Synergistic Effect to High-Performance Perovskite Solar Cells with

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Synergistic Effect to High Performance Perovskite Solar Cells with Reduced Hysteresis and Improved Stability by Introduction of Natreated TiO2 and Spraying-deposited CuI as Transport Layers Xin Li, Junyou Yang, Qinghui Jiang, Weijing Chu, Dan Zhang, Zhiwei Zhou, and Jiwu Xin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14926 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Applied Materials & Interfaces

Synergistic Effect to High Performance Perovskite Solar Cells with Reduced Hysteresis and Improved Stability by Introduction of Na-treated TiO2 and Spraying-deposited CuI as Transport Layers Xin Li1,2, Junyou Yang1,2*, Qinghui Jiang1,2*, Weijing Chu1,2, Dan Zhang1,2, Zhiwei Zhou1,2, Jiwu Xin1,2 1. State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, P.R. China 2. Shenzhen Institute of Huazhong University of Science & Technology, Shenzhen 51800, P.R. China

1

ACS Paragon Plus Environment


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ABSTRACT For a typical perovskite solar cell (PKSC), both the electron transport layers (ETLs) and hole transport materials (HTMs) play very important role in improving the device performance and long term stability. In this paper, we firstly improve the electron transport properties by modification of TiO2 ETL with Na species, and an enhanced PCE of 16.91% has been obtained with less hysteresis. Subsequently, inorganic CuI film prepared by a facile spray deposition method has been employed to replace the conventional spiro-OMeTAD as the HTM in perovskite solar cells. Due to the improved

transport

properties

at

the

ETL/perovskite

and

perovskite/HTM interfaces, a maximum photovoltaic efficiency of 17.6% with reduced hysteresis has been achieved in the PKSC with both the Na-modified TiO2 ETL and 60 nm-thick CuI layer HTM. To our knowledge, the PCE achieved in this paper is one of the highest values ever reported for the PKSC devices with inorganic HTMs. More significantly, the PKSCs manifest an outstanding device stability, the PCE keeps constant after storage in the dark for 50 days, and it can retain approximately 92% of their initial efficiency after storage even for 90 days. KEYWORDS: sodium doping, inorganic CuI, reduced hysteresis, perovskite, long term stability, high efficiency 2


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INTRODUCTION Hybrid organic-inorganic halide perovskite solar cells (PKSCs) have attracted considerable attention owing to their excellent photoelectric performance and relatively low fabrication costs over the past few years1-4. Many interesting properties have been reported in hybrid perovskite materials, such as suitable bandgap, high optical absorption coefficient, excellent carrier mobility, effective ambipolar charge transfer, and high tolerance of defects5-7. The efficiency of power conversion (PCE) of PKSC has been quickly improved from initial 3.8%8 to a certified value of 22.1%

9

by interface

engineering10-11, composition engineering3,12-13 and fabrication process optimizing14-19. Among the state of the art PKSC designs, electron transport layer (ETL) and hole transport material (HTM) are added in PKSCs between the photoactive layer and the electrodes in order to separate the generated charges and collect them efficiently20-22. Typically, a mesoporous TiO2 film23 has usually been used as the ETL while a p-type organic small-molecule spiro-OMeTAD film24 is most commonly used as the HTM in PKSCs. However, the electron recombination in TiO2 ETL is very serious due to the inferior electron mobility than the perovskite, resulting in unbalanced charge transfer and consequently low performance25-26. 3



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Moreover, it also suffers from low conductivity and carrier accumulation owing to the numerous trap states27. By now, many works10,28-32 have already been done to modify or dope the TiO2 ETL and reduce charge recombination and increase effective electron injection at the ETL/perovskite interface. Noteworthily, it is still not obsolete to modify the TiO2 ETL and thus improve the photovoltaic performance of PKSCs. And among these dopants, Li-modified TiO2 presents the best device performance and less hysteresis. However, Li is quite less abundant and more expensive than the same group of Na. In this light, modifying TiO2 with Na species is more favorable for PKSCs. Besides the ETL, the hole transport material (HTM), which transports holes to the counter electrode, is also very important to the device

performance

and

stability

of

PKSCs.

