An Insight into Atmospheric Plasma Jet Modified ZnO Quantum Dots

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An Insight into Atmospheric Plasma Jet Modified ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cell: Optoelectronic Transient and Charge Trapping Studies Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo, Mohammad Khaja Nazeeruddin, and Hyung-Shik Shin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00933 • Publication Date (Web): 22 Apr 2015 Downloaded from http://pubs.acs.org on April 26, 2015

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The Journal of Physical Chemistry

An Insight into Atmospheric Plasma Jet Modified ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cell: Optoelectronic Transient and Charge Trapping Studies Sadia Ameen†a, M. Shaheer Akhtar†b, Hyung-Kee Seoa, Mohammad K. Nazeeruddin*c Hyung-Shik Shin*a a

Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, 561-756, Republic of Korea b New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea c Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland _____________________________________________________________________________ ABSTRACT In this work, a new embodiment of flexible perovskite solar cell is based on graphene (Gr) as barrier layer and the atmospheric plasma jet (APjet) treated ZnO quantum dots (QDs) as mesoscopic metal oxide layer on ITO-PET substrates. ITO-PET flexible substrate was treated with oxygen (O2) plasma before creating an efficient barrier layer of Gr and thereafter, as synthesized ZnO QDs was deposited by spin coating on ITO-PET/Gr thin film. ITOPET/Gr/ZnO-QDs thin film substrates were finally subjected to APjet treatment using RF power of ~40 W with frequency of ~13.56 MHz which substantially improved the interfacial properties of the deposited layers. The fabricated ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiroMeOTAD/Ag flexible perovskite solar cell obtained the high conversion efficiency of ~9.73% along with high short circuit current (JSC) of ~16.8 mA/cm2, open circuit voltage (VOC) of ~0.935 V and high fill factor (FF) of ~0.62. The APjet treatment on ITO-PET/Gr/ZnO QDs thin film enhanced the performances and the photocurrent density as compared to other solar cells fabricated without APjet treated ITO-PET/Gr/ZnO QDs thin film. By analyzing the intensitymodulated photocurrent (IMPS)/photovoltage spectroscopy (IMVS), the fabricated flexible perovskite solar cell exhibited the good charge transfer rate and the reduction in the recombination rate. The APjet treatment and the introduction of low-cost Gr barrier layer are the promising prospects to approach the low cost photovoltaic devices. Keywords: Graphene, Atmospheric jet plasma, ZnO Quantum dots, Perovskite Solar cells, Electron transport. *Corresponding author. Tel.: +82 63 270 2438; Fax: +82 63 270 2306 E-mail address: [email protected] (H.S. Shin), [email protected] (M. K. Nazeeruddin) † These authors contributed equally to this work 1 ACS Paragon Plus Environment


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1. INTRODUCTION To find the cost effective photovoltaics, the thin film photovoltaic technology has emerged as the most promising technology compared to the conventional silicon solar cells. A thin film photovoltaic based on organometal halide perovskites has gained numerous attentions due to its high solar-to-electricity energy conversion efficiency.1-4 In the perovskite solar cells, the effective photogenerated charge separation and the light harvesting efficiency are greatly influenced by the properties like band gap, particle size, surface morphology, porosity, surface area, thickness of semiconducting nanomaterials and the nature of organometal halide perovskites.5 Among perovskite absorbers, the organometal halide perovskites such as methyl ammonium lead iodide (CH3NH3PbI3) possesses a direct band gap (~1.5 eV)6 with a large absorption coefficient (~1.5 × 104 cm-1 at 550 nm) and the high charge carrier mobility.7 The perovskite thin film solar cells have reached the high power conversion efficiency of over ~17% along with broad light absorption, and high open-circuit voltages (VOC) of over ~1.0 V.8 At present, the light weight photovoltaic devices such as flexible solar cells own a great deal of interest due to their low production cost, variable shapes and large-scale roll-to-roll processing.9,10 So far, various flexible substrates such as poly(ethylene terephthalate) (PET)/indium tin oxide (ITO), metal foils and metal sheets have already been employed for the fabrication of different flexible solar cells.11,12 Generally, the mesoporous TiO2 or other metal oxides thin layer are achieved by the sintering process at the high temperature,13 which substantially limits the usage of conducting plastic-film and malleable metal foils substrates for the perovskite solar cells.14 Hitherto, the configuration and the choice of mesoporous layer are essential factors to understand the physical electronic mechanisms in these solar cells which regulate the processes such as carrier separation, transport, extraction, and the recombination.

