UV-Sintered Low-Temperature Solution-Processed SnO2 as Robust

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UV-Sintered Low-Temperature Solution-Processed SnO2 as Robust Electron Transport Layer for Efficient Planar Heterojunction Perovskite Solar Cells Like Huang, Xiaoxiang Sun, Chang Li, Jie Xu, Rui Xu, Yangyang Du, Jian Ni, Hongkun Cai, Juan Li, Ziyang Hu, and Jianjun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 15, 2017

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ACS Applied Materials & Interfaces

UV-Sintered Low-Temperature Solution-Processed SnO2 as Robust Electron Transport Layer for Efficient Planar Heterojunction Perovskite Solar Cells

Like Huang,a Xiaoxiang Sun,a Chang Li,a Jie Xu,b Rui Xu,a Yangyang Du,a Jian Ni,a Hongkun Cai,a Juan Li,a Ziyang Hu*b and Jianjun Zhang*a

a. College of Electronic Information and Optical Engineering, Nankai University, The Tianjin Key Laboratory for Optical-Electronics Thin Film Devices and Technology, Tianjin, 300071, China. b. Department of Microelectronic Science and Engineering, Ningbo University, Zhejiang, 315211, China.

Keywords: Tin dioxide; UV irradiation; Low-temperature processing; Electron transport layer; Perovskite solar cells

Abstract: Recently, low-temperature solution-processed tin oxide (SnO2) as versatile electron transport layer (ETL) for efficient and robust planar heterojunction (PH) perovskite solar cells (PSCs) has attracted particular attention due to its outstanding properties such as high optical transparency, high electron mobility, and suitable band alignment. However, for most of the reported works, an annealing temperature of 180 °C is generally required. This temperature can reluctantly said to be a low temperature, especially with respect to the flexible application where 180 °C is still a little too high that PET flexible substrate cannot bear. In this contribution, low temperature (about 70 °C) UV/ozone treatment was applied to in situ synthesis of SnO2 films deposited on the FTO substrate as ETL. This method is a facile photochemical treatment, which is simple to operate and can easily eliminate the organic components. Accordingly, PH PSCs with UV-sintered SnO2 films as ETL

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were successfully fabricated for the first time. The device exhibited excellent photovoltaic performance as high as 16.21%, which is even higher than the value (11.49%) reported for counterpart device with solution-processed and high temperature annealed SnO2 films as ETL. These low temperature solution-processed and UV-sintered SnO2 films are suitable for the low-cost, large yield solution process on a flexible substrate for optoelectronic devices.

■INTRODUCTION Recently, organic-inorganic hybrid perovskite solar cells (PSCs) have attracted tremendous research attention in the photovoltaic field and highest power conversion efficiency (PCE) of 22.1% has been accomplished.1-9 Generally, high-efficient PSCs typically use electron transporting layers (ETLs) and hole transporting layers (HTLs) to separate and collect photo-carriers generated in perovskite layers. These layers are critical for achieving high-efficient cells as they prevent severe carrier recombination at interfaces, which ensures high the open-circuit voltages (Voc) and fill factors (FF) of devices.10,11 To further reduce the manufacturing cost and to be compatible with low-temperature solution large scale fabrication of PSCs, low temperature solution processed ETLs are widely proposed.12,13 Although TiO2 is the most widely used ETL material for PSCs and constantly record-breaking devices are based on it, the low mobility (0.1-1 cm2/V·s) which is even less than CH3NH3PbI3 (20-30 cm2/V·s) makes it not an ideal and ultimate ETL material of choice.14-16 Therefore, searching for electron transport material features higher electron mobility and conductivity is substantial to improve the electron transfer process of PSC thus the device performance and possibly stability. Among them, SnO2 has an electron mobility about 100-200 cm2/V·s, which is one and two order of magnitude higher than CH3NH3PbI3 and TiO2.16 SnO2 is a metal oxide semiconductor that has not only a much higher electron mobility but also a wider band gap (3.6 eV) than TiO2.17 Thus, SnO2 is more stable than TiO2 under UV illumination.10,15 For instance, Snaith et al. reported that both oxygen vacancies and


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UV light induced degradation in TiO2 are key factor to reduce the stability of TiO2-based photovoltaic devices.18 Miyasaka et al. found that PH PSC device based on TiO2 prepared with the same sequential deposition method degrades apparently faster than the SnO2-based devices.15 With its larger bandgap, SnO2 with little parasitic absorption also cause only a small ETL-induced current loss than TiO2. In fact, SnO2 can enhance the UV transmittance of the substrate as reported.10 Considering the favorable alignment of the conduction bands of the perovskite materials and the ETL, SnO2 is more suitable an ETL than TiO2. Hagfeldt et al. found that PH PSCs using TiO2 are inherently limited due to conduction band misalignment.19 By adopting atomic layer deposition (ALD) deposited SnO2 with a deeper conduction, they achieved a barrier-free energetic configuration in PH PSC that presents almost hysteresis-free PCEs of over 18% with record high voltages of up to 1.19 V. Ke et al. firstly reported low-temperature solution-processed nanocrystal SnO2 as ETL enabling an efficient PH PSC with highest efficiency of 17.21%.10 Jiang et al. reported a 19.9% certified efficiency of planar-structure HC(NH2)2PbI3-based PSCs using solution-processed SnO2 as an ETL at 150 °C.10 To summarize, the remarkable material properties of SnO2, such as wide-bandgap, suitable band edge positions, high charge carrier mobility, higher conductivity,20 good anti-reflection, coupled with low production cost and the compatibility with low temperature solution processing techniques for flexible applications, are the main driving force for the development. However, one of the key obstacles in using solution-processed metal-oxide (MO) semiconductors on low-cost plastic substrates arises from the precursor-to-MO conversion that typically requires a high temperature of 200-500 °C for attaining an impurity-free MO semiconductor.21 As for SnO2, an annealing temperature of 180 °C -190 °C is generally required in most of the reported works.10,14,15,22-24 This temperature can reluctantly said to be a low temperature, especially with respect to the flexible application where 180 °C is still a little too high that PET flexible substrate cannot bear. We also noticed that Jiang et al. reported 150 °C processed SnO2 with high degree of crystallinity as ETL for PSCs that presents a 19.9% certified efficiency. 3 ACS Paragon Plus Environment


