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Article

Efficient Perovskite Solar Cells by Temperature Controlin Single and Mixed Halide Precursor Solutions and Films

Devendra Khatiwada, Swaminathan Venkatesan, Nirmal Adhikari, Ashish Dubey, AbuFarzan Mitul, Lal Mohammad, Anastasiia Iefanova, Seth B Darling, and Qiquan Qiao

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08294 • Publication Date (Web): 28 Oct 2015

Downloaded from http://pubs.acs.org on November 3, 2015

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1

Efficient Perovskite Solar Cells by Temperature Control in Single and Mixed

Halide Precursor Solutions and Films

Devendra Khatiwada1, Swaminathan Venkatesan

1, Nirmal Adhikari

1, Ashish Dubey

1, Abu

Farzan Mitul1, Lal Mohammad

1, Anastasiia Iefanova

1, Seth B. Darling

2,3, and Qiquan Qiao

1*

1Center for Advanced Photovoltaics, Department of Electrical Engineering and Computer

Science, South Dakota State University, Brookings, SD, USA

Tel: 1-605-688-6965, qiquan.qiao@sdstate.edu 2Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne,

IL 60439

Abstract

Thermal annealing and precursor composition play critical roles in crystallinity control

and morphology formation of perovskite thin films for achieving higher photovoltaic

performance. In this study we have systematically studied the role of annealing temperature on

the crystallinity of perovskite (CHNH3PbI3) thin films cast from single (without PbCl2) and

mixed (with PbCl2) halide precursors. Higher annealing temperature leads to agglomeration of

perovskite crystals. The effects of annealing temperature on the performance of perovskite solar

cells are different in single and mixed halide processed films. It is observed that the perovskite

crystallinity and film formation can be altered with the addition of lead chloride in the precursor

solution. We report that single halide perovskite solar cells show no change in morphology and

crystal size with increase in annealing temperature, which was confirmed by UV-vis absorption

spectroscopy, x-ray diffraction (XRD) and atomic force microscopy (AFM). However, mixed

halide perovskite (CH3NH3PbI3-xClx) solar cells show significant change in crystal formation in

the active layer when increasing annealing temperature. In addition, heating perovskite precursor

solutions at 150 oC can lead to enhancement in solar cell efficiency for both single and mixed

halide systems. Perovskite solar cells fabricated using heated precursor solutions form dense film

morphology, thus significantly improved fill factor up to 80% with power conversion efficiency

exceeding 13% under AM 1.5 condition.

Keywords: Perovskite solar cells, temperature control, crystallinity and morphology

___________________________________________________________________________________________

3Institute for Molecular Engineering, University of Chicago, 5640 S Ellis Ave, Chicago, IL 60637

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Introduction

Hybrid organic-inorganic perovskite based solar cells have shown great promise with

different device architectures (meso/planar structure) 1, ideal bandgap (1.5 eV), long carrier

lifetimes, large diffusion length, high efficiency 1-5

, and rapid energy payback time 6. Adopting

device architecture similar to dye sensitized solar cells, perovskite solar cells were first

fabricated with mesoporous structure that required high processing temperature (> 400 oC)

7.

Low processing temperature, however, is necessary when using mechanically flexible substrates

thereby enabling roll-to-roll manufacturing 8. Temperature control in forming perovskite crystals

is crucial as it determines the crystal size and orientation as well as the film morphology 9-11

.

Methylammonium lead triiodide (CH3NH3PbI3) is a widely reported single halide

perovskite that has been used in fabricating efficient solar cells. Single halide perovskite

possesses various advantages including broad absorption range (350-1100 nm), high electron and

hole mobility (7.5 cm2V

-1S

-1 and 12.5-66 cm

2V

-1S

-1, respectively), and high charge carrier

diffusion length (100nm - 1µm) 12

. Mixed halide (CH3NH3PbI3-xClx ) perovskite solar cells have

also been of interest, possessing even higher electron mobility (~ 33 cm2V

-1S

-1) and longer

diffusion length (1000-1500 nm) 13

. The addition of chlorine in single halide perovskite

precursor leads to large scale crystalline domains (> 200nm), which are correlated with increased

charge transport with less charge recombination 5, 14-17

. In mixed halide perovskite solar cells,

different secondary phases (e.g., CH3NH3Cl, CH3NH3 PbCl3, and CH3NH3PbI3) of perovskite

may be formed during the crystal formation, which can hinder photovoltaic performance 8.

