Effect of substrate temperature on F and Al co-doped ZnO films deposited by radio frequency magnetron sputtering

Solar Energy 194 (2019) 471–477

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Effect of substrate temperature on F and Al co-doped ZnO films deposited by radio frequency magnetron sputtering

T

Yanfeng Wanga,1, Jianmin Songb,1, Jie Zhangc,d, Guoxi Zhenga, Xiaochen Duana, Xicheng Xiea, ⁎ ⁎ Bing Hana, Xudong Menga, Fu Yanga, Guangcai Wangc,d, , Ying Zhaoc,d, Junjie Lia, a

College of Sciences, Hebei North University, Photovoltaic Conductive Film Engineering Research Center of Hebei Province, Zhangjiakou 075000, China College of Sciences, Agriculture University of Hebei, Baoding 071001, China c Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, Tianjin 300350, China d Key Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin, Tianjin 300350, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Transparent conductive films F and Al co-doping ZnO film Magnetron sputtering Broadband spectral transmittance Thin film solar cells

In this study, F and Al co-doped ZnO (FAZO) films were fabricated by radio frequency (RF) magnetron sputtering technique using an AlF3-doped ZnO ceramic target. The effect of substrate temperature (Ts) on the structural, morphological, elemental, chemical states, electrical and optical properties of the FAZO films were systematically studied by X-ray diffraction, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectrometry, and ultraviolet–visible–near infrared spectrophotometry. The results show that the doping F and Al did not change the structure of the ZnO, and all films show a typical wurtzite structure with c-axis preferred orientation. As the Ts value increases, the surface morphology of the film changed from “pyramid” shape to “crater” shape and to smooth and dense. The doping effect of F and Al gradually appeared with the increase in Ts. The FAZO film exhibited good performance with mobility of 29.92 cm2/Vs, carrier concentration of 2.84 × 1020 cm−3, resistivity of 7.35 × 10−4 Ω cm, and average transmittance of nearly 90% in the optical spectral range of 400–1400 nm at Ts = 440 °C. When the FAZO films were used as front electrode material for perovskite thin film solar cells, a higher conversion efficiency was obtained compared with that of the AZO film used as reference.

1. Introduction Zinc oxide (ZnO) films have attracted wide attention owing to their low cost, non-toxicity, and high transmittance of visible light and have gained a wide application in femtosecond laser, enhanced photoresponse of self-powered perovskite photodetector, Cu2O solar cells, and fluorescence imaging performance (Zang et al., 2016; Li et al., 2017; Zang, 2018; Zang and Tang, 2015). Following appropriate doping, the electrical conductivity can be noticeably improved; this has become an important research direction in recent years (Özgür et al., 2005). As an electrode material, it has been successfully used in thinfilm solar cells and other fields, and it is expected to be an alternative to indium tin oxide (ITO) and fluorine doped tin oxide (FTO) films (Mansfield et al., 2018; Liu et al., 2017). At present, commonly used doping elements are IIIA group materials, such as Al, Ga, and In (Sarma et al., 2019; Mahdhi et al., 2017; Djessas et al., 2014). The ZnO transparent conductive film (TCF) with a resistivity in the range of

~2–5 × 10−4 Ωcm and visible light transmittance of more than 80% can be easily prepared by increasing the doping ratio of Al (Ga) (Sarma et al., 2019). It is known that the increase in carrier concentration inevitably results in the enhancement of light absorption and reflection in the near-infrared region of the film, resulting in a reduced in transmittance (Agashe et al., 2003). In recent years, thin-film solar cells have drawn significant attention owing to their advantages of low raw material requirement, flexibility, and high conversion efficiency (Wang et al., 2018; Guo et al., 2018; Heinemann et al., 2016). As an electrode material for thin-film solar cells, TCFs are simultaneously required to have good conductivity and high transmittance in the spectral range of thin-film solar cells (Mansfield et al., 2018; Liu et al., 2017). In particular, wide-spectrum tandem solar cells require front electrode materials with excellent transmittance not only in the visible, but also in the near-infrared region (Liu et al., 2017). This presents a new challenge to the currently available ZnO–TCF: the method of increasing the carrier concentration in itself is not sufficient to satisfy the specific

