An Efficient and Reproducible CH3NH3PbI3 Perovskite Layer

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An Efficient and Reproducible CH3NH3PbI3 Perovskite Layer Prepared Using a Binary Solvent Containing a Cyclic Urea Additive Lin Xie, An-Na Cho, Nam-Gyu Park, and Kyungkon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18761 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Applied Materials & Interfaces

1

An Efficient and Reproducible CH3NH3PbI3 Perovskite Layer Prepared

2

Using a Binary Solvent Containing a Cyclic Urea Additive

3

Lin Xie,† An-Na Cho,‡ Nam-Gyu Park,*,‡ and Kyungkon Kim*,†

4

†

Department of Chemistry and Nano Science, Ewha Womans University, Seoul, South Korea

5 6

‡

School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea

7 8

9

ABSTRACT

10

An efficient CH3NH3PbI3 perovskite solar cell (PSC) whose performance is reproducible and

11

shows reduced dependence on the processing conditions is fabricated using the cyclic urea

12

compound 1,3-dimethyl-2-imidazolidinone (DMI) as an additive to the precursor solution of

13

CH3NH3PbI3. X-ray diffraction analysis reveals that DMI weakly coordinates with PbI2 and

14

forms a CH3NH3PbI3 film (Film-DMI) with no intermediate phase. The surface of annealed

15

Film-DMI (Film-DMI-A) was smooth, with an average crystal size of 1 µm.

16

Photoluminescence and transient photovoltage measurements show that Film-DMI-A exhibits

17

a longer carrier lifetime than a CH3NH3PbI3 film prepared using the strongly coordinating

18

additive DMSO (Film-DMSO-A), because of the reduced number of defect sites in

19

Film-DMI-A. A solar cell based on Film-DMI-A exhibits a higher power conversion

20

efficiency (17.6%) than that of a cell based on Film-DMSO-A (15.8%). Furthermore, the

21

performance of the Film-DMI-A solar cell is less sensitive to the ratio between PbI2 and DMI,

22

and Film-DMI can be fabricated under a high relative humidity of 55%.

23

KEYWORD: perovskite, photovoltaics, urea additive, intermediate phase,

24

solar cells

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ACS Paragon Plus Environment

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INTRODUCTION

2

An organic-inorganic hybrid perovskite material based on methylammonium lead iodide

3

(MAPbI3) was first utilized as a photovoltaic sensitizer for a liquid-electrolyte-based

4

sensitized solar cell in 2009 by Miyasaka et al1, and delivered a power conversion efficiency

5

(PCE) of 3.8%. Park et al. introduced the perovskite material in a thin film solar cell,

6

achieving a milestone for the development of a promising next-generation solar cell.2 Over

7

the past 8 years, enormous efforts have been devoted to improving the performance of

8

MAPbI3-based PSCs 3-7 Until now, the PCEs of the PSCs fabricated using a one-step method

9

have increased by over 20%.8, 9

10

The addition of DMSO into a precursor solution of MAPbI3 used to fabricate PSCs via a

11

one-step method has greatly improved the quality of the MAPbI3 films.10 It was reported that

12

the DMSO formed the intermediate MAI·PbI2·DMSO species—this retards the reaction

13

between PbI2 and MAI due to its strong coordination capability, and is a critical consideration

14

factor in the preparation of high-quality MAPbI3 films.9,

15

PbI2:DMSO was found to be controlled in quite a narrow stoichiometric ratio around 1:1.11-13

16

In addition, we found that the intermediate of PbI2·DMSO is very sensitive to humidity

17

because the DMSO interacts strongly and quickly with H2O via hydrogen bonding.

18

Furthermore, the as-prepared intermediate phase should be subjected to thermal annealing

19

immediately because of the continuous vaporization of DMSO. Thus, it is required to control

20

the intermediate phase of PbI2·DMSO in a reproducible manner. Recently, Li et al. claimed

21

that a weak coordination additive (acetonitrile) showed a larger crystal size and better

22

photovoltaic performance due to the different crystallization kinetics of MAPbI3.14

23

Furthermore, Snaith’s group reported that the PSCs processed with a weakly coordinating

24

additive have great potential in the fabrication of large-scale solar cells.15 Since the study of

25

the intermediate phase is still at an early stage, further studies are required in terms of the

26

coordination abilities of the additives.

