1

Supporting Information

Toward high-efficiency, hysteresis-less, stable perovskite solar cells:

unusual doping of a hole-transporting material using a

fluorine-containing hydrophobic Lewis acid

Junsheng Luo,a Jianxing Xia,

a Hua Yang,

b Lingling Chen,

c Zhongquan Wan,

a Fei Han,

a Haseeb

Ashraf Malik,a Xuhui Zhu

c and Chunyang Jia

a

aState Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic

Science and Engineering, University of Electronic Science and Technology of China, Chengdu

610054, P. R. China.

bDongguan Neutron Science Center, Dongguan 523803, P. R. China.

cState Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic

Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China.

Electronic Supplementary Material (ESI) for Energy & Environmental Science.This journal is © The Royal Society of Chemistry 2018

2

Experimental

Materials preparation: Titanium isopropoxide (99.999%), lithium

bis(trifluoromethylsulfonyl)-imide (Li-TFSI, 99.95%) were achieved from Sigma-Aldrich.

18NR-T, CH3NH3I (99.5%), PTAA (P226, Mn: 10800 g/mol, Mw: 210000 g/mol, solubility: ~120

mg/mL in toluene at room temperature) and t-BP (96%) were purchased from Xi'an Polymer Light

Technology Corp. All other solvents and chemicals obtained from commercial sources and used as

received without further purification. The compact TiO2 (c-TiO2) layer solution was prepared

according to our previous work as following: 369 μL of titanium isopropoxide was added into 2.53

mL of ethanol, and 35 μL of 2 M HCl solution was added into 2.53 mL of ethanol in another vial

simultaneously. The second solution was then added dropwise to the first solution with fierce

stirring. The resulting solution was clear and transparent and was immediately discarded if cloudy.

The mixture was filtered with a 0.22 μm filter. The perovskite precursor solution was prepared by

mixing 1.2 mmol PbI2 and 1.2 mmol CH3NH3I in 1 mL of N,N-dimethylformamide (DMF) and

was stirred at 60 °C for 12 h. The control PTAA solution was prepared by dissolving 15 mg of

PTAA in 1 mL of toluene, in which 7.5 μL Li-bis(trifluoromethanesulfonyl) imide

(Li-TFSI)/acetonitrile (170 mg/1 mL) and 7.5 μL t-BP/acetonitrile (1 mL/1 mL) were added. The

PTAA (15 mg) and LAD were dissolved into 1 mL toluene and mixed by desired LAD/PTAA

ratios ranging from 1% to 10% (mole ratio with respect to the repeat unit mass). The HTL

solutions were stirred over night.

Fabrication of PSCs: FTO substrates were sequentially cleaned by ultrasonic bath with a

solution of detergent diluted in deionized water, alkaline ethanol solution, ethanol, acetone and

deionized water, each cleaning step lasted for 30 min and then dried with a nitrogen stream. The

3

substrates were treated with UV/ozone for 30 min to remove the last traces of organic residues.

The c-TiO2 layer was coated on the FTO glass by spin-coating a mildly acidic titanium

isopropoxide solution, followed by heating at 150 °C for 15 min, and then the c-TiO2 films were

gradually heated to 500 °C and baked at this temperature for 30 min. The mesoporous TiO2

(m-TiO2) layer was deposited on c-TiO2/FTO substrates via spin coating of 18NR-T diluted in

ethanol. The layers were then dried at 125 °C for 10 min and sintered at 500 °C for 30 min. Before

spin-coating perovskite solution, the TiO2 electrodes were treated with UV/ozone for 10 min and

transferred to glove box immediately to avoid any humidity effect. The perovskite precursor

solution was spin coated onto the FTO/c-TiO2/ m-TiO2 substrates at 3000 rpm for 55 s. During the

spin-coating, 100 μL of chlorobenzene was dropped at the center of the substrates after six

seconds. The spin-coated films were heated at 100 °C, resulting in the formation of dark brown

perovskite films. Subsequently, HTL was deposited on the CH3NH3PbI3 layers by spin coating at

3000 rpm for 30 s. Finally, Au electrode was thermally evaporated on the HTL-coated film and the

active area of each device was 0.09 cm2.