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It must be noted, however, that their SnO2 colloid precursor was obtained from Alfa Aesar (tin(IV) oxide, 15% in H2O colloidal dispersion), which tells us little about the synthesis information and the property control of SnO2 as well as its association with the performance of the final device.10 To circumvent the problem, a wide selection of novel annealing methods have been developed that allow the metal oxide network formation at a low external temperature, such as ultraviolet (UV)-assisted,25-27 ozone,28 vacuum,29 combustion,30 and microwave annealing methods.31 UV-assisted annealing of MO precursor films, as introduced earlier for dielectric MO films,32,33 and, recently, also for MO semiconductor films,25 has been shown to allow a rapid precursor-to-metal oxide conversion. Using UV-assisted annealing techniques, various dielectric MO films with high density and working IZO TFT devices have been obtained as swiftly as with a 5 min exposure.26,27 Recently there have been several reports concerning the application of ultraviolet irradiation-assisted sol-gel synthesis of TiO2 for DSSC and PSCs.34-39 With a high photon energy of about 4.04 and 5.54 eV, the UV light can cut off most of the common molecular bonds, decompose organic groups and promote the chemical transition of the precursor to the metal oxide desired. UV-curing tunnels are readily available in a wide range of irradiation power densities and spectral power distribution profiles and can be easily added to the deposition line of roll to roll devices.36 In this contribution, low temperature (about 70 °C) UV/ozone treatment was applied to in situ synthesis of SnO2 films deposited on the FTO substrate as ETL. PH PSCs with UV-sintered SnO2 films as ETL exhibited excellent PCE of 16.21%, which is even higher than the value (11.49%) of the counterpart device with solution-processed and a high temperature of 180 °C thermal annealed SnO2 films as ETL. These low temperature solution-processed and UV-sintered SnO2 films are suitable for the low-cost, large yield solution process on a flexible substrate for optoelectronic devices.


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■EXPERIMENTAL SECTION The experimental procedure is illustrated in Figure 1 with all the detail processes described in the following sections.

Materials All chemicals and reagents were purchased and used as received from commercial sources without purification, including SnCl2·2H2O (98%, Meryer, Shanghai), anhydrous ethanol (99.5%, Meryer, Shanghai), PbCl2 (99.999%, Sigma-Aldrich), CH3NH3I (Secondary crystallization purification, Materwin, Shanghai), dimethylforaormamide (DMF, 99.99%, J&K reagent), spiro-MeOTAD (99.9%, Shenzhen Feiming Science and Technology Co., Ltd.), Li-bis (trifluoromethanesulfonyl)imide (Li-TFSI, Acros), 4-tert-butylpyridine (4-tBP, Sigma-Aldrich), aceto-nitrile, and chlorobenzene. The purity of silver wire adopted is 99.99%.

Synthesis of SnO2 sol-gels The facile synthesis of SnO2 sol-gel was carried out as previously reported.10,24 Generally, SnCl2·2H2O was dissolved in ethanol to form a solution with a concentration of 0.1 M and the precursor solution was spun on FTO/glass substrate at 3000 rpm in ambient condition. The mechanism of forming SnO2 sol-gel during the synthesis can be expressed as follows:17 SnCl2·2H2O + 2C2H5OH → Sn(OH)2 + 2C2H5Cl

(1)

Sn(OH)2 → SnO + H2O

(2)

2SnO + O2 → 2SnO2

(3)

Processing of SnO2 Films. SnO2 films were prepared by spin-coating the as prepared SnO2 precursor solutions on clean fluorine-doped tin oxide glass substrates (FTO, ~14 Ω/sq., TEC10, Pilkington, NSG).



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For the preparation of thermal-annealed SnO2 films, the films were heated at 180 °C for 60 min on a hot plate. For the preparation of UV-processed SnO2 ETL, a UV ozone cleaner (YZUV-22C, Beijing Kenuo Instrument Co., Ltd) equipped with Hg lamp was used as the platform for in situ preparation of the SnO2 film. The UV lamp adopted can provide a power of 200 W with an irradiation area about 400 cm2. Thus the power density is about 0.5 W/cm2. The radiation height can be easily adjusted at the range from 20 to 40 mm. Simply, the as deposited SnO2 films on the FTO/glass substrates were placed in the UV processor drawer at a distance of about 40 mm from the UV lamp. The UV/ozone treatment unit can produce ultraviolet light at two wavelength of 253.7 nm and 184.9 nm with high-energy simultaneously. The corresponding photon energy are 472 and 647 kJ/mol, respectively. Both photon energy are higher than that of the Sn-Cl and O-H with a bond energy of 350 and 459 kJ/mol, respectively.40-42 They are also higher than the bond energy of C-C, C-H and C-O of about 346, 411 and 358 kJ/mol, respectively.42 Therefore, the UV light can easily break these chemical bonds so that the reaction continues to take place. While the 184.9 nm-wavelength of light can convert oxygen molecules O2 into active ozone molecules O3, which facilitates the formation of SnO2 and decomposition as well as oxidation of organic components. The final by-product will be released in the form of carbon dioxide (CO2) and water (H2O) vapor. Also, the UV/ozone instrument has the function of in-situ heating, and generally the temperature is below 100 °C. The temperature of the sample during UV treatment is about 70 °C as indicated by an infrared thermometer (Figure S1).

