Impact of Film Thickness of Ultrathin Dip-Coated Compact TiO2 Layers

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Impact of film thickness of ultra-thin dip-coated compact TiO2 layers on the performance of mesoscopic perovskite solar cells Muhammad Talha Masood, Christian Weinberger, Jawad Sarfraz, Emil Rosqvist, Simon Sandén, Oskar J Sandberg, Paola Vivo, Ghufran Hashmi, Peter D. Lund, Ronald Österbacka, and Jan-Henrik Smått ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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ACS Applied Materials & Interfaces

Impact of film thickness of ultra-thin dip-coated compact TiO2 layers on the performance of mesoscopic perovskite solar cells Muhammad T. Masood,1,2 Christian Weinberger,1 Jawad Sarfraz,1 Emil Rosqvist,1 Simon Sandén,3 Oskar J. Sandberg,3 Paola Vivo,4 Ghufran Hashmi,5 Peter D. Lund,5 Ronald Österbacka,3 Jan-Henrik Smått1,* 1

Laboratory of Physical Chemistry, Faculty of Science and Engineering and Center for

Functional Materials, Åbo Akademi University, Porthansgatan 3-5, 20500 Åbo, Finland. 2

Department of Materials Engineering, School of Chemical & Materials Engineering, National University of Science & Technology (NUST), H-12 sector, 44000, Islamabad, Pakistan 3

Physics, Faculty of Science and Engineering and Center for Functional Materials, Åbo Akademi University, Porthansgatan 3-5, FI-20500 Åbo, Finland.

4

Laboratory of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FI-33101 Tampere, Finland. 5

New Energy Technologies Group, Department of Applied Physics, Aalto University and School of Science, P.O. Box 15100, FI-00076, Aalto (Espoo), Finland.

*E-Mail: [email protected] KEYWORDS: perovskite solar cells, compact TiO2, TiCl4, pinhole free, dip coating,

ACS Paragon Plus Environment

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ABSTRACT

Uniform and pinhole-free electron selective TiO2 layers are of utmost importance for efficient perovskite solar cells. Here we used a scalable and low-cost dip coating method to prepare uniform and ultra-thin (5−50 nm) compact TiO2 films on fluorine doped tin oxide (FTO) glass substrates. The thickness of the film was tuned by changing the TiCl4 precursor concentration. The formed TiO2 follows the texture of the underlying FTO substrates, but at higher TiCl4 concentrations, the surface roughness is substantially decreased. This change occurs at a film thickness close to 20–30 nm. A similar TiCl4 concentration is needed to produce crystalline TiO2 films. Furthermore, below this film thickness, the underlying FTO might be exposed resulting in pinholes in the compact TiO2 layer. When integrated into mesoscopic perovskite solar cells, there appears to be a similar critical compact TiO2 layer thickness above which the devices perform more optimally. The power conversion efficiency was improved by more than 50% (from 5.5% to ~8.6%) when inserting a compact TiO2 layer. Devices without or with very thin compact TiO2 layers display J-V curves with an “s-shaped” feature in the negative voltage range, which could be attributed to immobilized negative ions at the electron-extracting interface. A strong correlation between the magnitude of the s-shape feature and the exposed FTO seen in the x-ray photoelectron spectroscopy measurements indicates that the s-shape is related to pinholes in the compact TiO2 layer when it is too thin.

INTRODUCTION Perovskite solar cells (PSCs) have received great attention due to their potentially low production costs and high efficiencies, which have skyrocketed since the first reports on solar


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cells containing perovskites as sensitizer with 3.8% efficiency in 2009

