SUPPORTING INFORMATION

Low-temperature SnO2-modified TiO2 yields record efficiency for normal planar

perovskite solar modules

Bin Ding, Shi-Yu Huang, Qian-Qian Chu, Yan Li, Cheng-Xin Li, Chang-Jiu Li and

Guan-Jun Yang*

School of Materials Science & Engineering, Xi'an Jiaotong University, No.28,

Xianning West Road, Xi'an, Shaanxi, 710049, P.R. China.

Corresponding author: G.-J. Yang, ygj@mail.xjtu.edu.cn.

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2018

Experimental section

N, N-dimethylformamide (DMF), titanium tetrachloride (TiCl4), and stannous

chloride dehydrate (SnCl2·2H2O) were purchased from Sinopharm Chemical Reagent

Co., Ltd (China). Methylammonium iodide (CH3NH3I, MAI), formamidinium iodide

(HC(NH2)2I, FAI), lead iodide (PbI2), and other chemicals for the preparation of

Spiro-OMeTAD solution were purchased from Xi'an Polymer Light Technology Corp.

(China). The transparent fluorine-doped tin oxide (SnO2:F, FTO) conductive glasses

(sheet resistance of 10 Ω sq-1) were patterned by laser from Weihua Solar Company

(China).

Fabrication of TiO2 compact layer

The FTO glass substrates were sequentially cleaned using acetone, ethanol, and

deionized water for 15 min via ultrasonic cleaning. The 2M aqueous TiCl4 solution

was firstly prepared by mixing TiCl4 with deionized water at 0 °C and then stored in a

freezer at the temperature of 5 °C, which remains stable for at least one year. The

cleaned FTO substrate was soaked in the dilute 2M aqueous TiCl4 solution, obtained

by mixing 2M aqueous TiCl4 solution and deionized water with the molar ratio of

1:10 and placed in a sealed glass container. Then the glass container was put in a

drying cabinet at the temperature of 70 °C for 1 h. After cooling, the FTO substrate

was repeatedly rinsed using ethanol and deionized water for three time, and dried at

120 °C for 1 h. The substrate was then treated at 70 °C for 30 min with the solution

prepared by 2M aqueous TiCl4 solution and deionized water with the molar ratio of

1:100. Finally the substrate was rinsed and dried by repeating the abovementioned

process.

Fabrication of SnO2 modified TiO2 (SnO2@TiO2) compact layer

2M SnCl2 ethanol solution was firstly prepared by using SnCl2·2H2O dissolved in

ethanol and then stored in the freezer at 5 °C. The FTO substrate coated with TiO2

nanoparticles by the abovementioned chemical bath was soaked in the solution

obtained by mixing 2M SnCl2 ethanol with deionized water with the molar ratio of

1:50 in a glass container. Then the glass container was placed in a drying cabinet at

the temperature of 70 °C for 1 h. After cooling, the FTO substrate was repeatedly

rinsed using ethanol and deionized water for three times. Finally the substrate was

annealed on a hotplate at 140 °C for 3h.

Fabrication of perovskite films and solar cells

The FTO substrate coated with the compact layer (TiO2 or SnO2@TiO2) was firstly

cleaned by UV-ozone for 15 min. A 40 wt % perovskite precursor solution was

prepared with molar ratios of PbI2/MAI/FAI fixed at 1:0.7:0.3 in DMF. The perovskite

films were deposited onto the TiO2 or SnO2@TiO2 substrates by using our previously

reported approach called gas-induced gas pump method.1 First, the perovskite

precursor solution was spin-coated onto the substrate at 2500 rpm for 10 s. Then, the

substrate was put into a gas pump system equipment. The gas pump equipment was

home-developed, mainly composed of a large vacuum tank and a sample chamber.

The vacuum tank maintaining at a constant pressure of 1500 Pa in this work and

sample chamber was connected with each other by a vacuum valve. At the bottom, the

sample chamber was attached to air by two symmetrical gas tubes further connecting

a gas flow controller, which allowing a certain amount of air flowing on the surface of

the wet perovskite film. After opening the valve, the DMF solvent evaporated

instantaneously. About 5 s later, closing the valve, a brown, somewhat transparent

perovskite film with a mirror-like surface was obtained. Subsequently, the film was

annealed at 120 °C for 20 min on a hot plate. Then the Spiro-OMeTAD solution (80

mg of Spiro-OMeTAD, 28.5 μL of 4-tert-butylpyridine, and 17.5 μL

lithium-bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg Li-TFSI) in

1mL acetonitrile) all dissolved in 1 mL of chlorobenzene) was spin-coated onto the

