Controlling the surface nanostructure of ZnO and Al-doped ZnO thin films using electrostatic spraying for their application in 12% efficient perovskite solar cells.

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Cite this: DOI: 10.1039/c4nr02065k

Controlling the surface nanostructure of ZnO and Al-doped ZnO thin films using electrostatic spraying for their application in 12% efficient perovskite solar cells† Khalid Mahmood,* Bhabani Sankar Swain and Hyun Suk Jung* In this paper, ZnO and Al-doped ZnO films were deposited using the electrospraying method and studied for the first time as photoanodes for efficient perovskite solar cells. Effects of substrate temperature, deposition time, applied voltage, substrate-to-nozzle distance and flow rate (droplet size) on the morphology of ZnO were studied with the help of FE-SEM images. The major factors such as the droplet size of the spray, substrate temperature and substrate-to-nozzle distance at deposition control the film morphology. Indeed, these factors determine the density of the film, its smoothness and the flow of solution over the substrate. The droplet size was controlled by the flow rate of the spray. The substrateto-nozzle distance and flow rate will both regulate the solution amount deposited on the surface of the substrate. The most favorable conditions for a good quality ZnO thin film were a long substrate-tonozzle distance and lower solution flow rates. In situ droplet size measurement shows that the size and dispersion of particles were narrowed. The method was shown to have a high deposition rate and efficiency relative to well-established thin film deposition techniques such as chemical and physical vapor deposition. In addition, it also allows easy control of the microstructure and stoichiometry of the deposits. The pure ZnO film produced under optimum conditions (440 nm thick) demonstrated a high power conversion efficiency (PCE) of 10.8% when used as a photoanode for perovskite solar cells, owing to its high porosity, uniform morphology and efficient electron transport. For thicker films a drastic decrease in PCE was observed due to their low porosity. We also observed that the open-circuit voltage

Received 16th April 2014 Accepted 15th May 2014

increases from 1010 mV to 1045 mV and also the PCE increases from 10.8% to 12.0% when pure ZnO

DOI: 10.1039/c4nr02065k

the reasonably uniform-sized droplets of smaller size, so the films have a smooth surface and are highly

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suited for optoelectronic applications.

films were doped with aluminum (Al). Under atmospheric pressure, the electrospraying system produces

1. Introduction Electrospraying has gained interest recently for the preparation of thin lms1–3 and polymer coatings,4,5 which were logically combined in electrospray studies for lms and coatings prepared using polymers.6–13 Thin lms of ZnO deposited by electrospraying are homogeneous and dense which enable them to be used in numerous applications such as biosensors,14 UV-luminescent devices,15 solar cells,16–18 and transparent conductive oxide (TCO) electrodes.19–22 One major advantage of the electrospraying method is that thin lms with nano-/ microscaled structures could be deposited. Moreover, the morphologies of the lms are easily controlled by changing the School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea. E-mail: [email protected]; [email protected]; Fax: +82-31-290-7410; Tel: +82-31-290-7403 † Electronic supplementary 10.1039/c4nr02065k

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deposition conditions such as (e.g.: solution properties, droplet size and applied voltage) and the thickness of the lms can be adjusted by controlling the deposition time. The morphology of ZnO lms depends directly on the particle size and the mode of spreading the solution on the substrate during the electrostatic spraying process. Smaller spray droplets will result in smaller particles and smoother lms. So, precise control of the interplay between droplet size (ow rate), substrate temperature and particle size is, therefore, the key to control the lm morphology. Various methods such as metal organic chemical vapor deposition (MOCVD), spray pyrolysis, pulse laser deposition and sol–gel have been used to control the morphology of ZnO thin lms.23–26 It is very difficult to deal with process parameters for controlling the morphology of lms in the above mentioned methods. Electrospraying is a very promising method for the preparation of continuous lms with controlled morphology at lower temperatures by the suitable adjustment of process parameters. In a recently reported

