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Ligand Engineering to Improve the Luminance Efficiency of CsPbBr Nanocrystal Based Light Emitting Diodes 3

Naresh K. Kumawat, Abhishek Swarnkar, Angshuman Nag, and Dinesh Kabra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00723 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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The Journal of Physical Chemistry

Ligand Engineering to Improve the Luminance Efficiency of CsPbBr3 Nanocrystal Based Light Emitting Diodes Naresh Kumar Kumawat1†, Abhishek Swarnkar2†, Angshuman Nag2,3* and Dinesh Kabra1* 1

2

3

Department of Physics, Indian Institute of Technology (IIT) Bombay, Mumbai400076 (India)

Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune-411008 (India)

Center for Energy Science, Indian Institute of Science Education and Research (IISER), Pune-411008, (India)

† Authors contributed equally

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Abstract In this paper, we present high-efficiency perovskite light emitting diodes (PeLEDs) using high quality green emissive colloidal CsPbBr3 nanocrystals (NCs) of size 11±0.7 nm. Surface chemistry of the NCs was optimized to achieve both reasonably high luminescence and charge transport in NC film. Thickness of active layer film was controlled by layer-by-layer deposition technique, where each layer is being deposited and washed using methyl acetate (MeOAc) before we put another layer of NCs. MeOAc being low surface tension solvent, enters into the NCs layers and partially removes the long-chain organic ligands oleylamine and oleic acid. The optical and structural properties were measured as prescreening for this device grade NCs. Steady-state photoluminescence (PL) and electroluminescence (EL) FWHM (full width at half maxima) is in the range of 20±0.5 nm suggest high color purity feature of these materials suitable for higher color gamut in the display applications. ITO/PEDOT:PSS/CsPbBr3-NCs/TPBi/Ca/Al (TPBi is 2,2',2"-(1,3,5Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) device structure is fabricated with various thickness of NCs layer and we demonstrated a maximum of ~ 5.2 cd/A luminance efficiency for optimal thickness of CsPbBr3 NC layer.


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Introduction Organic-inorganic hybrid lead halide perovskite (CH3NH3PbX3, X = Cl, Br and I) materials have become attractive in the scientific community because solar cell efficiency reached 22.1% within a few years.1,2 These materials have extraordinary optoelectronic properties, like long diffusion length (LD~1µm, in a thin film), high and balanced charge carrier mobility (1-10 cm2/V-s), low trap density, narrow PL emission (~18 nm), and higher photoluminescence quantum yield (PLQY).3,4 These unique properties make them suitable for efficient light emitting diodes (LEDs) and within a short time, external quantum efficiency (EQE) of perovskite LED (PeLED) reached ~14.36% (green emission). 5 This EQE is comparable to OLED (organic light-emitting diodes). 6 But low stability of PeLED is the main barrier for device operation. The major reason for poor thermal stability of organic-inorganic hybrid perovskites is the volatile nature of the organic component like CH3NH3+. Therefore, replacing organic CH3NH3+ with inorganic Cs+ provide better thermal stability (decomposition temperature ~500 oC) of CsPbBr3.7 This all-inorganic CsPbBr3 was employed for PeLED fabrication by developing a solution processed method but the thin film properties were not so great for device application due to coverage issue as shown by Yantara et al.8 In this regard, colloidal nanocrystals (NCs) or quantum dots (QDs) combine both solution processibility and excellent PLQY. Colloidal CsPbBr3 NCs show a PLQY 85±5% , with reduced PL blinking.9,10 The color of the PL can be tuned in the visible region just by varying the halide compositions of the NCs.11,12,13,14 Interstingly, Swarnkar et al. showed that not only thermal stability, the surface chemistry of NCs can be optimized to achieve cubic (α-) phase of CsPbI3 NCs at 3 ACS Paragon Plus Environment


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room temperature, unlike their bulk counterpart. 15 So colloidal perovskite NCs provide new properties for optoelectronic applications. Song

et

al.

