CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201301215
Perovskite Solar Cells Based on Nanocolumnar PlasmaDeposited ZnO Thin Films F. Javier Ramos,[a, c] Maria C. Lpez-Santos,[b] Elena Guilln,[a] Mohammad Khaja Nazeeruddin,[c] Michael Grtzel,[c] Agustin R. Gonzalez-Elipe,[b] and Shahzada Ahmad*[a] ZnO thin films having a nanocolumnar microstructure are grown by plasma-enhanced chemical vapor deposition at 423 K on pre-treated fluorine-doped tin oxide (FTO) substrates. The films consist of c-axis-oriented wurtzite ZnO nanocolumns with well-defined microstructure and crystallinity. By sensitizing CH3NH3PbI3 on these photoanodes a power conversion of 4.8 % is obtained for solid-state solar cells. Poly(triarylamine) is
found to be less effective when used as the hole-transport material, compared to 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD), while the higher annealing temperature of the perovskite leads to a better infiltration in the nanocolumnar structure and an enhancement of the cell efficiency.
1. Introduction The planet’s needs for cheap and abundant sources of clean energy, has fueled the research interest to find new materials which can be then exploited for advanced applications.[1] Third-generation excitonic solar cells are deemed as a credible advantageous alternative to existing cost-ineffective technologies.[1, 2] With respect to dye-sensitized solar cells (DSSCs), the processing of solid-state solar cells is industrially straightforward as it avoids hazards related from leakage and compact sealing. In this context, fabricating stable, efficient and cost-effective solar cells is of paramount importance to find alternative materials for sensitizers. Hybrid organic–inorganic perovskite structures are of enormous scientific and technological importance and are widely used in electroluminescence, light emitting diodes, photovoltaics (PVs) and for battery applications. Recently, their fascinating ability to act as an absorber in mesoscopic solar cells has revolutionized the approach to this technology. Perovskites combine extraordinary electron- and hole-transport properties providing an ambipolar behavior with a low-temperature processing which eases the synthesis procedure, two features that make them attractive for the manufacturing of a new generation of solid-state sensitized solar cells.[3] [a] F. J. Ramos, Dr. E. Guilln, Dr. S. Ahmad Abengoa Research, C/Energa Solar no 1 Campus Palmas Altas-41014, Sevilla (Spain) E-mail: [email protected] [b] Dr. M. C. Lpez-Santos, Prof. A. R. Gonzalez-Elipe Instituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla), c/Americo Vespucio 49 41092 Sevilla (Spain) [c] F. J. Ramos, Dr. M. K. Nazeeruddin, Prof. M. Grtzel Laboratory of Photonics and Interfaces Department of Chemistry and Chemical Engineering Swiss Federal Institute of Technology, Station 6 CH-1015 Lausanne (Switzerland)
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The CH3NH3PbI3 perovskite has attracted interest since the pioneering work by Miyasaka et al.,[4] proposing a new direction for DSSC fabrication. The ability of this compound to be deposited by either wet chemistry or vacuum evaporation techniques is another advantage for versatile device applications.[5, 6] The first work using methylammonium lead iodide as the sensitizer in a liquid (DSSCs) resulted in a power-conversion efficiency (PCE) of 3.8 %, although the device’s long-term stability remained an issue.[4] This poor stability seems to be mainly due to the dissolution of the perovskite in the electrolyte. This bottleneck was largely overcome by the fabrication of solid-state (ss) DSSCs. Perovskite-sensitized ss-DSSCs gave remarkable results, and a PCE over 15 % was measured when using TiO2 as the electron-conducting material.[5] This high PCE resulted from the replacement of liquid electrolytes by solidstate hole-transport materials (HTM), 2,2’,7,7’-tetrakis(N,N-di-pmethoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD).[5, 6] The ambipolar nature of perovskite also helped in fabricating heterojunction solar cells, without the use of any additional HTM and gave competitive PCE values.[7] Recent breakthroughs have led to rapid advancements in these cells and the initially used one-step process for perovskite deposition has been later modified by the introduction of a sequential deposition method[5] or vacuum sublimation techniques.[6] In most of these devices, mesoscopic TiO2 was employed, a material that has to be processed at high temperature for efficient chargecarrier transport. Insulating Al2O3 scaffold and recently ZrO2 have been also employed as photoanodes.[8, 9] During the submission process of this article, a ZnO-based photoanode was also reported for the fabrication of perovskite-based cells.[10, 11] The similar band gap (~ 3.37 eV at 25 8C) and the lower processing temperature of ZnO with respect to TiO2 is of interest for the realization of photoanodes in PV devices manufactured onto temperature-sensitive substrates. In liquid DSSCs, so far ChemPhysChem 0000, 00, 1 – 7
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a PCE of 7.5 % has been obtained using hierarchical ZnO aggregates,[12] while Ma et al.[13] obtained a 6.46 % PCE in a quasisolid-state cell. Recently, a PCE as low as 0.61 % was reported using spiro-OMeTAD as HTM in a ZnO-based ss-DSSCs.[14] Dye– oxide interactions have been pointed out as one of the main limitations for the performance of dye-sensitized ZnO solar cells, due to surface dissolution and poor injection yields.[15] The remarkable PCE obtained with perovskite-sensitized ZnO solar cells confirms the importance of sensitizer–semiconductor interactions for this semiconductor oxide, as well as the properties of these compounds. Much attention has been paid in the last decade to the fabrication by wet chemical routes of ZnO nanostructures such as nanoparticles, nanoribbons, nanowires, and nanorods.[16–20] Nanorods grown perpendicularly onto various substrates in the form of densely packed assemblies and a high aspect ratio with a high surface-to-volume ratio are suitable candidates for hybrid PV manufacturing. ZnO nanorods can be grown by difScheme 1. Schematic diagram of a device fabricated using ZnO nanocolumnar structures. ferent processes such as electrodeposition, pulsed laser deposition, hydrothermal routes, metal-organic vapor phase epitaxy (MOVPE), or by vapor–liquid–solid (VLS) growth.[18, 21–24] An altoanode in DSSCs or hybrid solar cells has not been investigatternative, but largely unexplored method, is plasma-enhanced ed. After ZnO deposition by PECVD, the CH3NH3PbI3 layer was deposited by a two-step method, and an HTM layer was spunchemical vapor deposition (PECVD) that allows the catalystcoated as shown in the device architecture (Scheme 1). These free growth of ZnO nanocolumnar films and nanorods at low low-temperature-processable ZnO devices resulted in a PCE of temperatures.[25] PECVD is an industrially adaptable method permitting the uniform coating of large areas of sensitive subabout 5 % at 100 mW cm 2, demonstrating its potential use for strates. By using appropriate precursors and plasma conditions, a wide variety of photovoltaic applications, including flexible this technique enables one to control the composition and solar cells. nanostructure of the deposited films. For example, it has been used to grow nanocolumnar TiO2 thin films or ZnO nanorods 2. Results and Discussion on various substrates.