⁎

Corresponding authors. E-mail addresses: [email protected] (G. Wang), [email protected] (J. Li). 1 Y. Wang and J. Song contributed equally to this study. https://doi.org/10.1016/j.solener.2019.09.095 Received 25 July 2019; Received in revised form 20 September 2019; Accepted 30 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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requirements (Coutts et al., 2000). Based on the relationship between resistivity, carrier concentration, and mobility, the electrical characteristics can also be improved by increasing the mobility. Furthermore, the improvement of mobility is also conducive to the transmission of TCF in the long wave region (Coutts et al., 2000). Therefore, to improve the carrier mobility in ZnO films has become an urgent problem. Generally, the doping metal elements replace the positions of Zn in the lattice structure and introduce an impurity level at the bottom of the conduction band, which scatters the electrons in the conduction band and restricts the improvement of carrier mobility (Wang et al., 2016). As a non-metallic element, fluorine (F) can be a substitute for O in ZnO film, which is located in the valence band of the energy band and introducing little disturbance to the conduction band, which is beneficial to prepare ZnO films with high mobility (Xu et al., 2005; Yoon et al., 2008). Xu et al. (2005) deposited F-doped ZnO (FZO) films with a mobility of 46.2 cm2/Vs, resistivity of 7.95 × 10−4 Ωcm, and carrier concentration of 1.73 × 1020 cm−3 by thermal oxidation of ZnF2. Yoon et al (Yoon et al., 2008) reported films with low resistivity of 3.6 × 10−3 Ωcm, Hall mobility of 30.5 cm2/Vs, and carrier concentration of 5.6 × 1019 cm−3 with post-vacuum annealing treatment of the magnetron sputtered FZO films. Although the mobility of the FZO films mentioned above has been improved, compared with Al-doped ZnO (AZO) films, their conductivity still needs to be improved owing to their low carrier concentration. Considering different F and Al doping mechanisms, the F and Al co-doped ZnO (FAZO) films was also studied. Wang et al. (Wang and Chang, 2016) prepared FAZO films with carrier concentration of 9.86 × 1020 cm−3, mobility of 22 cm2/Vs, and resistivity of 2.88 × 10−4 Ωcm using RF magnetron sputtering. In our previous experiment (Ji et al., 2019); a FAZO film with mobility of 39.33 cm2/Vs, carrier concentration of 4.50 × 1020 cm−3, and resistivity of 3.53 × 10−4 Ωcm were fabricated by RF magnetron sputtering using ZnF2 with low doping ratio and Al2O3-doped ZnO ceramic target. With post-annealing, the mobility, carrier concentration, and resistivity of the FAZO films were further improved to 53.97 cm2/Vs, 5.18 × 1020 cm−3, and 2.23 × 10−4 Ωcm, respectively (Wang et al., 2019). Compared with Al2O3 and ZnF2; codoping to ZnO to provide F and Al respectively, single element AlF3 doping can also achieve the same functionality. Furthermore, the preparation process of single-element doped targets is relatively simple. Therefore, FAZO films in this paper were deposited by AlF3-doped ZnO target using RF magnetron sputtering, and the effect of substrate temperature (Ts) on the electrical and optical characteristics of the ZnO film were systematically studied. Finally, an optimized FAZO and commercial AZO film were applied as electrodes in a perovskite thin film solar cell to verify their actual application performance.

Fig. 1. Schematic diagram of perovskite solar cell.

was spin coated with a two-step program at 1000 and 5000 rpm for 10 and 50 s. During the second step, 100 μL chlorobenzene was dripped on the spinning substrate 10 s prior to the end of the program. The substrates were then annealed at 130 °C for 30 min in a nitrogen filled glove box. And 35 μL Spiro-OMeTAD solution was spin coated on perovskite film at 5000 rpm for 30 s. Finally, a 100 nm gold layer was thermally evaporated on top of HTL to complete the device (Shi et al., 2018). The schematic diagram of perovskite solar cell on FAZO and AZO films is shown in Fig. 1. The structural, electrical, optical, and morphological characteristics of the FAZO thin films were characterized by X-ray diffraction (XRD, Cu-Kα1, λ = 0.154056 nm), Hall-effect measurement (Ecopia, HMS3000), ultraviolet–visible–near infrared spectrophotometry (UV-VisNIR, Shimadzu UV-3600 Plus), atomic force microscopy (AFM, NTMDT), and field-emission scanning electron microscopy (FESEM, ZEISS MERLIN Compact). The chemical states of O 1s, Zn 2p3, F 1s, and Al 2p on the film surface were detected by high-resolution X-ray photoelectron spectrometry (XPS, Kratos Axis Ultra DLD Multitechnique). Photocurrent density-voltage (J-V) curves of solar cells were measured at 25 °C in N2-filled glovebox. Unless specified, bias scan from 1200 mV to −200 mV firstly (SC-FB) and return back (FB-SC) with a voltage step of 40 mV and delay time 50 ms. The external quantum efficiency (EQE) spectral response was taken by QEX10, PV Measurement (Shi et al., 2018).

3. Results Fig. 2(a) shows the XRD patterns of the FAZO films prepared at different Ts values. As can be seen in Fig. 2(a), all films exhibit a strong and relatively weak diffraction peaks around 34.4° and 72.5°, respectively, which corresponds to the (0 0 2) and (0 0 4) diffraction peaks of ZnO film, respectively. This indicates that the incorporation of the Al and F ions did not change the wurtzite structure of ZnO, and all films grow preferentially along the c-axis perpendicular to the substrate (Wang and Chang, 2016). No Al2O3 and ZnF2 diffraction peaks could be detected in the diffraction pattern, indicating that the doping F and Al existed in the ZnO thin film in the form of substitution (Wang and Chang, 2016). With the increase in Ts, the (0 0 2) diffraction peak first increases and then decreases, reaching the maximum value at Ts = 440 °C, and then decreases slightly. The corresponding full width at half maximum (FWHM) and grain size (D) of the FAZO films are shown in Fig. 2(b). Consistent with the trend of variation shown in Fig. 2(a), the FWHM first decreases and then increases as the Ts increases, and the minimum value is obtained at Ts = 440 °C. At the same time, the D calculated by the Scherrer formula shows an inverse variation as it increases first and then decreases with the increase in Ts, with a maximum value of 39.74 nm at 440 °C, which is noticeably better

