Lithium and silver co-doped nickel oxide hole-transporting layer

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Lithium and silver co-doped nickel oxide hole-transporting layer boosting the efficiency and stability of inverted planar perovskite solar cells Xuefeng Xia, Yihua Jiang, Qixin Wan, Xiaofeng Wang, Li Wang, and Fan Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16649 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 21, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Lithium and Silver Co-Doped Nickel Oxide Hole-Transporting Layer Boosting the Efficiency and Stability of Inverted Planar Perovskite Solar Cells Xuefeng Xia#1, Yihua Jiang#1, Qixin Wan2, Xiaofeng Wang1, Li Wang1, Fan Li*1 1Department

of Materials Science and Engineering, Nanchang University, 999 Xuefu

Avenue, Nanchang 330031, China 2Key

Laboratory for Optoelectronics and Communication of Jiangxi Province, Jiangxi

Science and Technology Normal University, Nanchang 330013, China

Keywords: Perovskite solar cell; Nickel oxide; Co-doping; Hole-transporting layer; Efficiency; Stability

Abstract In this work, a lithium and silver co-doping strategy has been successfully implied to prepare NiOx films for high performance inverted planar PSCs. Compared to the pristine and single-doped NiOx, the Li and Ag co-doping approach exhibits the synergistic effect and can endow NiOx films with higher electrical conductivity, higher hole mobility and better interface energy band alignment with perovskite active layers. Moreover, the perovskite film with enhanced crystallinity can be obtained induced by the Li,Ag:NiOx film. The PSC with Li,Ag:NiOx HTL shows a high power conversion efficiency (PCE) up to 19.24% and less hysteresis effect, which outperforms the devices with the pristine NiOx or single-doped NiOx HTLs. Meanwhile, the Li,Ag:NiOx device can retain 95% of its initial PCE after storage at the relative humidity of 30 ± 2% in 30 days without encapsulation. Our work demonstrates that lithium and silver co-doping is a promising route for realizing efficient p-type NiOx HTL, which provides a simple way to boost the efficient and stable of inverted planar PSCs.

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1. Introduction As the new generation of solar cells, thanks to the excellent photo-physic performance of metal halide hybrid organic-inorganic perovskites (HOIPs), such as superior light absorption coefficient, long charge diffusion length, high charge carrier mobility, small exciton binding energy, low trap-state density and ambipolar transport nature,1-6 the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have exceeded 22%.7-13 Among different PSC device architectures, the inverted planar device has gained extensive attention due to its facial fabrication, simple device structure, low hysteresis and possibility of fabricating flexible devices.14-15

In inverted planar PSCs, hole transport materials (HTMs) play multiple roles, which not only extract/transport holes and block electrons effectively, but also have great impacts on the crystallinity and crystal growth of perovskite thin films, as well as the stability of PSCs.16-18 Many kinds of materials have been applied as hole transport layers (HTLs) in inverted planar PSCs, mainly including organic and inorganic p-type semiconductors.19

Compared

with

poly(3,4-ethylenedioxythiophene):poly(styrene

organic

HTMs,

sulfonate)

such

as

(PEDOT:PSS);20

poly-3-hexylthiophene

(P3HT),

poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl-alt-3-fluoro-2 -[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl]

(PTB7),

poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;.5b’]dithiophene-2,6-diyl-alt3-fluoro-2-[(2-ethyl-hexyl)-carbonyl]-thieno[3,4-b]thiophene-4,6-diyl]

(PTB7-Th)21

and poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA),22 inorganic HTMs are particularly attracting the interest of researchers because of their outstanding stability, higher carrier mobility, p-type semiconducting nature, low cost and easier processing via solution from precursors and nanoparticles.23-25 In particular, owing to the wide band gap, high optical transmittance, excellent energy level alignment with the perovskite absorbers and good chemical stability, NiOx HTLs have been widely employed in inverted planar PSCs and the NiOx-based PSCs have demonstrated very promising PCEs of over 20% along with higher fill factor (FF) and longer device


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lifetime.26-29 Despite these merits, the low intrinsic electrical conductivity of the pristine NiOx has seriously hindered further improvement of NiOx-based PSCs, which will result in the increased charge recombination and reduced hole extraction.30