demonstrated

first

time

green,

blue

and

orange

electroluminescence (EL) using these CsPbX3 NCs, and reported current efficiency of 0.43 cd/A (EQE~ 0.12%), 0.14 cd/A (EQE~0.07%) and 0.08 cd/A (EQE~0.09%), respectively. 16 Li et al. reported NC PeLEDs

using cross linking method and

achieved 5.7% EQE for red emission.17 Zhang et al. using mix phase of CsPbBr3CsPb2Br5 perovskite NCs and achieved 8.98 cd/A luminance efficiency with 2.2% EQE.18Zhang et al. treated CsPb(Br/I)3 NCs with polyethylenimine for better electron injection with ZnO and reported RGB (red, green and blue) emissive LED with 7.25%, 0.40% and 0.61% EQE respectively. 19 Similarly, Mg-doped ZnO (n-type), perfluorinated ionomer (PFI, p-type) and NiO (p-type) layers are useful for better charge injection in PeLEDs.20,21 A major problem of such colloidal NCs is the insulating organic ligands that are used during synthesis to dissolve the reaction precursors, to passivate the surface of NCs, 22 and to control the size of the NCs. These ligands inhibit the injection of charge carriers into NCs when deposited as film to fabricate an optoelectronic device. Partial or complete removal of these ligands from the surface of NCs may improve both the charge transfer and charge transport efficiencies, improving the efficiencies of solar cell,15 LEDs,23 and photodiodes.24 However, such surface modifications need to be carefully designed, such that the surface defects are reasonably well-passivated. Here, we report the effect of surface treatment of CsPbBr3 NCs with methylacetate (MeOAc) on PL and EL properties. We study the role of NC film thickness on the performance of PeLED. Optimization of reduced PL due to ligand washing versus charge-carrier balance demonstrates maximum 4 ACS Paragon Plus Environment

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luminance efficiency of ~5.2 cd/A (average efficiency of 2.32 cd/A over 6 devices) and EQE ~1.3% using ITO/PEDOT:PSS/CsPbBr3-NCs/ TPBi/Ca/Al PeLED device structure. Experimental Section Synthesis of Cs-oleate: Details of chemicals are given in the supporting information (SI). 0.5 g of Cs2CO3, 2 mL oleic acid (OA) and 100 mL 1-octadecene (ODE) were added to a 200 mL 3-necked round bottom flask and stirred under vacuum for 60 min at 120 °C. The reaction was considered complete when the solution was clear, indicating that the Cs2CO3 had reacted with the OA. The Cs-oleate solution in ODE was stored in nitrogen filled state until it was needed for the synthesis of NCs. Synthesis and purification of colloidal CsPbBr3 NCs: CsPbBr3 NCs were synthesized following the previously reported work of Protesescu et al.9 with some modifications in scale of synthesis and NC purification. The synthesis protocol was scaled up ~12 times to preparer larger amount of NCs. 11±0.7 nm size of colloidal CsPbBr3 NCs were synthesized. 0.8 g PbBr2 and 50 mL ODE were stirred in a 500 mL round bottom flask and degassed under vacuum at 120 °C for 1 hour. The flask was then filled with nitrogen and kept under constant flow of nitrogen. OA and oleylamine (OAm) (5 mL each, pre-heated at ~70 °C) were injected to the flask. The flask was put under vacuum again until the PbBr2 completely dissolved and the solution was no longer releasing gas bubbles (15 - 30 min). The temperature was then increased to 200 °C. The Cs-oleate (~0.033 M, 16 mL), pre-heated to ~70 °C under nitrogen atmosphere, was swiftly injected into the reaction mixture. The reaction mixture turned greenish yellow and the reaction was quenched after 2-3 min by dipping the reaction flask into an ice bath. The CsPbBr3 NCs were precipitated by addition 230



