Influence of different TiO2 blocking films on the photovoltaic performance of perovskite solar cells

Influence of different TiO2 blocking films on the photovoltaic performance of perovskite solar cells

Applied Surface Science 388 (2016) 82–88 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate...

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Applied Surface Science 388 (2016) 82–88

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Influence of different TiO2 blocking films on the photovoltaic performance of perovskite solar cells Chenxi Zhang, Yudan Luo, Xiaohong Chen ∗ , Wei Ou-Yang, Yiwei Chen, Zhuo Sun, Sumei Huang ∗ Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Department of Physics, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, PR China

a r t i c l e

i n f o

Article history: Received 24 September 2015 Received in revised form 23 February 2016 Accepted 11 March 2016 Available online 14 March 2016 Keywords: Perovskite solar cell Compact layer Titanium dioxide Chemical bath deposition Sol-gel Spin-coating

a b s t r a c t Organolead trihalide perovskite materials have been successfully used as light absorbers in efficient photovoltaic (PV) cells. Cell structures based on mesoscopic metal oxides and planar heterojunctions have already demonstrated very impressive and brisk advances, holding great potential to grow into a mature PV technology. High power conversion efficiency (PCE) values have been obtained from the mesoscopic configuration in which a few hundred nano-meter thick mesoporous scaffold (e.g. TiO2 or Al2 O3 ) infiltrated by perovskite absorber was sandwiched between the electron and hole transport layers. A uniform and compact hole-blocking layer is necessary for high efficient perovskite-based thin film solar cells. In this study, we investigated the characteristics of TiO2 compact layer using various methods and its effects on the PV performance of perovskite solar cells. TiO2 compact layer was prepared by a sol-gel method based on titanium isopropoxide and HCl, spin-coating of titanium diisopropoxide bis (acetylacetonate), screen-printing of Dyesol’s bocking layer titania paste, and a chemical bath deposition (CBD) technique via hydrolysis of TiCl4 , respectively. The morphological and micro-structural properties of the formed compact TiO2 layers were characterized by scanning electronic microscopy and X-ray diffraction. The analyses of devices performance characteristics showed that surface morphologies of TiO2 compact films played a critical role in affecting the efficiencies. The nanocrystalline TiO2 film deposited via the CBD route acts as the most efficient hole-blocking layer and achieves the best performance in perovskite solar cells. The CBD-based TiO2 compact and dense layer offers a small series resistance and a large recombination resistance inside the device, and makes it possible to achieve a high power conversion efficiency of 12.80%. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Since the first successful proof of concept of solid-state dyesensitized solar cell (ssDSC) in 1998, the photovoltaic power conversion efficiencies (PCE) have progressively improved to a maximum of 7% [1,2]. Recently, new class of absorbers based on solid state organic-inorganic hybrid perovskite structures possessing high absorption coefficients have shown significant potentials for low-cost and high-efficient solar cells [3–13]. In 2012, the advent of solid-state organic-inorganic hybrid perovskite solar cells (PSCs) has greatly motivated science and engineering researchers to redesign solar cells for improving PSC device effi-

∗ Corresponding authors. E-mail addresses: [email protected] (X. Chen), [email protected] (S. Huang). http://dx.doi.org/10.1016/j.apsusc.2016.03.093 0169-4332/© 2016 Elsevier B.V. All rights reserved.

ciencies 6–13. Over the recent two years, the efficiency of PSCs has sharply increased to 20.1% [13]. PSCs are typically composed of a fluorine-doped tin dioxide SnO2 (FTO) conductive substrate, compact layer or electron-selective layer (ESL), mesoporous scaffold layer (optional), organic–inorganic hybrid perovskite layer, holetransporting material (HTM), and metal electrode. In the structure of hybrid spiro-OMeTAD/perovskite/TiO2 cells, electron–hole pairs that are created in perovskite following light harvesters can possibly result in the formation of excitons after thermalization of the carriers. Charge separation can then occur through injection of photogenerated electrons into TiO2 nanoparticles and/or injection of holes into a HTM such as spiro-OMeTAD [14,15]. Thus, the compact layer is required to prevent direct contact between holes formed in the perovskite or HTM layer and fluorine-doped tin oxide (FTO) electrode [4–9] for high-efficiency PSCs with various configurations. The compact layer was com-