Device Fabrication Pre-patterned FTO were cleaned by washing with ethanol, acetone, and isopropanol for 15 min. To produce SnO2 ETLs, the sol-gel synthesized SnO2 precursor were deposited onto FTO substrates using the UV curing method mentioned above. For comparison, we also prepared a high-temperature (180 °C) sintered SnO2 film by spin coating the SnO2 precursor solution at 2000 rpm for 40 s, followed by heating at 180 °C for 60 min (Figure 1 (a)). Figure 1 (b) presents the schematic diagram of the 6 ACS Paragon Plus Environment

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proposed reaction mechanism of precursor-to-metal-oxide conversion.

Figure 1. Schematic diagram of (a) the device fabrication process and (b) the proposed reaction mechanism.

The perovskite CH3NH3PbI3-xClx films were fabricated by the one-step solution process and multi-step (MS) ramp annealing method as reported in our previous literature.43,44 Generally, a 1:3 ratio of PbCl2/CH3NH3I was mixed in DMF to form the perovskite precursor with concentrations of 0.73 and 2.2 M for PbCl2 and CH3NH3I, respectively. The mixture solution was stirred at 60 °C overnight and spin-coated on the SnO2/FTO substrates at 2500 rpm for 40s. Then the sample were placed on a hot plate to be annealed with MS ramp annealing method for about 90 min. To deposit the hole transport layer a spiro-MeOTAD and chlorobenzene solution was spin-coated on the as prepared perovskite films at 2800 rpm for 30 s. Specifically, 1 mL of spiro-MeOTAD/CB (72.3 mg/mL) solution was employed with addition of 18 µ L of Li-TFSI/acetonitrile (520 mg/mL), and 29 µL of 4-tBP. Lastly, a 120 nm thick silver layer was thermally evaporated on top of the device under a pressure of 5×10-6 Torr to form the back contact and complete the preparation of the device. All the device fabrication was processed in glove box filled with N2 except for the SnO2 covered substrates.

Characterization A scanning electron microscope (SEM, JEOL JSM-7800F) was used to confirm the top view images. A UV-Vis-NIR spectrophotometer (VARIAN Cary 5000) was used to record the UV-Vis absorption spectra or transmission spectrum of the SnO2 and perovskite films. An X-ray diffractometer (XRD, Bruker AXS D8 Advance) was used to obtain the X-ray diffraction (XRD) pattern (2θ scans) of perovskite films using Cu-Kα radiation (λ=1.54050 Å). Steady-state photo-luminescence spectroscopy (PL) measurements were acquired using a Cary Eclipse fluorescence spectrometer with an excitation wave length of 460 nm at room temperature. The binding energies in SnO2 7 ACS Paragon Plus Environment


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films were determined using X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250Xi). The J-V or I-V measurements of all devices were conducted under dark or simulated sunlight of 100 mW/cm2 using an AM 1.5G type filter (Abet sun 2000 solar simulator). The light intensity was adjusted by using a standard Si cell. I-V curves were obtained by applying an external bias to the devices and measurements were recorded with a Keithley model 2400 digital source meter at room temperature in the ambient air. The effective area of the cell was defined to be 0.06 cm2 by a non-reflective metal mask. The incident photo-current conversion efficiency (IPCE) spectra were measured in air on a Newport 2936-c power meter under the irradiation of a 300 W xenon light source with an Oriel Corner-stone 260 1/4 monochromator in DC mode. To allow the background to be subtracted, a reference scan with the Si detector was taken prior to the sample measurement.

■RESULTS AND DISCUSSION Figure 2. (a) Schematic device structure, (b) energy level diagram of the PH PSCs using UVLT SnO2 films as the electron transport layer and (c) the image of a complete device that contains 8 sub-cells.

Our device has a planar heterojunction structure as shown in Figure 2a. The corresponding band structure of the device at flat band condition is shown in Figure 2b. With suitable conduction band and wide band gap SnO2 is a suitable electron transport layer of choice enabling the device efficient electron transmission and hole blocking function. Figure 2c gives a photo of a completed device. In the previous work, an annealing temperature of 180 °C was widely adopted to sinter the SnO2 film. However, with only 180 °C of sintering, the infiltration of perovskite precursor solution on the SnO2 film surface is relatively poor in spite of large grain of 1-3 µm, as evidenced by the visible poor perovskite film coverage on


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such film (Figure S1 and Figure 3 (a, b)), indicating poor surface wettability of such films. As such, the 180 °C sintered SnO2 ETLs were further treated by UV-ozone for about 10 min before depositing the perovskite film. After UV-ozone treatment, the perovskite precursor solution can be well spread on the surface of the SnO2 film (Figure S1 and Figure 3 (c, d)). This enlightened us that UV light with high photon energy can efficiently modify the surface and direct SnO2 precursor-to-SnO2 conversion can be possible. Hereafter, unless specifically stated, all control device based on 180 °C thermal annealed SnO2 film is further treated with UV for 10 min before depositing the perovskite films. Figure 3. SEM images of the perovskite films deposited on different substrates: (a, b) on HT SnO2/FTO/glass without UV treatment, (c, d) on HT SnO2/FTO/glass with UV treatment, (e, f) on UVLT SnO2/FTO/glass and (g, h) on FTO/glass.