2. Experiments The FAZO films were deposited on a Corning XG glass substrate by RF magnetron sputtering using AlF3-doped ZnO ceramic target. Prior to the film deposition, the equipment was first vacuumized, and when a base pressure lower than 5 × 10−5 Pa was achieved, the chamber was filled with high-purity Ar gas. During film deposition, the pressure, power, distance between the target and substrate, and the Ar flow rate were fixed at 0.3 Pa, 180 W, 5 cm, and 30 sccm, respectively. The Ts values were varied at 320 °C, 360 °C, 400 °C, 440 °C, and 480 °C. Then, an optimized FAZO film was used as an electrode in a perovskite thin film solar cell. At the same time, a magnetron sputtered commercial AZO film (2 wt% Al2O3 doped ZnO) was also used for the perovskite thin film solar cell for comparison. FAZO (AZO)/glass substrates were coated with 100 μL SnO2 diluted solution, and rotated at 4000 r.p.m for 30 s, and then baked on a hot plate in ambient air at 150 °C for 30 min. The FAMACs perovskite precursor solution was spin coated with a twostep program at 1000 and 5000 rpm for 10 and 50 s. During the second step, 100 μL chlorobenzene was dripped on the spinning substrate 20 s prior to the end of the program. The FACs perovskite precursor solution 472

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(a)

(b) 40

0.30

(002) (004)

o

FWHM (o)

Intensity (a.u.)

0.28 36

FWHM D

0.26 0.24

32

D (nm)

480 C o 440 C o 400 C o 360 C o 320 C

0.22 28

0.20 20

30

40

50 o

2 ()

60

70

320

80

360

400

o

440

480

Temperature ( C)

Fig. 2. (a) XRD patterns and (b) FWHM and grain size (D) of the FAZO films prepared at different substrate temperatures.

30

than that of the AZO film (Oh et al., 2007). With the increase in Ts, the particles deposited on the substrate can obtain additional energy for migration and diffusion; thus, the crystal quality can be improved (Wang and Chang, 2016). However, too high Ts values result in an increase in the desorption rate of the deposited atoms on the substrate, resulting in an increase in the number of defects in the film (Wang and Chang, 2016). Thus, the crystal quality is deteriorated beyond the optimal value of 440 °C. The surface morphologies of the FAZO films prepared at different Ts values are shown in Fig. 3. As can be seen in Fig. 3, the surface of the FAZO films changes significantly with Ts. Specifically, they gradually change from a “pyramid” shape to a “crater” shape and then further evolving into a dense flat film, as the Ts increases. This is due to the difference of the additional adsorption energies of particles deposited on the surface of the substrates, which results in different migration and diffusion capabilities on the substrates (Wang and Chang, 2016). The corresponding root mean square (RMS) values increases first and then decreases sharply. The maximum value of 29.47 nm is obtained at Ts = 360 °C, and the minimum value of 2 nm can be found at 480 °C, as shown in Fig. 4.

(a)

RMS (nm)

25 20 15 10 5 0

360

400

440

480

o

Temperature ( C) Fig. 4. RMS values of the FAZO film at different substrate temperatures.

(b)

(c)

500 nm

(d)

320

500 nm

(e)

500 nm

(f)

Fig. 3. Two- and three-dimensional surface morphologies of the FAZO film with different substrate temperatures. SEM images at 320 °C (a), 440 °C (b), and 480 °C (c). AFM images at 320 °C (d), 440 °C (e), and 480 °C (f). 473

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(a1)

(a3)

(a2)

685.1 eV

684.9 eV

683.0

683.5

684.0

684.5

685.0

685.5

686.0

c/s

c/s

c/s

684.5 eV

683.0

683.5

684.0

Binding energy (eV)

(b1)

684.5

685.0

685.5

686.0

683.0

683.5

684.0

Binding energy (eV) (b2)

AlII, 73.9 eV

(b3)

685.0

685.5

686.0

AlII, 74.3 eV

c/s

c/s

c/s

AlII, 73.9 eV

684.5

Binding energy (eV)

AlI, 71.6 eV

AlI, 71.9 eV AlI, 71.7 eV

72

73

74

75

71

72

(c1)

c/s Smoothed Y1 Fit Peak 1 Fit Peak 2 Cumulative Fit Peak

74

(c2)

75

71

72

73

OI, 530.2 eV

74

75

Binding energy (eV)

c/s Smoothed Y1 Fit Peak 1 Fit Peak 2 Cumulative Fit Peak

c/s

OI, 530.3 eV

c/s

73

Binding energy (eV)