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monly named as blocking layer (BL) because it could obstruct the flow of electrons from the FTO to HTM or holes from the HTM to FTO, avoiding the heavy recombination of the holes which were generated in the hybrid perovskite absorber and the electrons, which were existed in both perovskite absorber and the electron transport layer at the surface of FTO [9,14–17]. As is well known, TiO2 BL is a key component in, for example, DSCs employing cobalt-based redox electrolytes [18] and also in iodine-based DSCs to prevent the recombination of electrons injected into the photoanaode with the oxidized redox species [19,20]. In general, the TiO2 compact layer is prepared by aerosol spray pyrolysis [21], atomic layer deposition [22], sol–gel [23], spin coating [24], DC-magnetron sputtering [25,26] or electrochemical deposition [27]. The role of BLs in DSCs was widely studied by these BL deposition methods in the past decades. Recently, optimization of BLs in PSCs has been drawing increasing attention as it can improve the performance of the photovoltaic (PV) devices. Yella et al. have reported low-temperature fabrication of a compact layer composed of small nanoparticles of rutile TiO2 compact layer on a FTO conducting glass substrate via hydrolysis of TiCl4 [28]. Highly conductive graphene nanoflakes were also introduced into the TiO2 blocking layer to improve the conductivity and increase the photovoltaic performance [29]. The morphological and structural properties of the compact TiO2 layers can have significant effects on the performance of perovskite solar cells. However, there have been few studies in which the effects of physical properties and nanostructures of compact TiO2 on solar cell performance were investigated systematically. In this study, we conduct a systematic investigation and study of the growth characteristic properties of TiO2 blocking layers by four commonly used methods and their effects on the photovoltaic performance of PSCs. TiO2 BLs were prepared by a sol-gel method based on titanium isopropoxide and HCl, spin-coating of titanium diisopropoxide bis (acetylacetonate), screen-printing of Dyesol’s blocking layer titania paste, and a chemical bath deposition (CBD) technique via hydrolysis of TiCl4 , respectively. The morphological and structural properties of the formed BLs were systemically characterized and analyzed. The four kinds of grown BLs were tested in complete PV devices with mesoporous TiO2 scaffold. It was found that surface morphologies of TiO2 blocking layers played a key role in affecting the device efficiencies. We show that the nanocrystalline TiO2 layer deposited via the CBD route works as the most efficient hole-blocking layer and achieves the best performance in PSCs. The CBD-based TiO2 compact layer provides a small series resistance and a large recombination resistance inside the PSC, and enables a high PCE of 12.80%.

2. Experimental 2.1. Preparation of blocking layers (BLs) FTO glass substrates (<15 /square, Nippon Sheet Glass Co., Ltd., Japan) were etched with zinc powders and HCl (2 M) to obtain the required electrode patterns. The etched sheets were then cleaned by ultrasonication in a glass detergent (Hui Jie washing Ltd., Shenzhen), rinsed with deionized water, ethanol and acetone, and subjected to an O3 /ultraviolet treatment for 30 min. A 25–50 nmthick TiO2 compact layer was then deposited on the cleaned FTO substrates by different methods.

