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Highly Efficient, Solution-processed CsPbI2Br Planar Heterojunction Perovskite Solar Cells via Flash Annealing Yaxin Gao, ya'nan dong, Keqing Huang, Chujun Zhang, Biao Liu, Shitan Wang, Jiao Shi, Haipeng Xie, Han Huang, Si Xiao, Jun He, Yongli Gao, Ross A Hatton, and Junliang Yang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00783 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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ACS Photonics

Highly Efficient, Solution-processed CsPbI2Br Planar Heterojunction Perovskite Solar Cells via Flash Annealing

Yaxin Gao,a,† Yanan Dong,a,† Keqing Huang,a Chujun Zhang,a Biao Liu,a Shitan Wang,a Jiao Shi,a Haipeng Xie,a Han Huang,a Si Xiao,a Jun He,a Yongli Gao,a,b Ross A. Hatton,c Junliang Yang a *

a

Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of

Physics and Electronics, Central South University, Changsha 410083, China. b

Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627,

USA. c

Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK

† These authors contributed equally to this work. * Corresponding author, Email: [email protected] (J. L. Yang) Tel.: +86-731-88660256

1

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ABSTRACT Highly efficient CsPbI2Br planar heterojunction (PHJ) perovskite solar cells (PSCs) with a structure of ITO/SnO2/CsPbI2Br/Spiro-OMeTAD/Ag are fabricated via low-temperature solution process. A flash annealing technique is used to produce high-quality CsPbI2Br perovskite films with high density and uniformity, as well as highly crystallized CsPbI2Br films. These CsPbI2Br films are used as the light harvesting layer in PHJ PSC devices using SnO2 as the electron transport layer (ETL) and Spiro-OMeTAD as the hole transport layer (HTL). Based on the measurement of the energy levels via ultraviolet photoelectron spectroscopy, it indicates that there is a very good interfacial band alignment between SnO2 and CsPbI2Br, supporting the efficient electron extraction from CsPbI2Br to SnO2. Thus, the simple-structural CsPbI2Br PHJ PSCs with power conversion efficiencies (PCE) up to 13.09% is achieved, which is comparable to the highest reported PCEs for all-inorganic PHJ PSCs. The findings show that the PHJ device architecture, instead of mesoporous structure, can be used to fabricate highly efficient inorganic PSCs with a simple structure, offering the advantages of well matching with low-cost, high throughout roll-to-roll (R2R) printing process.

Keywords: inorganic perovskite solar cells; planar heterojunction; interface; morphology; flash annealing

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INTRODUCTION Hybrid organic-inorganic halide perovskites are spotlighted as the promising candidates for efficient solar energy harvesting over the past 9 years,1-3 and the certified power conversion efficiency (PCE) of perovskite solar cells (PSCs) has reached 23.3%.4 The perovskite materials exhibit desirable properties such as strong optical absorption, very small exciton binding energy, ambipolar characteristic, good carrier mobility and lifetime, and long-rang diffusion length, which are advantageous for the fabrication of high-efficiency solar cells and other optoelectronic devices.5-7 Improvements in device structure,8 perovskite deposition techniques9-14 and compositional engineering15,16 further accelerate the great enhancement of photovoltaic performance and stability. Methylammonium (MA) and formamidinium (FA) are widely used in a hybrid organic-inorganic lead halide perovskite framework. However, organic cations in hybrid perovskites can give rise to undesirable thermal instability which is a serious drawback for long-term stably working device. For example, MA lead triiodide (MAPbI3) films show thermal decomposition at the temperature over 85oC.17,18 FA-based perovskites exhibit better thermal stability than MA-based perovskites and partial cation substitution with caesium (Cs) imparts further improvements in FA-based perovskites, it can remain both thermal and structural stability over 100 °C.16,19 Thus, it is expected that inorganic cation entirely substituting organic component in perovskites is possibly a way for achieving the thermally compositional stability.20 The Cs lead halide perovskites were first reported in 1893, and are gathering much interest since the discovery of PSCs.21 The PCE of inorganic PSCs based on CsPbI3 quantum dots has reached 13.4%,22 while CsPbI2Br based PSCs yield a highest PCE of 13.3%.23 Very recently, a PCE up to 14.4% is achieved using CsPbI2Br and CsPbI3 quantum dots to form a graded bandgap,24 and a record of 15.7% is achieved based on α-phase CsPbI3 film via solvent-controlled growth in a dry environment.25 As an alternative of hybrid organic-inorganic light absorber, Cs halide perovskite shows outstanding thermal stability. However, inorganic PCS devices 3