Binding energy (eV)

c/s Smoothed Y1 Fit Peak 1 Fit Peak 2 Cumulative Fit Peak

(c3)

OI, 530.4 eV

c/s

71

OII, 531.2 eV OII, 531.6 eV

OII, 531.3 eV 528

529

530

531

532

533

534

528

529

530

(d1)

(d2)

1021.6 eV

532

533

534

1020

1022

1024

Binding energy (eV)

528

529

530

1026

1018

531

532

533

534

Binding energy (eV)

(d3)

1021.5 eV

1021.7 eV

c/s

c/s

c/s 1018

531

Binding energy (eV)

Binding energy (eV)

1020

1022

1024

Binding energy (eV)

1026

1018

1020

1022

1024

1026

Binding energy (eV)

Fig. 5. XPS spectra of the FAZO film deposited with different substrate temperature: (a1), (a2), (a3), (b1), (b2), (b3), (c1), (c2), (c3), and (d1), (d2), (d3) are F 1 s, Al 2p, O 1 s, and Zn 2p3/2 peaks of the FAZO film fabricated at 320 °C, 440 °C, and 480 °C, respectively.

concentration in the film (Wagner, 1977; Gaarenstroom and Winograd, 1977). At the same time, a significant change of the peak of Al can be observed with Ts. At a lower temperature of 320 °C, there are two peaks, located at 72 ± 0.4 eV (AlI) and 74 ± 0.3 eV (AlII), as shown in Fig. 5(b1). The AlI peak corresponds to Al2O3/Al, while the higher AlII peak corresponds to the Al2O3 (Yan et al., 1989; Leinen et al., 1996). With the increase of Ts, the AlI peak gradually disappears, indicating that the oxidation state of Al in the film is gradually enhanced, as shown in Fig. 5(b2) and (b3); this indicates that the doping effect of Al is gradually improved, and more carriers are donated to the films. The bonding states of the O 1s spectra can be resolved into two peaks

To study the effect of Ts on the chemical states in FAZO films, an XPS study was performed to investigate the films deposited at 320 °C, 440 °C, and 480 °C. The XPS spectra of the FAZO film deposited with different Ts values are shown the Fig. 5. In Fig. 5(a1), a peak located around 684.9 ± 0.4 eV can be seen, which corresponds to the binding energy of F in ZnF2. This indicates that O is substituted by the doping F in ZnO films (Wagner, 1977; Gaarenstroom and Winograd, 1977). As Ts increases, the peak gradually becomes noticeable, while the peak position moves toward the high-energy direction, as shown in Fig. 5(a2) and (a3). It should be noted, that the increased Ts increases the displacement of F in the ZnO film and also increases the carrier 474

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3.0

15 Resistivity Mobility Carrier concentration

12 9 6

20 16 12

320

360

400

o

440

480

2.6 2.4 2.2 2.0

Sheet resistance ( /sq)

Resistivity (

24

20

2.8 2

18

(b)

-3

21 28

24

Carrier concentration (10 cm )

(a) 32

Mobility (cm /Vs)

cm)

24

21 18 15 12 9 6

320

360

400

o

440

480

Temperature (C)

Temperature ( C)

Fig. 6. Electrical properties of the FAZO films prepared at different substrate temperatures: (a) carrier concentration, mobility, and resistivity and (b) sheet resistance.

Transmitance (%)

100

(1021.6 ± 0.1 eV), which is corresponding to the Zn in ZnO, indicating that the Ts value has small effect on the valence state of Zn, and all Zn is in the form of oxidation state, as shown in Fig. 5(d1), 5(d2), and 5(d3) (Schoen, 1973). The electrical properties of the FAZO films prepared at different Ts temperatures are shown in Fig. 6. As can be seen in Fig. 6(a) both the mobility (m) and the carrier concentration (n) significantly increase with the increase in Ts. The maximum values of m = 31.9 cm2/Vs and n = 2.84 × 1020 cm−3 were achieved at the Ts values of 400 °C and 440 °C, respectively. Then the values of m and n decrease beyond the optimal value of Ts. This is owing to the change of the crystallization quality with Ts. With increase of Ts, the particles deposited on the substrate can obtain additional energy from the heated substrate and the crystal quality is improved, which decrease the defect and grain boundary scattering, and increase the m and n of the films, shown in Fig. 2. In addition, the effective donating effect of the co-doping F and Al, and the increased oxygen-deficiency provide extra carriers (Fig. 4) with increase in Ts. However, when the Ts value is too high, the crystallization quality of the film deteriorates, which increase the number of defects and the scattering of the carrier. So, the m and n decreased at higher Ts. Furthermore, the decrease in oxygen-deficiency is also results in the decrease of n at a higher Ts value of 480 °C. Owing to the inversely proportional relationship between ρ, n, and μ, ρ shows opposite changes, and it decreases first with the increase in Ts and then it increases. The lowest ρ value of 7.35 × 10−4 Ωcm is obtained at Ts = 440 °C. The trend of the variation of the sheet resistance is consistent with that of ρ, and the minimum value of 7.04 Ω/sq is also obtained at Ts = 440 °C, as shown in Fig. 6(b). Fig. 7 shows the optical properties of the FAZO films prepared at different Ts temperatures. As can be seen in Fig. 7, the transmittance of