2.1.1. Chemical bath deposition (CBD) method An aqueous stock solution of 2 M titanium tetrachloride TiCl4 (stored in the freezer) was diluted to 0.2 M. The cleaned FTO substrates were then immersed into the diluted TiCl4 solution and kept in an oven at 70 ◦ C for 1 h in a closed vessel. After 1hr the formed

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electrodes were washed with water and ethanol, and dried at 100 ◦ C in air for an hour. 2.1.2. Sol-gel method The titanium precursor solution consists of 0.23 M titanium isopropoxide (Sigma-Aldrich, 99.999%) and 0.013 M HCl solution in isopropanol (499.9% Fisher Chemicals). To prepare this solution, titanium isopropoxide (369 ml) was diluted in isopropanol (2.53 ml) at 0.46 M. Separately, a 2 M HCl solution (35 ml) was diluted down with isopropanol (2.53 ml) to achieve a 0.026 M concentration. Finally, the diluted acid solution was added dropwise to the titanium precursor solution under heavy stirring. The resulted mixture was filtered with a PTFE filter with 0.2 ␮m pore size before use. BLs were deposited on the etched and cleaned FTO substrates by spin coating the resulted sol at 2000 r.p.m. for 60 s and consequently heating at 500 ◦ C for 30 min. 2.1.3. Spin-coating method The titanium precursor solution is made up of 0.15 M titanium diisopropoxide bis(acetylacetonate) (TAA) (75 wt% in isopropanol, Aldrich) in 1-butanol (99.8%, Aldrich) solution. The TiO2 blocking layer was deposited on the cleaned FTO substrate by spin coating the prepared titanium precursor at 2000 r.p.m. for 20 s, and then heating at 125 ◦ C and 500 ◦ C for 5 min and 30 min, respectively. 2.1.4. Screen-printing method A dense TiO2 blocking layer was prepared by screen-printing a Dyesol’s blocking layer titania paste (Ti-Nanoxide BL/SP) and then heated at 500 ◦ C for 30 min. 2.2. Preparation of the PSCs The formed BL sample was treated in a 40 mMTiCl4 aqueous solution at 70 ◦ C for 30 min in order to improve the adhesion and mechanical strength of the TiO2 layer to the FTO layer. 0.5 ␮m-thick mesoporous TiO2 (p-TiO2 ) layer was deposited by spin-coating TiO2 paste (Dyesol 18NR-T) diluted in anhydrous ethanol at 1: 3.5 by weight at 2000 rpm for 50 s, and then heating in air at 500 ◦ C for 30 min. After cooling down to room temperature, the perovskite was deposited by spin-coating a N,N-dimethylformamide (DMF) solution of methylammonium iodide and PbCl2 (3:1 molar ratio) at 2000 r.p.m. for 50 s, and consequently heating at 100 ◦ C for 45 min in an oven. The hole-transport layer was deposited by spin-coating a spiro-OMeTAD solution at 4000 rpm for 30 s. The spin-coating solution was prepared by dissolving 0.0723 g spiro-MeOTAD, 28.8 ␮l 4-tertbutylpyridine,17.5 ␮l of a stock solution of 0.520 g/ ml lithium bis(trifluoromethylsulphonyl) imide in acetonitrile and 29 ␮l of a stock solution of 0.300 g/ml tris(2-(1H-pyrazol-1-yl)-4tert-butylpyridine) cobalt(III) bis(trifluoromethylsulphonyl) imide in acetonitrile in 1 ml chlorobenzene. Finally, about 100 nm-thick AgAl alloy was thermally evaporated on top of the device to form the back contact [12]. 2.3. Characterization of samples The morphologies and microstructures of the formed compacting layers were characterized by field emission scanning electron microscopy (FESEM, S4800, Hitachi and JSM-7001F, JEOL) and X-ray diffractometry (XRD, Rigaku ULTIMA IV, D/tex detector, Cu-Ka: ␭ = 0.15406 nm), respectively. The thicknesses of the formed compacting layers were measured by a surface profiler (Dektak 6). Electrochemical impedance spectroscopy (EIS) measurements of PSCs were recorded with a galvanostat (PG30.FRA2, Autolab, Eco Chemie B. V Utrecht, Netherlands) in the dark. The frequency range was from 10 to 100 KHz, and the applied bias voltage and ac amplitude were set at open-circuit voltage