mL anhydrous MeOAc (ratio of NC reaction mixture : MeOAc is 1:3) and then centrifuged at 8000 RPM for 5 min. The wet pellets of NCs in each centrifuge tube were re-dispersed in ~5 mL hexane, precipitated again with 15 mL MeOAc and centrifuged again at 8000 RPM for 5 min. CsPbBr3 Film Fabrication: CsPbBr3 NCs (~50 mg/mL in octane) were spin-coated on a desired substrate at 1000 RPM for 20 sec followed by 2000 RPM for 10 sec, and then dried with nitrogen flow. For removing the OAm and OA ligands, the NCs film was swiftly dipped 2-3 times in the neat, anhydrous MeOAc and then dried. The thickness of film can be increased by repeating this process of NC spin-coating and treating with MeOAc for several times which is termed as layer-by-layer film deposition method. Characterization: UV-visible absorption spectra were recorded using a Perkin Elmer, Lambda-45 UV/Vis spectrometer. Steady state PL and PL decay dynamics (time correlated single photon counting) were measured using FLS 980 (Edinburgh Instruments). Transmission electron microscopy (TEM) studies were carried out using a JEOL JEM 2100F field emission transmission electron microscope at 200 kV. Fourier-transform infrared (FTIR) spectroscopy was carried out in the transmission mode using a NICOLET 6700 FTIR spectrometer (Thermo scientific). Structural analysis was done using Bruker D8 Advance X-ray diffraction (XRD) machine using Cu Kα = 1.54 Å. NC-PeLED Fabrication and Characterization: ITO substrates were cleaned with the soap water, deionized (DI) water, acetone, and 2-isopropanol (IPA) using a sonicator. The samples were exposed to oxygen plasma to remove organic contaminants. PEDOT:PSS solution was spin coated at 5000 RPM for 60 sec and


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annealed at 150 °C for 30 minute in the nitrogen atmosphere. After that, CsPbBr3 NCs (dispersed in octane) were spin-coated and treated with MeOAc following, the procedures discussed above. The electron transport layer (TPBi (~50 nm)), Ca (~20 nm) and Ag (~100 nm) were evaporated in 1 × 10−6 mbar vacuum pressure using thermal evaporator. After that, the device was encapsulated using epoxy for measurement.

Steady

state

J-V-L

(current

density–voltage–luminance)

characteristics were measured using a Keithley 2400 source meter, 2000 multimeter, and the brightness obtained by placing the large-area calibrated Si photodiode (RS components), directly onto the surface of PeLEDs thus avoiding corrections needed to account for non-Lambertian spatial emission patterns. Device area is 1.5 mm x 3 mm = 4.5 mm2. Results and Discussion TEM image in Figure 1a shows cube shaped CsPbBr3 NCs with an edge-length of 11±0.7 nm (see Figure S1 of SI). Figure 1b compares XRD pattern of CsPbBr3 NCs with reference patterns of bulk CsPbBr3. This comparison suggest orthorhombic phase of CsPbBr3 NCs similar to prior report.11 Orthorhombic CsPbBr3 exhibits corner-shared [PbBr6]4- octahedra in 3-D fashion (see Figure S2 in SI), and is an optoelectronically active perovskite phase, unlike the orthorhombic CsPbI3, in which the octahedra are not corner shared.9 Detailed surface characterization of these NCs reported that OAm is the primary ligand on the NC surface, but a balanced amount of OA is also required for the stabilization of colloidal CsPbBr3 NCs.22 These NCs can be termed as OA-OAm capped CsPbBr3 NCs.



Figure 1: (a) TEM (transmission electron microscope) image of deposited CsPbBr3NCs on copper grid. Edge length of cube-shaped NCs is 11 ± 0.7 nm. (b) Comparison of XRD pattern of CsPbBr3-NCs with respect to its bulk references.


