parts of the impedance respectively. The dielectric permittivity found to decrease in composite nanostructures (CsPbBr 3-ZnO) as compared to pure CsPbBr3 nanostructures which can be attributed to improving carrier mobility of CsPbBr3 nanostructures after incorporation of ZnO QDs in the composite nanostructures. However, both the samples showed enhanced dielectric polarization and strong



frequency dependent at lower frequencies (< 104 Hz) which are due to hopping of

Permittivity (')

electrons/holes caused by Maxwell-Wagner-Sillars (MWS) effect and named as space charge/

100

tan  -3

A

CsPbBr3 CsPbBr3-ZnO

75 50 25 1.5

Modulus'' (10 )

1 2 3 4 5 6 7 8 9 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

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B

1.0 0.5

10 8

C

6 4 2

3

10

4

5

10 10 Frequency (Hz)

6

10

Figure 6: Frequency dependent (A) real part permittivity, (B) dielectric loss (tan and (C) imaginary part electric modulus of CsPbBr3 and CsPbBr3- ZnO composite nanostructures. interfacial polarization, thereafter decreased slowly at higher frequencies which are due to dominant orientation polarization.58,61,62 Here in at higher frequencies the dipoles do not have much time to polarize and lag behind the applied electric field, as a result decrease in dielectric permittivity was observed. On the other hand, Figure 6 B shows the dielectric loss tangent (tan 𝛿) with respect to frequency for both the systems. The dielectric loss (tan δ) of a material denotes quantitatively dissipation of the electrical energy due to different physical phenomena such as electrical conduction, dielectric relaxation, and losses occurred through non-linear processes. It’s extremely important to determine the dielectric loss in the materials for microelectronic and optoelectronic applications which show significant contribution in the


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device performance. The dielectric loss tangent or loss factor can be expressed as shown in equation 3: 59 𝑡𝑎𝑛 𝛿 =

ԑ

(3)

ԑ

Figure 6B shows that both the samples show high dielectric losses at lower frequency and with increasing the frequency the losses decreased drastically which is in-line with the permittivity results. Interestingly, the dielectric loss in composite nanostructures (CsPbBr 3-ZnO) is significantly less as compared to CsPbBr3 nanostructures which are due to strong interfacial polarization inducing relaxation effects caused by dipole impurities. Moreover, the higher dielectric loss in CsPbBr3 nanostructures suggests charge leakage caused by energy dissipation. The reduced dielectric loss in composite nanostructures is put forward for potential device applications. Further, to know the relaxation effects, the imaginary part of the electric modulus was recorded and is outlined as: 60,62

𝑀 ∗ = 𝑀 + 𝑖𝑀 =

𝑀 =

ԑ′ ԑ′ + ԑ′′

(4𝑎) ;

1 ԑ∗

𝑀 =

ԑ′′ ԑ′ + ԑ′′

(4)

(4𝑏)

Where 𝑀′ and 𝑀′′are the real and imaginary parts of the complex electric modulus. Figure 6C, shows frequency dependent imaginary part electric modulus plot of CsPbBr 3 nanostructures and CsPbBr3-ZnO composite nanostructures. It’s interesting to see that both the samples exhibited relaxation effect, however higher change is observed at lower frequencies in composite nanostructures as compared to pure CsPbBr 3 nanostructures. The shift of the relaxation peak towards lower frequency in composite nanostructures clearly suggests slower



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relaxation time (τ) of the generated charge carriers which has been determined by the position of maximum frequency (fm) peak using the relation τ = 1/2πfm.57 The life time of carrier relaxation is illustrated in the Table 2, which clearly indicates efficient charge separation thereby limiting relaxation process. Hence, the carriers with longer life time (τ) values have positive effect on the overall device performance. Further, temperature dependent dielectric studies were carried out to understand the carrier transportation and relaxation mechanism. Table 2. Dielectric loss and relaxation time and activation energy in different systems a Sample CsPbBr3 CsPbBr3-ZnO

Dielectric loss at 1 kHz ~1.1±0.03 ~0.5±0.013

Relaxation lifetime ( τ) ~8.9±0.27 s ~0.103±0.003 ms

Activation energy (Ea) ~0.4±0.008 eV ~0.38±0.009 eV

The variation in the dielectric properties of the nanostructures with respect to temperature (100 oC to 120 oC) were recorded and shown in supporting Figure S5. To know the carrier relaxation process frequency dependent imaginary part of electric modulus were plotted at various temperature (-100 oC to 225 oC) and shown in Figure 7. Relaxation peaks were observed in both the samples (CsPbBr3 and CsPbBr3-ZnO) and the peaks were found to be shifted towards higher frequency with increase in temperature, which signifies the hoping of charge carriers at lower frequencies. At the higher frequencies, the charge carriers are spatially confined at their potential wells and make short range mobility and the transition mobility from long range to short range was clearly observed in both the nanaostructures. 63 The shifting of the peaks was observed in both the nanostructures towards higher frequency with increase in temperature, and confirmed the typical characteristics of Maxwell-Wagner-Sillars (MWS) polarization. This observation can be attributed to distribution of relaxation peaks and due to the thermal fluctuations of the lattice.62 Also, variation in the dielectric modulus of the 24 ACS Paragon Plus Environment