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and 10 mV. Optical transmission spectra of blocking layers on glass substrate were examined and characterized by means of ultraviolet–visible light (UV–vis) spectrometer (Hitachi, U-3010). Photocurrent density–voltage (J-V) measurements were performed using an AM 1.5 solar simulator equipped with a 1000 W xenon lamp (Model no. 91 192, Oriel, USA). The solar simulator was calibrated using a standard silicon cell (Newport, USA). The light intensity was 100 mW cm−2 on the surface of the test cell. J-V curves were measured using a computer-controlled digital source meter (Keithley 2440) with the reverse direction. During device photovoltaic performance characterization, a metal aperture mask with an opening of about 0.1 cm2 was used. External quantum efficiency measurements (EQE) (74125, Oriel, USA) were also carried out for these cells.

3. Results and discussion Fig. 1 shows the XRD patterns of the TiO2 compact films prepared by different methods. The blocking layer grown by the sol-gel method shows a preferred orientation in the (101) direction, as indicated by the intense characteristic anatase peak at 2␪ = 25.4◦ (JCPDS card no. 84-1286) [30–32]. The BLs grown by the by spincoating and screen-printing methods display a single broad band, with a peak at approximately 25.4◦ . The X-ray diffraction pattern indicates that both BL films are of nanocrystalline anatase. This result is consisted with that of the photocatalytic TiO2 nanocrystalline film by a sol–gel method [33]. In contrast, the CBD grown TiO2 BL was well indexed as the rutile crystal structure of TiO2 (JCPDS card no. 75-1753), where the peaks at 27.4◦ , 36.1◦ , 41.3◦ and 54.4 ◦ Corresponded to the rutile crystal planes of (110), (101), (111), and (211), respectively [30–32]. The appearance of the rutile phases in the TiO2 BLs grown by CBD at a quite low temperature of 70 ◦ C is unanticipated, as the stable low-temperature TiO2 phase is anatase. The formation of rutile phased TiO2 can be associated with the use of TiCl4 as a precursor which is likely to produce rutile

Fig. 1. X-ray diffraction spectra of as-grown different TiO2 compact films on FTO slides.

nanoparticles on hydrolysis and induction of epitaxial growth of rutile nanoparticles by the FTO slide [34]. Fig. 2 exhibits the top–down SEM images of the TiO2 compact films prepared by various methods, which clearly displays their different grain size, surface roughness, and coverage. The CBD grown TiO2 thin films intimately follow the profile of the FTO crystals in Fig. 2(a). The result indicates controlled growth of TiO2 films and the thin film characteristic of the samples. CBD-grown rutile TiO2 nanoparticles are uniformly and densely distributed on the whole underlying FTO layer. The rutile particles fully and tightly cover the FTO substrate surface, exhibiting a compact and dense morphology, more clearly shown in the inset of Fig. 2(a). The thickness of the CBD-grown BL is approximately 25 nm, measured by a surface profiler. The crystalline rutile nanoparticles are approximately 4–8 nm in diameter. The thin overlayer of rutile nanoparticles is

Fig. 2. Top–down SEM images of the BLs by (a) CBD, (b) sol-gel, (c) spin-coating and (d) screen-printing methods.

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Fig. 3. Cross-sectional SEM image of glass/FTO/sol-gel grown BL TiO2 /p-TiO2.