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(later improved to a

stable solid-state PSC reported with 9.7% efficiency in 2012).2 Today, the record efficiencies reach above a remarkable 22%3,4. Perovskite solar cells (PSCs) have received great attention due to their potentially low production costs and high efficiencies (over 22%) achieved only within 5 years after the first PSC was reported with 3.8% efficiency in 2009.5 The hybrid organicinorganic perovskite materials used in PSCs (typically methylammonium lead halide, CH3NH3PbX3, with X=Cl, Br, I) exhibit high absorption coefficients, long charge carrier lifetimes, and are compatible with solution-based fabrication.6 Currently, the main obstacles the PSC development needs to overcome are related to the poor stability, reproducibility, and limited scalability of the currently applied deposition methods (such as spin coating). Additionally, the charge transport mechanisms and related features (e.g. hysteresis in the J-V curves) are still under investigation.7,8 Among the traditional and alternative device architectures6, the mesoscopic heterojunction PSC design (Figure 1) has been reported mostly for very high efficiencies9 which comprises of a 30-50 nm compact electron selective layer deposited on Fluorine doped tin oxide (FTO) coated transparent glass substrate, constituting the cathode. On top of this compact layer, a mesoporous scaffold layer (mostly based on TiO2,10,11 Al2O310 or ZnO12,13 nanoparticles) is then applied. This mesoporous structure is then infiltrated with the perovskite precursor solution in such a way that crystals of perovskite light absorbing layer grow within the porous scaffold followed by forming a uniform capping layer (50-300 nm) which prevents a direct contact between the mesoporous scaffold and the hole selective layer (typically spiro-OMeTAD, (2,2´,7,7´-tetrakis-(N,N-di-pmethoxyphenylamine) 9,9´-spirobifluorene)). Finally, gold is evaporated on top of the device to serve as the anode.11


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Figure 1. Scheme of a typical device structure of a mesoscopic perovskite solar cell (left) and a picture of a real sample (right).

Presently, theTiO2 (anatase) nano particle based mesoporous layer is the most commonly used electron selective contact material due to its high stability and electron mobility,14-17 whereas other materials (e.g. SnO218 and ZnO19) have also been investigated. The function of the compact TiO2 layer (c-TiO2) is to selectively allow the generated electrons to be extracted at the conductive FTO substrate, while effectively blocking the holes.20,21 Therefore, it is very important that the compact TiO2 layers (c-TiO2) have a well-defined and pinhole-free structure in order to avoid surface recombination at the contact interface.18,22 Compact TiO2 layers of different quality can be prepared using a range of deposition methods with varying complexity, including spray pyrolysis,11,22,23 atomic layer deposition,15,24,25 electrochemical deposition,26 thermal oxidation,25,26 and spin coating.15,27 Although the dip coating method would offer many benefits compared to these (e.g. solution processing, better scale-up possibilities, and less waste), there are surprisingly few studies on utilizing dip coating in the preparation of c-TiO2 layers for PSCs.8,28 In these studies, devices made by the dip coating


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method showed better performance and reproducibility when compared to devices with spincoated c-TiO2 layers due to a more uniform surface coverage of the underlying FTO layer. However, only relatively thick c-TiO2 layers (40−130 nm) were investigated and the conclusion was that the device performance worsened with increased layer thickness due to higher probability for charge trapping and subsequent recombination as well as increased light blocking.8 In the present study, we investigate how ultra-thin (5−50 nm) c-TiO2 layers prepared by the dip coating method influence the performance of PSCs. We are interested in knowing the minimum c-TiO2 layer thickness for efficient charge selectivity and when possible pinholes are eliminated. In contrast to the previous studies, we utilize titanium tetrachloride (TiCl4) as the TiO2 precursor, which has been shown to result in devices with superior performance compared to devices made from for instance titanium isopropoxide (e.g. less hysteresis and more stable devices).29 The produced c-TiO2 films are thoroughly characterized by x-ray reflectometry (XRR), atomic force microscopy (AFM), grazing incidence x-ray diffraction (GI-XRD), and xray photoelectron spectroscopy (XPS), and the results are related to the photovoltaic performance of PSCs made from these layers.