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Thus, the device structure configuration is essential to modify different mesoporous materials, electrode contacts, and the barrier layer for understanding these processes and mechanisms. Several mesoporous thin film nanomaterials such as TiO2, ZnO, Al2O3, fullerene derivatives etc.15-17 have been used for perovskite sensitized solar cells. Among them, zinc oxide (ZnO) nanomaterials could be the promising semiconducting metal oxide because of their wide band gap (~3.37 eV), large exciton binding energy (~60 meV), higher electron mobility,18 unique photoelectric properties, optical transparency, electric conductivity and piezoelectricity properties.19,20 Apart from other ZnO nanostructures, ZnO quantum dots (QDs) possess the higher stability and resistivity towards oxygen and water with tunable band gaps.21 ZnO QDs due to three-dimensional confinement of carriers and phonons are anticipated to improve the device performances by changing the optoelectronic properties.22 Additionally, 2D sp2-bonded carbon materials, graphene (Gr) has shown the high electrical conductivity, high charge mobility23-25 and large specific surface area of ∼2630 m2/g.26-28 The incorporation of Gr into polymer,29,30 ceramic materials31 and metal oxides have shown the remarkable improvements in the optoelectrical and electrochemical properties of the host materials. Recently, the combination of Gr and ZnO nanomaterials is seemed to be the most promising materials, as it could improve the carrier transport and the collection efficiency of ZnO based UV photodetectors, sensors, and electrochemical devices.32,33 Hwang and Kim et al.34,35 have recently grown the vertically aligned ZnO NWs on reduced graphene/PDMS substrates and fabricated a transparent and flexible optoelectronic material. Chang et al.36 reported the heterostructures of ZnO nanorods/Gr via facile in-situ solution growth method and demonstrated a highly sensitive visible-blind ultraviolet (UV) sensor. In the present work, Gr thin film as a barrier layer is deposited on O2 plasma treated ITO-PET substrate, and the as-synthesized ZnO QDs is coated on ITO-PET-Gr



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substrates by spin coating. The deposited ZnO QDs thin film is further subjected to APjet plasma treatment to enhance the interfacial contacts and modifying the surface properties of ITO-PETGr/ZnO QDs thin film substrate. An atmospheric plasma technology, also called as nonthermal or low-temperature plasma technology requires no vacuum systems and provides higher plasma density due to the large surface-to-volume ratio and low-discharge current of the plasma. ITOPET-Gr/ZnO QDs thin films substrate is used for the fabrication of flexible perovskite solar cell of

the

composition

ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag.

The

flexible perovskite solar cell presents reasonably high solar-to-electric conversion efficiency of ~9.73% with high photocurrent density and open-circuit potential. The incident-photon-tocurrent efficiency (IPCE) of ~59.2% in the wavelength range of ~400-700 nm is achieved by ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell.

2. EXPERIMENTAL SECTION 2.1 Synthesis of ZnO QDs ZnO QDs were synthesized by adopting our previous work.37 In brief, zinc acetate (Zn(CH3COO)2. 2H2O, 0.0658 g, Sigma-Aldrich, ≥99.9%) was dissolved in ethanol (30 ml, Sigma-Aldrich, 99.5%) and a separate ethanolic solution (10 ml) of tetramethyl ammonium hydroxide ((CH3)4NOH.5H2O, 5mmol, Sigma-Aldrich, ≥ 99.9%) was also prepared. Thereafter, the ethanolic solution of tetramethyl ammonium hydroxide was added dropwise into the ethanolic Zn (CH3COO)2. 2H2O solution and refluxed for 2 h to obtain a clear solution. The obtained ZnO QDs were centrifuged at ~5000 rpm, re-dispersed in ethanol and centrifuged again. The procedure was repeated for several times to ensure the removal of unreacted components or


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reactants. The finally obtained ZnO QDs were dispersed in ethanol by ultrasonic stirring for 10 min and then sealed at the room temperature.

2.2 Deposition of Gr thin film on O2 plasma treated ITO-PET substrates Prior to the deposition of Gr, the surface treatment of cleaned indium tin oxide poly (ethylene terephthalate) (ITO-PET) substrates was performed by O2 plasma at Radio Frequency (RF) power of ~60 W for 10 min under an oxygen flow of ~40 sccm and achieved the average Gr thickness of ~300 nm. The O2 plasma treatment considerably enhanced the hydrophilicity of ITO layer on PET substrates, which improved the effective adhesion of Gr thin film. Moreover, the sufficient oxygenated species were introduced and O2 plasma predominately removed the dust particles and traces of adsorbed impurities from the surface ITO-PET substrates. Herein, Gr was synthesized from graphite powder by the modified Hummers method as reported elsewhere.38 Afterward, a suspension (2 mg/ml) was prepared by dissolving Gr into the aqueous solution of 1% tert-butanol (C4H10O, Daejung, 99.5%) and kept under ultrasonication for 1h. The suspension was then centrifuged at ~3000 rpm for 30 min to remove inadequate exfoliated Gr stacks. The uniform Gr thin film was achieved by dipping O2 plasma treated ITO-PET substrates into the prepared Gr suspension. Subsequently, the ITO-PET/Gr substrates were dried under a gentle stream of nitrogen gas followed by oven drying at ~80 oC for 40 min.

