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High performance planar perovskite solar cells based on CH ₃ NH ₃ PbI _3-

x (SCN) _x perovskite fi lm and SnO ₂ electron transport layer prepared in ambient air with 70% humility

Yang Zhou

^a

, Zongbao Zhang

^a

, Yangyang Cai

^a

, Hui Liu

^a

, Qiqi Qin

^a

, Qidong Tai

^b

, Xubing Lu

^a

, Xingsen Gao

^a

, Lingling Shui

^a

, Sujuan Wu

^a,*

, Jun-Ming Liu

^a,c

aInstitute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, 510006, China

bInstitute for Interdisciplinary Research and Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, Jianghan University, Wuhan, 430056, China

cLaboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China

a r t i c l e i n f o

Article history:

Received 21 September 2017 Received in revised form 8 December 2017 Accepted 10 December 2017 Available online 12 December 2017

Keywords:

Low temperature

CH3NH3PbIx(SCN)3-xplanar perovskite solar cells

Tin oxide

Photoelectronic properties Long-term stability

a b s t r a c t

In this work, the efficient and stable planar perovskite solar cells (PSCs) based on CH3NH3PbI3-x(SCN)x perovskite layer and low-temperature processed SnO2electron transport layer (ETL) have been fabricated in ambient air with 70% humility. The maximum processing temperature of the PSCs is only 150C. The effect of SnO2solution concentrations on photoelectronic characteristics of the PSCs has been sym- metrically investigated. At the optimized SnO2concentration, the PSCs based on SnO2ETL (SnO2-PSCs) achieve the champion power conversion efficiency (PCE) of 16.23% and average PCE of 15.82%, being much higher than that of the reference PSCs based on TiO2ETL. The higher PCE of SnO2-PSCs can be attributed to the improved carrier transfer and collection, and the suppressed charge recombination.

Importantly, the SnO2-PSCs without encapsulation demonstrate an excellent long-term stability. After being stored in ambient air with 70% humidity for 30 days, the PCE of the unsealed SnO2-PSCs decay only 15%. This work presents a simple low-temperature solution method to fabricate efficient and stable PSCs in ambient air with high humidity, which is beneficial for future application of PSCs.

©2017 Elsevier Ltd. All rights reserved.

1. Introduction

In the past few years, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased from 3.8% to over 22.1%

[1,2]. This indicates the bright future of PSCs in photovoltaic in- dustry[3,4]. The excellent performance of PSCs is attributed to their remarkable photoelectric properties such as high absorption coefficient[5], large charge carrier mobility[6], and long carrier diffusion length [7,8]. Besides the improved PCE, the moisture stability of PSCs is also a very important characteristic for practical applications and industrialization in the future[9e11]. The mois- ture stability of PSCs addresses two issues. On one hand, whether the PSCs can be fabricated in ambient air. Until now, most of the PSCs have to be prepared under entirely inert atmosphere or

anhydrous environment to avoid the decomposition of perovskite film[12]. This will increase the cost and limit the practical appli- cations[13], which is still a big challenge for PSCs.

On the other hand, it is a challenge for PSCs to maintain a good long-term stability in ambient air. To date, there are not many practical methods to solve this problem although some works have been done. For example, tetraethyl orthosilicate (TEOS), butyl- phosphonic acid 4-ammonium chloride (4-ABPACl) or polyethylene glycol (PEG) has been used to modify the surface of perovskitefilm and improve its stability in ambient air [14e16]. Although the moisture stability of the modified PSCs have been improved by these strategies, the intrinsic instability of perovskitefilm in ambient air remains unchanged and the PSCs still have to be prepared in anhydrous environment[17]. Another strategy is to fabricate PSC with a two-dimensional perovskite structure by incorporating C6H5(CH2)2NH3þ (PEA) into MAPbI3 matrix [18]. The PSC shows better moisture stability, but the efficiency is only 4.73%. Therefore, it is necessary to exploit a new way to simultaneously improve the

*Corresponding author.

E-mail address:sujwu@scnu.edu.cn(S. Wu).

