Air-Induced High-Quality CH3NH3PbI3 Thin Film for Efficient Planar

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Air-Induced High-Quality CHNHPbI Thin Film for Efficient Planar Heterojunction Perovskite Solar Cells Chunhua Wang, Chujun Zhang, Sichao Tong, Jianqiang Shen, Can Wang, Youzhen Li, Si Xiao, Jun He, Jian Zhang, Yongli Gao, and Junliang Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00981 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 12, 2017

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The Journal of Physical Chemistry

Air-Induced High-Quality CH3NH3PbI3 Thin Film for Efficient Planar Heterojunction Perovskite Solar Cells Chunhua Wang, 1 Chujun Zhang, 1 Sichao Tong, 1 Jianqiang Shen, 1 Can Wang, 2 Youzhen Li, 1 Si Xiao, 1 Jun He, 1 Jian Zhang, 3 Yongli Gao, 1, 4 and Junliang Yang 1 * 1

Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of

Physics and Electronics, Central South University, Changsha 410083, China. 2 3

Light Alloy Research Institute, Central South University, Changsha 410083, China School of Material Science and Engineering, Guilin University of Electronic

Technology, Guilin 541004, China 4

Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627,

USA.

* Corresponding author email: [email protected] (J. L. Yang) Tel.: +86-731-88660256

1

ACS Paragon Plus Environment


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ABSTRACT Efficient planar heterojunction perovskite solar cells (PHJ-PSCs) with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al

were

fabricated

using

air-induced

high-quality CH3NH3PbI3 perovskite thin films, in which the air-inducing process was controlled with a humidity of ~ 40 %. The air exposure of CH3NH3PbI3 thin films could dramatically improve their properties with large grains and smooth surface, as well as uniform morphology, resulting in an impressive enhancement in carrier lifetime. The ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy results proved that the CH3NH3PbI3 film was n-doped by the absorption of H2O on the surface but was very stable without obvious degradation for 10 days’ exposure in air. The power conversion efficiency (PCE) of PHJ-PSCs with an air exposure process showed a significant increase up to 16.21% as compared to reference PHJ-PSCs with a PCE of 12.02%. The research work demonstrated that an air-exposure process with a suitable humidity could produce high-quality perovskite thin film for efficient PHJ-PSCs, which may pave a boulevard for fabricating high-efficiency PHJ-PSCs in atmospheric environment.

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1. INTRODUCTION Organic-metal halide perovskite solar cells (PSCs) as a bright star appear in the spotlight of photovoltaic community in recent years, and an explosion of PSCs studies experiences an unprecedented progress, resulting in the power conversion efficiency (PCE) skyrocket-increase rapidly from 3.8 % to over 20 %.1-5 In the process of pursuit of high-efficiency PSCs, morphology control, interface modification and compositional engineering are the fundamental and passionate research subjects for accelerating the development of PSCs.6-11 Especially, it is very interesting to develop low-cost, large-scale printing/coating methods to produce high-quality perovskite thin film for achieving efficient photovoltaic devices.12-14 Because of the moisture-intensive characteristic of perovskite materials, humidity has been generally regarded as an obstacle for perovskite crystallization process, and accordingly influences the photovoltaic performance. However, some recent studies have shown that suitable moisture would be favorable for improving the performance of PSCs.15-19 The low moisture could be used as the annealing vapor for improving perovskite thin film and enhancing PSCs performance.15,16 Furthermore, water could act as an additive and directly introduced into perovskite precursor solution for improving the formation process of perovskite thin film.17 Unfortunately, the incompatible phenomena and complex control have impeded their applications in the fabrication of PSCs. The potentially commercial PSCs with large-scale and low-cost fabrication process would require feasible methods under ambient environment with controllable humidity. Thus it is important to understand how the perovskite thin film influenced by ambient environment for processing highly efficient PSCs. Herein, high-quality perovskite thin film induced by air exposure with a humidity of ~40% was developed to fabricate highly efficient planar heterojunction perovskite solar cells (PHJ-PSCs) with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al. The results suggested that the air exposure of CH3NH3PbI3 thin films with a controllable humidity could dramatically improve the morphology with large grains 3



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and smooth surface, and accordingly enhance the photoluminescence (PL) lifetime. The PCE up to 16.21% could be achieved for CH3NH3PbI3 thin films exposed to air, which is much higher than that of the reference PHJ-PSCs without air exposure (12.02%). The work may provide the potentials to fabricate highly efficient PHJ-PSCs under ambient environment.