2.3 ZnO QDs coating on ITO-PET/Gr substrates and APjet treatment As synthesized ZnO QDs were spin coated on ITO-PET/Gr substrates at ~2500 rpm for 40 s, followed by subsequently drying at ~70 oC for 20 min. In order to improve the interfacial contacts between Gr and ZnO QDs layer, the ITO-PET/Gr/ZnO-QDs thin film was subjected to



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APJet treatment. In this treatment, RF power supply of 40 W and frequency of ~13.56 MHz were applied for generating the plasma from the tip of the 1/4 inch diameter quartz reaction tube for 5 min. Importantly, no heating was applied to ITO-PET/Gr/ZnO-QDs thin film but due to plasma and the distance from the discharge, the substrate temperature was itself raised to ~60 oC through the convective heat transfer. To lower the temperature, the ITO-PET/Gr/ZnO-QDs thin film substrates were kept away from the plasma tip at a specific distance of ~2.5 cm where the charged species in the plasma could easily interact with the thin films. Finally, the modified ITOPET/Gr/ZnO-QDs(APjet) thin film (with ZnO QDs thin film thickness of ~500 nm) was directly used for the fabrication of flexible perovskite solar cells.

2.4

Fabrication

of

flexible

perovskite

solar

cell

(ITO-PET/Gr/ZnO-

QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag) In the beginning, the perovskite solution of methyl lead ammonium iodide (CH3NH3PbI3) was synthesized as reported elsewhere39 and was spin coated at the speed of ~2000 rpm for 40s on ITO-PET/Gr/ZnO-QDs(APjet) flexible substrate using ~0.45 µm pore PVDF membrane syringe filter (Jet Biofil) at the ambient temperature. The perovskite deposited thin films were annealed at ~100 oC for 30 min to achieve ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3 thin films. A separate spiro-MeOTAD solution in chlorobenzene (~15mg/1ml) with ~13.6µl Li-bis (tri fluoro methane sulfonyl) imide (CF3SO2NLiSO2CF3, Li-TFSI, ~28.3 mg/1 ml, TCI, >98%) and ~6.8 µl TBP (C9H13N, Aldrich, 96%) as additives was further spin-coated on ITOPET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3 thin films at ~3000 rpm for 30 s and annealed at 100 oC for 15 min to get ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD thin films. Lastly, silver (Ag) contacts (thickness ~100 nm) were made by the thermal evaporation and achieved the


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final

flexible

device

structure

of

ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-

MeOTAD/Ag, as illustrated in the Figure 6 (b).

2.5 Characterizations The investigation of the morphology and the surface modifications of ITO-PET and thin films substrates were studied by the atomic force spectroscopy (AFM, Nanoscope IV, Digital Instruments, Santa Barbara, USA) and contact angle measurements (CTA480) respectively. The transmittances and sheet resistances of ITO-PET and thin films substrates were performed by UV-vis (UV-2550, Shimadzu, Japan) and CMT-100MP respectively. X-rays Photoelectron Spectroscopy (XPS) was performed by AXIS-NOVA CJ109, Kratos Inc., ranges 0-800 eV to study the bonding between ITO-PET/Gr and ITO-PET/Gr/ZnO-QDs(APjet) substrates. The current density (J)-voltage (V) measurements were performed for elucidating the performance of flexible perovskite solar cell using computerized digital multimeter (model 2000, Keithley) with a variable load under one sun (1.5 AM at 100 mW/cm2). A metal halide lamp with 1000 W power was used to supply the simulated light with the light intensity of 100 mW/cm2 (1.5 AM) using a Si photo detector (calibrated at NREL, USA) fitted with a Ka-5 filter as a reference. The incident photon-to-current conversion efficiency (IPCE) was elucidated by specially designed IPCE system for solar cell by PV measurements, Inc., USA. The calibration of the system was done using NIST-calibrated a silicon photodiode G425 as standard before every IPCE. The IPCE results of flexible perovskite solar cell were collected as a function of wavelength from ~400-800 nm using 75 W Xe lamp as a light source for generating monochromatic beam at a low chopping frequency. The charge collection efficiency and photoelectron density analysis were revealed by



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the intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) using IVIUM technologies (CompactStat.e20250, USA).