Currently,

spiro-OMeTAD is one of the most frequently used HTMs for high performance PKSCs14, 18, 27, 33. However, it is very expensive and less stable despite of the excellent hole transport properties34-36. Therefore, p-type inorganic semiconductors have been studied recently as potential HTMs for PKSC devices due to their low cost, high stability and mobility37. It was reported that MoS2 as well as FeS2 can be a potential candidate for HTM in PKSCs38-39. But the fabrication of those inorganic HTMs needs a series of complicated 4


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chemical procedures and is not conducive for large scale production. Some other inorganic HTMs, such as Cu-based compounds and transition metal oxide, have also been studied as HTMs for PKSCs. Among them, CuI, which is lost cost, stable, non-toxic and highly conductive, seems to be a very promising inorganic HTM for PKSC devices. Kamat and co-workers40 first reported a PKSC with a drop-casted CuI layer as the HTM and achieved a PCE about 6%, which is still much lower than that of those spiro-OMeTAD based PKSCs. The solvent to dissolve CuI for the typical drop casting process, which may damage the perovskite film, should be the main cause of the poor photovoltaic performance. Unfortunately, no suitable alternative but propyl sulfide has been known as the solvent for CuI. Therefore, new method should be developed to deposit CuI and reduce the damage of perovskite film. In order to fabricate CuI layer without any apparent damage of the underlying perovskite film, we developed a facile spray deposition technique to coat CuI film in this work, in addition, we further modified the TiO2 film with a CF3NaO2S/acetonitrile solution to form a Na-treated TiO2 ETL and enhance its electron transport property. By means of the synergistic effect, a maximum PCE of 17.60%, which is comparable to the PCE of spiro-OMeTAD based devices, has been achieved in the PKSCs with both the Na-TiO2 ETL 5



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and CuI HTM. Moreover, the fabricated PKSCs also show significant

long-term

stability,

the

unsealed

PKSCs

retain

approximately 92% of the initial efficiency after storage in the dark even for 90 days.

6


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RESULTS AND DISCUSSION

Figure 1. SEM images of the perovskite films deposited on a mesoporous TiO2 (a) and a Na-treated TiO2 (b), respectively. (c) XRD patterns of the perovskite films deposited on bare FTO, the pristine and Na-treated TiO2 substrates. (d) Absorption spectra of perovskite films deposited on the pristine and Na-treated TiO2 layers.

As shown in Figure 1a-b, SEM images of the high-quality perovskite films deposited on the pristine and Na-treated TiO2 7



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mesoporous layers manifest that Na treatment has no significant effect on the morphology of perovskite films. XRD patterns of all the perovskite films on different substrates can be well indexed to the perovskite phase with tiny amount of PbI2 remnants (Figure 1c). Figure 1d also indicates of no obvious difference between the UV-vis absorption spectra of perovskite films coated on the Na-treated TiO2 and the pristine TiO2, as consistent with the results of SEM and XRD.

Figure 2. The XPS spectra of FTO/TiO2 and FTO/Na-TiO2. (a) XPS spectra of Ti 2p peaks, (b) XPS spectra of Na 1s peaks.

XPS

spectra

were

collected

to

elucidate

the

chemical

compositions of pristine TiO2 and Na-TiO2 films. Figure 2a and 2b shows the XPS peaks of Ti 2p and Na 1s respectively for the two films. The Ti 2p1/2 and Ti 2p3/2 peaks of pristine TiO2 film, located at 8


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binding energies of 464.6 eV and 458.8 eV, are in good consistence with the reference values42. The Na 1s peak with a binding energy of 1071.4 eV has been detected in the Na-TiO2 film (Figure 2b), and the Ti 2p3/2 peak in the Na-TiO2 film shifts slightly lower to 458.5 eV, this shift can be explained by the Pauling electronegativity theory; the electronegativity value of Ti is 1.5 and Na is 0.9, which indicates weaker negative charge transfer toward sodium in the Ti-O-Na bond, thereby decreasing the Ti 2p core level binding energy, indicating the local electronic structure of Ti atoms is slightly different from that in the pristine TiO2, which should be ascribed to the modification effect of Na species.

Figure 3. (a) Schematic drawing and (b) energy level alignment of a 9



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typical device structure. (c) Transmittance spectra of the FTO/c-TiO2, FTO/c-TiO2/meso-TiO2

and

FTO/c-TiO2/meso-Na-TiO2

films,

respectively. (d) SEM cross-sectional image of a representative perovskite solar cell with Na-TiO2.

