Supporting information for

“High-performance CH3NH3PbI3 perovskite solar cells fabricated

under ambient conditions with high relative humidity”

Binglong Lei, Vincent Obiozo Eze, and Tatsuo Mori*

Department of Electrical and Electronics Engineering, Aichi Institute of Technology,

Yakusa-cho, Toyota, Aichi, 470-0392, Japan.

E-mail: t2mori@aitech.ac.jp

1 Film fluctuation:

Fig. S1 One-dimensional linear scanning of the GFA and w/o-GFA perovskite

surfaces with a TiO2 background dense layer of 50 nm in thickness (a) and the fluctuation

distributions thereof (b)

By virtue of the best GFA perovskite, we approximately calculated absorption

coefficient for the CH3NH3PbI3 perovskite by the formula of T=I/I0=Exp(-αd), where T, I, I0,

α and d represent transmittance, transmitted light intensity, incident light intensity,

absorption coefficient and layer thickness (300 nm), respectively. It is indicated in Fig. S2

that CH3NH3PbI3 exhibits high absorption coefficient of 5103-5104 cm-1 in the range of

500-800 nm and a specific value of 1.48104 cm-1 at 550 nm, as is much consistent with

previously reported data collected by the integrating sphere technique.1) This has laterally

proved that the amenable quality of the GFA perovskite.

Fig. S2 Absorption spectra of two kinds of perovskite layers constructing GFA and

w/o-GFA solar cells and an approximate calculation curve of absorption coefficient based

on the GFA perovskite of 300 nm in thickness.

2 Perovskite thickness of GFA and w/o-GFA films

Table S1 Correlation of perovskite thickness via spin velocity

Spin velocity (rpm/s) 6000 4000 3000 2000

GFA films (nm) 272±18 a 326±23 387±35 461±69

w/o-GFA films (nm) 285±58 335±67 406±73 494±125

(a: the minus-plus values represent film peak and valley fluctuation in a typical sample

instead of thickness variation from multiple samples. They can roughly reflect the film

quality as the RMS values)

Fig. S3 Correlation and data fitting of perovskite thickness via spin velocity

(Approximately, R2=0.9940, Chi2/DOF=45.69 for thickness fitting to GFA and w/o-GFA

films)

On account of the facts that air flow can significantly enhance film thinning2) and that

for perovskite solar cells, sufficient light-absorbing film thickness must be guaranteed (i.e.

for an absorption coefficient of 1.48104 cm-1 at 550 nm, 300-400 nm as half

incident-reflection absorption thickness is best required), we did not impose the GFA

immediately before or after the spinning was commenced but 2 s later after it. This

technique enabled us to produce the film thickness in the range of 250-500 nm, from which

we chose an ideal value of 4000 rpm for final perovskite fabrication when the surface

roughness was also taken into account.

According to the following empirical Formula (S1),3) we fitted the data sets of both

GFA and w/o-GFA perovskite film thickness versus spin velocity. It is demonstrated in

Table S1 and Fig. S3 that a thickness reduction of only 10-25 nm was successfully

guaranteed. This also indicates that the film thickness can also be expressed by the Formula

(1) as shown in the main manuscript, where m is approximately equal to 10-25 nm denoting

the function of thickness reduction produced by GFA. Therefore it is safe to conclude that

the gas flowing does not significantly enhance the film thinning by radial convection or

mass transfer but expedited solvent evaporation to generate more homogeneous films.

kh (S1)

where h is the film thickness, ω is the angular velocity, k denotes an empirical constant

relating to solution concentration, while we applied an α as -0.5.

3.1 Materials and Reagents

Methylammonium iodide (CH3NH3I) was synthesized according to the reported

procedures.4) Fluorine-doped tin oxide (FTO) glass substrates (sheet resistance 12 Ω /sq)

were purchased from Asahi Glass Co. Ltd. The Semicon detergent, acetone, isopropanol,

titanium diisopropoxide bis(acetylacetonate), absolute ethanol, acetonitrile, methyl

ammonium, hydroiodic acid, Spiro-OMeTAD (2,2’, 7,7’-tetrakis (N, N-di-p-methoxy-

phenyalmine) -9,9’ -spirobifuorene), dimethyl formamide (DMF) and lead iodide (99.99%,

Aldrich) were used without further treatment.