2. EXPERIMENTAL SECTION The fabrication scheme and structure of PHJ-PSCs are shown in Figure 1. The patterned indium tin oxide (ITO) glass substrate was ultrasonically cleaned in acetone, detergents, deionized water and isopropyl alcohol for 20 min, respectively. The cleaned glass/ITO substrate was dried by nitrogen flow and treated by UV-ozone for 20 min subsequently. The hole transport layer (HTL) material PEDOT:PSS solution was spin-coated onto the ITO at a speed of 3000 rpm for 30 s and then annealing at 150oC for 15 min. Solvent-induced-fast-crystallization deposition method20 was employed to deposit perovskite thin films with a precursor solution at a concentration of 550 mg/ml (CH3NH3I:PbI2=1:1) onto the PEDOT:PSS-based ITO substrate with anhydrous chlorobenzene (CB, ~ 65 µl) at a speed of 4000 rpm for 30 s. Then the deposited CH3NH3PbI3 thin films were treated on a hot plate at 100 oC for 10 min, resulting in a thickness of about 290 nm. After that, the prepared CH3NH3PbI3-coated samples were put in ambient environment with a humidity ~ 40 RH% at various time periods. The PCBM solution with a concentration of 15 mg/ml was spin-coated onto prepared films as the electron transport layer (ETL) at a speed of 3000 rpm for 30 s. Finally, a 100 nm Al electrode was deposited by thermal evaporation under a vacuum of about 8.0×10–6 mbar, resulting in an active area of 0.09 cm2. The absorption spectra and crystallographic properties of perovskite films that exposed to ambient environment for different time intervals were characterized by employing ultraviolet-visible spectrophotometer (UV-vis, Puxi, T9) and X-ray diffractometer (XRD, Rigaku D, Max 2500), respectively. The surface images were measured by scanning electron microscope (SEM, FEI Helios Nanolab 600i). The thickness and 4


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surface roughness of perovskite films were obtained by surface profilometer (Dektak 150,

Veeco)

and

atomic

force

microscope

(AFM,

Agilent

Technologies

5500AFM/SPM System). Current density-voltage (J-V) characteristics of PHJ-PSCs were measured by digital Source Meter (Keithley, model 2420) at 300 mV/s from -1.5 V to +1.5 V if without specified note, and the standard silicon solar cell was employed to calibrate the solar simulator (Newport 91160s, AM 1.5G) with a light intensity of 100 mW/cm2. Steady-state PL spectra and PL lifetime were achieved using intensified charge coupled device detector (ICCD, DH334T-18U-03) and time-correlated single photon counting (TCSPC, MS3504I) measurements. Ultraviolet photoelectron spectroscopy (UPS, He I, 21.22 eV)) and X-ray photoelectron spectroscopy (XPS, Al Ka X-ray source, 1486.6 eV) were employed to investigate the detail exposure effects on the surface.

Figure 1. The schematic illustration for the fabrication of CH3NH3PbI3 PHJ-PSCs.

3. RESULTS AND DISCUSSION It is generally considered that moisture is a lethal factor for PSCs due to the intrinsic moisture-sensitive characteristics of perovskite material, which would lead to 5



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the degradation of perovskite and result in sharp decline of PSCs performance.21-23 But, on the other hand, it has been demonstrated that the suitable humidity would improve perovskite thin-film quality and accordingly enhance PCEs.15-19 By contrast, other atmosphere factors such as O2 and N2 have been proved that they almost didn’t influence perovskite films and PSCs performance.15,21 It would be very interesting to understand how perovskite thin film exposed to air with a certain humidity influence the performance of PSCs, which is helpful to large-scale, low-cost fabrication of PSCs in air. Thus the deposited CH3NH3PbI3 perovskite films exposed to air with a controllable humidity ~ 40 % from beginning to 30 days were studied for probing the influence of humidity on the film properties and performance of PHJ-PSCs. The CH3NH3PbI3 perovskite films fabricated in N2-filled glovebox were exposed to air with a humidity ~ 40 % for the different time, which was fixed at 0 day, 2 days, 5 days, 10 days, 20 days and 30 days, respectively, and their film properties were characterized accordingly. It is clear that the dense, uniform and pin-hole-free CH3NH3PbI3 film was produced by solvent-induced-fast-crystallization deposition (Figure 2a). As exposed to air, the CH3NH3PbI3 films with the characteristics of compact and uniformity could be well maintained without pin holes under the humidity environment (Figure 2b-f), indicating that the CH3NH3PbI3 films should not decompose severely at a humidity ~ 40 %. Furthermore, it is very interesting that the grain size of perovskite films gradually grow larger and larger with increasing the exposure time, and the statistic grain size of perovskite films exposed to air for the different time are shown in Figure 2g. The average grain size is gradually enhanced from about 280 nm to about 460 nm as the exposure time goes to 10 days, and it almost doesn’t change any more for further longer exposure. The morphology and grain size were also characterized by AFM, as shown in Figure S1 in Supporting Information. The AFM results present the same tendency that the grains grow up with increasing the exposure time. One should notice that the surface roughness of perovskite films shows a little increase for the exposure time to 10 days (from about 5.08 nm to 7.67 nm) and an obvious increase for the further exposure time to 30 days 6