Contents lists available atScienceDirect

Electrochimica Acta

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / e le c t a c t a

https://doi.org/10.1016/j.electacta.2017.12.076 0013-4686/©2017 Elsevier Ltd. All rights reserved.

moisture stability and photovoltaic performance of PSCs. It has been reported that the mixed halide perovskites CH₃NH₃PbI_3-xBr_x has better moisture stability when 10e15mol% Iare substituted by Br in MAPbI3[19]. The enhanced stability is attributed to the stronger interaction between Brand CH₃NH₃^þ. Recently, Tai et al. have re- ported the stable PSCs based on MAPbI3-x(SCN)xperovskite layer. At the optimum process, the PSC yields an average efficiency of 13.49± 1.01% and a maximum PCE of 15.12%[20]. However, the CH3NH3PbI3-x(SCN)xlayer was deposited on mesoporous TiO2films annealed at 500C. It is well-known that electron transport layer (ETL) prepared at low temperature benefit to simplify the process and reduce the cost of PSCs, while mesoporous ETL needs high- temperature annealing (>450C) which hampers the application onflexible plastic substrate[21]. Moreover, planar PSCs can be easily prepared by low-temperature process.

It is well known that the ETL has a significant impact on photovoltaic performance and long term stability of PSC[22]. TiO2, SnO₂ and ZnO are usually used as ETL in solar cells [22e25].

However, PSCs based on ZnO ETL have thermal instability at tem- peratures above 80C[25], while PSCs based on TiO2ETL degrade faster under UV illumination[23]. These are a critical challenge for practical application in terms of long term stability. Compared with TiO2 and ZnO ETLs, low-temperature processed SnO2ETL shows advantages such as a wider band-gap, higher transparency, larger electron mobility and better stability [22,23,25e27]. SnO2 have been widely used to fabricate ETL of PSCs[23, 24, 28e32]. Planar PSCs based on SnO₂ETL and (FAPbI₃)_1-x(MAPbBr₃)_xperovskite layer demonstrate an efficiency close to 20%[30,31]. The PSCs based on SnO2ETL prepared by low-temperature plasma-enhanced atomic- layer deposition method and MA0.7FA0.3PbI3perovskite layer with 2% Pb(SCN)₂additive display a PCE of 20.3%[32]. However, all of the PSCs were prepared in the glove box and its moisture stability still needs further exploring. Until now, the efficient and stable planar PSCs based on CH₃NH₃PbI_3-x(SCN)_x perovskite layer and low- temperature processed SnO2 ETL fabricated in ambient air with high humility have not been reported.

Based on these considerations, we report low-temperature processed planar PSCs consist of ITO/SnO2/CH3NH3PbI3-x(SCN)x/ Spiro-OMeTAD/Ag structure fabricated in ambient air with 70%

humidity. The effect of SnO₂solution concentrations on the opto- electronic properties of PSCs has been systemically investigated. It is found that the SnO2concentrations have a significant impact on the microstructure of perovskitefilms and the performance of PSCs.

Two types of PSCs are discussed in this work: TiO₂-PSC as a refer- ence where the TiO2film is used as ETL, and SnO2-PSC where the SnO2 film is selected as ETL. Upon optimization, the SnO2-PSCs demonstrate the champion PCE of 16.23% and an average PCE of 15.82%. The TiO2-PSCs prepared in the same process show the highest PCE of only 12.15% and an average PCE of 11.2%. The better performance of the SnO₂-PSCs is ascribed to the promoted electron transport and collection, and the suppressed charge recombination.

Moreover, the SnO2-PSCs without any encapsulation show a su- perior long-term stability. Even stored in ambient air with 70%

humidity for 30 days, unsealed SnO2-PSC still retains the 85% of initial efficiency. It is worth noting that the maximum processing temperature is only 150C. This work provides a low-temperature solution method to fabricated efficient and stable PSCs in ambient air with 70% humility.