Moreover, perovskite solar cells often suffer strong hysteresis in current density-voltage (J-V)

measurements 2, 18

. Hysteresis strongly depends on the perovskite crystal interfaces, size and

defects in both mesoporous and planar structures. Researchers are actively pursuing low-

temperature processing with no hysteresis and high efficiency. Under low-temperature

processing, the temperature effects on perovskite crystal size in single and mixed halide films

has to be understood. In addition, temperature effects on perovskite precursor solutions and the

final device performance have not yet been clearly understood and correlated.

In this work, the effects of annealing temperature on perovskite crystal formation, film

morphology and their correlation with planar structure device performance in single and mixed

halide processed solar cells were investigated. A series of experiments correlating annealing

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temperatures to solar cell performance were conducted. The single halide solution consists of

methylammonium iodide and lead iodide whereas mixed halide contains methylammonium

iodide, lead iodide, and a small amount of lead chloride. For the mixed halide precursor solutions

heated at 60 oC, after processing into perovskite films, a trend in decreasing open circuit voltage

and short circuit current density was observed with increasing annealing temperature of the

perovskite film. However, for the single halide precursor solutions heated at 60 oC, no significant

changes were found in solar cell performance. After increasing the temperature of single and

mixed halide precursor solutions from 60 oC to 150

oC and processing into films a very high fill

factor—close to 80%—was achieved. Therefore, the processing temperatures for preparation of

both the precursor solutions and perovskite film are critical in achieving high performance cells

with large fill factor.

Experimental procedures

Preparation of single and mixed halide perovskite solutions

Methylammonium Iodide (CH3NH3I) was synthesized using a standard procedure 19

.

Hydro-iodic acid (10 mL, 0.227 molar) and methylamine (9.266 mL, 0.273 molar) were stirred

in ice bath in the air for 2 hr to obtain precipitate of CH3NH3I. Solvent was removed from the

precipitate by rotating in a rotary evaporator at 50 oC until all the solvent evaporated. Yellowish

raw product methylammonium iodide (CH3NH3I) was formed, which was purified by washing

with diethyl ether and was filtered using a paper filter 15 cm in diameter in the air. After

filtration, the solid precipitate CH3NH3I was collected and then dried in a vacuum oven at 60 °C

for 24 hr. The prepared CH3NH3I was kept inside a nitrogen filled glove box. PbI2 and PbCl2

were purchased from Acros Organic and γ- Butyrolactone was ordered from Sigma Aldrich.

Single halide perovskite was prepared from a precursor mixture solution of 209 mg

CH3NH3I and 581 mg PbI2 in a binary solvent mixture made by 0.7 ml γ- Butyrolactone and 0.3

ml dimethyl sulfoxide (DMSO) inside a glovebox. For mixed halide perovskite solution

preparation, a precursor mixture solution of 209 mg CH3NH3I, 581 mg PbI2 and 39 mg PbCl2

was prepared inside the glovebox with the same binary solvent as used in preparation of the

single halide perovskite solution. Both single and mixed halide precursor solutions were kept

stirring for 12 hr at 60 oC on a hot plate inside the nitrogen filled glove box. [6,6]-Phenyl-C61-

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butyric acid methyl ester (PC61BM) was purchased from Nano-C material. 20 mg PC61BM was

dissolved in1 ml chlorobenzene (CB) and kept stirring at 85 oC for 12 h. 0.5 mg Rhodamine 101

inner salt purchased from sigma Aldrich was dissolved in 1 ml iso-propane (IPA) and kept

stirring for 1 hr without heating before spin coating. PC61BM and Rhodamine solution were also

prepared inside the glove box in a nitrogen environment.

Device fabrication

Perovskite solar cells were fabricated on indium tin oxide (ITO) substrates. ITO

substrates were thoroughly cleaned in detergent, DI water, acetone and IPA for 20 min each.