3.2 Semi-product CH3NH3I Synthesis

CH3NH3I was synthesized by reacting 27.8 ml methylamine (about 0.273 mol, 40% in

methanol) and 30.0 ml HI (about 51.03 g, 0.228 mol, 57 wt% in water) in a 500 ml round

bottomed flask at 0 ℃ for 2 h while stirring. The latter solution was added into the former

one within 5 min (the addition was required to be finished in 10 min). Then the solution was

vaporized to receive the precipitate at 60 ℃ for about 1 hour and subsequently

recrystallized for two times in ethanol and washed by diethyl ether for another two times.

The semi product was dried at 60 ℃ in a vacuum oven for 2-4 hours. The synthesized

CH3NH3I weighed up to 34.3 g, with a recovery rate of about 94.62%, and it was kept in

2-propanol as stock solution (10 mg/ml) or stored in vacuum.

3.3 Perovskite solar cell fabrication

GFA and w/o-GFA cells are hereafter referred to the cells fabricated with and without

Gas Flowing Assisting, respectively.

Solar cells with a geometry of FTO glass /TiO2 dense-layer/ CH3NH3PbI3/ HTM/Au

have been applied by us. Fig. S4 demonstrates the schematic fabrication procedures for the

GFA and w/o-GFA cells. Complete conductions in detail will be listed as following seven

sections and all steps were conducted in air (room temperature, 23-25 ℃; RH, 42-48%)

without the assistance of glove box and the step for perovskite layer should be quickly

finished in 30 min.

Fig. S4 Schematic fabrication procedures for perovskite solar cells. DPS denotes

deposition; HTM, Spiro-OMeTAD.

(1) FTO pre-treating and cleaning

FTO substrates with a size of 23 cm2 were obtained by cutting from the original

3035 cm2 and then etched and treated by ultrasonication in Semicon cleaning solution,

deionized water, ethanol and acetone for one or two times. After drying in air, they were

treated in the O3/ultraviolet condition for 1.5 min and 15 min respectively.

(2) TiO2 dense layer

TiO2 dense blocking layers were completed by commonly used spin coating after the

anode part of about 0.7 cm was masked by a 3M scotch tape. A solution of 0.15 ml HCl

(36.5%)/2.25 ml Titanium diisopropoxide bis(acetylacetonate)/15 ml ethanol was prepared

and then spin-coated on the substrate samples at 2000 rpm/10s for the dense-layer with a

thickness of about 50 nm. All samples were moved onto a hotplate and kept at 125 ℃ for

20 min and then sintered at 500℃ for 30 min in a closed furnace.

(3) Treatment with TiCl4

Masking the anode part with tape was conducted beforehand, and then the substrates

obtained above with TiO2 dense layer were immersedly treated in a 0.04 M TiCl4 solution (3

ml/72 ml in water) at 70 ℃ for 20 min. After that the substrates were sufficiently rinsed

with purified water and then heated at 70 ℃ until they were dried and then they were again

heated at 500℃ for 30 min.

(4) PbI2 precursor layer by spin-coating

In the two consequential steps for light absorbing layer, the precursor PbI2 has a

decisive impact upon final perovskite layers and therefore delicate control must be

guaranteed. PbI2 (99.99%) was first dissolved in DMF at a concentration of 500 mg/ml at 70

℃ while stirring. While kept at this temperature, the PbI2 solution was spin-coated on the

substrates with speeds of 4000 rpm/20s for thickness of about 250 nm and no loading time

was applied. A vertical gas flowing assisting (GFA, with air employed) was additionally

applied at 2 s just after the spinning had been commenced. After the spin coating, the

samples were set on the hotplate for crystallization at 100 ℃ for 15 min. These samples

were labeled as GFA cells (semi), while for w-GAF (semi) cells as a control group, no gas

flowing had been implied but with the other conditions kept identical.

As for the flowing gas, we employed the high RH room air (in the clean room), which

possessed the same RH (42-48%) with the ambient conditions but much less dusts in its

content if compared with the outdoor unclear air. The outlet nozzle of the air flower was

about 2.52.5 cm2 and conveyed air with a rate of approximately 3.6 L/s (about 5.76 m/s).

The distance from the substrate to the outlet was controlled at 8-10 cm. According to the our

final results, we speculate that water or moisture from the high RH air had positively

functioned for mass transfer and crystallization of perovskite, since the existence of solvent

during crystallization is somewhat beneficial for perovskite quality.5)

It is worth a mention that the GFA PbI2 films, including our final GFA perovskite

layers, were all conducted at an optimized air flowing rate of approximately 3.6 L/s (about

5.76 m/s). When the rate we applied was lower than about 2.0 L/s, it was found that the

flowing impact upon PbI2 films was not very pronounced due to the high RH values (final

cells possessed adjacent PCEs to w/o-GFA cells but with larger variations), while when the

rate was elevated up to 3.2 L/s, the positive effect thereafter can not significantly increase

(Fig. S5). Therefore a rate of 3.6 L/s as preference were employed by us and also directly

used for the following transformation step from PbI2 to final perovskite transformation to

expedite evaporation of isopropanol as solvent, which is mainly due to the consideration that

the quality of PbI2 films dominates the final perovskite layers in the two-step sequential

procedures.