perovskite film by spin-coating at 4000 rpm for 30 s. The Spiro-OMeTAD-coated

substrates were stored in an auto-drying cabinet with temperature fixed at 20 °C and

relative humidity fixed at 15% for at least 8h. Finally, about100 nm thick gold layer

was deposited onto the Spiro-OMeTAD layer by thermal evaporation. As for the solar

module, after the substrate coated with Spiro-OMeTAD, the

Sipro-OMeTAD/perovskite on the non-device area was removed by DMF solvent in

order to forming a parallel module using the gold layer. All the films except gold layer

were prepared in air at the temperature of 22~25 °C and the relative humidity of

45~55 %.

Solar cell characterization

Photocurrent-voltage (J-V) characteristics of the devices were measured by applying a

sourcemeter (2400, Keithley) under the illumination of the solar simulator (Newport,

Class AAA) with an AM 1.5G filter (Sol3A, Oriel) at the light intensity of 100 mW

cm-2 calibrated with a standard Si reference cell (91150V, Oriel). After being taken out

of the chamber of the thermal evaporation, the devices were put in an auto-drying

cabinet for at least 4 h and then were directly measured. Normally, the devices were

measured at a scan step of about 23.7 mV (60 points in total) and a delay time of 1000

ms at the bias voltage range of -0.2 V to 1.2 V. For the champion cells, the devices

was also measured at different delay time, such as 100, 500 ,1000, and 1500 ms. For

the solar modules, the devices were measured at the scan step of 10 mV and the delay

time of 50 ms.

For the measurement of the maximum power point tracking in air, the device was

tested for 14 hours under the illumination of the solar simulator (Newport, Class AAA)

with an AM 1.5G filter (Sol3A, Oriel) at the light intensity of 100 mW cm-2 calibrated

with a standard Si reference cell (91150V, Oriel) with relative humidity of 45%-55%.

The current was updated every 4 s. For the measurement in ideal conditions, the

device was tested under continuous AM 1.5G illumination with intensity of one sun

from another solar simulator (CHF-XM500, Beijing perfectlight technology co. LTD)

equipped with a 420-nm cutoff UV-filter. The device was kept in a sealed home-made

steel box with a quartz glass window during the whole test. The box containing the

devices was purged with nitrogen flow for 3 hours to get rid of water and oxygen in

the whole space. The current-voltage characteristics were obtained every 24 hour.

The incident photon-to-current conversion efficiency (IPCE) was measured by Enli

tech (Taiwan) measurement system in AC mode in air without encapsulating the

devices. A home-developed system was applied to measure the transient photovoltage

and photocurrent decays. A white light bias with adjustable light intensity was

generated from an array of diodes. The voltage bias was maintained at open-circuit

voltage (VOC) by turning the intensity of the white light bias. The perturbation light

was generated from red light pulse diodes controlled by a fast solid-state switch with a

square pulse width, 100 ns rise and fall time. The intensity of perturbation light was

adjusted to a suitably amplitude of transient VOC below 5 mV in order for the voltage

decay kinetics to be mono-exponential. For the photovoltage decay measurement, the

open circuit condition was achieved by using a resistor of 5 MΩ controlled by a

resistance box. Similarly, the short circuit condition was achieved by using a 90

Ωresistor for the photocurrent decay measurement. The voltage dynamics were

recorded on a digital oscilloscope. The electrochemical impedance spectroscopy of

the perovskite devices was measured by using an electrochemical system (EIS,

Zennium IM6, Zahner). The devices were measured at the bias voltage from -0.7 V to

-1.0 V with step of 0.1 V and at a frequency ranging from 3 MHz to 100 mHz with an

AC amplitude of 20 mV under a LED light with the intensity of 10 mW/cm2.

Film characterizations

Field-emission scanning electron microscope (SEM, FEI Verios 460) was used to

characterize the surface and fracture morphologies of the ETLs, perovskite films and

