SnO2 Nanocomposites as Electron Transporting Layer for

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TiO2/SnO2 Nanocomposites as Electron Transporting Layer for Efficiency Enhancement in Planar CH3NH3PbI3-based Perovskite Solar Cells Heng Guo, Haiyan Zhang, Jian Yang, Haiyuan Chen, Yulan Li, Liping Wang, and Xiaobin Niu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01331 • Publication Date (Web): 16 Nov 2018 Downloaded from http://pubs.acs.org on November 17, 2018

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ACS Applied Energy Materials

TiO2/SnO2 Nanocomposites as Electron Transporting Layer for Efficiency Enhancement in Planar CH3NH3PbI3-based Perovskite Solar Cells Heng Guo, Haiyan Zhang, Jian Yang, Haiyuan Chen, Yulan Li, Liping Wang and Xiaobin Niu* State Key Laboratory of Electronic Thin Film and Integrated Devices, School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 610054, China Keywords: Titanium oxide (TiO2); tin oxide (SnO2); nanocomposites; electron transporting layer; perovskite solar cells; efficiency Abstract: Both TiO2 and SnO2 as electron transport layers (ETLs) are the most successful at producing high-efficiency perovskite solar cells (PSCs). Unfortunately, these layers usually require multiple treatments to improve device performance, especially for TiO2-based planar PSCs. Here, a simple, one-step, solution-based method is introduced for fabricating a TiO2/SnO2 nanocomposite ETL. Compared to pristine TiO2 layer, partial incorporation of SnO2 nanoparticles in a thin compact TiO2 layer contributes to high electrical conductivity and suitable band-level alignment at the ETL/perovskite interface. Using the optimized TiO2/SnO2 nanocomposite ETL, methylammonium lead halide perovskite CH3NH3PbI3-based PSCs exhibit the champion efficiency of 16.8 % (Area: 0.13 cm2) with less current density-voltage (J-V) hysteresis, which is significantly superior to that of TiO2-based PSCs. Our work highlights that the successful use of

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TiO2/SnO2 nanocomposite ETL can offer an efficient method for further improvement of the performance for planar PSCs. Introduction A new generation of photovoltaic devices, fully solidified organic-inorganic halide perovskite solar cells (PSCs),1 has drawn significant attention in both the academic and industrial communities, which is pioneered by Park et al. in 2012.2 Since then, the certified power conversion efficiency (PCE) of laboratory scale devices has dramatically increased up to 23.3 % with an incredible rate. 3-9 This renders them very promising candidates to make these competitive with the commercially mature Si-based photovoltaic technologies. The impressive merit of PSCs is high PCE, ease fabrication and scalability, which is thanks to the perovskite materials’ photovoltaic properties, such as high optical absorption coefficient, 10 a favourable direct band gap, 11-12 and high charge carrier mobility13. Despite of the large potential of PSCs, numerous problems not only appear in the halide perovskite itself but also in hole transport layers (HTLs) and electron transport layers (ETLs), as well as at the interfaces between the various layers of the device.14 Currently, the various PSCs are typically constructed with either planar heterojunction (p-i-n) or mesoporous structure (n-i-p).15-16

Generally,

state-of-the-art

planar

heterojunction

PSCs

using

the

ETL/perovskite/HTL structure achieved a record efficiency approaching 21 %. 17-19 Thus far, the most commonly top-performing planar PSCs were fabricated by using titanium dioxide (TiO2) as ETL material in such device configuration.

20

However, TiO2

often required a high processing temperature (> 450 oC) 21-22. Moreover, the surface of TiO2 suffers from ultraviolet (UV) illumination instability due to photocatalytic activity, which may hinder reproducibility over time and the stability of PSCs. In addition, TiO2 exhibits