3. RESULTS AND DISCUSSION The surface modifications are analyzed by the contact angles and three dimensional (3D) AFM images of bare ITO-PET substrate and modified ITO-PET substrates by O2 plasma and Apjet treatments. Figure 1 (a-b) shows the contact angles and 3D AFM images (Fig. 1(a1-b1)) of bare and O2 plasma treated ITO-PET substrates. The contact angle of ~89.6o is obtained for bare ITO-PET, while a substantive decrease in the contact angle of ~75.2o is recorded after O2 plasma treatment of ITO-PET substrate. The change in the contact angle imputes the increment of hydrophilicity and the surface energy of ITO-PET substrate due to the production of polar group moieties through the oxidation plasma process. The plasma treatment in the presence of O2 usually changes the structure of ITO-PET substrate by the incorporation of oxygen-containing functionalities due to the strong surface oxidation and therefore, improves the hydrophilicity of ITO-PET surface.40 Moreover, the molecular oxygen in the plasma ionization becomes active and dissociates into extremely reactive oxygen species, which could react immediately with the polymer surface to produce oxygen-containing polar groups such as C=O, O-C=O and OH.41,42 The enhancement in the contact angle of ~81.2o is further observed after the deposition of Gr thin film on O2 plasma treated ITO-PET substrates, as shown in Figure 1(c & c1). This increment might referable to the amphiphilic Gr thin film possessing both hydrophilic groups (–COOH, – OH and C=O) and hydrophobic (C–C, C–H) groups. From Figure 1(d), the further increase in the contact angle of ~96.2o of Apjet treated ITO-PET/Gr/ZnO-QDs thin film indicates the deformation of surfaces caused by the O2 plasma-treated hydrophilic ITO-PET surface and the


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amphiphilic Gr sheets. Importantly, the Apjet treatment might also improve the surface-tovolume ratio and shows good contact between hydrophilic ITO and amphiphilic Gr surfaces of thin film (As shown in (Fig. 1(d1)). Thus, it is believed that the enhanced hydrophilicity and amphiphilicity of the flexible substrates by the O2 plasma and Apjet treatments might deliver a suitable surface for the good deposition of new functionalities i.e. spiro-MeOTAD and CH3NH3PbI3. The topographic AFM images of bare ITO-PET, O2 plasma treated ITO-PET, Gr/O2 plasma treated-ITO-PET and Apjet treated ITO-PET/Gr/ZnO-QDs thin films are analyzed to understand the morphological and roughness factor of the thin films. From Figure 2(a), bare ITO-PET presents uniform and regular morphology however, O2 plasma treated ITO-PET substrate exhibits non-regular morphology (Figure 2(b)). On comparison to bare ITO-PET substrate (root mean square roughness, Rrms = ~7.7 nm), the low Rrms value of ~4.3 nm is recorded for O2 plasma treated ITO-PET substrate. In support, the surface analysis (AFM and contact angle results) reveals that both roughness and contact angle decrease upon the O2 plasma treatment of ITO-PET. The diminution in contact angle might due to the increment of hydrophilicity and the surface energy of ITO-PET substrate which occur due to the production of polar group moieties through the incorporation of oxygen-containing functionalities on ITO-PET substrate.42 In addition, the deposition of Gr and ZnO QDs reduce the roughness and contact angle which might suggest the case of photo-induce hydorphilicity in the obtained thin film. Interestingly as shown in Figure 2 (c, d), the low roughness (~1.1 nm) is obtained after the deposition of Gr and Apjet treatment of ZnO QDs which indicates the better interaction of O2 plasma treated ITO-PET substrate, Gr and ZnO QDs. These results are also consistent with the contact angles measurements. To check the porosity of ITO-PET/Gr/ZnO-QDs thin film, the surface area has 9 ACS Paragon Plus Environment


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been analyzed by a BET surface analysis. The ITO-PET/Gr/ZnO-QDs thin film records the high specific surface area of ~268.4 m2/g, however, the APjet treatment on ITO-PET/Gr/ZnO-QDs thin film enhances the surface area from ~268.4 to ~334.2 m2/g. This significant increment in surface area suggests the heightening in the porosity of the ITO-PET/Gr/ZnO-QDs thin film after APjet treatment. Figure 3 shows the transmittance spectra and sheet resistances of bare ITO-PET and modified ITO-PET substrates. A transmittance of maximum ~82% in the absorbance range of ~400-1000 nm is obtained by bare ITO-PET substrate as shown in Figure 3(a) which is consistent to the reported transparency of ITO-PET substrate.43 After O2 plasma treatment, the optical transmittance decreases to ~79.2% and further decreases to ~74.5% and ~62.2% after depositing Gr thin film on O2 plasma treated ITO-PET and Apjet treated ZnO-QDs deposition on Gr/O2 plasma treated ITO-PET substrates respectively. These observations suggest the slight change in the transparent nature of ITO-PET substrate. Furthermore, the ITO-PET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD on ITO-PET records the lowest transmittance of ~47.2%, indicating the interaction of the deposited layers on ITO-PET/Gr layer substrate. The transmittance properties are further supported by the measurement of sheet resistances of bare ITO-PET and the modified ITO-PET substrates. From Figure 3(b), the sheet resistance decreases with the increase of the transparency of bare ITO-PET and the modified ITO-PET substrates. It is visible that the low sheet resistance favors the strong interactions between Gr, ZnO-QDs and O2 plasma treated ITO-PET substrate which helps in restoring the inherently high electrical conductivity of Gr. In general, the low sheet resistance represents the good electrical conductivity and the charge transportation.44 In our case, the low resistance of ITO-PET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD

thin

film

substrate


supports

an

appropriate

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conductivity and the enhanced charge carriers and transportation, which might result to achieve the high performance of the flexible perovskite solar cell. The XPS analysis is employed to evaluate the interfacial interactions and the bonding between ITO-PET/Gr and ITO-PET/Gr/ZnO-QDs(APjet) substrates. Figure 4 shows C 1s, O1s and Zn 2p XPS plots of ITO-PET/Gr and ITO-PET/Gr/ZnO-QDs(APjet) substrates. Figure 4 (a) exhibits four resolved binding energies at ∼285.2, ∼286.3, ∼287.5 and ∼288.6 eV (weak) for Gr/ITO-PET substrate, which are assigned to C-H/C-C, C-O, C-O-C/C=O, and -O-C=O bonds.45,46 The existence of these bonds in Gr/ITO-PET confirms the uniform deposition of Gr on O2 plasma treated ITO-PET substrate. The weak -O-C=O bond occurs due to the chain-cutting of some ITO-PET molecules, especially the weakest bonds -O–C=O groups by O2 plasma treatment.47 After the Apjet treatment of ITO-PET/Gr/ZnO-QDs substrate, C 1s XPS (Figure 4(b) presents similar four resolved binding energies. Importantly, the binding energy at ~288.6 eV is prominent as compared to ITO-PET/Gr substrate, indicating the increased -O-C=O bond strength through the Apjet treatment. The high -O-C=O bond strength might due to the strong partial hydrogen bond formation of -O-C=O in Gr and Zn-O in ZnO via an ester linkage.

The

deconvoluted O1s XPS spectra of ITO-PET/Gr substrate and ITO-PET/Gr/ZnO-QDs(APjet) substrates are shown in Figure 4(c, d). ITO-PET/Gr substrate obtains the center peak at ~531.4 eV along with three resolved peaks at ~531.1 eV, ~532.1 eV and ~533.0 eV, correspond to O atom attached with sp2/sp3 C atom and moisture or hydroxyl contaminants respectively. The binding energies are shifted to lower binding energy in O1s XPS of ITO-PET/Gr/ZnOQDs(APjet) substrate, indicating the interaction between metal (Zn) and O atom. The binding energy at ~530.6 eV is ascribed to the presence of O2- ions on the hexagonal Zn2+ ion in wurtzite ZnO structure.48 The other resolved binding energies represent the oxygen deficiency or oxygen



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vacancies over the surface of ZnO and hydroxyl contaminants. Furthermore, the doublet peaks of Zn 2p3/2 at ~1022.1eV and Zn 2p1/2 at ~1045 eV in Zn 2p (Figure 4 (e)) have again confirmed the existence of Zn-O species and the binding energy difference of ~22.9 eV deduces the typical hexagonal ZnO wurzite structure.49 Thus, O2 plasma and Apjet treatment have considerably improved the bond strength of Gr with ITO-PET and increase the interaction between Gr and ZnO QDs in ITO-PET/Gr/ZnO-QDs(APjet) substrate, which might be beneficial for achieving the high performance of the device. Raman scattering spectroscopy is obtained for explaining the nature of Gr and the interaction with ZnO-QDs thin film on ITO-PET substrate, as shown in Figure 5. For ITOPET/Gr thin film, typically two Raman bands at ∼1349.2 and ∼1583.4 cm−1 are ascribed to D and G bands which are due to defects/out-of-plane breathing mode of sp2 atoms of carbon thin film and the E2g vibrational mode/double degenerate phonon mode at the Brillouin zone center respectively.50 Another Raman band at ∼2708.2 cm−1 represents the second order 2D peak due to fourth order phonon momentum exchange double resonance process.51-53 It is reported that the width of 2D peak and the I2D/IG ratio in the Raman analysis of Gr is used to estimate the number of layers and quality of Gr.54 From the Figure 5, the intensity ratios of ID/IG (~0.24)/I2D/IG (~0.64) and the large full width at half maximum (FWHM) of 2D (≈ 77 cm-1) peak are estimated which indicate few (non-interacting) layer graphene in non-AB stacking arrangement. However, ITOPET/Gr/ZnO-QDs(APjet) thin film has obtained the large I2D/IG (~0.93) and FWHM2D (>77cm-1) which again confirm few interlayers of graphene on the deposited thin film. Moreover, in the case of ITO-PET/Gr/ZnO-QDs(APjet) thin film, a strong Raman band at ∼436.7 cm−1 along with weak Raman bands of Gr are present. The Raman band at ∼436.7 cm−1 elucidates the main characteristic E2 mode of ZnO wurtzite hexagonal structure in ITO-PET/Gr/ZnO-QDs(APjet)