PSCs. By using an ultraviolet-visible spectrophotometer (U-3900, HITACHI), the

transmittance spectra of the ETLs on the FTO substrates were measured by using a

glass substrate that has the same thickness as the FTO substrate as the background

data and the absorption spectra of the perovskite films were measured by using a FTO

substrate as the background data. X-ray diffraction (XRD, D8 Advance, Bruker) was

used to characterize the phase composition of the ETLs and perovskite films with a

scanning range of 10-70° and a scanning speed of 0.05°/s. X-ray photoelectron

spectroscopy (XPS) was measured using a Thermo Scientific ESCALab 250Xi with

200W monochromated Al Kα (1,486.6eV) radiation. XPS analysis was conducted

with a 500 μm spot size, 20 eV pass energy and energy steps of 0.05 eV at the

pressure of 3 × 10-10 mbar. For the steady-state photoluminescence spectra

measurement, a compact steady-state spectrophotometer (Fluoromax-4, Horiba Jobin

Yvon) was used with the laser diode at a wavelength of 457 nm. A LabRAM HR800

(Horiba Jobin Yvon) was implied to measure the time-resolved photoluminescence

(TRPL) measurements of perovskite films on different substrates at 786 nm using an

excitation with a 478 nm light pulse from a HORIBA Scientific DeltaPro fluorimeter.

The electron diffusion coefficient was estimated by fitting TRPL with equation S1.2,3

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wherer N(t) is the total charge number generated in the active layer, L is the

thickness of the active layer, k is the TRPL decay rate without any acceptor layer, D is

the charge-carrier diffusion coefficient and α is the linear absorption coefficient of the

active layer at the excitation wavelength.

The cross-sectional sample of the perovskite film deposited on the

SnO2@TiO2/FTO glass substrate was prepared by using FEI Helios Nanolab 600i

FIB-SEM system. The sample was firstly coated with Cr layer by Gantan 682 and

then was coated with two layers of Pt by electron beam deposition followed by Ion

beam deposition. Finally, the sample was cut by Ga beam at a voltage of 30 kV with

the cross-section at a tilted angle of 52° relative to the Ga ion sputtering direction.

Supplementary Figures and Tables

Fig. S1 XPS, XRD and UV-vis characterization. (a) Survey scan XPS spectra and (b) XRD

patterns of FTO, TiO2/FTO and SnO2 deposited on TiO2/FTO substrates annealed at 80, 140

as well as 180 °C. (c) Transmission spectra of FTO substrates with and without the TiO2 film

or the SnO2@TiO2 film. (d) I-V curves of the FTO/ETL/Au devices with fixed area based on

the TiO2 (T) and SnO2@TiO2 (S@T) ETL.

Fig. S2 SEM images of perovskite films morphologies. Top-view images of perovskite films

deposited on TiO2 with low (a) and (c) high magnifications or SnO2@TiO2 with (b) low and

high (d) magnification. The corresponding cross-sectional view images of perovskite films

deposited on TiO2 (e and g) or on SnO2@TiO2 (f and h).

Fig. S3 XRD patterns of perovskite films deposited on TiO2 or SnO2@TiO2 with heat

treatment (W/ HT) and without heat treatment (W/O HT). The perovskite films were annealed

at 120 °C for 20 mins.

Fig. S4 Characterization of TEM. (a) High angle annular dark field (HAADF) scanning

cross-sectional view TEM image of the perovskite film deposited on the SnO2@TiO2/FTO

substrate obtained by using FIB. (b) HAADF scanning TEM image of the enlarged FTO/

SnO2@TiO2/perovskite interfaces. Individual elemental maps of (c) C, (d) N, (e) I, and (f) Pb

of the area indicated by the white box in Fig. 3b. (g) HAADF scanning TEM image of the

enlarged FTO/SnO2@TiO2/perovskite interfaces with high magnification. The energy

dispersive spectra of the point 1 (h) and 2 (i) indicated by the plus signs in Fig. S4g.

Fig. S5 SEM images of planar perovskite solar cells. Cross-view images of the TiO2-based

device with low (a) and (c) high magnification and the SnO2@TiO2-based device with (b) low

and high (d) magnification.

Fig. S6 Device with 0.1 cm2 masked area performance. (a) J-V curves (reverse scan) of the

planar perovskite solar cells based on TiO2 films treated by different concentration SnCl2

solution via chemical bath. Photovoltaic parameters (b) JSC, (c) VOC and (d) FF for perovskite

solar cells based on TiO2 or SnO2@TiO2 ETLs. (e) The output of current density and the

corresponding PCE at the maximum power point with bias voltage of 975 mV for the

SnO2@TiO2-based champion cell as shown in Fig. 4d. (f) IPCE spectra of the champion cells

with TiO2 (Fig. 4c) and SnO2@TiO2 (Fig. 4d) ETLs.

Fig. S7 The characteristics of impedance spectroscopy of the devices based on TiO2 and

SnO2@TiO2 ETLs. (a) The equivalent circuit consisting of a resistance and two lumped RC

elements in series to fit the impedance data. (b) The Nyquist plots of the device based the TiO2

ETL at different bias voltages. (c) The Nyquist plots of the device based the SnO2@TiO2 ETL at

different bias voltages. (d) The series resistance (RS), (e) the charge transfer resistance (RCT),

and (f) the charge recombination resistance (RCR).