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study, we studied the performance of boron and tantalumdoped ZnO thin lms and also investigated the effect of thermal treatment and dopant concentration on the performance (electrical and optical properties) of ZnO thin lms.27 In fact, thermal annealing and dopant concentrations have a signicant effect on the morphology of these lms, which will further inuence the performance of thin lm based devices Therefore, the surface morphology of lms is an important factor that can affect the performance of lm based devices. In this perspective, the present work is focused on controlling the surface nanostructure using the electrospraying method as a function of process parameters like the substrate temperature, applied voltage, the ow rate (droplet size) of the precursor solution, spraying time and nozzle-to-substrate distance. Recently, solid state perovskite (CH3NH3PbI3) solar cells have drawn great attention from researchers owing to their great benets like ease of fabrication, high efficiencies and cheaper technology.28–30 The solid-state solar cells composed of CH3NH3PbI3 layers fabricated over the mesoporous TiO2 lm exhibited a high power conversion efficiency (PCE) of 9.7% and also showed good stability for a long time.31 In addition, it was also demonstrated that in the absence of mesoporous TiO2 layers perovskite solar cells showed a high PCE of 10.9% by depositing the CH3NH3PbI2Cl layer onto the Al2O3 surface.32 The most recent research development in the area of perovskite solar cells has shown that the cell efficiency can even reach 15%,33 along with a proposed PCE of 20% to be achieved in the near future, which proves that these cells have great potential to be commercialized soon. Two basic approaches have been used to enhance the functionality and the performance of perovskite solar cells. One solution is to utilize a non-electron-injecting meso-superstructure planar layer and another way is based on an electrontransfer nanostructured oxide layer. In the case of electroninjecting oxide nanostructure lms, mostly mesoscopic TiO2 has been used, needing higher processing temperatures to enhance the charge transport. In addition to the TiO2 nanoparticle lms used as the electron-injecting layers in perovskite solar cells, the one-dimensional (1-D) TiO2 and ZnO nanorods were also studied.30,34–38 The ZnO nanostructures also possess numerous advantages over TiO2 owing to their excellent electron mobility and same band structure as TiO2.39 Furthermore, n-type characteristics like the conductivity of pure ZnO lms can be enhanced using metal doping which is also benecial for shiing the Fermi level (EF) in the direction of the conduction band (EC).40 Usually, group-III elements such as In, Ga, B and Al are highly preferable as n-type dopants for ZnO lms. Al-doped ZnO and Ga-doped ZnO lms are excellent candidates for TCO materials because they are easy to handle, less toxic, have suitable ionic radii, are inexpensive and show excellent optical transmission performance. Especially, for Al-doped ZnO nanostructures it has been proven that Al doping can greatly enhance the carrier concentration and optical transmittance of pure ZnO to achieve superior conductivity.41 The properties of ZnO nanoparticulate lms also depend mainly on the deposition techniques and their microstructures, which will nally play an important role in reducing the Nanoscale

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structural and electronic defects in the lms. Therefore, the development of a ZnO lm with a low temperature processed nanoporous structure will be very helpful to produce a lower concentration of defects and efficient interactions with the solid sensitizer. In order to achieve ZnO lms with these characteristics, various methods such as chemical bath deposition (CBD)36 and hydrothermal methods37,38 have been investigated to grow 1-D ZnO nanorod arrays for their application in the perovskite solar cells. In addition, other lm deposition methods such as plasma-enhanced chemical vapor deposition (PECVD)39 and spin coating42 have also been used to fabricate thin lms of ZnO as electron-injecting layers for use in perovskite solar cells. Although, these methods have shown a great capability to deposit ZnO nanostructures to obtain efficient perovskite solar cells, an alternative spray-coating method such as electrostatic spraying is also needed which can control the nanostructure of the ZnO lms in an easier and efficient manner for further enhancement of PCE of these cells. The key advantages of the electrostatic spraying method are mentioned above in comparison with other methods. So far, electrostatic spray deposited ZnO and Al-doped lms have not been investigated for application in solid state perovskite solar cells. The present report mainly focuses on the deposition of crack free ZnO and Al-doped lms as a function of various system parameters by utilizing electrospraying and they are studied as electron-transfer layers for perovskite solar cells in order to enhance the cell performance and develop a cost-effective alternative electron-transfer layer fabrication method. The morphology of the lms has been analyzed using eld emission scanning electron microscopy. The pure ZnO lm produced under optimum conditions with a thickness of 440 nm gave a high PCE of 10.8%. For Al-doped lms a higher PCE of 12.0% was observed due to an increase in open-circuit voltage. To the best of our knowledge this is the rst study which utilizes the electrospray deposited mesoporous ZnO and Al-doped lms as photoanodes to obtain highly efficient perovskite solar cells.

2.