[26, 27] The ZnO properties largely depend on the synthesis routes and microstructure, usually in the Figures 1 a–c illustrate scanning electron microscopy (SEM) top sense of having a high concentration of electronic and strucviews as well as cross-sectional views, showing respectively the tural defects for low-dimensional nanostructures of this materiin-depth and surface topology of films with three different al. Thus, the achievement of porous ZnO nanostructures, capable of efficiently interacting with sensitizers, with a low concentration of defects to avoid recombination or other electronscattering losses is critical for the manufacturing of efficient ssDSSC using this material as a photoanode. Recently,[27] a low-temperature procedure was reported for the preparation of crystalline, defect-free ZnO nanocolumnar thin films with a relatively low concentration of defects. Herein, we demonstrate the feasibility of incorporating nanocolumnar ZnO thin films prepared by PECVD as electron conductor and scaffold for CH3NH3PbI3 sensitized solar cells. So far, the utilization of these Figure 1. SEM images (cross-sectional and top view) of ZnO films of: a) 400, b) 500, and c) 600 nm thickness. PECVD-grown ZnO films as pho- d) Cross-sectional images of fabricated solar-cell devices containing 300 nm-thick ZnO films. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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thicknesses from 400 to 600 nm. The cross-sectional Table 1. Summary of the performances obtained using different experimental conviews show that the films present a well-aligned colditions under 1 Sun. umnar microstructure with the nanocolumns oriented HTM JSC [PbI2] CH3NH3I Annealing VOC FF PCE almost perpendicularly to the substrate. The rough lat[m] concentration temperature [mA cm 2] [mV] [%] eral faces and apparent width of the nanocolumns in[mg mL 1] [8C] creased from the interface with the substrate to the 1 8 70 Spiro12.5 792 0.391 3.9 film surface. The films were highly transparent and OMeTAD were characterized by a refraction index of 1.8, indica1.25 8 70 Spiro5.3 660 0.321 1.2 tive of a considerable porosity, as obtained by fitting OMeTAD 1.35 8 70 Spiro1.4 631 0.350 0.3 analysis of the interference oscillations determined by OMeTAD UV/Vis absorption spectroscopy when deposited on 1 10 70 Spiro6.0 761 0.406 1.9 a quartz substrate. The films were crystalline, correOMeTAD sponding to the wurzite structure and a high texturing 1.25 10 70 Spiro2.7 621 0.397 0.7 OMeTAD with the c-axis perpendicular to the substrate. From 1 8 100 Spiro16.0 718 0.412 4.8 the width of the (002) diffraction peak by the Scherrer OMeTAD method, we estimated a crystal size of approximately 70 nm. A prospective analysis of the performance of the cells showed that the best results were obtained for ZnO layers with a thickness of 300 nm and therefore in the rest of this work all the reported data will be referred to samples with this thickness. Figure 1 d illustrates the cross-section view of a 300 nm-thick ZnO nanocolumnar-based solar cell. It can be seen that the pores and intercolumnar space of the nanocolumnar film are infiltrated with the absorber CH3NH3PbI3, and an overlayer of CH3NH3PbI3 also forms on top of the film covering the nanocolumnar structure. The image shows that the nanocolumns are covered by a dense blanket layer formed by CH3NH3PbI3. This configuration is paramount for an efficient charge separation and is crucial to determine the filling fraction and infiltration depth of the CH3NH3PbI3. The formation of a CH3NH3PbI3 overlayer on the surface of the ZnO/CH3NH3PbI3 infiltrated film apFigure 2. J–V curves of the fabricated devices. The gray triangles show a cell annealed at 70 8C whereas the black dots represent a cell annealed at pears to be a key factor for effectively extracting hole carriers 100 8C. The corresponding dark currents are represented by dotted lines. through the interface with the HTM and up to the gold electrode. However, significant differences were found depending on the preparation protocol used for the deposition of the persome difficulties to hole transport when used in combination ovskite and the type of HTM layers infiltrated and deposited with ZnO. It is worth mentioning here that to the best of our on top of the ZnO nanocolumnar film. knowledge, the highest reported PCE value for ZnO in liquid ZnO films were infiltrated by the perovskite nanocrystals by DSSC is 7.5 %,[12] while only 0.61 % PCE was reported for ssDSSCs using Spiro-OMeTAD as HTM.[14] When comparing our using the two-step deposition strategy described in the Experiresults with those recently reported by Mathews et al.[11] conmental Section. Slight differences in the experimental protocol cerning perovskite ZnO cells, we observe that the photocuraffected the ratio between the two perovskite components, rent reported by these authors is very similar to ours, whereas the annealing temperature, and the type of HTM utilized, as the FF and specially the Voc are significantly higher. Here we summarized in Table 1. We measured current density–voltage (J–V) curves (Figure 2) wish to mention that we have applied a very low temperature for the solar cells under simulated air mass 1.5 global (AM1.5G) annealing to the ZnO samples. It is also worth noting that insolar irradiation and in the dark state. From these curves, we creasing the annealing temperature of the perovskite to 100 8C derived the values for the short-circuit photocurrent (Jsc), the drastically contributed to achieving higher Jsc values, as open-circuit voltage (Voc), and the fill factor (FF) that are suma result of a better infiltration of CH3NH3PbI3 through the ZnO marized in Table 1. The best-performance cells were obtained intercolumnar space. Statistical data suggests that the devices with Spiro-OMeTAD as HTM. Table 1 shows values of these pa(five devices) in which perovskites were annealed at 100 8C rameters, respectively, 16 mA cm 2, 718 mV, and 0.412, yielding gave over 4 % PCE whereas those which were annealed (five devices) at 70 8C gave less than 4 % PCE. It is well known that a measured PCE of 4.8 % (Table 1). Similar devices using poly(ZnO samples show relatively lower FF values compared to TiO2 triarylamine) [ PTAA] as HTM gave only 1.3 % of PCE (Jsc = 8.3 mA cm 2 ; Voc = 481 mV and FF = 0.327). The lower perdue to their higher recombination rate, and this was also obformance found when using PTAA compared to HTM can be served here. A similar decrease in FF when using ZnO was also ascribed to the large molecular structure, which may impose observed in DSSCs as well as ss-DSSC.[28, 29] This low FF has 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMPHYSCHEM ARTICLES been ascribed to the development of a series resistance linked to a higher recombination kinetics and to a poor injection from the absorber into this semiconductor.[15, 30] From the data gathered in Table 1, it is also possible to verify that higher loadings of PbI2 result in bad PV properties. A significant drop can be observed in both Voc and, more significantly, in Jsc. This drop in Jsc (from 12.46 mA cm 2 to 5.34 and 1.43 mA cm 2 for 1 m, 1.25 m, and 1.35 m loadings, respectively) was unexpected, since increasing the loading of PbI2 (chromophore) onto the ZnO nanocolumns should increase the light-harvesting capability of the system. It can be thus concluded that the amount of perovskite deposited onto the ZnO nanocolumns is not a crucial issue to fabricate efficient cells. Similarly, the strategy to further push the performance of the cells by increasing the concentration of methylammonium iodide(CH3NH3I) in the dipping solution (see Table 1) also led to bad PV performance. In this regard, the infiltration degree of the two solutes into the photoanode seems to be a much more critical issue to get high performances, very likely due to proper stoichiometry formation for the perovskite. The incident-photon-to-current efficiency (IPCE) as a function of the wavelength reaches a maximum 64 % at 500–600 nm in the visible spectral region (Figure 3), which is well in accordance with other devices based on ZnO.[10] Electrochemical impedance spectroscopy (EIS) was per-
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Figure 4. Nyquist plots of a perovskite solar cell under illumination and in the dark at an applied voltage of 0.6 V.
