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Supporting information

High-Efficiency and Stable Inverted Planar Perovskite Solar Cells with

Pulsed Laser Deposited Cu-Doped NiOx Hole Transport Layers

Menglei Feng,†, ‡ Ming Wang, §, ‡ Hongpeng Zhou,† Wei Li, § Shuangpeng

Wang*¶ Zhigang Zang,*§ Shijian Chen*†

†Chongqing Key Laboratory of Soft Condensed Matter Physics and Smart

Materials, College of Physics, Chongqing University, Chongqing 401331,

China.

§Key Laboratory of Optoelectronic Technology & Systems (Ministry of

Education), Chongqing University, Chongqing 400044, China.

¶Institute of Applied Physics and Materials Engineering, University of

Macau, Avenida da Universidade, Taipa 999078, Macau SAR.

Corresponding Authors

*Shuangpeng Wang Email: spwang@um.edu.mo

*Zhigang Zang Email: zangzg@cqu.edu.cn

*Shijian Chen Email: sjchen@cqu.edu.cn

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1. Experimental section

1.1 Materials

All solvents and reagents were used as received without further purification. High-purity green

nickel oxide powders (99.99%) were purchased from China New Metal Materials & Technology

Co., Ltd. (CNM). Methylammonium iodide (MAI, ≥99.5%), formamidinium iodide (FAI, ≥ 99.5%),

lead iodide (PbI2, >99.99%), lead chloride (PbCl2, >99.99%) were obtained from Xi'an Polymer

Light Technology Corp. Rhodamine 101 (RhB101), cuprous oxide (Cu2O, 99.99%) and the solvents

of isopropanol (IPA, 99.7% purity), γ-butyrplactone (GBL, >99%), dimethyl sulphoxide

(DMSO, >99.9%), chlorobenzene, and methylbenzene were purchased from Sigma-Aldrich.

1.2 Preparation of Cu-Doped NiOx Targets and Films

Pure NiO powders were pressed in a mould with one inch diameter under a pressure of 30 MPa

for 0.5 h, and then the NiOx target was sintered in a furnace at 900 ºC for 48 h under an oxygen-rich

atmosphere. For the Cu-doped NiOx target, Cu2O was used as the p-type dopant source material, the

Cu2O powder (mole ratio Cu/Ni = 1%, 3%, and 5%) was mixed with NiO powder for adequate

grinding in mortar. After being molded, pressed, and sintered, the Cu-doped NiOx targets were

achieved with the same preparation conditions for pure NiOx target. For the Cu doped NiOx powders,

the XRD patterns showed an obvious shift to the lower angle with the increased Cu doping

concentrations (Figure S1b), indicating that the Cu ions were successfully doped into NiOx lattice.

At the same time, the intensities of XRD peaks decreased as the doping concentrations increased

due to the lattice deformation resulted from Cu doping.

For the film deposition, firstly the FTO substrates (14 Ω sq−1) were ultrasonic cleaned with

detergent, and then consecutively washed by deionized water, absolute ethyl alcohol, acetone and

isopropanol for 0.5 h, then FTO substrates were dried by blowing with high-purity (99.99%) N2.

The NiOx (Cu:NiOx) films were deposited on the clear FTO substrates by pulsed laser deposition

(PLD) at room temperature, the oxygen partial pressure controlled at 11.0 Pa was favorable to form

intrinsic p-type NiOx films under oxygen-rich atmosphere, and then as the prepared NiOx (Cu:NiOx)

films were annealed at 500 oC in the air for 1 h for improving the crystalline qualities and enhancing

the optical transmittance of the films. The laser energy was set to 300 mJ with a 5 Hz pulse frequency,

and the distance between the target and substrate was about 5 cm.

