Low temperature processed ZnO thin film as electron transport layer for efficient perovskite solar cells

Solar Energy Materials & Solar Cells 159 (2017) 251–264

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Low temperature processed ZnO thin film as electron transport layer for efficient perovskite solar cells Md Arafat Mahmud, Naveen Kumar Elumalai n, Mushfika Baishakhi Upama, Dian Wang, Kah Howe Chan, Matthew Wright, Cheng Xu, Faiazul Haque, Ashraf Uddin n School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, 2052 Sydney, Australia

art ic l e i nf o

a b s t r a c t

Article history: Received 10 March 2016 Received in revised form 8 September 2016 Accepted 11 September 2016

Organic inorganic lead halide Perovskite photovoltaic devices are promising candidates for commercial application because of their high efficiency and low production cost. One integral part of these high efficiency solar cells is electron transport layer that provides the electron contact selectivity and mitigates recombination phenomena for enhanced device performance. However, high temperature sintering process of most widely used Titanium oxide electron transport layer or the sophisticated, time consuming processing with nanostructured electron extraction material is a fundamental barrier to mass production of Perovskite solar cell with roll-to-roll process. In this work, we have reported the application of simple, low temperature processed ( o150 °C) sol-gel ZnO thin film as electron transport layer with efficient (PCE: 8.77%), highly reproducible Perovskite solar cell. Consecutive spin coating process has been implemented to find a multi-layer ZnO film that ensures high optical absorption in photoactive Perovskite layer by acting as a highly transmitting, less reflective, transparent layer. The optimized ZnO film also provides coherent surface morphology for the proper crystalline growth of overlying Perovskite layer and suppresses the deep trap states existing at the ZnO/perovskite interface. A systematic impedance spectroscopy study has been presented in this work to comprehend the improved device performance with the optimized multilayer electron transport material. The electronic properties like contact resistance, recombination resistance, flat-band potential and depletion width of the best performing device have been investigated. The interfacial charge transfer characteristics between methyl ammonium lead triiodide perovskite and low temperature solgel ZnO have also been elaborated based on the interface electronic properties. & 2016 Elsevier B.V. All rights reserved.

Keywords: ZnO thin film Low temperature Deep trap states Electronic properties Mott-Schottky analysis Charge transport

1. Introduction Highly efficient methyl ammonium lead halide perovskite solar devices have agitated the photovoltaic research arena in the recent years due to their high power conversion efficiency and nominal fabrication cost compared to silicon solar cells [1,2]. Methyl ammonium lead halide Perovskite is a class of hybrid organic-inorganic material families whose general formula can be given as ABX3, where A, B and X stand for organic cation (methyl ammonium [2] or formamidinium cation [3]), divalent metal ion (Pb or Sn [4]) and an individual halogen element (Iodine, Chlorine, and Bromine) or a composition of them with a fixed molar ratio respectively. With efficiency already reported over 20% [3], Perovskite material has a unique set of semiconductor properties that n

Corresponding authors. E-mail addresses: [email protected] (N.K. Elumalai), [email protected] (A. Uddin). http://dx.doi.org/10.1016/j.solmat.2016.09.014 0927-0248/& 2016 Elsevier B.V. All rights reserved.

makes it compatible with high performance photovoltaic device fabrication. Methyl ammonium lead triiodide perovskite has a direct band gap of 1.57 eV [5] which only demands a photon with identical energy as the material bandgap for creating electron hole-pair without significant change in electron momentum. It is also blessed with high carrier mobility being as high as 10 cm2 V
1 s
1 [6]. Additionally, a considerably large exciton diffusion length of  100 nm [7,8] for pure tri-iodide perovskite material and more than 1 mm [9] for mixed halide perovskite has already been reported. The exciton binding energy for tri-iodide perovskite has been found to be 16 72 meV in a recent study [10] which explains its exciton dissociation even at room temperature [11]. This contributes to a significant boost in the short circuit current density and fill factor of the perovskite device structure [12]. Furthermore, the procedure of perovskite layer formation can be greatly simplified by virtue of its compatibility with solution-processing [9,13]. All these traits reflect the unprecedented potential of perovskite material in solar industry. Although perovskite solar cells do not require any

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heterojunction for the dissociation of the excitons [11] like organic photovoltaic cells (OPV), Electron transport layer (ETL) plays a vital role in perovskite solar cells for contact selectivity [14] and in optimizing device performance [15,16]. ETL works as electron selective contact that preferentially extracts electrons to one side of the device and blocks the direct contact between transparent conductive oxide and hole transporting layer [14]. Besides working as a HBL (Hole Blocking Layer), ETL enhances the fill factor [15] and the open circuit voltage [16] of a perovskite solar cell as the recombination rate is mitigated by electron selective contact between perovskite and ETL [15]. Additionally, ETL influences the perovskite layer morphology and its loading and controls the quality of perovskite/ETL interface and perovskite layer itself since ETL provides the intervention in the full perovskite transformation from the precursor [17]. For this reason, although some ETL free perovskite device structures have already been reported [18,19], none of them was as efficient as those with ETL. However, most of the highly efficient Perovskite devices with normal device structure need a high temperature sintering process of  500 °C [2,20– 23] for the compact TiO2 (titanium oxide) layer as electron transport layer (ETL). This high temperature is one of the fundamental barriers to mass production of perovskite solar cell since it does not satisfy the requirement of 100–150 °C [24] temperature range for flexible substrates with roll-to-roll process [25]. Although the inverted device structure having PEDOT: PSS [Poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate)] HTL underlying the perovskite layer is completely low temperature processed [26,27], due to the hygroscopic nature of PEDOT: PSS [28], the ambient device stability is expected to be lower with such device structure. Besides, the inverted structure includes PCBM ([6,6]Phenyl C61 butyric acid methyl ester) ETL on top of perovskite layer and in a recent study [29], it has been found that inverted structure has lower stability due to chemisorption of water and oxygen in PCBM layer. As such, replacing the high temperature process of TiO2 ETL with a low temperature alternative in a normal device structure can pave the way for large scale production of stable perovskite solar cells. As such, a number of research works have already been carried out to find the suitable substitute for Titania ETL. The studies used either ZnO nanorod [30–32] or ZnO nanocrystal [33] as alternative ETL to Titania. ZnO is a wide band gap semiconductor that can achieve high structural quality even at low temperature [34,35]. Along with the large bandgap, ZnO has a high exciton binding energy of 60 meV that contributes to its excellent chemical and thermal stability [36]. ZnO has a favorable conduction band energy level of 4.4 eV [37] which facilitates electron extraction from LUMO level (3.9 eV) [38] of methyl ammonium iodide perovskite. Similarly, the valence band energy level (7.6 eV) [37] of ZnO is at a conforming level to block holes from HOMO (highest occupied molecular orbital level) (5.4 eV) of perovskite [38]. Thus, ZnO can simultaneously play the role of ETL and HBL in a perovskite solar cell that makes it a befitting n-type conducting layer. The outstanding optical transparency of ZnO in the visible light spectrum [39] also makes it a worthy contender to be ETL in a normal device structured Perovskite solar cell. ZnO has nearly identical electrical affinity (4.2 eV) as TiO2 [17], but has higher conductivity ( 0.85 mS/cm) compared to Titania (  0.4mS/cm) [40,41]. Besides, the electron mobility in ZnO (  200–300 cm2 V
1 s
1) is several orders of higher than TiO2 (  0.1–4 cm2 V
1 s
1) [42]. All these optoelectronic properties of ZnO make them preferable alternative to TiO2 as ETL in Perovskite solar devices. But most of the reported ZnO ETL perovskite devices [30–33] involve around  400 0C annealing temperature for ZnO nanostructures. As a result, although the titania layer was replaced with ZnO nanostructures, the high temperature processing is still a concern. Of late, Timothy et al. [16] have reported ZnO nanoparticle processing which involves temperature lower than 70 °C. Pauport’e et al.