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low mobility, less than that of CH3NH3PbI3 perovskite, which also makes it not an ideal ETL for PSCs.23 Therefore, it is possible to search for a more likely candidate as ETL for use in highly efficient planar PSCs. More recently, some suitable ETL materials with high electron transfer mobility and conductivity have been reported, such as SnO2,24-26 WOx,27 ZnO,28 Nb2O5,29 BaSnO3,30 and La-BaSnO331-32. For example, SnO2 is the most promising alternative due to a wide band gap, a deeper conduction band and high electron mobility. More importantly, SnO2 as ETL not only has sufficient ability to improve charge extraction, but is also supposed to reduce charge accumulation.25 Further, SnO2-based planar PSCs show excellent durability in the ambient environment.33 However, pristine SnO2 ETL still suffers from inhomogeneous electrical properties and interfacial contact.34. To overcome these drawbacks, doping SnO2 ETL with low concentration metal aliovalent cations (Li+,35 Nb5+,36 Sb2+,37 and Al3+38) is an effective method. This method not only enhances the conductivity of the ETL films with good surface coverage, but also shifts the suitable energy level alignment of the ETL to be adjacent to the transparent electrodes. These factor result in the decrease of photocurrent hysteresis and improvement of device performance.34 Another effective way to modify the interfacial optoelectronic properties of the SnO2 surface is using fullerene derivatives, which improves charge transfer and passivation of the perovskite layer.

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It is therefore prudent to use the composite

SnO2/fullerene ETL to boost the performance of planar PSCs.40-41 Similarly, the SnO2 nanocomposite can improve the PCE of dye-sensitized solar cells with coating a thin layer of an isolation oxide, such as TiO2, ZnO, MgO and Al2O3. 42 Therefore, in the planar PSCs, these SnO2 ETLs still need to be covered by thin fullerene derivatives

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or metal oxide

layers 18 to reduce trap state density at the SnO2 surface and to facilitate interfacial charge


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transfer at the SnO2/perovskite interfaces. However, these multiple treatments are expense and unstable for the practical applications. Here, we report the effectiveness of the use of SnO2 nanoparticle as a dopant in TiO2 precursor to obtain TiO2/SnO2 nanocomposite film as an electron transport layer. Comparing to pristine TiO2-based PSCs (average PCE: 13.7 ± 0.35 %), the average PCE of TiO2/SnO2-based PSCs is improved to 15.9 ± 0.36 % with less hysteresis. Using the optimized TiO2/SnO2 ETL, the champion PCE of PSCs is 16.8 % measured under standard AM 1.5G illumination, and those devices exhibit high stability as well. Our study reveals that partial incorporation of SnO2 nanoparticle results in enhancing the performance of TiO2-based planar perovskite solar cells. Experimental section TiO2 and TiO2/SnO2 nanoparticle preparation. SnO2 nanoparticle was prepared via the evaporative crystallization of the SnO2 colloid precursor solution (15 % in H2O colloidal dispersion, Alfa Aesar). The TiO2 precursor solution was prepared by mixing a titanium isopropoxide (TIP, 99.99 %, Sigma-Aldrich)/ethanol solution (350 µL TIP in 2.5 ml of ethanol) in a weakly acidic solution (35 µL of 2 M HCl (36 w.t.%, Adamas) in 2.5 ml of ethanol). After stirring for 30 min, the SnO2 nanoparticle powder was added into the TiO2 precursor solution with different concentrations (CSnO2) of 0, 0.4, 0.8, 1.6 and 3.2 mg ml-1, respectively. The final TiO2/SnO2 precursor solutions were spin-coated (2,000 rpm, 60 s) onto the FTO-coated glass substrates (15 Ω sq-1, Zhuhai Kaivo Co., Ltd.). Finally, the TiO2 and TiO2/SnO2 coated FTO substrates were sintered at 500 °C for 30 min in air. Device Fabrication. Perovskite films were prepared by using a two-step solution method. Briefly, 400 mg ml-1 PbI2 (99.999 %, Alfa Aesar) in DMF (99.8 %, Sigma-Aldrich)