10

However, the molar ratio

27

The nucleation and crystallization process strongly influence the morphology, crystallinity, grain

28

size of the perovskite film. The Lewis acid–base adduct approach is one of effective methods to

29

control the process.15-18 In the Lewis acid–base adduct approach, PbI2 and polar aprotic solvent, such 2


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1

as dimethyl sulfoxide (DMSO), acts as a Lewis acid and Lewis base, respectively. It was found that

2

the existence of Lewis bases in the annealing process could significantly promote perovskite grain

3

growth.18 Therefore, it is more desirable to use Lewis base having high boiling point rather than using

4

volatile DMSO. It was also reported that utilization of weak coordinative Lewis base effectively

5

controls nucleation process of a perovskite film.18-23

6

The urea compound is one of promising candidates for the Lewis base because it is known to have

7

high boiling point and form weak Lewis acid-base adducts with PbI2. Recently, utilization of urea and

8

thiourea as a Lewis acid–base adduct has been reported.18-20 However, those urea compounds are solid

9

at the room temperature. The residual urea compound after annealing process negatively effects on the

10

performance

of

PSC.

In

this

work,

we

introduce

a

cyclic

urea

compound

11

1,3-dimethyl-2-imidazolidinone (DMI) which is liquid at room temperature with high boiling point

12

and would form weak Lewis acid-base adduct. We expect that the utilization of the DMI as Lewis base

13

additive could effectively control the growth of perovskite crystal and enhance the performance of

14

PSCs.

15

16

EXPERIMENTAL SECTION

17

Device fabrication

18

Methylammonium iodide (MAI, CH3NH3I) was synthesized by reacting 27.86 mL

19

methylamine (40% in methanol, TCI) and 30 mL of hydroiodic acid (57 wt.% in water,

20

Aldrich) in a 250 mL round bottom flask at 0 °C for 2 h with stirring. The precipitate was

21

generated by evaporation at 50 °C for 40 min. The product, CH3NH3I was recrystallized 3

22

times from a hot saturated ethanol solution and washed 2 times with diethyl ether. Finally, the

23

resultant white powder was dried in a vacuum oven for 12 h at 65 °C. Lead (II) iodide (PbI2)

24

(99.99%) was purchased from Sigma Aldrich.

25

Fluorine-doped tin oxide (FTO) glasses were cleaned with detergent, ethanol and acetone

26

with sonication for 20 min, respectively. After cleaning, the FTOs were treated with

27

UV/Ozone for 20 min. Then, a blocking TiO2 (bl-TiO2) layer was spin coated on top of the

28

FTO using a 0.1 M titanium diisopropoxide bis(acetylacetonate) solution (75 wt.% in 3



Page 4 of 24

1

isopropanol, Sigma) in 1-butanol, followed by an annealing treatment at 125 °C for 5 min. A

2

nanocrystalline TiO2 paste (40 nm) diluted in 1-butanol at a concentration of 100 mg/ml was

3

then deposited on the bl-TiO2 layer, spin coated at 2000 rpm for 20 s to from mesoporous

4

TiO2 (mp-TiO2), followed by an annealing treatment at 550 °C for 1 h. A 1.9 M MAPbI3 fresh

5

precursor solution was prepared (MAI, PbI2, and DMI were dissolved in dimethylformamide

6

(DMF) at a molar ratio of 1:1:0.5). The prepared perovskite precursor solution was deposited

7

on the mp-TiO2 at 6000 rpm for 25 s, then 0.7 mL diethyl ether was dripped onto the spinning

8

substrate at 10 s after starting the spin-coating process. The as-prepared film was pre-heated at

9

65 °C on a hot plate for 1 min and 100 °C for 1 h to form a dense perovskite photo-active

10

layer.

After

that,

the

hole

transport

solution

consisted

of

36

mg

of

11

2,29,7,79-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene

12

deposited by spin coating, 28.8 µl of 4-tert-butylpyridine, and 17.5 µl of 520 mg/ml lithium

13

bis(trifluoromethylsulfonyl) imide acetonitrile solution dissolved in 1 mL of chlorobenzene.

14

Finally, an Ag with a thickness of 100 nm was evaporated on top of the prepared film. For the

15

control device, the precursor was prepared with PbI2: MAI: DMSO with an optimum molar

16

ratio of 1:1:1, which was found to be the best ratio for the device. The control devices were

17

prepared in the same manner as for perovskite solar cells processed with DMI. For the

18

humidity tolerance test, the device fabrications were carried out under a fume hood at ~55%

19

humidity.