indicates that electron transport in the semiconductor is playing a role, and the response observed is due to the coupling of the recombination and transport processes. For these reasons, the complete transmission line, including the contribution of the hole-transport process, is necessary to obtain an accurate fitting. The fitting values obtained for the charge-transfer resistance (Rct) in the ZnO-nanocolumn-based electrode were ~ 30 and 46 W cm 2 under illumination and in the dark, respectively (see Table 2). For DSSCs, differences in the chargetransfer resistance in the dark or under illumination are usually
Table 2. EIS Rct and C data at 0.6 V under dark and illumination modes. Electrical parameters 2
Rct (W cm ) C (F cm 2)
Figure 3. Incident-photon-to-current efficiency (IPCE) of the ZnO nanocolumunar structure for CH3NH3PbI3-sensitized solar cells.
formed to analyze charge recombination in the cells with the best-performing configuration. Figure 4 shows the impedance response of the cells, in the dark and under illumination, for an applied voltage of 0.6 V. In both conditions, the impedance spectra are dominated by an arc at intermediate frequencies. Under illumination, the spectrum exhibits a feature at low frequencies, which is not observed in the dark. The spectra were fitted by using a similar circuit to the one proposed for ssDSSCs[31] and more recently for perovskite solar cells,[32] and in this case, the contribution of the blocking layer was ignored. In the past, a simpler circuit was employed which substituted the transmission line by a combination of two R–C elements connected in series.[11, 33] A clear separation in characteristic frequency was not observed for the transport in the hole conductor and recombination of electrons in the semiconductor layer. In addition, the spectra resemble Gerischer impedance, which 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Dark [0.6 V]
Illumination [0.6 V]
46 2.47 10
30 1.33 10
7
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related to a high hole density near the generation sites. Although a difference was observed between both conditions for ZnO perovskite cells, it was very small. It is well known that the deposition method greatly determines the ZnO structure and this will critically affect the recombination rate in the device. However, even when different deposition methods are employed, the resistance to recombination is similar to that recently reported for ZnO perovskite cells at an identical applied voltage under illumination.[11] As the measured voltage in our case was significantly lower, we speculate other factors apart from the recombination are affecting the performance of the devices. One possibility can be the use of the similar chemistry for the blocking and the electron conductor materials, which can be advantageous to obtain high voltages. The capacitance (C) was 2.47 10 7 F cm 2 in the dark, whereas under illumination it reached 1.33 10 7 F cm 2. Recently, Bisquert et al.[32] pointed out that at higher applied voltages, a capacitance at intermediate frequency is probably originated by the synergistic contribution of the semiconductor oxide and the perovskite to the charge accumulation. This result was also found to be in accordance with our kelvin probe force microscopy measurements.[34]
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CHEMPHYSCHEM ARTICLES 3. Conclusions By using plasma-enhanced chemical vapor deposition at low temperature, we demonstrated the growth of nanocolumnar ZnO thin films with a preferential c-axis orientation and a low concentration of defects. The microstructure, crystallinity, and minimum defect concentration of ZnO was optimized by adjusting the substrate temperature during deposition, the characteristics of the plasma used, and the film thickness. CH3NH3PbI3-sensitized solar cells were fabricated using the ZnO nanocolumunar structures and gave competitive photonto-light-conversion efficiencies. A power-conversion efficiency of 4.8 % was measured with this type of nanocolumunar structure, which is one of the highest reported to date using a tailored morphology for ZnO. Additionally, these results give us deep insight into the microstructure of the films, showing that these two techniques can be combined for a precise non-destructive characterization of ZnO films. Impedance spectroscopy results show that the main bottleneck in ZnO cells is the recombination, which is affecting the performance of the devices.
Experimental Section Materials All chemicals were obtained from Sigma Aldrich or Acros and were employed without any further treatment. Spiro-OMeTAD was acquired from Merck KGaA and (poly-triarylamine) (PTAA) from EMindex. CH3NH3I was synthetized following the procedure described in the literature.[35] Lead iodide, methyl ammonium iodide, and SpiroOMeTAD solutions were prepared inside an argon glove box under moisture- and oxygen-controlled conditions (H2O level: < 1 ppm and O2 level: < 10 ppm), while spin-coating and dip-coating were performed inside a dry box. The CH3NH3PbI3 layers were prepared by the sequential deposition method following the recipe using CH3NH3I and PbI2.
Preparation of ZnO Nanostructured Thin Films ZnO nanostructured thin films of varying thickness (400–700 nm) were deposited by PECVD inside a plasma reactor according to a method previously reported.[27] The films were directly grown on pre-treated FTO glass substrates in a plasma reactor with a remote configuration using diethyl zinc (ZnEt2) as zinc precursor. The precursor was dosed into the chamber through a line equipped with a mass flow controller that ended in a shower-type dispenser located 3 cm above the sample holder. The tubes assembly was heated at 375 K, and the mass flow controller at 315 K to prevent any condensation of precursor in the line walls. A thermocouple was placed at the sample holder to monitor its temperature during deposition. The microwave plasma source (SLAN, Plasma Consult GmbH) was coupled to the reaction chamber and separated by a grounded grid located 10 cm above the sample holder. This grid circumvents the microwave heating of the substrates and minimizes ion bombardment effects. In ref. [27] it was reported that the topology, crystallinity properties, and concentration of defects can be regulated by changing the temperature of the substrate, the plasma composition, and the film thickness. A low concentration of structural and electronic defects, determined by fluorescence analysis of the films, was achieved for thicknesses equal or lower 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org than 600 nm by plasma deposition at 423 K in a plasma of O2 and 400 W of power. The total pressure during deposition was maintained at 4 10 3 Torr. The samples were treated with pure O2 plasma before and after the film deposition for cleaning of the substrates. The thickness of the films was estimated ex situ by direct SEM observation and by fitting the optical reflectance spectra as measured in the 500–900 nm range at normal incidence with a Bruker spectrometer equipped with a confocal objective.