Characterization: The UV-Vis absorption spectra of PTAA and perovskite films were

measured by HITACHI (model U-2910) UV-Vis spectrophotometer. ESR (electron spin resonance)

spectra were analyzed by a Bruker-E500 spectrometer. Single carrier hole-only device

(ITO/PEDOT:PSS(40 nm)/PTAA/MoO3 (10 nm)/Al (100 nm)) was fabricated to measure hole

mobility. PEDOT:PSS was used as a hole-injection layer at the anode, and a vacuum-deposited

molybdenum trioxide (MoO3) layer was used as an electron-blocking layer at the cathode.

Ultraviolet photoemission spectroscopy (UPS) measurements were performed by AXIS ULTRA

DLD with a HeI monochromator (21.22 eV). The scanning electron microscopy (SEM) images

4

were observed by SEM (SEM, JEOL JSM-7600F). The J-V curves of the PSCs were measured

using an electrochemical workstation (CHI 660E, Shanghai Chenhua) under AM 1.5G simulated

solar light (100 mW cm-2

) (CHF-XM-500W, Trusttech Co. Ltd., Beijing, China) at ambient and

room temperature (about 25 ºC) without controlled atmosphere and not any followed

pre-conditioning protocols before the forward or reverse J-V scans. The incident light intensity

was calibrated with a standard Si solar cell. About the stability test, devices were kept in ambient

condition (50-70% RH and room temperature about 25 ºC) under dark condition for 70 days. We

just measured the stability at the end of the test. The incident photon-to-electron conversion

efficiency (IPCE) spectra were performed by using a commercial setup (QTest Station 2000 IPCE

Measurement System, CROWNTECH, USA). The time-resolved photoluminescence (TRPL)

spectra were measured at room temperature using of time-correlated single photon counting

(TCSPC) technique, employing a FluoroLog-3 Modular spectrofluorometer (HORIBA Jobin

Yvon). For TCSPC measurements, a pulsed laser source was laser diode with a wavelength of 474

nm, a repetition rate of 100 kHz, fluence of ~4 nJ cm-2

and a pulse width of 70 ps. Excitation

wavelength of 474 nm and an emission wavelength of 770 nm were used for measurement.

Femtosecond transient absorption (fs-TA) spectra were performed using a femotosecond

regenerative amplified Ti: sapphire laser system (Spectra Physics, Spitfire-Pro) and an automated

data acquisition system (ultrafast systems, Helios Fire). The samples of glass/perovskite/HTL

were excited at 365 nm. The bleaching kinetics were probed at 748 nm, and the delay curves were

fitted by double exponential functions. The electrochemical impedance spectroscopy (EIS) of the

PSCs were recorded under simulated AM1.5 illumination using an electrochemical workstation

(CHI 660E, Shanghai Chenhua, China) with a bias potential of 0.8 V. The water contact angles

5

were measured by using commercial setup (DSA25-Kruss). Time-of-flight secondary-ion mass

spectrometry (TOF-SIMS) was performed by TOF-SIMS 5 (ION-TOF GmbH, Germany). Two

dimensional grazing incidence X-ray diffraction (2D-GIXRD) images were conducted at BL14B1

beamline of Shanghai Synchrotron Radiation Facility (SSRF) (λ = 1.24 Å ). The incidence angle is

0.16 degree and the exposure time is 60 s.