2. Experimental section 2.1. Material and methods

ITO glass (15 oum/square, NSG) was etched with zinc powder and diluted hydrochloric acid, then cleaned successively with

detergent, deionized water, acetone and iso-propanol by ultra- sonication for 20 min. Then a SnO₂solution (15 wt% in H₂O colloidal dispersion, Alfa Aesar) was diluted by deionized water to 3%, 6%, 9%, and 12% with weight percentage, respectively. Compact SnO2films were prepared by spin-coating the SnO₂ precursor solution with different concentrations at 5000 rpm for 30 s and then annealed at 150C for 30 min in air. The TiO2compact layer was deposited on the ITO substrate which was treated by UV-ozone for 10 min and then was immersed in a TiCl4 solution (200 mM) at 70 C for 90 min. Thefinal TiO2films were annealed at 150C for 30 min in the air.

Fig. 1(a) shows the fabrication process of SnO2-PSCs. Firstly, Pb(SCN)2 powder (Sigma-Aldrich) was dissolved in dimethylsulf- oxide (DMSO, Sigma-Aldrich) at 500 mg/mL and stirred overnight at 60C on a hot plate, thenfiltered with 0.22m^{m nylon}filter to get a clear solution. Then the solution was spun on the SnO2coated ITO substrate (ITO/SnO2) and TiO2coated ITO substrate (ITO/TiO2) at 4000 rpm for 30 s in humid air, respectively. The Pb(SCN)₂layer was annealed at 90C for 30 min in ambient air with 70% humility. Then MAI (8 mg/mL dissolved in anhydrous isopropanol) solution was spin-coated on the Pb(SCN)₂film at 3000 rpm for 60s. Finally, the stacked precursors layer of Pb(SCN)2and CH3NH3I were annealed at 80C for 20 min in ambient air. After the annealing, a thin layer of Spiro-OMeTAD was deposited on the CH₃NH₃PbI_3-x(SCN)_xlayer by spin-coating a chlorobenzene solution containing 80 mM Spiro- OMeTAD, 64 mM tert-butylpyridine (TBP) and 24 mM Li- bis(trifluoromethanesulfonyl)-imide (Li-TFSI) (520 mg/mL in acetonitrile) at 4000 rpm for 30 s. Ultimately, an Ag electrode was evaporated on the sample surface through a shadow mask under a vacuum of 10⁴Pa. The sample size is 0.045 cm².

2.2. Characterizations

The morphology of the perovskitefilms and ETL was investi- gated by scanning electron microscopy (SEM, ZEISS ULTRA 55). The roughness and surface microstructure were characterized by atomic force microscopy (AFM, Asylum Research, Cypher). X-ray diffraction (XRD) pattern was measured by X'Pert PRO system with Cu Ka as the radiation source. The UVevis absorption spectra and transmittance spectra were measured by using a SHIMADZU UV- 2550 spectrophotometer. The current density-voltage (J-V) curves were measured using a Keithley 2420 source meter under an illu- mination of 100 mW cm²(Newport 91160). The light intensity was calibrated to AM1.5G by a standard silicon solar cell (certified by NREL) as the reference. The scan rate and delay time of the J-V measurement were 0.2 V/s and 100 ms, respectively. The external quantum efficiency (EQE) was measured using a standard EQE system (Newport 66902). The electrochemical impedance (EIS) spectra were obtained using a Zahner Zennium electrochemical workstation and a 50 mV ac-sinusoidal signal source was employed over the bias with the frequency ranging from 1 Hz to 1 MHz. The photoluminescence (PL) spectra were measured by afluorescence spectrophotometer (HITACHI F-5000) exited at 470 nm, which had been normalized to the absorbance and measured in the same circumstances.