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remain much challenging. Cs lead bromide (CsPbBr3) was reported in solar cells,26 but its bandgap is too large (up to 2.3eV) to absorb photons with long wavelengths. CsPbBr3 is compositionally stable up to the melting point beyond 460 °C. At room temperature, CsPbBr3 crystallizes in an orthorhombic phase, and it turns into tetragonal perovskite phase at 88 °C, then cubic perovskite phase at 130 °C.26 While Cs lead iodide (CsPbI3) has a bandgap of 1.73eV, and it is more suitable for solar energy conversion. Yet, it suffers from phase instability. At room temperature, CsPbI3 is stable in an orthorhombic non-perovskite structure (yellow phase), and it turns into cubic perovskite (black phase) while heating over approximately 300 °C. The cubic perovskite phase is not stable at ambient temperature, and it is easy to transit to the non-perovskite yellow phase rapidly. Its tolerance factor is barely over 0.8, and a Cs cation doesn’t have enough size for holding PbI6 octahedra. To overcome the perovskite phase instability, partial substitution of cation and incorporation of bromine into CsPbI3 are reported. For example, potassium incorporation in Cs lead halide PSCs to produce higher efficiency, better photo-excited charge carrier formation and transportation, as well as greatly improved phase stability.27 Moreover, well-functioning Cs lead mixed-halide (CsPbI3-xBrx) PSCs can realize excellent phase stabilization, which has been reported by several groups very recently.28-30 The results suggest that CsPbI2Br with a reasonable bandgap of 1.91eV is a stable and efficient light-conversion material. The inorganic metal oxide SnO2 has been reported as a superior electron transport layer (ETL) in efficient inorganic-organic hybrid planar heterojunction (PHJ) PSCs.31,32 Its peculiarities of deep conduction band and wide bandgap result in a more reasonable interfacial band alignment and a superior electronic contact between ETL and perovskite. Moreover, its electron mobility is very high and can form an efficient electron transfer and transport. These outstanding characteristics make SnO2 a promising and attracting material for high performance photovoltaic devices. Herein, highly efficient CsPbI2Br PSCs are fabricated via one-step solution process and flash annealing

technique

with

constructing

a

simple

PHJ

structure

of

ITO/SnO2/CsPbI2Br/Spiro-OMeTAD/Ag (Figure 1a), in which the SnO2 acts as the 4


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ETL and Spiro-OMeTAD acts as the hole transport layer (HTL). Because of the high-quality CsPbI2Br perovskite films with excellent denseness, uniformity and high crystallization, PHJ PSCs with a PCE up to 13.09% are achieved, which is one of the highest PCEs for all-inorganic Cs lead halide PSCs.

RESULTS AND DISCUSSION The schematic of PHJ PSCs with a structure of ITO/SnO2/CsPbI2Br/SpiroOMeTAD/Ag is shown in Figure 1a. The low-temperature, solution-processed SnO2 is used to substitute normally used TiO2 for simplifying the fabrication procedures of PSCs with excellent performance. Figure 1b shows the diagrammatic crystal structure of CsPbI2Br perovskite with a chemical formula of ABX3, in which the A site is occupied by Cs+ cation, B site is occupied by divalent cations of Pb2+, and X site is occupied by halide anions (I- or Br-). The crystal structure is the desired perovskite cubic phase with a space group of Pm3m, exhibiting the framework of octahedrons with Cs+ cation located between them.

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Figure 1. (a) Schematic of PHJ-PSCs with a structure of ITO/SnO2/CsPbI2Br/Spiro-OMeTAD/Ag. (b) Crystal structure of CsPbI2Br inorganic perovskite. (c-d) AFM images of SnO2 film deposited on ITO glass and CsPbI2Br film deposited on SnO2 layer, respectively. (e) Top-view SEM image of CsPbI2Br film. (f) Cross-sectional SEM image with a stack structure of ITO/SnO2/CsPbI2Br.