Cleaned ITO was then plasma cleaned for 25 min. PEDOT:PSS as hole transport layer was spin

coated on top of cleaned ITO at 4500rpm for 45 sec followed by annealing at 150 oC for 10 min

to remove any excess water in the air. PEDOT:PSS coated films were moved into the nitrogen

filled glove box and were again annealed at 100 oC. Both single and mixed halide perovskite

precursor solutions (yellow in color) were heated at 60 oC on a hot plate. Then these hot yellow

perovskite precursor solutions were spin coated on heated (100oC) PEDOT:PSS-coated ITO

substrates at 750/4000 rpm for 20s and 60s, respectively. After 40 sec, 160µl of toluene was

dripped onto the perovskite film. Toluene drip onto the film prepared from mixed solvent lead to

uniform and dense perovskite layer through an intermediate phase of CH3NH3I-PbI2-DMSO

resulting in low hysteresis 20

. Prior literature reveals that toluene reduces the solubility of

CH3NH3Pb3-xClx in a mixed solvent thereby promoting fast nucleation and crystal growth 21

.

Fabricated perovskite films from both single and mixed halide perovskite were then annealed on

different hot plates at temperatures 80 oC, 90

oC, 100

oC, 110

oC, and 120

oC respectively for 20

min to form perovskite crystals.

In the next step, single and mixed halide perovskite precursor solutions were again

prepared inside a glove box following the same process as mentioned above. These yellow color

perovskite precursor solutions inside the glove box were kept stirring overnight for 12 hr at

temperature of 60 oC. At 5 minute before spin coating, these yellow color perovskite precursor

solutions were further heated to a temperature of 150 oC in the glovebox till the color of

precursor solutions changed from yellow to reddish brown for both single and mixed halide

perovskite. Perovskite films were then fabricated using these reddish brown precursor solutions.

Prepared perovskite films were annealed on different hot plates at temperature 80 oC, 90

oC, 100

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oC, 110

oC, 120

oC, respectively, for 20 min. PC61BM was spin coated inside the glove box on

top of the perovskite active layer at 2000 rpm for 40 sec and then annealed at 100 oC for 10 min.

Rhodamine was then spin coated inside the glove box at 4000rpm for 40 sec. Finally, silver was

thermally evaporated as cathode with a thickness of 150 nm. The device cell area was 0.16cm2,

as defined by a mask.

Characterization

An Agilent 4155 semiconductor parameter analyzer was used to characterize current

density-voltage (J-V) characteristics. A silicon photo detector (NREL calibrated) was used as

reference cell to calibrate the intensity. Device J-V measurements were performed with a

Newport xenon lamp as a solar simulator (A.M 1.5). All perovskite solar cells were tested under

the same condition. Fast scan was performed with a voltage step of 10mV and scan rate 1V/s.

During fast reverse scan (open circuit to short circuit) capacitive charges together with photo-

generated charges are extracted that reduces hysteresis. However, during fast forward scan (short

circuit to open circuit), solar cells are partially charged by photo-generated charges, which

reduced the total charges. Furthermore, due to the high density of defect states in the perovskite

films, emptying and filling of trap states occur, which lead to hysteresis and are observed when

scanned at a slower rate 2. Chemical or structural changes in the material due to formation of

different secondary phases during fabrication also contributes to hysteresis 22

. External quantum

efficiency (EQE) measurements were performed using the same lamp that was attached to a

Newport monochromator.

An Agilent 8453 spectrophotometer was used to determine UV-Vis absorption. First, a

blank scan was performed on a PEDOT:PSS film followed by a scan on the perovskite film to

subtract absorbance due to glass/ITO and PEDOT:PSS. AFM images were obtained with an

Agilent 5500 SPM (scanning probe microscope) in tapping mode. Silicon tips coated with Cr/Pt

having a resonance frequency ~300 KHz were used.

Results and discussion

Perovskite solar cells were fabricated with device structure glass/ITO/PEDOT:PSS (hole

transport layer)/perovskite/PC61BM (electron transport layer)/Rhodamine/Ag(thermal

evaporated). Figures 1a and b show the device structure and energy level diagram of perovskite

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solar cells, respectively. Figure 1(c) and (d) show the absorbance spectra of perovskite solar cells

fabricated with single halide and mixed halide precursors. All perovskite films on PEDOT:PSS

at different temperatures 80 °C, 90 °C, 100 °C, 110 °C and 120 °C show broad absorbance. The

absorbance of single halide perovskite films in the range 400-750 nm slightly increases with

increase in annealing temperature. However, in mixed halide perovskite films, the absorbance in

the range 400-750nm significantly increases with higher annealing temperature.

