Controlled growth of CH3NH3PbI3 films towards efficient perovskite solar cells by varied-stoichiometric intermediate adduct

Applied Surface Science 403 (2017) 572–577

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Controlled growth of CH3 NH3 PbI3 films towards efficient perovskite solar cells by varied-stoichiometric intermediate adduct Yongguang Tu, Jihuai Wu ∗ , Xin He, Panfeng Guo, Hui Luo, Quanzhen Liu, Jianming Lin, Miaoliang Huang, Yunfang Huang, Leqing Fan, Zhang Lan Engineering Research Center of Environment-Friendly Functional Materials for Ministry of Education, Key Laboratory of Functional Materials for Fujian Higher Education, College of Material Science and Engineering, Huaqiao University, Xiamen 361021, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 5 November 2016 Received in revised form 26 December 2016 Accepted 23 January 2017 Available online 24 January 2017 Keywords: DMSO Intermediate Crystallization Perovskite solar cells

a b s t r a c t Lewis acid-base adduct approach with anti-solvent (diethyl ether) has been one of the efficient strategies to prepare high-quality perovskite films for top-performing perovskite solar cells. Conventionally, the molar ratio of CH3 NH3 I:PbI2 :DMSO in the precursor solution is 1:1:1, however, DMSO will be volatile alongside the extraction of DMF due to its miscibility with DMF solvent during the process of anti-solvent washing, which introduces a non-stoichiometric intermediate adduct CH3 NH3 I·PbI2 ·xDMSO (x < 1). In this work, we increased the DMSO content in the Lewis acid-base adduct approach to enhance the conversion efficiency of perovskite solar cells. More complete intermediate adduct CH3 NH3 I·PbI2 ·DMSO and wider window period of washing process ensure high-quality perovskite films and high reproducibility with raised content of DMSO. Furthermore, the devices prepared by the precursor solution of CH3 NH3 I:PbI2 :xDMSO (x = 2.0) exhibit high reproducibility and the best efficiency of perovskite solar cells is 17.84% under one-sun illumination. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Perovskite solar cells have aroused burgeoning interests for making transformative surge [1–7] due to its many merits such as broad and strong light absorption [8], longer carrier lifetimes [9], long charge carrier diffusion length [10], and low exciton binding energy [11]. The most-studied perovskite light absorbers are CH3 NH3 PbI3 [12], CH3 NH3 PbI3-x Clx [13] and (NHCH2 PbI3 )0.85 (CH3 NH3 PbBr3 )0.15 [14]. Perovskite solar cells are generally composed of transparent conducting oxide substrate (TCO), a n-type compact blocking layer, perovskite layer with or without scaffold layer (e.g TiO2 , ZnO or Al2 O3 ), a p-type holetransporting material (HTM) layer and metal back electrode [15]. The power conversion efficiency (PCE) have reached a high efficiency of 20.8% [16], leapfrogging every other solution-processed solar cell technology. More recently, an improved PCE of 22.1% was certified by NREL (http://www.nrel.gov/ncpv/images/efficiency chart.jpg). The optoelectronic properties of perovskite films are closely related to the film-quality [17,18], so perovskite deposition is

∗ Corresponding author. E-mail address: [email protected] (J. Wu). http://dx.doi.org/10.1016/j.apsusc.2017.01.240 0169-4332/© 2017 Elsevier B.V. All rights reserved.

crucial for fabricating high-performance perovskite solar cells. Perovskite film can be accomplished mainly using either one-step coating [19–21] or sequential two-step coating [22–25]. Topperforming perovskite solar cells usually are fabricated by a Lewis acid-base adduct approach [26,27] with anti-solvent such as diethyl ether, chlorobenzene or toluene to extract the solvents in the wet perovskite film. Typically, in the Lewis acid-base adduct approach to fabricate perovskite film, PbI2 and CH3 NH3 I is firstly dissolved by mixture polar aprotic solvents, such as ␥-butylolacone (GBL), N,Ndimethyl sulfoxide (DMSO), Thiourea, N,N-dimethylformamide (DMF). As is known, iodide (I− ) in CH3 NH3 I is a strong donor and could have a strong interaction with PbI2 , also DMSO is deemed to be a Lewis base with O-donor, so intermediate adduct of CH3 NH3 I·PbI2 ·DMSO is formed in the perovskite precursor solution [21]. Then while the perovskite solution is spin-coated on the mesoporous TiO2 layer, the anti-solvent (diethyl ether) drips onto the rotating substrate, which leads to a transparent CH3 NH3 I·PbI2 ·DMSO adduct film. Trace its root, diethyl ether is miscible with DMF, but is immiscible with DMSO, so the solvent DMF can be extracted quickly by diethyl ether, leading to a uniform and homogeneous intermediate adduct film. However, to obtain a high-quality perovskite film, the dropping rate and time of diethyl ether is extremely crucial. Usually, the molar ratio of CH3 NH3 I:PbI2 :DMSO in the precursor solution is 1:1:1,

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Fig. 1. Schematics of the process of fabricating perovskite films by ameliorated Lewis acid-base adduct approach.