The AFM images of SnO2 film deposited on ITO and CsPbI2Br perovskite film deposited on SnO2 layer after annealing process are shown in Figure 1c-d. The root mean square (RMS) roughness values are measured to be about 5.0 nm and 16.0 nm for the scanning area of 20 µm×20 µm, respectively. Figure 1c shows a uniform and dense SnO2 film, which serves as the ETL and would be beneficial for the charge 6


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transfer. The X-ray diffraction (XRD) patterns and transmission spectrum of SnO2 film deposited on ITO substrate are shown in Figure S1 and Figure S2, separately. It can be seen that SnO2 film shows superior transparency, and the transmission value can exceed 97% in the most of visible region. Transmission spectrum of ITO/SnO2 with ITO as a reference is shown in Figure S3, which indicates that the SnO2 layer can enhance the light transmission in a narrow wavelength range and minimize the optical losses.31 The AFM image of CsPbI2Br film deposited on SnO2 layer shown in Figure 1d reveals the perovskite film morphology, which looks dense and uniform as well, although the RMS roughness values is larger than that of SnO2 layer. As compared with organic perovskites, normally a much higher annealing temperature process, for example, larger than 300 °C, is required to support the phase transition of inorganic Cs lead iodide perovskite from orthorhombic non-perovskite phase to cubic black perovskite phase.33 Meanwhile, the phase transition temperature of Cs lead halide perovskites decreases as the content of Br increases.27 The optimized flash annealing process, i.e., 240 °C for 10 s, is developed to fabricate the desired CsPbI2Br black phase. As shown in Figure S4, the film is dark brown. The optimization data, including XRD patterns of perovskite films, J-V curves and the detailed photovoltaic parameters of fabricated PHJ PSCs via the thermal annealing at the different conditions, are shown in Figure S5 and Table S1. The top-view scanning electron microscopy (SEM) image of CsPbI2Br film is shown in Figure 1e. The HTL-free morphology is very uniform, and the crystal domain size varies from about 100 nm to 800 nm. The highly crystalline, uniform CsPbI2Br perovskite film is surely beneficial for photo-induced charge carrier generation and transport, indicating the good quality is achieved from flash annealing. The SEM image of SnO2 film deposited on ITO substrate is shown in Figure S6 as well, which is dense and pinhole-free. The cross-sectional SEM image of ITO/SnO2/CsPbI2Br is presented in Figure 1f, indicating the thicknesses of SnO2 and CsPbI2Br layers are approximately 45 nm and 210 nm, respectively, while the ITO is 150 nm. Besides, the vertical perovskite is also well-distributed. The optical absorbance spectra of the black phase CsPbI2Br film is recorded 7


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using UV-vis spectroscopy, as shown in Figure 2a. The spectrum exhibits an absorbance onset at 627 nm, and the absorption band edge is at 650 nm, indicating an optical bandgap about 1.91 eV, which can also be derived from the Tauc Plot, as shown in inset in Figure 2a. X-ray diffraction (XRD) pattern of the black phase CsPbI2Br film is shown in Figure 2b. The peaks at 14.9° and 29.8° can be assigned to the (100) and (200) planes, respectively.28,29 The pattern does not exhibit other more peaks corresponding to the intermediate yellow phase or PbI2, indicating that the CsPbI2Br film fabricated by flash annealing is highly crystallized and predominantly consisted of cubic black perovskite phase. The results indicate that high-quality perovskite films could be achieved by flash-annealing technique. As shown in Figure 2c, the steady photoluminescence (PL) decay profiles of CsPbI2Br films deposited on ITO glass and SnO2 film reveal the charge carrier injection changes because a significant PL quench is observed, which show an emission peak at 650 nm. When depositing perovskite on ITO glass, a relatively high photoluminescence

peak

is

observed,

indicating

a

predominant

radiative

recombination behavior and illustrating few defects in the perovskite films. It is beneficial for charge transportation and the reduction of non-radiative recombination. While depositing perovskite layer on SnO2 film, the peak intensity is much lower, demonstrating that the charge carriers could be effectively extracted to the electron transport layer. The time-resolved photoluminescence (TRPL) result is shown in Figure 2d, in which the curves are fitted with a bi-exponential function below to determine the decay times.32,33 y=A1*exp (-t/τ1) + A2*exp (-t/τ1) + y0