[17,43] have used electrodeposition of ZnO ETL at around 60 °C for perovskite devices. Seok et al. [44] have implemented highly dispersed Zinc stannate (Zn2SnO4) nanoparticles as ETL in a perovskite photovoltaic device at less than 100 °C temperature. Despite being promising endeavors in terms of low temperature processing, these processes [16,17,43,45] comprise of time consuming precipitation, dilution and washing processes coupled with a long reaction time or the need for sophisticated handling and delicate electrochemical machinery. A simple, low temperature processing technique devoid of these nanoparticle synthesis complexities, thus, can be a potential candidate for ETL in mass production of perovskite solar devices. Likewise, low temperature, easily processable sol-gel processed ZnO bids fair to be a promising contender for its nanoparticle counterparts. There has been only one study [46] on sol-gel ZnO as ETL for perovskite device. Sol-gel ZnO process enabled them to drop down the ETL processing temperature to 290 °C [46] from a higher temperature of 500 °C normally used with TiO2 compact layer or scaffolding structure. However, the process temperature (290 °C) in this work [46] still does not satisfy the constraint of 100–150 °C [24] temperature range for flexible substrates with roll-to-roll process. In this work, we have reported a simple, low temperature (o150 °C) processed sol-gel ZnO thin film as ETL for methyl ammonium lead triiodide perovskite solar device. An optimized thickness has been found out for the sol-gel ZnO ETL to be applied in highly reproducible perovskite photovoltaic devices. Two step dipping technique [22] has been chosen to form the perovskite layer over the sol-gel ZnO ETL in a normal device structure on ITO/ glass substrate. Two step dipping technique has been successfully implemented previously for perovskite formation on flexible PET (Polyethylene terephthalate) substrates as well [16,24]. Thus the two step dip-coating technique in our work can be easily scalable to roll-to-roll process [47] for mass production of perovskite solar devices. In our work, we have also ensured the complete conversion of PbI2 to CH3NH3PbI3 in the dipping process by the incorporation [48] of 4-tert-Butylpyridine (4-TBP) which improves the device performance by enhancing the Fill Factor of the device. Doped P3HT [49] has been implemented as a cheap and readily available hole transport layer (HTL) instead of commonly used Spiro-OMeTAD which is quite expensive. Combining all these thin film layers, we have reported a low temperature processed device structure, which, to the best of our knowledge, has never been reported so far with a CH3NH3PbI3 Perovskite device. Another special feature of our work lies in the in-depth study conducted by means of Electrochemical Impedance Spectroscopy (EIS) that enables us to dig into the physical phenomena like charge transport, carrier recombination, interfacial charge transfer etc. taking place inside the device those directly affect the device performance. A number of research studies have already been conducted for perovskite solar cell with EIS, focusing on mesoscopic [50] or thinfilm structure with TiO2 compact layer [45,51,52] or TiO2 and ZnO nanostructures [18,52]. In this work, we have used EIS study to extract the perovskite/solgel ZnO interface contact resistance, device recombination resistance, carrier density, flat-band potential and depletion width at zero bias using the fitted data from Nyquist plot and Mott-Schottky curve. Interpreting these basic device parameters, we have explored how the trap states present in the low temperature processed sol-gel ZnO ETL affect charge extraction and thus the overall device performance in a perovskite device. Using the EIS study, we have also explained how the optimized sol-gel ZnO ETL in our work suppresses the trap states for enhanced device performance. Thus the optimization process of low temperature sol-gel ZnO presented in our work can be very useful for the future endeavors regarding low temperature ETL processed perovskite solar cells.