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Figure 2: Change in (a) schematic step before and after MeOAc treatment on CsPbBr3 NCs, (b) FTIR spectra, (c) PL intensity, UV-vis absorption (inset) and; (d) PL decay profile of CsPbBr3 NC film before (black line) and after (red line) MeOAc treatment. Both OAm and OA have 18 carbon-long insulating hydrocarbon chain, and are expected to inhibit charge injection (or extraction) into (or from) NCs. To remove these long-chain hydrocarbons from the surface of NCs without affecting the high PLQY, films of CsPbBr3 NCs were treated with MeOAc (schematic shown in Figure 2a). Figure 2b shows comparison of FTIR spectra of CsPbBr3 NCs before and after MeOAc treatement. Similar amount of NCs from both (before and after MeOAc treatment) the films were taken for FTIR measurements. The doublet bands at ~2900 cm-1 correspond to C-H bond stretching of the hydrocarbon. A significant decrease in the transmittance at ~2900 cm-1 after MeOAc treatment suggest removal of long hydrocarbon chain (OAm, OA, or any residual ODE) from the NC film. However, the extent of decrease in transmittance is less for these CsPbBr3 NCs compared to the previous report of similar treatment in case of CsPbI3 NCs.15 This difference might arise from stronger hard acid-hard base interaction of oleylammonium with surface bromide in CsPbBr3 NCs, compared to the softer surface iodide in CsPbI3 NCs (hard acid-soft base interaction).25 In case of MeOAc treated film, the broad IR peak in the range of 3200-3600 cm-1 probably corresponds to O-H bond stretching due to the adsorption of moisture from the ambient surrounding, while the untreated film do not exhibit this peak due to the hydrophobic nature of OA, OAm and residual ODE. Figure 2c show the effect of MeOAc treatment (partial removal of OAm and OA) on the PL and UV-visible properties of CsPbBr3 NC film. The absorption spectra in the inset of Figure 2c remain similar before and after the MeOAc treatment suggesting that the CsPbBr3 NCs remain unchanged during the MeOAc treatment. Interestingly,



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the sharpness of the first excitonic peak at ~500 nm decreases after the MeOAc treatment. This change might arise due to the improved electronic coupling between adjacent NCs in the film after MeOAc treatment.15 PL spectra in Figure 2c show the peak position remains unchanged: however a decrease in PL intensity of CsPbBr3 NCs is observed after MeOAc treatment. This decrease in PL intensity is expected, as the organic ligands passivate the dangling bonds on the surface of NCs, and partial removal of such ligands can gives rise to non-radiative trap states. To probe it further, we used PL decay dynamics. Figure 2d shows a faster decay after the ligand treatment. While the PL decay without MeOAc treatment can be fitted with biexponential decay, a tri-exponential decay is required to fit the decay profile obtained after MeOAc treatment. The best fit parameters are shown in Table 1. The lifetimes, ~4 and 19 ns are observed for both with and without MeOAc treated NCs, are attributed to the radiative recombination of electron-hole pair. However, the NCs after MeOAc treatment show contribution from ~1 ns lifetime, which attribute nonradiative recombination of electron-hole pair. This faster decay, along with a decrease in PL (Figure 2c) suggests the formation of surface defects after the MeOAc treatment. Table 1: The best fit parameters of PL decay profiles of CsPbBr3 nanocrystals film measured at emission peak energies shown in Figure 2d using bi-exponential decay for before MeOAc treatment and tri-exponential decay for after MeOAc treatment respectively. A1, A2 and A3 are percent contributions of lifetimes from, , and respectively. CsPbBr3 NC film

A1 (%) (ns)

A2 (%)

(ns)

A3 (%)

(ns)

w/o MeOAc treatment 6±0.3

18.6±0.9 94±4.7 4.3±0.2 -

MeOAc treated

19.3±1.0 50±2.5 4.1±0.2 44±2.2 1.4±0.1

6±0.3


-

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MeOAc treatment decreases the PL intensity of CsPbBr3 NCs, though it is expected to improve the charge transport in NCs. To study the effect of these two opposing aspects on EL, we prepared CsPbBr3 NC PeLED. Figure 3a shows the PeLED device structure glass/ITO/PEDOT:PSS/CsPbBr3-NCs/TPBi/Ca/Al; here TPBi and PEDOT:PSS are 2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) and Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate),

respectively,

and

PEDOT:PSS and TPBi/Ca are hole and electron injection layers. Figure 3b shows flat energy level diagram of the NC-PeLED with hole and electron injection layers.13

