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Cite this: Energy Environ. Sci., 2016, 9, 3128 Received 16th August 2016, Accepted 15th September 2016
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Highly efficient and stable planar perovskite solar cells by solution-processed tin oxide† Elham Halvani Anaraki,ab Ahmad Kermanpur,b Ludmilla Steier,c Konrad Domanski,c Taisuke Matsui,cd Wolfgang Tress,c Michael Saliba,c Antonio Abate,ce Michael Gra¨tzel,c Anders Hagfeldt*a and Juan-Pablo Correa-Baena*a
DOI: 10.1039/c6ee02390h www.rsc.org/ees
Perovskite solar cells (PSCs) are one of the most promising lab-scale technologies to deliver inexpensive solar electricity. Low-temperature
Broader context
planar PSCs are particularly suited for large-scale manufacturing.
Perovskite-based solar cells have emerged as a promising technology for highly efficient and low-cost photovoltaics. The high efficiencies so far reported that go beyond 20% have been fabricated by using high temperature mesoporous thin layers (ca. 200 nm) infiltrated and capped (ca. 500 nm composed of large crystals) by a tailored-ion Pb-based perovskite material. Mesoporous-free ‘‘planar’’ perovskite solar cells (achieving up to 19.6%) have struggled to keep up with the progress of the mesoporous counter parts. Our work shows that efficiencies close to 21% are possible for the planar configuration, by a simple, low temperature, solution processed SnO2 method with suitable energetics and the additional advantage of improvements in series resistance at the electron selective layer/perovskite interface. In addition, the solar cells show great stability under 1 sun illumination and constant maximum power point tracking. Therefore, the newly proposed approach represents an important contribution on the way towards industrialization of perovskite photovoltaics that can withstand sustained operation under working conditions.
Here, we propose a simple, solution-processed technological approach for depositing SnO2 layers. The use of these layers in planar PSCs yields a high stabilized power conversion efficiency close to 21%, exhibiting stable performance under real operating conditions for over 60 hours. In addition, this method yielded remarkable voltages of 1214 mV at a band gap of 1.62 eV (approaching the thermodynamic limit of 1.32 V) confirming the high selectivity of the solution-processed layers. PSCs aged under 1 sun illumination and maximum power point tracking showed a final PCE of 20.7% after ageing and dark storage, which is slightly higher than the original efficiency. This approach represents an advancement in the understanding of the role of electron selective layers on the efficiency and stability of PSCs. Therefore, the newly proposed approach constitutes a simple, scalable method paving the way for industrialization of perovskite solar cells.
Introduction PSCs composed of organic-metal-halide materials have made impressive progress in just a few years with maximum power conversion efficiencies (PCEs) jumping from 3.8%1 in 2009 to a certified 22.1%2 in 2016. Such rapid progress is unprecedented for any photovoltaic (PV) material2 causing much excitement in a
Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, ´cole Polytechnique Fe´de´rale de Lausanne, CH-1015-Lausanne, Switzerland. E E-mail:
[email protected],
[email protected] b Department of Materials Engineering, Isfahan University of Technology, Isfahan, 84156-83111, Iran c Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, ´cole Polytechnique Fe´de´rale de Lausanne, CH-1015-Lausanne, Switzerland E d Advanced Research Division, Panasonic Corporation, 1006, (Oaza Kadoma), Kadoma City, Osaka 571-8501, Japan e Adolphe Merkle Institute, University of Fribourg, CH-1700 Fribourg, Switzerland † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c6ee02390h
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the photovoltaic field. The general formula of the commonly used perovskite is ABX3 containing an organic cation A, such as methylammonium (MA) or formamidinium (FA),1,3 a divalent metal B, such as Pb or Sn,4 and a halide X, such as Br or I. Perovskite materials can be processed by a large number of techniques, mostly low temperature, solution-based, and therefore they are ideal for commercialization. These techniques are comprised of spin coating,5 dip coating,6 2-step interdiffusion,7 thermal evaporation,8,9 and vacuum-induced crystallization.10 The impressive performances achieved to date have been attributed to exceptional material properties such as low charge recombination, high light absorption over the visible spectrum and charge carrier diffusion lengths in the micrometre range.11–13 In the past year, tremendous progress has been made for different device configurations, including the classic mesoporousinfiltrated and mesoporous-free ‘‘planar’’ configurations. For the latter, we have recently shown planar devices employing atomic layer deposited (ALD) SnO2 as the electron selective layer yielding efficiencies of 19.5%.14,15 SnO2 was shown to exhibit well-aligned conduction bands of the perovskite and SnO2 materials in a planar configuration, which is critical for making PSCs with high