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2. Experimental detail 2.1. Device fabrication Patterned ITO/glass substrates were washed with Hellmanex III soap, DI water, Acetone and Isopropanol respectively with duration of 10 min each. A 0.4 M sol-gel ZnO solution was prepared by dissolving Zinc Acetate Dihydrate (Zn(CH3COO)2  2H2O, SigmaAldrich, 499.0%) in 2-Methoxyethanol (CH3OCH2CH2OH, SigmaAldrich, 99.8%, anhydrous) with an additive Ethanolamine (NH2CH2CH2OH, Sigma-Aldrich, 499.5%) and by vigorous stirring for 24 h. The sol-gel ZnO was spin coated on ITO/glass substrate with 4000 rpm for 60 s and then annealed on a hotplate at 140 °C for half an hour. For double or triple coated device, the spin coating and the annealing process were repeated once or twice. 1 M PbI2 solution in DMF (N, N –dimethylformamide, Sigma Aldrich, Anhydrous) with 120 mL 4-TBP (4-tert-Butylpyridine, Sigma Aldrich) was prepared at a constant temperature of 70 °C. The PbI2 solution was spin cast on ZnO coated substrate at 3000 rpm for 30 s. Before spin coating, both the substrates and the PbI2 solution were kept at 70 °C. After the PbI2 coating, the substrates were annealed at 100 °C for one hour and then dipped in 10 mg/ml methyl ammonium iodide (LUMTEC) solution in 2-Propanol (Sigma Aldrich, Anhydrous) for 1 min. Within this time, the full conversion of PbI2 into CH3NH3PbI3 perovskite takes place and then the film is dried quickly by blowing with N2 gun. 20 mg/ml P3HT (Poly (3-hexylthiophene), 1-Material) in Chlorobenzene was doped with and 10 mL Li-TFSI (170 mg/ml in Acetronitrile) and 10 mL 4-TBP. The doped P3HT layer was spin coated on the perovskite layer with 1200 rpm for 30 s. Finally, 100 nm Ag layer was deposited on the doped P3HT film by thermal evaporation with a evaporation rate of 2 Å-s under a vacuum condition of 1  10
6 mBar. The device area was fixed to be 0.045 cm2 with the use of a metal mask. 2.2. Characterization The current-voltage characteristics of the devices were measured with a NREL calibrated Keithley 2400 Source Meter under 100 mW/cm2 (AM 1.5 G) simulated sunlight. For optical characterization like transmittance, reflectance and absorbance measurement, a UV–VIS-NIR spectrometer (Perkin Elmer – Lambda 950) was used. X-ray diffraction (XRD) with CuKα radiation was performed by step-scanning with a step size of 0.02 degree. Surface topology and device cross sectional view were captured by Carl Zeiss AURIGA CrossBeam SEM (Scanning Electron Microscopy) and the surface roughness was measured with Bruker Dimension ICON SPM AFM (Atomic Force Microscopy) machine. The impedance analysis was conducted with an Autolab PGSTAT-30 equipped with a frequency analyzer module in the frequency range from 1 MHz to 10 Hz. AC oscillating amplitude was as low as 20 mV (RMS) to maintain the linearity of the response. External quantum efficiency (EQE) measurements were performed using a QEX10 spectral response system from PV measurements Inc.

3. Results and discussion In the present work, low temperature processed Sol-gel ZnO film has been optimized for the application as ETL in a normal perovskite solar device structure. In the normal perovskite device structure under study, the rudimentary optical role of the ZnO ETL layer is to perform as a highly transmitting antireflective layer [48] for higher light absorption of the perovskite film lying on top of it. So, at first, consecutive spin coating process has been applied to create multi-layer ZnO ETL film for finding out the optimum

Fig. 1. (A) Estimation of approximate material bandgap for low temperature processed sol-gel ZnO Pristine Perovskite using Tauc plot (B) XRD pattern of sol-gel processed ZnO annealed at 140 Deg. C (The * signs denote the peaks for ITO).

thickness to ensure the highest light transmittance from the ZnO ETL for the highest light absorption in the photoactive perovskite layer. Table S1 shows the number of spin coating layer and the corresponding thickness of the sol-gel ZnO film used in our investigation. The thicknesses for the single, double and triple coated ZnO film have been found to be 25, 45 and 60 nm respectively. Although, the ZnO ETL used in this work is low temperature processed, the formation of basic ZnO structure has been ensured by observing the material bandgap from Tauc plot and performing the XRD measurement. From the Tauc plot shown in Fig. 1(A), the band gap of the sol-gel ZnO film has been found to be 3.53 eV which is indicative of the ZnO formation despite the low temperature annealing used in the ETL processing. For further clarification, we have also conducted XRD measurement to ascertain the ZnO construction on ITO/glass substrate. Formation of hexagonal Wurtzite ZnO phase is indicated by the (002) and (101) diffraction peaks [53] shown in Fig. 1(B). Along the (002) plane, a strong preferential growth is visible which denotes the film orientation along c-axis, which is consistent with a ZnO precursor having concentration within the 0.3–0.6 M range [54]. Along the

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(101) plane, the film is weakly oriented which is expected with a ZnO precursor concentration much smaller than 1.3 M [54]. However, no distinct (100) diffraction peak was observed in the low temperature processed ZnO in our study. The (100) peak is not visible even with higher annealing temperature up to 340 °C (Fig. S1). This proves the ZnO precursor concentration is such that the constructed ZnO film is non-oriented along a-axis irrespective of annealing temperature. Since, the orientation along a-axis is less kinetically favorable [54], the produced ZnO ETL film is compatible for efficient charge extraction despite having less crystallinity [55] compared to high temperature processed ZnO films. After the formation of ZnO thin films on ITO/glass substrate, their optical properties have been investigated with absorbance, transmittance and reflectance measurement. From the Beer-Lambert's law, the absorbance, A of a thin film can be expressed as:

⎛I ⎞ A = log ⎜ 0 ⎟ = εlc ⎝ I⎠

(1)

where, I, Io, ε, l and c denote sample light intensity, reference light intensity, material dielectric constant, length of optical light path and precursor concentration respectively. Since, in our work, all the three ZnO thin films have been deposited on ITO/glass substrate from the same ZnO precursor, the dielectric constants and precursor concentration can be treated as proportionality constants to bring out a proportional relation between film absorbance and film thickness. We can observe the similar trend in the absorbance pattern (Fig. S2) for ZnO films with three different thicknesses. Within the limited absorption range [55] (400– 500 nm) of the transparent ZnO film, the 60 nm ZnO film has the highest light absorption while the 25 nm ZnO layer has the lowest. Fig. S3(A) and (B) illustrate the transmittance and the reflectance pattern of the three ZnO ETL films under our investigation. Though the 25 nm ZnO has the lowest absorption from the midrange UV to lower wavelength region of the visible light spectrum, the 45 nm ZnO has the highest transmittance and the lowest reflectance over a wide spectrum region extending from around 450 nm to the onset of perovskite absorption (800 nm) [16]. Thus the 45 nm ZnO film is superior to 25 nm or 60 nm ZnO layer as a highly transmitting antireflective ETL in a normal device structure with perovskite material. On top of all the three ZnO coated substrates, the perovskite layer has been fabricated in an identical manner with two step dipping technique. In our work, the complete conversion of PbI2 into CH3NH3PbI3 perovskite has been ensured by the incorporation of an additive 4-tert-Butylpyridine [56] in the PbI2 precursor in DMF. In the two-step dipping process, the elimination of remnant PbI2 lying underneath the partially formed perovskite film is very crucial for enhanced device performance and stability [56]. From the tauc plot shown in Fig. S4, the material bandgap found for the pristine CH3NH3PbI3 Perovskite is 1.56 eV which is consistent with the previous literature [5] and no band edge is visible in the tauc plot that can be extrapolated to around 2.3 eV [57] at the positive x-axis denoting the full conversion of PbI2 into the perovskite film. For further clarification, XRD measurement was done for the pristine perovskite film. The XRD patterns of Perovskite and PbI2 have been presented in the same plot to lift up the full PbI2 conversion (Fig. S5(A)). From the amplified view of the XRD pattern in Fig. S5(B), it is obvious that the (001) diffraction peak in PbI2 is absent in the XRD pattern for CH3NH3PbI3 perovskite, rather there are prominent (110), (220), (310), (224) and (314) diffraction peaks those ensure that the tetragonal perovskite structure [58,59] has been properly formed. As the perovskite film was prepared at ambient air condition (35–40% relative humidity), a characteristic peak of dihydrate [(CH3NH3)4PbI6  2H2O] at 11.4° [60] is also observed in XRD pattern of CH3NH3PbI3 perovskite (Fig. S5). Under