Supplementary Figures

2012 2013 2014 2015 2016 2017 2018

8

12

16

20

24

PTAA: Li-TFSI/t-BP

Spiro-OMeTAD: Li-TFSI/t-BP

PTAA: Li-TFSI/t-BP

Spiro-OMeTAD: Li-TFSI/t-BP

PTAA: Li-TFSI/t-BP

Spiro-OMeTAD: Li-TFSI/t-BP

Spiro-OMeTAD: Li-TFSI/t-BP

PTAA: Li-TFSI/t-BP

Spiro-OMeTAD: Li-TFSI/t-BP

PC

E /

%

Year

Spiro-OMeTAD: Li-TFSI/t-BP9.7

10.9

12.3

15.0

15.416.2

19.320.1

21.122.1

Fig. S1 Trends in record PSCs with focus on the hole-transporting materials and corresponding

dopants. All the PSCs are using Li-TFSI/t-BP doped Spiro-MeOTAD or PTAA as HTL

Fig. S2 Reversible formation of adduct between Lewis basic PTAA and Lewis acidic LAD in

solution.

6

Fig. S3 Images of the LAD solution, dopant-free PTAA solution, a series of LAD doped PTAA

solutions and Li-TFSI/t-BP doped PTAA solution.

Fig. S4 Optimized structure and electron distributions in highest occupied molecular orbital

(HOMO) and lowest unoccupied molecular orbital (LUMO) levels of LAD.

7

2 3 4 5 6

48

64

80

96

112

128

J1/2 (

A1/2 m

-1)

Vappl

- Vbi - V

S / V

Dopant-free PTAA

Fig. S5 J1/2

-V characteristics of the dopant-free PTAA based on hole-only devices:

ITO/PEDOT:PSS/PTAA/MoO3/Al.

The hole mobilities (μ) were extrapolated from the J1/2

-V curves by using space charge

limited current (SCLC) method with an equation as follow:

𝐽 = 9

8𝜀r𝜀0𝜇

𝑉2

𝐿3

where εr is the dielectric constant ( εr = 3), ε0 is the permittivity of free space, L is the thickness of

active layer, μ is the hole mobility and V is the effective voltage corrected by subtracting from the

applied voltage (Vappl) the built-in voltage (Vbi) and the voltage drop (Vs) resulting from the series

resistance.

8

Fig. S6 Energy levels for PTAA based on different dopants with respect to the perovskite material

and TiO2 electrode.

Fig. S6 shows that the Voc of PSC is defined by the energy level difference between the Fermi

level of the TiO2 photoelectrode ( ) and the HOMO level of HTL ( ) as shown in

Equation:1, 2

𝑉 = -

where is constant for PSCs due to the same TiO2 was used in all the cases. Therefore, Voc

depends only on . was shifted downward away from by using

the novel 5% LAD, correspondingly leading to improve Voc of PSCs.

9

400 500 600 700 800 900

PL

Ab

so

rba

nce

Wavelength / nm

(b)

10 15 20 25 30

CH3NH

3PbI

3

CH3NH

3PbI

3CH

3NH

3PbI

3

Inte

nsity

2 / degree

CH3NH

3PbI

3

TiO2

FTO

(c)

Fig. S7 (a) Top-view SEM image of perovskite film, (b) UV-Vis and PL spectra of perovskite film

and (c) XRD of the perovskite film deposited on TiO2/FTO substrate.

The top-view SEM image of CH3NH3PbI3 film (Fig. S7a) reveals that CH3NH3PbI3 exhibits good

coverage without pinholes. For optical characterization of CH3NH3PbI3 films, optical absorption

and PL spectroscopy were performed (Fig. S7b), revealing good correspondence between optical

absorption and PL emission features. The XRD pattern (Fig. S7c) shows strong intensity for the

CH3NH3PbI3 peaks, pointing to good crystallinity of the CH3NH3PbI3 film. Meanwhile, no extra

peaks could be found beyond those for CH3NH3PbI3 and the substrate.

10

Fig. S8 Statistical distribution of the photovoltaic parameters for PSCs with 5% LAD and

Li-TFSI/t-BP doped PTAA as HTL, with a structure of

FTO/c-TiO2/m-TiO2/CH3NH3PbI3/HTL/Au. a) Distribution of Jsc, b) distribution of Voc, c)

distribution of FF. Results are shown for 45 devices, all devices were measured at a 100 mV/s

reverse scan rate.

