www.sciencemag.org/cgi/content/full/science.aam5655/DC1

Supplementary Materials for

Perovskite solar cells with CuSCN hole extraction layers yield stabilized

efficiencies greater than 20%

Neha Arora, M. Ibrahim Dar,* Alexander Hinderhofer, Norman Pellet, Frank Schreiber, Shaik Mohammed Zakeeruddin, Michael Grätzel*

*Corresponding author. Email: ibrahim.dar@epfl.ch (M.I.D.); michael.graetzel@epfl.ch (M.G.)

Published 28 September 2017 on Science First Release DOI: 10.1126/science.aam5655

This PDF file includes: Materials and Methods

Supplementary Text

Figs. S1 to S11

Table S1

References

S2

Materials and Methods

All materials were purchased from Sigma-Aldrich and were used as received unless

stated otherwise.

Preparation of TiO2 photoanode

Fluorine-doped tin oxide (FTO)-glass substrates (transparent conducting oxide

(TCO) glass, NSG 10, Nippon sheet glass, Japan) were cleaned by ultrasonication in

Hellmanex (2%, deionized water) for 30 min, rinsed thoroughly with de-ionized water

and ethanol, and then treated in oxygen plasma for 15 min. A compact layer of TiO2 of ca

30 nm was subsequently deposited via spray pyrolysis at 450 °C from a precursor

solution of commercial titanium diisopropoxide bis(acetylacetonate) (75% in 2-propanol,

Sigma-Aldrich) diluted in anhydrous ethanol (1:9, volume ratio) and oxygen as a carrier

gas. A mesoporous TiO2 layer was then deposited by spin coating a diluted paste at a spin

speed of 4000 rpm, acceleration 2000 rpm for 20 s (the weight ratio of TiO2 (Dyesol

paste, 30NRD) and ethanol is 1:6), onto the substrate containing TiO2 compact layer.

This was followed by sintering the substrates at 450 °C for 30 min in dry air flow. For Li

treatment of the mesoporous TiO2 scaffold, 150 µL of LiTFSI

(bis(trifluoromethylsulfonyl)imide lithium salt) solution in acetonitrile (10mg/mL, freshly

prepared in argon atmosphere) was spin coated (3000 rpm, acceleration 2000 rpm for 20

s) after a loading time of 10 s. Thereafter, Li-treated substrates were subjected to a

second sintering step at 450 °C for 30 min in dry air. The substrates were transferred in a

dry air glove box (humidity< 1%) after cooling down to 200 °C for perovskite deposition.

Preparation of perovskite

The perovskite films were deposited using single-step deposition method from the

precursor solution containing FAI (1.0 M) (Dyesol), PbI2 (1.1 M) (TCI), MABr (0.2 M)

(Dyesol) and PbBr2 (0.2 M) (TCI) in anhydrous dimethylformamide (99.8%, Acros) /

dimethylsulphoxide (99.7%, Acros) (4:1 (v:v)). Thereafter, CsI (abcr, GmbH, ultra dry;

99.998%), (5% volume, 1.5 M DMSO) was added to the precursor solution (33). The

precursor solution was spin-coated onto the mesoporous TiO2 films in a two-step

programme at 1000 and 6000 r.p.m. for 10 and 30 s, respectively. During the second step,

100 μl of chlorobenzene (99.8%, Acros) was dropped on the spinning substrate 10 s prior

to the end of the programme. This was followed by annealing the films at 100 °C for 45

min. The perovskite deposition was carried out in a dry air glove box under controlled

atmospheric conditions with humidity <1%.

Deposition of hole transporting layer

To complete the fabrication of devices, 2,2’,7,7’-tetrakis(N,N-di-p-

methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD, 70 mM in chlorobenzene) as

a hole-transporting material (HTM) was deposited by spin coating 40 μL of the prepared

solution at 4000 rpm for 30 s. The spiro-OMeTAD (Merck) was doped with

bis(trifluoromethylsulfonyl)imide lithium salt, tris(2-(1H-pyrazol-1-yl)-4-tert-

butylpyridine)- cobalt(III) tris(bis(trifluoromethylsulfonyl) imide) (FK 209, from

Dyenamo) and 4-tert-Butylpyridine (96%, Sigma-Aldrich) in a molar ratio of 0.5, 0.03

and 3.3, respectively.

CuSCN solution was prepared by dissolving 35 mg CuSCN salt (99%, Sigma-

Aldrich) in 1 mL of diethyl sulfide (98%, Sigma-Aldrich) after constant stirring at room

S3

temperature for 30 min. To deposit a thin and uniform film of CuSCN, 35 μL of a

CuSCN solution were drop casted within 2-3 seconds on the substrate containing

perovskite film spinning at 5000 rpm, and the substrate was allowed to spin for 30

seconds. Like in case of spiro-OMeTAD deposition, no post heat treatment was applied.