humidity controlled ambient environment, moisture assists the ion diffusion of unreacted methyl ammonium iodide to form CH3NH3PbI3 perovskite and dihydrate [60]. Thus moisture aids to mobilize methyl ammonium iodide and thus help crystallization of CH3NH3PbI3 perovskite. However, the hydrate peak disappears after 11 days of perovskite formation [60]. Since, the XRD characterization of perovskite film was conducted within that time range, the dihydrate peak was visible in the XRD pattern. The perovskite XRD pattern in our work is thus consistent with the previous report [60]. After the formation of perovskite layer on top of ZnO films, we have checked the optical properties exhibited by perovskite/ZnO films on ITO/glass substrate. Keeping in line with the high transmittance through the transparent 45 nm ZnO film, the perovskite located on top of this film shows higher absorption compared to the identically fabricated perovskite film overlying on 25 nm or 60 nm ZnO layer over a wide region of light spectrum starting from around 550 nm (Fig. S6(A)). The corresponding transmittance and reflectance pattern have been illustrated in Fig. S7. The attainment of the highest absorption of the Perovskite film coupled with the 45 nm ZnO layer underneath enunciates the superiority of this twice-coated ETL film in contrast with too thin single layer or too thick triple layer. The absorbance pattern of pristine perovskite film has been presented in Fig. S6(B) demonstrating that the insertion of highly transmitting 45 nm ZnO film causes negligible decline in photo-active perovskite absorption. The transmittance and the reflectance pattern of the Pristine Perovskite layer have also been presented in Fig. S8. Along with the optical properties of perovskite/ZnO films, we have probed into the growth of perovskite film over three different ZnO films by means of surface morphology analysis with SEM. Because, it is well-established from the previous study that the surface morphology of the perovskite layer is not only affected by processing conditions [23,61–64], but also significantly modified based on the underlying layer [65,66] the perovskite crystal grows on. Fig. 2 shows the SEM topographic image of perovskite film on top of 25 nm, 45 nm and 60 nm ZnO layer on ITO/glass substrate. The top SEM images of pristine ZnO films have also been presented in Fig. S9. The perovskite film overlying on 25 nm ZnO film does not have a uniform surface coverage owing to the mutually isolated, multiple-sized crystalline topography (Fig. 2(A), (B)), since the underneath 25 nm ZnO is too thin to grow [17] the perovskite structure on it properly. As a result, the perovskite surface constructed on 25 nm ZnO is devoid of close packing of identicallysized perovskite grain that can hinder the charge transport properties in the photovoltaic device [61]. Conversely, with the 45 nm ZnO layer underneath, the perovskite film shows much improved surface coverage, increased grain size and significantly reduced interfacial area due to well-defined grain boundaries (Fig. 2(C), (D)). The pin-holes on this film are significantly reduced due to uniform surface coverage. The enhanced surface coverage with the 45 nm ZnO film can be attributed to the optimum film-substrate interaction energy [67] that can be attained by the application of a thicker metal-oxide layer underneath the perovskite structure. Besides, the larger grain size in this film contributes to much smaller interfacial area in between the grain boundaries [65] that minimizes the charge trapping probability and thus reduces anomalous hysteresis due to variation in scan rate or direction [64]. The photo-generated carriers in this film are expected to encounter less defect and impurities which ensures higher carrier mobility [64]. The characterization results with impedance spectroscopy also support this assumption which will be explained subsequently in details. However, the perovskite film on 60 nm ZnO film has degraded surface morphology compared to the similarly fabricated perovskite film on 45 nm ZnO. As shown in (Fig. 2(E), (F)), the topography of perovskite layer on 60 nm ZnO

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Fig. 2. Top view Scanning Electron Microscopy (SEM) images of Perovskite layer on top of (A–B) 25 nm, (C–D) 45 nm and (E–F) 60 nm sol-gel ZnO film on ITO/glass substrate.

ETL suffers from the build-up of visible pinholes as it replicates the underneath ZnO layer with increased interfacial area amid the grain boundaries owing to less closely packed ZnO grains (Fig. S9 (E), (F)). The emergence of such pinholes in a 60 nm ZnO film is consistent with the observations in an earlier literature [55] with sol-gel ZnO. The 60 nm ZnO film has to undergo three consecutive annealing sessions lasting up to 1.5 h in total while Uddin et al. [55] have demonstrated that the sol-gel ZnO film develops pinholes on the surface if it is annealed longer than an optimum timeline of 1 h and the device performance is degraded likewise. Thus, although the grain size of 60 nm ZnO is comparable to that of 45 nm ZnO, from the perspective of providing conforming surface morphology for the perovskite structure, the 45 nm ZnO ETL outraces the 60 nm and let alone the much thinner single coated 25 nm ZnO film. With a view to gaining more insights on the surface topography, the Atomic force microscopy (AFM) images of the similarly fabricated perovskite layer on 25 nm, 45 nm and 60 nm ZnO film were examined (Fig. 3). Consistent with the SEM top view analysis, the RMS surface roughness (68 nm) of perovskite on 25 nm ZnO