11

550 600 650 700 750 800

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04(c)Perovskite/5% LAD doped PTAA

O

D

Wavelength / nm

50 ps

100 ps

200 ps

500 ps

800 ps

1000 ps

550 600 650 700 750 800

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04Perovskite/Li-TFSI and t-BP doped PTAA

(d)

O

D

Wavelength / nm

50 ps

100 ps

200 ps

500 ps

800 ps

1000 ps

Fig. S9 3D fs-TA spectra of bilayered (a) perovskite/5% LAD doped PTAA and (b)

perovskite/Li-TFSI and t-BP doped PTAA films, excited at 365 nm. ΔOD versus wavelength plots

at different delay times for (c) perovskite/5% LAD doped PTAA and (d) perovskite/Li-TFSI and

t-BP doped PTAA films.

Fig. S10 Equivalent circuit employed to fit the Nyquist plots.

12

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

25Dopant-free

Curr

ent

Density /

mA

cm

-2

Voltage / V

Forward 10 mV/s

Forward 50 mV/s

Forward 100 mV/s

Reverse 10 mV/s

Reverse 50 mV/s

Reverse 100 mV/s

(a)

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

251% LAD

Cu

rre

nt

De

nsit

y /

mA

cm-2

Voltage / V

Forward 10 mV/s

Forward 50 mV/s

Forward 100 mV/s

Reverse 10 mV/s

Reverse 50 mV/s

Reverse 100 mV/s

(b)

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

25

Curr

ent

Density /

mA

cm

-2

Voltage / V

Forward 10 mV/s

Forward 50 mV/s

Forward 100 mV/s

Reverse 10 mV/s

Reverse 50 mV/s

Reverse 100 mV/s

(c)2% LAD

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

257% LAD

Cu

rre

nt

De

nsit

y /

mA

cm-2

Voltage / V

Forward 10 mV/s

Forward 50 mV/s

Forward 100 mV/s

Reverse 10 mV/s

Reverse 50 mV/s

Reverse 100 mV/s

(d)

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

25(e)10% LAD

Curr

ent

Density /

mA

cm

-2

Voltage / V

Forward 10 mV/s

Forward 50 mV/s

Forward 100 mV/s

Reverse 10 mV/s

Reverse 50 mV/s

Reverse 100 mV/s

Fig. S11 J-V curves of PSCs based on different ratios LAD doped PTAA as HTL. Measurements

were performed under AM 1.5 solar illumination (100 mW cm-2

) with different scan rates (10, 50

and 100 mV/s) and directions (reverse and forward).

13

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

25

Curr

ent

Density /

mA

cm

-2

Voltage / V

Fresh

After 70 days

(a)5% LAD

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

25(b)

Cu

rre

nt

De

nsit

y /

mA

cm-2

Voltage / V

Fresh

After 70 days

Li-TFSI and t-BP

Fig. S12 The time-lapsed efficiency curves of PSCs with (a) 5% LAD and (b) Li-TFSI/t-BP as

dopants, inset shows the image of aged PSCs (without encapsulation and under ambient conditions

for 70 days).

Fig. S13 Water contact angles of (a) 5% LAD and (b) Li-TFSI/t-BP doped PTAA films (the 5%

LAD doped PTAA film shows a larger water contact angle of 113°as compared to 96° of Li-TFSI

and t-BP doped PTAA).

14

Fig. S14 The images of perovskite films added with different dopants leaved at ambient air.

Deliquescent characteristics of Li-TFSI, absorbing the moisture in the air and detrimental effect on

perovskite film,3, 4

t-BP tends to corrode perovskite to form a new complex with PbI2, leading to

chemical decomposition of the perovskite film;5 LAD was placed and removed from perovskite

film without any visible effect on quality and morphology of film.

Fig. S15 (a) Degradation mechanism of perovskite film under humid environments with dopant

Li-TFSI. (b) The mechanism of t-BP diffuses to the perovskite and form new complexes at the

perovskite/HTL interface.

15

Fig. S16 Top view SEM images of LAD doped PTAA films: (a) fresh and (b) after long-term

aging for 70 days in ambient conditions without encapsulation.