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stabilized power output. This method proved useful for application of the perovskite solar cell as a top cell in a monolithic tandem device with an amorphous silicon (a-Si) sub-cell and an ITO tunneling recombination layer, which requires low temperature processing to avoid heat-induced damage of the stack. The fabricated tandem a-Si/perovskite solar cells yielded efficiencies of ca. 20%.16 Because ALD is costly and not easily scalable, it renders the method more suited for establishing and prototyping proof-of-concept architectures. Therefore, the lack of a simple method to deposit electron selective layers is an obstacle for widespread implementation and large-scale commercialization of PSCs, and there is a need to find alternative methods available to every research group. On the other hand, while the PCE of devices has rapidly achieved remarkable values, this has not been matched by equal developments in long-term stability.17 At this stage, operational stability is considered one of the main challenges for PSCs to become a serious contender on the PV market.18 Recent breakthroughs on stability in the field have shown promise for devices using a mesoporous layer and mixed ion perovskites,19 as well as the use of more robust contacts for high temperature tests.20–22 However, there are hardly any studies on the stability of planar PSCs that go beyond dark ‘‘shelf stability,’’14,23 raising the question of whether this device configuration will be able to rival its mesoporous counterpart. Here, we report highly efficient planar perovskite solar cells based on SnO2 electron selective layers prepared through a simple solution-processing method, reaching a PCE close to 21% which is the highest efficiency for any planar configuration reported to date. Briefly, our chemical bath deposition (CBD) yields SnO2 films in short times, resulting in blocking layers with high selectivity supporting the highest reported voltages so far of 1214 mV (at a band gap of 1.62 eV).24 In addition to high voltages, this new method showed high fill factors which is key to matching the efficiencies of the high-temperature, mesoporous counterparts. In addition, PSCs prepared with this facile method showed great stability retaining more than 82%
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of the initial efficiency over 60 hours. Remarkably, the devices showed a final PCE of 20.7% after ageing and several hours of dark, dry air storage, being higher than the initial pre-aged measurement of 20.4%. This novel deposition method represents a breakthrough as it opens up a scalable and inexpensive way to deposit high-quality SnO2 electron selective contacts for remarkably stable PSCs with efficiencies close to 21%.
Results and discussion Schematics of the simple, low temperature, solution processed methods are shown in Fig. 1. The two methods and combinations of them were used to understand their homogeneous blocking properties. The first method is comprised of spin coating (SC) of a SnCl4 precursor solution in isopropyl alcohol, as shown by Ke et al.25 and depicted in Fig. 1a. In a second method a combination of SnO2 SC and a post-treatment chemical bath deposition (SC-CBD),26 as shown in Fig. 1b, was used to improve the conformality and thus blocking capabilities of the SnO2 layer, important for shunt-free PSCs. The layers were further optimized by modifying the solution concentration. More details can be found in ESI,† note 1. SnO2 layers have been prepared on FTO-coated glass substrates using the above methods and an ALD reference. Top-view scanning electron micrographs (SEM) of the different layers are shown in Fig. 1c–e. ALD layers (Fig. 1c) exhibit no clear features compared to the FTO substrate (Fig. S1f, ESI†), due to the conformal nature of this deposition method. The SnO2 layers by SC and SC-CBD, presented in Fig. 1d and e, display a clear change in the morphology of the substrate, indicating a rougher surface than the ALD-deposited controls. The typical cassiterite SnO2 structure is detectable for all thin films in the XRD patterns in Fig. S2 (ESI†). X-ray photoelectron spectroscopy (XPS, Fig. S3, ESI†) exhibits the survey of elements and the high resolution of Sn and O confirms the formation of pure SnO2 observing the oxygen peak O 1s at 531.0 eV and Sn4+ peaks at 495.3 eV as well as at 487.0 eV.