Fig. 2. Optical microscope of perovskite films after heat treatment in group of x = 1.0 at different dropwise time. Magnification of the microscope is 100.

DMSO will be volatile alongside the extraction of DMF due to its miscibility with DMF during the washing process of antisolvent, which introduces a nonstoichiometric intermediate adduct CH3 NH3 I·PbI2 ·xDMSO (x < 1). Intermediate adduct film will turn to light brown owing to rapid crystallization of not quantitative perovskite. Also, the window period of washing process of diethyl ether is very narrow, which makes difficult to manipulate and reduces reproducibility. Herein, based on the Lewis acid-base adduct approach to fabricate perovskite film, we improved the DMSO content in the precursor solution to regulate the crystallization of perovskite film by varied-stoichiometric intermediate adduct. The effects of DMSO content on the perovskite film morphology, crystallization, and device characteristics are discussed. Firstly, the degree of red brown is obviously reduced, intimating more intermediate CH3 NH3 I·PbI2 ·DMSO adduct. Secondly, the time of film changes turbid delays, the window period of the washing process is broaden. Thirdly, the morphology and crystallization of perovskite films can be fine-adjusted by tuning the content of DMSO. Furthermore, the devices prepared by the precursor solution of PbI2 :CH3 NH3 I:xDMSO (x = 2.0) exhibit high reproducibility and the best efficiency of perovskite solar cells is 17.84% under one-sun illumination. 2. Results and discussion Fig. 1 shows the schematics of the process of fabricating perovskite films by ameliorated Lewis acid-base adduct approach. Perovskite films were fabricated by controlling different kinds of precursor solutions with varying DMSO content. In first-step program, removing excess precursor solution is a major process [19], so introduction of diethyl ether should require an appropriate point in time. In second-step program, most DMF could be extracted by rapid evaporation of ether, and then the intermediate CH3 NH3 I·PbI2 ·DMSO will be crystalized by the supersaturation from which a smooth and uniform film is formed [28]. To determine the precise time of introduction of the diethyl ether, we studied three dropwise times in group of x = 1.0:4000 rpm, 5th s; 4000 rpm, 7th s, 4000 rpm, 9th s, respec-

tively. Fig. 2 shows the optical microscope of perovskite films at different dropwise time. For the sample of 5th s, there is a big crack in the film, indicating that a large amount of solvent is remained in the wet film and diethyl ether lashes the wet film. And the introduction of diethyl ether is premature. For the sample of 9th s, there are so many aggregates in the film, indicating that the intermediate has been crystalized by the supersaturation and the introduction of diethyl ether is too late. In this case, the window period would be very narrow. Then the time of films in six groups changed turbid were census, 4000 rpm, 7th s for x = 0.5; 4000 rpm, 9th s for x = 1.0; 4000 rpm, 11th s for x = 1.5; 4000 rpm, 12th s for x = 2.0; 4000 rpm, 20th s for x = 4.0; 4000 rpm, 28th s for x = 6.0, respectively. So the window period becomes wide for the washing process as the content of DMSO increased. We recorded the color change of the films versus different DMSO content to further scrutinize the crystal growth behavior. Fig. 3 shows visual images of the perovskite film at different time at room temperature. As shown in Fig. 3(a), the films show different degree of hyperchromicity at the end of spin-coating. As is known to all, the color of CH3 NH3 PbI3 is dark red, while the color of CH3 NH3 I·PbI2 ·DMSO adduct is light white [21]. For sample of x = 0.5, dark red-brown is the characteristic color, suggesting the formation of perovskite. The degree of hyperchromicity gradually decreased from group of x = 0.5 to group of x = 6.0. For sample of x = 1.5, a brown transparent film was obtained. When x > = 2.0, the film showed colorless transparent. So there are variedstoichiometric intermediate adducts in the incipient films. So a conclusion can be drawn, with the increase of DMSO content in precursor solution, the more complete CH3 NH3 I·PbI2 ·DMSO intermediate adduct remained. Afterwards, when the time was extended from 0 min to 60 min at room temperature, the color of perovskite films of all groups was gradually deepened, indicating the slow crystallization introduced by the slow evaporation of DMSO. And the effective regulation of crystal growth can be achieved. For x = 6.0, the original colorless transparent also turned brown. Fig. 4 shows the waterfall XRD patterns of perovskite films versus different DMSO content. The XRD patterns confirm the

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Fig. 3. Photos of different group of perovskite films at different time at room temperature.

x=6.0 x=4.0 x=2.0 x=1.5 x=1.0 x=0.5

10

11

12

13

14

2 Theta (degree)

15

16

Fig. 4. The waterfall XRD patterns of perovskite films versus different DMSO content.

