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Low Temperature Solution-Processed ZnSe Electron Transport Layer for Efficient Planar Perovskite Solar Cells with Negligible Hysteresis and Improved Photostability Xin Li, Junyou Yang, Qinghui Jiang, Hui Lai, Shuiping Li, Jiwu Xin, Weijing Chu, and Jingdi Hou ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01351 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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Low Temperature Solution-Processed ZnSe Electron Transport Layer for Efficient Planar Perovskite Solar Cells with Negligible Hysteresis and Improved Photostability Xin Li,1,2 Junyou Yang,*,1,2 Qinghui Jiang,1,2 Hui Lai,1,2,3 Shuiping Li,1,2 Jiwu Xin,1,2 Weijing Chu,1,2 Jingdi Hou1,2

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 3. China-Eu Institute for Clean and Renewable Energy, Huazhong University of Science & Technology, Wuhan 430074, P.R. China

1

ACS Paragon Plus Environment

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ABSTRACT For a typical perovskite solar cell (PKSC), electron transport layer (ETL) has a great effect on device performance and stability. Herein, we manifest that low-temperature solution-processed ZnSe can be used as a potential ETL for PKSCs. Our optimized device with ZnSe ETL has achieved high power conversion efficiency (PCE) of 17.78 % with negligible hysteresis, compared with the TiO2 based cell (13.76%). This enhanced photovoltaic performance is attributed the suitable band alignment, high electron mobility and reduced charge

accumulation

at

the

interface

of

ETL/perovskite.

Encouraging results were obtained when the thin layer of ZnSe cooperated with TiO2. It shows that the device based on the TiO2/ZnSe ETL with cascade conduction band level can effectively reduce the interfacial charge recombination and promote carrier transfer with the champion PCE of 18.57%. In addition, the ZnSe-based device exhibits a better photostability than the control device due to the greater ultraviolet (UV) light-harvesting of the ZnSe layer, which can efficiently prevent the perovskite film from intense UV light exposure to avoid associated degradation. Consequently, our results present that a promising ETL can be a potential candidate of the n-type ETL for commercialization of efficient and photostable PKSCs. 2


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KEYWORDS: ZnSe, low-temperature solution process, perovskite solar cell, hysteresis, photostability

During the past several years, organic-inorganic metal halide perovkite solar cells (PKSCs) have attracted much attention due to their apparent combination of high efficiency, low-cost and easy fabrication with power conversion efficiency (PCE) rapidly increased from an initial 3.8% to 22.1%

1-9

On one hand, such

superior photovoltaic performance of PKSCs is ascribed to the intriguing features of perovskite materials, including strong light absorption, transport,

14

10, 11

high extinction coefficient,

12, 13

ambipolar charge

and long charge carrier diffusion length;

15

on the other

hand, as an important component of PKSCs, the electron transport layer (ETL) plays an indispensable role in charge separation, electron transportation and hole-blocking layer together.

16-18

Moreover, the stability issue of PKSCs under ambient and illumination condition has still been the biggest obstacle on the pathway of devices toward commercial applications. 19 Therefore, the optimization of ETL and the photostability of device are both vital to improve the overall performance of the cell devices, especially for planar PKSCs. Up to now, various materials have been studied as ETLs: e.g. TiO2, 3


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ZnO, SrTiO3, SnO2, Zn2SnO4. 20 Among these ETL materials, TiO2 is the most commonly used ETL in a typical n-i-p PKSC architecture. However, some demerits like high-temperature process intrinsic low electron mobility (0.1-1 cm2V-1s-1),

22

21

and

makes it not so

cost-effective and incompatible with flexible substrate. ZnO is also an important candidate for ETL, however, it is chemical unstable and sensitive to weak acids or alkali, that is harmful to the stability of PKSCs.

23, 24

Moreover, n-i-p planar devices based on TiO2 or ZnO

ETL always suffer from serious hysteresis due to the changes in absorber or contact conductivity carrier accumulation at the interface of perovskite/ETL.

25-27

In order to overcome these problems, low

temperature solution-processed metal sulphides, such as CdS, CdSe,

31

and In2S3,

17

28-30

have been employed as the ETL in PKSCs.