Fig. 1 Schematic illustration of different steps in all-solution processed methods of SnO2 thin film deposition. (a) Spin coating (SC) and post-annealing at 180 1C and (b) chemical bath deposition (CBD) by heating in 70 1C lab oven followed by post-annealing at 180 1C. Top view scanning electron micrographs are presented for SnO2 layers deposited by (c) atomic layer deposition (d) spin coating and (e) spin coating and chemical bath deposition. All scale bars are 200 nm.
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Fig. 2 Fabrication of PSCs based on solution-processed low temperature SnO2. (a) Top-view SEM images of the low temperature layer by layer deposition process: Bare FTO, SnO2 deposited by spin coating and a chemical bath deposition (SC-CBD) on FTO and the perovskite crystals grown atop. (b) Schematic illustration and (c) cross-sectional SEM of a typical planar perovskite solar cell architecture FTO/ESL/perovskite/spiro-MeOTAD/Gold, scale bar in all SEM images is 200 nm. (d) Photograph of a typical device with a two-pixel configuration and an active area of each pixel of 0.25 cm2.
Various SnO2-coated substrates were used to fabricate PSCs to compare the influence of the different deposition methods of the ESL on the device performance. The ESL deposition is followed by coating of a solution-processed perovskite layer based on the Cs-containing mixed triple cation and iodide/ bromide formulation, and an antisolvent procedure.19,27 The perovskite layers are composed of large crystals in the range of 300 nm as seen in Fig. 2a. No change in the perovskite morphology (Fig. S4, ESI,† SEM) and optical properties (Fig. S5, ESI†) can be observed with respect to the different ESLs. The devices were completed by depositing a 200 nm layer of doped Spiro-OMeTAD as the hole transporting material (HTM), followed by the thermal evaporation of an 80 nm gold layer, as depicted in the schematic in Fig. 2b. Fig. 2c exhibits a crosssectional SEM image of a typical device, employing a thin layer (ca. 30 nm) of SnO2 by SC-CBD, a perovskite layer as absorber and charge transporter (ca. 450 nm), with single grains sandwiched between ESL and HTM, and a gold electrode. Corresponding cross-sectional images of devices employing all studied ESLs are shown in Fig. S6 (ESI†). Regardless of the deposition method, a thin SnO2 layer no thicker than 40 nm is clearly visible. ALD layers are ultra thin, smooth and compact at around 10–15 nm, whereas the solution-processed layers are thicker and rougher. A photograph is exhibited in Fig. 2d (back-side view) of a completed device containing two devices with active areas defined by the square Au electrode (0.25 cm2). Current–voltage hysteresis (forward and backward scans) and maximum power point tracking (MPPT) measurements at one sun illumination for ALD devices are shown in Fig. 3a and Table S1 (ESI†). Mild hysteresis (slightly lower voltage and fill factor, FF, in the forward compared to the backward scan) is shown, however the devices exhibited a high PCE of 19% under MPPT. Devices prepared with SC-CBD SnO2 showed consistently higher PCEs, with the champion device reaching 20.8% (Fig. 3b and Table S1, ESI†), surpassing the ALD controls. In addition, a
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high open-circuit voltage was found for the unmasked28 champion device (Fig. 3c) achieving a record 1214 mV at the time of writing, showing the high level of selectivity of this layer. The increase in VOC over the first 40 seconds is ascribed to the hysteresis effect, as ions reorganize after a J–V curve is taken.29,30 Given a bandgap of 1.62 eV for this material,19 and a thermodynamic maximum VOC of around 1.32 V, the voltages achieved for these solution-processed materials are remarkable.13 Because reproducibility is key for future development, we optimized the solution processing to avoid poor coverage of the SnO2 layers. The use of SC and the dilute CBD solution showed more robust blocking