Experimental

2.1. Deposition of mesoporous pure and Al-doped ZnO thin lms ZnO and Al-doped ZnO thin lms were deposited using the electrospraying method as shown in Fig. 1. The spraying solution was prepared using 0.05 M zinc acetate dihydrate in an ethanol–water (30 : 70 v/v) mixture. Al doping can be achieved by adding an Al dopant in the form of aluminum chloride (0.5 at.%) into the precursor solution. A syringe pump (KD200, KD Scientic Inc., USA) was used to transfer the precursor solution to a small stainless steel nozzle. The precursor solution ow rate ranges from 0.001 mL min1 to 0.007 mL min1. The high voltage (0 to 7 kV) was varied between the needle tip and the hot plate, using a dc power supply (Korea switching, Inc., Korea). The distance between the needle tip and the substrate was varied between 1 cm and 4 cm. Fluorine-doped tin oxide (FTO) substrates were washed with isopropyl alcohol (IPA), ethanol, acetone and water for proper cleaning. The deposition time ranged from 40 to 120 minutes and the temperature of the


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Fig. 1

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Schematic illustration of the electrostatic spraying deposition setup.

heating plate ranged from 100 to 170 C. A conventional digital camera (model no. 7309P-1) assembled with a lens and a light source was used to capture the spray patterns.

dissolved in 1 mL chlorobenzene) solution for 30 s at 3000 rpm. Finally, approximately a 90 nm thick Ag lm was deposited using thermal evaporation over this layer.

2.2. Fabrication of CH3NH3PbI3 perovskite solar cells

2.3. Characterization

Perovskite solar cells were prepared using the sequential deposition method as reported elsewhere.28 Firstly, a very thin compact layer of ZnO or Al-doped ZnO named the blocking layer was deposited onto FTO substrates by spin coating the above mentioned precursor solution at 4000 rpm for 30 s and annealed in air at 350 C for 30 minutes. Then 1 M PbI2 solution was made in DMF while stirring overnight, at a constant temperature of 70 C. The as-deposited photoelectrodes contained ZnO and Aldoped ZnO thin lms were spin coated at 6000 rpm for 5 s, using the above solution. Then the PbI2 coating was allowed to dry for 30 minutes at 70 C. Aerwards, the lms were dipped into a 10 mg mL1 solution of CH3NH3I in IPA for about 20 minutes, followed by rinsing in IPA and drying at 70 C for 30 minutes. The hole transporting layer was made by spin coating the spiroOMeTAD Hole (80 mg of 2,20 ,7,70 -tetrakis(N,N-di-p-methoxyphenyl-amine)-9,90 -spirobiuorene, 8.4 mL of 4-tert-butylpyridine, and 51.6 mL of bis(triuoromethane) sulfonamide lithium salt (Li-TFSI) solution (154 mg mL1 in acetonitrile), all mixtures were

Cross-sectional and surface morphologies were obtained using scanning electron microscopy (SEM, JSM-7600F, JEOL). The elemental composition of doped lms was conrmed using energy dispersive X-ray spectra (EDS; DX-4). The crystallinity of lms was checked using an X-ray diffractometer (XRD; model M18XHF, Macscience Instruments). X-ray photoelectron spectra (XPS) were used to check the purity and composition of lms using X-ray photoelectron spectroscopy (Thermo VG Scientic, Sigma Probe). The optical transmittance spectra were obtained using a UV-visible spectrometer (UV-3101PC). Photovoltaic properties were measured by using a potentiostat (CHI660, CHI instrument). The solar spectrum under AM 1.5 conditions was simulated with a solar simulator (Oriel Sol 3A class AAA, Newport) to measure the current voltage characteristics. The incident photon to current conversion efficiency was measured with an IPCE measurement system (PV Measurements). A phase Doppler particle analyzer (PDPA, TSI., USA) was used to observe the size distribution of sprayed droplets.


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3.