Figure 4. (a) Optical transmission, (b) UV-vis absorption and (c) room temperature PL spectra of SnO2 films coated on glass substrates with different treatment.

Fortunately, we have noticed recently that a wide selection of novel annealing methods have been developed that allow the precursor-to-MO conversion and thus MO network formation at a low external temperature as mentioned above. So we first tried to treat the SnO2 precursor film with UV for one hour to get SnO2 film and compared its properties with that of the one hour and 180 °C sintered SnO2 film. The experimental results are shown in Figure 4. From Figure 4 (a), the UV processed film and the thermal annealed film present a same shape of transmission curve in the entire UV-visible wavelength range. The transmission intensity is consistent with the value reported in other literature.10 The substrate deposited with SnO2 film has a higher transmittance across the visible wavelength compared to the substrate itself, so SnO2 also severs as efficient anti-reflection film. The UV-vis absorption spectra of the two SnO2 films (Figure 4 (b)) reveals a strong ultraviolet absorption peak at 285.3 and 280.3 nm for 180 °C sintered SnO2 film and UV processed one, respectively, which 9 ACS Paragon Plus Environment


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corresponds to an excitation energy of 3.59 and 3.65 eV, respectively. Figure 4 (c) gives the PL emission spectra of SnO2 film coated on glass substrates. The corresponding emission peak were located at 483.7 and 484.8 nm, respectively. The distinctive blue emission has been previously ascribed to oxygen vacancy-related defects, Sn interstitial, and Sn vacancies that occur during the synthesis.45-50 However, taking into account the precursors and synthetic pathways we use, for our situation here, oxygen vacancy is unlikely possible, it is more likely to be due to the hydroxyl group, which will be discussed in the following. Anyway, it could be expected that such defects in metal oxides act as recombination centers in solar cells and reduce power conversion efficiencies. Then, we try to directly use UV light as sintering technique to prepare SnO2 film and then the perovskite film was deposited on such SnO2 film. With a same processing time of 60 min just like the 180 °C thermal annealing, the perovskite film on such SnO2 film presents dense and uniform property (Figure 3 (e, f)), similar with those perovskite film we previously deposited on the FTO/glass substrate (Figure 3 (g, h)). Figure 5. UV-visible absorption spectra (a), PL spectra (b) and XRD patterns (c) of the perovskite films deposited on different substrates.

Figure 5 gives the UV-visible absorption spectra (a), PL spectra (b) and XRD patterns (c) of the perovskite films deposited on above mentioned different substrates. Generally, the perovskite films deposited on FTO or UVLT SnO2 film presents slightly stronger UV absorbance and crystallinity with respect to that deposited on the 180 °C thermal annealed one. PL spectra suggests more efficient interface charge transfer between perovskite film and UVLT SnO2 film than between perovskite film and 180 °C thermal annealed one.

Figure 6. Device performance of the PSCs using different ETL: (a) J-V curves under AM 1.5 G irradiation (100 mW/cm2) and (b) the corresponding IPCE spectrum. 10 ACS Paragon Plus Environment

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Figure 6 gives the J-V curves and IPCE spectrum of the corresponding device. Without ETL, the device presents much lower PCE of 7.31% with low Voc, Jsc and FF (Table 1), possibly due to inferior interface charge transfer as depicted in Figure 5 (b) and/or severe interface recombination due to the large number of interface defects. This is further demonstrated by the increased Rsh by inserting ETL (Table 1) as a smaller Rsh means greater leakage current. With SnO2 as ETL, the device performance was obviously promoted to be exceeding 10%. The UVLT SnO2 based device performed better than the HT SnO2 based one in terms of all performance parameters (Figure 6 (a) and Table 1), which coupled with the gradually increased IPCE value among the whole visible-wavelength (Figure 6 (b)), suggest that the potential of UV sintering as an effective means for preparing efficient ETLs thus efficient PSCs.

Table 1. The corresponding photovoltaic performance parameters of the PSCs. Device

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Rsh (Ω cm2)

Rs (Ω cm2)

ETL-free

0.96

14.89

51.19

7.31

548.75

26.37

HT-SnO2

1.01

19.82

57.40

11.49

582.93

9.81

UVLT-SnO2

1.06

21.94

61.73

14.36

769.23

6.19

So far we have found that the UV processed SnO2 film exhibit comparable optical and photonic properties to the 180 °C thermal annealed one that enable close and even higher device photovoltaic performance. Next, we will systematically study the effect of UV treatment on the SnO2 precursor-to-SnO2 conversion process and its effect on the final device performance.

Figure 7. Top-view SEM images of SnO2 films with (a) 0 min, (b) 30 min, (c) 60 min, (d) 90 min, (e) 120 min of UV treatment and (f) 60 min of 180 °C thermal annealing. The white scale bar in the figure represents 1 µm.