EXPERIMENTAL Materials FTO substrates (TEC 7) and microscope slides were purchased from Solaronix and VWR International, respectively. TiCl4 (>99%) was obtained from Fluka, while Pluronic F-127 block co-polymer, bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI), 2-propanol (anhydrous, 99.5%), chlorobenzene (anhydrous, 99.8%), acetonitrile (anhydrous, 99.8%), and N’N-


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dimethylformamide (DMF, anhydrous, 99.8%) were all purchased from Sigma-Aldrich. PbI2 (99.99%) and methylammonium iodide (MAI, >98%) were purchased from TCI Europe, while 30 NR-D TiO2 nanoparticle paste and tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] (FK209 Co III, > 98%) from Dyesol and spiro-OMeTAD (99%) from Feiming Chemical Ltd. Furthermore, ethanol (>99.5%, ALTIA Oyj), tetrahydrofuran (THF, > 99%, Sigma-Aldrich), and Millipore water were used as solvents in the c-TiO2 film preparation.

Preparation of TiCl4 dip coating solutions For the preparation of the compact TiO2 layer, TiCl4 (18.97 g) was added dropwise into ethanol (23.04 g) while stirring at 0 °C. Another solution containing ethanol (13.44 g), water (0.21 g), THF (1.72 g) and Pluronic F127 block co-polymer (15.2 mg) was prepared at room temperature. The first solution (TiCl4/ethanol; 0.50 g) was added in to the second, giving a TiO2 layer with a thickness of about 5 nm after dip coating. The solutions for the thicker films were prepared by increasing the molar ratio of TiCl4 precursor while keeping the same amounts of other species in the final solution. The molar ratios of all the species in the five different solutions and the names of the corresponding solutions are shown in Table 1.


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Table 1. Molar composition of the TiCl4 solutions to prepare dense TiO2 layers with varying thickness. solution

TiCl4

EtOH

water

THF

Pluronic F127

[mol]

[mol]

[mol]

[mol]

[mol]

TiCl4-1

1.0

250.0

9.8

20.0

1.0·10-4

TiCl4-2

2.0

250.0

9.8

20.0

1.0·10-4

TiCl4-3

3.0

250.0

9.8

20.0

1.0·10-4

TiCl4-5

5.0

250.0

9.8

20.0

1.0·10-4

TiCl4-7

7.0

250.0

9.8

20.0

1.0·10-4

Preparation of c-TiO2 layers 4 cm x 2 cm FTO glass substrates were selectively laser-etched using Cencorp 300 LM setup with 20 W ytterbium fiber laser (speed: 500 mm/s, power: 10%, frequency: 20 kHz, line spacing 25 µm). The etched region was 1.5 x 2 cm2 from one end of the FTO glass. The etched FTO substrates were then sonicated using an aqueous solution of Hellmanex III solution (2%), acetone, and 2-propanol for 10 min each at room temperature. Substrates were dried with nitrogen. Prior to deposition, the substrates were plasma treated for 5 min each.

The substrates were dip-coated with the solutions summarized in Table 1 using a withdrawal speed of 85 mm min-1 while keeping the relative humidity between 8-12%. To avoid film deposition on the back side and the anode region of the FTO substrates, these parts were covered with Kapton tape during the coating process. After dipping, the substrates were kept in the dipping chamber for 15 min and then transferred to a hot plate at 120 °C for about 10 min and then annealed at 500 °C for 30 min in air.


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In order to further characterize the compact TiO2 layers with x-ray reflectivity (XRR) measurements, 5 cm x 2 cm microscope glass slides were dip-coated with the same solutions and parameters as used for the FTO substrates (Table 1). It should be noted that the thickness of the compact TiO2 layers on top of FTO substrates cannot be determined by XRR measurements due to the high surface roughness of the substrates.