Figure 3a shows a schematic illustration of the standard device structure for mesoscopic MAPbI3 perovskite solar cells. As shown in Figure S1, the change of energy levels of TiO2 and Na-TiO2 ETLs were confirmed by UPS measurement and absorbance spectra. Obviously, the Na-TiO2 has a lower conduction band than pure TiO2, as depicted in Figure 3b. The lower conduction band of Na-TiO2 can enhance the driving force of electron injection from perovskite to mesoscopic ETL, thus facilitate the charge transport ability of the ETL layer30. As shown in the Figure S2, you can see, there is no trace of sulphur or fluorine from CF3NaO2S precursor for the Na-treated TiO2. Therefore, we focus on the effect of Na on the electron transport properties of TiO2, regardless of F- and S2-. Figure 3c presents that the FTO/c-TiO2/meso-Na-TiO2 sample has almost identical transmittance to the FTO/c-TiO2/meso-TiO2 sample. In addition, some other experiments have also been performed to further characterize the pristine TiO2 and Na-TiO2 samples (Figure S3-4). In general, all these results confirm that Na treatment could 10


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improve the band level alignment between TiO2 and perovskite but with no significant effect on other aspects compared with pure TiO2. Figure 3d exhibits a cross-sectional SEM picture of a typical PKSC device with a Na-TiO2 ETL, the thickness of mesoscopic TiO2/perovskite layer is about 500 nm.

Figure 4. (a) J-V curves of mesoscopic MAPbI3 perovskite solar cells with and without Na treatment with respect to forward and reverse scan direction. (b) IPCE spectra of the devices using TiO2 and Na-TiO2 mesoscopic layer. (c) Photoluminescence (PL) and (d) 11



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time-resolved photoluminescence (TRPL) spectra of bare perovskite film and perovskite film on pristine TiO2 and Na-treated TiO2.

Figure 4a shows the J-V curves of PKSCs in forward and reverse scans and the corresponding photovoltaic parameters were listed in Table 1. As can be seen from the data, the maximum power conversion efficiency was greatly enhanced from 13.84% to 16.91% by modification of the mesoscopic TiO2 with Na species. The enhancement in device efficiency should be mainly attributed to the increase of Jsc and FF due to the decrease of conduction band minimum of TiO2 by Na treatment. Moreover, resulting from the improved charge injection and transport efficiency, the device with Na-treated TiO2 ETL also exhibits greatly reduced hysteresis than that with pristine TiO2 ETL, which has further been confirmed by the J-V curves with various scanning rate in forward and reverse scan direction plotted in Figure S5.

Table 1 The photovoltaic parameters of PKSCs with and without Na treatment with respect to forward and reverse scan direction.

Na-TiO2(forward)

Voc (V)

Jsc (mA cm-2)

FF

η (%)

1.00

22.09

0.73

16.13

12


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Na-TiO2(reverse)

1.02

22.40

0.74

16.91

TiO2(forward)

0.98

17.92

0.62

10.89

TiO2(reverse)

1.04

19.01

0.70

13.84

Figure 4b shows the incident photon-to-current conversion efficiency (IPCE) spectra of the devices with mesoscopic TiO2 and Na-TiO2 ETLs. Obviously, the device based on a Na-TiO2 ETL exhibits higher IPCE than that on pristine TiO2 ETL almost over the entire wavelength range, confirming a superior charge injection and transport of the Na-TiO2 ETL over the pristine TiO2. Moreover, the integrated current densities derived from the IPCE curves are 17.75 and 20.89 mA cm-2 for the cells with pristine TiO2 and Na-TiO2 respectively, which are in good consistence with the Jsc obtained from the corresponding J-V measurements within deviation range. The steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) intensity decay spectra were acquired to explore the charge extraction and electron transport kinetics. As presented in Figure 4c, the PL intensity of the perovskite film on the FTO/Na-TiO2 is greatly quenched as compared to that of the perovskite film on the FTO/TiO2 substrate or just on the FTO glass, indicating that there is an efficient charge carrier extraction from perovskite to Na-treated mesoscopic TiO2. To further check the 13