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(from about 7.67 nm to 16.90 nm), of which the very rough surface resulted from the longer exposure time probably influences the performance of PHJ-PSCs. The results suggest that high-quality perovskite films with large-size grains and smooth surface could be achieved through the exposure to air with controllable humidity at suitable time.

(a)

(b)

(c)

(d)

(e)

(f)

1 µm 1000

(h) (g) 800

Size (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600 400 200 0 0 day

2 days 5 days 10 days 20 days 30 days

Figure 2. SEM morphology images of CH3NH3PbI3 films exposed to air with a humidity of ~ 40 % for (a) 0 day, (b) 2 days, (c) 5 days, (d) 10 days, (e) 20 days, and (f) 30 days, respectively. (g) The average grain size distribution for CH3NH3PbI3 films exposed to air with the different exposure time.

7



The XRD patterns of CH3NH3PbI3 films exposed to ambient environment with a controlled humidity of ~ 40 % for specific time are shown in Figure 3a. The strong diffraction peaks at 2θ = 14.25 o, 28.56 o and 32.00 o appear for all samples, which can be assigned to the (110), (220) and (310) peaks, respectively. More importantly, the CH3NH3PbI3 films, no matter exposed to ambient environment or not, show the similar XRD patterns without the diffraction peak resulted from PbI2 crystals. It suggests that the CH3NH3PbI3 films exposed to ambient environment with a humidity of ~ 40 % for the time less than 30 days would not show noticeable change in crystal

(a)

5

30 days

4

Absorbance

(224) (314)

(112) (211) (202) (220) (310) (312)

Intensity (a.u)

(110)

structure, which should be helpful for achieving good PHJ-PSCs.

20 days 10 days 5 days 2 days

(b)

3

0 day 2 days 5 days 10 days 20 days 30 days

2 1

0 day

0 20

30 40 o 2-theta ( )

50

(c) 0 day 5 days 10 days 20 days 30 days

650

700

750 800 850 Wavelength (nm)

400

60

900

Normalized PL signal (a.u.)

10

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500 600 700 Wavelength (nm)

1.0

800

(d)

0.8 0 day 5 days 10 days 20 days 30 days

0.6 0.4 0.2 0.0

0

10

20 30 40 Decay time (ns)

50

Figure 3. (a) XRD patterns, (b) absorption spectra, (c) steady-state PL spectra and (d) PL lifetime of CH3NH3PbI3 films exposed to air with the time of 0 day, 2 days, 5 days, 10 days, 20 days and 30 days, respectively.