Device fabrication A mesoporous TiO2 scaffold layer was prepared by spin coating an ethanolic suspension (150 mg mL-1) of 30 NRD TiO2 nanoparticle paste at 4000 rpm.29 The films were dried on a hot plate at 120 °C for 10 min and then calcinated at 450 °C for 30 min (the detailed program is summarized in the Table S1 of Electronic Supplementary Information). Afterwards, the substrates were transferred into a glove box to prepare the rest of the device in dry nitrogen atmosphere. The perovskite layer was obtained with two-step sequential deposition method.11 1 M solution of PbI2 in anhydrous N,N-dimethylformamide was prepared by stirring at 100 °C for 2 h to ensure complete dissolution of PbI2. The solution was spin-coated onto the mesoporous TiO2 layer at 6000 rpm followed by heat treatment at 100 °C for 30 min on a hot plate. Afterwards, the samples were dipped in 10 mg mL-1 solution of methylammonium iodide in anhydrous 2propanol for 1 minutes to grow the perovskite crystalline layer. The samples were rinsed with 2propanol to remove excess CH3NH3I and were then dried on preheated hotplate at 100 °C for 30 minutes to ensure the complete growth of perovskite layer. The estimated values of the mesoporous TiO2 layer including the perovskite is roughly 150 nm and the capping layer is expected to be around 70 nm.


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Spiro-OMeTAD and 4-tert-butyilpyridine were added into chlorobenzene and mixed with lithium bis(trifluoromethylsulphonyl)imide and FK209 Co (III) which were previously dissolved in acetonitrile to give a molar composition of 1.0: 0.5: 2.5·10-2: 3.3: 131.5: 7.2 (spiro-OMeTAD: Li-TFSI: FK 209 Co III: 4-tert-butyilpyridine: chlorobenzene: acetonitrile). The final solution was stirred for about 10 min and then spin-coated at a speed of 4000 rpm for 30 s.11 Finally, a 60 nm thick gold film was deposited by thermal evaporation on top of the spiroOMeTAD layer to form the back contact. The evaporation was carried out in vacuum at 2.5·10-5 bar with the evaporation rate of 0.1 Å/s. The shape of the gold contacts was circular with the diameter of 0.6 cm, i.e. the area of each contact was 0.28 cm2.

CHARACTERIZATION Characterization of c-TiO2 layers Grazing incidence x-ray diffraction (GI-XRD) patterns of the compact TiO2 layers on FTO substrates were collected on a Bruker AXS D8 Discover instrument. The data were collected in the 2θ range between 24-40° using a step size of 0.04° and a grazing incidence angle of 0.3°. TOPAS P software (v.4.2) was used for calculating the TiO2 crystallite size based on the Scherrer equation. The same instrument was used in the x-ray reflectivity measurements (XRR) of the compact TiO2 layers on microscope glass slides. The reason for utilization of the glass slides is that the roughness of the FTO substrates is too high to observe any clear Kiessig fringes. A 2θ/ω scan was conducted with an increment of 0.002°. The experimental data was fitted using LEPTOS software (v.7.03). The surface structure of the c-TiO2 layers on FTO substrates was characterized with a Prima (NT-MDT) atomic force microscope (AFM) using intermittent-contact mode. Imaging was done


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under ambient conditions in air. NSG10 (NT-MDT) cantilevers with a nominal tip radius of 10 nm were used. A scan speed of 0.300 Hz or 0.391 Hz was used to obtain images with a 1024 × 1024 pixel resolution.. SPIP 5.1.3 image analysis software (Image Metrology) was used to process and analyze the obtained images. In order to quantify the morphology and roughness, the following parameters were used: the RMS (root-mean-square) surface roughness (Sq), surface area ratio (Sdr), autocorrelation length (Scl37), which is defined as the length for which the autocorrelation coefficient has decayed to 1/e (∼37%), as well as the correlation length normalized RMS roughness (Sq/Scl) that gives a length-scale independent roughness measure.30 Void volume (Vv), is the volume in the valleys of the surface void of matter when excluding the topmost 10 % of the height (bearing area) in the Abbott curve.31

X-ray photoelectron spectroscopy (XPS) spectra were obtained with a PHI Quantum 2000 scanning spectrometer, using monochromatic Al Kα x-ray source (1486.6 eV) excitation and charge neutralization by using electron filament and an electron gun. The photoelectrons were collected at 45° in relation to the sample surface with a hemispherical analyzer. The penetration depth was approximately ~10 nm. The pass energy was 187.85 and 29.35 eV for survey and high resolution spectra, respectively. The measurements were carried out on three different spots for each sample. The atomic concentration (at-%) of the different elements was derived by calculating the area of the peaks and correcting for the sensitivity factors (using the software MultiPak v6.1A from Physical Electronics). The binding energies acquired in the XPS spectra were corrected using C1s photoelectron peak at 284.8 eV as a reference.