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thin film. Additionally, the Raman band at ~1110 cm-1 is assigned to second-order signals in the 2xLO phonon which is usually due to the presence of band disorder in ZnO nanomaterials.55 The weakening of Gr Raman band might due to the bonding formation between -O-C=O of Gr and Zn-O of ZnO-QDs, resulting to well disperse, regular and smooth thin film on ITO-PET substrate. Figure 6 shows the photograph of ITO-PET/Gr thin film substrate and the schematic illustration of the fabricated flexible perovskite solar cells. Figure 6(a) explains the good flexibility with inherent transparency of ITO-PET/Gr substrate. The structure of the fabricated flexible perovskite solar cell of composition ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiroMeOTAD/Ag is illustrated in Figure 6(b). Herein, the introduction of Gr layer on ITO-PET prevents the direct contact of ITO-PET substrate with perovskite and hole transporting layer (HTL). The performances of the fabricated flexible perovskite solar cells have been evaluated by measuring the current density (J)-Voltage (V) curves under the light illumination of 100 mW/cm2 (1.5AM). Figure 7 (A) shows the J-V curves of the fabricated flexible perovskite solar cells with different thin films such as ITO-PET/Gr, ITO-PET/Gr/ZnO-QDs and ITOPET/Gr/ZnO-QDs(APjet) and the photovoltaic results are summarized in Table 1. The highest conversion efficiency of ~9.73% along with high short circuit current (JSC) of ~16.8 mA/cm2, open circuit voltage (VOC) of ~0.935 V and high fill factor (FF) of ~0.62 are accounted by the fabricated ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell. However, the flexible perovskite solar cells with ITO-PET/Gr/ZnO-QDs and ITOPET/Gr thin films present the low conversion efficiencies of ~5.28% and ~2.89% respectively along with inferior JSC and VOC. The brilliant performance with ITO-PET/Gr/ZnO-QDs(APjet) thin film might be rationalized with respect to an effective interaction of the perovskite with



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Apjet treated ZnO-QDs. In support, the Apjet treatment has significantly increased the porosity of ITO-PET/Gr/ZnO-QDs(APjet) thin film which might favor a good adsorption and the penetration of perovskite (CH3NH3PbI3) into ZnO-QDs and results to low roughness of ITOPET/Gr/ZnO-QDs(APjet) thin film, as evidenced from AFM results. Moreover, Apjet treated ZnO QDs are aggregated randomly to form a loose matrix, which might beneficiary for the reflection and the refraction of incident light, leading to a multiple scattering effects which might capture huge amount of incident light and improve the generation of photoelectrons.56,57 The high JSC with ZnO-QDs(APjet) might due to the inhibition of the leakage current between the electrodes through the uniform and high penetration of perovskite and spiro-MeOTAD on ITOPET/Gr/ZnO-QDs(APjet) thin film substrate, resulting to the enhancement in the charge transfer rate through less generation of pin-holes and shunting paths.58 The considerable increase in VOC and FF with ITO-PET/Gr/ZnO-QDs(APjet) thin film is related to the improved interfacial contact conducting layer, ZnO-QDs and Gr layer via Apjet treatment followed by O2 plasma treatment. The performances of the fabricated flexible perovskite solar cell are further investigated in terms of the incident photon to current conversion (IPCE) efficiency to explain the light harvesting, charge collection and the photocurrent. Figure 7(B) shows the IPCE graphs of the fabricated flexible perovskite solar cells with ITO-PET/Gr, ITO-PET/Gr/ZnO-QDs and ITO-PET/Gr/ZnO-QDs(APjet) thin films. The highest IPCE of ~59.2% in the wavelength range of ~400-700 nm is observed by the fabricated flexible perovskite solar cell with ITOPET/Gr/ZnO-QDs(APjet) thin film, as well correlated with JSC value. Whereas, the other flexible perovskite solar cells with ITO-PET/Gr, ITO-PET/Gr/ZnO-QDs thin film present the low IPCE values. In ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag solar cell, APjet treated ZnO-QDs with improved porosity and the surface area might significantly increase the