However, these compounds contain toxic element (Cd, In), which are not green environmental protection and cannot be used in commercialization. Moreover, Liu et al. reported that ZnS could be used as the ETL in PKSCs, but the band alignment of ZnS is not suitable with that of perovskite, leading to low PCE.

28

Therefore, it

is of great significance to explore an alternative ETL for high performance planar perovskite solar cells. As a versatile compound semiconductor, ZnSe has been extensively applied in many important areas. 4


32

It exhibits a high

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electron mobility (summarized in Table S1), 33, 34 which are just ideal properties to ETL for perovskite solar cells. More importantly, the direct bandgap of ZnSe is about 2.8 eV, enabling a broad absorbance in the UV region. This phenomenon is of great significance as a UV-blocking undelayer to retard the degradation of perovskite materials without obviously decaying the light harvesting in the rest of solar spectrum. However, to the best of our knowledge, no related work has been reported on adopting ZnSe as ETL in PKSCs yet. In this work, we has developed a low temperature chemical bath deposition (CBD) method to prepare ZnSe films as the ETL of PKSCs. Due to the optimized band alignment between the ZnSe ETL and CH3NH3PbI3, a significantly enhanced photovoltaic performance together with less hysteresis has been achieved in the PKSCs with such ZnSe ETL. The champion device with ZnSe ETL presents a high power conversion efficiency of 17.78%. Encouraging results were obtained when the thin layer of ZnSe cooperated with TiO2. It shows that the device based on the TiO2/ZnSe ETL with cascade conduction band level can effectively reduce the interfacial charge recombination and promote carrier transfer with the champion PCE of 18.57%. In addition, the ZnSe-based device exhibits an outstanding photostability than the control device under UV radiation. 5


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Overall, our results present that a promising ZnSe ETL can be a potential

candidate

of

the

n-type

collection

layer

for

commercialization of efficient and photostable PKSCs.

RESULTS AND DISCUSSION

Figure 1. (a) Top-view SEM image, (b) TEM image, (c) HRTEM image and (d) XRD pattern of the ZnSe film.

Figure 1a presents the SEM morphology of the as-deposited ZnSe 6


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film on an FTO substrate, which is quite different from that of FTO shown in Figure S1. Obviously, it presents a very smooth and compact surface and the nanoparticles distribute on the substrate very homogenously. The XRD pattern of the as-deposited film, as shown in Figure 1b, can be well indexed as ZnSe with zinc blende structure (JCPDS card no. 03-065-9602) shown in Figure S2a. Figure 1c shows the TEM image of some particles scraped from the as-deposited ZnSe film, the high resolution lattice fringes (Figure 1d) with interplanar spacing of d=0.33 nm is also in good consistence with the (111) crystal plane of ZnSe, further confirming the zinc blende ZnSe nature of the film. More results on the composition and structure analysis to the as-deposited ZnSe film were presented in Figure S2b-d, and the coexistence of Zn and Se elements was confirmed by energy-dispersive X-ray spectroscopy (EDX) mapping (Figure S2c-d). Moreover, they are also in good agreement with the nominal value of ZnSe. The SEM images of as-deposited ZnSe films on FTO with different reaction time are presented in Figure 2a-d. It can be seen that the morphology of ZnSe deposits changes significantly with the reaction time. At the beginning, very small ZnSe nuclei about several nanometers in size were formed on the FTO substrate after keeping the reaction pot at 353 K for 0.5 h, leading to some pinholes 7


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existed in the surface of ZnSe film, which is marked in yellow circle shown in Figure 2a. Then, the ZnSe nanoparticles grow bigger with better crystallinity after reaction for 1 h. Further increasing the reaction time to 2 h, the nanoparticles grow up to form a very compact and uniform film.

Figure 2. SEM images of the ZnSe films processed by low-temperature CBD method with various reaction duration of (a) 0.5 h, (b) 1 h, (c) 2 h and (d) 3 h.

However, when the reaction time further extends to 3 h, some 8


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cracks appear in the grain boundaries due to the overgrowth of ZnSe nanoparticles marked in red circle shown in Figure 2d, which is not conducive to electron transfer due to increased shunt pathways at the interface of ETL/perovskite.