3. Results and discussion

Fig. 1(b) and (c) show the structure of SnO2-PSC and the XRD pattern of SnO₂ ETL spin-coated on a slide glass substrate after annealing at 150 C for 30 min in air, respectively. The four diffraction peaks in the XRD pattern at 27, 34, 38 and 52 are assigned to the (110), (101), (200) and (211) planes, respectively [30]. The peaks closely match the tetragonal crystal structure (JCPDS card: 41e1445), implying the pure SnO2crystals and high

crystallinity[33].Fig. 1(d) shows the SEM image of SnO2film pre- pared by spin-coating 9% SnO2solution. It can be seen that the SnO2

film is dense and uniform, which will benefit to form high-quality perovskitefilm.Fig. S1(Supporting Information) demonstrates the SEM image of TiO2film. Compared with the TiO2film, the SnO2film is more uniform and the grain size is smaller. It is well-known that the ETL has a significant influence on the morphology of perovskite film and the performance of PSCs. To explore the effect of SnO2ETL, Fig. 2(a) shows the J-V curves of SnO₂-PSCs. The detailed photo- voltaic parameters are listed inTable S1(Supporting Information).

Fig. 2(b and c) show the detailed photovoltaic parameters including the short-circuit current density (J_sc), open-circuit voltage (V_oc),fill factor (FF) and PCE as functions of SnO2 concentrations. At the beginning, the PCE of SnO2-PSCs increase with the increase of SnO2

concentrations. When the concentration is increased to 9%, the PSC shows the best performance, yielding a J_scof 22.82 mA cm², a V_oc of 0.944 V, a FF of 0.754 and a PCE of 16.23%. Then the PCE decreases with the increase of SnO2 concentration when it is over 9%.

Fig. S2(a)shows the J-V curves of the SnO₂-PSCs with different SnO₂ concentrations measured under both forward and reverse scan directions. Hysteresis index can be used to describe the hysteresis behavior of PSCs.Fig. S2(b)shows the hysteresis index of SnO₂-PSCs with different SnO2concentrations. Here hysteresis index of PSCs was calculated by the following equation[34]:

Hysteresis index¼

PCE_reversePCE_forward.

PCE_reverse

It can be seen that the hysteresis index of SnO₂-PSCs decreases with the increase of SnO2concentrations at the beginning. When the concentration is increased to 9%, the SnO2-PSC exhibits the lowest hysteresis index. Then the hysteresis index increases with the increase of SnO2 concentration. It is clear that the SnO2-PSC deposited on the ETL prepared by 9% SnO2concentration show the

smallest hysteresis. This is in agreement with the results ofFig. 2.

Thus the optimum SnO2concentration is 9%, which is the condition of SnO₂-PSCs in all of our other experiments, including the J-V, EQE, EIS, PL and stability samples shown fromFigs. 4e6,Fig. S4 and Fig. S5, respectively.

Fig. 2(eei) show the top view SEM images of perovskitefilms spin-coated on SnO2ETL prepared by SnO2solution with different concentrations. It can be seen that the grain size in the perovskite films gradually increases and the pinholes decrease with the in- crease of SnO2concentrations. The uniform and dense perovskite films with the grain size of about 400 nm can be obtained at the 9%

SnO₂ concentration, as shown inFig. 6(g). When the SnO₂ con- centration is higher than 9%, the pinholes appear in perovskitefilm.

The compact and pinhole-free perovskitefilm with large crystal size will promote charge transport due to fewer crystal boundaries and weaker carrier transport scattering[13]. As shown inFig. 2(g), the pinhole-free perovskitefilm can be obtained at the 9% SnO2

concentration. In this SnO2-PSC, the direct contact between SnO2

ETL and Spiro-OMeTAD can be avoided which will benefit to reduce the interface recombination, resulting in the improved Vocand FF [5]. These results are consistent with the variation of Vocand FF observed in J-V measurements discussed above.

The AFM images also show a significant change in the surface feature of SnO2ETL and the root mean square roughness (RMS) can be obtained from the AFM images, as shown in Figs. S3(eei).

Fig. 2(d) shows the RMS values of SnO2ETL as a function of SnO2

concentrations. At the beginning, the RMS values decrease with the increase of SnO2concentrations and achieve the minimum value of 0.847 nm at the 9% SnO₂concentration. Then the RMS values in- crease with the increase of SnO2concentrations when it is over 9%.