To overcome this drawback, considerable efforts have been successfully put forth to increase the electric conductivity of NiOx. Among these, one effective approach is metal ion doping. It is well-known that metal ion doping is an effective method to modify the electrical properties of semiconductor thin films, which can conveniently adjust the bandgap and electrical resistivity, thereby, acquiring higher conductivity. Many kinds of metal doping elements have been used to improve the conductivity of NiOx, such as Cu31-32, Co33-34, Cs25 and Li35-36. In various metal ion dopants, the lithium (Li) dopant draws our attention. Interestingly, except for enhancing the electrical conductivity of NiOx, lithium doped NiOx (Li:NiOx) can control the growth of hybrid perovskites, obtaining perovskite thin films with high-crystallinity and orientation.16 Meanwhile, lithium dopant has also been successfully applied in co-doping approach and it has been figured out that co-doping of metal ions has a synergistic effect on the improvement of the NiOx properties and thus boosts the performance of corresponding PSCs.37-38 Efficient PSCs have been demonstrated for Li, Mg (18.3%)37 and Li, Cu (14.53%)38 co-doped NiOx. Moreover, in our previous work, silver (Ag) has been proved to be an excellent candidate of acceptor dopants for NiOx lattice and Ag:NiOx HTL can effectively improve the efficiency and stability of PSCs.39 However, Li, Ag-codoped NiOx has not been applied to PSCs so far.

In this work, we have successfully prepared Li, Ag-codoped NiOx (Li,Ag:NiOx) film and applied it as an excellent HTL for inverted planar PSCs. The efficiency and stability of PSCs are dramatically improved by Li, Ag-codoped NiOx films. Compared to the pristine NiOx and single-doped NiOx (Li:NiOx and Ag:NiOx), Li and Ag co-doping can effectively increase the electrical conductivity, hole mobility and work function of NiOx films, resulting in the improved hole extraction. Meanwhile, the high-quality and uniform MAPbI3 (MA = CH3NH3+) perovskite films with better



crystallinity and larger grain size can be fabricated on the Li,Ag:NiOx film. As a consequence, the novel Li, Ag:NiOx HTL boosts the PCE of inverted planar MAPbI3 PSCs to be over 19% with less hysteresis effect. Furthermore, PSCs with Li, Ag co-doped NiOx show improved stability in comparison with the pristine NiOx-based devices. Our work demonstrates that the Li, Ag-codoped NiOx HTL is a potential candidate for high-performance inverted planar PSCs.

2. Results and Discussion The pristine NiOx and doped NiOx films (Li:NiOx, Ag:NiOx and Li,Ag:NiOx) were prepared according to the reported literature with minor modifications (see more details in the Supporting Information).17 In this work, for the convenience of comparison, the doping amount of each metal ion dopant in doped NiOx films has been optimized in our preliminary experiment or selected according to the literatures. For Li:NiOx films, the optimal doping amount of the lithium dopant is 5 at.%.16 The optimal doping amount of silver in Ag:NiOx films is 2 at.%.39 In Li,Ag:NiOx films, the doping amount of Li+ and Ag+ is selected to be 0.5 at.% and 1.5 at.%, respectively. X-ray diffraction (XRD) spectra of the NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx film in Figure S1 show that all films are amorphous. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to characterize the surface morphologies of the NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx films. As shown in Figure 1, high-quality films with continuous, dense and smooth surface are obtained, which will facilitate the hole transport and electron block, as well as the formation of perovskite films with full coverage. Moreover, the surface root mean square (RMS) of the NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx film is 9.27 nm, 6.38 nm, 7.62 nm and 7.95 nm, respectively. It is clear that the doped NiOx film exhibits a more uniform and smooth surface compared to the pristine NiOx film, which is also confirmed by the laser scanning confocal microscopy images shown in Figure S2. Similar results are also reported in the Cs and Co doped NiOx films.25,34


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a

b

Li:NiOx

NiOx d

c

Ag:NiOx

Li,Ag:NiOx

Figure 1. SEM and AFM (inset) images of the (a) NiOx, (b) Li:NiOx, (c) Ag:NiOx and (d) Li, Ag:NiOx film.

For inverted planar perovskite photovoltaic devices, the optical transmittance of HTLs is vital for high-performance PSCs. High transparency of HTLs will minimize the optical losses and ensure the high photocurrent generation. As illustrated in Figure 2a, whether the undoped NiOx or the doped NiOx films on indium tin oxide (ITO) substrates reveal high transmittance (> 80%) in the range from 300 to 800 nm while the metal ion doping can slightly narrow the band gap (inset of Figure 2a), mainly originating from the creation of gap states due to the doping effect.27 Apart from the optical properties, electrical properties of NiOx HTLs are also critical to the PSCs performance. We initially perform the conductive atomic force microscopy (c-AFM) measurements to characterize and compare the electrical conductivity of the pristine NiOx and doped NiOx films. The I-V curves presented in Figure 2b demonstrate that the conductivity of NiOx films is obviously improved by metal ion doping. More strikingly, significantly higher vertical current is observed in the Li,Ag:NiOx film,