(1)

Ai is the decay amplitude, τi is the decay time, τave is the average PL decay time, and the derived parameters are listed in Table 1. The τave is calculated by the formula: τave = ∑ iτi

(2)

The average PL decay time of CsPbI2Br layer is significantly reduced as it is deposited on SnO2 film, i.e., from 4.99 ns to 2.39 ns, indicating an efficient electron transfer at the SnO2/perovskite interface, which is beneficial for suppressing charge 8


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recombination. The evident changes of PL intensity and charge carrier lifetime of perovskite layer mean that electrons can be efficiently transferred into the SnO2 layer, which could give credit to reduced energy barrier as shown by the results of ultraviolet photoelectron spectroscopy (UPS) below.

(a)

(b)

3.0 CsPbI2Br

CsPbI2Br film

Intensity (a.u.)

(200) (α hν) 2

Absorbance

2.5 2.0 1.5 1.0

1.86 1.88 1.90 1.92 1.94 1.96 1.98 2.00

Energy (eV)

0.5

(100)

CsPbI2Br film

0.0 400

450

500

550

600

650

700

750

10

800

15

20

(c)

25

30

35

40

2θ (degree)

Wavelength (nm)

(d) PL intensity (norm.)

Perovskite SnO2/Perovskite

PL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

0

10

perovskite SnO2/perovskite

-1

10

-2

10

-3

10 575

600

625

650

675

700

725

750

0

3

6

9

12

15

18

Time (ns)

Wavelength (nm)

Figure 2. (a) UV-vis absorbance spectrum and Tauc plot for CsPbI2Br films. Inset is the Tauc plot. (b) XRD pattern of CsPbI2Br films fabricated via flashing annealing at 240 °C for 10 s. (c) Typical PL decay profiles of CsPbI2Br films deposited on glass (black) and SnO2 (red), respectively. (d) TRPL decay profiles of CsPbI2Br films deposited on glass (black) and SnO2 (red), respectively.

Table 1. Fitted parameters for time-resolved PL of CsPbI2Br film and SnO2/CsPbI2Br film. Samples

τ1(ns)

A1(%)

τ2(ns)

A2(%)

τave(ns)

CsPbI2Br

5.47

87.21

1.74

12.79

4.99

SnO2/CsPbI2Br

2.61

75.48

1.73

24.52

2.39 9


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The interface electronic states between SnO2 and CsPbI2Br are characterized using UPS. The work function (Wf) values of SnO2 and perovskite CsPbI2Br are calculated to be 4.0 eV and 3.86 eV (Figure 3a), respectively, resulting from Wf = 21.22 eV － Binding energy.30 The valence band maximum (VBM) values of SnO2 and perovskite layer are measured to be 3.77 eV and 1.82 eV, as shown in Figure 3b. Based on semiconductor band structure of Ec = Wf + VBM - Eg,34 the conduction band (Ec) values of SnO2 and CsPbI2Br are calculated to be 3.98 eV and 3.77 eV, respectively. Thus, the possible band diagram of CsPbI2Br-based PHJ PSCs is exhibited in Figure 3c. It is obvious that the electrons can be easily extracted from the CsPbI2Br conduction band to SnO2 conduction band, exhibiting a reasonable interfacial band alignment. While the holes could be transferred efficiently from the valence band of CsPbI2Br to the valence band of Spiro-OMeTAD, and it should contribute to the relatively high current density. The chemical composition of CsPbI2Br film is evaluated by X-ray photoelectron spectroscopy (XPS), and it is basically consistent with the molecular formula, of which the atomic ratios of Pb/Cs and Br/I are 1:1.02 and 1:1.86, respectively. The detail parameters and analysis are shown in Table S2. Meanwhile, Figure S7 shows the XPS spectra for each element of CsPbI2Br, and the C 1s peak is set to be 284.8 eV. The peak positions at binding energy of 725.1 eV, 138.8 eV, 619.6 eV and 68.9 eV are corresponded to Cs 3d, Pb 4f, I 3d and Br 3d, respectively.34,35 Figure S8 shows the Sn 3d and O 1s core level spectra of SnO2 layers, and they are similar to Figure S7, indicating that there are not additional chemical states.