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Characterization of devices The current density against voltage (J-V) curves were measured using 2636 Series SourceMeter (Keithley instruments) under simulated AM 1.5 sunlight at 100 mW cm-2 irradiance using an Oriel Class ABB solar simulator,150W, 2x2". The devices were masked (aperture 0.126 cm2) and scanned in a range between -1.0 to 1.5 V (forward sweep) and 1.5 V to -1.0 (reverse sweep) with a scan speed of 10 mV s-1 in both cases. For each series of samples, 4−12 devices were characterized to determine the photovoltaic parameters which are summarized in Table 3.

RESULTS AND DISCUSSION Compact TiO2 layer XRR measurements (Figure 2) show that the thickness of the c-TiO2 thin films on glass substrates is increasing superlinearly from 5 to 48 nm when the TiCl4 concentration is increased 7-fold. The superlinear increase is likely due to the higher viscosity/density of the dip coating solutions with larger TiCl4 molar ratio. It should be pointed out that the corresponding c-TiO2 layers prepared on FTO substrates could be of slightly different thickness and uniformity due to the larger surface roughness of the FTO glass (see discussion below).


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Figure 2. Thickness (black circles, left axis) and density (blue triangles, right axis) of the c-TiO2 layers as a function of the TiCl4 concentration in the dip coating solution.

In conjunction to the increased film thickness, the density of the TiO2 layers decreases gradually from 3.7 to 3.4 g cm-3. These values are lower in comparison to the bulk density of anatase (3.79 g cm-3 32). This may be attributed to defects (e.g. pores or structural disorder and vacancies) that are generated due to shrinkage while converting the precursor to TiO2 at elevated temperature or due to a non-crystalline nature of the TiO2 structure.33 The XRR data is shown in detail in Figure S1. It is interesting to note that films prepared using TiCl4 as the precursor are typically denser than when using other precursors, which might be beneficial to avoid pinholes, and, thus, increase the performance of the devices.29

Furthermore, Figure S2 depicts the grazing incidence x-ray diffraction patterns of FTO substrates with TiO2 layers with varying thickness. The FTO substrate without a compact TiO2 layer (TiO2-0) exhibits 4 reflections which can be attributed to the tetragonal structure of SnO2 (JCPDS 072-1147). Similar diffractograms can also be observed for the samples with very thin


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c-TiO2 layers (i.e. TiO2-1, TiO2-2, and TiO2-3), suggesting that the thin TiO2 film structure in this case is amorphous. However, for the TiO2-5 and TiO2-7 samples, a clear peak from the 101 reflection of the tetragonal anatase phase can be discerned at ~25.5° 2θ (JCPDS 03-065-5714). Thus, a minimum film thickness has to be reached before crystallization starts. The crystallite size of the anatase phase can be calculated using the Scherrer equation from the full width half maximum of the 101 reflection. The estimated crystallite size is about 20 nm for samples TiO2-5 and TiO2-7.

Table 2. Thickness, density, surface roughness of the FTO (TEC 7) and of the compact TiO2 layers as a function of the TiCl4 concentration. thickness a

density a

crystallite size b

Sq c

[nm]

[g cm-3]

[nm]

[nm] [%]

[µm3]

TiO2-0

-

-

-

34

39

3.5

TiO2-1

5

3.7

-

36

45

3.6

TiO2-2

10

3.7

-

32

28

3.5

TiO2-3

16

3.5

-

30

27

3.6

TiO2-5

31

3.5

20

30

20

3.0

TiO2-7

48

3.4

22

19

8

0.7

sample

Sdr d

Vv e

a

Determined by XRR on glass substrates. b Calculated by Scherrer equation from 101 reflection of the GI-XRD pattern. c Root-mean-square surface roughness, d surface area ratio, and e void volume roughness parameters determined from AFM images.