Fig. S5 Performance of solar cells fabricated by the GFA route with varying air flowing

rates (reverse scanned PCE are statistically shown from 10 solar cells, while 15 or 20 cells

are calculated for 0 L/s (w/o-GFA) and the best GFA cells at 3.6 L/s)

(5) Transformation of PbI2 into CH3NH3PbI3

For the semi GAF samples fabricated above, they were endowed with a structure of

FTO glass /TiO2 dense-layer/PbI2, and the current step will transform the PbI2 layer into

perovskite by the following way: spin coating of a thermal CH3NH3I solution (70 ℃) with

GFA assisting as it is mentioned above. The CH3NH3I solution of 200 µl, for every single

sample, was loaded on the substrates with a duration of 20 s before the spin coating was

conducted at 4000 rpm/30 s (air flowing started 2 s later after spinning) and then desiccation

and crystallization were done at 100 ℃ for 15 min. For the w-GAF samples, identical

conductions had been finished but without gas flowing assisting.

It is worth a mention that attempts had been made to extend the soaking duration to 120

s, however the remnant PbI2 still can not be completely eliminated in all perovskite samples.

Further observation reveals that some PbI2 substrates with a slight hazy surface fabricated

by a spin-coating rate of less than 3000 rpm (without GFA assisting) can be more quickly

converted into perovskite than the GFA-based PbI2 layers. This auxiliarily demonstrates that

GFA can impair preferential crystallization and enhance the compactness of PbI2 layer,

which to some extent has also hindered the transformation in terms of dynamics.

(6) Formation of HTM layer

The samples achieved above were deposited by the Spiro-OMeTAD solution (the

composition was list in Table S2) by spin coating the solution at 2000 rpm for 60s for

thickness of about 200 nm. After that, they were set in air for 2-4 hours for HTM oxidation

before the back gold deposition was conducted in the next step.

Table S2 Composition of Spiro-OMeTAD solution for CH3NH3PbI3 solar cells

Ingredients Contents

Spiro-MeOTAD (98%) 144.6 mg

4-tert-butylpridine (TBP) 57.6 ul

520 mg/ml LiTFSI in acetonitrile 35 ul

Chlorobenzene 2 ml

(7) Gold back contact deposition in vacuum

The final solar cell devices were completed by thermal evaporation of gold with a

thickness of 100 nm at 3.4 10-4 Pa. A metal mask has been used to define the active area as

0.15 cm2.

3 Testing and characterization

The morphology of perovskite layer was observed by a field emission scanning

electron microscope (FE-SEM, JSM-6335FM, JEOL, accelerating voltage of 10 KV). The

voltage-current J–V measurement was recorded by applying external potential biases to the

cells and recording the output photocurrent with a digital source meter (Model B2901A,

Agilent Corp.). A 150 W xenon lamp (Model Otento-SUN3, Xe-S150, Bunkoukeiki) was

applied as light source and the output irradiation intensity was adjusted to the AM 1.5G

condition (100 mW/cm2). The voltage step length and delay time were set at 2-40 mV and

0.5-1000 ms, respectively. Incident photon conversion efficiency (IPCE) measurement was

conducted in the wavelength range of 350-830 nm with a 300 W xenon light adjusted by a

monochromator (PVL 3300, Asahi Spectra). X-Ray diffraction (XRD) patterns were

recorded by in the 2-theta range of 5°-65° on an X-ray diffractometer (RINT2500V/PC,

Rigaku) by Cu Kα radiation (40 kV, 100 mA). The absorption spectra of our solar devices

were measured with (Model UV-2450, Shimadzu) in the wavelength of 400 nm to 900 nm.

Surface morphologies and roughness demonstration were scanned using an atomic force

microscopy (AFM, Keyence VN-HIV8). For thickness testing and surface one-dimensional

scanning and comparison, we applied a thickness tester (Surfcorder ET200, Kosaka

Labortory Ltd.) with a step length of 0.03 um.

For the J–V scanning, we had tried series of step delays from 0.1 ms to 1000 ms and

found that it was most favorable at 50 ms for forward-reverse balance to drastically suppress

the hysteresis effect. A step delay of 50 ms was therefore applied for testing all our cells.

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