Nano Res

1

Facile preparation of organometallic perovskite films

and high-efficiency solar cells using solid-state

chemistry

Lei Chen1,2,§, Feng Tang2, §, Yixin Wang2,3, Shan Gao2, Jinhua Cai2() and Liwei Chen2()

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0662-1

http://www.thenanoresearch.com on December 1 2014

© Tsinghua University Press 2014

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Nano Research

DOI 10.1007/s12274-014-0662-1

1

TABLE OF CONTENTS (TOC)

Facile preparation of organometallic perovskite

films and high-efficiency solar cells using

solid-state chemistry

Lei Chen1,2,§, Feng Tang2, §, Yixin Wang2,3, Shan

Gao2, Jinhua Cai2* and Liwei Chen2*

1Shanghai University, China

2Suzhou Institute of Nano-Tech and Nano-Bionics,

Chinese Academy of Sciences, China

3University of Science and Technology of China,

China

§These authors contributed equally to this work.

Page Numbers. The font is

ArialMT 16 (automatically

inserted by the publisher)

A solid-state reaction is developed for the preparation of organometallic

perovskite thin films and perovskite solar cells. The method involves facile

annealing of precursor films in contact and yields solar cells with the best

efficiency reaching 10%.

http://oil.sinano.ac.cn/

2

Facile preparation of organometallic perovskite films and

high-efficiency solar cells using solid-state chemistry

Lei Chen1,2,§, Feng Tang2, §, Yixin Wang2,3, Shan Gao2, Jinhua Cai2() and Liwei Chen2()

1Department of Chemistry, Shanghai University, Shanghai 200444, China

2i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China

3Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, Jiangsu 215123, China

§These authors contributed equally to this work.

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)

© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

ABSTRACT The power conversion efficiency of organometallic perovskite-based solar cells has skyrocketed in recent

years. Intensive efforts have been made to prepare high-quality perovskite films that are tailored to various

device configurations. Planar heterojunction devices have achieved record efficiencies; however, the

preparation of perovskite films for planar junction devices requires the use of expensive vacuum facilities

and/or the fine control of experimental conditions. Here, we demonstrate a facile preparation for perovskite

films using solid-state chemistry. Solid-state precursor thin films of CH3NH3I and PbI2 are brought into

contact with each other and allowed to react via thermally accelerated diffusion. The resulting perovskite

film displays good optical absorption and a smooth morphology. Solar cells based on these films show an

average efficiency of 8.7% and a maximum efficiency of 10%. The solid-state synthesis of organometallic

perovskite can also be applied to flexible plastic substrates. Using this method on a PET/ITO substrate

produces devices with an efficiency of 3.2%. Unlike existing synthetic methods for organometallic perovskite

films, the solid-state reaction method does not require the use of orthogonal solvents or careful adjustment of

reaction conditions, and thus shows good potential for mass production in the future.

KEYWORDS

solid-state chemistry, perovskite solar cells, planar heterojunction, flexible substrates

Nano Res DOI (automatically inserted by the publisher)

Review Article/Research Article Research Article

————————————

Address correspondence to Jinhua Cai, jhcai2013@sinano.ac.cn; Liwei Chen, lwchen2008@sinano.ac.cn

3

There has been tremendous development in

organometallic perovskite solar cells in recent years

[1-5]. The power conversion efficiency of

perovskite-based planar heterojunction solar cells

has reached 19.3% [6]. Organometallic perovskites,

such as CH3NH3PbI3, are excellent photovoltaic

materials because of their strong absorption

coefficients, long carrier diffusion lengths [7, 8], and

appropriate energy gap (~1.5 eV), which is very

close to the ideal gap of 1.3 eV from detailed

balance theory [9]. The preparation of a

high-quality perovskite film is a critical step in

fabricating planar junction solar cells because only

high-quality films can enable the excellent intrinsic

material characteristics of perovskites to be fully

exploited in solar cells [10-15]. Researchers in this

field have developed many methods to achieve

high-quality perovskite films in recent years. These

methods fall into two categories, vacuum vapor

deposition and solution processes. High-quality

perovskite films and high efficiency (15%) solar

cells have been obtained using vacuum vapor

deposition; however, the deposition process

requires the use of expensive vacuum facilities [16].