Results and discussion


3.1. Size distribution of sprayed droplets Fig. S1a† exhibits the size distribution of droplets in real time. By controlling the solution ow rate, the diameter of sprayed droplets was adjusted accordingly. The diameter of droplets increased as the ow rate increased under a constant applied voltage (5.9 to 6 kV). Geometric standard deviation (GSD) was used to identify and describe the sprayed droplets' uniformity. The GSD and geometric mean diameter (GMD) as a function of different ow rates are exhibited in Fig. S1b and c,† respectively. GSD values range from 1.39 to 1.47, which describe the uniform-sized droplets, and the GMD values change from 2.73 to 2.94 mm for all ow rates. Since the electrospraying technique generates droplets of uniform size which will result in the formation of better quality lms with uniform-sized particles. 3.2. Operating envelope for the atomization of solution and the formation of a charged jet To control the lm morphology the solution properties in terms of electrospraying are very crucial to be optimized. Typically, for the electrospraying technique the understanding of the operating envelope and other system parameters such as the solution ow rate, substrate temperature, applied voltage and substrate-to-nozzle distance is very important to control the lm morphology. Lord Rayleigh rstly explained the development of charged jets as follows: 1=3 2=3 93g F rj ¼ (1) 2p2 I where F is the solution ow rate, I is the injection current, g is the surface tension, and 3 is the permittivity of the solution.43 Especially, electrospraying officiates by activation of surface charges on the sprayed uid with the help of applied voltage.44–47 The generation of a ne spray of solvents with low dielectric constant (3) might be tough. On the other hand, by utilization of a sharp nozzle, electrons can be easily injected into or removed from the uid (led-injection) for generating an ionized solution with enhanced capacity to transfer the surface charge in a process called as eld ionization. To obtain the stable cone-jet mode with monodispersed electrospray droplet generation, a minimum and maximum range of ow rate should be adjusted using a specic nozzle type. The minimum and the maximum ow rates used to obtain the stable cone-jet mode were 0.001 mL min1 and 0.025 mL min1 along with the applied voltage, and various modes of spraying such as dripping, micro-dripping, spindle, unstable cone-jet, stable cone-jet and the multi-jet modes are exhibited in Fig. S2.† It was observed that at zero voltage and a ow rate of 0.003 mL min1, the dripping mode was obtained. The surface tension results in the formation of large drops until the droplets' weight becomes too high for the surface tension and capillary force to dominate. The small increase in voltage from zero to 3.8 kV micro-causes the dripping mode to appear. At this

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stage, more charges were collected at the meniscus surface providing the intense electric tension forces away from the nozzle. Consequently, the size of droplets dripping off of the nozzle decreases and the density of drops increases. When the applied voltage was further enhanced to 5.5 kV, unstable conejet mode was obtained. Once the electric force was great enough (6 kV), the charged surface was disturbed into a smooth thin jet, which nally breaks-up into small (2.8 mm) fairly uniform sized droplets. This is called as stable cone-jet mode. At a much higher voltage of about 7 kV multi-jet spray mode was obtained. 3.3. Effect of applied voltage Rayleigh's equation (eqn (1)) describes that the enhancement of current (I) present on the precursor solution will decrease the diameter of the jet. Upon increasing the applied voltage, the current can be efficiently controlled since V is dependent on I, by keeping other parameters constant. In order to study the effect of applied voltage on the lm morphology, the zinc acetate dihydrate precursor solution at a ow rate of 0.003 mL min1 was deposited on the FTO substrate with a 4 cm distance at 150 C for 60 min. The applied voltage is varied from 3.8 kV to 7 kV. Fig. 2 shows clearly that the lm morphology was inuenced by increasing the applied voltage by keeping other parameters constant. A continuous and dense thin lm was obtained only at a high voltage of 6 kV, where the stable cone-jet mode was observed because the increase of applied voltage leads to the formation of a thin jet and reasonably distributed droplets. While at lower voltages, the larger droplet formation takes place that still contained the solvent to evaporate upon deposition. 3.4. Effect of substrate temperature The deposition of lms using the electrospraying technique is the result of co-action of evaporation of the solvent in the spraying droplets and salt precipitation in precursor solution as soon as it reaches over the heated substrate. Therefore, the temperature of the substrate is a crucial factor since it decides the precursor decomposition and the rate of solvent evaporation. This study also emphasizes the discovery of appropriate process parameters for the fabrication of uniform and dense ZnO lms. As mentioned in the Experimental section, the zinc acetate dihydrate precursor solution has been prepared and sprayed onto a glass substrate from a distance of 4 cm at a ow rate of 0.003 mL min1 for 60 min at an applied voltage of 6 kV. The substrate temperature was varied from 100 to 170 C. Fig. 3 shows the surface morphologies of ZnO thin lms deposited at substrate temperatures of 100, 130, 150 and 170 C, respectively. It was observed that the increase in substrate temperature has little inuence on the morphology of ZnO lms. At very low temperature such as 100 C, porous spherical shaped microstructures can be observed while at the optimal temperature of 150 C the pores disappeared and the lm is dense without any cracks. The heat of the substrate inuences the lm surface and the radiative heat transfer causes the sprayed droplet temperature to increase before they strike the


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Influence of the applied voltage on the morphology of ZnO thin films deposited on the FTO substrate at a flow rate of 0.003 mL min1 from a distance of 4 cm at 150 C for 60 min: (a) 3.8 kV, (b) 5.5 kV, (c) 6 kV, and (d) 7 kV, respectively. Fig. 2

surface of the substrate. Therefore, solvent evaporation in the droplets takes place before they approach the substrate surface and higher temperature will increase the solvent evaporation

rate. Moreover, the solvent in droplets smaller in size will evaporate faster before they reach the substrate and eventually strike over the substrate surface in the form of solid dried