36-39

AFM measurements in Figure S3

show that the 2 h CBD deposited ZnSe layer has a very smooth and compact morphology with root mean square (RMS) of 7-8 nm, which is more beneficial to the formation of perovskite layer. 40

Figure 3. SEM images of the perovskite layers coated on ZnSe ETLs with different reaction time of (a) 0.5 h, (b) 1 h, (c) 2 h, (d) 3 h, 9


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respectively.

Figure 3a-d show SEM morphologies of the perovskite films grown on various ZnSe ETLs with different reaction time. Obviously, the quality of CH3NH3PbI3 layer is greatly influenced by the reaction time of ZnSe layers. For the perovskite layer coated onto the ZnSe films with deposition time less than 2 h, there are many pinholes in the perovskite overlayer with low coverage, which may accelerate the recombination of carriers between ETL and HTL, resulting in a poor device performance. 3, 41-42

Table 1. Average J-V parameters of the PKSCs based on ZnSe films with different reaction time. Reaction time (h)

Voc (V)

Jsc (mA cm-2)

FF

η (%)

0.5 1 2 3

0.98±0.0086 1.02±0.0103 1.05±0.0046 1.04±0.0073

17.24±0.358 19.23±0.334 21.96±0.235 20.80±0.316

0.58±0.0075 0.68±0.0668 0.74±0.0062 0.70±0.0106

09.79±0.491 13.34±0.226 17.06±0.382 15.36±0.389

10


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Figure 4. (a) Schematic view of the typical cell architecture. (b) Energy band diagram of the PKSC, showing the separation and collection of photogenerated electrons and holes. (c) J-V curves of the perovskite solar cells based on various ZnSe ETLs at a scan rate of 0.1 V/s under 100 mW cm-2. (d) Absorbance spectra of ZnSe films under different reaction time and CH3NH3PbI3-coated ZnSe ETL films on FTO, respectively. (e) IPCE spectra of various ZnSe ETLs based PKSCs. (f) Photoluminescence (PL) spectra of the perovskite 11


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films deposited on various ZnSe films.

Along with the various as-deposited ZnSe films as the ETLs, devices with the structure of FTO/ETL/Perovskite/HTM/Au were assembled and a schematic model was shown in Figure 4a. As is shown in Figure S4, the conduction band of ZnSe was confirmed by UPS spectra. Then, the band diagram of the typical PKSCs is presented in Figure 4b, in which ZnSe exhibits a more favorable energy level alignment with the perovskite layer, resulting in a higher open circuit voltage Voc, which will be discussed below. Figure 4c shows the J-V curves of representative planar PKSCs using ZnSe ETLs deposited under various reaction time. The average photovoltaic parameters are also summarized in Table 1. It can be seen that the solar cell with the 2h-deposited-ZnSe ETL shows the highest PCE of 17.06% with an open circuit voltage (Voc) of 1.05 V, a short-circuit current density (Jsc) of 21.96 mA cm-2, a fill factor (FF) of 0.74. The relatively low photovoltaic parameters in the PKSCs with ZnSe ETLs of 0.5 h, 1 h and 3 h reaction time should be attributed to the insufficient coverage of ZnSe ETL and perovskite (0.5 h), high recombination at the interface of rough ETL and perovskite with many pinholes (1 h), and increased shunt pathways at the cracks appeared in the ZnSe grain boundaries (3 h), 12


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respectively. Furthermore, 30 cells based on ZnSe under different reaction time were measured and the statistical box charts and histograms of the photovoltaic parameters are illustrated in Figure S5 and Figure S6, which exhibits a good reproducibility with relatively low SD. Evidently, too many pinholes in the perovskite grown on the ZnSe ETL with short deposition time (0.5 h and 1 h) can reduce the absorbance of the active layers, as shown in Figure 4d, which is not favorable for achieving high Jsc or FF.