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Figure 3. (a) UPS cutoff edge of SnO2 and CsPbI2Br layer. (b) Valence band spectra of SnO2 and CsPbI2Br layer from UPS measurements. (c) Energy band diagram of CsPbI2Br-based PHJ PSC devices.

The high-quality CsPbI2Br film achieved by flash annealing is used to fabricate PHJ PSCs with a simple structure of ITO/SnO2/CsPbI2Br/Spiro-OMeTAD/Ag. The photocurrent density-voltage curves of typical PSCs are shown in Figure 4a, exhibiting a maximum efficiency of 13.09%, with a VOC of 1.06 V, a JSC of 15.99 mA cm-2 and an FF of 77.12%. Furthermore, a forward bias at 0.84 V under AM 1.5 simulated sun light is added, and the photocurrent rise quickly to the maximum value. It yields a current density of 14.97 mA cm-2 and the stabilized power output (SPO) is demonstrated to be ~12.6 % (Figure 4b), manifesting that the devices are stable and 11


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the efficiency result is credible. It’s necessary to point out that a little hysteresis from the different scan directions can be observed in CsPbI2Br PHJ PSCs, as shown in Figure S9. Normally, the origin of hysteresis comes from ion migration and capacitive effect.36-38. In order to alleviate the hysteresis behavior resulted from the ion migration, compositional engineering could be applied to precisely control the stoichiometric composition of perovskite for achieving a lower density of ionic vacancies. Meanwhile, the passivation is an effective method to constrain the pathways of ionic migration for achieving less defective grain boundaries and lower density of defects, as well as inhibiting the mobile ions.39,40 In addition, a suitable charge extraction layer or interfacial modification could improve charge extraction efficiency at the interface and passivate the interface ionic charges, which could suppress the interfacial electronic dipole polarization and ionic accumulation, reduce capacity effect and suppress or eliminate hysteresis behavior.41 Thus, the hysteresis in CsPbI2Br PHJ PSCs would be further alleviated or fully eliminated by compositional and interface engineering, such as surface passivation or interface modification. Furthermore, the incident-photon-to-current conversion efficiency (IPCE) spectra is measured and shown in Figure 4c, where the integrated JSC is calculated to be 14.84 mA cm-2, which is approximately 93% of the JSC values straightly from the J-V scans (Figure 4a). The aberration may be attributed to the miscellaneous factors like the ambient atmosphere condition with the humidity influence. Both the J-V and IPCE measurements are proceed in ambient conditions with the relative humidity of about 45%. It is known that CsPbI2Br perovskite would transform to the yellow phase rapidly as the relative humidity is at about 50%.28,33 Thus, it is reasonable that the integrated JSC is a little smaller than the value measured from the J-V scan. The statistical data of PCEs obtained from 25 PHJ PSC devices are shown Figure 4d. The obtained devices are well reproducible with a narrow distribution range at the average value of 12.12%. The detailed photovoltaic parameters of each cell are shown in Table S3 in the supporting information, and the summarized statistics diagrams of JSC, VOC, FF and PCEs for CsPbI2Br PSCs are shown in Figure S10. The CsPbI2Br based solar cell shows good stability, it can retain the efficiency of ~12% after storing in nitrogen 12


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glove box for 20 days without any encapsulation, maintaining 96% of initial efficiency, as shown in Figure S11. 14

(a)

-2

15

(b)

12 10

12

SPO (%)

Current Density (mA cm )

18

9

reverse scan -2

VOC = 1.06 V JSC = 15.99 mA cm FF = 77.12 % PCE = 13.09 %

6

8 6 4 2

3

0 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

100

Voltage (V)

12

60

9

40

6

20

3

0 300

0 400

500

600

700

8

Numbers

(c)

-2

15

Integrated Jsc (mA cm )

80

200

300

400

500

Time (s) 10

100

IPCE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

(d)

6

4

2

0 10

Wavelength (nm)

11

12

13

14

PCE (%)

Figure 4. (a) J-V curve for a typical CsPbI2Br PHJ PSC device. (b) Stabilized power output (SPO) as a function of time held at 0.84 V forward bias under one sun illumination. (c) IPCE spectra and integrated JSC for the corresponding CsPbI2Br device. (d) Statistical PCEs of 25 CsPbI2Br PHJ PSC devices.