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was spin-casted on the resultant TiO2 and TiO2/SnO2-coated FTO substrates with a speed of 3500 rpm for 20 s at 75 ºC. The PbI2 film was dried at 100 oC for 10 min. Subsequently, 45 mg ml-1 CH3NH3I (99 %, Xi’an Polymer Light Technology Corp.) in ethanol was spincoated on the PbI2 layer at 3500 rpm for 30 s. Then the films were annealed at 100 oC for 60 min to form CH3NH3PbI3 perovskite layer. 72 mg ml-1 Spiro-OMeTAD (98.5 %, Lumtec) in 1 ml of chlorobenzene (99.8 %, Sigma) with addition of 18 μL of lithium bis(trifluoromethylsulfonyl)imide (99.95 %, Sigma-Aldrich) solution (520 mg in 1 ml acetonitrile) and 28.8 μL of 4-tert-butylpyridine (99 %, Sigma-Aldrich) was spin-coated on the perovskite layer at 5000 rpm for 50 s. Finally, 100 nm Au as an electrode was thermally evaporated through a shadow mask (Area: 0.13 cm2). Characterization. The morphologies were measured by emission scanning electron microscope (SEM, JEOL, JSM-5900 LV) and atomic force microscopy (AFM, a NanoScope NS3A system). Transmission electron microscopy (TEM, JEM2100) was used to characterize to the morphology and lattice spacing of the nanoparticles at 100 kV. The SnO2 sample is the diluent of the SnO2 colloid precursor solution. The TiO2/SnO2 samples is prepared by mixing ethanol and TiO2/SnO2 nanoparticles powders scraped off the sintered TiO2/SnO2 layers on glass/FTO substrates. X -ray diffraction (XRD) patterns of the samples were measured with an X-ray diffractometer (RINT2400, Japan) with Cu Kα radiation. Photoluminescence (PL) spectroscopy was measured by NOVA photoluminescence spectroscopy at room temperature. Time-resolved photoluminescence (TRPL) was obtained by exciting the perovskite samples deposited on TiO2 and TiO2/SnO2 ETLs at 420 nm. Ultraviolet photoelectron spectroscopy (UPS) measurements of the samples were carried with an Omicron SPHERA hemispherical analyzer under He-I excitation (21.2 eV)


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of gas discharge lamp operated at 90 W, a pass energy of 10 eV, and a channel width of 25 meV. Current-voltage (J-V) curves, were carried out using a Keithley 2400 Source Meter under simulated one-sun illumination (100 mW cm-2, AM 1.5G, 25oC) with a solar simulator. The light intensity was calibrated with a calibrated Si diode as reference. All JV curves were tested at a 100 mV s-1 scan rate. EQE measurement was carried out with a solar cell quantum efficiency measurement system (QEX10). Results and discussion a

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100 nm 0

e

g

f

h (101) d (101) = 3.52 Å

20 nm

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Figure 1. (a) Schematic illustration of the TiO2/SnO2 nanoparticles preparation procedures. SEM images of (b) TiO2 (CSnO2 = 0 mg ml-1) and (c) TiO2/SnO2 (CSnO2 = 0.8 mg ml-1) films on FTO/ glass substrates. (d) AFM images of TiO2/SnO2 film on FTO/ glass substrates. TEM images of (e) SnO2 and (f) TiO2/SnO2 nanoparticles. (g) High-resolution TEM image and (h) electron diffraction of TiO2/SnO2 nanoparticles.


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We produced the TiO2/SnO2 nanoparticle films in a single step from a solution of TiO2 precursor. Figure 1a depicts a schematic procedure of the TiO2 and TiO2/SnO2 nanoparticle thin film. The SnO2 nanoparticle was dispersed in the TiO2 precursor solution, forming the SnO2-TiO2 precursor solution. During the sintering step, the titanium isopropoxide on the SnO2 surface comes to undergo hydrolysis and shrinkage in its volume to the formation of Ti-O-Ti bonds.

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Figure 1b

and 1c illustrate SEM top-view images of the TiO2 (CSnO2 = 0 mg ml-1) and TiO2/SnO2 (CSnO2 = 0.8 mg ml-1) nanoparticle films on the FTO substrate. The pristine TiO2 nanoparticle displays a compact layer, uniformly coating the uneven surface of FTO. In contrast, the TiO2/SnO2 nanoparticle forms a continuous and pinhole-free film, albeit with some aggregates and a relatively moderate surface roughness (Root-mean-square roughness, Rms = 2.2 nm), as shown by the respective atomic force microscopy (AFM) image of Figure 1d. Transmission electron microscopy (TEM) observations were carried out to further investigate the as-prepared TiO2/SnO2 nanoparticle. Figure 1e is a typical TEM image of the pristine SnO2 nanoparticle we used, clearly showing the SnO2 nanoparticles are very uniform with a particle size of 2-4 nm. The highresolution TEM image (Supplementary Figure S1) results reveal the pristine SnO2 nanoparticles show the polycrystalline structure, which helps to reduce the number of defect traps in SnO2. Besides, this kind of SnO2 nanoparticle used as an ETL can enhance charge transfer at the ETL/perovskite interface. 25 TEM and high-resolution TEM images of the TiO2/SnO2 nanoparticle are shown in Figure 1f and 1g, respectively. An overview image of the TiO2/SnO2 nanoparticles illustrates that these tiny nanosized particles are compactly stacked together, and making them possible to form a uniform TiO2/SnO2 nanoparticle film (Figure 1f). Interestingly, the observed electron diffraction pattern (Figure 1h) can be readily indexed to the (101) planes of anatase TiO244 with a d spacing of 3.52 Å (Figure 1g), confirming the polycrystalline structure of TiO2 appears