500 600 700 800 900 1000 1100

0.0

0.2

0.4

0.6

0.8

1.0(c)

Absorb

an

ce

(A

U)

Wavelength (nm)

80oC

90oC

100oC

110oC

120oC

400 500 600 700 800 900 1000 1100

0.0

0.2

0.4

0.6

0.8

1.0(d)

Absorb

an

ce

(A

U)

wavelength (nm)

80oC

90oC

100oC

110oC

120oC

Figure 1. (a) Device structure and (b) energy level diagram of perovskite solar cells. Normalized

absorbance spectra of (c) single halide (without PbCl2) and (d) mixed halide (with PbCl2)

perovskite films at different annealing temperatures, 80 °C, 90 °C, 100 °C, 110 °C, and 120 °C

processed from precursor solutions heated at 60 °C.

(a) (b)

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400 500 600 700 8000

20

40

60

80

(a)E

QE

(%

)

Waelength (nm)

80oC

90oC

100oC

110oC

120oC

400 500 600 700 800

0

20

40

60

80(b)

EQ

E (

%)

Wavelength (nm)

80oC

90oC

100oC

110oC

120oC

Figure 2. EQE of (a) single halide (without PbCl2) and (b) mixed halide (with PbCl2) perovskite

solar cells at different film annealing temperatures 80 °C, 90 °C, 100 °C, 110 °C, and 120 °C

processed from precursor solutions heated at 60 °C.

Figures 2a and b show external quantum efficiency (EQE) measurement for single and

mixed halide perovskite solar cells, respectively. EQE results show broad spectral range from

350 – 800 nm. Integrated EQE for single and mixed halide perovskite solar cells are found to be

75% in average. The short circuit current density (Jsc) obtained from EQE integration over the

AM 1.5 solar simulator spectrum is very close to the experimentally obtained J–V curves.

10 20 30 40 50 60

700

1400

700

1400

700

1400

700

1400

700

1400

2 θ (degree)

80 oC

90 oC

Inte

nsity (

a.u

.)

100 oC

110 oC

(a)

120 oC

10 20 30 40 50 60

1000

1000

2000

1000

2000

1000

2000

1000

2000 (b)

2 θ

80 oC

(degree)

90 oC

Inte

nsity (

a.u

.)

100 oC

110 oC

120 oC

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Figure 3. XRD spectra of (a) single halide (without PbCl2) and (b) mixed halide (with PbCl2)

perovskite films at different annealing temperatures 80 °C, 90 °C, 100 °C, 110 °C and 120 °C

processed from precursor solutions heated at 60 °C.

Figures 3a and b show XRD measurements of single and mixed halide perovskite films

respectively annealed each at different temperatures (80 °C, 90 °C, 100 °C, 110 °C and 120 °C).

Strong peaks at 14.08°, 24.8°, 28.41°, 31.85°, and 43.19° are attributed to the formation of

orthorhombic crystal structure of halide perovskite (CH3NH3PbI3/CH3NH3PbI3-xClx) with high

crystallinity. The peak at 12.65° corresponds to lead iodide (PbI2). Single halide perovskite

precursor solution resulted in complete formation of perovskite film, as evidenced by the absence

of the PbI2 peak. However, mixed halide perovskite precursor solution resulted in incomplete

formation of perovskite film with a prominent PbI2 peak at 12.65°. Intensity of the PbI2 peak for

mixed halide perovskite films was found to increase with increase in annealing temperature. This

trend is attributed to incomplete conversion of PbI2 to perovskite at higher temperature resulting

in some residual PbI2 phase. The presence of a PbI2 phase is attributable to phase decomposition

of CH3NH3PbCl3 and the precursor reaction 23

. Thus, perovskite solar cells processed from

mixed halide precursor solution under the above mentioned experimental condition exhibit

higher photovoltaic performance at lower temperatures. Annealing temperature had little effect

on single halide perovskite film morphology.