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Meanwhile, the absorption spectra of CH3NH3PbI3 films exposed to ambient environment with a humidity of ~ 40 % for the different time are shown in Figure 3b. It is obvious that all perovskite films display the absorption curves with the similar peaks, in which the strong photon harvesting capability is ranged from 350 nm to 780 nm. It is interesting that the intensity of absorption curves between 350 nm and 500 nm shows a little increase as exposed to ambient environment. It then decreases a little as the exposure time increase to more than 5 days and shows the similar intensity as the original one for the exposure time more than 20 days. The enhancement of absorption intensity should result from the large increase in grain size induced by air exposure (Figure 2g). Meanwhile, slight PbI2 maybe be generated as well during the exposure process. The amount of PbI2 is too low to be detected by normal XRD. The slight PbI2 would passivate the surface and play a critical role in stabilizing the perovskite films and improve the performance of PHJ-PSCs.24 Furthermore, the steady-state PL spectra CH3NH3PbI3 films exposed to ambient environment with a humidity of ~ 40 % at the fixed time intervals are shown Figure 3c. It obviously shows that the PL intensity of CH3NH3PbI3 films enhances as increasing the exposure time. It should be associated with the non-radiative decay, which is restrained tremendously after the CH3NH3PbI3 films exposed to ambient environment.15 At the same time, time-resolved PL were measured and the results are shown in Figure 3d. The PL lifetime of CH3NH3PbI3 films exposed to ambient environment is improved dramatically to 23.9 ns as the exposure time reaches to 10 days as compared with the pristine one of 11.5 ns. Then the PL lifetime decreases gradually when the exposure time exceeds 10 days, but it is still much longer than pristine lifetime. The PL increase and slower PL decay are possibly related to the slight PbI2 formation, which could inhibit the charge injection to the selective contacts.24 A longer PL lifetime suggests a longer diffusion length, contributing the decrease of recombination and the increase of the photocurrent. On the other hand, the decrease of non-radiative recombination is helpful to improve the open-circuit voltage (Voc) as well.25 The results from PL spectra and PL lifetime indicate that non-radiative 9



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recombination routes are restrained significantly for CH3NH3PbI3 films exposed to ambient environment, which would substantially reduce the trap numbers and make photo-generated carries efficiently migrate from the active layer to the electrodes.8,15 The properties of CH3NH3PbI3 films discussed above suggest that the CH3NH3PbI3 films exposed to ambient environment should produce better PHJ-PSCs than that without air exposure. Thus PHJ-PSCs were fabricated based on CH3NH3PbI3 films with and without air exposure, and the typical J-V curves are shown in Figure 4a for the CH3NH3PbI3 films exposed to ambient environment for 0 day, 2 days, 5 days, 10 days, 20 days and 30 days, respectively. The detail performance parameters of typical PHJ-PSCs are presented in Table 1 as well. The change trend of average PCEs as the function of air-exposure time is shown in Figure 4b. The detailed statistic performance parameters are shown in Figure S2, and the typical PHJ-PSC devices of J-V characteristic curves under forward and reverse scanning measurements with the specified air-exposure time are presented in Figure S3. It is amazing that the average PCEs of PHJ-PSCs for the CH3NH3PbI3 films exposed to ambient environment increase dramatically as compared to the one without the exposure. The average PCEs of 10 PHJ-PSC devices with the exposure time for 0 day, 2 days, 5 days, 10 days, 20 days and 30 days increase from 10.80 % to 13.82 %, 13.54 %, 14.04 %, 12.06 % and 11.74%, respectively. It should notice that the champion of PHJ-PSC device for the exposure in air for 10 days shows the PCE up to 16.21 % with Voc of 0.94 V, short-circuit current (Jsc) of 23.3 mA/cm2 and fill factor (FF) of 73.8 %, respectively. It is very interesting that the PHJ-PSC devices with an average PCE of about 11.74 % could still be achieved even the exposure time increases to 30 days.

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0

18

(a)

15

(b)

-6 0 day 5 days 20 days

-12

PCE (%)

Current Density (mA/cm 2 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 days 10 days 30 days

-18

12 9 6 3

-24 0.0

0.2

0.4 0.6 Voltage (V)

0.8

1.0

0

0

5

10

15

20

25

30

Time (day)

Figure 4. (a) The typical J-V curves and (b) the average PCEs of PHJ-PSCs for CH3NH3PbI3 films exposed to ambient environment with a humidity ~ 40 % for the time of 0 day, 2 days, 5 days,10 days, 20 days and 30 days, respectively.

Table 1. The typical performance parameters of PHJ-PSCs obtained from J-V curves in Fig. 3a. The average PCEs of ten PHJ-PSCs are shown in the bracket. Exposure time

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

0 day

0.89

21.18

76.46

12.02 (10.80 ± 0.70)

2 days

0.95

22.51

79.06

14.57 (13.82 ± 0.76)

5 days

0.94

22.57

70.72

14.74 (13.54 ± 0.63)

10 days

0.94

23.72

75.15

16.21 (14.04 ± 1.56)

20 days

0.94

22.91

75.07

13.53 (12.06 ± 1.02)

30 days

0.92

22.90

70.52

12.77 (11.74 ± 1.36)