Figure 3 visualizes the 3D AFM topography measurements on bare FTO substrate (Figure 3a) and with an increasing thickness of c-TiO2 layer (Figure 3b-f). The corresponding AFM height profiles are depicted in Figure S3 and selected roughness parameters are summarized in


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Table 2. The measurements show that the bare FTO (TiO2-0) exhibits a rough surface with a root mean square height (Sq, i.e. equivalent to the standard deviation of heights) of 34 nm and a developed interfacial area ratio (Sdr, i.e. surface area contributed by the texture compared to the planar surface) of 39 %. The void volume (Vv) with a value of 3.5 µm3 is also large. For the TiO2-1 sample, the Sq value slightly increases (by 7 %), as does Sdr (14 %), while Vv increases only by 2 %. By further increasing the TiO2 layer thickness, the roughness parameters are gradually decreasing down to Sq = 19 nm and Scl = 8 % for the TiO2-7 sample. More interestingly, the void volume remains nearly constant (3.5-3.6 µm3) up to TiO2-3 and then drastically decreases to 3.0 µm3 and 0.7 µm3 for the TiO2-5 and TiO2-7 samples, respectively. The AFM measurements clearly show that the TiO2 layer covers the rough surface of the FTO substrate. The small reduction in roughness values at low TiCl4 concentrations indicates that the TiO2 is quite evenly distributed. However, for the two solutions with the highest concentration (i.e. TiCl4-5 and TiCl4-7) the smoothening of the surface and the filling of the voids by TiO2 become particularly evident. These findings are supported by the AFM height profiles which are shown in the Figure S3 and the additional roughness parameters listed in Table S2. It is worth mentioning that the crystallization of the TiO2 seems to start just after the void volume starts to reduce drastically (i.e. for sample TiO2-5). Thus, it seems that the TiO2 within the voids need to exceed a certain volume (in other words a certain film thickness) until the crystallization starts.


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Figure 3. 3D AFM topography images of FTO with increasing TiO2 layer thickness: a) TiO2-0, b) TiO2-1, c) TiO2-2, d) TiO2-3, e) TiO2-5, and f) TiO2-7.

Figure 4 shows quantitative XPS measurements (summarized in Table S3 and Table S4) where the characteristic 3 d3/2 and 3 d5/2 peaks of Sn can be seen at 495 eV and 486 eV, while the 2 p1/2 and 2 p3/2 peaks of Ti are at 464 eV and 459 eV, respectively. These reveal that there is a significant decrease in the intensity of the Sn signals from 17.5 at-% for TiO2-0 (i.e. bare FTO substrate) to 1.24 at-% for TiO2-3 attributed to an increasing thickness of the TiO2 layer on top of the FTO substrate. For even thicker films (e.g. TiO2-5) no residual Sn signals can be detected. At the same time the atomic ratio of Ti gradually increases from zero for the bare FTO substrate to 18.6 at-% for TiO2-5. As the penetration depth of the XPS measurement is in the range of 5– 10 nm, it is possible that a complete surface coverage of the FTO layer has already been reached also for films thinner than TiO2-5 even if small amounts of Sn can be detected in the XPS spectra. Nonetheless, it is more likely that these signals originate from exposed FTO or so called pinholes in the c-TiO2 layer. This will be further discussed in the device characterization section


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below as the exposed FTO has a direct effect on the shapes of the J-V curves and the overall solar cells performance.

Figure 4 XPS high resolution characteristic binding energy peaks for a) Sn and b) Ti from the FTO substrates with compact layers of different thickness and without compact TiO2 (i.e. TiO2-0). c) XPS data in terms of atomic percentage of Sn (blue circles) and Ti (black squares) showing the effect of precursor concentration on TiO2 coverage over FTO (mean values and standard deviation of 3 measurements for each data point; for some points the error bar is smaller than the symbol itself).