80 o

320 C o 360 C o 400 C o 440 C o 480 C

60 40 20 0

400

600

800

1000

1200

1400

Wavelentg (nm) Fig. 7. Optical properties of the FAZO films prepared at different substrate temperatures.

centered at 530.3 ± 0.1 eV (OI), and 531.3 ± 0.3 eV (OII), as shown in Fig. 5(c1)–(c3) (Chen et al., 2000; Schoen, 1973). The OI peak is due to the Zn–O bonds and the OII peak can be associated with the O2− ions in the oxygen-deficient regions within the ZnO matrix (Schoen, 1973). More carriers can be provided when the O2− ions are in an oxygendeficient state. The area under the OII peak increases with the increase in Ts; and then it decreases. The maximum area ratio of 61.23% is obtained at Ts = 440 °C, which indicates that more oxygen-deficient state is produced at this Ts value and more redundant carriers are provided to the film. There is no significant change in the peak of Zn Table 1 The electrical and optical properties of the doped ZnO films. Material

Deposition method

n (1020 cm−3)

μ (cm2/Vs)

ρ (10−4 Ω cm)

T (%)

Ref.

ZnO: AlF3 ZnO: Al(NO3)3·9H2O: NH4F ZnO:Al2O3:ZnF2 ZnO: F: Al Zn(CH3COO)2·2H2O: NH4F: H3BO3 ZnO: ZnF2: B2O3 ZnO: ZnF2: Ga2O3 ZnCl2: InCl3: NH4F ZnO: F ZnO: F ZnO:CF4 ZnO: F: Al ZnO: F: Al

RF magnetron sputtering Sol-gel technique RF magnetron sputtering RF sputtering technique Ultrasonic Spray Pyrolysis Magnetron sputtering Mid-frequency sputtering Spray pyrolysis Thermal oxidation of ZnF2 Pulsed laser deposition RF magnetron sputtering RF sputtering technique RF sputtering technique

4.06 0.3 9.86 8.58 6.35 7.99 6.8 0.022 1.70 5.43 1.53 4.50 2.84

0.62 27 22.0 6.14 13.22 5.18 13.4 55 46.2 23.8 17 39.33 29.92

360 56 2.88 11.8 3.13 16.4 6.4 520 7.95 4.83 24 3.53 7.35

85% Visible region 90% (500–1100 nm) 92% (400–800 nm) 90% (Visible and NIR) 90% (Visible region) 80% (300–2000 nm) > 90% (Visible region) > 70% (400–800 nm) > 85% (Visible region) > 90% (Visible region) 80% (Visible region) 80% (400–1200 nm) 90% (400–1400 nm)

Lin et al.(2012) Altamirano-Juárez et al. (2004) Wang and Chang (2016) Mallick and Basak (2017) Karakaya (2018) Li et al. (2017) Shi et al. (2014) Hadri et al. (2016) Xu et al. (2005) Cao et al. (2011) Yoon et al. (2008) Ji et al. (2019) This paper

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Table 2 The electrical properties of the FAZO and AZO films. Films

Carrier concentration (1020 cm−3)

Mobility (cm2/Vs)

Resistivity (10−4 Ωcm)

Sheet resistance (Ω/sqr)

FAZO AZO

2.84 3.47

29.92 25.05

7.35 7.18

7.04 7.98

(a)

80

(b)

20 15 10 5

Films

Jsc Voc FF 2 (mA/cm ) (mV) FAZO 24.07 1020.06 0.71 AZO 21.49 1020.05 0.74

Eff (%) 17.37 16.24

100

60

Transmittance (%)

FAZO AZO

EQE (%)

2

Current Density (mA/cm )

25

40

20

80 60

FAZO AZO

40 20 0

400 450 500 550 600 650 700 750 800 850

Wavelength (nm)

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Voltage (V)

0

400

450

500

550

600

650

700

Wavelength (nm)

750

800

850

Fig. 8. Characteristics of perovskite solar cells using FAZO and AZO electrodes. (a) Current density–voltage characteristics and (b) external quantum efficiency. The inset of panels (a) and (b) show conversion characteristics and transmittance of perovskite solar cells prepared using FAZO and AZO films, respectively.