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charge transfer rate and the charge separation efficiency, the TRPL decay spectra of those samples were shown in Figure 4d. The transient PL decay plots can be well fitted by the following tri-exponential decay kinetics equation: PL intensity=A1exp(-t/τ1)+ A2exp(-t/τ2)+ A3exp(-t/τ3) Where A1, A2 and A3 are time independent coefficients of amplitude fraction for each decay component, τ1, τ2 and τ3 are decay time of fast, intermediate and slow component, respectively. The fitted time coefficients (τi) for each sample were shown in Table 2 with the corresponding relative amplitudes (Ai) shown in parentheses. Furthermore, the average PL lifetimes (τavg) determined with an intensity-averaged method reported elsewhere43, were also listed in Table 2. As we can see, the τavg for MAPbI3 deposited directly on FTO is 18.60 ns but decreases to 12.83 ns, 6.96 ns when MAPbI3 is in contact with the ETL of pristine TiO2 and Na-treated TiO2, respectively. These results further confirm that the efficient electron injection and transportation of perovskite solar cells are improved by the Na-treatment. In order to further confirm the enhanced electrical properties of Na-TiO2 films, Hall measurements were performed and the results of conductivity and electron mobility were shown in Table S1. The electron mobility of mesoscopic TiO2 increases from 2.09×10-5 to 14


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7.25×10-5 cm2 V-1 s-1 and the conductivity increases from 1.85×10-6 to 3.45×10-5 S cm-1 after Na treatment, which are well consistent with the results discussed above. In a word, Na-treatment improves both the conductivity and the band alignment at the ETL/perovskite interface compared to pure TiO2. Then, the Na-treated TiO2 could enhance the charge injection from perovskite to mesoscopic ETL due to the increased driving force at the interface of perovskite/ETL. Moreover, the conductivity of Na-TiO2 is higher than that of TiO2 employed by Hall Effect measurement, which is possibly caused by the increased carrier density and hall mobility, thus reducing the series resistance in Na-TiO2 based cells.

Table 2 Summary of measured fast decay time (τ1), intermediate decay time (τ2), slow decay time (τ3), and PL average decay (τavg) for the FTO/MAPbI3, FTO/meso-TiO2/MAPbI3 and FTO/ meso-TiO2 with Na treatment/MAPbI3, respectively. Sample name

A1

τ1(ns)

A2

τ2(ns)

FTO/MAPbI3

3.56

1.75

43.32

7.91

53.12 28.43 18.60

TiO2/MAPbI3

2.47

1.42

37.35

4.71

60.18 18.33 12.83

Na-TiO2/MAPbI3 4.23

0.59

48.38

3.22

47.39 11.27

15


A3

τ3(ns)

τavg

6.92


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Apparently, the photon-induced electrons transport faster and more effective from perovskite to mesoscopic Na-TiO2 ETL than that from perovskite to mesoscopic TiO2 layer. Therefore, we take Na-treated TiO2 as the electron transport layer hereinafter and replace the conventional spiro-OMeTAD organic HTM with inorganic CuI HTL to investigate the effect of CuI HTM on the photovoltaic performance of PKSCs.

Figure 5. (a) J-V curves of PKSCs with different thickness of CuI layers prepared by various spraying cycles, (b) J-V curves of the PKSCs employing various HTMs, (c) Stabilized power output of the 16


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PKSC devices based on spiro-OMeTAD and CuI, (d) J-V curves of CuI and spiro-OMeTAD based perovskite solar cells measured by reverse and forward scans.

As depicted above, the CuI HTMs were fabricated by a spraying deposition method, and they were confirmed to be CuI by XRD (Figure S6) and XPS analysis (Figure S7). As demonstrated in Figure S8, the CuI film deposits can fully cover the perovskite layer very homogeneously. The cross-sectional SEM images of CuI layers fabricated with different spraying cycles were shown in Figure S9a, from which the thickness of CuI layer was estimated and depicted as a function of coating cycles shown in Figure S9b. The J-V curves of PKSC devices with different thickness of CuI films were measured and shown in Figure 5a, and the detailed photovoltaic parameters were derived and displayed in Table 3. Apparently, the PKSC with CuI layer deposited by three spray coating cycles about 60 nm in thickness, achieves an optimized PCE of 17.60% with Jsc of 22.78 mA cm-2, Voc of 1.03 V and FF of 0.75, respectively. Further increasing the spraying cycle is not good for improving PCE of the related PKSCs because too thick CuI layer is detrimental to carrier transportation. The integrated current densities 17