43

It is

noteworthy that the absorbance of ZnSe films under different reaction time are almost equal to each other. As can be seen, the absorbance shows almost no variation because the thickness of ZnSe changes very slightly with reaction time shown in Figure S7. That is to say, for the absorbance of perovskite, the contribution from ZnSe is negligible. Meanwhile, Figure 4e shows the measured IPCE spectra of the cells with different ZnSe films. The integrated current densities from these curves are 17.02, 19.13, 21.12 and 20.39 mA cm-2 for the devices based on ZnSe ETL of 0.5 h, 1 h, 2 h and 3 h reaction time, respectively, which are in good consistence with the Jsc obtained from the J-V curves. To further explore the recombination

behavior

of

photogenerated

carriers,

the

photoluminescence (PL) spectra of the cells based on ZnSe ETLs were measured and shown in Figure 4f. It can be seen that the PL 13


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intensity decreases from the 0.5 h sample to 2 h sample and then increases slightly from the 2 h to 3 h sample. Apparently, the 2 h sample exhibits the weakest PL intensity, indicating the minimum recombination of carriers and thus the best device performance. These PL spectra also confirm the results of J-V curves as shown in Figure 4c. As a contrast experiment, the J-V curves of best-performing devices employing ZnSe ETL and TiO2 ETL with respect to scan direction were both shown in Figure 5a. The surface morphology and cross-sectional SEM images of TiO2 film are shown in Figure S8, respectively. Moreover, the J-V curves of the devices based on c-TiO2, c-TiO2/Meso-TiO2 and ZnSe are also demonstrated in Figure S9, respectively. The solar cell based on the ZnSe ETL not only presents more superior photovoltaic performance over the device based on the TiO2 ETL, but also shows a negligible hysteresis in J-V curves. As can be seen from the corresponding device parameters in Table 2, the ZnSe-based cell demonstrates a PCE of 17.78% (17.28%) with a Voc of 1.06 (1.05) V, a Jsc of 22.36 (22.24) mA cm-2, and a FF of 0.75 (0.74) when measured under the reverse (forward) voltage scan. Figure S10 exhibits the SEM cross sectional image of a typical

planar

heterojunction

device

with

ZnSe

as

ETL.

Correspondingly, the solar cell based on conventional TiO2 ETL 14


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shows a PCE of 13.76% (11.19%) with a Voc of 1.03 (0.99) V, a Jsc of 19.08 (18.83) mA cm-2, and a FF of 0.70 (0.60) when measured under the reverse (forward) scan. Due to the negligible hysteresis of device based on ZnSe as ETL, the J-V curves of the sample device based on ZnSe ETL with different step delay times and voltage step sizes are also given for comparison in Figure S11. It can be seen that the J-V curves of device based on ZnSe are almost the same, independent on scan rate, confirming that the less hysteresis phenomenon in device based ZnSe ETL. As shown in Figure 5b, the ZnSe-based cell presents almost the same light absorption as that of the device with TiO2 ETL, that is to say, the different photovoltaic performance of two devices based on ZnSe and TiO2 ETLs should be mainly attributed to the charge injection and transfer at the ETL/perovskite interface. As shown in Figure 5c, ZnSe ETL based device shown a faster photoresponse of current compared to TiO2 ETL based cell. This can be ascribed to the fast trap filling process or the low density of charge traps in perovskite layer deposited on ZnSe ETL. 17

15


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Table 2. Summary of best-performing device parameters obtained in different scan directions for ZnSe and TiO2, respectively. Sample TiO2 ZnSe

Scan direction Reverse Forward Reverse Forward

Voc (V) 1.04 1.02 1.06 1.05

Jsc (mA cm-2) 20.15 19.72 22.35 22.04

16


FF 0.72 0.62 0.75 0.74

η (%) 15.09 12.47 17.78 17.13

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Figure 5. (a) J-V curves of the best performing PKSCs using ZnSe and TiO2 ETLs measured under forward and reverse voltage scans at a scan rate of 0.1 V/s under 100 mW cm-2. (b) UV-vis absorbance spectra of the perovskite coated ZnSe and TiO2 ETLs. (c) Photocurrent rising process on turning on and turning off the incident light for PKSCs with ZnSe and TiO2 ETLs. (d) I-V curves for ZnSe and TiO2 thin films obtained under dark condition. (e) Steady-state efficiencies of PKSCs using TiO2 and ZnSe ETLs measured at constant bias voltages of 0.80 and 0.85V, respectively. (f) Histograms of PCEs for 20 cells with ZnSe ETLs and 20 cells with TiO2 ETLs measured under reverse voltage scan.