Fig. 5. Magnified views of XRD patterns of perovskite films between 10◦ and 16◦ .

presence of the tetragonal perovskite phase of CH3 NH3 PbI3 in all of the prepared films. The two main peaks located at 14.1◦ and 28.4◦ can be indexed to the (110) and (220) planes [29]. The additional increase in the intensity indicates that the crystallinity enhances as the DMSO content increases, which is ascribed to strong Lewis basicity of DMSO. From x = 4.0 to x = 6.0, the intensity shows dramatic downward, too much DMSO inhibits the crystallization of the perovskite. So less DMSO is applied, the intermediate CH3 NH3 I·PbI2 ·DMSO will be incomplete and perovskite crystals fast, producing many traps and recombination centers. If too much DMSO is applied, the excess DMSO remained in the wet film will retard the crystallization. Fig. 5 shows the magnified views of XRD

patterns between 10◦ and 16◦ . And there is no excess PbI2 residue in the film. In order to investigate the surface morphologies of CH3 NH3 PbI3 films, we studied the top-view SEM images. As shown in Fig. 6, the DMSO content has an obvious influence on the film’s grain size. In group of x = 2.0 and group of x = 4.0, it presents an enlarged average grain size, which would minimize the grain boundary energy and be beneficial for the charge transmission. Time resolved photoluminescence (TRPL) intensity decay measurements of CH3 NH3 PbI3 perovskite films can be used to explore the charge carrier extraction at the interface between TiO2 layer and the perovskite layer [30], the samples were composed of FTO/CLTiO2 /mp-TiO2 /perovskite, and corresponding results are presented

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Fig. 6. Top-view SEM images of perovskite films versus different DMSO content.

104

25

Current Density (mA cm-2)

PL intensity (arb.units)

20

103

15

X=0.5 X=1.0 X=1.5 X=2.0 X=4.0 X=6.0

10

102

101

50

100

150

200

Time (ns)

250

300

5

0 0.0

350

0.2

0.4

0.6

0.8

1.0

1.2

Potential (V)

Fig. 7. Time-resolved photoluminescence intensity decay of CH3 NH3 PbI3 detected at peak emission wavelength of 760 nm.

Table 1 Fitting parameters for the time-resolved PL measurements shown in Fig. 7. PbI2 :CH3 NH3 I:xDMSO

A1

␶1 /ns

A2

␶2 /ns

Average␶/ns

x = 0.5 x = 1.0 x = 1.5 x = 2.0 x = 4.0 x = 6.0

0.58 0.63 0.83 0.77 0.37 0.27

49.31 16.23 7.73 5.05 39.68 44.10

0.42 0.37 0.17 0.23 0.63 0.73

14.25 43.98 38.99 34.70 13.84 10.27

34.58 26.50 13.04 11.86 23.40 19.41

in Fig. 7. And the curves were fitted with a two exponential decay function and the corresponding lifetimes were obtained, as listed in Table 1. And the charge carrier lifetime (␶) are 34.58 ns, 26.50 ns, 13.04 ns, 11.86 ns, 23.40 ns, 19.41 ns, corresponding to the groups, x = 0.5, 1.0, 1.5, 2.0, 4.0 and 6.0, respectively. The shorter ␶ value means that the charge more fast transfer from the perovskite to TiO2 layer [31–35], which may be attributed to the high crystallinity and few grain boundaries. J–V measurements of the perovskite solar cells based on different DMSO content are shown in Fig. 8 and the corresponding photovoltaic parameters are listed in Table 2. For x = 0.5, a PCE

Fig. 8. J–V curves of the PSCs based on based on different DMSO content under AM 1.5 G illumination. The devices were measured by reverse (2.0 v to −0.1 v) voltage scanning at a scan step of about 21.2 mV (100 data points in total).