Methods based on solution processes, such as the

two-step method and the vapor-assisted solution

process, offer the advantage of low cost over

expensive vacuum vapor-deposition, but careful

control of experimental conditions and parameters

is necessary to ensure that the experimental results

were reproduced as consistently as possible [17-19].

In this paper, we develop a solid-state reaction

method for the preparation of a high-quality

perovskite film. The structure, optical absorption,

and surface morphology of the resulting films are

characterized using X-ray diffractometry (XRD),

UV-vis absorption spectroscopy, photoluminescence

(PL) spectra, atomic force microscopy (AFM) and

scanning electron microscopy (SEM). The prepared

perovskite films are used to fabricate planar

heterojunction solar cells with a stacking

ITO/PEDOT:PSS/perovskite/PC61BM/Al structure.

The current density-voltage (J-V) curves and the

external quantum efficiency (EQE) spectra are used

to evaluate the photovoltaic performance of the

prepared cells. The experimental conditions,

including the film thickness (which is adjusted by

the thickness of precursor film), the annealing

temperature and the time, are optimized to produce

the highest photovoltaic performance of the

corresponding devices. The highest conversion

efficiency attained by our cells is 10%.

Figure 1a is a schematic of the primary

procedures in our method. A lead iodide (PbI2) film

and a CH3NH3I film are deposited on two

independent substrates; the CH3NH3I film is then

placed face-to-face on top of the PbI2 film. The two

contacting films are annealed over a certain period,

after which a CH3NH3PbI3 film is obtained by

removing the top substrate. CH3NH3I vaporizes

easily at 1 atm (140 °C is a sufficiently high

temperature to drive CH3NH3I molecules into the

PbI2 film to form perovskite [15]); therefore, the

perovskite film forms by a rapid solid-state

chemical reaction. The photographs in Figure 1b

show a clear color change after the annealing. The

stoichiometric ratio of the reaction is difficult to

determine because the mass of both the reactant

and product thin films are too small to be accurately

weighted. Nevertheless, we believe the PbI2 film is

fully converted to perovskite as long as there is

enough CH3NH3I on the top substrate based on

AFM height measurements (see Figure S1 and S2

for the detailed information).

Spectroscopic techniques including XRD, PL,

UV-vis absorption and SEM imaging further

confirm the completion of the solid-state reaction.

Figure 2 shows the evolution of XRD pattern with

the annealing time for a PbI2 film on a

ITO/PEDOT:PSS substrate after the film is covered

by CH3NH3I film at an optimized annealing

temperature of 135 °C. Five samples are prepared

and annealed for a series of timeration of a

high-quality perovskite film. The structure, optical

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absorption, and surface morphology of the resulting

films are characterized using X-ray diffractometry

(XRD), UV-vis absorption spectroscopy,

photoluminescence (PL) spectra, atomic force

microscopy (AFM) and scanning electron

microscopy (SEM). The prepared perovskite films

are used to fabricate planar heterojunction solar

cells with a stacking

ITO/PEDOT:PSS/perovskite/PC61BM/Al structure.

The current density-voltage (J-V) curves and the

external quantum efficiency (EQE) spectra are used

to evaluate the photovoltaic performance of the

prepared cells. The experimental conditions,

including the film thickness (which is adjusted by

the thickness of precursor film), the annealing

temperature and the time, are optimized to produce

the highest photovoltaic performance of the

corresponding devices. The highest conversion

efficiency attained by our cells is 10%.

Figure 1a is a schematic of the primary

procedures in our method. A lead iodide (PbI2) film

and a CH3NH3I film are deposited on two

independent substrates; the CH3NH3I film is then

placed face-to-face on top of the PbI2 film. The two

contacting films are annealed over a certain period,

after which a CH3NH3PbI3 film is obtained by

removing the top substrate. CH3NH3I vaporizes

easily at 1 atm (140 °C is a sufficiently high

temperature to drive CH3NH3I molecules into the

PbI2 film to form perovskite [15]); therefore, the

perovskite film forms by a rapid solid-state

chemical reaction. The photographs in Figure 1b

show a clear color change after the annealing. The

stoichiometric ratio of the reaction is difficult to

determine because the mass of both the reactant

and product thin films are too small to be accurately

weighted. Nevertheless, we believe the PbI2 film is

fully converted to perovskite as long as there is

enough CH3NH3I on the top substrate based on

AFM height measurements (see Figure S1 and S2

for the detailed information).