Fig. S17 Top view SEM images of Li-TFSI/t-BP doped PTAA films: (a) fresh and (b) after

long-term aging for 70 days in ambient conditions without encapsulation.

16

Supplementary Tables

Table S1 The TRPL data were obtained by fitting the PL delaying curves through an exponential

diffusion model = 0 (

) 2 (

).

Sample A1 τ1 (ns) A2 τ2 (ns) τave (ns)

Glass/CH3NH3PbI3/5%

LAD doped PTAA 0.44 1.13 0.09 7.51 4.83

Glass/CH3NH3PbI3/Li-TFSI

and t-BP doped PTAA 0.51 2.53 0.16 16.52 11.93

Table S2 EIS parameters of the PSCs.

HTL RREC (Ω) R2 (Ω)

PTAA + 5% LAD 79.29 91.84

PTAA + Li-TFSI/t-BP 54.93 41.23

Table S3 Photovoltaic parameters and hysteresis index (HI) of PSCs with 5% LAD and

Li-TFSI/t-BP doped PTAA. Measurements were performed under AM 1.5 solar illumination (100

mW cm-2

) with different scan rates and directions.

Dopant

Scan rate /

mV/s

Scan

direction

Jsc /

mA cm-2

Voc / V FF PCE / % HI

5% LAD

100

Reverse 22.35 1.05 0.81 19.01

0.073

Forward 22.79 1.01 0.72 16.57

50

Reverse 22.55 1.04 0.78 18.29

0.038

Forward 22.77 1.01 0.72 16.68

10

Reverse 22.84 1.04 0.76 18.11

0.036

Forward 22.51 1.02 0.73 16.76

17

Li-TFSI and t-BP

100

Reverse 22.34 1.02 0.78 17.77

0.08

Forward 23.38 0.98 0.66 15.28

50

Reverse 22.46 1.02 0.76 17.41

0.067

Forward 23.29 0.99 0.68 15.68

10

Reverse 22.46 1.02 0.75 17.25

0.040

Forward 23.19 1.00 0.70 16.23

Table S4 Photovoltaic parameters of PSCs with different LAD doping levels in PTAA.

Measurements were performed under AM 1.5 solar illumination (100 mW cm-2

) with different

scan rates and directions.

PTAA

Scan rate

mV/s

Scan

direction

Jsc /

mA cm-2

Voc/V FF PCE/%

Dopant-free

100

Reverse 19.79 0.99 0.65 12.73

Forward 20.9 0.95 0.57 11.2

50

Reverse 19.64 0.99 0.64 12.42

Forward 20.84 0.95 0.57 11.28

10

Reverse 19.63 1 0.6 11.85

Forward 20.89 0.97 0.55 11.12

1% LAD

100

Reverse 21.59 1.01 0.71 15.48

Forward 22.35 0.99 0.64 14.22

50

Reverse 21.2 1.01 0.72 15.47

Forward 21.96 1 0.65 14.34

18

10

Reverse 21.12 1 0.71 15

Forward 22.06 1 0.64 14.2

2% LAD

100

Reverse 21.71 1.03 0.77 17.22

Forward 22.13 0.99 0.71 15.61

50

Reverse 21.75 1.03 0.76 16.89

Forward 22.51 1 0.71 16

10

Reverse 21.79 1.03 0.75 16.75

Forward 22.05 1.01 0.73 16.24

7% ALD

100

Reverse 21.89 1.03 0.8 18.03

Forward 23 1.01 0.73 16.81

50

Reverse 22.1 1.03 0.77 17.53

Forward 22.91 1.01 0.73 16.92

10

Reverse 22.43 1.03 0.75 17.21

Forward 22.86 1.01 0.74 17.03

10% ALD

100

Reverse 21.71 1.02 0.75 16.61

Forward 21.79 1 0.71 15.39

50

Reverse 21.78 1.02 0.74 16.46

Forward 21.72 1.01 0.72 15.63

10

Reverse 21.69 1.03 0.74 16.48

Forward 21.88 1.01 0.73 16.07

19

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