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1.3 Fabrication of Perovskite Solar Cells (PSCs)

The prepared NiOx films were firstly treated with oxygen plasma for 30 s to increase the

wettability for further deposition. Perovskite precursor solution was mixed with 37.1 mg FAI, 172

mg MAI, 581 mg PbI2, and 39 mg PbCl2 (molar ratio = 0.27:1.08:1.26:0.14) with desired molar

ratio and dissolved in a mixture of DMSO (300 μL) and GBL (700 μL) solution. Then the prepared

FA0.2MA0.8PbI3-xClx perovskite precursor solution was stirred for at least 12 hours at 70 ºC before

using. The perovskite precursor solution was spin-coated on the NiOx films deposited substrates at

1000 rpm for 20 s and 3500 rpm for 45s, and 1000 μL methylbenzene was injected onto the center

of spinning perovskite films as an antisolvent during the last 15 s, and then the perovskite films

were annealed at 105 ºC on a hot plate for 6 min. Sequentially, Phenyl-C61-Butyric-Acid-Methyl-

Ester (PCBM) solution (23 mg/mL in chlorobenzene) as electron transport layer was spin-coated

onto the top of the perovskite layer at 2500 rpm for 40 s, following by an annealing at 105 ºC for 8

min. After that, 0.6 mg/mL RhB101 solution was spin-coated at 1500 rpm for 40 s. Finally, the 1.2

nm LiF and 100 nm Ag electrodes were sequentially thermally evaporated through a shadow mask

under an ultra-low vacuum degree (7.5× 10-5 Pa). The active area of the solar cell was 0.11 cm2.

2. Device Characterization

The current-voltage curves of perovskite solar cells were measured using a Sourcemeter (USA

Keithley 2400) under AM1.5G irradiation with a power density of 100 mW/cm2 from a solar light

simulator, which was calibrated by a standard silicon cell. The forward (-0.1 to 1.2 V) and reverse

(1.2 to -0.1 V) scan ranges were applied with a delay time of 20 ms. All of the PSC devices had no

encapsulation and were tested in ambient air. Incident photon-to-current density efficiency (IPCE)

was recorded on a computer-controlled solar cell quantum efficiency measurement system (Newport,

USA) with data collected at DC mode. XRD patterns were performed by an X’Pert PRO

diffractometer (PANalytical). The transmittance and absorption spectras were recorded by an UV-

vis spectrophotometer (Shimadzu UV3600) with the range from 300 to 900 nm. A hall measurement

system (775 HMS Matrix Lake Shore) was used in the Vander-Pauw mode to measure the room

temperature conductivity, carrier concentration and mobility. X-ray photoelectron spectroscopy

(XPS, Kratos AXIS UltraDLD) with Al Kα radiation (1486 eV) was performed to investigate the

elemental composition and valence state, which were calibrated by C 1s peak position (284.8 eV).

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The work function and valence band level were calculated by the data collected from an Ultraviolet

photoelectron spectrum (UPS ESCALAB 250Xi, Thermo Fisher) with He source of incident energy

of 21.21 eV. Atomic force microscope (AFM MFP-3D-Stand Alone) was employed to investigate

the film surface morphology and conductivity. The surface morphologies and energy-dispersive X-

ray spectroscopy (EDS) of the films were obtained from Environmental Scanning Electron

Microscope (ESEM Quattro S). The cross-section and corresponding EDS of the Cu-doped NiOx

films were performed by a Focused Ion Beam Scanning Electron Microscope (FIB-SEM) workhorse

(Zeiss Crossbeam 540). The steady-state photoluminescence and time-resolved photoluminescence

were collected from a fluorescence spectrophotometer (FLS1000 Edinburgh Instruments Ltd.) with

an excitation wavelength of 470 nm.

3. Calculation method

The Vienna Ab-initio Simulation Package (VASP) with the framework of plane wave method

was used to calculate the electronic structure and density of state (DOS) of Cu-doped NiO.1 The

projector-augmented wave (PAW) method together with Perdew-Burke-Erzernhof (PBE)

exchange-correlation function was used.2-3 The cut-off energy was set to 500 eV, and the Hellman-

Feynman forces acting on each atom was less than 0.01 eV/Å. The PBE+U method with Ueff = 4 eV

on Ni 3d orbitals was adopted to deal with the underestimation of electron localization effects. NiO

has a cubic structure with Fm-3m space group, the lattice parameter for NiO was calculated to a =

4.18 Å,4 which agreed with the experimental value of a = 4.17 Å.5 The supercell contained 64 Ni

and 64 O atoms. Due to the nonstoichiometric NiOx, we constructed the supercell of Ni62O64

containing 2 VNi defects. For the 3% Cu doped NiOx, we constructed the supercell of Cu2Ni61O64,

in which one Cu atom substituted Ni2+ site and one Cu atom occupied VNi site, and one VNi defect

existed.