rough on a nanoscale but nonporous. Moreover, the rutile nanoparticles formed via CBD very strongly bonds to the FTO, and hence no further sintering step is required for the formation of the highquality electron transport TiO2 contact. From Fig. 2(b), the sol-gel grown BL consists of larger particles (about 18 nm in diameter) compared to the case of CBD due to the calcination step at 500 ◦ C for the former. The anatase nanoparticles formed via sol-gel also exhibit good adhesion to the FTO and almost completely cover the FTO substrate, and the formed BL is with slight porous nature. The thickness of the former is about 50 nm as shown in Fig. 3. The BL by the sol-gel method is rougher, thicker and more massive than the CBD grown BL. From Fig. 2(c) and its inset, the spin-coated TiO2 compact layer shows an obvious porous structure on top of the FTO. The spin-coated BL samples for the SEM analysis were sintered for 30 min at 500 ◦ C, as is a common practice for fabricating a mesoscopic photoanode for perovskite solar cells. The spin-coating process normally induces amorphous TiOx films, which crystallize upon heating, leading to the formation of pores and pin-holes in the TiO2 layer. The BL sample prepared by the spin-coating method shows a porous sponge-like structure and exposes some fractions of the FTO electrode, exhibiting a low TiO2 coverage. The thickness of the spin-coated BL is about 40 nm. The BL made by screen-printing shows a rough and porous structure as shown in Fig. 2(d). Its thickness is approximately 30 nm. In this BL, the nanocrystalline anatase nanoparticles were self-assembled parallel to the substrate (FTO) to form large aggregates from tens to hundreds of nanometers. Voids and cracks appeared between the large aggregates in the screenprinted and sintered BL, as shown in the inset of Fig. 2(d). The sizes of the pin-holes and cracks are up to 50 nm. In PSCs with TiO2 mesoporous scaffolds, the blocking layer should provide good adhesive properties between the transparent conducting oxide (TCO) and the p-TiO2 and/or the perovskite layers to facilitate electron transport from the p-TiO2 layer and/or the perovskite to the TCO layers. On the other hand, the BL should prevent holes formed in the perovskite or HTM layer from reaching the TCO electrode, as this would cause short-circuit problems in the cell. Thus, the blocking layer not only promotes charge transport but also blocks electron recombination. Of the four BL samples, conformal pin-hole free ultra-thin oxide films were deposited on the profile of the FTO crystals by the CBD method, and the resulted oxide shows the most homogeneous morphology, the best-consolidated dense and most highly blocking TiO2 layer on the FTO, due to the conformal growth and the smallest TiO2 particle size. The smallest particles can be most efficiently packed on the FTO crystal surface, and as a result the size of the voids

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Fig. 4. J–V curves for perovskite solar cells with various TiO2 blocking layers.

present among the particles decreases to the most degree, exhibiting the best network interconnectivity as shown in Fig. 2(a). The CBD grown high-quality BL enhances the surface area, increases the contact points between the TiO2 film and the FTO and improves the electronic interconnection, so that the photoelectrons can be collected efficiently and the probability of electron recombination will be reduced. But the high-temperature calcinations with the other three methods reduced the quality of the BL due to the formation of pores, pin-holes, voids or cracks in the TiO2 film during the crystallization process of titanium oxide, decreasing the blocking capability and electron collecting efficiency of the photoanade, especially for the case of the screen-printing method. The screenprinting grown BL sample shows a very rough structure with large aggregates, voids and cracks which are evident from the SEM picture of this BL in Fig. 2(d). A low blocking capability can be expected from this sample. The differences in the morphological properties of compact TiO2 films on the FTO layer likely affect the device characteristics. We tested the different BLs embedded in a complete photovoltaic device. The mesoporous TiO2 film was infiltrated with the CH3 NH3 PbI3 − x Clx perovskite nanocrystals using the single step procedure. J-V characteristics of perovskite solar cells with different BLs under a light intensity of 100 mW cm−2 are presented in Fig. 4. Device parameters including PCE (), open-circuit voltage (VOC ), short-circuit current density (JSC ) and fill factor (FF) are summarized in Table 1. As shown from Fig. 4 and Table 1, the PSC based on the screen-printed BL displays a Jsc of 16.79 mA cm−2 , FF of 0.64, VOC of 0.83 V and  of 8.92%. The device based on the spincoated BL shows a quite similar PV performance ( = 9.05%) to the cell based the screen-printed BL. When the sol-gel grown BL was used, the photovoltaic performance was obviously improved compare to the case of screen-printing. The former device exhibits a JSC of 17.67 mA cm−2 , VOC of 0.86 V, FF of 0.69, and thus,  of 10.48%. Surprisingly, applying the thinnest (25 nm) compact TiO2 layer, prepared by low-temperature CBD method, substantially improved the JSC , the VOC and the fill factor, resulting in the highest  of 12.80%. With improved JSC , VOC and FF values, the PCE of PSC based on the Table 1 Photovoltaic performance of perovskite solar cells with various TiO2 compact films. Device