The trend is consistent with the result of SEM images, indicating that the SnO₂ETL with the minimum RMS value benefits to grow the dense and pinhole-free perovskitefilm.

Fig. 1. (a)Schematic illustration for the fabrication of SnO2ETL and CH3NH3PbI3-x(SCN)xfilms.(b)Schematic diagram of the SnO2-PSC structure. (c)XRD pattern of SnO2ETL. (d) SEM image of SnO2film deposited on the ITO substrate derived from SnO2solution with a concentration of 9%.

Fig. 2. (a)J-V curves of SnO2-PSCs.(b)and(c)photovoltaic parameters (Jsc, PCE, Vocand FF) of SnO2-PSCs as a function of SnO2concentrations. Error bars represent standard deviation calculated from ten devices prepared in the same process.(d)The RMS of SnO2films prepared by SnO2solution with different concentrations. Top view SEM images of perovskitefilms deposited on the SnO2films derived from SnO2solution with different concentrations(e) 3%,(f)6%,(g)9%,(h)12% and(i)15%, respectively. Scale bar is 1mm.

Fig. 3. (a)Transmittance spectra of ITO and ITO/SnO2films prepared by SnO2solution with different concentrations,(b)Absorbance spectra of perovskitefilms deposited on ITO/

SnO2prepared by SnO2solution with different concentrations,(c)EQE curves of SnO2-PSCs and(d)Rsh/Rsand FF as a function of SnO2concentrations. Error bars represent standard deviation calculated from ten devices prepared in the same process.

To further investigate the origin of the improved performance of the SnO2-PSCs,Fig. 3(a) illustrates the transmittance spectra of ITO/

SnO₂prepared by SnO₂solution with different concentrations. The transmittance of ITO/SnO2 is obviously higher than that of ITO substrate. This can be attributed to the antireflection of SnO2film. It is noted that the SnO₂ concentrations have little effect on the transmittance of SnO₂ ETL, which is due to the number of SnO₂ nanocrystals increasing and scattering reinforcement with the in- crease of the SnO₂thickness[33].Fig. 3(b) shows the absorbance of perovskite films deposited on ITO/SnO₂ with different SnO₂

concentrations. It can be seen that the absorption increases with the increase of the SnO₂concentrations. When the concentration increases to 9%, the absorbance achieves the maximum. The increased light absorption can be ascribed to the intensified surface coverage and the increased grain size of perovskitefilm[13,35], which will benefit to enhance Jsc. The absorption becomes worse when the concentration is over 9%. The variation of Jscfor SnO2- PSCs with the different SnO₂concentrations is further confirmed by EQE measurement.Fig. 3(c) compares the EQE curves of SnO2-PSCs at different SnO2concentrations. Obviously, the EQE increases with the increase of SnO₂ concentrations in the range of about 350e750 nm and reach its maximum when the concentration is 9%.

Then the EQE decreases when the concentration continues to in- crease. This is consistent with the varying of J_scobserved in J-V measurements and light absorption discussed above. Fig. 3(d) shows the plots for FF and the ratio of shunt resistance (Rsh) to series resistance (Rs), Rsh/Rs, as functions of SnO2concentrations.

The FF and R_sh/R_sincrease with the increase of SnO₂concentrations.

When the concentration is increased to 9%, the value of FF and Rsh/ Rsachieve maximum. Then they decrease with the increase of SnO2

concentrations. It is reported that the FF value depends on the R_sh/ Rs[36,37]. The higher FF is attributed to the larger Rsh/Rs. This can explain the variation of FF as functions of SnO2concentrations.

To further investigate and confirm the contribution of the SnO₂ ETL to the high performance of SnO2-PSCs, TiO2-PSCs have been prepared at the same process.Fig. 4(a) shows the J-V curves for the two PSCs. The TiO₂-PSC gives a J_scof 18.82 mA cm², a V_ocof 0.913 V, a FF of 0.707 and a PCE of 12.15%. The four parameters (Jsc, Voc, FF and PCE) of SnO2-PSC are increased to a 22.82 mA cm², 0.944 V, 0.754 and 16.23%, respectively. Obviously, SnO2-PSC demonstrates the higher cell parameters, thus larger PCE.Fig. S4shows the J-V curves of SnO2-PSC and TiO2-PSC measured under both reverse and forward scan directions. It is noted that the SnO2-PSC demonstrates the lower hysteresis index (~0.127) than that of TiO₂-PSC (~0.246).