properties than all other solution-processed deposition methods (Fig. S7, ESI†), and the lowest number of shunts, thus the highest yield of working solar cells. Devices with the more reproducible, dilute SC-CBD deposition method also yielded efficiencies of 20% as shown in Fig. S8 and Table S1 (ESI†). The higher PCE of SC-CBD SnO2 devices compared to ALDbased devices is confirmed by statistics of at least 18 devices as shown in Fig. 3d. The lower performance for ALD PSCs is mainly due to low FFs (Fig. 3e), which can be ascribed to high series resistance (Fig. 3f) possibly due to a more resistive and very homogenous conformal SnO2 layer compared to SC and SC-CBD. For the latter, higher roughness might allow for an increased contact of the ESL with the perovskite material during processing. On the other hand, the higher series resistance for ALD PSCs could be due to a lack of contact between the perovskite layer and the ESL during processing. This effect could be ruled out given the good contact between the two layers as seen from cross-sectional images (Fig. S6, ESI†). However, it is possible that this effect in morphology (improved contact between perovskite and ESL) is not visible in the magnifications studied here, and therefore, we cannot exclude completely that morphological changes in the solution processed ESLs are responsible for the higher FFs. Clearly, while ALD SnO2 can yield as high open-circuit voltages as SC-CBD (approaching 1.2 V; Fig. 3g),14
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Fig. 3 Current–voltage, stabilized power output characteristics and statistics of planar PSCs. The J–V curves and the maximum power point (MPP) tracking (as the inset) of the champion planar perovskite solar cells based on (a) ALD SnO2 (b) spin coated SnO2 and chemical bath (SC-CBD) post-treatment. (c) Open-circuit voltage (VOC) tracking of an unmasked SC-CBD champion device measured at 20 1C and normalized for the JSC of the masked measurement. Statistical parameters of (d) power conversion efficiency (PCE), (e) fill factor, (f) series resistance approximated by the slope of the J–V curves at 1.15–1.25 V, (g) open-circuit voltage, and (h) hysteresis (defined as the difference in efficiency between the backward and forward scan) for devices employing SnO2 layers fabricated by 3 different deposition methods. Except where specified, all measurements were performed using an aperture mask of 0.16 cm2 at room temperature in air.
the PCE of ALD PSCs is limited, in general, by the lower FFs (Fig. 3e), making it difficult to compete with high efficiency mesoporous devices.19,31,32 ALD-based PSCs also showed more variability in PCE, as shown in the spread of values in Fig. 3d, mostly due to fill factor and attributed to the higher series resistance for this group. Whereas all devices with solution-processed SnO2 layers benefit from an increased FF approaching 80%, the opencircuit voltage of the SC-only sample is lower than for its ALD analogues (Fig. 3g), suggesting faster recombination dynamics for these PSCs. Hence, the post-treatment implemented in this work by CBD proves essential to reducing the variability in the properties of the SC-blocking layer (seen at the series resistance) and achieving high voltages while not compromising on JSC or FF. No major difference in JSC was measured for devices employing ESLs by the different methods, as seen in Fig. S9c (ESI†). Good agreement of the external quantum efficiency (EQE) and measured JSC was found for devices employing SC-CBD ESLs (Fig. S10, ESI†). While the best devices presented reduced hysteresis with almost no difference in the backward and forward scan, devices with lower performance showed more hysteresis behavior in