VOC (V)

JSC (mA/cm2 )

FF

␩ (%)

RS ( cm2 )

CBD Sol gel Spin-coating Screen-printing

0.89 0.86 0.84 0.83

21.46 17.67 15.61 16.79

0.67 0.69 0.69 0.64

12.80 10.48 9.05 8.92

60 78 94 91

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Fig. 5. EQE spectra for perovskite solar cells with various TiO2 compact films.

CBD grown BL is increased by 43.5% relative to the device based on the screen-printed BL. The relatively poor photovoltaic performance of devices based on the spin-coated and screen-printed BLs can be associated with the bad morphological features of both BLs shown in Fig. 2. The increase of JSC value in the order spin-coating < screenprinting < sol-gel < CBD based cell can be explained by the better network interconnectivity and the higher blocking capability of the corresponding TiO2 BL shown in Fig. 2. The better network interconnectivity on the FTO substrate surface ensured the formation of the higher-quality electron-selective contact in the device, which more fostered the electron transport. The higher photocurrent density could be generated due to an increase in the electron collection efficiency [28]. The JSC drop in the order CBD > solgel > screen-printing > spin-coating based cell can be ascribed to the poorer coverage of the FTO surface by the TiO2 . The TiO2 poorer surface coverage increased the contact chance between FTO and spiro-MeOTAD, and as a consequent, increased charge carrier recombination and suppressed charge extraction. This is evident from the SEM images shown in Fig. 2. For solar cells, the shunt resistance is due to p-n junction nonidealities and impurities near the junction, but nevertheless the major contributors to the series resistance (RS ) are the bulk resistance of the semiconductor material (the active layer), the contact resistance at the semiconductor—conductive interfaces and the resistance of the conductive contacts [35]. In our work, except the BL, other functional layers of the devices were made under the same conditions. Therefore, RS can be expected to change with the compact TiO2 deposition method. RS resistance was also estimated from the J–V characteristics shown in Fig. 4 and summarized in Table 1. The RS from the PSC based on the CBD compact layer is 60  cm2 , which is smaller than 78  cm2 , 91  cm2 and 94  cm2 from the device based on the compact layer by sol-gel, screen-printing and spin coating method, respectively. This RS value increase could also be the result of the deceasing of the connectivity between the TiO2 particles or the lessening of the TiO2 coverage in the BL prepared by CBD, sol-gel, screen-printing and spin-coating methods as shown in Fig. 2. The lowest RS can be associated with most efficient charge transport or most negligible charge accumulation and recombination [36], resulting in the highest JSC value in the device based on CBD compact layer shown in Fig. 4 and Table 1. The corresponding EQE spectra of the devices based on the four types of BLs are shown in Fig. 5. EQE depends on both the absorption of light and the collection of charges. PSCs based on CBD and sol-gel grown compact films achieved higher EQE values

Fig. 6. Optical transmission spectra of different compact films on FTO slides.