7


in the TiO2/SnO2 nanoparticles prepared by this method. Moreover, we can find that the appearance of only one weak diffraction peak at 25.5o responding to the (101) crystal plane for pristine TiO2 and TiO2/SnO2 nanoparticle (Figure S2, Supporting Information), which results

Au Spiro-OMeTAD

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further confirmed this conclusion.

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Figure 2. (a) A coloured high-resolution cross-section SEM image of the completed devices, with the structure glass/FTO/TiO2/SnO2/CH3NH3PbI3/Spiro-OMeTAD/Au. (b) Photovoltaic metrics of devices plotted as a function of SnO2 concentrations (CSnO2, mg ml-1) in the TiO2 precursor solution, the data points with the same colour correspond to the same SnO2 concentration. (c) The representative current-density (J-V) characteristics, (d) series resistances (RS) and (e) shunt resistances (RSH) for the devices based on TiO2 /SnO2 ETLs prepared from different CSnO2s in the TiO2 precursor solution. By using the above-mentioned technique for ETL deposition we fabricated PSCs with a planar structure. The cross-sectional illustrative SEM image is shown in Figure 2a. The detailed photovoltaic performance parameters of these devices employing this configuration are as follows:


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open-circuit voltage (VOC), short-circuit current density (JSC), power conversion efficiency (PCE), and fill factor (FF) displayed in Figure 2b. We tested the effects of varying the concentration of SnO2 nanoparticles (CSnO2, mg ml-1) in the TiO2/SnO2 ETL. The PCE first augments and subsequently decreases with a peak at CSnO2 = 0.8 mg ml-1. The corresponding data was summarized in Supplementary Table S1. Upon increasing the CSnO2 from 0 to 0.8 mg ml-1, the VOC rises from 1.01 ± 0.03 V to 1.04 ± 0.02 V, the JSC from 22.5 ± 0.44 mA cm-2 to 23.2 ± 0.64 mA cm-2, the FF from 60 ± 2.8 % to 66 ± 2.2 % and the PCE from 13.7 ± 0.35 % to 15.9 ± 0.36 %. The representative current density-voltage (J-V) curves for perovskite solar cells using TiO2 and TiO2/SnO2 ETLs with different SnO2 concentrations are shown in Figure 2c. It is apparent that there is an optimum SnO2 concentration for the device performance at 0.8 mg ml-1. Figure 2d and 2e present the series resistances (RS) and shunt resistances (RSH) of the devices based on TiO2/SnO2 ETLs prepared from different CSnO2s in the TiO2 precursor solution. It is conspicuous that the SnO2 concentration at 0.8 mg ml-1 results in the largest decrease of RS. On the contrary, the RSH has the maximum value thanks to the SnO2 concentration of 0.8 mg ml-1. Such result elucidates the decrease of JSC and increase of FF in the series of PSCs.23 To investigate the efficacy of the use of the ETLs in PSCs, we characterized the band structures and optoelectronic properties of both TiO2 and TiO2/SnO2 nanocomposite films. As shown in Figure S3, the optical transmittance of the TiO2 and TiO2/SnO2 nanocomposite films shows resembling spectra with good transparency, where the transmittance of TiO2/SnO2 nanoparticle film is slightly lower in the range from 419 to 800 nm, compared with that of TiO2 film. The optical band gaps and absorption edges were estimated using the Tauc’s equation: 45 𝛼ℎ𝑣 = A(ℎ𝑣 ― 𝐸𝑔)𝑛

(1)