(spiro-MeOTAD)

20

The current density-voltage (J-V) curves of the devices were measured using a Keithley

21

2400 Source Measure Unit. An AM 1.5 G simulator was used (McScience K201 LAB50,

22

Oriel) to simulate the solar spectrum. UV-visible absorption spectra were measured using a

23

UV-2450 (SHIMADZU, Japan). Field emission scanning electron microscopy (FE-SEM)

24

images were obtained using a JSM-6700F (JEOL, Japan). The PL spectra of the films were

25

obtained using an LS 55 (Perkin Elmer, USA). Atomic force microscopy (AFM) images were

26

obtained using an XE-70 (Park Systems Corp., Korea). The MAPbI3 film properties were

27

characterized by powder X-ray diffraction (XRD) (Rigaku D/Max 2200), using Ni-filtered

28

Cu/Kα1 radiation (λ =1.54184 Å). The external quantum efficiency (EQE) was measured with

29

monochromatic light generated from a 300 W Xenon lamp in the range 300-900 nm using a

30

K3100 EQX (McScience, Korea). The steady-state, transient photovoltage (TPV) 4


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1

measurements were conducted under continuous illumination from an intensity-adjustable

2

white LED. The resulting voltage transient was acquired by a TDS3054B Tektronix digital

3

oscilloscope with the 1 M input impedance. TPV results were fitted to a monoexponential

4

decay function to find the carrier recombination lifetime.

5

6

RESULTS AND DISCUSSION

7

The impact of additives on the intermediate phase composition and microstructure of the

8

films was investigated by XRD. Figure 1a shows the XRD patterns of as-prepared films

9

processed with DMSO or DMI additives and thermal annealing treatment, and Figure 1b

10

shows that of perovskite films after annealing at 100 °C. As shown in Figure 1a, the film

11

processed with DMSO (Film-DMSO) exhibited diffraction peaks at 7°, 7.9°, and 9.6° which

12

to the (002), (021), and (022) diffraction peaks and originate from the (MA)2Pb3I8·2DMSO

13

intermediate phase, respectively.24-26 The XRD peaks (Figure 1b Film-DMSO-A) at 14.5°,

14

28.8° and 32.3°, corresponding to (110), (220), and (310) planes of MAPbI3, respectively,

15

appeared after thermal annealing, implying that the I-deficient structure in the

16

(MA)2Pb3I8·2DMSO intermediate had been replaced by MAI to form MAPbI3 crystals.18, 21

17

The film processed using the DMI additive (Film-DMI) did not show diffraction peaks

18

corresponding to the intermediate but showed identical XRD peaks of a pure tetragonal

19

MAPbI3, which indicates no intermediate phase is formed in case of DMI additive. This result

20

implies that the perovskite crystals were formed immediately after spin coating without any

21

further thermal annealing process.

22

The diffraction peak intensity of MAPbI3 was further enhanced by thermal annealing

23

treatment of the Film-DMI. In addition, the XRD peak intensity was higher than that of

24

annealed Film-DMSO (described as Film-DMSO-A). The peaks ratio at (110), (220), and

25

(310) planes of the MAPbI3 for annealed Film-DMI (described as Film-DMI-A) and

26

Film-DMSO-A were 1:0.53:0.22 and 1:0:45:0.43, respectively, which suggests that the

27

orientation of MAPbI3 crystal along the (110) and (220) facets were more prominent in

28

Film-DMI-A than Film-DMSO-A. Mosconi et al. reported that binding between the adjacent 5



1

MAPbI3 between under-coordinated Ti (IV) atoms of the mesoporous TiO2 layer would lead

2

the MAPbI3 to grow preferentially in the (110) direction.22, 27 Based on the XRD results, the

3

final orientation of the MAPbI3 crystal was assumed to be strongly dependent on the crystal

4

orientation of as-prepared films before annealing. In other words, the fact that the Film-DMI

5

and its annealed Film-DMI-A have the same MAPbI3 crystal orientation means the DMI

6

additive is useful for growing the MAPbI3 crystals in a desired direction.

7

We checked the solubility of the PbI2 in the pure DMI and DMSO solvents, respectively,

8

to study the non-intermediate phase caused by the DMI. The results are shown in Figure S1 in

9

the Supporting Information. PbI2 showed very poor solubility (28 mg/ml) and dissolved very

10

slowly in pure DMI solvent to give a yellow colored solution. However, the PbI2 dissolved in

11

DMSO very quickly and exhibited high solubility (922 mg/ml) to form a transparent solution.