The results above powerfully prove that efficient PHJ-PSCs could be fabricated through exposing CH3NH3PbI3 films in ambient environment with a suitable humidity, which results in the great improvement in CH3NH3PbI3 film properties. The suitable humidity is beneficial for improving the CH3NH3PbI3 film with large size grains, resulting in less interfacial area for suppressing charge trapping, as well as producing lower bulk defects and higher mobility.8 Thus, they are potential to obtain excellent 11



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photovoltaic performance. Furthermore, the large-size grains can enhance the charge extraction as well because it is indispensable for the photo-generated charges to transport and pass through grains boundaries, and they ultimately are collected by the electrodes, in which the large-size grains diminish the spaces between perovskite crystals and the shallow pin-holes of perovskite films.17, 26 As the exposure time is more than 10 days, the surface of CH3NH3PbI3 films become rough, of which the RMS increases from about 7.67 nm to 16.90 nm. Meanwhile, the carrier lifetime decrease as well (Figure 3d). Thus the performance of PHJ-PSCs shows a little decrease gradually. But the performance parameters of PHJ-PSCs with the air-exposure time up to 30 days are still better than the devices without the air-exposure (Table 1). The results also suggest that the CH3NH3PbI3 films could be stably stored for more than 30 days in air with a humidity of ~ 40 %. Furthermore, UPS and XPS were employed to better understand the internal mechanism for the improvement of photovoltaic parameters. The UPS data of CH3NH3PbI3 films exposed to the predetermined humidity environment for specific time are shown in Figure 5. Figure 5a presents the evolution of secondary photoemission cut-off for different CH3NH3PbI3 films, from which the work function (WF) of the films can be achieved from the energy difference between the secondary cut-off and the Fermi level (EF). For original CH3NH3PbI3 film, the WF was measured to be 4.01 eV, which is in accordance with the reported results.27 As increasing the exposure time, the WF increases gradually and arrives at 4.53 eV with an exposure of 10 days. The increase of WF might be related with the change of surface morphology and the adsorption. Figure 5b shows the valence band maximum (VBM), which is determined by the linear extrapolation of the leading edge and the background. The VBM of pristine CH3NH3PbI3 film is observed to be 1.40 eV below the EF, indicating that pristine CH3NH3PbI3 film is n-type as its band gap is about 1.6 eV.6,8 The VBM values increase with the augment of exposure time, resulting in the 1.57 eV after 10 days’ exposure. The rigid shift indicates that the CH3NH3PbI3 film with exposure in air is more n-type than the pristine CH3NH3PbI3 film, which would increase the 12


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electron intensity and accordingly improve the performance of PHJ-PSCs. Our previous results suggested that the CH3NH3PbI3 film was not sensitive to oxygen but could be n-doped by H2O.21 Thus the n-doping of CH3NH3PbI3 film results from the air humidity and makes the EF very close to the bottom of conduction band. The typical energy-level alignment diagram for CH3NH3PbI3 films that exposed to air before and after 10 days is shown in Figure 5c. The n-doping would be attributed to improve the Voc and charge extraction, resulting in the performance improvement of PHJ-PSCs.

(b) VBM

(a)

(c)

1.57 eV

in air for 10 days Pristine film

1.57 eV

Intensity (a.u)

1.51 eV

4.31 eV 4.20 eV

1.40 eV 1.46 eV

1.6

4.01 eV

3

2.5

(d) Pb 4f 10 days

5 days

1.0 2 days

0.5 0.0

Intensity (a.u)

1.5

2

1.4

1.3

EF 1

0

-1

Binding Energy (eV)

3.0

2.0

1.5

1.40 eV

2 4 6 8 Work Function (eV) 2.5

1.46 eV

1.51 eV

2.0 10 days

1.5 1.0 0.5

2.5

(e) I 3d

5 days 2 days

10 days

1.5 5 days

1.0 0.5

2 days 0 day

0.0

0.0

Binding Energy (eV)

N 1s

0 day

0 day 146 144 142 140 138 136 134

(f)

2.0

Intensity (a.u)

Intensity (a.u)

4.53 eV

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


632 628 624 620 616

405 404 403 402 401 400 399

Binding Energy (eV)

Binding Energy (eV)

Figure 5. The UPS data of CH3NH3PbI3 films showing (a) the cut-off region and (b) the HOMO region for the CH3NH3PbI3 films exposed to air with 40% humidity for 0 days (black), 2 days (orange), 5 days (blue) and 10 days (magenta), respectively. (c) Energy-level alignment diagram of CH3NH3PbI3 films before and after exposure to air for 10 days. (d-f) The XPS spectra of Pb

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4f7/2, I 3d5/2 and N 1s core levels of CH3NH3PbI3 films as a function of exposure time range from 0 day to 10 days, respectively.