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Device characterization The J-V curves of representative devices are compiled in Figure 6, while the photovoltaic parameters and the standard deviations for all devices are summarized in Table 3. From timedependent current and power conversion efficiency plots it can be seen that a maximum current is reached within 70 s and is stable for at least 5 minutes (see Figure S4). The results from the forward sweeps (from -1.0 V to 1.5 V) and the reverse sweeps (from 1.5 V to -1.0 V) at a constant sweep rate of 10 mVs-1 are quite similar (i.e. the hysteresis is small due to a low ion movement, see Figure S4). Thus, in the following paragraph, only the results from the reverse sweeps will be discussed as representative for both measurement directions. The short-circuit current density (Jsc) gradually increases from 9.5 mA cm-2 for devices without a c-TiO2 layer (i.e. TiO2-0) up to 13.2 mA cm-2 for TiO2-7. Likewise, the open-circuit voltage (Voc) increases slightly from 0.93 V to 0.98 V. The fill factor (FF) remains nearly constant around 0.62. The power conversion efficiency (PCE) increases from 5.5% for the TiO2-0 devices up to 8.6% for TiO2-5 devices, respectively. The reason for the improved PCE can be directly linked to the increased Jsc, as all other parameters show no or just a small increase with increasing c-TiO2 layer thickness. It can be observed that the photovoltaic parameters improve up to TiO2-5 and then remain roughly constant for TiO2-7. Hence, it can be concluded that from TiO2-5 and onwards the produced compact TiO2 layer completely covers the surface without any pinholes. Thus, a further increase of the thickness of the c-TiO2 layer would not be beneficial for improved photovoltaic performance, which is consistent with the previously discussed GI-XRD and XPS results, as well as literature values.8 The Voc remains high and is nearly independent of the c-TiO2 layer thickness. This suggests that the surface recombination of holes at the FTO contact is low even without a c-TiO2 layer.34


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As we know that the TiO2-0 sample has an exposed FTO surface, the low recombination could be explained by an unreacted hole-blocking PbI2 layer formed between the perovskite and the FTO.35,36 Thus, from the Voc values we cannot deduce if the thinnest c-TiO2 layers possibly contain pinholes. However, we note that the J-V curves of samples TiO2-0, TiO2-1, and TiO2-2 have a strong “s-shaped” character in the voltage range between -0.70 V and -0.25 V, which also reduces the JSC values for these samples. Based on numerical simulations, the distinct shape of the J-V curves of the TiO2-0 devices is consistent with that negative immobile ions are present at the electron-extracting (i.e. the FTO) interface.37 These ions screen the internal electric field at small positive voltages (i.e. between 0 V and 1 V), which inhibits the collection (of some) of the generated electrons and explains the reduced Jsc seen for samples TiO2-0 to TiO2-3. However, at negative voltages, these electrons are fully extracted as the current is almost the same for all devices at -1 V (~13 mA cm-2). When comparing the magnitude of the s-shape feature in Figure 6 and the atomic ratio of the exposed Sn in the XPS measurements (Figure 5), a strong correlation can be seen indicating that the s-shape is related to pinholes in the c-TiO2 layer. Thus, it appears that a poorly defined interface with trapped ions between the perovskite and the exposed FTO surface is the culprit for this phenomenon. Nonetheless, when introducing the cTiO2 layer, the surface coverage (and quality) is gradually improved, the electron collection is enhanced, and the s-shape diminishes.


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Figure 5. J-V curves of the reverse sweep (from 1.5 V to -1.0 V) of the devices without (TiO2-0) and with increasing c-TiO2 layer thickness (see Table 2).