film is higher than that (57.5 nm) on 45 nm ZnO layer. Because of multiple sized crystallographies, the Perovskite on shallow, single coated 25 nm ZnO film has expansive interfacial areas and discrete grain boundaries. These cause larger surface coarseness in Perovskite film on mono-coated ZnO film. Whereas, the perovskite on double layer 60 nm ZnO enjoys the privilege of smoother surface topography, courtesy of the more conforming surface coverage provided by the twice- coated ETL film underneath. Moving to the perovskite with triple coated 60 nm ETL layer, we observe degraded surface topography with higher RMS surface roughness (69.8 nm) which can be attributed to the surface pinhole formation in the underlying ZnO film owing to consecutively conducted, long annealing sessions described earlier. However, commonly the surface roughness in two step process is inherently higher (even up to 66.56 nm) [68] as the surface topology is significantly influenced by the drying of 2-Propanol used as the precursor or dipping solution for CH3NH3I. For a CH3NH3I dipping solution with concentration of 10 mg/ml, the surface roughness has been reported over 40 nm [56] in the previous literature. In line with the previous literature [68], the cross sectional SEM image presented

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Fig. 3. Two and Three dimensional Atomic Force Microscopy (AFM) images of Perovskite layer on (A–B) 25 nm, (C–D) 45 nm and (E–F) 60 nm ZnO film.

in Fig. S11 also shows inhomogeneity in the perovskite-P3HT interface (marked as “good” and “bad” regions on Fig. S11) which can be ascribed to its comparatively large surface roughness. The large surface roughness can be attributed to the less conforming wetting of perovskite on underlying layer [65] which causes island formation. The quality of the perovskite layer thus can be improved by forming a more homogeneous perovskite film, devoid of any island formation. In this regard, a highly relevant approach can be adopted from a recent study [56] which reveals that increasing the concentration of CH3NH3I dipping solution over 15 mg/ml (rather than using the conventional concentration of 10 mg/ml [16,22,49]) can be an effective way of decreasing the surface roughness of CH3NH3I layer. The device performance is expected to be even higher if this approach is undertaken with our reported study. However, it has been stated in the earlier works that the surface roughness decreases drastically once another carrier transport layer [69–71] is deposited on top of the perovskite layer. Regarding this, our observation also matches well with the literatures once

we deposit doped P3HT as HTL on all the perovskite films. The lowest RMS surface roughness of doped P3HT was found to be 35.7 nm with Perovskite/45 nm ZnO ETL underneath (Fig. S10). Besides, since the thickness of doped P3HT is  130 nm, the surface roughness of the underlying perovskite layer does not pose any threat of providing shunting path with the overlying cathode electrode (Ag in our case). To ascertain these unexpected shunting phenomena do not take place in our device structure, we have done the cross-section SEM of the fully fabricated device. The complete device structure was Glass/ITO/sol-gel ZnO (three different films)/CH3NH3PbI3 Perovskite/Doped P3HT/Ag (Fig. 4(A)). The cross-section image of the device (Fig. S11) confirms that the individual thin film layer has well-defined layer boundary with no sign of overlapping or shunting. Thus the overall device configuration has energetically favorable cascaded structure (Fig. 4(B)) for enhanced device performance. To evaluate the device performance with 25 nm, 45 nm and 60 nm ZnO ETL, corresponding IV measurement was conducted.

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Fig. 5. I-V curve of the champion device with 45 nm ZnO ETL.

Fig. 4. (A) Schematic representation of the fabricated device structure (B) Energy band diagram of the device showing the individual HOMO and LUMO levels with work function of the electrodes.

The device performance of the three devices has been tabulated in Table 1. The best performing double coated 45 nm ETL device shows a PCE of 8.77% with the JSC, VOC and FF value of 14.99 mA/ cm2, 0.932 V and 62.67%. The average values of JSC, VOC and FF of this device structure are 13.83 mA/cm2, 0.817 V and 54.72% respectively. The average PCE of the 45 nm ZnO ETL device is almost equal (  95%) to the average PCE (  6.7%)of a previously reported FTO/TiO2/CH3NH3PbI3 perovskite /Doped P3HT/Au device [72], although the processing temperature for ZnO ETL in our device is 360 °C lower. The high reproducibility of our 45 nm ZnO ETL device is evident from the histogram shown on Fig. S12. The I-V curve and the EQE plot for the champion device have been shown in Fig. 5 and Fig. S13 respectively. As seen from the I-V curve in Fig. S14, there is some hysteresis relative to scan direction even in the best performing device. The presence of hysteresis is understandable with the existence of trap state [33,66] due to low temperature processing adopted for device fabrication. To ensure suppressed hysteresis, fullerene based interlayers can be

incorporated for surface passivation [69,70,73], which is beyond the scope of our present work. In the subsequent sections, we have investigated into the better performance of 45 nm ZnO ETL device in terms of the electronic properties of the device with the aid of impedance analysis. For attaining in-depth understanding of the device performance in terms of carrier transport, recombination and interfacial charge transfer phenomena, we have conducted electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis of the fabricated devices in our study. Since the ZnO based devices have lower stability compared to TiO2 based devices [74], the electronic characterization (EIS and Mott-Schottky analysis) of the devices have been conducted just after the device has been fabricated. Thus it has been ensured that the electrical response is not attained rather from a degraded device, which can cause large perovskite/ETL or perovskite/HTL contact resistance [29]. The device photovoltaic performance has been found almost identical before and after the electrical measurement (Table S5) which ascertains the electrical measurements are not influenced by the degradation process. Fig. 6(A) shows the Nyquist plots for the Perovskite device with 25 nm, 45 nm and 60 nm solgel ZnO ETL at 700 mV bias under light illumination. From the amplified view of the high frequency portion of the curves (Fig. S15), we clearly observe a small radius arc in high frequency which is followed by a larger radius arc for each device. Our experimental data fit nicely with a previously reported model [18] that used ZnO nanostructures as ETL in a perovskite solar device. The equivalent circuit (Fig. S16) consists of a series combination of one parallel R-C element and another parallel R-CPE (Constant Phase Element) element combined with a series resistance, RS. In Fig. S16, RC, CC, RRec, CPE and RS denote contact resistance and capacitance at ETL/perovskite or HTL/perovskite interface, recombination resistance, constant phase element originating from heterogeneity [51,75] and resistance incorporating metal contact and wire respectively. Here, the device

Table 1 Device performance of Perovskite devices with 25 nm, 45 nm and 60 nm sol-gel ZnO ETL. Efficiency (%) RS (Ω cm2) RSh (Ω cm2)

ZnO layer thickness (nm)

Average/ best value

Open circuit voltage, VOC (mV)

Short circuit current density, JSC Fill factor, FF (mA/cm2) (%)

25

Average

774.83

7.47

43.68

2.53

35.33

6580.00

45

Average Best Average

816.15 932.89 803.56

13.93 14.99 9.61

56.16 62.67 48.9

6.42 8.77 3.84

15.26 10.17 23.76

11308.89 18200.00 8650.00

60

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Fig. 6. (A) Nyquist plot of Perovskite device with 25 nm, 45 nm and 60 nm ZnO ETL at a bias of 700 mV under light illumination and Bias dependence of the Nyquist plot for Perovskite device with (B) 25 nm, (C) 45 nm and (D) 60 nm ZnO ETL (make the axis and font larger).