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light scattering capacities and the interaction between the CH3NH3PbI3 sensitizer and ZnO-QDs interfaces. The O2 plasma treatment, Gr layer, and Apjet treatment during the preparation of desired thin films are essential for the charge transfer, charge collection and charge recombination. The Nyquist plots, as shown in Figure 8 (a) investigate the series and charge transfer resistances of the fabricated flexible perovskite solar cells with ITO-PET/Gr, ITO-PET/Gr/ZnO-QDs and ITOPET/Gr/ZnO-QDs(APjet) thin film under a frequency range from 100 kHz - 1 Hz. Figure 8 (b) depicts the Nyquist plots of the fabricated flexible perovskite solar cells with ITO-PET/Gr/ZnOQDs(APjet) thin film at different applied voltages under a frequency range from 100 kHz - 1 Hz. The corresponding equivalent circuit (Figure 8(c)) illustrates several resistances i.e. the resistance related to diffusion of holes through HTM (R1) with HTM capacitance (C1), and a recombination resistance (Rrec) at lower frequency with a chemical capacitance, (Cµ) which is related to the interface of HTM (spiro-MeOTAD) and ITO-PET/Gr/ZnO-QDs(APjet). It is visible that the appearance of main arc occurs due to the existence of the recombination resistance (Rrec) at the interface of HTM/perovskite and ZnO-QDs(APjet) layer.59 The fabricated flexible perovskite solar cells with ITO-PET/Gr/ZnO-QDs(APjet) thin film exhibits the large Rrec as compared to other fabricated solar cells. The observed large Rrec might govern few surface recombination sites by the generation of oxygenated species on ZnO-QDs via Apjet treatment, resulting to high VOC. From Figure 8(b), Rrec value continuously decreases with the increase of the applied voltage, suggesting the retardation of the recombination sites. In other words, the Apjet treatment of ITO-PET/Gr/ZnO-QDs thin film improves the interfacial contact between CH3NH3PbI3/spiro-MeOTAD layer and ZnO-QDs layer which might create surface charge recombination, resulting to increased Rrec and high VOC of the device. In support, O2 plasma



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treatment of ITO-PET substrate and Gr coating might reduce the charge recombination rate and surplus the charge collection efficiency because the Fermi level of Gr is more positive than the conduction band energy of ZnO-QDs.60 The intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) are further studied for the characterization of diffusion transit time (τtr), the electron recombination time (τR), charge collection efficiency and diffusion coefficient under the fixed light intensity at different voltages of light. Figure 9(a, b) presents gradual decrease in IMPS and IMVS of the fabricated flexible perovskite solar cells with the increase of voltage of light from 1.0V to 2.5V. The minimum frequency from IMPS and IMVS plots of the fabricated flexible perovskite solar cell at 2.5V is selected to evaluate τtr and τR values.61 The fabricated ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag solar cell demonstrates the faster τtr and smaller τR, which might attribute to less trapping sites and the recombination centers along with enhanced charge transport rate during the operation of the device, leading to high JSC and high VOC respectively. τtr and τR as a function of the incident light intensity is shown in Figure 10(a, b). The τtr and τR values gradually decrease with the increase of the photon fluxes, corresponding to the high penetration of perovskite and HTM to the closely connected ZnO-QDs(APjet) thin film. The charge collection efficiency of the fabricated flexible perovskite solar cell could be calculated by the following relation:62 ηCC = 1–τtr/τR

(1)

where τtr and τR values are estimated from IMPS and IMVS plots of the fabricated flexible perovskite solar cell.63 From Figure 10(c), the high ηCC value is appraised for the fabricated ITOPET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag solar cell, representing the high charge generation and the collection under the illumination, tending to the fast electron-transport


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rate and the high photocurrent density. Furthermore, the diffusion coefficient and diffusion length in the flexible perovskite solar cell could be estimated by using the following expressions: Dn = d2/2.35.τtr DL = (Dn·τR)1/2, Where Dn is the diffusion coefficient obtained by IMPS plot and d is the film thickness.55 Dn value

of

~9.18

x

10-8 cm2.s-1

is

obtained

by the

fabricated

ITO-PET/Gr/ZnO-

QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag solar cell, as presented in Figure 10(d). In general, the DL defines the average distance of an electron travels before it recombines with either the absorber (perovskite) or the hole conductor.64 In our case, the fabricated ITO-PET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag solar cell exhibits a good DL value of ~1.46 µm which is attributed to the probability of large electrons to enter from the ITO-PET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD thin film layers to the top Ag layer electrode and significantly improves the charge collection efficiency, as also evidenced in EIS and IPCE results.

Therefore,

the

fabricated

ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-

MeOTAD/Ag flexible perovskite solar cell presents the improved electron transport rate, high charge collection, and low DL value, which considerably lead to achieve high JSC, VOC and the high photovoltaic performance. In spites of all the above factors, the improved surface-to-volume ratio, porosity and the structure of ZnO-QDs by APjet treatment might intensify the adsorption capability of the perovskite molecules and facilitates the high light-harvesting efficiency, resulting to the high charge carrier generation and collections.