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Table 3. Photovoltaic parameters of the devices with varying thickness of the c-TiO2 layer, determined from the J-V curves for the forward (fwd, from -1.0 V to 1.5 V) and reverse sweep (rev, from 1.5 V to -1.0 V). sample

TiO2-0

TiO2-1

TiO2-2

TiO2-3

TiO2-5

TiO2-7

Na

10

6

12

9

12

4

JSC b

VOC c

FF d

PCE e

[mA cm-2]

[V]

fwd: 9.4±0.9

0.91±0.03

0.56±0.05

4.7±0.6

rev: 9.5±0.9

0.93±0.03

0.62±0.03

5.5±0.8

fwd: 12.7±1.3

0.95±0.01

0.57±0.06

7.0±1.3

rev: 12.5±1.2

0.95±0.02

0.62±0.04

7.4±1.1

fwd: 12.4±1.0

0.96±0.02

0.60±0.02

7.1±0.7

rev: 12.4±0.8

0.96±0.02

0.63±0.01

7.5±0.6

fwd: 13.4±0.4

0.95±0.03

0.62±0.02

7.9±0.5

rev: 13.3±0.4

0.94±0.03

0.61±0.03

7.8±0.6

fwd: 13.2±0.7

0.98±0.02

0.63±0.01

8.2±0.5

rev: 13.2±0.7

0.97±0.02

0.67±0.02

8.6±0.4

fwd: 13.5±0.5

0.99±0.01

0.61±0.04

8.2±0.6

rev: 13.2±0.4

0.98±0.03

0.62±0.05

8.0±0.7

[%]

a

Number of measured devices. b Short-circuit current density. voltage. d Fill factor. e Power conversion efficiency.

c

Open-circuit

CONCLUSION


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The influence of ultra-thin c-TiO2 layers on the performance of mesoscopic perovskite solar cells has been investigated. Reference samples of the TiO2 layers on glass substrates revealed that the thickness can easily be tuned between 5 nm and 50 nm by adjusting the TiCl4 concentration in the dip coating solution. According to AFM measurements, the c-TiO2 film seems to cover the surface of the FTO substrate quite well and with increasing precursor concentration, it even starts to fill up the voids leading to a reduced surface roughness. This seems to occur in conjunction with the onset of the anatase crystallization as seen in the GI-XRD results. Furthermore, the small amounts of Sn seen in the XPS measurements for the thinnest films seem to be associated with pinholes in the compact TiO2 layers. For devices without (or with a thin) c-TiO2 layer, s-shaped features J-V curve can be observed at negative voltage. This study supports the fact that immobilized negative ions at the exposed FTO surface are responsible for this shape. This means that not all of the electrons generated in the perovskite layer can be extracted in the positive voltage regime, which lowers the JSC and the PCE of these devices. The study highlights that a certain critical c-TiO2 film thickness is required to produce pinhole-free c-TiO2 layers, although functioning mesoscopic devices can be fabricated even without a compact TiO2 layer. By introducing the compact c-TiO2 layer the PCE increases from 5.5% to 8.6%. Furthermore, it should be noted that also the type of underlying FTO substrate might influence the quality of the c-TiO2 layer. In this particular case, we used the TEC-7 type, which is typically quite rough, while other smoother types might require thinner compact TiO2 layers to fully cover the substrate.


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AUTHOR INFORMATION Corresponding Author Jan-Henrik Smått, [email protected]

AUTHOR CONTRIBUTIONS The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT M.T.M. thanks National University of Science & Technology (NUST) of Pakistan for funding. C. W thanks the Deutsche Forschungsgemeinschaft (DFG, WE 6127/1-1) for a Post-Doctoral Fellowship. J-H.S. also acknowledges financial support from the Academy of Finland (decisions No. 259310 and No. 279055) and Magnus Ehrnrooth Foundation. P.V. and S.G.H are grateful to Academy of Finland for the post-doctoral fellowships (decision No. 268672 and 287641, respectively).

SUPPORTING INFORMATION Temperature program for the preparation of mesoporous TiO2 layer, X-ray reflectivity measurements, grazing incidence x-ray diffraction patterns, AFM height profiles, roughness summary and XPS results of FTO substrates with compact TiO2 layer. Time-dependent photocurrent density and J-V measurements of solar cell devices.


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TOC


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Impact of Film Thickness of Ultrathin Dip-Coated Compact TiO2 Layers

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