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Where α is the absorbance coefficient, hν is the photon energy, A is a constant, Eg is the band-gap of the film and n = 1/2. According to the Tauc polt of the absorption spectrum (Supplementary Figure S4), an indirect bandgap of ~ 3.58 eV for TiO2 film is obtained, consistent with that of anatase TiO2.46 For comparison, the bandgap of TiO2/SnO2 film reduces to 3.65 eV, showing a slight decrease of optical band gap with the addition of pure SnO2 nanoparticles (3.76 eV). The work function (WF) is calculated by subtracting the secondary-electron cut-off banding energy (Ecut-off, Figure 3a) with photo energy (21.19 eV), changed from 3.96 eV (TiO2) to 4.03 eV (TiO2/SnO2). It can be calculated that the valence band (EVB) of TiO2 and TiO2/SnO2 nanoparticles is 7.38 eV and 7.56 eV, respectively, as obtained from EVB = WF + VBM (Figure 3b) based on the semiconductor band structure 25, 46. Finally, the conduction band (ECB) is determined from EVB and Eg, which was found to be 3.80 eV (TiO2) to 3.91 eV (TiO2/SnO2), respectively. Obviously, TiO2/SnO2 nanoparticles have a deeper conduction band than that of TiO2, which should facilitate more efficient electron injection from the conduction band (3.90 eV)47 of CH3NH3PbI3 perovskite to the ETLs. Figure 3c presents energy levels of each constituent layer in the device. 47 In addition to the downward shift of the energy levels, the addition of SnO2 can slightly promote the conductivity of TiO2. Figure 3e shows electrical conductivity of TiO2 and TiO2/SnO2 ETLs with the structure (Figure 3d) of glass/FTO/ETL/Au in dark. It is apparent that the electrical conductivity of TiO2/SnO2 ETL is significantly higher than that of TiO2 ETL regardless of light conditions. This is most likely a result of increased carrier concentration37. To investigate and further confirm the SnO2-incorporation, the steady-state J-V curves of the cells with the TiO2/SnO2 (CSnO2 = 0 mg ml-1) and TiO2/SnO2 (CSnO2 = 0.8 mg ml-1) ETL measured at a constant bias of 1.0 V are displayed in Figure 3f. The photocurrents of the corresponding cells are rapidly stabilized to


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1.2 and 3.9 A cm-2, with 27 and 35 s, respectively. The adding of SnO2 nanoparticles in the TiO2 precursor solution can enhance the steady-state photocurrent of the ETL for PSCs. b

0 0.8

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Figure 3. (a) UPS cut-off edge spectra and (b) valence band spectra of TiO2 (CSnO2 = 0 mg ml-1) and TiO2/SnO2 (CSnO2 = 0.8 mg ml-1) films on FTO/glass substrate. (c) Energy-level diagram of the studied perovskite device. (d) Schematic structure, (e) J-V characteristics and (f) evolution of the photocurrent of the devices FTO/TiO2/Au and FTO/TiO2/SnO2/Au. A voltage of 1.0 V is biased. Figure 4a shows AFM topography image of the CH3NH3PbI3 perovskite film, which presents a large average grain size (300-1380 nm) with a root-mean-square roughness value of 27.9 nm. This can be confirmed by the cross-sectional SEM image of the perovskite film (thickness ~500 nm) deposited on TiO2/SnO2/FTO/Glass substrate shown in Figure 4b. However, the CH3NH3PbI3 perovskite films grown on the two ETLs show this surface morphological difference, which is evident from low magnification SEM images (See Figure S5). In addition, these XRD peaks of the


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perovskite deposited on TiO2/SnO2 ETL become much stronger and sharper, which agree well with the improved perovskite quality discussed in morphological analysis (Figure S6). The observation suggests that a high-quality polycrystalline perovskite film with large grain sizes and few grain boundaries can be grown on the studied TiO2/SnO2 nanoparticle film. The large grain sizes with high aspect ratio are desirable for reducing hysteresis because grain boundaries in perovskite films are known to trap centers. 48 Moreover, the small amount of grain boundaries can retard nonradiative charge carrier recombination, lead to reduced dark current and, therefore, improve VOC and FF of the cells.