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15

A

CsPbBr3

B

CsPbBr3-ZnO

o

-100 C o -75 C o -50 C o -25 C o 0 C o 25 C o 50 C o 75 C o 100 C o 125 C o 150 C o 175 C o 200 C o 250 C

10

Modulus'' (x10 -3)

1 2 3 4 5 6 7 8 9 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


5 0

10 5 0

0

10

1

10

2

10

3

10

4

10

Frequency (Hz)

5

10

6

10

Figure 7: Frequency dependent imaginary part electric modulus plot of (A) CsPbBr 3 nanostructures and (B) CsPbBr3- ZnO composite nanostructures at various temperatures. nanostructures with respect to temperature (-100 oC to 120 oC) were recorded and shown in supporting Figure S5. Both the samples show relaxation behavior as a function of temperature at fixed frequency (1kHz). Moreover, the observed relaxation peak is shifted to higher temperature in composite nanostructures. Also, frequency dependent studies at various temperatures (Figure S6) suggests, both the samples (CsPbBr 3 and CsPbBr3-ZnO) exhibited frequency independent behavior at higher temperatures. The conductivity increases with temperature due to the increase of charge carrier mobility, indicating the contribution of a dielectric conduction mechanism (such as bulk-limited conduction or electrode-limited conduction). In addition, the temperature dependent carrier relaxation mechanism process is analyzed by determining the activation energies for the newly synthesized nanostructures and shown in Figure 8. The activation energies are determined by using the Arrhenius equation: 58



(5)

𝑓 = 𝑓 𝑒𝑥𝑝 −

Where f and T are the frequency maximum and temperature in electric modulus, Ea is the activation energy determined from the slope and k is the Boltzmann’s constant.

15

CsPbBr3 CsPbBr3-ZnO

12 ln(fmax)

1 2 3 4 5 6 7 8 9 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

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9 6 3 0 0.002

0.003

1/K

0.004

0.005

Figure 8: Arrhenius plots with activation energies of CsPbBr3 nanostructures and CsPbBr3ZnO composite nanostructures. The activation energies (Ea) for CsPbBr3 nanostructure and CsPbBr3- ZnO composite nanostructure are determined to be 0.40 eV and 0.38 eV respectively. The activation energy describes the minimum energy required for the charge carriers to cross the barriers and accumulated at the interfacial regions. The lower Ea in composite nanostructures as compared to pure CsPbBr3 nanostructure can be attributed due to the higher contribution of MWS polarization which enhances carrier mobility with reduced charge leakage. Further, to understand the device characteristics in the newly synthesized nanostructures I-V measurements were carried out in dark and under illumination of light and shown in Figure 9.


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2.0

1.6

Current (A)

1 2 3 4 5 6 7 8 9 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


CsPbBr3 ZnO_Dark

1.2

CsPbBr3 ZnO_Light CsPbBr3_Dark

0.8

CsPbBr3_Light

0.4

0.0 0

2

4

6

8

10

Voltage (V)

Figure 9: Current–voltage (I–V) measurement of CsPbBr3 and CsPbBr3-ZnO composite nanostructures thin films under dark/light conditions. Inset shows the schematic diagram of IV measurements. Here in the photocurrent behavior of the nanostructures plays a key role in the device performances. Initially, the photocurrent was measured in the range of -2 to +2 V and observed linear increment in the current for both the nanostructures (CsPbBr3 and CsPbBr3-ZnO samples) to the applied voltage as shown in Figure S7. Thereafter, the measurements were carried out by applying the positive bias (0 to 10 V) and shown in Figure 9. The dark current of the composite nanostructures with ZnO showed ~ 0.21 A under an applied voltage of 10 V, which is slightly higher as compared to CsPbBr3 nanostructures (~0.18 A). However, interestingly the photocurrent is enhanced in composite nanostructures (~1.8A) as compared to CsPbBr3 nanostructures (~0.9 A) at 10 V. The sharp increase in photocurrent is due to higher yield of electron-hole pairs under similar incident light on the devices. In case of 27 ACS Paragon Plus Environment