Figure 3: (a) CsPbBr3 NCs light emitting device structure. (b) Corresponding flat energy level diagram and energy levels taken from literature.13 Figure 4a shows J-V characteristics of our NC-PeLEDs. CsPbBr3 NCs, without MeOAc treatment, do not show any significant current density because of the presence of excess organic ligands. But the same NC films after MeOAc treatment show reasonably good diode behavior. Removal of organic ligands can result to significant weight loss from the film, generating cracks and pin-holes in the films. So to improve the film quality, and therefore improving the charge transport, multiple NC-layers were deposited in a layer-by-layer fashion (see the experimental section). One-layer of NC film correspond to ~50 nm thick film (see Figure S3 of SI). We are able to grow at most 140 ± 10 nm thick film of uniform thickness by employing layer11 ACS Paragon Plus Environment


by-layer deposition for four times (Figure S4 of SI). Consequently, current on-set bias voltage reduces as we increased the number of NCs layer. But with increase in layer number, we also increased the thickness of active layer, hence overall currentdensity remains low in NC-PeLED, if we compare it with respect to bulk systems whether inorganic perovskite or hybrid perovskite as active layer.4,5

(c)

(b)

4

10 3 10 2 10 1 10 0 10 -1 10 -2 10 -3 10 -4 10 -5 10 -2 0

w/o MeOAc wash 2 NCs layer 3 NCs Layer 4 NCs layer

2

4

6

Luminance (cd/m2)

J (mA/cm2)

(a)

8 10 12 14 16 18 20 Voltage (V)

(d)

10

1 2

4

6

8 10 Voltage (V)

12

14

16

1

10

0

10 0

10

EQE (%)

Luminance Efficiency (cd/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

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-1

10

-2

10

-2

10

-1

10

4

-3

6

8

10

12

10

14

4

6

Voltage (V)

Figure 4:

8

10 12 Voltage (V)

14

16

Champion NC-PeLEDs characteristics (a) J-V (current-voltage) (b)

Luminance (brightness cd/m2) (c) current efficiency (cd/A) and (d) External quantum efficiency (EQE) with applied voltage for ITO/PEDOT:PSS/CsPbBr3-NCs/TPBi/Ca/Al device structure. Table 2: Champion and average performance parameters of NCs PeLEDs with variation from device to device (approximately 6 devices).


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Number

of Luminance (cd/m2)

CsPbBr3 layers

Luminance

EQE (%)

efficiency (cd/A) Average

Max.

Average

Max.

Average

Max.

Two Layer

31.8 ±9.2

40

2.56 ±1.1

3.75

0.51±0.39

0.90

Three Layer

37±17

44

0.54 ± 0.8

1.34

0.11± 0.21

0.32

Four Layer

44.70± 9.3

54

2.32 ± 3

5.20

0.52 ± 0.78

1.30

Figure 4b shows luminance versus applied voltage. Luminance gets saturated beyond 10 V, and the maximum luminance achieved is ~ 54 cd/m2 (average luminance value is 44 cd/m2, Table 2 and Table S3) for NC-PeLED with four-layer NC film as an active medium. Luminance increases with respect to thickness of NC films. This increase should be solely due to improved film quality improving the charge carrier balance. Inset of figure 4b shows EL spectra (dotted line) of NCPeLED. The EL spectra of NCs matches well with PL spectra and EL is uniform from the complete area of NCs LEDs, see in Figure S5. Figure 4c and 4d show luminance efficiency and EQE with applied voltage. Our MeOAc treated 4-layer CsPbBr3 NC film shows maximum luminance efficiency of ~5.2 cd/A (average eff. 2.32 cd/A over 6 devices) and EQE of ~1.3% at 6.5 V, Table 2 shows six device average data with deviation from mean value and with maximum efficiency of PeLED (also see Table S1 to S3 in SI).Variation in device parameters were found to be least in two-layer NCs based PeLEDs and variation gets higher as we increase the thickness of active layer, which can be due to changes in interfacial properties between layers of these NCs. We observed efficiency roll-off after 6.5 V applied voltage. Such efficiency droop is commonly observed phenomena in NC-PeLEDs,18,20 however, not studied 13 ACS Paragon Plus Environment