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of the device with spiro-OMeTAD and CuI calculated from IPCE spectra are 20.89 and 21.95 mA cm-2 shown in Figure S10, respectively, confirming the Jsc values with deviations within ~10% between the two current density values. Moreover, 30 devices with the different coating cycles were fabricated and tested, the statistical data of the photovoltaic parameters were plotted in Figure S11, which demonstrates a good reproducibility of photovoltaic performance with relatively low standard deviations. Figure 5b presents a comparison of J-V curves of the PKSCs with various HTMs. It can be seen that the PKSC based on CuI-spray HTM manifests a superior photovoltaic performance with a remarkable PCE of 17.6% higher than those of all other devices. Detailed photovoltaic parameters were also listed in Table 3.

Table 3 Photovoltaic parameters of the PKSCs with CuI layers prepared by various numbers of coating cycles and the PKSCs employing various HTMs. Voc (V)

Jsc (mA cm-2)

FF

η (%)

No HTM

0.86

17.48

0.49

7.36

CuI /1cycle

0.99

20.25

0.72

14.43

CuI /2cycles

1.02

21.12

0.73

15.73

18


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CuI /3cycles

1.03

22.78

0.75

17.60

CuI /4cycles

1.00

20.80

0.72

14.98

CuI /5cycles

0.97

19.03

0.71

13.11

Spiro-MeOTAD

1.02

22.40

0.74

16.91

CuI (TiO2 ETL)

1.03

20.12

0.72

14.92

To check the reliability of the J-V curves, the steady state PCEs of the devices corresponding to the spiro-OMeTAD and CuI based devices shown in Figure 5b have been achieved and presented in Figure 5c, the steady state PCEs are 16.17% and 17.20% for the spiro-OMeTAD and CuI based devices at its maximum power point, respectively. Apparently, they are in good accordance with their corresponding J-V curves. Moreover, the CuI-based PKSC also shows the less hysteresis effect shown in Figure 5d. There are two important factors in determining the strength of the hysteresis effect. One is perovskite itself and the other is charge transport across interface at ETL/perovskite/HTM. Perovskite is prone to ionic diffusion causing charge accumulation and leading to subsequent severe recombination at the interface region. As is shown in Figure 1, SEM images of the high-quality perovskite films deposited on the pristine and Na-treated TiO2 mesoporous layers manifest that Na treatment has no significant effect on the morphology of perovskite 19



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films. Thus, in this work, the interface does play an important role and the electron transport interface must provide sufficient charge transfer rates under buildup filed. As discussed above, we introduce Na-treated TiO2 as ETL to improve the electron injection and transfer with higher conductivity and better band alignment. Therefore, both devices based on spiro-OMeTAD and CuI with Na-TiO2 as ETL have negligible hysteresis. In order to achieve higher PCE and long term stability, we use spray-CuI because the organic spiro-OMeTAD can be easily decomposed in the air, which will lead to severe hysteresis over a long period of time. To best of our knowledge, this is also one of the highest values among the PCEs of the PKSCs based on inorganic HTMs reported so far.

Figure 6. Nyquist plots of (a) the electrochemical impedance spectra for the PKSC devices based on spiro-OMeTAD and CuI at 0.8 V under AM 1.5 illumination (100 mW cm-2) and (b) the Rrec for 20


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devices based on spiro-OMeTAD and CuI at various bias voltages, respectively.

In order to better understand the performance differences between the devices based on the spiro-OMeTAD and CuI HTMs, electrochemical impedance spectra (EIS) were collected under AM 1.5 illumination with a bias voltage of 0.8 V to compare their charge recombination properties. According to the previous reports44, the lower frequency semicircle of the impedance spectrum is commonly attributed to the recombination resistance (Rrec) which can be determined by fitting with the equivalent circuit in the inset image of Figure 6a. Further analysis in Figure 6b indicated that the recombination resistance is closely related with the applied bias voltage for the spiro-OMeTAD and CuI based PKSCs. It can be seen that the CuI-based PKSC presents higher recombination resistance than that of the spiro-OMeTAD based PKSC in the applied bias voltage range, which indicates that a lower recombination at the interface between MAPbI3 and CuI HTM than that of the MAPbI3/spiro-OMeTAD based PKSCs. Hence, it is a very important factor in increasing Jsc of the CuI-based device. Moreover, as shown in Figure S12, the PL quenching behavior between the perovskite active absorber and hole transport layer further confirms that more 21