Device Fabrication FTO-coated glass, either TEC7 or TEC15 from Pilkington, were patterned by laser etching. Then, the substrates were cleaned and brushed using Hellmanex solution and rinsed with deionized water and ethanol; subsequently they were ultrasonicated in 2-propanol and then rinsed using ethanol and acetone and finally dried with compressed air. Prior to the compact layer deposition, the samples were cleaned with an ultraviolet/O3 treatment during 30 min. A TiO2 compact layer was deposited by spray pyrolysis at 450 8C using 0.5 mL of a titanium diisopropoxide bis(acetyl acetonate) precursor solution (75 % in 2-propanol, Sigma Aldrich) in 19.5 mL of pure ethanol using O2 as the carrier gas. After spray pyrolysis, the samples were kept for 30 min at 450 8C to facilitate anatase formation. After reaching room temperature, they were immersed in a 0.02 m TiCl4 solution in deionized water at 70 8C for 30 min. Following this, the samples were washed again with deionized water, annealed at 500 8C for 15 min and left to cool down slowly. The electron-transporting layers (ZnO nanocolumns) were then deposited by PECVD following the procedure mentioned above. The perovskite was prepared by the sequential deposition method, as reported for high-performance perovskite cells.[5] In summary, a lead iodide (PbI2) film was deposited by spin-coating (6500 rpm for 30 s) using a solution of N,N-dimethylformamide (DMF) and kept on a hot plate at 70–80 8C under vigorous stirring to avoid lead iodide precipitation. Subsequently, this solution was cooled down to 70 8C, a temperature which was maintained during the whole PbI2 deposition process. After spin-coating, the cells were placed onto a hot plate at 100 8C for 30 min for annealing, and after cooling down the samples, the photoanodes were dipped in a methyl ammonium iodide (MAI) solution in 2-propanol for 20 s and rinsed in pure 2-propanol and dried using the spin coater at 4000 rpm for 30 s. Subsequently, the samples were annealed again at 100 8C for 30 min. For the fabrication of the best-performing cell, both annealing steps were developed at 100 8C. Both SpiroOMeTAD or PTAA (Mw = 17 500 g mol 1) were selected as hole-transporting materials. Spiro-OMeTAD was spun-coated at 4000 rpm for 30 s by dissolving 72.3 mg of Spiro-OMeTAD in 1 mL of chlorobenzene; 21.9 mL of tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyrydine)cobalt(III) bis(trifluoromethylsulphonyl)imide (FK209) from a stock solution (400 mg of FK209 in 1 mL of acetonitrile), 17.5 mL of a lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) stock solution (520 mg of LiTFSI in 1 mL of acetonitrile), and 28.8 mL of 4-tert-butylpyridine were also added to the solution as dopants. While PTAA was spuncoated at 2000 rpm for 30 s and the PTAA solution was prepared by dissolving 30 mg of PTAA in 1 mL of toluene, 18.8 mL of LiTFSI stock solution (170 mg of LiTFSI in 1 mL of acetonitrile) and 9.4 mL of 4-tert-butylpyridine were also added. Finally, 80 nm of gold (Au) were thermally evaporated on top of the cell (as the cathode) under a vacuum level between 1 10 6 and 1 10 5 torr. ChemPhysChem 0000, 00, 1 – 7
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CHEMPHYSCHEM ARTICLES Characterization For the J–V curves, a 450 W Xe lamp (Oriel) with a Schott K113 Tempax sunlight filter (Praezisions Glas & Optik GmbH) was employed as the light source. A digital source-meter (Keithly 2400) was used to apply the voltage to the cell while current was recorded. For the IPCE measurements, a 300 W Xe lamp (ILC Technology) was connected to a Gemini-180 double monochromator (JobinYvon Ltd.) as the light beam. A constant white-light bias (5 % of the received light) was found recommendable to measure perovskite cells, so an array of white LEDS was used for that purpose. A Model SR830 DSP lock-in Amplifier (Standford Research Systems) was utilized to record the performance. For both J–V and IPCE measurements, the active area was fixed to 0.285 cm2 using a mask, unless otherwise specified. Impedance measurements were carried out in the dark and under illumination at a 0.6 V forward bias. The sample was illuminated with the help of a 540 nm light-emitting diode (LUXEON), and the AC modulation was 20 mV. A response analyzer module (PGSTAT302N/FRA2, AutoLab) was used to analyze the frequency response of the devices in the 106–10 1 Hz range. The spectra were fitted using the Zview software. The surface and in-depth microstructure of the films were observed by plan-view and cross-sectional SEM, respectively, using a Hitachi S5200 field-emission microscope operated at 5.0 keV.
Acknowledgements A.R.G. thanks the Junta de Andaluca (projects TEP08067 and P12-FQM-2265) and the Spanish Ministry of Economy and Competitiveness (projects CONSOLIDER CSD2008-00023, MAT201021228, and MAT2010-18447) for financial support. S.A. acknowledges a grant from Torres Quevedo, Ministry of Spain, and thanks Peng Gao for the CH3NH3I synthesis and Manuel Oliva for the SEM images. Keywords: perovskite · photovoltaics · plasma-enhanced chemical vapor deposition · solid-state solar cells · ZnO [1] M. Grtzel, R. A. J. Janssen, D. B. Mitzi, E. H. Sargent, Nature 2012, 488, 304 – 312. [2] S. Ahmad, E. Guilln, L. Kavan, M. Gratzel, M. K. Nazeeruddin, Energy Environ. Sci. 2013, 6, 3439 – 3466. [3] S. Kazim, M. K. Nazeeruddin, M. Gratzel, S. Ahmad, Angew. Chem. Int. Ed. 2014, 53, 2812 – 2824; Angew. Chem. 2014, 126, 2854 – 2867. [4] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050 – 6051. [5] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Grtzel, Nature 2013, 499, 316 – 319. [6] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395 – 398. [7] W. A. Laban, L. Etgar, Energy Environ. Sci. 2013, 6, 3249 – 3253.
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www.chemphyschem.org [8] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643 – 647. [9] D. Bi, S.-J. Moon, L. Haggman, G. Boschloo, L. Yang, E. M. J. Johansson, M. K. Nazeeruddin, M. Gratzel, A. Hagfeldt, RSC Adv. 2013, 3, 18762 – 18766. [10] D. Bi, G. Boschloo, S. Schwarzmuller, L. Yang, E. M. J. Johansson, A. Hagfeldt, Nanoscale 2013, 5, 11686 – 11691. [11] M. H. Kumar, N. Yantara, S. Dharani, M. Grtzel, S. Mhaisalkar, P. P. Boix, N. Mathews, Chem. Commun. 2013, 49, 11089 – 11091. [12] N. Memarian, I. Concina, A. Braga, S. M. Rozati, A. Vomiero, G. Sberveglieri, Angew. Chem. 2011, 123, 12529 – 12533; Angew. Chem. Int. Ed. 2011, 50, 12321 – 12325. [13] Y. Shi, K. Wang, Y. Du, H. Zhang, J. Gu, C. Zhu, L. Wang, W. Guo, A. Hagfeldt, N. Wang, T. Ma, Adv. Mater. 2013, 25, 4413 – 4419. [14] L. Schlur, A. Carton, P. LvÞque, D. Guillon, G. Pourroy, J. Phys. Chem. C 2013, 117, 2993 – 3001. [15] J. A. Anta, E. Guilln, R. Tena-Zaera, J. Phys. Chem. C 2012, 116, 11413 – 11425. [16] S. B. Ambade, R. S. Mane, S.-H. Han, S.-H. Lee, M.-M. Sung, O.-S. Joo, J. Photochem. Photobiol. A 2011, 222, 366 – 369. [17] W.-H. Chiu, C.-H. Lee, H.-M. Cheng, H.-F. Lin, S.-C. Liao, J.-M. Wu, W.-F. Hsieh, Energy Environ. Sci. 2009, 2, 694 – 698. [18] C. Xu, J. Wu, U. V. Desai, D. Gao, J. Am. Chem. Soc. 2011, 133, 8122 – 8125. [19] E. Guilln, E. Azaceta, L. M. Peter, A. Zukal, R. Tena-Zaera, J. A. Anta, Energy Environ. Sci. 2011, 4, 3400 – 3407. [20] Y. Xie, P. Joshi, S. B. Darling, Q. Chen, T. Zhang, D. Galipeau, Q. Qiao, J. Phys. Chem. C 2010, 114, 17880 – 17888. [21] Z. W. Liu, C. K. Ong, T. Yu, Z. X. Shen, Appl. Phys. Lett. 2006, 88, 053110. [22] J. Elias, R. Tena-Zaera, C. Lvy-Clment, J. Electroanal. Chem. 2008, 621, 171 – 177. [23] H. J. Fan, F. Bertram, A. Dadgar, J. Christen, A. Krost, M. Zacharias, Nanotechnology 2004, 15, 1401. [24] S. Kar, B. N. Pal, S. Chaudhuri, D. Chakravorty, J. Phys. Chem. B 2006, 110, 4605 – 4611. [25] A. Martı’n, J. P. Espins, A. Justo, J. P. Holgado, F. Yubero, A. R. GonzlezElipe, Surf. Coat. Technol. 