This indicates that the SnO2ETL with faster charge extraction than TiO2ETL can reduce hysteresis induced by charge accumulation at the perovskite/ETL interface. This result is consistent with the recent reports[30].Fig. S5(Supporting Information) demonstrates the transmittance spectra of ITO/SnO2and ITO/TiO2films. It can be seen that the transmittance of ITO/SnO₂is obviously higher than that of ITO/TiO2. This will benefit to enhance the absorbance and performance of SnO2-PSCs.Fig. 4(b) shows the EQE (solid line) and the integrated J_sc(dash-dot line) curves of SnO₂-PSC and TiO₂-PSC.

It is can be seen that EQE in the range of 300e800 nm and inte- grated Jscfor SnO2-PSC are higher than that of TiO2-PSCs. This result is consistent with the Jsc obtained in J-V curves discussed above (error is about 4.3% and 6.9%, respectively). In order to investigate the reproducibility of the device performance, we have fabricated 50 devices for every batch.Fig. 4(c) shows the statistical histogram of the PCE for TiO₂-PSCs and SnO₂-PSCs. The PCE of TiO₂-PSCs and SnO2-PSCs were 10.2%e12.15% and 14.5%e16.23%, respectively. The average PCE for TiO2-PSCs and SnO2-PSCs is 11.2% and 15.82%, respectively. Obviously, the SnO₂-PSCs show the higher PCE. The average PCE of our planar SnO2-PSCs prepared by low-temperature process in ambient air with 70% humility is higher than that of the best PCE of the PSC based on the mesoporous TiO₂ETL annealed at 500C[20].

To understand the origin of the different performance of TiO2- PSCs and SnO2-PSCs, the electrochemical impedance (EIS) and steady state photoluminescence (PL) spectra were measured to investigate the charge transfer and recombination process.Fig. 5(a) shows Nyquist plots for TiO2- and SnO2-PSC measured at a bias of 0.8 V under light illumination. The solid lines inFig. 5(a) are the fitting results of experimental data using the inserted model, which shows the good fitting by the model [35,37e41]. The fitted Fig. 4.J-V curves(a),and EQE (solid line) and integrated Jsc(dash-dot line)(b)of SnO2-

PSC and TiO2-PSC.(c)Efficiency statistics histograms measured from 50 PSCs for each batch. The blue curve represents a Gaussian functionfitting to the data. (For inter- pretation of the references to colour in thisfigure legend, the reader is referred to the Web version of this article.)

equivalent circuit model is composed of series resistance (R_s), two parts of transfer resistance (Rtr) at the ETL/perovskite and the perovskite/HTL interfaces, and recombination resistance (Rrec) comprising a parallel circuit with capacitors (CPE_trand CPE_rec). For more reliablefitting, the constant phase angle element (CPE) instead of ideal capacitance (C) is used to consider the spatial inhomogeneity induced by defects and impurities on the interfaces[37]. The R_trand Rrec are ascribed to high-frequency arc and low-frequency arc, respectively.Table 1lists thefitted values of Rs, Rtrand Rrecobtained from the Nyquist plots. As shown inTable 1, the value of R_sfor TiO₂- PSC and SnO₂-PSC are similar. However, the R_trin TiO₂-PSC is about 82% higher that of the SnO2-PSC (6.52 and 3.58U^{cm2for TiO}2and SnO2-PSC, respectively). The value of Rtr is related to the carrier extraction and transport properties on the ETL/perovskite and perovskite/HTL interfaces[40,42e44]. Given that the only difference in TiO2-PSC and SnO2-PSC is the ETL, the Rtr and Rrec is mainly associated with the ETL/perovskite interface[42]. The larger R_trof TiO2-PSC originates from the larger interfacial trap states, which will result in the unfavorable carrier extraction to charge selective con- tacts[44]. Compared to that of the TiO₂-PSC, the lower R_trin SnO₂- PSC implies more efficient carrier extraction and transport at the SnO2/perovskite interface. Here the Rrecis mainly related to the defect densities and carrier recombination at the ETL/perovskite interface[25,37e39,44]. The R_recof SnO₂-PSC is about three times higher than that of the TiO2-PSC (24.54 and 66.74U^{cm2for TiO}2and SnO2-PSC, respectively). Moreover, the SnO2-PSCs show the same trend of higher R_recirrespective of different bias under light illu- mination (Fig. 5(b)) and in the dark (Fig. 5(c)). In the dark, carriers