12

A solubility test indicated that the DMI could form weak coordination with PbI2 and be

13

completely removed during the spin coating process, resulting in direct crystallization of the

14

MAPbI3 in as-prepared Film-DMI. In addition, the dielectric constant for DMSO and DMI are

15

46.7 and 37.5, respectively. The polarity of DMSO is larger than that of DMI. As a result, the

16

lone pair of S=O is more activated than that of the C=O bonding, which means that the lone

17

pairs of DMSO are more strongly coordinated with Pb (II) than DMI. The addition of DMI to

18

the DMF solvent reduces the vapor pressure of the solution, and slows down the growth rate

19

of MAPbI3 without forming intermediate with precursor materials. The growth mechanism is

20

different from the Film-DMSO-A, since it first becomes the (MA)2Pb3I8·2DMSO

21

intermediate phase before it is converted into MAPbI3.

22

The surface morphology and film quality were investigated by SEM (Figure 2). Figure 2a

23

and b show the surface of the as-prepared Film-DMSO and Film-DMI samples, respectively.

24

The average grain sizes of the Film-DMI and the Film-DMSO were 128.2 nm ± 42.9 nm and

25

244.3 nm ± 109.8 nm, respectively. Although the size of the Film-DMSO was larger than that

26

of the Film-DMI, Film-DMI showed a narrow size distribution. The values of film roughness

27

determined by the AFM were 5.1 nm and 5.4 nm for Film-DMI and Film-DMSO,

28

respectively (Figure S2a and b in Supporting Information). Figure 2c and d show the surface

29

of Film-DMSO-A and Film-DMI-A, respectively. The grain size of the Film-DMI-A was

30

determined to be ~1 µm, which is 3-4 times larger than that of the Film-DMSO-A showing 6


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1

~300 nm of grain size. From the SEM images obtained at the large scale (Figure S3a and b in

2

the Supporting Information), both films showed excellent surface coverage. The surface

3

morphology was further investigated by using the AFM (Figure 3). The images of surface

4

morphology obtained from the AFM experiments agreed well with the SEM results. In

5

addition, the roughness values of the perovskite films were evaluated by the AFM as shown in

6

the height profile. Zheng et al. reported that film surface roughness is linearly related to the

7

crystal size of perovskite due to the void between the crystals and grain boundary.28 The

8

surface roughness of Film-DMSO-A is evaluated at 25.1 nm with an average crystal size

9

353.4 nm ± 143.9 nm by AFM. However, the surface roughness of the Film-DMI-A is

10

extremely smooth with an RMS of 7.3 nm and a large crystal size of 968 nm ± 247nm. This

11

result is thought to be due to the introduction of the weakly coordinating DMI as an additive.

12

As shown in the AFM images, a significant change in the surface roughness of the

13

Film-DMSO was observed during the thermal annealing. It was reported that the formation of

14

(MA)2Pb3I8·2DMSO intermediate is attributed to the strong coordination between the DMSO

15

and PbI2 and the intermediate liberates the DMSO molecule by thermal annealing followed by

16

the reaction with the residual MAI in the thin film, which resulted in the formation of the

17

tetragonal perovskite MAPbI3.24, 25 Rong et al. and Cao et al. claimed that the intermediate

18

phase caused by DMSO (donated as Pb3I8) can be described as triple-chains of edge-sharing

19

PbI6 octahedra with all unshared apexes at their sides with disordered orientation, and the

20

reported XRD pattern is consistent with our results as shown in Figure 1a.18, 26 Thus, it is

21

thought that thermal annealing initiated a secondary reaction between the Pb3I8 and MAI by

22

liberating the strong coordinative DMSO from the intermediate, resulting in a rough surface

23

of the Film-DMSO-A. The tetragonal MAPbI3 domains in Film-DMI were formed in the as

24

prepared samples and grew in size without changing the surface roughness of the

25

Film-DMI-A during the thermal annealing since there would be no secondary reaction during

26

the thermal annealing.