Figure 7 displays top-view SEM images of the SnO2 films. The surface 11 ACS Paragon Plus Environment


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morphology of all films retained to some extent the FTO morphology underneath, suggesting that due to its small thickness SnO2 film is conformally covered on FTO surface (Figure S2 and Figure S3). With a short time of UV treatment (Figure 7 (a, b)), many suspected pinholes are found on these films. Obviously, the presence of these pinholes will weaken the hole blocking function of SnO2 as ETL. The 60-min-UV-sintered SnO2 film (Figure 7 (c)) show good coverage as no visible pinholes is found. With UV treatment time continues to increase, FTO surface-like morphology becomes more apparent, it seems that SnO2 films became thinner because of excessive UV irradiation (Figure 7 (d, e), this will be discussed further in detail in the Supporting Information section). In this case, local FTO exposure thus device short circuit is very possible. For the 180 °C thermal annealed SnO2 film (Figure 7 (f)), it also contains many pinholes which is detrimental to the performance of the device, which will be involved in the subsequent discussion. Figure 8. Optical transmission of the UVLT-SnO2 film (a), I-V linear sweeping curve (b) and I-V curve as a function of the scanning direction and speed of the FTO/UVLT-SnO2/Ag device with different UV treatment time (c). The inset of pane (b) gives the schematic structure and photographs as well as the corresponding band diagram of the device.

Figure 8(a) gives the optical transmission spectra of the SnO2 films deposited with different UV treatment time. With a shorter UV treatment time (0 and 30 min), these films show inferior transmittance. Extending the UV treatment time, the transmittance of these films get better. It is interesting to note that both 180 °C thermal annealed and 60 min UV-sintered SnO2 films coated substrates present optical transmission higher than that of the FTO substrates. This interesting phenomenon is also reported in other reference, which is considered to be because a smoother surface due to SnO2 film coating can be beneficial to the light transmission.10 Electrical conductivity is a critical figure of merit for the SnO2 film as ETL. Figure 8(b) shows the linear sweep I-V curves of the SnO2 film deposited on FTO/glass substrates with different UV 12 ACS Paragon Plus Environment

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treatment time. The measurement was conducted on a longitudinal device that is similar to a solar cell by collecting the longitudinal current under a linear sweep bias. The inset of Figure 8(b) gives the sample structure for this measurement. Inset (b, c) give the device photos and corresponding band structure. In the I-V coordinate system, the slope directly reflects the conductivity of the film. It is apparent that the electrical conductivity of the SnO2 film is significantly increased by extending the UV processing time until the UV treatment reaches 120 min. The conductivity of the 60 min UV-radiated SnO2 film is very close to that of the 180 °C annealed one. In view of the fact that the trap state of ETL may lead to the hysteresis effect of PH PSCs,51 we investigated the dependence of I-V curves of the ITO/SnO2/Ag device on the scanning rate and direction (Figure 8(c)). We did observed a dependency of I-V curve on the scanning rate and direction. This phenomenon may be due to the defects existed in the low temperature solution processed metal oxide semiconductor. However, we did not found a clear regular dependency relationship of I-V curve on the scanning rate and direction. Anyway, such a scan direction and speed dependent I-V response are likely to contribute partially to the hysteresis effect of the device just like the TiO2 based PH device.51

Figure 9. XPS spectra of SnO2 films coated on glass substrates with different treatment: (a, b, c) survey, (d, e, f) Sn 3d, (g, h, i) O 1s, (a, d, g) 0 min, (b, e, h) 60 min and (c, f, i) 90 min, respectively.

The compositional nature of the precursor-to-metal oxide conversion is further studied by X-ray photoelectron spectroscopy (XPS, Thermo Scientific). The measurements were conducted on an ESCALAB 250Xi system that is equipped with monochromated Al Kα source and a hemispherical electron analyzer. XPS data were collected at a photoelectron takeoff angle of 90°. Figure 9 shows the survey, Sn 3d and O 1s core shell photoelectron spectrum taken on SnO2 films sample treated with different time of UVO. The raw data were fitted by peak to analyze changes in different components. 13 ACS Paragon Plus Environment


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The full XPS spectrum survey given in Figure 9(a, b, c) shows the presence of O, Sn and different levels of Cl in those SnO2 films with 0, 60 and 90 min of UV treatment, revealing that the composition of the films prepared by SnCl2·2H2O precursor at a low temperature is SnO2. Generally, for SnO2 the Sn 3d5/2 and Sn 3d3/2 peaks present binding energies of about 487 and 495 eV, respectively, as is reported in several works.10,17,24 From Figure 9(d), the SnO2 films without UV sintering gives Sn 3d5/2 and Sn 3d3/2 peaks with binding energies at 487.8 and 496.4 eV, respectively. This chemical shift implies that without UV radiation, the precursor-to-MO conversion process mainly stay in the reaction process (1) as mentioned in the Experimental section. This hypothesis is also confirmed by the huge chlorine content exists in such film as shown in Figure S4. Extending the UV treatment time to 60 or 90 min, the binding energies for Sn 3d5/2 and Sn 3d3/2 peaks move toward to 487.2 and 495.6 eV, respectively, which values are very close to those ascribed to SnO2 as reported in the reference.10,52 Therefore, we can claim that SnO2 has begun to generate at about 60 min. Figure S4 gives the XPS Cl 2p core level spectra of corresponding SnO2 films. The dashed lines are the two Cl 2p spin-orbit split (p3/2 and p1/2) peaks.53 The solid line is the sum of the Cl 2 p3/2, 1/2 peaks. This excellent fit to the raw data indicates a single Cl species on the surface. It can be seen from Figure S5 (a) that the intensity of Cl 2p core level decreases almost linearly with UV treatment time during the observation time of period, indicating that the Cl loss with the reaction takes place according to equation (1) as shown in the main text. It should be noted that the final Cl residue should be very little, as indicated by the strength of Cl 2p core level approaching zero when treated for 90 min. This suggests that most of the precursor has been chemically reacted in accordance with process (1). However, it is intriguing that the intensity of -OH bond maintained at a high intensity and decreases a little within the observation time range (Figure 9 (g, h, i) and Figure S5 (b)). The main binding energy of about 531.1 eV is attributed to the O 1s, which is the O2- state (or O-Sn bonds) in SnO2. The higher binding energy of about 352.5 eV can be assigned to the hydroxyl groups (-OH).10,54 This finding can be 14 ACS Paragon Plus Environment