Spectroscopic techniques including XRD, PL,

UV-vis absorption and SEM imaging further

confirm the completion of the solid-state reaction.

Figure 2 shows the evolution of XRD pattern with

the annealing time for a PbI2 film on a

ITO/PEDOT:PSS substrate after the film is covered

by CH3NH3I film at an optimized annealing tems, 0,

5, 15, 20, and 30 min. The film clearly initially

consists of the PbI2

Figure 1 (a) Schematic illustration of the fabrication process of perovskite film using solid-state reaction; and (b) corresponding

photographs of the real films.

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Figure 2 Evolution of XRD patterns with time for a PbI2 film

on a ITO/PEDOT:PSS substrate, where a CH3NH3I film is

placed on the PbI2 film.

phase (0 min). After the CH3NH3I film has covered

the PbI2 film for only 5 min, peaks appear at

2θ=14.16°, 28.50°, 31.94° and 43.19°, corresponding

to the (110), (220), (310) and (330) reflections of the

CH3NH3PbI3 phase (the diffraction peaks at 21.4°,

30.4°, 35.3° and 50.6° indicate the crystallinity of

ITO) [1, 15, 20], thereby indicating the formation of

the tetragonal perovskite structure. The gradual

disappearance of the signature peak of the PbI2 at

12.71° indicates the progress of the rapid reaction of

CH3NH3I with PbI2. After 20 min of annealing, the

PbI2 peaks disappear, after which the XRD pattern

remains almost unchanged, indicating the complete

conversion of PbI2. Figure S3 shows theXRD pattern

of the top thin film (CH3NH3I on glass) before (0

min) and after (30 min) the reaction and the bottom

film (perovskite on ITO glass) after 30 min of

reaction. The results indicate that excessive

CH3NH3I is left on the top substrate. Considering

that the top film is removed right after the reaction

while the temperature of the bottom thin film is still

at about 135°C, any remaining CH3NH3I at the

bottom film shall evaporate and may not

contaminate the perovskite film.

Figure 3a shows the UV-vis absorption spectra

for a series of annealing times. At the initial stage (0

min), the spectrum shows a pronounced absorption

edge at 500 nm, agreeing well with the fact that the

energy gap of PbI2 is 2.5 eV [21, 22]. The absorbance

of the film increases with the reaction time and the

characteristic absorption edge of CH3NH3PbI3 at 780

nm appears after 15 min [23]. The absorbance

saturates after 20 min. Figure 3b shows the PL

spectra of bottom film at different reaction time

measured at low temperature (3.8 K) [24]. The

Figure 3 (a) UV-vis absorption spectra of perovskite film

prepared from PbI2 films on ITO/PEDOT:PSS substrate, as

covered with a CH3NH3I film and annealed at 135 °C for

different time periods; and (b) PL spectra of these films

obtained at low temperature (3.8 K) with 405 nm excitation.

emission peak from PbI2 at 514 nm disappears after

20 min. This result indicates that the reaction of

CH3NH3I and PbI2 is nearly complete after 20 min.

The evolution of the absorption and PL spectra is

consistent with the XRD patterns.

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The SEM image clearly shows the change in the

grain structure of the corresponding deposited film.

Figure 4 shows that voids of different sizes are

scattered among polygonal PbI2 grains at the

beginning of the procedure. After the PbI2 film is

covered by the CH3NH3I film and annealed at

135 °C for 5 min, the voids in the PbI2 film become

smaller and fewer. After 20 min, the voids almost

disappear, and a uniform film forms. The grain size

is almost unchanged for an annealing time of 5-20

min, but the grain size of the film that has been

annealed for 30 min is significantly larger than

obtained with a 20-min annealing time. Although

no significant changes can be observed in the XRD

(Figure2) or optical absorption spectrum (Figure 3)

by increasing the annealing time from 20 min to 30

min, the SEM result clearly shows that the grains

become larger, which should facilitate carrier

transport in the film.