27

Considering the XRD, AFM and SEM results, we concluded that the MAPbI3 crystals

28

were homogeneously grown along the (110)-preferred orientation in the Film-DMI. This

29

conclusion accounts for the highly smooth and large MAPbI3 domains in the Film-DMI-A. On

30

the other hand, the existence of the strong coordinative DMSO refrained the homogeneous 7



1

growth of the MAPbI3 domains. This finding reflects that it is more appropriate to use a weak

2

coordinative DMI as an additive in terms of MAPbI3 nucleation than using strongly

3

coordinating DMSO as an additive.29-31

4

Steady PL measurements and VU-vis absorption spectra were conducted for Film-DMI-A

5

and Film-DMSO-A to investigate the trap states and the charge extraction ability of those

6

films (Figure 4a, b). The films were prepared on two different substrates, glass and

7

glass/FTO/TiO2. Generally, the spontaneous radiative recombination between trap states leads

8

to a red-shifted emission peak compared with that from the band edge transition or

9

passivation of these trap states can blue shift the PL peak.32 Comparing the PL spectra of

10

Film-DMSO-A and Film-DMI-A on the glass substrate, the wavelength of the maximum PL

11

peak of annealed Film-DMI was ~7 nm shorter than that of Film-DMSO-A, and the full width

12

at half maximum (FWHM) was narrower. These results indicate a less sharp band edge,

13

which might be attributed to the reduced shallow trap density or defect state in

14

Film-DMI-A.33

15

The extraction efficiency of photo-generated charge at TiO2/MAPbI3 interface was

16

investigated by measuring PL quenching efficiency of films (Figure 4a). Since the absorption

17

intensities of Film-DMI-A and Film-DMSO-A were almost the same, the degree of PL

18

quenching mainly depended on the charge extraction efficiency at TiO2/MAPbI3 interface.

19

The charge extraction efficiency of FTO/TiO2/Film-DMI-A (quenching efficiency 73.7%)

20

was significantly higher than that of FTO/TiO2/Film-DMSO-A (quenching efficiency 65.9%),

21

which implies that photo-generated charges were extracted more efficiently at the

22

TiO2/Film-DMI-A interface than at the TiO2/Film-DMSO-A interface. The efficient charge

23

extraction in Film-DMI-A was possibly due to the presence of fewer MAPbI3 defects in

24

Film-DMI-A and the better interaction between Film-DMI-A and TiO2. As a result, the

25

bimolecular recombination at the interface between the photoactive layer and TiO2 layer was

26

suppressed.

27

Solar cell devices were fabricated using Film-DMI-A and Film-DMSO-A, and the J-V

28

curves and external quantum efficiency (EQE) spectrum of devices are presented in the Figure

29

5a and b. Table 1 summarizes the extracted parameters of open circuit voltage (Voc), short

30

circuit current (JSC), fill factor (FF), and PCE. The Film-DMSO-A achieved a PCE of 15.8% 8


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1

with a Voc, Jsc, and FF of 1.00 V, 20.8 mA/cm2 and 76.0%, respectively, as measured under

2

a reverse voltage scan. A significantly improved Voc, Jsc, FF, and PCE of 1.09 V, 20.9

3

mA/cm2, 77.6%, and 17.6%, respectively, were obtained using Film-DMI-A. Figure 5b shows

4

the EQE spectrum and its integrated Jsc over a 100 mW/cm2 AM 1.5 G solar spectrum. The

5

integrated Jsc values of Film-DMSO-A and Film-DMI-A were 21.5 mA/cm2 and 20.6

6

mA/cm2, respectively, which were in good agreement with the Jsc obtained from the J-V

7

measurements.

8

In order to analyze and understand the enhanced Voc, the charge recombination behavior

9

of Film-DMSO-A and Film-DMI-A was investigated by analyzing the light intensity

10

dependence of Jsc and Voc. In addition, TPV measurements were conducted for both devices.

11

As shown in the Figure 6a and b, a linear dependence of Jsc on incident light intensity were

12

found for both devices indicating the charge collection efficiency was independent of the light

13

intensity. The α values of devices were obtained by fitting the obtained data with the power

14

law dependence of Jsc with the light intensity (J∝Iα). The α value should be close to 1 when

15

there is no recombination by the space charge effects.34-36 X The α value of Film-DMSO-A

16

and Film-DMI-A were obtained to be 0.93 and 0.97, respectively. Although the value of

17

Film-DMI-A is closer to 1 than that of Film-DMSO-A, values of both devices were close to 1,

18

which reflects that there was no significant charge recombination due to space charge.