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related to a more hydroxylated surface after UV treatment. In their previous work, Hirsch et al. found that the UV-processed films contained much more hydroxyl groups than the HT-treated ones which was consistent with FTIR measurement that indicates a higher contribution of the stretching vibration bands of -OH and Sn-O bonds of Sn-OH groups in the spectrum of UV-treated films than that of its HT-processed danalogue.54 High levels of -OH may be derived from the incomplete reaction of process (2), which may cause trap states, as evidenced by the PL measurement mentioned above. This also implies the presence of amorphous regions in such film with amorphous structure, as is often the case for the most low temperature solution processed metal oxides (Figure S6). Of course, there is another possibility, that is, the intensity of these peaks concludes the contribution of the chemisorbed oxygen atoms, but not only -OH. This is possible because of the fact that the UV ozone cleaner can produce a large amount of reactive oxygen species or ozone during the treatment of the substrate. While, it is worth mentioning that a similar case was reported by Liu et al. in their recent published work on Nb doped SnO2 ETL where a high -OH intensity also occurs.24 In spite of the high intensity of such a side peak, the device still gives an amazing efficiency of 17.57%. While the author assigned it to the chemisorbed oxygen atoms but not only hydroxyl groups. As such, we attribute this to the “electron highway” (Figure S7) built by the crystalline or quasi-crystalline part in the amorphous SnO2 film considering that SnO2 has an astonishing intrinsic mobility as high as 100-200 cm2 V-1s-1 as mentioned above. In order to better compare the UVLT SnO2 films and the HT ones (especially for comparing with the -OH bond intensity), the XPS data for HT-SnO2 films is further provided in Figure S5 (c-f). From Figure S5 (c-f), there is no residual Cl observed in the HT SnO2 films while the intensity of -OH is approaching 75k, which is higher than that of the SnO2 films. These results combined with the PL spectrum (Figure 4) indicate that the UVLT SnO2 films and the HT SnO2 films contain almost equivalent amount of -OH (Strictly speaking, the HT SnO2 films contain higher content of -OH with a -OH intensity of 75k relative to < 75k of the UVLT SnO2 films). Therefore, in the case of nearly similar defects, the reasons for the difference between the UVLT 15 ACS Paragon Plus Environment


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SnO2 based device and the HT SnO2 based one should be attributed to other factor. We believe that this factor is the difference in coverage between the two kind of SnO2 films as shown in Figure 7 (c, f) and as discussed above.

Figure 10. SEM images of the perovskite films deposited UVLT-SnO2 film with different UV treatment time: 0 min (a, b), 30 min (c, d), 60 min (e, f), 90 min (g, h) and 120 min (i, j). The inset of each pane gives the corresponding high resolution SEM images.

Figure 10 displays the perovskite films deposited on the as prepared SnO2 film. The CH3NH3PbI3-xClx perovskite films were prepared by one-step deposition and multi-step annealing method (Figure S8). For the 0-30 min-UV-treated SnO2 film (Figure 10 (a-d)), the perovskite films are unsatisfactory with small grain and poor coverage. With 60-120 min of UV treatment (Figure 10 (e-j)), the resulted perovskite films are dense and uniform, which can ensure efficient UV-vis light absorption and avoid possible short-circuit thus high efficient devices. We also note here that the perovskite films grown on 60-120 min-UV-treated SnO2 film or directly on FTO55,56 have different texture as compared with the perovskite film we previously grown on TiO2.43,44 It is difficult to determine the grain size of perovskite films grown on FTO and SnO2. This may be due to the fact that different substrate has different orientation induction effects on the growth of perovskite grain. Anyway, uniform and pinhole-free perovskite with grain of larger than 1 µm in diameter is still obtained, which can ensure a considerable PCE.57

Figure 11. XRD and PL of perovskite films on SnO2 film with different time of UV treatment.

Figure 11 (a) gives the XRD patterns of the perovskite films on SnO2 film with different time of UV treatment. It can be seen that the intensity of the diffraction peak of perovskite at 14.2° get higher and sharper with UV treatment time extending, 16 ACS Paragon Plus Environment

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suggesting better crystallinity and coverage as evidenced by the SEM images above. Figure 11(b) shows the steady-state PL spectra of SnO2/perovskite and the independent perovskite deposited on blank glass. Obviously, the perovskite film shows a considerably greater degree of PL quenching when on 60 min-UV-treated SnO2 film as compared to other ones. We attribute this to the low coverage and poor crystallinity of other SnO2 films as well as poor coverage of perovskite films on them.

Figure 12. Device performance of the PSCs with UVLT SnO2 electron transport layer with different time of UV treatment: (a) J-V curves under AM 1.5 G irradiation (100 mW/cm2), (b) EQE spectrum.