2002, 151, 289 – 293. [26] A. Borrs, J. Cotrino, A. R. Gonzlez-Elipe, J. Electrochem. Soc. 2007, 154, P152 – P157. [27] P. Romero-Gmez, J. Toudert, J. R. Snchez-Valencia, A. Borrs, A. Barranco, A. R. Gonzalez-Elipe, J. Phys. Chem. C 2010, 114, 20932 – 20940. [28] G. R. A. Kumara, J. K. Tiskumara, C. S. K. Ranasinghe, I. S. Rathnayake, W. M. N. M. B. Wanninayake, E. N. Jayaweera, L. R. A. K. Bandara, R. M. G. Rajapakse, Electrochim. Acta 2013, 94, 34 – 37. [29] U. V. Desai, C. Xu, J. Wu, D. Gao, Nanotechnology 2012, 23, 205401. [30] J. R. Jennings, Y. Liu, Q. Wang, J. Phys. Chem. C 2011, 115, 15109 – 15120. [31] F. Fabregat-Santiago, J. Bisquert, L. Cevey, P. Chen, M. Wang, S. M. Zakeeruddin, M. Grtzel, J. Am. Chem. Soc. 2009, 131, 558 – 562. [32] H.-S. Kim, I. Mora-Sero, V. Gonzlez-Pedro, F. Fabregat-Santiago, E. J. Jurez-Prez, N.-G. Park, J. Bisquert, Nat. Commun. 2013, 4, 2242. [33] H.-S. Kim, J.-W. Lee, N. Yantara, P. P. Boix, S. A. Kulkarni, S. Mhaisalkar, M. Grtzel, N.-G. Park, Nano Lett. 2013, 13, 2412 – 2417. [34] P. Qin, A. L. Domanski, A. K. Chandiran, R. Berger, H.-J. Butt, M. I. Dar, T. Moehl, N. Tetreault, P. Gao, S. Ahmad, M. K. Nazeeruddin, M. Grtzel, Nanoscale 2014, 6, 1508 – 1514. [35] J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Park, N.-G. Park, Nanoscale 2011, 3, 4088 – 4093. Received: December 22, 2013 Published online on && &&, 0000
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ARTICLES ZnO can work as an electron conduit in the fabrication of perovskite-based solid-state solar cells. Nanocolumnar ZnO thin films synthesized by plasmaenhanced chemical vapor deposition are effective candidates for large-area deposition.
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F. J. Ramos, M. C. Lpez-Santos, E. Guilln, M. K. Nazeeruddin, M. Grtzel, A. R. Gonzalez-Elipe, S. Ahmad* && – && Perovskite Solar Cells Based on Nanocolumnar Plasma-Deposited ZnO Thin Films
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Perovskite Solar Cells Based on Nanocolumnar PlasmaDeposited ZnO Thin Films F. Javier Ramos,[a, c] Maria C. Lpez-Santos,[b] Elena Guilln,[a] Mohammad Khaja Nazeeruddin,[c] Michael Grtzel,[c] Agustin R. Gonzalez-Elipe,[b] and Shahzada Ahmad*[a] ZnO thin films having a nanocolumnar microstructure are grown by plasma-enhanced chemical vapor deposition at 423 K on pre-treated fluorine-doped tin oxide (FTO) substrates. The films consist of c-axis-oriented wurtzite ZnO nanocolumns with well-defined microstructure and crystallinity. By sensitizing CH3NH3PbI3 on these photoanodes a power conversion of 4.8 % is obtained for solid-state solar cells. Poly(triarylamine) is
found to be less effective when used as the hole-transport material, compared to 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD), while the higher annealing temperature of the perovskite leads to a better infiltration in the nanocolumnar structure and an enhancement of the cell efficiency.
1. Introduction The planet’s needs for cheap and abundant sources of clean energy, has fueled the research interest to find new materials which can be then exploited for advanced applications.[1] Third-generation excitonic solar cells are deemed as a credible advantageous alternative to existing cost-ineffective technologies.[1, 2] With respect to dye-sensitized solar cells (DSSCs), the processing of solid-state solar cells is industrially straightforward as it avoids hazards related from leakage and compact sealing. In this context, fabricating stable, efficient and cost-effective solar cells is of paramount importance to find alternative materials for sensitizers. Hybrid organic–inorganic perovskite structures are of enormous scientific and technological importance and are widely used in electroluminescence, light emitting diodes, photovoltaics (PVs) and for battery applications. Recently, their fascinating ability to act as an absorber in mesoscopic solar cells has revolutionized the approach to this technology. Perovskites combine extraordinary electron- and hole-transport properties providing an ambipolar behavior with a low-temperature processing which eases the synthesis procedure, two features that make them attractive for the manufacturing of a new generation of solid-state sensitized solar cells.[3] [a] F. J. Ramos, Dr. E. Guilln, Dr. S. Ahmad Abengoa Research, C/Energa Solar no 1 Campus Palmas Altas-41014, Sevilla (Spain) E-mail: [email protected] [b] Dr. M. C. Lpez-Santos, Prof. A. R. Gonzalez-Elipe Instituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla), c/Americo Vespucio 49 41092 Sevilla (Spain) [c] F. J. Ramos, Dr. M. K. Nazeeruddin, Prof. M. Grtzel Laboratory of Photonics and Interfaces Department of Chemistry and Chemical Engineering Swiss Federal Institute of Technology, Station 6 CH-1015 Lausanne (Switzerland)
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The CH3NH3PbI3 perovskite has attracted interest since the pioneering work by Miyasaka et al.,[4] proposing a new direction for DSSC fabrication. The ability of this compound to be deposited by either wet chemistry or vacuum evaporation techniques is another advantage for versatile device applications.[5, 6] The first work using methylammonium lead iodide as the sensitizer in a liquid (DSSCs) resulted in a power-conversion efficiency (PCE) of 3.8 %, although the device’s long-term stability remained an issue.[4] This poor stability seems to be mainly due to the dissolution of the perovskite in the electrolyte. This bottleneck was largely overcome by the fabrication of solid-state (ss) DSSCs. Perovskite-sensitized ss-DSSCs gave remarkable results, and a PCE over 15 % was measured when using TiO2 as the electron-conducting material.[5] This high PCE resulted from the replacement of liquid electrolytes by solidstate hole-transport materials (HTM), 2,2’,7,7’-tetrakis(N,N-di-pmethoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD).[5, 6] The ambipolar nature of perovskite also helped in fabricating heterojunction solar cells, without the use of any additional HTM and gave competitive PCE values.[7] Recent breakthroughs have led to rapid advancements in these cells and the initially used one-step process for perovskite deposition has been later modified by the introduction of a sequential deposition method[5] or vacuum sublimation techniques.[6] In most of these devices, mesoscopic TiO2 was employed, a material that has to be processed at high temperature for efficient chargecarrier transport. Insulating Al2O3 scaffold and recently ZrO2 have been also employed as photoanodes.[8, 9] During the submission process of this article, a ZnO-based photoanode was also reported for the fabrication of perovskite-based cells.[10, 11] The similar band gap (~ 3.37 eV at 25 8C) and the lower processing temperature of ZnO with respect to TiO2 is of interest for the realization of photoanodes in PV devices manufactured onto temperature-sensitive substrates. In liquid DSSCs, so far ChemPhysChem 0000, 00, 1 – 7