Influence of substrate temperature on the morphology of ZnO thin films deposited on the FTO substrate at a flow rate of 0.003 mL min1 from a distance of 4 cm for 60 min and an applied voltage of 6 kV: (a) 100 C, (b) 130 C, (c) 150 C, and (d) 170 C, respectively. Fig. 3


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particles. Furthermore, when the substrate temperature was between 100 C and 130 C, pores appeared over the lm surface revealing intense contraction during the lm formation process. Indeed, while spraying is in process, some quantity of the solution gets to the surface of the substrate and a thin layer of liquid was formed on the top. As the spraying is completed the temperature of the thin liquid layer increases sharply which causes the solvent to evaporate at a faster rate which results in the stress development in the lm. Thus, the lm deposited at low temperature was not dense and continuous. Moreover, if the substrate temperature is increased further, the rate of lm drying and the degree of damage decreased. The optimized temperature of about 150 C allows a dense and uniform sized lm. In addition, the effect of substrate temperature should be connected with the solution ow rate or the substrate-to-nozzle distance, since these factors effectively control the solution quantity reaching the surface of the substrate. 3.5. Effect of precursor solution ow rate The ow rate of precursor solution decides the amount of liquid that approaches the surface of the substrate for a specied time. On the other hand, the droplet size can be controlled by adjusting the ow rate, expressed by the following equation:48 1=3 Q d 3r 1=6 (2) k where Q is the precursor solution ow rate, 3r is the relative solution permittivity and k is the electrical conductivity of the precursor solution. According to it, the larger the ow rate, the

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larger will be the droplet size which is also conrmed by Fig. S1a.† Moreover, for larger size droplets the solvent will evaporate slowly during the spraying process. Thus, at higher ow rates a larger amount of solution will reach the substrate surface. To evaluate the effect of ow rate on the lm morphology, the precursor solution was sprayed onto the FTO substrate using 4 cm distance at 150 C for 60 min and an applied voltage of 6 kV. The precursor solution ow rate was varied from 0.001 mL min1 to 0.007 mL min1. Fig. 4 reveals that the lm morphology was inuenced by increasing the ow rate. A continuous and dense thin lm was obtained only at a low ow rate of 0.003 mL min1. As mentioned before, a liquid will accumulate over the substrate surface at higher ow rates. If it gathered over the substrate surface in a larger amount then cracks and pores will be observed in the lms because a longer drying period will produce stress in the lms. Thus, the lm forming at larger ow rates will contain larger particles and pores as exhibited in Fig. 4. 3.6. Effect of substrate-to-nozzle distance Variation in the distance between substrate and nozzle will modify the transition of matter which approaches the substrate. Specically, the same amount of solution is sprayed over a larger substrate area, if the substrate-to-nozzle distance is increased and vice versa. By keeping in mind the relationship between the amount of liquid remains on the surface of the substrate and the quality of lms in terms of morphology as

Fig. 4 Influence of flow rate on the morphology of ZnO thin films deposited on the FTO substrate at 150 C from a distance of 4 cm for 60 min and an applied voltage of 6 kV: (a) 0.001 mL min1, (b) 0.003 mL min1, (c) 0.005 mL min1, and (d) 0.007 mL min1, respectively.

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Fig. 5 Influence of substrate-to-nozzle distance on the morphology of ZnO thin films deposited on the FTO substrate at a flow rate of 0.003 mL min1 at 150 C for 60 min and an applied voltage of 6 kV: (a) 1 cm, (b) 2 cm, (c) 3 cm, and (d) 4 cm, respectively.

discussed previously, a longer distance will yield a better morphology while a shorter distance produces defects in the lms.

To investigate the effect of substrate-to-nozzle distance on lm morphology, the precursor solution was sprayed onto the FTO substrate at 150 C, at a ow rate of 0.003 mL min1 for 60

Fig. 6 Influence of deposition time on the morphology of ZnO thin films deposited on the FTO substrate from a distance of 4 cm at 150 C, at a flow rate of 0.003 mL min1 and an applied voltage of 6 kV: (a) 60 min, (b) 80 min, (c) 100 min, and (d) 120 min, respectively.