are only injected from the supplied external voltage. The lower R_rec for TiO2-PSC irrespective of the applied bias can be related to the increased electron trapping at the defect states in the TiO2/perov- skite interface [38,41,44,45]. Under light illumination, the R_recfor both PSCs are reduced due to the photo-generated carriers. How- ever, even under illumination, the values of Rrecfor SnO2-PSC are significantly higher than that of TiO₂-PSC. This indicates that the charge recombination rates in the TiO2-PSC are higher than that of SnO2-PSC[35,46,47]. The lower Rtrand higher Rrecin the SnO2-PSC imply the more efficient carrier transfer and suppressed charge recombination in SnO₂-PSC, which will result in higher cell param- eter[22,29,44,48,49]. This is consistent with our J-V results inFig. 4.

Fig. 5(d) shows the PL spectra of perovskitefilms deposited on the ITO, ITO/TiO₂and ITO/SnO₂, respectively. It can be seen that the PL intensity of ITO/perovskite is obviously reduced upon the inserting of SnO2or TiO2ETL. Moreover, the ITO/SnO2/perovskitefilm shows lower PL intensity than that of the ITO/TiO₂/perovskite, indicating that SnO2 ETL show the enhanced charge collection efficiency compared to the TiO2ETL[24,37,38,50]. The results are consistent with the EIS characterization. This confirms that SnO₂ ETL does reduce the charge recombination rates in the PSCs, which contrib- utes to the enhanced charge extraction efficiency at the ETL/perov- skite interface and thus the improved performance of SnO2-PSC[30].

Finally, the stability of SnO₂-PSCs against humid circumstance is investigated. In our experiments, all of the SnO2-PSCs without encapsulation are stored in a sealed box with 70% humidity. As seen inFig. 6(a), the PCE of SnO₂-PSCs retains to be about 85% of the initial value after exposure in such circumstance for 30 days.

Moreover, the PCE decreases by 10% with aging time from 0 to 15th day while the PCE decreases by only 5.6% from the 15th to the 30^th day. To further understand the origin of stability, the samples after exposure for different days in ambient air were investigated by XRD patterns and SEM images. Fig. 6(b) shows the variations in XRD patterns at several days. Obviously, the intensity of (110) and (220) crystal planes for perovskitefilm decrease with time. The (001) Fig. 5. (a)Nyquist plots of SnO2-PSC and TiO2-PSC measured at a bias of 0.8 V under light illumination. The open symbol represents the experimental data and the solid line is the fitting results. Insert: the equivalent circuit model which is used tofit the data of the Nyquist plots. Rrecas a function of applied bias for SnO2and TiO2-PSCs extracted from the Nyquist plot under light illumination(b)and in the dark(c). (d) PL spectra of ITO/perovskite, ITO/TiO2/perovskite and ITO/SnO2/perovskite, respectively.

Table 1

Fitted value of Rs, Rtrand Rrecobtained from Nyquist plot of SnO2-PSC and TiO2-PSC measured at 0.8 V under illumination.