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a PCE of 7.5 % has been obtained using hierarchical ZnO aggregates,[12] while Ma et al.[13] obtained a 6.46 % PCE in a quasisolid-state cell. Recently, a PCE as low as 0.61 % was reported using spiro-OMeTAD as HTM in a ZnO-based ss-DSSCs.[14] Dye– oxide interactions have been pointed out as one of the main limitations for the performance of dye-sensitized ZnO solar cells, due to surface dissolution and poor injection yields.[15] The remarkable PCE obtained with perovskite-sensitized ZnO solar cells confirms the importance of sensitizer–semiconductor interactions for this semiconductor oxide, as well as the properties of these compounds. Much attention has been paid in the last decade to the fabrication by wet chemical routes of ZnO nanostructures such as nanoparticles, nanoribbons, nanowires, and nanorods.[16–20] Nanorods grown perpendicularly onto various substrates in the form of densely packed assemblies and a high aspect ratio with a high surface-to-volume ratio are suitable candidates for hybrid PV manufacturing. ZnO nanorods can be grown by difScheme 1. Schematic diagram of a device fabricated using ZnO nanocolumnar structures. ferent processes such as electrodeposition, pulsed laser deposition, hydrothermal routes, metal-organic vapor phase epitaxy (MOVPE), or by vapor–liquid–solid (VLS) growth.[18, 21–24] An altoanode in DSSCs or hybrid solar cells has not been investigatternative, but largely unexplored method, is plasma-enhanced ed. After ZnO deposition by PECVD, the CH3NH3PbI3 layer was deposited by a two-step method, and an HTM layer was spunchemical vapor deposition (PECVD) that allows the catalystcoated as shown in the device architecture (Scheme 1). These free growth of ZnO nanocolumnar films and nanorods at low low-temperature-processable ZnO devices resulted in a PCE of temperatures.[25] PECVD is an industrially adaptable method permitting the uniform coating of large areas of sensitive subabout 5 % at 100 mW cm 2, demonstrating its potential use for strates. By using appropriate precursors and plasma conditions, a wide variety of photovoltaic applications, including flexible this technique enables one to control the composition and solar cells. nanostructure of the deposited films. For example, it has been used to grow nanocolumnar TiO2 thin films or ZnO nanorods 2. Results and Discussion on various substrates.[26, 27] The ZnO properties largely depend on the synthesis routes and microstructure, usually in the Figures 1 a–c illustrate scanning electron microscopy (SEM) top sense of having a high concentration of electronic and strucviews as well as cross-sectional views, showing respectively the tural defects for low-dimensional nanostructures of this materiin-depth and surface topology of films with three different al. Thus, the achievement of porous ZnO nanostructures, capable of efficiently interacting with sensitizers, with a low concentration of defects to avoid recombination or other electronscattering losses is critical for the manufacturing of efficient ssDSSC using this material as a photoanode. Recently,[27] a low-temperature procedure was reported for the preparation of crystalline, defect-free ZnO nanocolumnar thin films with a relatively low concentration of defects. Herein, we demonstrate the feasibility of incorporating nanocolumnar ZnO thin films prepared by PECVD as electron conductor and scaffold for CH3NH3PbI3 sensitized solar cells. So far, the utilization of these Figure 1. SEM images (cross-sectional and top view) of ZnO films of: a) 400, b) 500, and c) 600 nm thickness. PECVD-grown ZnO films as pho- d) Cross-sectional images of fabricated solar-cell devices containing 300 nm-thick ZnO films. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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thicknesses from 400 to 600 nm. The cross-sectional Table 1. Summary of the performances obtained using different experimental conviews show that the films present a well-aligned colditions under 1 Sun. umnar microstructure with the nanocolumns oriented HTM JSC [PbI2] CH3NH3I Annealing VOC FF PCE almost perpendicularly to the substrate. The rough lat[m] concentration temperature [mA cm 2] [mV] [%] eral faces and apparent width of the nanocolumns in[mg mL 1] [8C] creased from the interface with the substrate to the 1 8 70 Spiro12.5 792 0.391 3.9 film surface. The films were highly transparent and OMeTAD were characterized by a refraction index of 1.8, indica1.25 8 70 Spiro5.3 660 0.321 1.2 tive of a considerable porosity, as obtained by fitting OMeTAD 1.35 8 70 Spiro1.4 631 0.350 0.3 analysis of the interference oscillations determined by OMeTAD UV/Vis absorption spectroscopy when deposited on 1 10 70 Spiro6.0 761 0.406 1.9 a quartz substrate. The films were crystalline, correOMeTAD sponding to the wurzite structure and a high texturing 1.25 10 70 Spiro2.7 621 0.397 0.7 OMeTAD with the c-axis perpendicular to the substrate. From 1 8 100 Spiro16.0 718 0.412 4.8 the width of the (002) diffraction peak by the Scherrer OMeTAD method, we estimated a crystal size of approximately 70 nm. A prospective analysis of the performance of the cells showed that the best results were obtained for ZnO layers with a thickness of 300 nm and therefore in the rest of this work all the reported data will be referred to samples with this thickness. Figure 1 d illustrates the cross-section view of a 300 nm-thick ZnO nanocolumnar-based solar cell. It can be seen that the pores and intercolumnar space of the nanocolumnar film are infiltrated with the absorber CH3NH3PbI3, and an overlayer of CH3NH3PbI3 also forms on top of the film covering the nanocolumnar structure. The image shows that the nanocolumns are covered by a dense blanket layer formed by CH3NH3PbI3. This configuration is paramount for an efficient charge separation and is crucial to determine the filling fraction and infiltration depth of the CH3NH3PbI3. The formation of a CH3NH3PbI3 overlayer on the surface of the ZnO/CH3NH3PbI3 infiltrated film apFigure 2. J–V curves of the fabricated devices. The gray triangles show a cell annealed at 70 8C whereas the black dots represent a cell annealed at pears to be a key factor for effectively extracting hole carriers 100 8C. The corresponding dark currents are represented by dotted lines. through the interface with the HTM and up to the gold electrode. However, significant differences were found depending on the preparation protocol used for the deposition of the persome difficulties to hole transport when used in combination ovskite and the type of HTM layers infiltrated and deposited with ZnO. It is worth mentioning here that to the best of our on top of the ZnO nanocolumnar film. knowledge, the highest reported PCE value for ZnO in liquid ZnO films were infiltrated by the perovskite nanocrystals by DSSC is 7.5 %,[12] while only 0.61 % PCE was reported for ssDSSCs using Spiro-OMeTAD as HTM.[14] When comparing our using the two-step deposition strategy described in the Experiresults with those recently reported by Mathews et al.[11] conmental Section. Slight differences in the experimental protocol cerning perovskite ZnO cells, we observe that the photocuraffected the ratio between the two perovskite components, rent reported by these authors is very similar to ours, whereas the annealing temperature, and the type of HTM utilized, as the FF and specially the Voc are significantly higher. Here we summarized in Table 1. We measured current density–voltage (J–V) curves (Figure 2) wish to mention that we have applied a very low temperature for the solar cells under simulated air mass 1.5 global (AM1.5G) annealing to the ZnO samples. It is also worth noting that insolar irradiation and in the dark state. From these curves, we creasing the annealing temperature of the perovskite to 100 8C derived the values for the short-circuit photocurrent (Jsc), the drastically contributed to achieving higher Jsc values, as open-circuit voltage (Voc), and the fill factor (FF) that are suma result of a better infiltration of CH3NH3PbI3 through the ZnO marized in Table 1. The best-performance cells were obtained intercolumnar space. Statistical data suggests that the devices with Spiro-OMeTAD as HTM. Table 1 shows values of these pa(five devices) in which perovskites were annealed at 100 8C rameters, respectively, 16 mA cm 2, 718 mV, and 0.412, yielding gave over 4 % PCE whereas those which were annealed (five devices) at 70 8C gave less than 4 % PCE. It is well known that a measured PCE of 4.8 % (Table 1). Similar devices using poly(ZnO samples show relatively lower FF values compared to TiO2 triarylamine) [ PTAA] as HTM gave only 1.3 % of PCE (Jsc = 8.3 mA cm 2 ; Voc = 481 mV and FF = 0.327). The lower perdue to their higher recombination rate, and this was also obformance found when using PTAA compared to HTM can be served here. A similar decrease in FF when using ZnO was also ascribed to the large molecular structure, which may impose observed in DSSCs as well as ss-DSSC.