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each group. Interestingly, hysteresis (calculated here as the difference between the efficiency of the backward and forward scans) varied substantially for the different deposition methods as reported in Fig. 3h. Devices using ALD and SC SnO2 showed a larger distribution in the difference between the backward and forward scans, whereas the SC-CBD exhibited less hysteresis. The trend in hysteresis correlates with the trend in fill factor and the effective series resistance. For low series resistance, the FF is barely limited by charge transport but approaches its limit defined by recombination (81–83%), as is almost the case for the best SC-CBD devices. In this case, charge collection does not depend on the electric field and thus it’s voltage dependent. Then, the JV curve is less prone to hysteresis whose origin is a modified charge collection efficiency depending on the electric field in the device that is influenced by the biasing history.29 Devices based on the SnO2 deposited by SC-CBD, with PCEs above 20% (Fig. 4a and Table S2, ESI†) were rigorously aged under MPPT and visible light illumination at 100 mW cm 2 intensity for over 60 hours at 20 1C, to avoid thermally induced gold diffusion21 (we are currently investigating alternative HTM/back contact combinations for high temperature testing).21 As seen in Fig. 4b, device performance dropped from 20.4% to
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Fig. 4 Ageing of a solution processed SnO2 high performance device. (a) J–V hysteresis characteristics before aging measured in air at room temperature. (b) Long-term ageing under MPPT and constant 1 sun equivalent, with N2 flow at 20 1C. (c) J–V hysteresis characteristics after aging measured in air at room temperature. J–V curves shown were measured at 10 mV s 1.
ca. 17% in 60 hours, similar to the ageing behavior of hightemperature mesoporous PSCs.19 Similarly, devices employing ALD and different solution-processed ESLs did not show significant degradation (Fig. S11, ESI†). During the ageing procedure, J–V curves were taken every 60 min and their parameters are plotted against time in Fig. S11 (ESI†). For all devices VOC and JSC remain constant over time, with FF showing the greatest deterioration. After ageing, the devices were stored in dark conditions and dry air and remeasured after 8 hours. The J–V curves are presented in Fig. S11 and Table S2 (ESI†). All devices, show recovery with respect to the efficiency measured before the ageing test. However, the SC-CBD sample shows complete recovery from a PCE of 20.4% to 20.7% (Fig. 4c) after being aged for over 60 hours, suggesting that the devices can recover under operational conditions of day/night cycles. This phenomenon has been recently reported by Huang and coworkers33 and is the subject of an ongoing study by our group. In addition, dark, shelf stability testing of devices stored in dry air and measured periodically exhibited impressive stability, with SC-CBD devices maintaining over 20% efficiency after more than 90 days (Fig. S12, ESI†).
Conclusions We developed a simple, fast, low temperature, industry-scalable method to deposit SnO2 layers. The best combination of spincoating and a chemical bath post-treatment (SC-CBD) PSCs yielded efficiencies close to 21%, gaining in FF when compared to the conformal blocking layers deposited by ALD. Compared to the SC-only method, higher VOC values could be achieved. The ALD method exhibited consistently high VOC’s, which is a testament to the reproducibility of conformal layers by this technique, whereas the SC-CBD showed consistently higher FFs. The combined SC-CBD deposition method yielded an ideal combination of all photovoltaic parameters with improved hysteresis and promising long-term stability under continuous visible light irradiation, when compared to all other deposition methods. This work elucidates the importance of the ESL as a
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means to achieve high efficiencies and stability of planar PSCs. Therefore, the newly proposed all-solution based approach represents a key contribution and a scalable method on the way towards PSC industrialization.