0.0 0.2 0.4 0.6 0.8

-20

-10

0

10(a)

Cu

rre

nt d

en

sity (

mA

/cm

2)

Voltage (V)

80 oC

90 oC

100 oC

110 oC

120 oC

0.0 0.2 0.4 0.6 0.8 1.0

-20

-10

0

10

(b)

Cu

rre

nt

de

nsity (

mA

/cm

2)

Voltage (V)

800C

900C

1000C

1100C

1200C

Figure 4. Current density-voltage characteristics in reverse scan (scan rate of 1V/s with step of

10mV) of (a) single halide (without PbCl2) and (b) mixed halide (with PbCl2) perovskite solar

cells at different film annealing temperatures 80 °C, 90 °C, 100 °C, 110 °C and 120 °C processed

from precursor solutions heated at 60 °C.

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Table 1. Photovoltaic parameters in reverse scan for single and mixed halide perovskite solar

cells with films annealed at different temperatures 80 °C, 90 °C, 100 °C, 110 °C and 120 °C

processed from precursor solutions heated at 60 °C. J-V characteristics were performed under the

same scan rate (1V/s with step of 10mV) for single and mixed halide systems.

Figure 4a and b shows J-V characteristics for single and mixed halide perovskite solar

cells respectively at different annealing temperatures of 80 °C, 90 °C, 100 °C, 110 °C and 120

°C, respectively. The J-V characteristics were collected at the same conditions with the same

scan rate for all the cells. J-V curves with the forward and reverse scan for both single and mixed

halide solar cells at different annealing temperature of perovskite films are shown in Figure S1

(supporting information). Single halide perovskite solar cells did not show much change in the J-

V curves between the forward and reverse scans at the same annealing temperature indicating

negligible hysteresis (supporting information). However, the mixed halide processed devices

show variations between the forward and reverse scans at the same annealing temperature

indicating significant hysteresis. All the devices processed from single halide perovskite

precursor exhibited good reproducibility. Efficiency for mixed halide perovskite solar cell

decreased with increase in annealing temperature. In addition, perovskite solar cells made by

mixed halide do not show consistent efficiency at higher temperature. Table 1 shows

photovoltaic parameters for perovskite solar cells fabricated from single and mixed halide

precursors at different film annealing temperatures of 80 °C, 90 °C, 100 °C, 110 °C and 120 °C,

respectively.

Perovskite solution

at 150oC

Annealing

temperature (°C) Voc (Volt)

Jsc

(mA/cm2)

Fill factor

(FF)

Efficiency

(η)

Single halide

(without PbCl2)

80 °C 0.88±0.03 17.56±0.10 0.76 11.74±0.12

90 °C 0.79±0.02 17.61±0.11 0.62 11.05±0.13

100 °C 0.80±0.03 21.08±0.10 0.67 11.32±0.10

110 °C 0.87±0.05 18.65±0.11 0.70 11.28±0.11

120 °C 0.84±0.01 19.79±0.08 0.73 12.23±0.10

Mixed halide (with

PbCl2)

80 °C 0.92±0.06 15.46±0.12 0.61 8.97±0.12

90 °C 0.84±0.07 10.94±0.10 0.70 6.47±0.16

100 °C 0.89±0.04 15.94±0.15 0.40 5.69±0.18

110 °C 0.76±0.05 10.56±0.16 0.71 5.73±0.15

120 °C 0.71±0.08 11.93±0.17 0.69 5.92±0.14

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Figure 5. AFM topography for (a) single halide (without PbCl2) and (b) mixed halide (with

PbCl2) perovskite films at different annealing temperatures of 80 °C , 90 °C, 100 °C, 110 °C, and

120 °C processed with precursor solutions heated at 60 °C.