Table 2 Fitted values of different parameters from Nyquist plot of the Perovskite solar devices at 700 mV under illumination. ZnO layer thickness RC (Ω cm2) RRec (nm) (Ω cm2)

CC (nF) CPE-Y (nMho)

CPE-N

25 45 60

2.53 5.80 3.22

0.79 0.92 0.83

6.30 4.96 5.20

40.37 74.25 47.25

64.30 34.70 37.00

series resistance incorporates the contact resistance between ETL/ perovskite or HTL/perovskite interface and the resistance arising from the wire connection and metal contact. Since, in our work, the doped P3HT HTL layer remains constant for all the device structures and the resistances incorporating from wire and metal contact are almost identical ( 5.4 Ω cm2), we can explain the high frequency features to be associated with the varying ETL/perovskite interface which is an assumption consistent with the previous literature [18]. From Fig. S16, the perovskite device with 25 nm ZnO film has the first high frequency arc at the highest impedance value while it is at the lowest with 60 nm ZnO layer. Keeping in line with the qualitative observation, the contact resistance, RC for the 25 nm ETL device is the largest while that for the triple layer is the smallest with the 45 nm ZnO ETL device value being in between them (Table 2). The highest contact resistance (6.3 Ω cm2) with 25 nm ETL is very obvious as the shallow ZnO ETL cannot provide efficient charge extraction from perovskite due to electron hopping in deep localized trap states [76]. Despite being an inorganic semiconductor, the low temperature, ambient air annealed ZnO has a high order of energetic disorder at grain boundaries due to chemisorption of oxygen [76]. As a result, a large density of trap states is existent in such ZnO film compared

to more crystalline, well-ordered ones. These trap states can be very deep extending up to 200 meV below the conduction band [77]. Thus, the thin single-coated 25 nm ZnO ETL does not have a well-defined hole and electron quasi fermi level because of the gap tail states induced by disorder which is more commonly prevalent in organic semiconductor [78,79]. Consequently, the electron from perovskite LUMO will hop in between these trap states [80] before being finally collected by the ITO. This hopping between trap states adversely affects carrier extraction owing to the unfavorable energy barrier introduced between deepest trap state and ITO work function (Fig. 7). Thus, the device series resistance (RS) increases and short circuit current density, JSC decreases likewise. However, as the ZnO film thickness increases, the grain orientation effect takes place and the trap density reduces rapidly [76]. Therefore, perovskite device with 45 nm ZnO layer has lesser trap states compared to that with 25 nm ZnO film, resulting in more favorable carrier extraction and decline in contact resistance as observed from impedance spectroscopy. Thus the series resistance of the device with 45 nm ETL is over two times smaller than that for 25 nm ETL device. Consequently, JSC and fill factor also have ameliorated numbers [68] for 45 nm ZnO with the best device rendering JSC close to 15 mA/cm2 (Table 1). For 60 nm ZnO ETL, contact resistance (5.2 Ω cm2) from fitted model is comparable to 25 nm ZnO device ( 5 Ω cm2). Nevertheless, because of the degraded surface morphology and increased surface roughness owing to visible pinhole formation, efficient charge extraction is hindered by leakage current [69] that brings about the downfall for JSC and fill factor for 60 nm ZnO device. The device performance of the optimized 45 nm ZnO device can further be improved if the contact resistance at the perovskite/charge selective contact interface can be reduced by forming more

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259

Fig. 7. Energy band diagram demonstrating electron-hole recombination due to electron hopping in deep trap states in 25 nm ZnO ETL device (Device-A) and less carrier recombination and favorable charge extraction in 45 nm ZnO ETL device (Device-B) due to less trap states.

homogeneous perovskite film having less surface roughness. From the fitted model, the recombination resistance, RRec for the 45 nm ZnO ETL device is the largest (74.25 Ω cm2) and it is the smallest (40.37 Ω cm2) for mono coated 25 nm ETL device (Table 2). These values indicate the highest carrier recombination take place in 25 nm ETL device, while the least recombination phenomenon is experienced in 45 nm ETL device. The largest radius arc in the low frequency region of the Nyquist plot also lifts up the enhanced carrier recombination with 45 nm ZnO ETL device qualitatively [59,68] (Fig. 6(A)). The high value of recombination resistance is also consistent with the large value of shunt resistance, RSh from the I-V data for twice coated ETL devices. Since the non-radiative recombination due to deep trap states [80] in a well-formed perovskite is very rare [12], the recombination mechanism in double ETL coated Perovskite film is mostly bimolecular recombination [12]. However, the large recombination resistance in the device hints significant reduction in bimolecular recombination that explains the enhanced Voc in double coated cell. In contrast, both the 25 nm and 60 nm ZnO device suffer from higher recombination phenomena as indicated by low RRec value from impedance spectroscopy and low RSh value from I-V data. Thus, the degraded surface morphology of perovskite on 25 nm and 60 nm ZnO, as detected from SEM morphological analysis, deteriorates the cell Voc due to increased recombination. In addition to that, the spatial variation in quasi fermi level [12] due to deep trap states in less crystalline ZnO thin film also contributes to the decrease in Voc. This will be explained elaborately in the next paragraph. Fig. 6 (B–D) lift up the bias dependence of the Nyquist plots for 25 nm, 45 nm and 60 nm ZnO device. With the increase in applied bias, higher recombination causes downfall in recombination resistance for all the devices. However, as observed from Fig. 8, recombination resistance is higher in 45 nm ZnO device for all the bias compared to its single and triple layer counterparts, demonstrating the superior performance of 45 nm ETL device. Careful observation of Fig. 8(A), (B) reveals that the recombination resistance under illumination is significantly lower than that under dark condition, which is coherent with the effect of photo-generation with light illumination

Fig. 8. Recombination resistance as a function of bias voltage for Perovskite device with 25 nm, 45 nm and 60 nm ZnO ETL (A) under illumination (B) at dark condition.