Experimental section Glass preparation F:SnO2 (FTO) substrates (NSG-10) were chemically etched with zinc powder and 4 M HCl solution and then cleaned through immersing in piranha solution (H2SO4/H2O2 = 3 : 1) for 10 min. All substrates were further cleaned by UV-ozone for 15 min before deposition of SnO2. Preparation of SnO2 by ALD SnO2 control planar devices were deposited through atomic layer deposition of Tetrakis(dimethylamino)tin(IV) (TDMASn, 99.99%-Sn, Strem Chemicals INC, heated at 55 1C) and ozone in a Savannah ALD 100 instrument (Cambridge Nanotech Inc.), at 118 1C. Oxygen gas (99.9995% pure, Carbagas) was used for production of ozone (13% in O2) by a generator (AC-2025, IN USA Incorporated). The carrier gas was Nitrogen (99.9999% pure, Carbagas) with a flow rate of 10 sccm. The growth rate (0.065 nm per cycle) was measured by ellipsometry. Preparation of SnO2 by spin coating (SC) A 0.05 M was prepared by dissolution of SnCl4 5H2O (Sigma) in isopropyl alcohol (Sigma). After 30 min stirring at room temperature it was deposited on cleaned FTO substrates with 3000 rpm spin rate for 30 s with a 200 rpm s 1 ramp, followed by pre-drying at 100 1C for 10 min and then heat-treated at 180 1C for 1 h. Preparation of SnO2 by chemical bath (CBD) SnO2 layers were grown by chemical bath deposition either on FTO directly or as a post-treatment on spin-coated layers. 0.5 g urea was dissolved in 40 ml deionized water, followed by the
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addition of 10 ml mercaptoacetic acid and 0.5 ml HCl (37 wt%). Finally, SnCl22H2O was dissolved in the solution at 0.012 M (labeled in the text as CBD) and 0.002 M (dilute CBD, where the 0.012 M was diluted 6 times) followed by stirring for 2 min. The deposition was made by putting the substrates vertically in a designed glass container filled with the above solution, in a 70 1C lab oven for 3 h. To optimize thickness, uniformity and final device performance of the CBD layer, solution concentration was varied. The treated substrates were rinsed in a sonication bath of deionized water for 2 min to remove any loosely bound material, dried in a stream of dry air and annealed for 1 h at 180 1C. All SnO2 layers were cleaned with an UV-ozone treatment for 15 min immediately before deposition of the perovskite films.
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Mixed-cation lead mixed-halide perovskite solution was prepared from a precursor solution made of FAI (1 M, Dyesol), PbI2 (1.1 M, TCl), MABr (0.2 M, Dyesol) and PbBr2 (0.22 M, TCl) in a 4 : 1 (v : v) mixture of anhydrous DMF : DMSO (Acros). Then 1.5 M stock solution of CsI (abcr GmbH) in DMSO was added to above solution in 5 : 95 volume ratio. This triple cation perovskite solution was deposited through a two step spin coating program (10 s at 1000 rpm and 20 s at 6000 rpm) with dripping of chlorobenzene as anti-solvent during the second step, 5 s before the end. All the perovskite layers annealed at 100 1C for 45 min.
was applied before starting the measurement, such as light soaking or forward voltage bias. The starting voltage was determined as the potential at which the cells furnished 1 mA in forward bias (normally between 1.2 and 1.3 V), no equilibration time was used. The cells were masked with a black metal mask limiting the active area to 0.16 cm2 and reducing the influence of the scattered light. The photocurrent density was scaled to 1000 W m 2. Aging under maximum power point tracking was performed on masked devices which were mounted on a temperature controlled plate. The aging was performed under nitrogen atmosphere and 1-sun equivalent illumination provided by an array of white LEDs. During the aging time the devices were kept under maximum load under illumination. The maximum power point was updated every 4 s by recording the current response to a small perturbation in potential. Additionally, a full J–V scan was taken every 60 min (at a scan rate of 100 mV s 1 starting from forward bias) which was used to extract the displayed parameters for the aging data. The EQE spectra were measured under constant white light bias with an intensity of 10 mW cm 2 supplied by a LED array. The superimposed monochromatic light was chopped at 2 Hz. The homemade system comprises a 300 W Xenon lamp (ICL Technology), a Gemini-180 double-monochromator with 1200 grooves per mm grating (Jobin Yvon Ltd) and a lock-in amplifier (SR830 DSP, Stanford Research System).