19

The trap-assisted recombination can be predicted from the dependence of Voc on the light

20

intensity. A slope of KBT/q could be obtained from the plot of the Voc versus the Plight, where

21

KB is Boltzmann’s constant, T is temperature, and q is the elementary charge. When

22

trap-assisted or Shockley-Read-Hall recombination occur, a stronger dependence of Voc on

23

light intensity with a slope greater than KBT/q is observed. The obtained slopes of devices

24

were between 1 and 2, which indicate the trap-assisted recombination influence on the device

25

performance.37 The slope of the Film-DMSO-A was higher than that of the Film-DMI-A,

26

implying that trap-assisted recombination was more significant in Film-DMSO-A than in

27

Film-DMI-A. The recombination mechanism was also proven using TPV measurements. As

28

shown in Figure 6c, the charge collection efficiency of Film-DMSO-A was similar to that of

29

Film-DMI-A, which was consistent with the result of the Jsc dependence on the light 9



1

intensity. The charge carrier lifetime (τ) was extracted by fitting monoexponential decay

2

function (Figure 6d). As revealed in the Table S1 (Supporting Information), the τ of the

3

Film-DMI-A was longer than that of the Film-DMI-A at the different light intensity. This

4

finding implies that Film-DMI-A had fewer charge recombination sites than Film-DMSO-A,

5

which is attributed to the smoother film surface and reduced number of defect in the MAPbI3

6

crystals induced by the DMI.

7

We showed that the weak coordination capability of DMI to PbI2 allow the DMI and DMF

8

to be easily removed during the spin coating process. Therefore, it is expected that the

9

fabrication of perovskite solar cells with the DMI additive would be less sensitive to the

10

stoichiometric ratio between the DMI and PbI2. To prove our postulation, devices with the

11

different molar ratio between the PbI2 and DMSO/DMI were fabricated, and the results are

12

shown in the Figure 7a and b, and the averaged photovoltage parameters are summarized in

13

Table 2. The Film-DMI-A prepared with the molar ratio of PbI2: DMI from 1:0.5 to 1:1.5

14

shows similar PCE above 17%. As shown in Figure S4a, b in the Supporting Information, the

15

peak intensity at 2θ = 12.1 o was increased with increasing the concentration of DMI. Thus,

16

we concluded that the peaks at 12.1o belong to the intermediate phases existed in a trace

17

amount in prepared films, and those peaks were disappeared after annealing. Based on the

18

XRD results, the amount of intermediate phase in Flim-DMI is smaller than that in

19

Film-DMSO. Therefore, it is thought that the CH3NH3PbI3 is the primary phase existing in

20

Film-DMI. It is indicated that DMI is weakly coordinated with PbI2, and the DMI is able to

21

accelerate the crystallization of CH3NH3PbI3. Thus, the performance of Film-DMI is less

22

sensitive to the concentration of DMI. Whereas, the PCE of the Film-DMSO-A showed a

23

strong dependence on ratio between PbI2 and DMSO. This could be attributed that the

24

different amount of PbI2:DMSO lead to a different molecular structure of intermediate phase

25

in the as-prepared film of Film-DMSO.24

26

Furthermore, the reproducibility of the device performance under humid condition was

27

investigated. Devices were fabricated under a high relative humidity (~55%), and the results

28

are shown in Figure 7c and d. The inset photographs show the perovskite films after thermal

29

annealing. Film-DMSO-A always exhibited a white color when it was fabricated under

30

humidity conditions above ~30%, whereas the Film-DMI-A film was brown under the same 10


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1

condition. It is known that DMSO is highly hygroscopic because of the hydrophilic S=O

2

group, which quickly forms strong hydrogen bonds with H2O in the air.38 It is predicted that

3

the DMSO or DMSO containing intermediates would interact with H2O under high humidity

4

resulted in the unreproducible and poor device performance of Film-DMSO-A with a PCE of

5

11.05%. In contrast, the cyclic urea additive of DMI containing C=O bonding has a lower

6

possibility of forming a hydrogen bond with H2O. We suggest that the high humidity

7

tolerance of the Film-DMI during the fabrication process was due to the termination of the

8

perovskite crystal with PbI2 and I atoms at the early stage of the fabrication. For the

9

as-prepared film of Film-DMSO, Film-DMSO intermediate phase terminated with DMSO is

10

very sensitive to the fabrication condition.