[28, 29] This low FF has 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMPHYSCHEM ARTICLES been ascribed to the development of a series resistance linked to a higher recombination kinetics and to a poor injection from the absorber into this semiconductor.[15, 30] From the data gathered in Table 1, it is also possible to verify that higher loadings of PbI2 result in bad PV properties. A significant drop can be observed in both Voc and, more significantly, in Jsc. This drop in Jsc (from 12.46 mA cm 2 to 5.34 and 1.43 mA cm 2 for 1 m, 1.25 m, and 1.35 m loadings, respectively) was unexpected, since increasing the loading of PbI2 (chromophore) onto the ZnO nanocolumns should increase the light-harvesting capability of the system. It can be thus concluded that the amount of perovskite deposited onto the ZnO nanocolumns is not a crucial issue to fabricate efficient cells. Similarly, the strategy to further push the performance of the cells by increasing the concentration of methylammonium iodide(CH3NH3I) in the dipping solution (see Table 1) also led to bad PV performance. In this regard, the infiltration degree of the two solutes into the photoanode seems to be a much more critical issue to get high performances, very likely due to proper stoichiometry formation for the perovskite. The incident-photon-to-current efficiency (IPCE) as a function of the wavelength reaches a maximum 64 % at 500–600 nm in the visible spectral region (Figure 3), which is well in accordance with other devices based on ZnO.[10] Electrochemical impedance spectroscopy (EIS) was per-
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Figure 4. Nyquist plots of a perovskite solar cell under illumination and in the dark at an applied voltage of 0.6 V.
indicates that electron transport in the semiconductor is playing a role, and the response observed is due to the coupling of the recombination and transport processes. For these reasons, the complete transmission line, including the contribution of the hole-transport process, is necessary to obtain an accurate fitting. The fitting values obtained for the charge-transfer resistance (Rct) in the ZnO-nanocolumn-based electrode were ~ 30 and 46 W cm 2 under illumination and in the dark, respectively (see Table 2). For DSSCs, differences in the chargetransfer resistance in the dark or under illumination are usually
Table 2. EIS Rct and C data at 0.6 V under dark and illumination modes. Electrical parameters 2
Rct (W cm ) C (F cm 2)
Figure 3. Incident-photon-to-current efficiency (IPCE) of the ZnO nanocolumunar structure for CH3NH3PbI3-sensitized solar cells.
formed to analyze charge recombination in the cells with the best-performing configuration. Figure 4 shows the impedance response of the cells, in the dark and under illumination, for an applied voltage of 0.6 V. In both conditions, the impedance spectra are dominated by an arc at intermediate frequencies. Under illumination, the spectrum exhibits a feature at low frequencies, which is not observed in the dark. The spectra were fitted by using a similar circuit to the one proposed for ssDSSCs[31] and more recently for perovskite solar cells,[32] and in this case, the contribution of the blocking layer was ignored. In the past, a simpler circuit was employed which substituted the transmission line by a combination of two R–C elements connected in series.[11, 33] A clear separation in characteristic frequency was not observed for the transport in the hole conductor and recombination of electrons in the semiconductor layer. In addition, the spectra resemble Gerischer impedance, which 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Dark [0.6 V]
Illumination [0.6 V]
46 2.47 10
30 1.33 10
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related to a high hole density near the generation sites. Although a difference was observed between both conditions for ZnO perovskite cells, it was very small. It is well known that the deposition method greatly determines the ZnO structure and this will critically affect the recombination rate in the device. However, even when different deposition methods are employed, the resistance to recombination is similar to that recently reported for ZnO perovskite cells at an identical applied voltage under illumination.[11] As the measured voltage in our case was significantly lower, we speculate other factors apart from the recombination are affecting the performance of the devices. One possibility can be the use of the similar chemistry for the blocking and the electron conductor materials, which can be advantageous to obtain high voltages. The capacitance (C) was 2.47 10 7 F cm 2 in the dark, whereas under illumination it reached 1.33 10 7 F cm 2. Recently, Bisquert et al.[32] pointed out that at higher applied voltages, a capacitance at intermediate frequency is probably originated by the synergistic contribution of the semiconductor oxide and the perovskite to the charge accumulation. This result was also found to be in accordance with our kelvin probe force microscopy measurements.[34]
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CHEMPHYSCHEM ARTICLES 3. Conclusions By using plasma-enhanced chemical vapor deposition at low temperature, we demonstrated the growth of nanocolumnar ZnO thin films with a preferential c-axis orientation and a low concentration of defects. The microstructure, crystallinity, and minimum defect concentration of ZnO was optimized by adjusting the substrate temperature during deposition, the characteristics of the plasma used, and the film thickness. CH3NH3PbI3-sensitized solar cells were fabricated using the ZnO nanocolumunar structures and gave competitive photonto-light-conversion efficiencies. A power-conversion efficiency of 4.8 % was measured with this type of nanocolumunar structure, which is one of the highest reported to date using a tailored morphology for ZnO. Additionally, these results give us deep insight into the microstructure of the films, showing that these two techniques can be combined for a precise non-destructive characterization of ZnO films. Impedance spectroscopy results show that the main bottleneck in ZnO cells is the recombination, which is affecting the performance of the devices.
Experimental Section Materials All chemicals were obtained from Sigma Aldrich or Acros and were employed without any further treatment. Spiro-OMeTAD was acquired from Merck KGaA and (poly-triarylamine) (PTAA) from EMindex. CH3NH3I was synthetized following the procedure described in the literature.[35] Lead iodide, methyl ammonium iodide, and SpiroOMeTAD solutions were prepared inside an argon glove box under moisture- and oxygen-controlled conditions (H2O level: < 1 ppm and O2 level: < 10 ppm), while spin-coating and dip-coating were performed inside a dry box. The CH3NH3PbI3 layers were prepared by the sequential deposition method following the recipe using CH3NH3I and PbI2.