the FAZO film in the short-range and visible light region significantly increases with the increase in Ts, which can be attributed to the improvement of the crystal quality of the film. There are many oscillations were presented in Fig. 6, especially when the Ts over 360 °C. The strong oscillations in the transmittance spectrum of the FAZO film on different Ts is due to the constructive or destructive interference caused by multiple reflections of the light at interfaces of the air/FAZO film and FAZO film/Glass substrate (Leem et al., 2011). Normally, the thicker and flatter of the film, the more oscillating the transmittance is. At the same time, the variation in the surface appearance of the FAZO films prepared as different Ts temperatures is another reason for the variation of transmittance in the short wavelength region. A textured surface is often used in thin film solar cells for scattering the incident light (Wang et al., 2012; Wang et al., 2018). Therefore, in our present experiment, in the short wavelength region, the lower transmittance of the FAZO films fabricated at Ts temperatures of 320 °C and 360 °C is due to the “crater” shape surface morphology, which increases the light scattering and decreases the transmittance (Wang et al., 2012; Wang et al., 2018). When Ts is higher than 400 °C, the surface of the film becomes flat, and the scattering effect is negligible; thus, the transmittance of the film is nearly unchanged, and the average transmittance in the range of 400–1400 nm is nearly 90%. The photoelectric properties of FAZO films prepared under the best condition is better than those of the reported ZnO films shown in Table 1. These excellent optical properties enable application of FAZO films as electrode materials in wide-spectrum highefficiency thin film solar cells. To further verify the photoelectric properties of FAZO films, the optimized film was used as a front electrode in perovskite thin film solar cells. For comparison, a commercial AZO film was also used for perovskite thin film solar cells at the same time. The electrical properties of the FAZO and AZO films are listed in Table 2. As can be seen from Table 1, both the mobility and sheet resistance of the FAZO film is better than referenced AZO film. Furthermore, lower carrier concentration of the FAZO film is also benefit to the transmittance especially in the long wavelength region. Higher electrical and optical properties of FAZO film manifesting that has higher performance than AZO film in solar cell application. The current density–voltage (J–V) characteristics and the external

quantum efficiency (EQE) of the perovskite solar cells using FAZO and AZO are shown in Fig. 8. The corresponding conversion characteristics of the solar cells are listed in the inset of Fig. 8(a). The perovskite solar cells fabricated on the FAZO film exhibited better performance than the reference devices employing AZO transparent electrodes, with a shortcircuit current density (Jsc) of 24.07 mA/cm2, open-circuit voltage (Voc) of 1020.06 mV, and power conversion efficiency (PCE) of 17.37%. Although the fill factor (FF) was slightly lower than that of the reference devices, the FAZO-based device exhibited higher Jsc and PCE than solar cell based on the AZO film (Jsc of 21.49 mA/cm2, Voc of 1020.06 mV and PCE of 16.24%). An improvement of 12.01% in Jsc and 6.96% in the PCE was observed in the FAZO-based solar cell, and these results can be attributed to the relatively high EQE, which results in the high transmittance of the FAZO electrode, as shown in Fig. 8(b). 4. Conclusions In this study, F and Al co-doped ZnO films were fabricated on Corning XG glass substrates at different Ts temperatures by RF magnetron sputtering technique using a 1.0 wt% AlF3-doped ZnO ceramic target. All FAZO films exhibited a wurtzite structure with c-axis preferred orientation. As the Ts increases, the crystal quality and the D of the films improve, and the surface appearance changed from “pyramid” shape to “crater” shape and to flat and dense. The donating effect of the doping F and Al improves with the increase in Ts, and the oxygen-deficient state increases the carrier concentration. The carrier concentration and mobility increases with the increase in the Ts values, and then decreases further. A low resistivity of 7.35 × 10−4 Ω cm and an average transmittance of nearly 90% was achieved in the optical spectral range of 400–1400 nm at Ts = 440 °C. The excellent photoelectric properties and the high performance of the perovskite solar cells application demonstrate that the FAZO film has a broad application possibility in wide-spectral high-efficiency thin film solar cells. Acknowledgement The work was supported by the National Natural Science Foundation of China (Grant No. 11404088), Natural Science 476