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Figure S1. (a) XRD patterns and (b) enlarged high resolution XRD patterns of Cu-doped NiOx

powders with different doping concentrations (0%, 1%, 3%, 5%).

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Figure S2. (a) The cross-section SEM images of NiOx film deposited on FTO substrate, and (b) the

corresponding EDS spectra, inset is the electron image.

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Figure S3. The carrier concentration and mobility of Cu:NiOx films as different Cu/Ni ratios (0, 1%,

3% and 5%).

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Figure S4. Conductive atomic force microscopy (c-AFM) characteristics of (a) 0%, (b) 1%, (c) 3%,

and (d) 5% Cu:NiOx on FTO substrates.

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Figure S5. High resolution O 1s XPS spectras for (a) 0%, (b) 1%, (c) 3%, and (d) 5% Cu:NiOx

films.

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Figure S6. Atomic model of (a) NiOx and (b) 3% Cu:NiOx. The corresponding total DOS of (c)

NiOx and (d) 3% Cu:NiOx. The Fermi level for the calculation is set to zero.

The density of state (DOS) of the undoped NiOx (Ni62O64) and 3% Cu:NiOx (Cu2Ni61O64) are

shown in Figure S6. The atomic model of undoped NiOx contains two Ni vacancies, which are

located as far from each other as possible and doped NiOx contains two Cu atoms, in which one Cu

atom occupies VNi site and one Cu atom substitutes lattice Ni2+ site, respectively (Figures S6a-b).

Figures S6c-d indicate that the both Ni vacancies and Cu dopants create gap states close to the

valence bands in the band gap which can act as acceptor levels for providing hole carriers in the

system. Furthermore, the gap states created by Cu dopant are closer to valence band maximum than

the VNi-induced gap states, which reflects that the Cu dopant plays the more important role than VNi

defect for improving the conductivity of Cu-doped NiOx films.6

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Figure S7. (a) SEM images of the 3% Cu:NiOx films and the Energy-dispersive X-ray spectroscopy

(EDS) element mapping of the corresponding elements (O, Ni, Cu). (b) EDS spectra of 3% Cu:NiOx

films.

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Figure S8. SEM images of (a) 1% and (b) 5% Cu:NiOx films deposited on FTO. Scale bars are 100

nm. AFM images of (c) 1% and (b) 5% Cu:NiOx films deposited on FTO. The scan areas are 5 μm

× 5 μm.

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Figure S9. Top view SEM images of perovskite films grown on the underlayer (a) NiOx and (b) 3%

Cu:NiOx HTLs.The scale bars are 500 nm.

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Figure S10. The structure of inverted planar PSCs with NiOx (Cu:NiOx) HTLs.

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Figure S11. Ultraviolet photoelectron spectra (UPS) of the NiOx and 3% Cu:NiOx films deposited

on FTO substrate. Fermi level was set to 0.

The work functions of NiOx and Cu:NiOx films are calculated by the formula WF = 21.2- (Ecutoff-

EF)7, the work function of NiOx after Cu doping increases from 4.71 eV to 4.76 eV, indicating higher

open-voltage for inverted PSCs, which are attributed to the introduction of Cu+ ions. Favorable

decrease in the valence band level of Cu:NiOx (-5.33 eV) comparing to NiOx (-5.26 eV) result in

better energy alignment with perovskite layer and the improved charge transport ability.

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Figure S12. Statistic distributions of (a) Voc, (b) Jsc, (c) FF, and (d) PCE for 25 devices based on

NiOx and Cu:NiOx HTLs.

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