(a) Single halide (without PbCl2) (b) Mixed halide (with PbCl2)

10

0 °

C a

nnea

led

fil

m

80

°C a

nnea

led

fil

m

90

°C

annea

led

fil

m

12

0 °

C a

nnea

led

fil

m

11

0 °

C a

nnea

led

fil

m

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AFM was conducted to investigate nanoscale interfacial morphology in the active layer

of the perovskite solar cells. Figures 5a and b show the AFM topography images of single and

mixed halide perovskite films respectively obtained using different annealing temperatures. For

single halide perovskite films, the topography images exhibit almost no difference in grain size

with variation in temperature. This uniformity reveals that annealing temperature has little effect

in single halide perovskite films. However, the grain size of mixed halide perovskite films were

observed to increase as temperature gets higher. The RMS roughness for single halide perovskite

film at 80 oC, 90

oC, 100

oC, 110

oC and 120

oC are 18.2nm, 19.8nm, 23.4nm, 23.8nm, 29.4nm,

respectively. Grain size for single halide perovskite films is in the range 200-300nm and forms

agglomeration. The RMS roughness for mixed halide perovskite films at 80 oC , 90

oC, 100

oC,

110 oC, 120

oC are 13.8nm, 30.9nm, 44.2nm, 31.0nm,31nm, respectively, with grain size of 200-

300nm at 800C and increased up to 1µm with increase in temperature.

10 20 30 40 50 60

500

1000

1500

500

1000

1500

Inte

nsity (

AU

)

2 Theta (degree)

with PbCl2/80

0C

(b)

without PbCl2/80

0C

(a)

Figure 6. XRD scans of perovskite films from (a) single halide (without PbCl2) and (b)

mixed halide (with PbCl2) processed with precursor solutions heated at 150 °C. The films were

annealed at 80 °C.

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Perovskite solar cells fabricated from the heated perovskite precursor solution at 150 oC

showed broad absorbance from 400 to 1100 nm (Figure S3 in supporting information). Figs. 6a

and b show XRD scans of single and mixed halide perovskite films respectively with each

annealed at 80 oC fabricated from perovskite precursor solutions heated at 150

oC. XRD data

show an absence of a PbI2 peak indicating complete formation of perovskite structure. Sharp

peaks of perovskite crystal are observed at 14.08°, 24.8°, 28.41°, 31.85°, and 43.19°, which are

attributed to higher crystallinity.

J-V characteristics for perovskite solar cells fabricated from perovskite precursor

solutions heated at 150 oC are shown in Figure 7. Table 2 summarizes photovoltaic parameters

for single and mixed halide perovskite solar cells under illumination. Perovskite films prepared

from single and mixed halide precursor solutions were annealed at 80 oC. J-V curves show

increase in fill factor up to 80% with device efficiency greater than 13% in comparison to

perovskite solar cells prepared from perovskite precursor solution heated at 60 oC. J-V

characteristics were performed under identical forward and reversed scan rate (1V/s with step of

10mV) for both single and mixed halide systems. Single halide perovskite solar cells showed

higher efficiency than mixed halide due to larger short circuit current density. Higher fill factor

was observed during reverse scan corresponding to comparable photocurrent between the

forward and reversed scans (supporting information, table S2). This suggested that fill factor also

depends on the scan direction.

0.0 0.2 0.4 0.6 0.8

-20

-15

-10

-5

0

5

10

Cu

rre

nt d

en

sity (

mA

/cm

2)

Voltage (V)

PbCl2

No PbCl2

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Figure 7. J-V characteristics in reverse scan (scan rate of 1V/s with step of 10mV) of perovskite

solar cells for single halide (without PbCl2) and mixed halide (with PbCl2) processed with

precursor solutions heated at 150 °C. The films were annealed at 80 °C.

Table 2 Photovoltaic parameters in reverse scan (scan rate of 1V/s with step of 10mV) for

perovskite solar cells fabricated from heated perovskite precursor (150 °C).

Perovskite solution

at 150oC

Film annealing

temperature (oC)

Voc(Volt) Jsc(mA/cm2)

Fill factor

(FF)

Efficiency

(η)

Single halide

(without PbCl2) 80 °C 0.90±0.01 18.65±0.01 0.78 13.14±0.01

Mixed halide (with

PbCl2) 80 °C 0.88±0.02 14.77±0.02 0.81 10.56±0.02

Figures 8a and b show AFM topography images from single halide (without PbCl2) and

mixed halide (with PbCl2), respectively. These images reveal dense morphology with compact

grain size. Thus, the films fabricated from the 150 °C heated, color-changed precursor solution

(yellow to light brown) resulting in effective charge transport and increase in fill factor, which

correlate with observed XRD and UV-Vis absorbance data. The RMS roughness for single and

mixed halide perovskite films are 23.4 nm and 9.95 nm, respectively.