By means of the I-V curves (Figure 5d) of ZnSe and TiO2 films in the structure of Au/ZnSe (or TiO2)/Au, 44 the conductivity of ZnSe is higher than that of TiO2. The higher conductivity of ZnSe can lower the contact resistance and facilitate the carrier transfer, benefiting the improvement of Jsc and FF. Hall-effect measurement (Table S3) further confirms that the optimal ZnSe film has better electrical properties than TiO2, where electron mobility and conductivity are 13.95 cm2 V-1 s-1 and 1.98×10-3 S cm-1, respectively. Moreover, the higher conduction band minimum of ZnSe (shown in Figure 4b) can also lead to higher Voc. Figure 5e shows the variation of PCE of two 17


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devices at a constant bias of maximum output power point, it can be seen that both the devices exhibit stable PCE of 17.25 % (ZnSe ETL) and 13.37 % (the device with TiO2 ETL) throughout almost 300 s duration, which are in good consistence with the average PCEs obtained from the J-V curves measured in different voltage scans within the margin of error. Figure 5f presents a statistics comparison of the averaged PCE of 20 separate cells with TiO2 and ZnSe ETLs and the average photovoltaic parameters of the corresponding 20 devices are summarized in Table S2. Moreover, the SD of the measurements has been shown in Figure S12. On one hand, it further confirms that the device with ZnSe ETL has more excellent PCE than that of the device with TiO2 ETL, on the other hand, it also indicates the good reproducibility of our solar cells. To further understand the internal mechanism of the charge transfer and recombination in PKSCs, the steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra were collected to reveal the charge transport behavior at the interface of ETL/perovskite.

45, 46

As shown in Figure S13a, the ZnSe-based

perovskite film presents more efficient PL quenching than the TiO2-based perovskite film, indicating enhanced electron collection and carrier transport at the ZnSe ETL/perovskite interface. Moreover, the TRPL spectrum, as shown in Figure S13b, also 18


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demonstrates a shorter carrier lifetime in the ZnSe-based perovskite layer than that of TiO2 ETL. Apparently, the perovskite/ZnSe system can provide a faster electron transfer process and lower interface recombination in compared with the perovskite/TiO2 system. Therefore, the charge transportation and injection get improved and less charge accumulation occurs at the interface of ETL/perovskite, thus leading to the reduced hysteresis behavior in the device with ZnSe ETL. Furthermore, EIS was also employed to investigate the interfacial carrier transfer and recombination kinetics in PKSCs. 3, 26-27, 47 Figure S14 shows the Nyquist plots measured in dark condition with the equivalent circuit in the inset for the devices with the ZnSe and TiO2 ETLs, respectively. Generally, the high frequency semicircle is attributed to the transport resistance (Rtr), whereas the low frequency part is determined by the recombination resistance (Rrec). the simplified transmission line model, semicircle is almost indistinguishable.

17

47

Due to

the high frequency

Thus, the Nyquist plots in

Figure S14a show a main semicircle at low frequency, which is associated with the recombination processes. As shown in Figure S14b, the recombination resistance is closely related with the applied bias voltage. Correspondingly, the fitted Rrec values of the ZnSe-based cell are much larger than those of the TiO2-based cell. 19


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That is to say, the ZnSe-based PKSCs can inhibit the recombination of charge carriers and improve the device performance more effectively. To further investigate the properties of the interface, the capacitance-voltage (C-V) characteristics of the devices based on TiO2 and ZnSe have been measured in the dark, shown in Figure S15. It is indicated that the built-in potential (Vbi) of the device based ZnSe (0.94 V) obtained from the Mott-Schottky plots is slightly higher than the device based TiO2 (0.87 V), which directly confirms that the device based ZnSe exhibits fast charge transportation and reduced charge accumulation at the interface. More importantly, encouraging results were obtained when the thin layer of ZnSe cooperated with TiO2. It shows that the device based on the TiO2/ZnSe ETL with cascade conduction band level can effectively reduce the interfacial charge recombination and promote carrier transfer, shown in Figure S16a. As is shown in Figure S16b, PKSC based on ZnSe/TiO2 cascade ETL yielded a PCE of 18.57% with a Voc of 1.09 V, a Jsc of 22.45 mA cm-2, and a FF of 0.76, when measured under reverse voltage scanning. Since the PCE of the device is closely to that of traditional Si-based solar cell, the commercial applications are mainly hampered by the stability of perovskite solar cell, especially for UV-induced stability. In order to find out how the various ETLs 20