11

Based on the XRD in the Figure 1, it is implied that CH3NH3PbI3 films processed with DMI

12

can be prepared at room temperature, thus, the performance of Film-DMSO and Film-DMI

13

were investigated, as shown in the Figure 8. The device of Film-DMI with Jsc, Voc, FF and

14

PCE of 18.2mA/cm2, 1.05V, 0.74% and 14.2%, respectively, which is significant higher than

15

the device of Film-DMSO with Jsc, Voc, FF and PCE of 10.2 mA/cm2, 0.92V, 54.9% and

16

5.2%, respectively. The poor performance of Film-DMSO is attributed to DMSO-induced

17

intermediate phases existing inside the film. Compared to the Film-DMI-A, the Film-DMI

18

showed low JSC, which was mainly due to the smaller size of perovskite grains in the

19

Film-DMI (Figure 2b). We believed that the device performance of Film-DMI will be further

20

enhanced after sophisticated optimization of the process. It is indicated that the utilization of

21

cycle urea have a potential to fabricate efficient PSCs at room temperature.

22

A photograph of the as-prepared films confirmed the above explanation (Figure S5 in the

23

Supporting Information). A prepared sample of Film-DMSO was transparent yellow due to

24

the existence of the intermediate of Pb3I8. In contrast, the as-prepared film of the Film-DMI

25

was dark brown, which was similar to the perovskite film after thermal annealing. These

26

results agreed well with the XRD results. The as-prepared Film-DMSO requires immediate

27

heating to prevent contact with H2O, which may be a hindrance to commercialization.

28

However, the as-prepared Film-DMI was very stable in the air due to the fast formation of the

29

perovskite crystals without intermediates step that may come with the H2O or other else

30

uncertain factor. 11



1

CONCLUSIONS

2

In conclusion, MAPbI3 films with smooth and regular crystal size were obtained using the

3

cyclic urea compound DMI as an additive in the precursor solution of MAPbI3. The DMI

4

additive weakly coordinated with PbI2, which nucleated MAPbI3 through a mechanism that

5

was different from that involving the strongly coordinating DMSO additive. A solar cell

6

fabricated with DMI exhibited an efficient and reproducible performance even under

7

conditions of high relative humidity. It is expected that the perovskite solar cells processed

8

with weakly coordinating additives have a great potential for large-scale fabrication at room

9

temperature.

10

11

ACKNOWLEDGEMENTS

12

This research was supported by NRF under the program number NRF-2015M1A2A2057506

13

and 2016M1A2A2940914. This research was also supported by the New & Renewable

14

Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation

15

and Planning (KETEP), and by a financial grant from the Ministry of Trade, Industry &

16

Energy, Republic of Korea (No. 20163030013900). A.-N.C. and N.-G.P. acknowledge

17

financial support from the National Research Foundation of Korea (NRF) grants funded by

18

the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contracts No.

19

NRF-2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy

20

System) and NRF-2015M1A2A2053004 (Climate Change Management Program).

21

12


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1

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Planar Heterojunction Solar Cells. Nanoscale 2015, 7, 10595-10599.

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TiO2/Organohalide Perovskites Interface: The Role of Interfacial Chlorine. J. Phys. Chem. Lett.

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Molecular Structures of Intermediates for Optimizing the Fabrication of High-Quality

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Perovskite Films. J. Am. Chem. Soc. 2016, 138, 9919-9926.

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of Planar Perovskite Solar Cells Produced from PbI2(DMSO) and PbI2(NMP) Complexes by

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Intramolecular Exchange. Adv. Mater. Interfaces 2016, 3, No. 1500768.

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26. Chen, J.; Xiong, Y.; Rong, Y.; Mei, A.; Sheng, Y.; Jiang, P.; Hu, Y.; Li, X.; Han,H., Solvent

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Effect on the Hole-Conductor-Free Fully Printable Perovskite Solar Cells. Nano Energy 2016,

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27. Wang, Q.; Lyu, M.; Zhang, M.; Yun, J.-H.; Chen, H.; Wang, L., Transition from the

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Tetragonal to Cubic Phase of Organohalide Perovskite: The Role of Chlorine in Crystal

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Formation of CH3NH3PbI3 on TiO2 Substrates. J. Phys. Chem. Lett. 2015, 6, 4379-4384.

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Interfaces by Sequential Deposition. Nanoscale 2014, 6, 8171-8176. 15



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K. Y., Additive Enhanced Crystallization of Solution-Processed Perovskite for Highly Efficient

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Planar-Heterojunction Solar Cells. Adv. Mater. 2014, 26, 3748-3754.

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31. Xie, L.; Hwang, H.; Kim, M.; Kim, K., Ternary Solvent for CH3NH3PbI3 Perovskite Films

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Polythiophene Based Bulk Heterojunction Photodetectors. Appl. Phys. Lett. 2002, 81,

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38. LeBel, R. G.; Goring, D. A. I., Density, Viscosity, Refractive Index, and Hygroscopicity of

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Mixtures of Water and Dimethyl Sulfoxide. J. Chem. Eng. Data 1962, 7, 100-101.