suggesting that Li, Ag co-doping is an more effective way to increase the conductivity of NiOx films compared to the single-doped NiOx (Li:NiOx and Ag:NiOx). Hole mobility is another important characteristics of HTLs. Therefore, the hole mobility of the pristine NiOx and doped NiOx HTLs is further measured by the space-charge-limited-current (SCLC) method. The mobility is calculated by fitting the I-V curves of hole-only devices with the ITO/NiOx or doped NiOx/Au device configuration (Figure 2c) by the Mott-Gurney equation:40 J = qε0εrμV2/8L3 Where J is the current density, μ is the hole mobility, ε0 is the vacuum permittivity, εr is the dielectric permittivity of NiOx, L is the thickness of the pristine NiOx or doped NiOx films and V is the applied voltage of devices. The calculated hole mobility of the NiOx, Li:NiOx, Ag:NiOx and Li, Ag:NiOx film is 1.72 ×10-2 cm2 V-1 s-1, 2.25 ×10-2 cm2 V-1 s-1, 2.44 ×10-2 cm2 V-1 s-1, and 3.21 ×10-2 cm2 V-1 s-1, respectively. The obtained values are within the ranges for NiOx HTLs reported in literatures depending on the film compositions and deposition conditions.41 Clearly, the hole mobility of NiOx films can be improved by metal ion doping, especially in the case of the Li,Ag:NiOx film. In addition, ultraviolet photoelectron spectroscopy (UPS) is carried out to probe the surface electronic structure and work functions (WFs) change of NiOx films upon doping (Figure 2d). It can be observed that all samples show typical p-type nature with the same difference between valence band and Fermi level (0.3 eV). In the case of the Li:NiOx film, there is a slight decrease in the WF compared to the pristine NiOx film.37 However, the WF is increased to 4.87 eV after Ag doping and further reaches 4.96 eV upon Li and Ag co-doping, demonstrating that Li and Ag co-doping results in more favorable WF for hole extraction and provides better band alignment between NiOx HTLs and perovskite active layers (Figure S3). Meanwhile, scanning Kelvin probe microscopy (SKPM) is applied to measure the surface potential variations (Figure S4). Similarly, the lowest surface potential profile is observed in the Li,Ag:NiOx film, revealing a higher work function compared to other samples. All these results show that lithium and silver (Li, Ag) co-doping can render NiOx HTLs better electrical properties for efficient hole extraction and transport,


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which will promote the PSCs performance.

b

100

60 40 20

Ag: NiOx Li, Ag: NiOx

50

3.0

3.2

3.4

3.6

3.8

4.0

500

600

700

d

NiOx Ag:NiOx Li,Ag:NiOx

J

102

Au NiOx Films

ITO 10-2

10-1

100

-0.5

0.0

0.5

1.0

V (V) 3.5x105 3.0x105

Li:NiOx

101 10-3

0

-4 -1.0

800

Intensity (cps)

)

103

Li,Ag:NiOx

4.2

Energy (eV)

Wavelength (nm)

c

Ag:NiOx

-2

0

400

Li:NiOx

2

Li: NiOx

100

0 300

-2

NiOx

NiOx

2.5x10

5

2.0x105

3.5x10

5

NiOx

3.0x105

Intensity (cps)

h2 1010 cm-1eV2

150

4

I (nA)

80



Transmittance (%)

a

Current Density (mA cm

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


Li:NiOx

2.5x105

Ag:NiOx

2.0x105

Li, Ag:NiOx

1.5x105 1.0x105 5.0x104 0.0 14

1.5x105

16

Binding Energy (eV)

18

EF = 4.96 eV

1.0x105

EF = 4.87 eV

EVB - EF = 0.3 eV

EF = 4.84 eV

5.0x104 0.0

EF = 4.76 eV

0

Voltage (V)

4

8

12

16

Binding Energy (eV)

20

Figure 2. a) Transmission spectra of NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx films on ITO substracts (Inset: the corresponding Tauc plot of each film), b) I-V curves of NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx films on ITO glass measured using c-AFM mode (Inset: schematic diagram of c-AFM measurements, c) Log J vs log V plots for Mott-Gurney SCLC fitting of the hole-only devices using NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx films (Inset: the architecture of hole-only device), d) UPS spectra of NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx films (Inset: Enlarged UPS spectra from 14 eV to 18 eV).