The evolution of the XRD patterns, the

UV-vis absorption spectra, and the SEM images

with time shows that 30 min is an appropriate

annealing time at a temperature of 135 °C. We use

the films to prepare solar cells with a stacking

ITO/PEDOT:PSS/perovskite/PC61BM/Al structure,

which is a classical structure in the field of organic

solar cells [25, 26]. After the perovskite film is

prepared using the solid state reaction, a layer of

PC61BM is deposited by spin-coating a

chlorobenzene solution of PC61BM onto the film,

followed by vacuum evaporation of an Al layer

with a thickness of approximately 100 nm. The

active area of the cell is 0.12 cm2 and is defined as

the overlap between the ITO electrode and the Al

electrode. For one batch of 30 samples, the mean

efficiency of the cells is 8.7%, and the maximum

efficiency of the cells is 10.0%. In addition to

optimizing the annealing temperature (135 °C) and

the annealing time (30 min), the perovskite layer

thickness is optimized at approximately 300 nm, as

estimated from the SEM image (see Figure 4f). This

optimized thickness is in good agreement with the

reported optimized value (300 nm) [15, 27].

Figure 5 shows the J-V curves and the EQE

spectrum for our best performing device, which has

a conversion efficiency 10.0%, a short-circuit current

(Jsc) of 17.9 mA/cm2, an open-circuit voltage (Voc) of

0.87 V, and a fill factor (FF) of 64.3%. The EQE

spectrum shows that photocurrent generation starts

at 780 nm, which is in accordance with the bandgap

of CH3NH3PbI3 [23], and reaches peak values of

over 80% in the visible spectrum. Integrating the

Figure 4 Top view SEM images of a PbI2 film on a ITO/PEDOT:PSS substrate after the film has been covered by a CH3NH3I film for

7

(a) 0 min, (b) 5 min, (c) 15 min, (d) 20 min and (e) 30 min at 135 °C; (f) cross-sectional SEM image of a PbI2 film on a

ITO/PEDOT:PSS substrate after the PbI2 film has been covered with a CH3NH3I film for 20 min at 135 °C.

Figure 5 (a) The J-V curves of the perovskite solar cells prepared using the solid-state chemical reaction method under AM 1.5G

irradiation and in dark. (b) EQE spectrum (black) and the integrated photocurrent (red) under AM 1.5G irradiation of the device.

overlap of the EQE spectrum with the AM 1.5G

solar photon flux yields a current density of 18.0

mA/cm2, which is similar to that obtained from J-V

measurements. Although the Voc is lower than that

obtained for devices prepared using other methods,

the efficiency is not lower than the value that has

been reported in the literature for the same

structure [12, 28-30].

The devices that are prepared by the solid-state

reaction method also exhibit good consistency.

Figure 6 shows the statistics of the device

performance for 30 cells that are prepared using our

method. The conversion efficiency of 80% of the

samples ranges from 8% to 10%, the Voc of 80% of

the samples is above 0.83 V, the Jsc of 80% of the

samples is above 16 mA/cm2, and the fill factors of

80% of the samples are above 0.6. This high

consistency should translate into high yields in

mass production.

Our method can be applied to both rigid and

flexible substrates. Figure 7 shows a device on a

flexible PET substrate. The conversion efficiency in

a cell with 0.09 cm2 area is 3.2%, which is

considerably lower than the conversion efficiency of

a cell prepared on an ITO glass substrate. This low

efficiency mainly manifests in lower Jsc and fill

factor values. The low fill factor results from a low

shunt resistance and a high serial resistance.

Possible causes for low Jsc values are the low

annealing temperature (125 °C) and the long

Figure 6 Statistics of device performance for 30 cells prepared

using our solid-state reaction method: (a) Voc, (b) Jsc, (c) FF,

and (d) PCE.

8

Figure 7 J-V characteristics for the best-performing flexible

device with the ITO/PEDOT:PSS/CH3NH3PbI3/PC61BM/Al

stacking configuration on PET substrate. Inset: photograph of

the device.

annealing time (45 min) that are used to prepare the

perovskite films on the PET substrate. Although the

current efficiency is as low as 3.2%, the method

shows potential for application to mass production.

The solid-state chemistry method presented

here offers significant advantages over existing

methods, such as the vapor-assisted solution

process and the inter-layer diffusion method [15, 27].