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min and an applied voltage of 6 kV. In addition, the substrateto-nozzle distance was altered from 1 to 4 cm. The microstructural changes are shown in Fig. 5, agreed well with our assumption: the lm formation using short distances is not continuous and particles are not uniform in size and the intermediate one also shows a porous structure, while the longer distance of 4 cm produces a dense and uniformly sized lm. It is remarkable that a longer substrate-to-nozzle distance is complement to the lower ow rates. Because, most of the solvent evaporates during the ight of droplets from the nozzle to the surface of the substrate as shown in Fig. 5. Furthermore, it also decreases the quantity of solution reaching over the surface of the substrate in the same way as the decrease of ow rate effects. 3.7. Effect of deposition time In order to inquire the effect of deposition time on the lm morphology, the zinc acetate dihydrate solution was sprayed on the FTO substrate with 4 cm distance at 150 C, at a ow rate of 0.003 mL min1 and an applied voltage of 6 kV. The deposition time has been varied from 60 to 120 minutes. Fig. 6 shows the FESEM images of ZnO thin lms as a function of deposition time. With the increase of deposition time from 60 to 120 minutes the particle size increased. From a previous report by Chen and co-workers,49 the formation of a dense lm was clear evidence that the solvent in the droplets has not been totally evaporated, when it approached the surface of the substrate. Moreover, the droplet liquid might spread on the surface of the substrate and form a continuous layer. At deposition temperatures, where the incoming sprayed droplets are still wet but provide adequately high decomposition and evaporation rates, the dispersion of sprayed droplets on the surface of the substrate will form a continuous layer. By increasing the deposition time, the spreading of sprayed droplets will occur on the surface of the layer made which has a surface tension different from that of the substrate. In addition, the solution wettability on the deposited layer is less than that on the substrate. Thus, by keeping the substrate temperature constant, the increase of deposition time will lead to slower spreading of sprayed droplets and individual particles might be formed that can increase the surface roughness.50 Furthermore, our results agree well with a previously reported study.51 3.8. Correlation between the electrospraying process parameters Aer controlling the surface nanostructure using various process parameters systematically, a rational image about the electrospraying technique for preparing a dense and crack free ZnO lm is developed using the correlation between solution ow rate, substrate-to-nozzle distances and substrate temperature. To obtain better quality ZnO lms, the relationship between the substrate-to-nozzle distance and the substrate temperature as a function of different ow rates is shown in Fig. 7a. It does not describe a linear dependency owing to the quadratic

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Fig. 7 Correlations between the process parameters for optimized ZnO and AZO thin films: (a) substrate temperature vs. precursor solution flow rate for different distances; (b) substrate temperature vs. substrate-to-nozzle distance for different flow rates.

relationship between the area covered by the spray and the substrate-to-nozzle distance. For shorter distances, more amounts of liquid will reach the surface. Therefore, to evaporate the solvent completely, a higher temperature is needed. Fig. 7b exhibits the dependence of substrate temperature on the precursor solution ow rate as a function of various substrateto-nozzle distances to attain a dense ZnO lm. The trend shows a linear dependency because a linear relationship exists between the quantities of solution reaching the substrate and the ow rate. In the case of higher ow rates, complete evaporation of the solvent can be accomplished using higher substrate temperatures. The ow rate and substrate-to-nozzle distance behave in a similar fashion but in opposite directions since both of them control the amount of solution that reaches the substrate surface. The substrate temperature can control the changes in these two variables to maintain the quality of the lm. 3.9. Solar cell performance for optimized pure and Al-doped ZnO thin lms Fig. 8a–c show the cross-sectional FE-SEM images of the optimized ZnO lm fabricated at a distance of 4 cm at 150 C, at a ow rate of 0.003 mL min1 and an applied voltage of 6 kV, with This journal is © The Royal Society of Chemistry 2014

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Fig. 8 Cross-sectional FE-SEM images of ZnO thin films with: (a) 440 nm, (b) 625 nm, and (c) 745 nm thickness, respectively and (d) fabricated perovskite solar cell with 440 nm thick ZnO film.