The deposition of hole transporting layer was carried out in a dry air glove box with

humidity <1%. Finally, device fabrication was completed by thermally evaporating

~60 nm of gold layer as a back contact.

Deposition of Al2O3 spacer layer.

Al2O3 was deposited from trimethylaluminium (Al(CH3)3) (Sigma Aldrich) and

deionized H2O using Savannah ALD system from Cambridge Nanotech, USA. The

precursors were pulsed into the reactor using N2 (99.999%) as an inert carrier gas. The

water was pulsed for 0.015 s and confined in the reactor for 60 s followed by purging of

TMA for 0.015 s, and wait another 60 s. 20 ALD cycles were applied to achieve the

thickness of ~2 nm.

Deposition of rGO spacer layer.

A thin layer (~10 nm thickness) of reduced graphene oxide (Ossila (flake size 0.1-1 μm,

flake thickness <1nm, purity>99%)) was deposited by spin coating the solution (filtered)

containing reduced graphene oxide (1 mg/mL in chlorobenzene, sonicated for 30 min) at

3000 rpm onto the CuSCN layer.

Structural characterization

X-ray scattering experiments were done at beamline ID10 EH1 of the ESRF with a

photon energy of 22 keV under nitrogen atmosphere. The beam size was 10 µm in

vertical direction and 120 µm in horizontal direction. GIXD data was measured under an

angle of incidence of 0.08° with a point detector. GIWAXS data was measured with a

PILATUS 300k area detector under the same angle of incidence. Calculation of powder

diffraction data was done with mercury (34).

Morphological characterization

A field-emission scanning electron microscope (Merlin) was used to examine the

morphology of the perovskite films and the thickness of various layers. An electron beam

accelerated to 3 kV was used with an in-lens detector.

X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was performed on an X-ray

photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher SCIENTIFIC INC., USA)

with Al Kα radiation (hν = 1486.6 eV) as the source.

Charge extraction by linearly increasing voltage

Fully assembled devices were connected to a function generator and measured with

an oscilloscope across a small load resistance to minimize the RC effects of the circuit.

The mobilities were measured across a range of electric fields, which are determined by

the field at the time where the maximum current is extracted.

S4

Photoluminescence studies

Photoluminescence and time-resolved photoluminescence spectra were recorded on

a spectrofluorometer Fluorolog 322. Photoluminescence spectra were recorded by

exciting the samples with 450 W Xenon lamp at a fixed wavelength of 450 nm

and scanning the emission monochromator from 625 to 850 nm. The same spectrometer

working in a time-correlated single-photon counting mode was used for the

measurements of photoluminescence decay kinetics with sub-nanosecond time resolution.

Picosecond pulsed diode laser head NanoLED-405LH (Horiba) emitting <200 ps duration

pulses at 408 nm with repetition rate of 1 MHz was used as an excitation source. For

electron injection study, the samples were excited from the TiO2 side while as to study

the photoluminescence decay kinetics in pristine and HTM containing samples, the

samples were excited from the top.

Photovoltaic studies

The current-voltage (J-V) characteristics of the perovskite devices were recorded

with a digital source meter (Keithley model 2400, USA). A 450 W xenon lamp (Oriel,

USA) was used as the light source for photovoltaic (J-V) measurements. The light source

was equipped with a Schott K113 Tempax sunlight filter (Praezisions Glas and Optik

GmbH) to match the emission spectrum of the lamp to the AM1.5G standard. Before

each measurement, the exact light intensity was determined using a calibrated Si

reference diode equipped with an infrared cutoff filter (KG-3, Schott) The photo-active

area of 0.16 cm2 was defined using a dark-colored metal mask. External quantum

efficiency (EQE) measurements were made using a LED light source (Ariadne EQE from

Cicci Research).

Stability Test

Photo-stability tests were carried out at a maximum power point under one-sun

illumination at 60 °C using a home-built electronic board with an eight-channel

maximum power point capability. The channels were equipped with DACs (DAC7563),

level shifters (INA2132), and an output line driver (OPA2192). The driving line had a 12

bit resolution in the ±2.048 V range (1 mV per bit). The buffer output was connected to

the cells through a 0.5 Ohm sense resistor. The voltage drop was amplified (INA188) to

sense the current. The voltage was buffered (OPA2188) to sense the voltage. The signal

lines were multiplexed (CD54HC4051) into a fourth order active Butterworth filter with

its pole set at 500 Hz. Data conversion was performed at 430 SPS by an analog to digital

converter (ADS1118). The DACs and the ADC were interfaced by an Atmega328

microcontroller. The light source consisted of an array of white LEDs was powered by a

constant current and no filters (UV) were used. Equivalent sun intensities were calibrated

using a calibrated Si reference diode equipped with a KG-3 filter. The setup was

calibrated periodically using a Keithley 2602B source-measuring unit. For thermal

stability tests, CuSCN based devices were coated with a thin layer (30 nm) of

poly(methyl methacrylate) (PMMA) (20mg/mL in chlorobenzene) atop the gold

layer. Thermal stability tests were performed by ageing the devices at 85 °C in an oven

and the photovoltaic efficiency under AM1.5 simulated sun light was periodically

recorded.