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affect the stability of devices under UV light, it was set upon the FTO substrate side before J-V measurement. The unsealed devices based on ZnSe and TiO2 ETLs were stored in a glove box to exclude other environment factors, such as humidity and temperature. Please note that, UV-light accounts for about 5% of the whole sunlight intensity. Thus, the devices were continuously irradiated by simulated UV light under just 5 mW cm-2 intensity condition.

Figure 6. (a) UV-vis spectra of the optimized TiO2 and ZnSe film, inset shows the normalized PCE decay of devices based on ZnSe and TiO2 ETLs as a function of storage time upon UV irradiation. (b) The normalized PCE decay of devices based on ZnSe and TiO2 21


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ETLs as a function of storage time upon 1 sun irradiation. The XRD patterns of MAPbI3 coated on ZnSe and TiO2 before (c) after (d) UV radiation, respectively.

As is shown in Figure 6a, ZnSe enhanced a broad absorbance in the UV region compared with TiO2. Therefore, it is of great significance for ZnSe as a UV-blocking layer to retard the degradation of perovskite materials without obviously decaying the light harvesting in the rest of solar spectrum. Moreover, the UV-induced stability of devices is shown in the inset of Figure 6a. The device based on ZnSe manifests an outstanding UV-light induced stability, the PCE keeps 90% of their initial efficiency after aging for 500 h. However, for device based on TiO2, it can only retain approximately 45% of their initial efficiency after storage even for 30 days. Furthermore, the stability of the devices based on ZnSe and TiO2 under 1 sun illumination has also been measured, shown in Figure 6b. Generally, the UV-light refers to the wavelength range less than 400 nm. For TiO2 (3.2 eV), it can absorb UV-light less than 387 nm. Therefore, we use ZnSe with a bandgap of 2.8 eV as ETL because it can absorb all ultraviolet light to avoid the perovskite degraded by UV light radiation, thus, the stability of device based on ZnSe is higher than that of device based on TiO2 22


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under 1 sun illumination. In addition, it also can be used as light absorbing layer to produce photogenerated electron-hole pair to enhance photocurrent,

48

confirmed by the relevant result shown in

Figure S17. These results can be explained and evidenced by the XRD patterns of MAPbI3 coated on ZnSe and TiO2 before and after UV radiation, shown in Figure 6c and 6d, respectively. In Figure 6c, before UV aging, the perovskite manifests the typical peaks of the tetragonal phase and no PbI2 peaks are observed in both samples. However, after UV aging for 30 days, perovskite coated on ZnSe and TiO2 ETLs demonstrates a PbI2 (001) diffraction peak shown in Figure 6d. Furthermore, the perovskite based on ZnSe exhibits much lower (001) PbI2 peak than that based on TiO2, which is mainly ascribed to alleviating the decomposition of perovskite material via organic component removal under UV radiation.

49

Overall, the low

temperature solution-processed ZnSe film, as a potential ETL, can also act as a UV barrier layer to improve the device stability.

CONCLUSION In summary, we have demonstrated the use of zinc selenide by low-temperature solution-process deposition as an efficient ETL in planar perovkite solar cells. As a result, a significantly enhanced steady-state PCE and negligible hysteresis performance of PKSC 23


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with a ZnSe thin film as the ETL than that using a conventional TiO2 ETL. The champion device using ZnSe ETL achieved an efficiency of 17.78%. More importantly, the device based on the TiO2/ZnSe ETL with cascade conduction band level can effectively reduce the interfacial charge recombination and promote carrier transfer with the champion PCE of 18.57%. In addition, the ZnSe-based device exhibits an outstanding photostability than the control device under UV radiation.