[78]. Tables S2 and S3 contain the recombination resistance values of the three different device structures at different bias under light illumination or at dark condition for facilitating numeric comparison.

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Fig. 9. Capacitive response of three different ETL devices (25, 45 and 60 nm ZnO/ perovskite device) as a function of frequency under dark (The inset displays the capacitance value at intermediate and high frequency regions for further clarification).

To further investigate whether the trend in recombination resistance is related to the variation in perovskite/ETL interface, capacitive response of the three different device structures has been explored in our work. Fig. 9 shows the capacitive response of the three device structures as a function of frequency under dark. In general, the low frequency (below 1 kHz frequency) capacitance under dark is related to the electrode polarization process associated with the ion migration to the external electrodes [81]. The intermediate frequency (1–10 kHz frequency) capacitance is associated with the accumulation of charge carriers at the interface while the high frequency (over 10 kHz) capacitance is connected with the bulk properties of intrinsic perovskite layer [81,82]. As observed from Fig. 9, there is a significant difference in the capacitive response of the three different device structures at low and intermediate frequency region, whereas the high frequency capacitive response is very close-ranged for all the devices (inset of Fig. 9 for more clarification). Since, the high frequency response is almost identical for the three devices, it can be suggested that the diverse dipolar mechanisms like ionic off-centering or CH3NH3 þ or PbI6 octahedra reorientation are almost similar in all of those devices [82], rendering similar intrinsic bulk perovskite property in them. Thus, it is anticipated that the variation in electrical response (like recombination resistance) is originated from the perovskite/ETL interfacial contact, rather than from the intrinsic bulk perovskite property. In the low and intermediate frequency region, the capacitance value varies maintaining an inverse relation with the ZnO layer thickness (Fig. 9). Since, the capacitance value is inversely proportional to the thickness [81] and the perovskite layer thickness remains constant for all the device structures, thus the variation in the capacitive response in the low and intermediate frequency region can be attributed to the variation in ZnO layer thickness. This observation is also consistent with a previous literature [81], where the variation in ETL layer thickness has been reported to change the low and intermediate frequency capacitive response for a perovskite device. Moreover, the absence of any inductive feature (indicated by a loop in the intermediate frequency region at the Nyquist plot (Fig. 6)) [81] also supports that the interfacial charge accumulation at perovskite/ETL interface is existent in our reported devices which influences the device performance [81,82]. This interfacial charge accumulation can lead to recombination phenomena which have been explained in the subsequent paragraph along with energy band diagram. Under light illumination, the photo-generated carrier causes

Quasi Fermi level splitting [78]. However, Quasi fermi level is a combined property of charge carriers and it is strongly modified by energetic disorder in the material [83]. As described earlier, low temperature processed, less crystalline ZnO ETL possesses disorders which is more acute for very shallow ZnO film. So, any electron from any unoccupied trap state in the distribution will contribute to the Fermi level [83] and there will be spatial variation in Quasi Fermi level [12]. Fig. 7 lifts up the visual representation of this shift in Quasi Fermi level. With deep trap state in 25 nm ZnO device (Device-A in Fig. 7), the Quasi Fermi level is * from the ideal posiexpected to drop down to some position EFn tion EFn for a perfectly crystalline ZnO film. Thus there will be severe electron-hole recombination in 25 nm ZnO device. Courtesy of less trap states owing to crystal oriental effect discussed earlier, * position for 45 nm ZnO ETL will be closer to the theoretical EFn EFn position compared to that for single and triple layer ZnO ETL (Device-B in Fig. 7). Thus, the 45 nm coated ETL device has lower recombination phenomena and a higher effective density of states than the other devices under study, which ensures more carrier injection sites for the electrons injected from perovskite LUMO. Now, the actual open circuit voltage, VOC can be described as the minimum Quasi Fermi level splitting [12] in the device under illumination, that is the difference between the Quasi Fermi level of * at ETL/Cathode interface and Quasi Fermi level of electron, EFn hole, EFp at HTL/Anode interface [84,85]. So, VOC can be written as:

* − EFp qVOC = EFn

(2)

Since, all the devices have been fabricated with the identically fabricated HTL (Doped P3HT), a constant value of EFp can be con* − EFp) value for sidered to determine VOC value. The lowest (EFn single layer ETL device explains its smallest VOC value from I-V data. The higher VOC from the other two devices can be elucidated in the similar fashion. There is a profound impact of flat-band potential and depletion width on device ultimate output. We can relate the device performance to these key device parameters with the information retrieved from Mott-Schottky plot under dark condition [78,83,86]. Fig. 10(A–C) show the Mott-Schottky plot for perovskite device with 25 nm, 45 nm and 60 nm ZnO ETL respectively. The x-axis intercept of the extrapolated linear section of Mott-Schottky curve gives the flat-band potential of the device while the charge carrier density or doping density can be found from the slope of the curve using the following Eq. (3) [87]:

N=

2(Vfb − V )C2 qA2 ε

(3)

where, Vfb , V , C, A, q and ε are flat-band potential, applied bias, junction capacitance appearing due to the modulation in depletion width, active device surface area, elementary charge and permittivity for Perovskite respectively. The p-doped behaviour of perovskite is evident [83,88] from the Mott-Schottky curve lines shown in Fig. 10(A–C). Since, the dipping two step fabrication technique of perovskite does not involve any annealing, the observation of p-type behaviour from Mott-Schottky analysis complies with the report from the earlier literature [88,89]. Thus, the perovskite layer is expected to form a p-n junction with the adjacent ZnO layer and the Mott-Schottky behaviour should be contributed by both the perovskite and ZnO [90]. So, the flat-band potential can be expressed as: Vfb = Vfb(perovskite) + Vfb(ZnO ). Fig. 10(D) illustrates the flat-band potential and overall charge carrier density retrieved from Mott-Schottky curve. Both the charge carrier density and flat-band potential are higher for the perovskite solar cells employing 45 nm ZnO ETL compared to the other two devices under study. The overall charge carrier density for 45 nm ZnO device has been found to be 2.88  1016 cm
3 while