Hole transporting layer and top electrode deposition
Scanning electron microscopy (SEM)
The spiro-OMeTAD (Merk) (70 mM solution in chlorobenzene) doped at a molar ratio of 3.3, 0.5 and 0.03 with 4-tert-butylpyridine (TBP, Sigma Aldrich), bis(trifluoromethylsulfonyl)imide lithium salt (Li-TFSI, Sigma Aldrich) and tris(2-(1H-pyrazol-1-yl)4-tert-butylpyridine)-cobalt(III) tris(bis(trifluoromethylsulfonyl)imide) (FK209, Dyenamo), respectively and deposited by spin coating (4000 rpm for 20 s) as hole transport layer on top. An 80 nm-thick gold top electrode was deposited as top contact electrode by thermal evaporation under high vacuum.
Characterization of device and ESL morphology was performed with a ZEISS Merlin HR-SEM.
Cs/MA/FA perovskite precursor solution and film deposition
Optical measurements The photoluminescence spectra were measured with a Fluorolog 322 (Horiba Jobin Ybon Ltd) with a wavelength range from 470 nm to 850 nm by an exciting wavelength of 460 nm. The samples were mounted at 601 and the emission recorded at 901 from the incident beam path. UV-vis measurements were performed on a Varian Cary 5 in the wavelength range of 300 to 800 nm. Solar cell characterization The solar cells were measured using a 450 W xenon light source (Oriel). The spectral mismatch between AM 1.5G and the simulated illumination was reduced by the use of a Schott ¨zisions Glas & Optik GmbH). The light K113 Tempax filter (Pra intensity was calibrated with a Si photodiode equipped with an IR-cutoff filter (KG3, Schott) and it was recorded during each measurement. Current–voltage characteristics of the cells were obtained by applying an external voltage bias while measuring the current response with a digital source meter (Keithley 2400). The voltage scan rate was 10 mV s 1 and no device preconditioning
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Cyclic voltammetry (CV) measurements were carried out with a SP-200 (BioLogic) in a 3-electrode configuration in the dark. The SnO2 working electrode was contacted via a soldered Cu-wire that was insulated with epoxy. The epoxy also determined the surface area exposed to the electrolyte solution. The area was normally 1 cm2. A Pt-wire was used as the counter electrode and a Ag/AgCl (KCl sat’d) as the reference electrode. The electrolyte solution was 1 mM K4Fe(CN)6 + 1 mM K3Fe(CN)6 in aqueous 0.5 M KCl, pH 7 and was thoroughly purged with argon prior to the CV measurements. 3–5 cycles were recorded whereof usually the 3rd cycle is reported showing an equilibrated and stabilized CV scan. Fe(CN)63 /4 was shown to be a suitable redox couple for the testing of the presence of pinholes in TiO234 and SnO2 (work has been submitted for publication) thin films. X-ray diffraction (XRD) The samples were analyzed by XRD using a Bruker D8 Advance X-ray diffractometer using Cu Ka radiation (l = 0.154178 nm) at a scanning rate of 0.021 s 1 in the 2y range from 101 to 801.
Acknowledgements E. H. A. acknowledges the Ministry of Science, research and technology of Iran and Iran Nanotechnology Initiative Council
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for the financial support. M. S. acknowledges support from the co-funded Marie Skłodowska Curie fellowship, H2020 Grant agreement no. 665667. A. A. received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no. 291771. Financial support is acknowledged from the Swiss National Science Foundation (SNSF), funding from the framework of Umbrella project (Grant Agreement no. 407040-153952 and 407040-153990); the NRP 70 ‘‘Energy Turnaround’’; the in the 9th call proposal 906: CONNECT PV as well as from SNF-NanoTera and Swiss Federal Office of Energy (SYNERGY). M. G. and W. T. thank the King Abdulaziz City for Science and Technology (KACST) for financial support under a joint research project. K. D. thanks the SNF for funding within the framework of Umbrella project (grant agreement no. 407040-153952 and 407040-153990).
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