Figure 8. AFM topography images of perovskite films (a) single halide (without PbCl2) and (b)

mixed halide (with PbCl2) processed from precursor solutions heated at 150 °C. The films were

annealed at 80 °C.

80

o

C a

nnea

led

fil

m

Single halide (without PbCl2) Mixed halide (with PbCl2)

(a) (b)

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Conclusions

We have reported the effects of annealing temperatures on crystallinity and morphology

formation of perovskite thin films using single and mixed halide precursors. Single halide

processed perovskite solar cells do not show morphological changes and device characteristic

difference with increasing perovskite film processing temperature. This type of perovskite solar

cell can be fabricated across a broad range of annealing temperatures from 80 °C to 120 °C.

However, mixed halide perovskite solar cells give higher device efficiency at lower film

annealing temperatures, while higher annealing temperatures lead to decrease in device

efficiency and morphologically increase in grain size with increasing temperatures. Our findings

are supported by XRD measurements that show complete formation of perovskite structure for

single halide perovskite solar cells. However, the peak intensity of PbI2 increases as the

temperature gets higher in the mixed halide processed films. A new finding here is that the solar

cell fill factor can be significantly improved by increasing temperature of the perovskite

precursor solution prior to deposition.

Acknowledgements

This research was benefited from the grants including NASA EPSCoR (NNX13AD31A),

Pakistan-US Science and Technology Cooperation Program, and NSF MRI (grant no.1428992).

This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of

Energy Office of Science User Facility under Contract No. DE-AC02-06CH11357.

Supporting Information

J-V characteristics and photovoltaic parameters for singe and mixed halide perovskite solar cells

at different film annealing temperature 80 °C, 90 °C, 100 °C, 110 °C and 120 °C at both forward

(/F) and reverse (/R) scan. J-V characteristics and photovoltaic parameters of perovskite solar

cells processed with and without PbCl2 using heated solution (>150 °C) at both forward (/F) and

reverse (/R) scan. UV-vis absorbance and EQE for perovskite solar cells fabricated from heated

solution (150 °C). Photo of heated solution at 60 °C and 150 °C. This information is available

free of charge via the Internet at http://pubs.acs.org

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Author Information

Corresponding Author

*Email: qiquan.qiao@sdstate.edu

Tel: 1-605-688-6965

References

1. Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar

Cells by Vapour Deposition. Nature 2013, 501, 395-398.

2. Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S.

D.; Wang, J. T.-W.; Wojciechowski, K.; Zhang, W. Anomalous Hysteresis in Perovskite Solar

Cells. J. Phys. Chem. Lett. 2014, 5, 1511-1515.

3. Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.;

Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345,

542-546.

4. Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat.

Photonics 2014, 8, 506-514.

5. Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid

Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338,

643-647.

6. Gong, J.; Darling, S. B.; You, F. Perovskite Photovoltaics: Life-Cycle Assessment of

Energy and Environmental Impacts. Energy Environ. Sci. 2015, 8, 1953-1968.

7. Kim, H.-S.; Lee, J.-W.; Yantara, N.; Boix, P. P.; Kulkarni, S. A.; Mhaisalkar, S.; Grätzel,

M.; Park, N.-G. High Efficiency Solid-State Sensitized Solar Cell-Based on Submicrometer

Rutile Tio2 Nanorod and Ch3nh3pbi3 Perovskite Sensitizer. Nano Lett. 2013, 13, 2412-2417.

8. Ball, J. M.; Lee, M. M.; Hey, A.; Snaith, H. J. Low-Temperature Processed Meso-

Superstructured to Thin-Film Perovskite Solar Cells. Energy Environ. Sci. 2013, 6, 1739-1743.

9. Laban, W. A.; Etgar, L. Depleted Hole Conductor-Free Lead Halide Iodide

Heterojunction Solar Cells. Energy Environ. Sci. 2013, 6, 3249-3253.

10. Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.; Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y. A

Hole-Conductor–Free, Fully Printable Mesoscopic Perovskite Solar Cell with High Stability.

Science 2014, 345, 295-298.