49-50

This provides one explanation for the performance

improvement induced by SnO2 nanoparticle additive. 148.5

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Figure 4. (a) AFM image and (b) a cross-sectional SEM image of the perovskite film deposited on TiO2/SnO2 (CSnO2 = 0.8 mg ml-1) film substrates. (c) Steady-state PL spectra and (d) timeresolved PL decay of the representative perovskite films deposited on TiO2 (CSnO2 = 0 mg ml-1) and TiO2/SnO2 (CSnO2 = 0.8 mg ml-1) ETL substrates.


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Figure 4c shows steady-state photoluminescence (PL) spectra of CH3NH3PbI3 perovskite films on these substrates. An obvious emission peak centered around 772 nm is observed for all samples, which corresponds well to the UV-vis absorption onset (Figure S7). Also, in the presence of TiO2 and TiO2/SnO2 ETLs, the PL intensity of the perovskite films shows a stronger quenching effect in comparison to the pristine film on glass/FTO. This promotes a more rapid extraction of electrons or holes across the perovskite/ETL interfaces.

51

Moreover, the maximum PL quench is shown

when CH3NH3PbI3 perovskite film was deposited on TiO2/SnO2 ETL. This observation means that the charge-carrier created in perovskite absorbers could travel through TiO2/SnO2 ETL more efficiently. We further investigated the interfacial charge-carrier-transport process using the timeresolve photoluminescence spectra (TRPL) decay, conducted in Figure 4d. The TRPL decay were fitted by a bi-exponential decay function: 52 PLintensity = A1e -t/τ1 + A2e -t/τ2

(2)

where A1 and A2 are time independent coefficients of amplitude fraction for each decay component, τ1 and τ2 are decay time of fast and slow component, respectively. The fitted parameters are depicted in Table S2. We can observe a significant decrease in PL intensity as well as PL lifetime for the perovskites. Note that the charge-carrier lifetime depends on morphology, including surface states, grain boundaries, defects, etc., induce recombination.53 Apparently, a decrease in the PL lifetime for the perovskite/TiO2/SnO2 (15.98 ns) as compared to the perovskite/TiO2 (35.79 ns), means that charge carriers are more efficiently transferred by TiO2/SnO2 ETL. These measurements suggest that the improved photo-induced carrier transfer and charge separation/injection process can avoid accumulated charge in perovskite/ETL interface, which results in a significant gain in the performance of PSCs with a reduced J-V hysteresis. 54


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0 800

15 10 Jsc

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0 0.8 0

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PCE

Bias 0.74 V 0.83 V

40

60

80

Time (s)

Figure 5. J-V characteristics of devices with (a) TiO2 (CSnO2 = 0 mg ml-1) and (b) TiO2/SnO2 (CSnO2 = 0.8 mg ml-1) ETLs measured at: forward scan (from 0 V to 1.1 V) and reverse scan (from 1.1 V to 0 V) at the scan rate 1.25 V s-1. (c) External quantum efficiency (EQE) and integrated shortcircuit current density of devices with TiO2 (CSnO2 = 0 mg ml-1) and TiO2/SnO2 (CSnO2 = 0.8 mg ml-1) ETLs. (d) The steady-state measurement of the photocurrent and PCE of devices with TiO2 (CSnO2 = 0 mg ml-1) and TiO2/SnO2 (CSnO2 = 0.8 mg ml-1) ETLs measured near the maximum power point at 0.74 V and 0.83 V, respectively. As shown in Figure 5a and 5b, the best PCE achieved in the TiO2 and TiO2/SnO2 ETL-based devices was 14.2 % and 16.8 %, respectively for the reverse scan, and 11.9 % and 16.5 %, respectively, for the forward scan. The detailed photovoltaic parameters of PSCs measured under reverse and forward scan directions are summarized in Table S3. In addition, we calculated the hysteresis index H of each J-V scan given by equation:


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PCEreverse - PCEforward

Hysteresis index (H) = a

(3)

PCEreverse

b

16

1.2 1.0

Voc (V)

PCE (%)

12 8

0.8 0.6 0.4

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400

600

800

1000

1200

0

0 0.8 0

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Figure 6. The long-term stability test of the devices with TiO2 (CSnO2 = 0 mg ml-1) and TiO2/SnO2 (CSnO2 = 0.8 mg ml-1) ETLs as a function of storage time: (a) PCE, (b) VOC, (c) JSC and (d) FF. Based on the mentioned equation, the hysteresis index of the devices based on TiO2/SnO2 ETL was lower than that of the device based on TiO2/SnO2 ETL (Figure S8), revealing TiO2/SnO2 ETL helps to improve the device performance with a much improved FF and reduce the degree of hysteresis as compared with TiO2 ETL. It is clear that with the use of SnO2 nanoparticles a fast charge separation and transfer can reduce charge accumulation, and reduce J-V hysteresis.