To elucidate the effect of UV treatment time thus SnO2 electron extracting and transporting property on the final PSC performance, the cells based on different SnO2 ETL are investigated. From Figure 12 (a), with a shorter time of UV treatment to SnO2 film the device gives very poor photovoltaic performance with inferior Jsc, Voc and FF (Table S2). A UV treatment time of 60 min gives the best device performance with PCE of 16.21%, Jsc of 21.95 mA/cm2, Voc of 1.07 V and FF of 69%. A longer UV treatment time of 90-120 min gives no further performance improvement, possibly due to the local FTO exposure thus FTO/perovskite direct contact which cause short circuit, as evidenced by the SEM measurement (Figure 7). The main factor that limits the efficiency of the devices here is the FF, which could be due to the considerable number of defective states exist in such low temperature solution processed film with amorphous structure, as evidenced by the PL measurement above. Figure 12 (b) gives the IPCE and the integrated current density of the corresponding device, which is close to those valve of Figure 12 (a) and Table S2, confirming to some extent the accuracy of our test. It is worth mentioning here that Dong et al reported a pseudo Jsc exceeding 30 mA/cm2 of SnO2 based PH PSC without a mask during their measurement which is attributed to the large lateral current caused by the large conductivity of SnO2.20 For our case here, we think it is unlikely to be so given the amorphous film properties of SnO2 we fabricated. Anyway, to ensure the accuracy 17 ACS Paragon Plus Environment


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of the J-V test, we used a mask during the measurement.

Figure 13. Photovoltaic performance of PSCs as a function of the UV treatment time of SnO2 film.

The photovoltaic parameters of devices using the SnO2 ETL for various UV irradiation times are shown in Figure 13. We found that 60 min of UV irradiation time provides sufficient dose for ~ 40 nm thick layers and that further longer time of treatment did not cause any other effect or further improvement on the performance of the prepared photovoltaic devices. Figure 14. Photovoltaic metrics and stability of the PSC devices based on the SnO2 films by UV-sintering or the conventional thermal annealing process as the ETL. For the stability test, the devices without encapsulation were saved in an ambient environment with 30% relative humidity.

The statistical photovoltaic parameters measured for each set of PSCs based on the UV sintered and 180 °C thermal annealed SnO2 are reported in Figure 14(a), which suggests an overall improvement of all device parameters of the PSCs based on the UV-sintered SnO2 as the ETL. Figure 14(b) shows the results of stability tests of the corresponding PSCs in an ambient environment with 30% relative humidity without encapsulation. We noted that the silver electrode damage caused by the test clip during the test is an important reason for the device's efficiency decay. Anyway, excluding this common factor, it is clear from these tests that the UV-sintered SnO2-based device gives better stability than that of the control device with the conventional thermal annealed SnO2. Lastly, the hysteresis effect of the UV-sintered SnO2-based device and the conventional thermal annealed SnO2 based device is simply examined as shown in Figure S9. At a fixed scanning speed for the two kinds of devices, forward and reverse scan gives different device parameters. According quantitative calculation, the 18 ACS Paragon Plus Environment

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conventional thermal annealed SnO2 based device presents higher degree of hysteresis with a hysteresis index value of 0.059 than the UV-sintered SnO2 based device with a hysteresis index value of 0.021. We infer that in addition to ion migration in perovskite,58 the defect state in defective SnO2 itself processed from low temperature solution technique is also a possible reason as confirmed by the voltage scan direction and rate dependent I-V curves of the SnO2 films (Figure 8(b)). Thus, further deep research addressing these trap state in SnO2 is necessary to further enhance the performance of such device.

■CONCLUSION In this contribution we reported on the preparation of SnO2 films via a low-temperature UV curing process and their application as the electron-transporting layer in planer perovskite solar cells. These SnO2 ETLs are formed by direct espousing the as deposited precursor films to UV irradiation with extreme low temperature (70 °C) of heating, without any further thermal or chemical treatment. The photovoltaic devices with these films as ETLs showed a highest efficiency of 16.21%, which was much higher than the devices containing SnO2 ETLs prepared by a conventional thermal annealing process (11.49%). Slower recombination rates along with more hydroxylated SnO2 layers would be at the origin of the improved performances of the UV-processed SnO2 films. Considering its simplicity and low cost, this strategy therefore can promote the development for manufacturing relatively high-performance, low temperature solution processed PH PSCs on flexible substrate for commercial applications.

■ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Optical images of the perovskite films (before annealing) deposited on SnO2 treated with different condition; Top SEM images of FTO and UV-sintered SnO2 film 19 ACS Paragon Plus Environment


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on FTO; SEM image and the corresponding EDS elemental mapping of the UV-sintered SnO2 film; The XPS Cl 2p core level spectra of SnO2 films coated on glass substrates; The intensity of Cl 2p core level and O-H of SnO2 films coated on glass substrates with different treatment; XRD patterns of the 180 °C thermal annealed SnO2 film and UV-sintered SnO2 film; Electronic process in the defective SnO2; Optical images of the perovskite films; Hysteresis analysis; AFM images of SnO2 films ( PDF ).

■AUTHOR INFORMATION Corresponding Authors * Phone: +8602223508046. E-mail: [email protected]. * E-mail: [email protected]. Notes The authors declare no competing financial interest.

■ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 11304170, 61504068, and 61377031). The author L. Huang would like to greatly thank his wife Ms. Xiaohong Cui for constant encouragement and constructive discussion during the excited preparing but somewhat painful revising process of this work.