three different lm thicknesses of 440 nm, 625 nm and 745 nm by varying the spraying time of 40, 50 and 60 minutes, respectively. The cross-sectional FE-SEM images show that the ZnO lms deposited under the optimized conditions with different thicknesses presented a mesoporous structure to achieve both the optimum charge transport characteristics and better perovskite loading. Moreover, along with porosity considerations the total thickness of the lm also plays a crucial role in the cell performance.39 In this regard, a 440 nm thick ZnO thin lm based perovskite solar cell is shown in Fig. 8d. It is observed that the pores of the ZnO lm are inltrated with the absorbance CH3NH3PbI3, and a thin overlayer of CH3NH3PbI3 also exists on the top of the surface covering the ZnO lm surface. This type of structure is highly suitable for efficient charge separation and very important for estimating the lling fraction of the CH3NH3PbI3. The existence of CH3NH3PbI3 as an overlayer on the top of the ZnO/CH3NH3PbI3 inltrated lm is very benecial for efficient extraction of charges through the interface with the hole transporting layer and the top most Ag layer.39 A comparison of the XRD pattern for pure and Al-doped ZnO (440 nm thick) lms is exhibited in Fig. S3a.† The lms have a strong (002) peak intensity, indicating that the lms are highly crystalline in nature, corresponding to a hexagonal wurtzite structure along the c-axis. Due to the substitution of Al into ZnO sites, the diffraction peaks shied towards the larger or smaller angle side which conrms the presence of Al doping into the pure ZnO lms (Fig. S3b†). The chemical composition of Aldoped ZnO lms is further conrmed using EDS (Fig. S4†). The


presence of Al, Zn and O peaks in the EDS spectrum ensures the presence of Al in the ZnO lm. The substitution of Al into the ZnO lm is further conrmed through XPS analysis (Fig. S5†). The existence of the Al 2p peak at 73.50 eV in the ZnO lm revealed that Al is present in the lm (Fig. S5a†). Moreover, for Zn 2p the two peaks corresponding to Zn 2p3/2 and Zn 2p1/2 at 1022.6 and 1045.4 eV (Fig. S5b†) demonstrate that Zn atoms are present in the lm. Furthermore, the scan of the O 1s spectrum shows a peak around 532.1 eV (Fig. S5c†), attributed to the metal ions which are oxidized in the lm, specically O–Zn and O–Al in the ZnO lattice. The pure and Al-doped ZnO lms produced under optimum conditions are highly transparent (greater than 94%) in the visible region (Fig. S6†). The Al-doped ZnO lm shows slightly higher transmittance compared to the pure ZnO lms, because the substitution of Al atoms into the ZnO sites provides an increase in Fermi level in the conduction band of semiconductors due to enhancement in carriers which will nally help to widen the optical bandgap. Therefore, a larger bandgap would increase the optical transmittance in the visible region.41 The photovoltaic characteristics of different ZnO thin lm thicknesses are shown in Fig. 9a and the detailed photovoltaic parameters such as short-circuit photocurrent (Jsc), ll factor (FF) and open-circuit voltage (Voc) are summarized in Table 1. The optimal thickness of ZnO lms to obtain the enhanced cell performance is about 440 nm. However, for thicker lms of 625 nm and 745 nm, the Voc reduces from 0.86 V to 0.83 V, respectively. The decrease of Voc on increasing the lm thickness is attributed to the higher recombination rate caused by

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Paper Photovoltaic parameters for the perovskite solar cells fabricated with various ZnO film thicknesses at optimized process parameters under one sun illumination (AM 1.5G, 100 mW cm2)


Table 1

PCE*a (%)

ZnO lm thickness (nm)

Jsc (mA cm2)

Voc (mV)

Fill factor

PCE (%)

10.8 0.1 4.4 0.2 3.4 0.2

440 625 745

16.0 7.48 5.98

1010 860 830

67.0 69.0 70.0

10.8 4.4 3.4

a

PCE* is the power conversion efficiency calculated for 6 different devices.

by other researchers for planner ZnO lm based devices,39 and also for nanober devices.35 The comparison of photovoltaic characteristics of the best performing 440 nm thick pure and Al-doped ZnO lm based cells is presented in Fig. 10a and Table 2. The pure ZnO lm based perovskite cell shows a higher Jsc of 16 mA cm2. On the other hand, a reduction of Jsc (15.1 mA cm2) is observed for Aldoped ZnO lm based cells. However, the Voc increases from 1010 mV to 1045 mV aer doping with Al which will nally increase the PCE up to 12%. It is well known that Voc is dened by the difference between the n-type semiconductor's quasi

Fig. 9 (a) Photovoltaic performance as a function of pure ZnO film thickness and (b) IPCE spectrum for 440 nm thick pure ZnO thin films.