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effects on electrical and optical properties of ZnO nanocrystalline films. Appl. Phys. Lett. 86, 123107-1–1231071-3. Yoon, H.S., Lee, K.S., Lee, T.S., Cheong, B., Choi, D.K., Kim, D.H., Kim, W.M., 2008. Properties of fluorine doped ZnO thin films deposited by magnetron sputtering. Sol. Energy Mater. Sol. Cells 92, 1366–1372. Wang, F.H., Chang, C.L., 2016. Effect of substrate temperature on transparent conducting Al and F co-doped ZnO thin films prepared by rf magnetron sputtering. Appl. Surf. Sci. 370, 83–91. Ji, X.Z., Song, J.M., Wu, T.T., Tian, Y., Han, B., Liu, X.N., Wang, H.W., Gui, Y.B., Ding, Y., Wang, Y.F., 2019. Fabrication of high-performance F and Al co-doped ZnO transparent conductive films for use in perovskite solar cells. Sol. Energy Mater. Sol. Cells 190, 6–11. Wang, Y.F., Song, J.M., Song, W.Y., Tian, Y., Han, B., Meng, X.D., Yang, F., Ding, Y., Li, J.J., 2019. Effects of thickness on photoelectric properties and perovskite solar cells application of transparent conductive F and Al co-doped ZnO films. Solar Energy 186, 126–135. Shi, B., Yao, X., Hou, F.H., Guo, S., Li, Y.C., Wei, C.C., Ding, Y., Li, Y.L., Zhao, Y., Zhang, X.D., 2018. Unraveling the passivation process of PbI2 to enhance the efficiency of planar perovskite solar cells. J. Phys. Chem. C 122, 21269–21276. Oh, B.Y., Jeong, M.C., Myoung, J.M., 2007. Stabilization in electrical characteristics of hydrogen-annealed ZnO: Al films. Appl. Surf. Sci. 253, 7157–7161. C.D. Wagner Chapter 7 In 'Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy', D. Briggs, Editor (1977). Gaarenstroom, S.W., Winograd, N., 1977. Initial and final state effects in the ESCA spectra of cadmium and silver oxides. J. Chem. Phys. 67, 3500–3506. Yan, Y.L., Helfand, M.A., Clayton, C.R., 1989. Evaluation of the effect of surface roughness on thin film thickness measurements using variable angle XPS. Appl. Surf. Sci. 37, 395–405. Leinen, D., Lassaletta, G., Fernandez, A., Caballero, A., Gonzalez-Elipe, A.R., Martin, J., Vacher, B., 1996. Ion beam induced chemical vapor deposition procedure for the preparation of oxide thin films. II. preparation and characterization of AlxTiyOz thin films. J. Vac. Sci. Technol. A 14, 2842–2848. Chen, M., Wang, X., Yu, Y.H., Pei, Z.L., Bai, X.D., Sun, C., 2000. X-ray photoelectron spectroscopy and auger electron spectroscopy studies of Al-doped ZnO films. Appl. Surf. Sci. 158, 134–140. Schoen, G., 1973. Auger and direct electron spectra in X-ray photoelectron studies of zinc, zinc oxide, gallium and gallium oxide. J. Electron Spectrosc. Relat. Phenom. 2, 75–86. Leem, J.W., Joo, D.H., Yu, J.S., 2011. Biomimetic parabola-shaped AZO subwavelength grating structures for efficient antireflection of Si-based solar cells. Sol. Energy Mater. Sol. Cells 95, 2221–2227. Wang, Y.F., Zhang, X.D., Bai, L.S., Huang, Q., Wei, C.C., Zhao, Y., 2012. Effective light trapping in thin film silicon solar cells from textured Al doped ZnO substrates with broad surface feature distributions. Appl. Phys. Lett. 100, 263508-1–263508-4. Wang, Y.F., Song, J.M., Bai, L.S., Yang, F., Han, B., Guo, Y.J., Dai, B.T., Zhao, Y., Zhang, X.D., 2018. Management of light trapping capability of AZO film for Si thin film solar cells-via tailoring surface texture. Sol. Energy Mater. Sol. Cells 179, 401–408. Lin, T.C., Huang, W.C., Liu, C.H., Chang, S.C., 2012. Structural, electrical and optical properties of ZnO:AlF3 thin films deposited by RF magnetron sputtering. Appl. Surf. Sci. 258, 3302–3308. Altamirano-Juárez, D.C., Torres-Delgado, G., Jiménez-Sandoval, S., Jiménez-Sandoval, O., Castanedo-Pérez, R., 2004. Low-resistivity ZnO:F: Al transparent thin films. Sol. Energy Mater. Sol. Cells 82, 35–43. Mallick, A., Basak, D., 2017. Comparative investigation on cation-cation (Al-Sn) and cation-anion (Al-F) co-doping in RF sputtered ZnO thin films: mechanistic insight. Appl. Surf. Sci. 410, 540–546. Karakaya, S., 2018. Effect of fluorine and boron co-doping on ZnO thin films: structural, fluminescence properties and Hall effect measurements. J. Mater. Sci.-Mater. Electron. 11, 1–9. Li, Q., Zhu, L.P., Li, Y.G., Zhang, X.Y., Niu, W.Z., Guo, Y.M., Ye, Z.Z., 2017. Highly conductive thin films of nonmetal F and B co-doped ZnO on flexible substrates: experiment and first-principles calculations. J. Alloys Compd. 697, 156–160. Shi, Q., Zhou, K., Dai, M.J., Lin, S.S., Hou, H.J., Wei, C.B., Hu, F., 2014. Growth of highquality Ga-F codoped ZnO thin films by mid-frequency sputtering. Ceram. Int. 40, 211–216. Hadri, A., Taibi, M., loghmarti, M., Nassiri, C., Slimani Tlemҫani, T., Mzerd, A., 2016. Development of transparent conductive indium and fluorine co-doped ZnO thin films: Effect of F concentration and post-annealing temperature. Thin Solid Films 601, 7–12. Cao, L., Zhu, L.P., Jiang, J., Zhao, R., Ye, Z.Z., Zhao, B.H., 2011. Highly transparent and conducting fluorine-doped ZnO thin films prepared by pulsed laser deposition. Sol. Energy Mater. Sol. Cells 95, 894–898.