CONCLUSION In summary, CsPbI2Br perovskite films and consequently PHJ PSC devices are fabricated using flash annealing and one-step solution process. The high-quality desired black-phase CsPbI2Br films with good density, uniformity and high crystallization could be achieved. Furthermore, high-efficiency CsPbI2Br PHJ PSCs are fabricated with a simple architecture of ITO/SnO2/CsPbI2Br/Spiro-OMeTAD/Ag. The SnO2 layer acts as the ETL in CsPbI2Br-based PSCs, resulting in a remarkable improvement in JSC due to the good energy level positions and efficient charge extraction. Thus, the PCE up to 13.09 % is achieved with high reproducibility, which is one of the highest PCEs for inorganic PHJ PSCs. This work demonstrates that 13


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highly efficient and reproducible inorganic CsPbI2Br PSCs could be achieved via constructing simple PHJ using one-step solution process and flash annealing treatment.

EXPERIMENTAL SECTION Materials All materials were used directly without any purification, including: lead iodide (PbI2, 99.99%, Xi'an Polymer Light Technology Corp.), Cs bromide (CsBr, 99.9%, Alfa Aesar), N, N-dimethylformamide (99.8%, J&K Scientific), Dimethyl sulfoxide (99.9%, J&K Scientific), tin oxide (SnO2, tin(IV) oxide 15% in H2O colloidal dispersion,

acetonitrile,

isopropanol

(IPA),

Alfa

Aesar),

2,20,7,70-tetrakis-(N,N-di-4-methoxyphenylamino)-9,90-spirobifluorene

(Spiro-

OMeTAD, 99%, Wuhan Zhuojia Technology Co., Ltd), chlorobenzene (CB, 99.8%, J&K

Scientific),

lithium

bis(trifluoromethanesulfonyl)imide

(Li-TFSI),

4-tert-butylpyridine (4-TBP).

Device Fabrication The fabrication scheme and structure of PHJ PSCs are exhibited in Figure 1. Patterned indium tin oxide (ITO) glass was used as the initial substrate, and was ultrasonically cleaned with acetone, detergents/H2O, distilled water, and isopropyl alcohol for 20 min sequentially, and then dried using high purity N2 flow. The substrate was treated with UV-ozone for 20 min afterwards. The SnO2 colloid precursor was diluted from 15% to 2.67% by distilled water and was spin-coated on glass/ITO substrate at the speed of 3000 rpm for 30 s, followed by a post-annealing procedure at 150 °C for 15 min. Then the substrates were loaded into a N2-filled glovebox. To form the 0.5M CsPbI2Br precursor solution, CsBr and PbI2 were dissolved in DMF and DMSO (the volume ratio DMF: DMSO = 4:1) in a 1:1 molar ratio. It was vigorously stirred at 65 °C for 12 hours and filtered with a 0.22 µm PVDF filter before the use. Then, the CsPbI2Br precursor solution was spin-coated onto 14


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SnO2-coated ITO substrates at 2000 rpm for 30 s to prepare the perovskite layer, followed by a post-annealing procedure at the temperature of 240 oC for 10 s on a hot plate in the glovebox. During the heat treatment, the color of the films changed from transparent light yellow to transparent dark brown. Spiro-OMeTAD was dissolved in chlorobenzene to prepare a 90 mg mL-1 solution with additive of 45 ul Li-TFSI/acetonitrile (170 mg mL-1) and 10 ul tBP. After the substrates cooling to room temperature, Spiro-OMeTAD solution was spin-coated on top of the perovskite layer at 3000 rpm for 30 s to form hole transport layer. Finally, a 100 nm Ag back electrode was deposited by thermal evaporation with a mask at a constant rate of 0.1 nm s-1, resulting in an active area of 0.09 cm2.