the higher surface area and as well as the increase of trap promoting recombination in TiO2 lms.39 On the other hand, a larger reduction in Jsc values is observed by increasing the ZnO lm thickness. In fact, the increase of lm thickness will probably hinder the passage of perovskite inltration into the ZnO lm which eventually results in a lower loading of perovskite. In addition, thicker lms with irregular shaped particles and cracks on their surface hindered the efficient electron transport thus showing a lower PCE. Remarkably, the cells based on thinner ZnO lms demonstrated the highest PCE of 10.8%, which is much better than that (4.8%) of the cells based on nanocolumnar plasma deposited ZnO thin lms.39 Incident photon-to-current conversion efficiency measurement (IPCE, Fig. 9b) is also performed to further investigate the efficient electron transportation within the optimized ZnO lm having a thickness of 440 nm. The efficiency of around 80% is observed along the entire spectrum (370–750 nm) which demonstrates the excellent cell performance. Aer integrating the product of AM 1.5G photon ux with the IPCE spectrum, the calculated Jsc is found to be 15.6 mA cm2 which is in good agreement with measured Jsc values of 16.0 mA cm2 obtained for a thinner device. The shape of the IPCE spectrum for thinner efficient devices agreed well with the spectrum shapes reported

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Fig. 10 Comparison of (a) photovoltaic performance and (b) IPCE spectra of 440 nm thick pure and Al-doped ZnO thin films.


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Table 2 Comparison of photovoltaic parameters of the perovskite solar cells based on 440 nm thick pure and Al-doped ZnO films


Film type PCE*a (%)

Jsc (mA cm2)

Voc (%)

Fill factor (%)

PCE (%)

Pure ZnO lm 10.8 0.1 Al-doped ZnO lm 12.0 0.2

16.0

1010

67.0

10.8

15.1

1045

76.0

12.0

a PCE* is the power conversion efficiency calculated for 6 different devices.

Fermi-level and hole transporting material's (HTM) HOMO level in nanostructured perovskite solar cells. Thus, the Al-doped ZnO lm demonstrated the lower recombination rate of the conduction band electrons and faster transport of electrons. Therefore, the higher electron concentration in the conduction band and the reduction of recombination rate resulted in the improvement of Voc. In addition, for doped ZnO lms higher values of ll factor are obtained due to the suppressed charge recombination at the perovskite/ZnO lm or HTM/ZnO lm interface. Fig. 10b shows the comparison of IPCE spectra for pure and Al-doped ZnO lms. Jsc for both cells accurately corresponds to integrating the IPCE spectrum. The cells based on Al-doped ZnO lms show a slightly enhanced square spectral response compared to cells with pure ZnO lms having an efficiency of greater than 80% in the entire wavelength ranging from 370 nm to 750 nm. These results conrm that the Aldoping into the ZnO sites is helpful to enhance the PCE due to the reduced charge recombination at the ZnO lm interface.

4. Conclusions The basic process variables in the electrospraying method which can control the morphology of ZnO thin lms are substrate temperature, precursor solution ow rate (droplet size), substrate-to-nozzle distance, applied voltage and deposition time. The parameters such as solution ow rate, substrate temperature and substrate-to-nozzle distance are dependent on each other since they all affect the quantity and evaporation of spraying solution over the surface of the substrate. Both the substrate-to-nozzle distance and ow rate control the solution amount approaching the substrate surface. In addition, the substrate temperature is very helpful to nally control the morphology of the lm by the adjustment of variation between these two parameters. The most appropriate conditions for good quality of ZnO lms are either a long distance or low solution ow rates. Moreover, the substrate temperature should be controlled accordingly. For our case, continuous and dense lms have been obtained at an optimized ow rate of 0.003 mL min1 and at a distance of 4 cm which can produce a higher PCE compared with other lms. For these lms the substrate temperature ranged from 150 to 170 C. Practically, the temperature window for the given deposition parameters is approximately 10 to 20 C and the temperature window


becomes narrow in the case where a larger distance and/or larger ow rates are used. The ZnO lms produced under the optimized process conditions are employed as photoanodes for efficient perovskite solar cells. An enhanced PCE of 10.8% is achieved for a 440 nm thick ZnO lm which is the rst study about using an optimized electrosprayed ZnO lm to the best of our knowledge. The efficiency is mainly dependent on the lm porosity and smoothness which change by increasing the lm thickness and ZnO particle size. Furthermore, Al-doped ZnO lms suppress charge recombination at the ZnO/perovskite interface which leads to an increase in PCE to 12.0%. Since the present report emphasizes on the proper utilization of deposition parameters, the same solution was used for all the cases. Therefore, the discussed lm formation process is usually applicable; the optimized parameter values are relevant to this typical precursor solution. The deposition parameters can be modied when the concentration or the nature of precursor salts becomes different.

Acknowledgements This work was supported by grants from the National Research Foundation (NRF), which is funded by the Korean government (MEST) (2012M3A7B4049967, 2012M3A6A7054861 and 2014R1A4A1008474).

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