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261

Fig. 10. Mott Schottky curve at 10 kHz frequency for Perovskite device with (A) 25 nm (Single coated) ETL, (B) 45 nm (Double coated) ETL and (C) 60 nm (Triple coated) ETL. The flat-band potential and acceptor density of the devices extracted from Mott-Schottky curve are presented in (D).

those for the 25 nm and 60 nm ZnO device are 1.06  1016 cm
3 and 1.64  1016 cm
3 respectively as mentioned in Table S4. The variation in the ZnO ETL thickness is attributed to the observed change in overall charge carrier density of the devices; since the adjacent perovskite layer remains constant and the bulk

perovskite property is similar in all the three devices. The similarity in perovskite characteristics is also confirmed by the identical high frequency capacitive response from capacitance vs frequency curve presented in Fig. 9. Similar trend is observed in perovskite devices employing other metal oxide ETLs reported in

Fig. 11. Energy band diagram of the Perovskite device (A) before contact, (B) after contact with the 45 nm ZnO ETL device and (C) after contact with the 25 nm ETL device at zero bias under dark. 45 nm ZnO ETL devices exhibit higher flat band potential and lower depletion width at zero bias compared to 25 nm ZnO ETL device (Vfb1, W1, Vfb2 and W2 denote flatband potential and depletion width at zero bias for 45 nm ZnO ETL and 25 nm ZnO ETL device respectively).

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literature [81,82]. In addition, the charge carrier density of sol-gel derived ZnO thin films in our work is expected to be in the range 1  1017 cm
3 to  9  1017 cm
3 in accordance with the previously reported literatures [91,92]. The charge carrier density is therefore expected to vary with different thicknesses of ZnO and concomitantly influencing the overall charge carrier density of the devices. The flat band potentials for 25 nm, 45 nm and 60 nm ZnO devices are 0.42 V, 0.63 V and 0.52 V respectively. To relate these parameters to the superior device performance shown by 45 nm ZnO ETL device, we would like to elaborate our discussion with reference to the energy band diagram of the devices under investigation. Fig. 11(A) illustrates the generic energy band diagram of the devices before the contacting of the thin film layers. Since, our work concentrates on the impact of the variation in ZnO ETL, for simplicity of the analytical discussion; we will consider no spatial variation in Perovskite Fermi level. In Fig. 11(A), Ep denotes the * denote the ideal Fermi level position of perovskite. EFn and EFn Fermi level position and the shifted Fermi level position of ZnO/ Cathode owing to trap states respectively. For the isolated layers, * − EP ), which is the highest the difference in work function is (EFn for 45 nm ZnO device and the lowest for the 25 nm ZnO one, as described earlier. After contacting, the fermi-levels are aligned and the work-function mismatch is compensated by a band bending [78] (Fig. 11(B,C)). The flat-band potential, Vfb can be represented as [78]:

systematic study utilizing impedance spectroscopy to retrieve useful information regarding carrier generation, recombination, charge transport and extraction in a perovskite solar device with sol-gel ZnO ETL. In our study, low contact resistance found from the Nyquist plot and thinner depletion width owing to high flatband potential and larger carrier density from Mott-Schottky curve indicate conforming ETL/perovskite interface for most efficient charge extraction and hence improved FF and JSC in 45 nm ZnO ETL device compared to mono coated (25 nm) or triple coated (60 nm) ZnO ETL ones. The higher flat band potential in 45 nm ZnO device also proves the existence of shallower trap states and smaller spatial variation in ETL Quasi Fermi level under illumination, which provides more favorable cascaded structure between ETL and ITO. Consequently, it ascertains ameliorated charge collection and elucidates the fill factor enhancement in double layer ETL device. The higher recombination resistance from the Nyquist plot, irrespective of the applied bias, under both light illumination and at dark condition also explicates the lower carrier recombination and enhanced VOC in the best performing double coated ETL device. Thus, we have found out the most optimum ETL film thickness for solgel ZnO which is also scalable to flexible devices with roll-to-roll process. So, our present work can play a vital role in giving an impetus to the ongoing research with low temperature processed ETL in Perovskite solar device.

* − Ep Vfb = EFn

Acknowledgements

(4)

Due to the deeper trap state, in accordance with Eq. (4), Vfb should be the lowest in 25 nm ZnO device and the highest in 45 nm ETL device, which is consistent with our data from MottSchottky curve. Now, the depletion layer width at zero bias for the devices can be expressed as [78]:

W=

2εVfb qN

The authors gratefully acknowledge the financial support provided by Future Solar Technologies Pty. Ltd. for this research work. The authors would also like to acknowledge the endless support from the staffs of Photovoltaic and Renewable Energy Engineering School, UNSW, Dr. Jakaria Quadir, Scientific Officer from Electron Microscope Unit (EMU) and Solid State and Elemental Analysis Unit under Mark Wainwright Analytical Center, UNSW.

(5)

The calculated depletion widths for the three devices have been presented in Table S4. The depletion layer width thickness (  305 nm) in 25 nm ETL device is close to 90% of the photo-active perovskite layer thickness ( 350 nm). On the contrary, the perovskite device with 45 nm ZnO layer has lower depletion width (  226 nm). Moving on to the triple coated device, we again observe increased depletion zone (  273 nm). Hence, all the results obtained from our various characterization techniques are consistent with the overall device performance and bear testimony to the proper optimization of low temperature processed ZnO ETL to be applied in Lead Iodide Perovskite solar cell.

4. Conclusion In summary, we have reported the low temperature, (o 150 °C) sol-gel processed ZnO thin film as electron transport layer with efficient, highly reproducible perovskite solar cell. Sol-gel processed ZnO provides a much simpler, low temperature fabrication technique for ETL compared to time-consuming, sophisticated dealing with other nanostructured ETLs reported in the literature. Using the consecutive spin coating process, we have found that the double coated ZnO (45 nm film) provides the most suitable ZnO ETL by acting as a highly transmitting, antireflective electron extraction layer, providing conforming surface morphology to the overlying perovskite layer, suppressing deep trap states prevalent in very shallow ZnO thin film and thus rendering a superior device performance (PCE: 8.77%). In this work, we have presented a

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2016.09. 014.

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