4. CONCLUSIONS



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For the first time, the modification of ZnO QDs in terms of surface area, pore size and porosity has been performed using highly advanced APjet technology for the high infiltration of perovskite and HTL for the fabrication of flexible perovskite solar cells. O2 plasma treatment of ITO-PET substrate considerably enhances the hydrophilicity and surface energy of ITO-PET substrate due to the production of polar group moieties through the oxidation plasma process. The increased contact angle in ITO-PET/Gr/ZnO-QDs(APjet) thin film deduces the improvement in the surface-to-volume ratio and the contact between hydrophilic ITO and amphiphilic Gr surfaces of thin film. The ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell attains the high conversion efficiency of ~9.73% along with high short circuit current (JSC) of ~16.8 mA/cm2, open circuit voltage (VOC) of ~0.935 V and high fill factor (FF) of ~0.62, which is higher than other flexible perovskite solar cells fabricated with ITO-PET/Gr and ZnO-QDs/Gr/ITO-PET thin films. IMPS and IMVS studies reveal that the fabricated ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell shows good charge transfer rate and the reasonable recombination rate. The APjet treatment and O2 plasma treatment on ITO-PET substrate are the promising approaches to enhance the surface-to-volume ratio, porosity and structure of ZnO-QDs, which substantially intensifies the adsorption capability of the perovskite/HTM molecules and facilitates the high light-harvesting efficiency. The simple fabrication process and the long-term stability open up new avenues for the future development of low-cost photovoltaic cells. ACKNOWLEDGMENTS This paper is fully supported by the Research Funds of Chonbuk National University in 2012. The NRF Project #2011-0029527 is acknowledged. We also acknowledge the Korea Basic Science Institute, Jeonju branch, for utilizing their research supportive facilities.


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Table 1 Photovoltaic parameters Thin films

JSC (mA/cm2)

VOC (V)

FF

η(%)

ITO-PET/Gr

12.03

0.840

0.29

2.89

ITO-PET/Gr/ZnO QDs

14.97

0.830

0.43

5.28

ITO-PET/Gr/ZnO QDs(APjet)

16.8

0.935

0.62

9.73



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Table and Figure Captions Table: Photovoltaic parameters of the fabricated flexible perovskite solar cells with ITO-PET/Gr, ITO-PET/Gr/ZnO-QDs and ITO-PET/Gr/ZnO-QDs(APjet) thin films. Figure 1. Contact angles and the corresponding 3D AFM images of (a, a1) ITO-PET, (b, b1) O2 plasma treated ITO-PET, (c, c1) ITO-PET/Gr and (d, d1) ITO-PET/Gr/ZnO-QDs(APjet) thin films. Figure 2. Topographic AFM images of (a) ITO-PET, (b) O2 plasma treated ITO-PET, (c) ITOPET/Gr and (d) ITO-PET/Gr/ZnO-QDs(APjet) thin films. Figure 3. Transmittance spectrum (a) and variation plot of sheet resistance and transmittance (b) of ITO-PET, F-1(O2 plasma treated ITO-PET), F-2 (ITO-PET/Gr), F-3 (ITOPET/Gr/ZnO-QDs),

F-4

(ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD

thin films. Figure 4. C1s XPS spectra of (a) ITO-PET/Gr, (b) ITO-PET/Gr/ZnO-QDs(APjet), and O1s XPS spectra of (c) ITO-PET/Gr, (d) ITO-PET/Gr/ZnO-QDs(APjet) and (e) Zn 2p XPS of ITOPET/Gr/ZnO-QDs(APjet) thin films. Figure 5. Raman spectra of ITO-PET/Gr and ITO-PET/Gr/ZnO-QDs(APjet) thin films. Figure 6. (a) Photograph of ITO-PET/Gr and (b) a schematic illustration of the fabricated ITOPET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell. Figure 7. J-V curves (A) and IPCE spectra (B) of the fabricated flexible perovskite solar cells with (a) ITO-PET/Gr, (b) ITO-PET/Gr/ZnO-QDs and (c) ITO-PET/Gr/ZnO-QDs(APjet) thin films.


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Figure 8. (a) Nyquist plots of the fabricated flexible perovskite solar cells with different thin films and (b) Nyquist plots of ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiroMeOTAD/Ag flexible perovskite solar cell at different voltages. Figure 9. IMVS (a) and IMPS (b) measurement plots of the fabricated ITO-PET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell. Figure 10. The electron transport (a), the recombination lifetime of electrons (b), charge collection efficiency (c) and the diffusion coefficient (d) of the fabricated ITOPET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell with respect to different incident photon fluxes.



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Figure 1


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Figure 3


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Figure 5


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Figure 7


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Figure 9


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Figure 10



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TOC graphic


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An Insight into Atmospheric Plasma Jet Modified ZnO Quantum Dots

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