in great details so far and it might be non-radiative Auger recombination.26 We have checked this with multiple batches of devices (Figure S6 to S8) and agree that these NCs PeLEDs have improved charge transport balance versus poor PL in four layer NC PeLEDs whereas there is poor charge balance and higher PL in two layer NCs PeLEDs. This brings the average efficiency similar for both; however, four layer shows relatively higher luminance due to improved charge transport. Though, two layer and four layer NC PeLEDs have more or less similar average efficiencies, however, four layers have more injection current than two layer devices, which allow us to get relatively higher luminance, but at the cost of relatively higher efficiency droop as a function of injection current (Figure S9). Three layer NC PeLEDs are limited by both carrier balance factor and poor PL, hence, shows efficiencies lower than either of the systems. We note that PL efficiency decreases with MeOAc treatment, but still luminance efficiencies of NC-PeLEDs increases after MeOAc treatment. These results indicate that we actually gained on charge carrier balance factor mainly and may be little effect of out-coupling factor, which are in general expected to be poor in perovskite LEDs.27 In an approach of getting high PL and EL, Li et al. optimized the ligand passivation and charge balance via ligand density control in solution phase before depositing the NC film for device and reported 6.27% EQE.23 In this method the PLQY, colloidal stability and size of NCs are highly sensitive to ratio of good and bad solvent (hexane/ethyl acetate) used during the ligand density optimization. Also, the film thickness is difficult to increase by using layer-by-layer deposition, because attempt to deposit second layer is expected to partially dissolve the NCs in first layer, disturbing the film uniformity. In NCs PeLED device performance can be improved by ligand density optimization and better charge injection combination. We note that 14 ACS Paragon Plus Environment

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efficiency number of these NC LEDs are not as high as previous report due to poor hole-injection, PL quenching PEDOT:PSS hole-injection layer (HIL). Further fine tuning of active layer thickness and alternative HIL can facilitate to achieve higher efficiency.23, 28 Ligand optimization can take care for both charge carrier balance while keeping optical properties optimally good to get high internal quantum efficiency of these PeLEDs. In addition, by the use of right photonics geometry photon escape rate can also be enhanced. The most easy approach would be reduce the size of the device,29 as current density in these devices are not too high, these efforts are underway in our laboratory.

Conclusions In conclusions, we demonstrated layer-by-layer deposition of CsPbBr3 NC film for use in NC-PeLEDs with luminance efficiency of ~5.2 cd/A (average efficiency of 2.32 cd/A over 6 devices). CsPbBr3 NCs exhibit orthorhombic phase with size 11±0.7 nm. These NCs are capped with insulating OAm and OA ligands that inhibit charge transport. Consequently, PeLED prepared from OAm-OA capped NC films does not show a measurable current. To overcome this problem, we treated the NC films with MeOAc that significantly removes these long-chain organic ligands from the NC surface. This removal of ligands reduces the PL efficiency, but enhances the charge transport in NC films.

Thickness of the NC films was increased by layer-by-layer

method, where each layer corresponds to a thickness ~50 nm. NC-PeLEDs with multilayer (higher thickness) device showed higher current density and lower turn-on operating voltages compared to that single-layer device. Multilayer device gets rid of pin-holes and cracks in the NC film, improving the charge-carrier-balance factor



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significantly to boost the efficiency number from ~1 cd/A (average efficiency of 0.54 cd/A over 6 devices) for single-layer to ~5.2 cd/A (average efficiency of 2.32 cd/A over 6 devices) four-layer NC film. Our future work is focused on better charge injection by removing the residual ligand density for better performance and stability. This approach can be further utilized for waveguide lasers and optical-detectors using same class of materials.

ASSOCIATED CONTENT Supporting Information: The Supporting Information (SI) is available free of charge on the ACS Publications website. List of chemicals; size distribution plot, schematics of crystal structure, AFM images for film thickness and film morphology, device reproducibility data and comparison of EL and PL spectra are provided.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS A.N.

thanks

DST

Nanomission

(SR/NM/NS-1474/2014

and

Thematic

Unit

SR/NM/TP-13/2016). A.S. thanks IISER Pune, for student fellowships. D. K. thanks to the Department of Science & Technology (DST/TM/SERI/FR/186(G)) and National Centre for Photovoltaic Research and Education (NCPRE). N. K. K. thanks to IITB


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for scholarship. DK thanks Centre of Excellence in Nanoelectronics (CEN) for deposition facility.

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