Fig. S8 device with active area of 1.13 cm2 performance. Photovoltaic parameters (a) JSC, (b)

VOC and (c) FF for 1.13 cm2 perovskite solar cells based on SnO2@TiO2 ETLs. (d) The output

of current density and the corresponding PCE at the maximum power point with bias voltage

of 939 mV for the SnO2@TiO2-based champion cell with active area of 1.13 cm2 as shown in

Fig. 5b.

Fig. S9 The illustration and photographs of the mini-module. (a) The size of the module in

detail. (b) The photograph of the module from the Au side. (c) The photograph of the module

from the FTO side which is the light incident surface.

Fig. S10 (a) The photograph of the modules on the 5 × 5.5 cm substrates from Au side. (b)

The photograph of the module from the FTO side which is the light incident surface.

Fig. S11 The certified result of the normal planar perovskite solar module. The device has a

masked area of 10.55 cm2 and an average PCE of 15.65% (VOC=5.25 V, ISC=45.58 mAcm-2,

and FF=69.0%) with less hysteresis.

Fig. S12 The certified results of the normal planar perovskite solar module in Fig. S17 at (a)

reverse scan (PCE=15.87%, VOC=5.25 V, ISC=45.41 mA/cm2 and FF= 70.20%) and (b)

forward scan (PCE=15.43%, VOC=5.26 V, ISC=45.75 mA/cm2 and FF= 67.70%).

Fig. S13. Normalized EQE spectra of the certified solar module with the integrated short

circuit current density of 20.86 mA/cm2.

Fig. S14 (a) Sol3A Class A spectral distribution and (b) EQE of Newport secondary solar cell

10510-0054 KG1 for the certification body.

Table S1 Photovoltaic parameters of the planar perovskite solar cells based on TiO2

films treated by different concentration of SnCl2 solution via chemical bath.

Concentration

(mM)

JSC

(mA/cm2)

VOC

(mV)

FF

(%)

PCE

(%)

0 22.50 1113 78.90 19.76

20 23.20 1113 79.03 20.42

40 24.79 1114 79.35 21.92

60 24.85 1067 79.73 21.15

80 24.91 1002 79.50 19.83

Table S2 Photovoltaic parameters of the TiO2-based champion cell. The device was

measured by reverse and forward scans at a scan step of 23.7 mV and delay time of

100, 500, 1000 as well as 1500 ms under a simulated AM 1.5G solar illumination of

100 mW/cm2.

Scan direction Delay time

(ms)

JSC

(mA/cm2)

VOC

(mV)

FF

(%)

PCE

(%)

Forward 100 22.26 1079 49.32 11.85

Reverse 100 22.97 1112 80.69 20.61

Forward 500 22.64 1075 64.86 15.79

Reverse 500 22.96 1107 81.31 20.66

Forward 1000 22.77 1074 70.12 17.15

Reverse 1000 22.83 1114 81.45 20.72

Forward 1500 22.79 1074 71.00 17.38

Reverse 1500 22.74 1114 81.50 20.65

Table S3 Photovoltaic parameters of the SnO2@TiO2-based champion cell. The device

was measured by reverse and forward scans at a scan step of 23.7 mV and delay time

of 100, 500, 1000 as well as 1500 ms under a simulated AM 1.5G solar illumination

of 100 mW/cm2.

Scan

direction

Delay time

(ms)

JSC

(mA/cm2)

VOC

(mV)

FF

(%)

PCE

(%)

Forward 100 24.47 1.10 75.76 20.38

Reverse 100 24.56 1.11 80.49 21.96

Forward 500 24.32 1.10 76.88 20.59

Reverse 500 24.54 1.11 80.52 21.95

Forward 1000 24.34 1.10 76.09 20.35

Reverse 1000 24.54 1.11 80.46 21.94

Forward 1500 24.56 1.10 76.47 20.57

Reverse 1500 24.61 1.11 80.38 21.96

Notes and references

1 B. Ding, Y. Li, S. -Y. Huang, Q. -Q. Chu, C. -X. Li, C. -J. Li, G. -J. Yang, J. Mater.

Chem. A, 2017, 5, 6840-6848.

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

L. M. Herz, A. Petrozza, H. J. Snaith, Science, 2013, 342, 341-342.

3 G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Gratzel, S. Mhaisalkar, T. C.

Sum, Science, 2013, 342, 344-347.