S5

Supplementary Text

The intensity dependence of VOC curves (Fig. 3A, B; insets) were fitted using the

following equation:

𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝐒𝟏: V𝑂𝐶 =𝑛𝑘𝑇

𝑞ln (

𝐽𝑠𝑐

𝐽0)

Fig. S1. Intensity distribution of β-CuSCN. Angular distribution of intensity from the

CuSCN (002) reflection on glass and perovskite after background subtraction. (0° is

parallel to the substrate plane; 90° is perpendicular).

S6

Fig. S2. Morphological characterization of spiro-OMeTAD coated perovskite film

and spiro-OMeTAD based PSC. (A) Top-view SEM micrograph of spiro-OMeTAD

layer deposited onto perovskite film showing the presence of pin holes. (B) Cross section

SEM micrograph displaying the thickness of different layers of the complete device.

Fig. S3. Photovoltaic characterization. (A) J-V characteristics of the best devices

obtained after employing concentration of 15, 25, 35 and 45 mg/mL of CuSCN in diethyl

sulfide recorded at a scan rate of 0.05 V/s (reverse scans). (Inset shows the photovoltaic

parameters extracted from the JV curves). (B) J-V curves of the best CuSCN based

device involving conventional deposition of CuSCN recorded at a scan rate of 0.05V/s.

(Inset shows the photovoltaic parameters extracted from the JV curve).

S7

Fig. S4. The JSC (log scale) as a function of illumination intensity. (A) Light intensity

dependence of JSC in spiro-OMeTAD based device (Black solid squares) red color solid

line: linear fit. (B) Light intensity dependence of JSC in CuSCN based device (Green solid

squares) red color solid line: linear fit.

Fig. S5. Stability test at 85 °C. Thermal stability test of spiro-OMeTAD (sealed device)

and CuSCN (coated with a thin layer of PMMA) based perovskite solar cells aged at 85

°C in air for 1000 h in the dark. The error bars represent the standard deviation of a batch

of four solar cells.

S8

Fig. S6. Photovoltaic characterization of device based on CuSCN/Au. (A) J-V

characteristic of the CuSCN/Au based device recorded at a scan rate of 10 mV/s (reverse

scan, JSC of 23.40 mA cm-2, VOC of 1103 mV, and a FF of 77.2% resulting in a PCE of

20.3%); the inset shows the maximum power point (MPP) tracking for 60 s yielding a

stabilized efficiency of 20.3%. (B) EQE as a function of monochromatic wavelength

recorded for CuSCN/Au based device and the integrated current density obtained from the

EQE spectrum.

Fig. S7. Operational stability of CuSCN based devices with Al2O3 (20 cycles). Photo-

stability examined at a maximum-power-point under continuous full-sun illumination at

60 °C in nitrogen atmosphere.

S9

Fig. S8. Morphological characterization of CuSCN/rGO/Au sample. Cross-sectional

SEM micrograph showing the formation of a thin layer of rGO onto CuSCN layer.

Fig. S9. Operational stability of un-encapsulated spiro-OMeTAD and CuSCN based

device (containing a thin layer of rGO as a spacer layer between CuSCN and gold layers).

Photo-stability examined at a maximum-power-point under continuous full-sun

illumination at 60 °C in nitrogen atmosphere.

S10

Fig. S10. Stability of CuSCN based device under applied potential. The stability tests were

performed by biasing the devices under a constant potential of 0.8 V in the dark and the

photovoltaic efficiency was periodically recorded under AM1.5 simulated sun light.

Fig. S11. X-ray photoelectron spectroscopy. Core shell Au 4f spectra recorded from

freshly deposited gold layer and the gold layer removed from CuSCN based device

illuminated at a maximum-power-point at 60 °C for 50 h.

S11

Table S1. Photovoltaic parameters extracted from the JV curves (Figure 3A, B) of the

best performing spiro-OMeTAD/Au and CuSCN/rGO/Au based devices measured at

reverse and forward scans.

Scan direction Voc

(mV)

Jsc

(mA/cm2)

FF

(%)

PCE

(%)

Spiro/Au Reverse 1137 23.35 77.5 20.8

Forward 1129 23.31 75.5 20.1

CuSCN/rGO/Au Reverse 1112 23.24 78.2 20.4

Forward 1107 23.24 76.9 20.0

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