11. Wang, J. T.-W.; Ball, J. M.; Barea, E. M.; Abate, A.; Alexander-Webber, J. A.; Huang,

J.; Saliba, M.; Mora-Sero, I. n.; Bisquert, J.; Snaith, H. J. Low-Temperature Processed Electron

Collection Layers of Graphene/Tio2 Nanocomposites in Thin Film Perovskite Solar Cells. Nano

Lett. 2013, 14, 724-730.

12. Grätzel, M. The Light and Shade of Perovskite Solar Cells. Nat. Mater. 2014, 13, 838-

842.

13. Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; Leijtens, T.;

Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1

Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344.

14. Edri, E.; Kirmayer, S.; Kulbak, M.; Hodes, G.; Cahen, D. Chloride Inclusion and Hole

Transport Material Doping to Improve Methyl Ammonium Lead Bromide Perovskite-Based

High Open-Circuit Voltage Solar Cells. J. Phys. Chem. Lett. 2014, 5, 429-433.

15. Colella, S.; Mosconi, E.; Fedeli, P.; Listorti, A.; Gazza, F.; Orlandi, F.; Ferro, P.;

Besagni, T.; Rizzo, A.; Calestani, G. Mapbi3-Xcl X Mixed Halide Perovskite for Hybrid Solar

Page 15 of 17

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The Journal of Physical Chemistry

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

16

Cells: The Role of Chloride as Dopant on the Transport and Structural Properties. Chem. Mater.

2013, 25, 4613-4618.

16. Suarez, B.; Gonzalez-Pedro, V.; Ripolles, T. S.; Sanchez, R. S.; Otero, L.; Mora-Sero, I.

Recombination Study of Combined Halides (Cl, Br, I) Perovskite Solar Cells. J. Phys. Chem.

Lett. 2014, 5, 1628-1635.

17. Kitazawa, N.; Watanabe, Y.; Nakamura, Y. Optical Properties of Ch3nh3pbx3 (X=

Halogen) and Their Mixed-Halide Crystals. J. Mater. Sci. 2002, 37, 3585-3587.

18. Wei, J.; Zhao, Y.; Li, H.; Li, G.; Pan, J.; Xu, D.; Zhao, Q.; Yu, D. Hysteresis Analysis

Based on the Ferroelectric Effect in Hybrid Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5,

3937-3945.

19. Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Graetzel,

M.; White, T. J. Synthesis and Crystal Chemistry of the Hybrid Perovskite (Ch 3 Nh 3) Pbi 3 for

Solid-State Sensitised Solar Cell Applications. J. Mater. Chem. A 2013, 1, 5628-5641.

20. Kim, H.-B.; Choi, H.; Jeong, J.; Kim, S.; Walker, B.; Song, S.; Kim, J. Y. Mixed

Solvents for the Optimization of Morphology in Solution-Processed, Inverted-Type

Perovskite/Fullerene Hybrid Solar Cells. Nanoscale 2014, 6, 6679-6683.

21. Sun, K.; Chang, J.; Isikgor, F. H.; Li, P.; Ouyang, J. Efficiency Enhancement of Planar

Perovskite Solar Cells by Adding Zwitterion/Lif Double Interlayers for Electron Collection.

Nanoscale 2015, 7, 896-900.

22. Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S.; Nazeeruddin, M. K.; Grätzel, M.

Understanding the Rate-Dependent J–V Hysteresis, Slow Time Component, and Aging in Ch 3

Nh 3 Pbi 3 Perovskite Solar Cells: The Role of a Compensated Electric Field. Energy Environ.

Sci. 2015, 8, 995-1004.

23. Song, T.-B.; Chen, Q.; Zhou, H.; Luo, S.; Yang, Y. M.; You, J.; Yang, Y. Unraveling

Film Transformations and Device Performance of Planar Perovskite Solar Cells. Nano Energy

2015, 12, 494-500.

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Graphic abstract

0.0 0.2 0.4 0.6 0.8

-20

-15

-10

-5

0

5

10

Curr

en

t d

en

sity (

mA

/cm

2)

Voltage (V)

PbCl2

No PbCl2

Thermal annealing and precursor composition play critical roles in crystallinity control and

morphology formation of perovskite thin films for achieving higher photovoltaic performance.

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