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efficient charge transfer occurs at the CuI/MAPbI3 interface, therefore leading to the high Jsc of PKSCs which is also consistent with the results of EIS discussed above. In addition, the CuI film itself also exhibits 2 orders of magnitude higher intrinsic electrical conductivity in comparison to spiro-OMeTAD HTM37. That is why the CuI-based PKSCs present more efficient hole transport ability and higher photovoltaic performance than the spiro-OMeTAD based PKSCs.

Figure 7. Normalized PCE changes of both spiro-OMeTAD and CuI based PKSCs.

22


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The long-term stability of devices has also been studied and shown in Figure 7. Apparently, the spiro-OMeTAD based PKSC degrades very severely under the humidity (40%-50%), resulting in about 40% loss of the initial PCE after 20 days, and almost 60% loss after 30 days. Comparatively, the PKSC with spraying-deposited CuI HTM shows a very good long-term stability, its PCE keeps almost constant after exposure over 50 days, and it loses only about 8% PCE after storage even for 3 months. To best of our knowledge, the PCE achieved in this paper is the highest value among the PKSC devices with inorganic HTMs, which can be comparable with the traditional device based on spiro-OMeTAD as HTM.

CONCLUSION In summary, the photovoltaic performance of PKSCs have firstly been enhanced by employing Na-modified TiO2 with improved electron conductivity and mobility as ETL, and then the PCE has further been improved by introducing inorganic CuI film fabricated by a facile spray deposition method as the HTM in perovskite solar cells based on the Na-modified TiO2 ETL. Due to the improved transport properties at the ETL/perovskite and perovskite/HTM interfaces, a maximum PCE of 17.6% has been achieved in the PKSC with Na-treated TiO2 ETL and 60 nm-thick CuI layer HTL 23



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simultaneously. Furthermore, the device based on Na-treated TiO2 ETL and CuI HTM demonstrates a remarkable long-term stability at ambient condition, the PCE decreases only about 8% even after storage in the dark for 90 days.

EXPERIMENTAL SECTION Materials. All chemicals were purchased from commercial suppliers and used without further purification. Unless otherwise stated, all of materials were purchased from Alfa Aesar. CH3NH3I (MAI) was synthesized according to the literature2. Formation of CuI films by spray deposition method. CuI layers were deposited on the FTO/TiO2/MAPbI3 substrate by the spray deposition technique as follows. 1.25 g copper iodide was dissolved 1 mL propyl sulfide, and diluted to 6 mL with chlorobenzene. Then the solution was deposited onto the perovskite layer by spray process similar to that reported by Lee et al. for CuSCN deposition41. And the thickness of CuI layer can be increased by repeating the coating cycles. After the final deposition, the coated CuI film was baked at 353 K for 5 min. Device Fabrication. The patterned FTO glass substrates were washed sequentially with detergent and deionized water, acetone, and isopropanol with ultrasonication for 10 min each and were then 24


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dried and treated by O2 plasma. Then, to make a compact TiO2 layer, it was sprayed coated onto the hot FTO substrate (450 ℃) using 0.2 M solution of Ti(IV) bis(ethyl acetylacetonate)-diisopropoxide in 1-butanol and annealed at 723 K for 30 min. Afterward, mesoporous TiO2 film was prepared by spin coating of TiO2 paste ( 18NR-T Dyesol) under 5000 rpm for 50 s, and then annealed at 773 K for 30 min. To make Na-treated mesoporous TiO2, we spin-coated 0.2 mL CF3NaO2S/acetonitrile solution (15 mg/1 mL) on the untreated mesoporous TiO2 film at 4500 rpm for 45 s. The CF3NaO2S deposited mesoscopic TiO2 film was then heat-treated at 773 K for 30 min under atmosphere condition. MAPbI3 perovskite layer was deposited via an anti-solvent method. Here, the perovskite precursor solution containing 0.461 g PbI2 and 0.159 mg MAI in a 1 mL mixture of DMF and DMSO with a ratio of 8:2 (4:1, volume ratio) was spin-coated at 1000 rpm for 10 s and then at 4000 rpm for 30s. During the second step, 120 µL chlorobenzene was dropped onto the spinning substrates at the first 10 s. The substrates were then annealed at 373 K for 15 min. The spiro-MeOTAD solution prepared according to the literature2 was used as the hole transport material and deposited on the perovksite film at 3000 rpm for 40 s. Finally, a 80 nm Au electrode was 25