Preparation of ZnO Nanostructured Thin Films ZnO nanostructured thin films of varying thickness (400–700 nm) were deposited by PECVD inside a plasma reactor according to a method previously reported.[27] The films were directly grown on pre-treated FTO glass substrates in a plasma reactor with a remote configuration using diethyl zinc (ZnEt2) as zinc precursor. The precursor was dosed into the chamber through a line equipped with a mass flow controller that ended in a shower-type dispenser located 3 cm above the sample holder. The tubes assembly was heated at 375 K, and the mass flow controller at 315 K to prevent any condensation of precursor in the line walls. A thermocouple was placed at the sample holder to monitor its temperature during deposition. The microwave plasma source (SLAN, Plasma Consult GmbH) was coupled to the reaction chamber and separated by a grounded grid located 10 cm above the sample holder. This grid circumvents the microwave heating of the substrates and minimizes ion bombardment effects. In ref. [27] it was reported that the topology, crystallinity properties, and concentration of defects can be regulated by changing the temperature of the substrate, the plasma composition, and the film thickness. A low concentration of structural and electronic defects, determined by fluorescence analysis of the films, was achieved for thicknesses equal or lower 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org than 600 nm by plasma deposition at 423 K in a plasma of O2 and 400 W of power. The total pressure during deposition was maintained at 4 10 3 Torr. The samples were treated with pure O2 plasma before and after the film deposition for cleaning of the substrates. The thickness of the films was estimated ex situ by direct SEM observation and by fitting the optical reflectance spectra as measured in the 500–900 nm range at normal incidence with a Bruker spectrometer equipped with a confocal objective.
Device Fabrication FTO-coated glass, either TEC7 or TEC15 from Pilkington, were patterned by laser etching. Then, the substrates were cleaned and brushed using Hellmanex solution and rinsed with deionized water and ethanol; subsequently they were ultrasonicated in 2-propanol and then rinsed using ethanol and acetone and finally dried with compressed air. Prior to the compact layer deposition, the samples were cleaned with an ultraviolet/O3 treatment during 30 min. A TiO2 compact layer was deposited by spray pyrolysis at 450 8C using 0.5 mL of a titanium diisopropoxide bis(acetyl acetonate) precursor solution (75 % in 2-propanol, Sigma Aldrich) in 19.5 mL of pure ethanol using O2 as the carrier gas. After spray pyrolysis, the samples were kept for 30 min at 450 8C to facilitate anatase formation. After reaching room temperature, they were immersed in a 0.02 m TiCl4 solution in deionized water at 70 8C for 30 min. Following this, the samples were washed again with deionized water, annealed at 500 8C for 15 min and left to cool down slowly. The electron-transporting layers (ZnO nanocolumns) were then deposited by PECVD following the procedure mentioned above. The perovskite was prepared by the sequential deposition method, as reported for high-performance perovskite cells.[5] In summary, a lead iodide (PbI2) film was deposited by spin-coating (6500 rpm for 30 s) using a solution of N,N-dimethylformamide (DMF) and kept on a hot plate at 70–80 8C under vigorous stirring to avoid lead iodide precipitation. Subsequently, this solution was cooled down to 70 8C, a temperature which was maintained during the whole PbI2 deposition process. After spin-coating, the cells were placed onto a hot plate at 100 8C for 30 min for annealing, and after cooling down the samples, the photoanodes were dipped in a methyl ammonium iodide (MAI) solution in 2-propanol for 20 s and rinsed in pure 2-propanol and dried using the spin coater at 4000 rpm for 30 s. Subsequently, the samples were annealed again at 100 8C for 30 min. For the fabrication of the best-performing cell, both annealing steps were developed at 100 8C. Both SpiroOMeTAD or PTAA (Mw = 17 500 g mol 1) were selected as hole-transporting materials. Spiro-OMeTAD was spun-coated at 4000 rpm for 30 s by dissolving 72.3 mg of Spiro-OMeTAD in 1 mL of chlorobenzene; 21.9 mL of tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyrydine)cobalt(III) bis(trifluoromethylsulphonyl)imide (FK209) from a stock solution (400 mg of FK209 in 1 mL of acetonitrile), 17.5 mL of a lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) stock solution (520 mg of LiTFSI in 1 mL of acetonitrile), and 28.8 mL of 4-tert-butylpyridine were also added to the solution as dopants. While PTAA was spuncoated at 2000 rpm for 30 s and the PTAA solution was prepared by dissolving 30 mg of PTAA in 1 mL of toluene, 18.8 mL of LiTFSI stock solution (170 mg of LiTFSI in 1 mL of acetonitrile) and 9.4 mL of 4-tert-butylpyridine were also added. Finally, 80 nm of gold (Au) were thermally evaporated on top of the cell (as the cathode) under a vacuum level between 1 10 6 and 1 10 5 torr. ChemPhysChem 0000, 00, 1 – 7
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CHEMPHYSCHEM ARTICLES Characterization For the J–V curves, a 450 W Xe lamp (Oriel) with a Schott K113 Tempax sunlight filter (Praezisions Glas & Optik GmbH) was employed as the light source. A digital source-meter (Keithly 2400) was used to apply the voltage to the cell while current was recorded. For the IPCE measurements, a 300 W Xe lamp (ILC Technology) was connected to a Gemini-180 double monochromator (JobinYvon Ltd.) as the light beam. A constant white-light bias (5 % of the received light) was found recommendable to measure perovskite cells, so an array of white LEDS was used for that purpose. A Model SR830 DSP lock-in Amplifier (Standford Research Systems) was utilized to record the performance. For both J–V and IPCE measurements, the active area was fixed to 0.285 cm2 using a mask, unless otherwise specified. Impedance measurements were carried out in the dark and under illumination at a 0.6 V forward bias. The sample was illuminated with the help of a 540 nm light-emitting diode (LUXEON), and the AC modulation was 20 mV. A response analyzer module (PGSTAT302N/FRA2, AutoLab) was used to analyze the frequency response of the devices in the 106–10 1 Hz range. The spectra were fitted using the Zview software. The surface and in-depth microstructure of the films were observed by plan-view and cross-sectional SEM, respectively, using a Hitachi S5200 field-emission microscope operated at 5.0 keV.
Acknowledgements A.R.G. thanks the Junta de Andaluca (projects TEP08067 and P12-FQM-2265) and the Spanish Ministry of Economy and Competitiveness (projects CONSOLIDER CSD2008-00023, MAT201021228, and MAT2010-18447) for financial support. S.A. acknowledges a grant from Torres Quevedo, Ministry of Spain, and thanks Peng Gao for the CH3NH3I synthesis and Manuel Oliva for the SEM images. Keywords: perovskite · photovoltaics · plasma-enhanced chemical vapor deposition · solid-state solar cells · ZnO [1] M. Grtzel, R. A. J. Janssen, D. B. Mitzi, E. H. Sargent, Nature 2012, 488, 304 – 312. [2] S. Ahmad, E. Guilln, L. Kavan, M. Gratzel, M. K. Nazeeruddin, Energy Environ. Sci. 2013, 6, 3439 – 3466. [3] S. Kazim, M. K. Nazeeruddin, M. Gratzel, S. Ahmad, Angew. Chem. Int. Ed. 2014, 53, 2812 – 2824; Angew. Chem. 2014, 126, 2854 – 2867. [4] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050 – 6051. [5] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Grtzel, Nature 2013, 499, 316 – 319. [6] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395 – 398. [7] W. A. Laban, L. Etgar, Energy Environ. Sci. 2013, 6, 3249 – 3253.
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ARTICLES ZnO can work as an electron conduit in the fabrication of perovskite-based solid-state solar cells. Nanocolumnar ZnO thin films synthesized by plasmaenhanced chemical vapor deposition are effective candidates for large-area deposition.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
F. J. Ramos, M. C. Lpez-Santos, E. Guilln, M. K. Nazeeruddin, M. Grtzel, A. R. Gonzalez-Elipe, S. Ahmad* && – && Perovskite Solar Cells Based on Nanocolumnar Plasma-Deposited ZnO Thin Films
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