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High-Temperature Persistent Photoconductivity Experiments in SnO2 Nanobelts. J. Phys. Chem. C 2013, 117, 7844-7849. (50) Gu, F.; Wang, S. F.; Lü, M. K.; Zhou, G. J.; Xu, D.; Yuan, D. R. Photoluminescence Properties of SnO2 Nanoparticles Synthesized by Sol-Gel Method. J. Phys. Chem. B 2004, 108, 8119-8123. (51) Zhang, F.; Ma, W.; Guo, H.; Zhao, Y.; Shan, X.; Jin, K.; Tian, H.; Zhao, Q.; Yu, D.; Lu, X. Interfacial Oxygen Vacancies as a Potential Cause of Hysteresis in Perovskite Solar Cells. Chem. Mater. 2016, 28, 802-812. (52) Wei, H.; Xia, Z.; Xia, D. One Step Synthesis of Uniform SnO2 Electrode by UV Curing Technology toward Enhanced Lithium-Ion Storage. ACS Appl. Mater. Interfaces 2017, 9, 7169-7176. (53) Lu, Z. Air-stable Cl-terminated Ge (111). Appl. Phys. Lett. 1996, 68, 520-522. (54) Tebby, Z.; Uddin, T.; Nicolas, Y.; Olivier, C.; Toupance, T.; Labrugère, C.; Hirsch, L. Low-Temperature UV Processing of Nanoporous SnO2 Layers for Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2011, 3, 1485-1491. (55) Huang, L.; Hu, Z.; Xu, J.; Sun, X.; Du, Y.; Ni, J.; Cai, H.; Li, J.; Zhang, J. Efficient Planar Perovskite Solar Cells without a High Temperature Processed Titanium Dioxide Electron Transport Layer. Sol. Energy Mater. Sol. Cells 2016, 149, 1-8. (56) Huang, L.; Xu, J.; Sun, X.; Du, Y.; Cai, H.; Ni, J.; Li, J.; Hu, Z.; Zhang, J. Toward Revealing the Critical Role of Perovskite Coverage in Highly Efficient Electron-Transport Layer-Free Perovskite Solar Cells: An Energy Band and Equivalent Circuit Model Perspective. Appl. Mater. Interfaces 2016, 8, 9811-9820. (57) Huang, L.; Sun, X.; Li, C.; Xu, R.; Xu, J.; Du, Y.; Wu, Y.; Ni, J.; Cai, H.; Li, J. Electron Transport Layer-Free Planar Perovskite Solar Cells: Further Performance Enhancement Perspective from Device Simulation. Sol. Energy Mater. Sol. Cells 2016, 157, 1038-1047. (58) Yuan, Y.; Huang, J. Ion Migration in Organometal Trihalide Perovskite and its Impact on Photovoltaic Efficiency and Stability. Acc. Chem. Res. 2016, 49, 286-293. 26 ACS Paragon Plus Environment

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Figure 1. Schematic diagram of (a) the device fabrication process and (b) the proposed reaction mechanism. 482x268mm (150 x 150 DPI)

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Figure 2. (a) Schematic device structure and (b) energy level diagram of the PH PSCs using UVLT SnO2 films as the electron transport layer.

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Figure 3. SEM images of the perovskite films deposited on different substrates: (a, b) on HT SnO2/FTO/glass without UV treatment, (c, d) on HT SnO2/FTO/glass with UV treatment, (e, f) on UVLT SnO2/FTO/glass and (g, h) on FTO/glass. 482x181mm (300 x 300 DPI)


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Figure 4. (a) Optical transmission, (b) UV-vis absorption and (c) room temperature PL spectra of SnO2 films coated on glass substrates with different treatment. 482x152mm (150 x 150 DPI)



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Figure 5. UV-visible absorption spectra (a), PL spectra (b) and XRD patterns (c) of the perovskite films deposited on different substrates.

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Figure 6. Device performance of the PSCs using different ETL: (a) J-V curves under AM 1.5 G irradiation (100 mW/cm2) and (b) the corresponding IPCE spectrum. 482x170mm (150 x 150 DPI)



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Figure 7. Top-view SEM images of SnO2 films with (a) 0 min, (b) 30 min, (c) 60 min, (d) 90 min, (e) 120 min of UV treatment and (f) 60 min of 180 °C thermal annealing. The white scale bar in the figure represents 1 µm. 482x241mm (300 x 300 DPI)


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Figure 8. Optical transmission of the UVLT-SnO2 film (a), I-V linear sweeping curve (b) and I-V curve as a function of the scanning direction and speed of the FTO/UVLT-SnO2/Ag device with different UV treatment time. The inset of pane (b) gives the schematic structure and photographs as well as the corresponding band diagram of the device. 482x152mm (150 x 150 DPI)



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Figure 9. XPS spectra of SnO2 films coated on glass substrates with different treatment: (a, b, c) survey, (d, e, f) Sn 3d, (g, h, i) O 1s, (a, d, g) 0 min, (b, e, h) 60 min and (c, f, i) 90 min, respectively. 482x328mm (150 x 150 DPI)


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Figure 10. SEM images of the perovskite films deposited UVLT-SnO2 film with different UV treatment time: 0 min (a, b), 30 min (c, d), 60 min (e, f), 90 min (g, h) and 120 min (i, j). The inset of each pane gives the corresponding high resolution SEM images. 482x904mm (96 x 96 DPI)



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Figure 11. XRD and PL of perovskite films on SnO2 film with different time of UV treatment. 482x1045mm (150 x 150 DPI)


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Figure 12. Device performance of the PSCs with UVLT SnO2 electron transport layer with different time of UV treatment: (a) J-V curves under AM 1.5 G irradiation (100 mW/cm2), (b) EQE spectrum. 482x170mm (150 x 150 DPI)



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Figure 13. Photovoltaic performance of PSCs as a function of the UV treatment time of SnO2 film. 482x472mm (150 x 150 DPI)


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Figure 14. Photovoltaic metrics and stability of the PSC devices based on the SnO2 films by UV-sintering or the conventional thermal annealing process as the ETL. For the stability test, the devices without encapsulation were saved in an ambient environment with 30% relative humidity. 482x178mm (150 x 150 DPI)


UV-Sintered Low-Temperature Solution-Processed SnO2 as Robust

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