In the vapor-assisted solution process, CH3NH3I

powder is cast around the PbI2 film, such that

CH3NH3I is transferred onto the surface of the PbI2

film in the vapor phase, and a perovskite film forms

via thermal diffusion [15]. The CH3NH3I powder

does not come into direct contact with PbI2; thus, a

long reaction time is required in this process. The

inter-layer diffusion method is similar to the

traditional solid-state diffusion method [27], which

is conventionally used to deposit alloy films, e.g.,

CuInGaSe [31], in the field of solar energy. Two

layers of precursor films are sequentially deposited

onto a substrate, and the perovskite film forms via

thermal diffusion. Limitations of this method are

that orthogonal solvents are required for the

sequential deposition of the bilayer precursor

film，and accurate feed ratio of the two precursors

are required to obtain the perovskite film with the

intended stoichiometry.

In the solid-state chemistry method, the

CH3NH3I film and PbI2 film are deposited on two

different substrates, and thus orthogonal solvents

are not required. The excessive CH3NH3I is

removed before the perovskite film is used in

successive preparation steps for solar cells,

therefore, accurate ratio between CH3NH3I and PbI2

is not necessary. The two precursor films are placed

in contact with each other during the thermally

assisted reaction, resulting in a rapid reaction.

In summary, a solid-state reaction method is

developed to prepare perovskite thin films with

smooth morphology and good optical absorption

property. The corresponding solar cell devices show

an average efficiency of 8.7%, a maximum efficiency

of 10%, and good consistency among prepared

batches. The solid-state synthesis of organometallic

perovskite can also be applied to flexible plastic

substrates. The advantages of this method, such as

fast reaction speed, flexibility in selecting solvent

and feeding of precursor films, render it much

potential for future large-scale production.

Materials and Methods

CH3NH3I synthesis

CH3NH3I is prepared according to previous

reports [3]. 30 ml of methylamine (40% in methanol,

Sinopharm Chemical Reagent Co., Ltd) and 32.3 ml

of hydroiodic acid (57 wt% in water, Sinopharm

Chemical Reagent Co., Ltd) are reacted in a 250-ml

round-bottom flask at 0 °C for 2 h with stirring. The

precipitate is recovered by placing the solution into

a rotary evaporator and carefully removing the

solvents at 50 °C. The white (slightly yellow) raw

product of methylammonium iodide (CH3NH3I) is

washed with diethyl ether three times. After

filtration, the solid is collected and dried at 60 °C in

a vacuum oven for 24 h. The resulting white solid is

used without further purification.

Solar cell fabrication

ITO-coated glass substrates and pure glass

9

substrates are cleaned with detergent, deionized

water, ethanol, acetone, and isopropyl alcohol by

ultrasonication for 10 min and then dried with clean

nitrogen. All of the ITO-coated glass substrates and

pure glass substrates are treated with oxygen

plasma for 5 min. A buffer layer of PEDOT:PSS-4083

(Clevios PVPAI4083, Heraeus, Germany) with a

thickness of ≈35 nm is spin-coated at 3500 rpm onto

the ITO-coated glass substrates under ambient

conditions and annealed at 120 °C for 30 min in a

N2-filled glove box. The PbI2 layer is deposited onto

the ITO/PEDOT:PSS substrate by spin-coating a

230-mg/ml solution of PbI2 in

N,N-dimethylformamide (DMF) at 1500 rpm for 40

s and annealed at 80 °C for 15 min. The CH3NH3I

film is spin-coated onto the pure glass substrate

using a 200-mg/ml solution of CH3NH3I in DMF at

3000 rpm for 20 s. The CH3NH3I film is placed on

the PbI2 layer and baked at 135 °C for 30 min in case

of glass substrate, but baked at 120 °C for 45 min in

case of PET substrate. After removing the top

substrate, a uniform CH3NH3PbI3 thin film appears

on the underlying substrate. The PbI2 films, the

CH3NH3I films and the CH3NH3PbI3 films are

prepared in a glove box. PC61BM (purchased from

Luminescence Technology Corp.) is dissolved in

chlorobenzene to form a 20 mg/ml solution, and the

solution is spin-coated onto the

ITO/PEDOT:PSS/CH3NH3PbI3 substrate at 1200 rpm

for 1 min. No heat treatment is applied to the

PC61BM layer. A 100-nm Al layer is subsequently

evaporated under a pressure of 4×10-4 Pa through a

shadow mask to define the active area of the

devices (0.12 cm2).