The XPS spectra of Pb 4f7/2, I 3d5/2, and N 1s core levels are shown in Fig. 5d-f. For visual clarity, the spectra are normalized to be the same height. It is obvious that negligible variation could be found, strongly indicating that the CH3NH3PbI3 films do not encounter lethal damage with the exposure in air. In addition, the full XPS spectra of the CH3NH3PbI3 films before and after the exposure in air are shown in Figure S4. There is also not detectable difference which supports the good stability of CH3NH3PbI3 films exposed to air. Inset in Figure S4 exhibits a slightly increase of oxygen peak, It should come from the adsorption of H2O on the surface of CH3NH3PbI3 films. Our previous results indicated that H2O molecules could be absorbed onto the surface of CH3NH3PbI3 film and form the n-doping as the H2O exposure under a threshold value.21 If the H2O exposure is above the threshold, the CH3NH3PbI3 film decomposes and accelerates the formation of PbI2. It was reported that the CH3NH3PbI3 film dramatically decomposed into PbI2 after the exposure in ambient with 60% relatively humidity.28 In our case, after the exposure in ambient with a relative humidity of 40% for even 10 days, all experiment results proved that the CH3NH3PbI3 film is stable and its crystal structure is well-maintained, resulting in outstanding device performance. More importantly, the exposure in air with controlled humidity could increase the grain size and form the n-doping, which are greatly helpful to improve the performance of PHJ-PSCs. In other words, the controlled humidity (under a threshold, ~ 40 % humidity) would not do harm to the CH3NH3PbI3 film, on the contrary, it would greatly improve the quality of CH3NH3PbI3 film, and accordingly improve the device performance.

4. CONSLUSIONS In summary, high-efficiency PHJ-PSC devices with an architecture of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al were fabricated by one-step solution 14


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deposition with an air-induced process. High-quality CH3NH3PbI3 films with large-size domains and uniform surface could be achieved through the exposure in ambient environment with humidity of ~ 40 %, resulting in the great improvement in morphology, absorption, PL spectra, and PL lifetime. Furthermore, UPS and XPS results proved that the CH3NH3PbI3 film could be n-doped by the absorption of H2O on the surface, but the CH3NH3PbI3 film is very stable due to the passivation of slight PbI2. Thus, the PHJ-PSCs with CH3NH3PbI3 films exposed to ambient environment for 10 days produced the PCE up to 16.21 %, and the average PCE was over 14.00 %. As compared to reference PHJ-PSCs without the exposure to ambient environment, the PCEs showed an improvement of over 30 %. The study demonstrated that ambient environment with controlled humidity would greatly improve the perovskite film morphology and contribute to enhance the PCEs of PHJ-PSCs. It provides a potential method for the fabrication of high-efficiency and large-scale PHJ-PSCs in ambient environment. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51673214), the Hunan Provincial Natural Science Foundation of China (2015JJ1015), and the Project of Innovation-driven Plan in Central South University (2015CXS036). Y.L.G. acknowledges the support by National Science Foundation CBET-1437656. REFERENCES (1) Kojima, A.; Teshima K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050. (2) Shao, Y. C.; Yuan, Y. B.; Huang, J. S. Correlation of energy disorder and open-circuit voltage in hybrid perovskite solar cells. Nat. Energy 2016, 1, 15001. (3) Wu, R. S.; Yang, B. C.; Zhang, C. J.; Huang, Y. L.; Cui, Y. X.; Liu, P.; Zhou, C. H.; Hao, Y. Y.; Gao, Y. L.; Yang, J. L. Prominent efficiency enhancement in perovskite solar cells employing silica-coated gold nanorods. J. Phys. Chem. C 2016, 120, 6996-7004.

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Rau,

U.;

Reciprocity

relation

between

photovoltaic

quantum

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and

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12 9

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6 3 0

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Air-Induced High-Quality CH3NH3PbI3 Thin Film for Efficient Planar

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