than the other two types of devices at 450–750 nm, but a contrary result was obtained at 300–450 nm. The EQE results of the four cells can be associated with the optical properties of their corresponding compact films. Fig. 6 shows the optical transmission measurements of the four types of TiO2 compact films. During the measurements, the radiation went through the glass, FTO, and the coated compact layer as they do in a real device. The spincoating and screen-printing grown BLs show the higher optical transmission than CBD and sol-gel grown ones in short wavelength range of 300–450 nm, but the four samples show similar transmission properties in the 450–800 nm region from Fig. 6. The increased transmission in the front contacts could help increase the light absorption in the absorbers of the solar cells. Therefore, the higher EQE values of the devices based on the spin-coating and screen-printing grown BLs can be attributed to the distinctly higher absorption in short wavelength range of 300–450 nm. The lower EQE efficiency of the PSC based on the compact layer by spincoating or screen-printing method at the longer wavelength range arose from inefficient charge extraction or recombination effects due to the lower TiO2 surface coverage shown in Fig. 2, resulting in the relative lower JSC of solar cells shown in Table 1. The improved photovoltaic performance can also be attributed to the increased light absorption, the enhanced charge injection and/or charge collection efficiency. The highest EQE values of the PSC based on the CBD compact layer in the longer wavelength region can be associated with the most efficient charge extraction due to the most highly blocking action of charge carrier recombination between FTO and spiro-MeOTAD. The IPCE results of the devices with different BLs are in good agreement with those of the J-V measurements of these cells shown in Fig. 4. To further understand the influence of the different BL on the cell performance, electrochemical impedance spectroscopy (EIS) was carried out, in which the potential bias was applied at 800 mV under the dark in the frequency range from 105 to 0.1 Hz. Fig. 7 shows the EIS results for the devices based on the different BLs. EIS was used to characterize the internal resistance and charge transfer kinetics of the solar cell [37–40]. Under no illumination, the structure of the PSC could be considered as a leaking capacitor [40]. From Fig. 7, the semicircle represents the recombination resistance (Rrec ) at the interface of TiO2 /perovskite layer and TiO2 /HTM layer. The bigger the diameter of the semicircle is, the less electron recombination at the interfaces. The CBD BL based cells showed the highest Rrec value, the device based on the sol-gel grown BL displayed the second largest Rrec, and the cell based on the screen-printed BL showed the lowest Rrec. The lowest Rrec value of this device can be attributed

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[2]

[3]

[4]

[5]

[6]

[7]

[8] Fig. 7. Nyquist plots of perovskite solar cells assembled with different TiO2 compact films.

to the nonconnectivity between the large TiO2 aggregates and the appearance of large voids and cracks in the TiO2 layer made by the screen-printing shown in Fig. 2(d). The high recombination resistance resulted in a significantly increase in current loss through recombination and an increase in the FF factor. The lowest recombination resistance of the solar cell based on the screen-printed BL induced the lowest FF factor of this cell. The highest recombination resistance of the CBD based cell most effectively retards the charge recombination, which results in the high FF and the highest JSC , and thus, the highest PCE shown in Fig. 4 and Table 1. 4. Conclusions We have investigated the characteristics of TiO2 blocking layers using various methods and their effects on the PV performance of perovskite solar cells. BLs were prepared by a sol-gel method based on titanium isopropoxide and HCl, spin coating of titanium diisopropoxide bis (acetylacetonate), screen-printing of Dyesol’s bocking layer titania paste, and a CBD technique via hydrolysis of TiCl4 , respectively. It is found that the surface morphology of TiO2 compact layer played a critical role in the devices performance. The interconnectivity between the TiO2 particles and the TiO2 coverage in the blocking layers is far higher for the CBD-based method compared with other three methods. The rutile nanocrystalline TiO2 film deposited via the CBD route exhibited outstanding morphological properties and acted both as an effective electron collector and as effective blockade of interfacial charge recombination inside the device. The highly compact and dense CBD TiO2 layer exhibits perfect network interconnectivity, greatly decrease the series resistance inside the device and significantly increases the recombination resistance at the interface of TiO2 /perovskite layer and TiO2 /HTM layer, enabling a high-energy conversion efficiency of 12.80%. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 11274119 and 61275038) and the Large Instruments Open Foundation of East China Normal University.

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