Perovskite film and device characterization

XRD analysis of the crystal structures of the

perovskite films is conducted using a Bruker D8

Advance X-ray diffractometer (with Cu Kα

radiation at 0.15418 nm). An UV-Vis

spectrophotometer (Perkin Elmer Lambda 750) is

used to get the UV-Vis absorption spectra of the

planar perovskite films. PL spectra are

characterized using a home-made system with a

light souce (56RCS001/HV, MellesGriot, US), a

cryogenic refrigerator (PT403, Cryomech Inc., US)

and a detector (Spectra Pro 2500i, Princeton

Instrument, US). The surface morphologyis

obtained with an FEI Quanta 400 FEG Field

Emission Environment SEM and model 5500

AFM/SPM from Agilent Technologies, US.

The J-V characteristics of the solar cells are

measured in the dark and under 100 mW/cm2 white

light from a Hg-Xe lamp that is filtered by a

Newport 81094 Air Mass Filter using a

Keithley2635A source meter. The EQE measurement

is performed using a Merlin radiometer (Newport)

with a monochromator-calibrated wavelength

controller. A calibrated silicon photodiode is used as

a reference device to count the incident photons. All

of the measurements are performed under the

ambient atmosphere at room temperature.

Acknowledgements

This work was supported by the National

Natural Science Foundationof China (Nos: 91233104,

and 61376063), andthe National Basic Research

Programof China (2010CB934700). L. C.

acknowledges the support from Jiangsu Provincial

Natural Science Foundation (Grant No.

BK20130006).

Electronic Supplementary Material: Supplementary

material (further details of the chemically

strochiometry, AFM measurements, optical

micrograph imaging and XRD measurements) is

available in the online version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

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12

Electronic Supplementary Material

Facile preparation of organometallic perovskite films and

high-efficiency solar cells using solid-state chemistry

Lei Chen1,2,§, Feng Tang2, §, Yixin Wang2,3, Shan Gao2, Jinhua Cai2() and Liwei Chen2()

1Department of Chemistry, Shanghai University, Shanghai 200444, China

2i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China

3Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, Jiangsu 215123, China

§These authors contributed equally to this work.

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

Figure S1 AFM topographical image and height profile of (a) PbI2 and (b) perovskite films. Subtracting the thickness of the

PEDOT:PSS layer (35 nm), we obtain that the thickness of the perovskite is about 245 nm, and that of the original PbI2 film is about 145

nm. Considering the density of PbI2 (6.16 g/cm3) and CH3NH3PbI3 (4.09 g/cm3) [S1, S2], the film

thickness of perovskite should be 290 nm if the 145 nm thick PbI2 film is completely converted to perovskite, assuming film area is not

changed. We think the difference is mainly due to the porous state of PbI2 film.

————————————

Address correspondence to Jinhua Cai, jhcai2013@sinano.ac.cn; Liwei Chen, lwchen2008@sinano.ac.cn

13

Figure S2 Optical micrographs of CH3NH3I on the top substrate. The top film spin-coated with 100 mg/ml CH3NH3I solution (a)

before and (b) after the reaction; those with 200 mg/ml solution (c) before and (d) after the reaction; those with 300 mg/ml solution (e)

before and (f) after the reaction. The spin coating speed is 3000 rpm in all preparation. The amount of remaining CH3NH3I increased

with the increase of the CH3NH3I concentration.

Figure S3 XRD pattern of the top film (CH3NH3I on glass) before (0 min) and after (30 min) the reaction. For comparison, the

bottom film after 30 min of reaction (perovskite on ITO/glass) is also included. The signal intensity of the top film before reaction has

been reduced (multiply by a factor of 0.4).

14

Supporting References

[S1] http://en.wikipedia.org/wiki/Lead(II)_iodide

[S2] Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G., Semiconducting tin and lead iodide perovskites with organic cations: phas

e transitions, high mobilities, and near-infrared photoluminescent properties. Inorg. Chem. 2013, 52, 9019-9038.