EXPERIMENTAL DETAILS Preparation of ZnSe and TiO2 ETLs on FTO substrates. The etched FTO substrate was cleaned sequentially with detergent and deionized water, acetone, and isopropanol by ultrasonication for 30 min each and then dried and followed by UV-ozone treatment for 15 min. According to the previous literature,

34

zinc sulphate, 20 mL,

was mixed with 20 mL of 50% hydrazine hydrate solution with constant stirring. Then, 20 mL of 0.7 M ammonia solution was added which made the solution clear and transparent. To this, 100 mL of distilled water was added with constant stirring. Next, the solution bath was heated to 353 K and 20 mL of selenourea solution was added. Finally, the cleaned substrates were attached to a home-made holder and mounted vertically in the bath. After the 24


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deposition, the films were taken out of the bath, rinsed with distilled water and dried with nitrogen gas. To prepare the TiO2 ETL, a TiO2 compact layer was spin-coated on FTO at 3500 rpm for 25 s using 15 mM of titanium diisopropoxide bis(acetylacetonate) (75 wt% in isopropanol, Aladdin) solution at 398 K for 5 min. Afterward, mesoporous TiO2 film was prepared by spin coating of TiO2 paste ( 18NR-T Dyesol) under 5000 rpm for 30s, and then annealed at 773 K for 30 min. Device Fabrication. The CH3NH3PbI3 thin film was prepared on ETLs using a conventional two-step method. Snow-white CH3NH3I crystals were purchased (Ying Kou You Xuan Trade Co., Ltd) without further purification. PbI2 was dissolved in DMF at a concentration of 462 mg mL-1 under constant stirring at 343 K. The solution was spin-coated on the ETL films at 3500 rpm for 30 s. And the PbI2 layer coated ETLs were dried at room temperature for 30 min. Then, the films were dipped in a solution of CH3NH3I in 2-propanol (10 mg ml-1) for 5 min, rinsed with 2-propanol and dried at 373 K for 15 min. Spiro-MeOTAD 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 deposited on the top of HTM via thermal evaporation. Characterization. Crystal structure of the films was analyzed by 25


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using X-ray diffraction (XRD) with a Philip X’Pert PRO X-ray diffractometer (Cu Kα irradiation, λ=1.5406 Å). A photoelectron spectrometer (AXIS-ULTRA DLD-600W, Kratos Company) was employed to record the X-ray photoelectron spectroscopy (XPS) and the

ultraviolet

photoelectron

spectroscopy

(UPS).

Surface

morphology and RMS roughness were characterized by Field emission scanning electron microscopy (FE-SEM, Nova NanoSEM 450, FEI Company) and atomic force microscopy (AFM, SPM9700, Shimadzu Company), respectively. The ZnSe films were scraped, dispersed in ethanol and then observed by a transmission electron microscopy (TEM, JEM-2100, JEOL Ltd). The transmittance spectra were recorded using a UV-Vis spectrophotometer (Lambda 950, PerkinElmer). The J-V curves were measured using a digital source meter (Keithley 2400) under one-sun illumination with a solar light simulator (Oriel, Model 71675-71580). Photoluminescence and time-resolved photoluminescence spectra were obtained with an Edinburgh Instruments Ltd FLS 980 spectrometer. 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 (IVIUM, IviumStat 10800) in a dark condition and a frequency range was from 1 MHz to 100 mHz. Z-View analyst software was 26


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used to model the Nyquist plots obtained from the EIS measurements.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. SEM images, XPS survey spectra, AFM images, UPS spectra, Box chart of photovoltaic parameters, J-V curves, Steady-state and time-resolved PL spectra, Nyquist plots of the EIS, Mott-Schottky plots, Photocurrent plots with and without UV illumination.

The authors declare no competing financial interest.

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

ORCID Junyou Yang: 0000-0003-0849-1492

27


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ACKNOWLEDGMENTS This work is co-financed by National Natural Science Foundation of China (Grant No. 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), Graduates' Innovation Fund, Huazhong University of Science and Technology, 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.

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GRAPHICAL ABSTRACT

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