composite nanostructures the photo-exited charge carriers overcome the barrier and get accumulated at the interfaces which limits the recombination process as a result carrier transportation and tunneling become easy. Similar type of phenomenon was reported in photocurrent measurements of various perovskite based nanostructures. 39,64 The observed change in photocurrent and dark-current (I ) ~ 0.5 A is in good agreement with reported MAPbI3 based system.39 Overall, both the nanostructures showed considerable increase in photocurrent behavior under illumination of light than measured under dark conditions. Under the applied voltage the carriers drift towards respective terminals and the photocurrent enhancement has been observed, which is due to the efficient photo-excited charge (electron and hole pair) separation and reduced charge leakage in the perovskites composite nanostructures. Our observation suggests that CsPbBr 3-ZnO composite nanostructures is better materials for both in terms of photoconductivity and dielectric behavior as compared to pure CsPbBr3. This observation provides a clear idea of direct correlation between ultrafast charge carrier dynamics as well as photoconductivity and dielectric behavior in CsPbBr3-ZnO composite nanostructures. Conclusions: In summary, CsPbBr3 based nanostructures were synthesized by simple and facile route. The charge transfer mechanism of CsPbBr3–ZnO composite nanostructures has been demonstrated by using various spectroscopic techniques. Steady state and time-resolved luminescence spectroscopy suggests the nanostructures absorbs in visible region and the photo-excited electrons in composite nanostructures are transferred from conduction band (CB) of CsPbBr 3 to CB ZnO as the process thermodynamically viable. Dielectric studies reveal efficient charge


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separation with reduced loss and slower carrier relaxation time (~0.103 ms) with longer life time (τ) in the composite nanostructures. Furthermore, charge carrier dynamics was monitored through ultrafast transient spectroscopy and has been correlated with the broadband dielectric studies. The electron transfer time was found to be ~500 fs and slow charge recombination was observed in the composite nanostructures which overall improved the photocurrent behavior. It has been realized that the photocurrent enhancement is due to the efficient charge separation, which has been achieved by limiting the recombination of photoexcited electron–hole pairs and also due to reduced charge leakage in the composite nanostructures. To the best of our knowledge, for the first time we are reporting direct correlation between ultrafast charge carrier dynamics and both dielectric loss and photocurrent behaviour of any photo-detector materials. It has clearly been realized that to design and develop of efficient photodetector and other optoelectronic devices its utmost important to understand carrier dynamics in photodetector materials. SUPPORTING INFORMATION: X-ray diffraction, TEM image of ZnO QDs and steady state absorption spectra of ZnO QDs. TEM image of CsPbBr3 nanostructures. HRTEM and EDX images of composite nanostructures. Imaginary part of electrical modulus w.r.t. temperature and real part of conductivity at various temperatures of CsPbBr3 nanostructures and CsPbBr3-ZnO composite nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION: Corresponding Author *E-mail: [email protected], [email protected]



ACKNOWLEDGMENT: The authors acknowledge the SERB, Department of Science and Technology, Govt. of India for financial support under “National Postdoctoral Fellowship” (Project Sanction No.: PDF/2017/002124). The authors thank Dr. Jayanta Dana and Dr. Sourav Maiti for their support during the work. REFERENCES: (1) Zhang, W.; Eperon, G. E.; Snaith, H. J. Metal Halide Perovskites for Energy Applications. Nat. Energy 2016, 1, 16048 1-8. (2) Chris de, W.; Leyre, G.; Hong, Z.; Wybren J. B.; Georgian, N.; Maksym V. K.; Tom, G. Energy Transfer between Inorganic Perovskite Nanocrystals. J. Phys. Chem. C, 2016, 120, 13310–13315. (3) Zhou, H.; Shi, Y.; Wang, K.; Dong, Q.; Bai, X.; Xing, Y.; Du, Y.; Ma, T. LowTemperature Processed and Carbon-Based ZnO/CH3NH3PbI3/C Planar Heterojunction Perovskite Solar Cells. J. Phys. Chem. C, 2015, 119, 4600–4605. (4) Tang, X.; Zu, Z.; Shao, H.; Hu, W.; Zhou, M.; Deng, M.; Chen, W.; Zang, Z.; Zhu, T.; Xue, J. All-Inorganic Perovskite CsPb(Br/I)3 Nanorods for Optoelectronic Application. Nanoscale 2016, 8, 15158–15161. (5) Huang, J.; Yuan, Y.; Shao, Y.; Yan, Y. Understanding the Physical Properties of Hybrid Perovskites for Photovoltaic Applications. Nat. Rev. Mater. 2017, 2, 1-19. (6) Maity, P.; Dana, J.; Ghosh, H. N. Multiple Charge Transfer Dynamics in Colloidal CsPbBr 3 Perovskite Quantum Dots Sensitized Molecular Adsorbate. J. Phys. Chem. C 2016, 120, 18348–18354. 30 ACS Paragon Plus Environment

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