Table 2 Photovoltaic performances of the devices based on different DMSO content. PbI2 :CH3 NH3 I:xDMSO

Voc (V)

Jsc (mA cm−2 )

FF (%)

PCE (%)

x = 0.5 x = 1.0 x = 1.5 x = 2.0 x = 4.0 x = 6.0

1.012 1.035 1.052 1.076 1.049 0.997

20.89 22.13 22.35 23.03 22.80 21.74

67 69 69 72 71 67

14.16 15.80 16.22 17.84 16.98 14.52

of 14.16% was achieved with short-circuit current density (Jsc) of 20.89 mA cm−2 , open-circuit photovoltage (Voc) of 1.012 V and fill factor (FF) of 0.67. And the performance continuously improved as the amount of DMSO increased. Compared to the devices from group of x = 1.0 to group x = 4.0, the performance improvement mainly lies in the increase of the Voc and the FF, depended largely on the high-quality perovskite film. Best performance was achieved in group of x = 2.0, yielding a Jsc of 23.03 mA cm−2 , Voc of 1.076 V, and FF of 0.72, resulting in a PCE of 17.84%. Further increasing the amount of DMSO in group of x = 6.0 did not improve device per-

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24

22

22

Jsc=19.79mA·cm-2

20

100

25

80

20

60

15

40

10

20 18

PCE=17.25%

16

14

14

12

12

10

10

8

8

6

6

4

4

2

2 100

200

20

300

5

Integrated Jsc=21.20mA·cm-2 0 300

0 400

0 0

IPCE (%)

16

Integrated Jsc (mA·cm-2)

18

PCE (%)

Currenr Density (mA·cm-2)

576

400

500

600

0 800

700

Wavelength (nm)

Time (s) Fig. 9. The steady-state photocurrent and output PCE of the devices at the maximum power points.

Fig. 11. IPCE spectrum (black line) and integrated photocurrent density Jsc (red line) for the champion cell. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

25 24

20

17.24% 16.92%

x=1.0 x=2.0 x=4.0

22 20 18 16

15

15.46%

14

Count

Current Density (mA cm-2)

Reverse Forward

Voc(V) Jsc(mA cm-2) FF(%) PCE(%)

10

Reverse 1.076

23.03

72

17.84

Forward 1.035

23.05

59

14.08

12 10 8 6

5

4 2

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Potential (V) Fig. 10. J–V curves of the best device measured by reverse (open circuit → short circuit) and forward (short circuit → open circuit) scans under AM 1.5 G illumination.

formance, resulting from a lower Jsc of 21.74 mA cm−2 , which was ascribed to the reduced crystallinity. Fig. 9 shows the steady-state photocurrent and output PCE of the devices in group of x = 2.0 at the maximum power points with a stabilized current density output of 19.79 mA cm−2 (at the voltage of 0.87 V), yielding a PCE of 17.25%. Fig. 10 shows the J-V curve of the best device under both reverse and forward bias scans, while there is a noticeable hysteresis behavior. For the reverse scan, the device shows a power conversion efficiency (PCE) of 17.84%, with open-circuit voltage (Voc) of 1.076 V, short-circuit current density (Jsc) of 23.03 mA cm−2 , and fill factor (FF) of 0.72. For the forward scan, the device shows a PCE of 14.08%, with Voc of 1.035 V, Jsc of 23.05 mA cm−2 , and FF of 0.59. Fig. 11 shows the incident-photon-to-current conversion efficiency (IPCE) for the best performing perovskite solar cell in group of x = 2.0. IPCE reaches peak values of near 90% in the shortwavelength region of the visible spectrum. The integrated Jsc is calculated to be 21.20 mA cm−2 . This confirms that any mismatch between the simulated sunlight and the AM1.5 G standard is negligibly small [5]. Statistic results of the cell performance are provided in Fig. 12 as histogram charts. It can be found that the devices in group of x = 2.0 show better performance and the average PCE is 17.24%. Meanwhile, the histogram chart demonstrates the high reproducibility of the devices (Each team is calculated from a batch of 50 cells).

0 13.5

14.0

14.5

15.0

15.5

16.0

16.5

17.0

17.5

18.0

18.5

Efficiency (%) Fig. 12. Histograms of the PCEs of the devices in groups of x = 1.0, x = 2.0, x = 4.0.

3. Conclusions In conclusion, we have prepared high-quality CH3 NH3 PbI3 film by increased the DMSO content in the Lewis acid-base adduct approach to regulate the crystallization of perovskite by variedstoichiometric intermediate adduct. Firstly, the degree of red brown is obviously reduced, intimating more complete intermediate adduct CH3 NH3 I·PbI2 ·DMSO. Secondly, the time of film changed turbid delays, the window period of the washing process is broaden. Thirdly, the morphology and crystallization of perovskite films can be fine-adjusted by tuning the content of DMSO. Furthermore, the devices prepared by the precursor solution of PbI2 : CH3 NH3 I: xDMSO (x = 2.0) exhibit high reproducibility and the best efficiency of perovskite solar cells is 17.84% under one-sun illumination.

Acknowledgements The authors acknowledge the financial joint support by the National Natural Science Foundation of China (Nos. 91422301, 51472094, 61474047, 21301060, 61306077, U1205112), and the Cultivation Program for Postgraduate in scientific Research Innovation Ability of Huaqiao University (No. 1400102002).

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