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16


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1 2

Figure 1. XRD patterns of the (a) as-prepared films prepared with DMI and DMSO

3

respectively, and (b) thermally annealed perovskite films prepared with DMI and DMSO.

4

17



1

2 3

Figure 2. Top-view SEM images of as-prepared and annealed perovskite films deposited on

4

top of FTO/mp-TiO2 substrates (a) Film-DMSO, (b) Film-DMI, (c) Film-DMSO-A, and (d)

5

Film-DMI-A.

6 7

18


Page 18 of 24

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1 2

Figure 3. The 3D AFM images of (a) Film-DMSO-A, (b) Film-DMI-A prepared on

3

FTO/mp-TiO2 substrates, along with the corresponding height profiles of each film.

19



1 2

Figure 4. (a) PL spectra of perovskite films prepared with glass/Film-DMSO-A (black line),

3

glass/Film-DMI-A (red line), FTO/mp-TiO2/Film-DMSO-A (black dot), and FTO/mp-TiO2/

4

Film-DMI-A (red dot), (b) the UV-vis absorption spectra of the Film-DMSO-A (black) and

5

Film-DMI-A (red) films prepared on FTO/mp-TiO2 substrates.

20


Page 20 of 24

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1

2 3

Figure 5. (a) J-V curves and (b) EQE spectrum of the Film-DMSO-A and Film-DMI-A

4

devices.

5 6

Table 1. Photovoltaic parameters of the PSCs processed with DMSO and DMI. The parameters are av

7

erage values of 20 devices. Jsc (mA/cm2)

Device

Film-DMSO-A Film-DMI-A

Avg. Best Avg. Best

20.4 ± 0.5 20.8 20.3 ± 0.2 20.9

Calc. Jsc by EQE (mA/cm2)

Voc (V)

FF (%)

PCE (%)

21.5

1.01 ± 0.02 72.4 ± 5.7 14.5 ± 2.3 1.00 76 15.8

20.6

1.06 ±0.01 73.6 ± 4.3 16.1 ± 0.9 1.09 77.6 17.6

8 9 10 11 12

21



1 2

Figure 6. Light intensity dependent (a) Jsc and (b) Voc of the Film-DMSO-A and

3

Film-DMI-A devices. Plots of (c) charge density versus light intensity and (d) charge lifetime

4

versus charge density of Film-DMSO-A (black square) and Film-DMI-A (red circle).

5

22


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1 2

Figure 7. J-V curves of perovskite devices processed using different molar ratios between

3

PbI2 and (a) DMSO, (b) DMI respectively; J-V curves of perovskite devices fabricated at

4

~55% humidity with (c) DMSO and (d) DMI.

5 6

Table 2. Summary of the photovoltaic parameters of PSCs fabricated with different molar ratios of

7

DMI and DMSO (relative to PbI2). The parameters are average values of 20 devices.

8

Device

Film-DMSO-A

Film-DMI-A

Molar ratio

Jsc (mA/cm2)

Voc (V)

1:0.5

14.2 ± 0.38

0.99 ± 0.01

69.3 ± 5.4 9.0 ± 3.4

1:1

18.8 ± 0.73

1.03 ± 0.02

71.4 ± 4.9 13.5 ± 2.8

1:1.5

17.6 ± 0.53

0.97 ± 0.02

74.3 ± 2.4 11.2 ± 2.9

1:0.5

20.3 ± 0.21

1.07 ± 0.01

73.4 ± 2.5 16.7 ± 0.6

1:1

20.8 ± 0.33

1.06 ± 0.01

76.5 ± 3.1 16.4 ± 0.4

1:1.5

20.1 ± 0.29

1.08 ± 0.02

76.9 ± 2.2 16.8 ± 1.1

(PbI2: DMSO or DMI)

9 10 23


FF (%)

PCE (%)


1 25 Current density (mA/cm2)

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

Page 24 of 24

20 15 10 Film-DMSO Film-DMI

5 0 0.0

2 3

0.2

0.4 0.6 0.8 Voltage (V)

1.0

Figure 8. The device performance of Film-DMSO and Film-DMI under room temperature.

4 5

1.2

TOC:

6

24


An Efficient and Reproducible CH3NH3PbI3 Perovskite Layer

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