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deposited on the top of HTM via thermal evaporation. Characterization. Crystalline structures of the films were analyzed using X-ray diffraction (XRD) with a Philip X’Pert PRO X-ray diffractometer ( Cu Kα irradiation (λ=1.5406 Å)). X-ray photoelectron spectroscopy (XPS) was carried out using a photoelectron spectrometer (AXIS-ULTRA DLD-600W, Kratos Company). Surface morphology and RMS roughness of TiO2 and Na-TiO2 thin films were characterized by Atomic force microscopy (AFM, SPM9700, Shimadzu Company). Field emission scanning electron microscopy and energy-dispersive spectroscopy (FE-SEM and EDS, Nova NanoSEM 450, FEI Company) were utilized to investigate the morphology of PKSC devices and the amount of sodium dopants. The light absorption measurement was performed using a UV-vis spectrophotometer (Lambda 950, PerkinElmer). The Hall measurements were accomplished by Van der Pauw method in a Hall effect measurement system (HMS 5500). The photocurrent density-voltage (J-V) characteristics were measured using a Keithley 2400 source meter under one-sun AM 1.5G (100 mW cm-2) illumination with a solar light simulator (Oriel, Model 71675-71580). Photoluminescence (PL, excitation at 325 nm) and time-resolved photoluminescence (TRPL, excitation at 325 nm and emission at 760 m) spectra were obtained with an Edinburgh Instruments Ltd FLS 26


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980 spectometer. The IPCE with wavelengths ranging from 300 nm to 800 nm was measured using Newport-74125 system (Newport Instrument). Electrochemical impedance spectroscopy (EIS) was performed

using

a

potentiostat

(IviumStat

10800,

Ivium

Technologies) under AM 1.5 illumination (100 mW cm-2) and a frequency range was from 1 MHz to 100 mHz.

ASSOCIATED CONTENT Supporting Information Figures S1-S12 and Table S1. This material is available free of charge on the ACS Publications website at http://pubs.acs.org. Absorbance spectra and UPS spectra of mesoscopic TiO2 and Na-TiO2 based on FTO substrates, XPS survey spectra of pure TiO2 and Na-treated TiO2 electrodes showing Ti 2p, O 1s and C 1s peaks, SEM images of TiO2 and Na-treated TiO2 films, EDX spectra of Na-treated mesoscopic TiO2 films, J-V curves of Na-treated mesoscopic MAPbI3 perovksite solar cells under different delay times, XRD patterns of a powdered CuI sample and a thin CuI film, XPS of survey spectra of the CuI film coated onto perovskite layer, SEM images of the CuI films, Cross-sectional SEM images of the CuI layers, Plots of the CuI thickness and normalized PCE of PKSC devices as a function of the number of coating cycles, IPCE spectra of the devices using CuI and spiro-OMeTAD layer, Box chart of photovoltaic parameters comparison of perovskite solar cells based on different coating cycles, Steady-state photoluminescence (PL) spectra of the bare MAPbI3, MAPbI3/Spiro-OMeTAD and MAPbI3/CuI based HTL, and Table summarizing the conductivity and mobility of pristine the TiO2 and Na-treated TiO2 layers. 27



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AUTHOR INFORMATION Corresponding Author *Email: [email protected]. *Email: [email protected].

ORCID Junyou Yang: 0000-0003-0849-1492 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is co-financed by National Natural Science Foundation of China (Grant Nos. 51572098 and 51272080), Fund for Strategy Emerging Industries of Shenzhen (No. JCY20150630155150208), Open Fund of State Key Laboratory of Advanced Technology for Materials

Synthesis

and

Processing,

Wuhan

University

of

Technology (Grant No. 2016-KF-5), Technology innovation fund project of Huazhong University of Science and Technology Innovation Research College. The technical assistance from the Analytical and Testing Center of HUST is likewise gratefully acknowledged. 28


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Graphical Abstract

33


Synergistic Effect to High-Performance Perovskite Solar Cells with

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