Characterization The UV-vis spectrophotometer (Puxi, T9, China) and X-ray diffraction (XRD, Rigaku D, Max 2500, Japan) were used to characterize the absorption properties and crystallographic properties of CsPbI2Br thin films, respectively. The root-mean-square (RMS) roughness and morphologies of perovskite thin films were measured with an atomic force microscopy (AFM, Agilent Technologies 5500AFM/SPM System). Scanning electron microscope (FEI Helios Nano lab 600i SEM, USA) was employed to characterize surface morphology and cross-sectional appearances. Steady-state PL spectra were achieved using an intensified charge-coupled device detector (ICCD, DH334T-18U-03). Time-resolved PL was achieved through

time-correlated

single-photon counting (TCSPC, MS3504I) measurements. Ultraviolet photoelectron spectroscopy (UPS, He I, 21.22 eV) and X-ray photoelectron spectroscopy (XPS, Al Kα X-ray source, 1486.6 eV) were employed to investigate the detailed energy-level and element distribution. Current density-voltage (J-V) characteristics of PHJ-PSCs were measured by a digital Source Meter (Keithley, model 2420, USA), and the solar simulator (Newport 91160s, AM 1.5G, USA) was calibrate using standard silicon simulator with the standard value 100mW cm-2.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD pattern and transmission spectrum of SnO2 film; Transmission spectrum of ITO/SnO2 with ITO as a reference; Photograph of CsPbI2Br film; XRD patterns, J-V curves, and detailed parameters of CsPbI2Br films fabricated via thermal annealing at the different conditions; SEM image of SnO2 films; Detailed parameters deduced from XPS analysis; XPS spectra of CsPbI2Br film and SnO2 films; Hysteresis behavior of CsPbI2Br-based solar cells; Statistic data for 25 devices; Stability measurement; Detailed photovoltaic parameters of 25 devices.

AUTHOR INFORMATION Corresponding author *Email: [email protected]

ORCID Junliang Yang: 0000-0002-5553-0186

Author Contributions †

Y.X.G. and Y.N.D. contributed equally to this work. J.L.Y. conceived the idea. Y.X.G.

and Y.N.D. performed the fabrication and characterization on PHJ PSCs. Y.X.G., Y.N.D., K.Q.H., B.L. and J.L.Y. analyzed all the performance of perovskite films and PHJ PSCs, as well as wrote the manuscript. C.J.Z. conducted and analyzed the SEM measurements. T.S.W, S.X. and J.H. conducted and analyzed the PL lifetime measurements. J.S. and H.H. conducted AFM measurements. H.P.X. and Y.L.G. conducted and analyzed the UPS and XPS measurements. R.A.H. discussed the experiment results and revised the manuscript. J.L.Y. directed the overall research plan. All the authors read and commented on the manuscript.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51673214) and the National Key Research and Development Program of China (2017YFA0206600). Y.L.G. acknowledges the support by National Science Foundation CBET-1437656.

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For Table of Contents Use Only

Highly Efficient, Solution-processed CsPbI2Br Planar Heterojunction Perovskite Solar Cells via Flash Annealing

Yaxin Gao,a,† Yanan Dong,a,† Keqing Huang,a Chujun Zhang,a Biao Liu,a Shitan Wang,a Jiao Shi,a Haipeng Xie,a Han Huang,a Si Xiao,a Jun He,a Yongli Gao,a,b Ross A. Hatton,c Junliang Yang a *

Highly efficient CsPbI2Br planar heterojunction (PHJ) perovskite solar cells (PSCs) with a structure of ITO/SnO2/CsPbI2Br/Spiro-OMeTAD/Ag are fabricated via low-temperature solution process. A flash annealing technique, i.e., 240 °C for 10 s, is used to produce high-quality CsPbI2Br perovskite films with high density and uniformity, as well as highly crystallized CsPbI2Br films. This simple-structural CsPbI2Br PHJ-PSCs with power conversion efficiencies (PCE) up to 13.09% is achieved, which is comparable to the highest reported PCEs for all-inorganic PHJ PSCs. The findings show that the PHJ device architecture, instead of mesoporous structure, can be used to fabricate highly efficient inorganic PSCs with a simple structure, offering the advantages of well matching with low-cost, high throughout roll-to-roll (R2R) printing process.

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