Foundation of Hebei Province (Grant No. A2019405059), the Third Batch of Young Top-notch Talent Fund of Hebei Province (China), Talent Training Funding for Scientific Research Project of Hebei Province (Grant No. A2016002020), Basic scientific research project of Hebei north university (Grant No. JYT2019001), Key research and development project of Hebei province (Grant No. 19214301D), General Projects of Hebei North University (Grant No. YB2018014), and the Doctoral Scientific Research Foundation of Hebei North University. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.09.095. References Zang, Z.G., Zeng, X.F., Du, J.H., Wang, M., Tang, X.S., 2016. Femtosecond laser direct writing of microholes on roughened ZnO for output power enhancement of InGaN light-emitting diodes. Opt. Lett. 41, 3463–3466. Li, C.L., Han, C., Zhang, Y.B., Zang, Z.G., Wang, M., Tang, X.S., Du, J.H., 2017. Enhanced photoresponse of self-powered perovskite photodetector based on ZnO nanoparticles decorated CsPbBr 3 films. Sol. Energy Mater. Sol. Cells 172, 341–346. Zang, Z.G., 2018. Efficiency enhancement of ZnO/Cu2O solar cells with well oriented and micrometer grain sized Cu2O films. Appl. Phys. Lett. 112, 042106-1–042106-5. Zang, Z.G., Tang, X.S., 2015. Enhanced fluorescence imaging performance of hydrophobic colloidal ZnO nanoparticles by a facile method. J. Alloys Compd. 619, 98–101. Özgür, Ü., Alivov, Y.I., Liu, C., Teke, A., Reshchikov, M.A., Doğan, S., Avrutin, V., Cho, S.J., Morkoç, H., 2005. A comprehensive review of ZnO materials and devices. J. Appl. Phys. 98, 041301-1–041301-103. Mansfield, L.M., Kanevce, A., Harvey, S.P., Bowers, K., Beall, C., Glynn, S., Repins, I.L., 2018. Efficiency increased to 15.2% for ultra-thin Cu(In, Ga)Se2 solar Cells. Prog Photovolt Res Appl. 26, 949–954. Liu, B.F., Bai, L.S., Li, T.T., Wei, C.C., Li, B.Z., Huang, Q., Zhang, D.K., Wang, G.C., Zhao, Y., Zhang, X.D., 2017. High efficiency and high open circuit voltage quadruplejunction silicon thin film solar cells for tomorrow’s electronic applications. Energy Environ. Sci. 10, 1134–1141. Sarma, B., Barman, D., Sarma, B.K., 2019. AZO (Al:ZnO) thin films with high figure of merit as stable indium free transparent conducting oxide. Appl. Surf. Sci. 479, 786–795. Mahdhi, H., Alaya, S., Gauffier, J.L., Djessas, K., Ben, Z., 2017. Ayadi, Influence of thickness on the structural, optical and electrical properties of Ga-doped ZnO thin films deposited by sputtering magnetron. J. Alloys Compd. 695, 697–703. Djessas, K., Bouchama, I., Gauffier, J.L., Ben, Z., 2014. Ayadi, Effects of indium concentration on the properties of In-doped ZnO films: applications to silicon wafer solar cells. Thin Solid Films 555, 28–32. Agashe, C., Kluth, O., Schöpe, G., Siekmann, H., Hüpkes, J., Rech, B., 2003. Optimization of the electrical properties of magnetron sputtered aluminum-doped zinc oxide films for opto-electronic applications. Thin Solid Films 442, 167–172. Wang, F.Y., Yang, M.F., Zhang, Y.H., Yang, L.L., Fan, L., Lv, S.Q., Liu, X.Y., Han, D.L., Yang, J.H., 2018. Activating old Materials with new architecture: Boosting performance of perovskite solar cells with H2O-assisted hierarchical electron transporting layers. Adv. Sci. 6, 1801170-1–1801170-9. Guo, X.Z., Tan, Q.X., Liu, S.W., Qin, D.H., Mo, Y.Q., Hou, L.T., Liu, A.L., Wu, H.B., Ma, Y.G., 2018. High-efficiency solution-processed CdTe nanocrystal solar cells incorporating a novel crosslinkable conjugated polymer as the hole transport layer. Nano Energy 46, 150–157. Heinemann, M.D., Ruske, F., Greiner, D., Jeong, A.R., Rusu, M., Rech, B., Schlatmann, R., Kaufmann, C.A., 2016. Advantageous light management in Cu(In, Ga)Se2 superstrate solar cells. Sol. Energy Mater. Sol. Cells 150, 76–81. Coutts, T.J., Young, D.L., Li, X.N., 2000. Characterization of transparent conducting oxides. MRS Bull. 58–65. Wang, Y.F., Zhang, X.D., Meng, X.D., Cao, Y., Yang, F., Nan, J.Y., Song, Q.G., Huang, Q., Wei, C.C., Zhang, J.J., Zhao, Y., 2016. Simulation, fabrication, and application of transparent conductive Mo-doped ZnO film in a solar cell. Sol. Energy Mater. Sol. Cells 145, 171–179. Xu, H.Y., Liu, Y.C., Mu, R., Shao, C.L., Lu, Y.M., Shen, D.Z., Fan, X.W., 2005. F-doping

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