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Meanwhile, the measured external quantum efficiency (EQE) and integrated short-circuit current density (Jsc) of these TiO2/SnO2-based devices are displayed in Figure 5c. Clearly, the EQE values of the TiO2/SnO2-based device are higher than that of the TiO2-based cell in the whole wavelength from 340 to 750 nm, yielding the Jsc values of 17.8 and 19.9 mA cm-2, respectively. As shown in Figure 5d, the steady stabilized output photocurrent of the TiO2 and TiO2/SnO2-based PSCs


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measured at a constant bias of 0.74 V and 0.83 V near the maximum power point are 17.8 mA cm-2 and 19.9 mA cm-2, respectively. The corresponding steady-state PCEs are 13.1 % and 16.4 %, respectively. It is clear that adding the SnO2 nanoparticles in the TiO2 precursor solution significantly improves the steady-state efficiency of the perovskite solar cells. In addition, to investigate the long-term stability of high-performance devices using these TiO2/SnO2 ETLs, we preformed stability tests of perovskite solar cells without encapsulation under identical storage conditions. Figure 6a presents the PCE of the fabricated devices from the J-V curves as a function of storage time. It is clearly seen that the PCE of TiO2/SnO2 (CSnO2 = 0.8 mg ml-1)-based PSCs drops from 16.2 % to 15.4 % in 500 hours, whereas that of TiO2/SnO2 (CSnO2 = 0 mg ml-1)-based PSCs decays from 14.0 % to 12.8 % with similar to the ageing behavior. Despite these devices employing both TiO2 and TiO2/SnO2 ETLs doesn’t show significant degradation in the initial 500 h of storage time, the PCE of TiO2/SnO2(CSnO2 = 0 mg ml-1)-based PSCs significantly reduces as the storage time increases. Similarly, the JSC and FF values for both TiO2 and TiO2/SnO2-based devices suffer a substantial decrease during the aging process as shown in Figure 6c and 6d. It was previously reported that CH3NH3PbI3 perovskite absorber undergo went fast thermal degradation on the TiO2 ETL surface. These results show the advantages of the use of TiO2/SnO2 ETL to enhance the lifetime of the obtained devices, revealing that such stability can be ascribed to the robustness introduced by incorporation-SnO2 nanoparticle. Conclusions In summary, we find that partial incorporation of SnO2 nanoparticle in TiO2 precursor solution leads to a new TiO2/SnO2 nanocomposite particles under one-step sintering process. Comparing with pristine TiO2 ETL, TiO2/SnO2 nanocomposite ETLs can improve the efficiency of PSCs because deeper band (3.91 eV), larger conductivity (3.9 A cm-2). Moreover, PL and TRPL studies


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reveal faster charge extraction and J-V measurements implying better performances and reduced J-V hysteresis. Consequently, TiO2/SnO2-based PSCs show a maximal PCE value of 16.8 %. More importantly, it is found the employment of the TiO2/SnO2 ETL significantly improves the longterm stability of PSCs. Therefore, the composite TiO2/SnO2 nanoparticle film is a very promising ETL for fabricating efficient and stable perovskite solar cells, potentially boosting the already high efficiency. ASSOCIATED CONTENT Supporting Information Supporting Information Available: TEM images, XRD measurements, Transmission and UV-vis absorption spectra of TiO2 and TiO2/SnO2 nanocomposite films, SEM images; the statistics of hysteresis index of the PSCs; the average photovoltaic parameters of the PSCs; fitted parameters of TRPL decay curves in perovskite films; photovoltaic parameters of the PSCs measured under different scan directions. Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. Acknowledgments We acknowledge the financial support from the Recruitment Program of Global Young Experts of China and Sichuan one thousand Talents Plan. References


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83x35mm (300 x 300 DPI)


SnO2 Nanocomposites as Electron Transporting Layer for

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