Rs/U.cm2 Rtr/U^.cm² Rrec/U.cm²

SnO2 1.32 3.58 66.74

TiO2 1.57 6.52 24.54

plane corresponding to the characteristic peak of PbI₂appear on the sample exposed for 15 days in ambient air and the intensity in- crease when the time prolongs to 30 days[51]. Besides, we inves- tigate the full width at half maximum (FWHM) value of the (110) peak of CH₃NH₃Pb_3-x(SCN)_xperovskitefilm at different time. The FWHM value for the sample exposed in the ambient air for 0^th, 15th and 30^thday is 0.368, 0.370 and 0.379, respectively. The increased FWHM value indicates the grain size of perovskitefilm decreases with time [52]. Fig. 6(c) demonstrates the relative intensity for (110) peak of perovskitefilm to (001) peak of PbI2.It is noted that the relative intensity decreases with time, indicating the obvious decomposition of the perovskite films upon the exposure in ambient air. To further confirm the XRD results,Fig. 6(def) shows the SEM top view images of perovskite films upon exposed in ambient air for 0, 15 and 30 days, respectively.Fig. 6(gei) shows the grain size distribution spectra of the perovskitefilms correspond- ing toFig. 6(def), respectively. It can be seen that the uniform, compact and pinhole-free of CH₃NH₃Pb_3-x(SCN)_x film have been obtained at the beginning. However, pinholes in the perovskite films after 30-day exposure are observed obviously. As shown in Fig. 6(gei), the average grain size of the perovskitefilms exposed in ambient air for 0, 15 and 30 days are about 400, 300 and 250 nm, respectively. Obviously, the average grain size of the perovskite films gradually decreases over time. The results are in good consistent with the result of XRD pattern. This can explain the performance and stability variation in the SnO2-PSC.

4. Conclusions

In this work, we have reported a low-temperature solution processed method to fabricate efficient and stable SnO₂-PSC in ambient air with 70% humility. The maximum processing temper- ature of SnO2-PSC is 150C. The effect of SnO2concentrations on

the micrograph of perovskitefilms and the performance of PSCs have been investigated. Upon optimization, it is found that the SnO₂ ETL with smallest roughness is beneficial to form a dense and pinhole-free perovskitefilm, resulting in the best performance. The champion and average efficiency for SnO₂and TiO₂-PSCs are 16.23%

and 12.0%, and 15.82% and 11.2%, respectively. Obviously, the SnO2- PSC show higher PCE than that of TiO2-PSC prepared in the same process. The results indicate that the higher PCE of SnO₂-PSC is attributed to the more efficient electron collection and transfer, and suppressed charge recombination. Importantly, the SnO2-PSCs demonstrate a good long-term stability. After being exposed to humid circumstance with 70% humidity for 30 days, the PCE of unsealed SnO2-PSC can retain to be about 85% of the initial value.

The reduced PCE over time is attributed to the decomposition of perovskitefilms. Our work provides a low-temperature method to prepare efficient and stable PSCs in ambient air with high humility.

Acknowledgements

We acknowledge thefinancial support of the Natural Science Foundation of Guangdong Province (No. 2016A030313421), the Characteristic Innovation Project of Guangdong Provincial Depart- ment of Education (Science 2016, 22), the National Natural Science Foundation of China (Grant No. 51431006, 51472093, 61574065, 21403089), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016), the Science and Technology Planning Project of Guangdong Province (Grant No.

2016B090907001, 2016B090906004, 2015B090927006), the Pro- gram for Changjiang Scholars and Innovative Research Team in University (IRT_17R70), the National Key R&D Program of China (2016YFA0201002), the Guangdong Innovative Research Team Program (No. 2011D039) and the MOE International Laboratory for Optical Information Technologies.

Fig. 6.Evolution of the PCE(a), XRD patterns(b)and the relative intensity of (110) peak of perovskitefilm to (001) peak of PbI2(110/001)(c)over time; top view SEM images of perovskitefilms at different time0day(d), 15 days(e)and 30 days(f). Scale bar is 1m^{m.(g),(h)and(i)}are the corresponding grain size distribution spectra of(def), respectively.

Appendix A. Supplementary data

Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2017.12.076.

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