To figure out the mechanism behind the improvement of electrical properties upon the Li and Ag co-doping, X-ray photoelectron spectroscopy (XPS) measurements are performed to analyze the chemical states and surface element compositions of the NiOx and Li,Ag:NiOx films. The successful incorporation of Li and Ag in the Li,Ag:NiOx film is confirmed, as shown in Figure S5, Figure 3a and Figure 3b. Uniform inclusion of Ag can be further confirmed by energy-dispersive X-ray



spectroscopy (EDS) mapping of the Li,Ag:NiOx film shown in Figure S7 (it is difficult to detect the Li element signals in EDS and the corresponding images for the pristine NiOx film are depicted in Figure S6). According to the XPS results, the obtained atomic ratio of Li:Ag:Ni is 0.53:1.59:97.88 for the Li,Ag:NiOx film, which agrees with the initial precursor ratio, revealing that the Li and Ag can be readily doped in NiOx lattice. Typically, the Ni 2p spectra consists of the Ni 2p3/2 (850-865 eV) and Ni 2p1/2 (870-880 eV) spin-orbit levels (Figure 3c). We can observe that there is a distinct change for the Ni 2p3/2 peaks due to the Li and Ag co-doping. Figure 3d presents the high resolution Ni 2p3/2 peaks of the NiOx and Li,Ag:NiOx films. By deconvolving with the Lorentzian-Gaussian function, the Ni 2p3/2 peak can be separated to two peaks. Generally, the peak at 854 eV is assigned to Ni2+ in cubic NiOx rock salt,42-43 while the peak at 856 eV has previously been ascribed to the Ni2+ vacancy-induced Ni3+ ion44. The appearance of Ni2+ and Ni3+ states is an indicative of nonstoichiometric nature for both doped and undoped films. Specially, after Li and Ag co-doping, the intensity of Ni3+ is higher than that of Ni2+. The higher Ni3+ ratio in the Li,Ag:NiOx film may contribute to the improvement of hole conductivity45, which is in agreement with previous works on Li-doped and Ag-doped NiOx.39, 46


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a

b Li, Ag:NiOx

NiOx

58

57

56

55

54

53

52

7

Ni 2p

NiOx

7

Ni 2p3/2

6

Intensity (cps/104)

4 3

12

Ni 2p

NiOx

10 8

372

370

368

366

Li, Ag: NiOx

Raw data Fitting Background

Ni2+

5 4 3 14

Ni 2p3/2

Raw data Fitting Background

NiOx

12 10 8

6 4

374

Ni3+

6

5

376

Binding Energy (eV)

d

Li, Ag: NiOx

Li, Ag: NiOx

378

Binding Energy (eV)

c

Ag 3d

Intensity (a.u.)

Intensity (a.u.)

Li 1s

Intensity (cps/104)

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


6

880

870

860

Binding Energy (eV)

850

4

858

857

856

855

854

853

852

851

Binding Energy (eV)

Figure 3. XPS spectra of (a) Li 1s peaks, (b) Ag 3d peaks, (c) Ni 2p peaks and (d) high resolution Ni 2p3/2 peaks of the NiOx and Li,Ag:NiOx films.

Furthermore, the first-principle density functional theory (DFT) calculations are performed to elucidate the role of Li and Ag in the NiOx lattice and the electrical structures change resulting from Li and Ag co-doping. To investigate the location of Li and Ag in the NiOx lattice, the formation energies of related defects in Li,Ag-codoped NiOx are calculated. As shown in Table S1, the formation energy of AgNi-LiNi is the smallest among all related defects, indicating that the Ag and Li prefer to occupy the substitutional Ni site (Figure 4). Figure 4b and 4d show the density of states (DOS) for the pristine NiOx and Li,Ag:NiOx, respectively. The calculated densities of states reveal that Li and Ag co-doping (i.e. AgNi-LiNi) will result in the creation of shallower acceptor levels compared to the pristine NiOx, which might contribute to the enhancement of the hole concentration of Li,Ag:NiOx.



b

Ef

100

Total DOS (counts)

a

50 0 -50 -100

Ni

-10

O

c

d

Ni

-5

0

Energy (eV)

5

10

5

10

Ef

40

Total DOS (counts)

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

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20

0

-20

-40

O Li

-10

-5

Ag

0

Energy (eV)

Figure 4. (a) Atomic model and (b) the total DOS of undoped NiOx, (c) atomic model and (d) the total DOS of Li,Ag-codoped NiOx (AgNi-LiNi). The Fermi level for the calculation is set at zero.

In vertical planar PSCs, the perovskite active layers are prepared on the HTLs during the device fabrication. Therefore, the HTL will play an important role in the crystal growth of perovskite and determine the ultimate crystal quality of perovskite active layers, which is pivotal for the device performance. Here, the influences of NiOx HTLs (undoped and doped) on the perovskite film formation, surface morphology and crystallization are systematically investigated via field-effect scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) measurements. As can be seen in Figure 5a, the MAPbI3 films grown on various HTLs (whether the pristine NiOx or the doped NiOx) all show the smooth and dense polycrystalline surface with high coverage. In spite of this, the perovskite films grown on the doped NiOx show larger grain size compared to the film fabricated on the pristine NiOx (100-200 nm), especially for the Li and Ag co-doped NiOx (300-500 nm). The crystallinity of the


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MAPbI3 perovskite films on different HTLs is then evaluated using XRD. As illustrated in Figure 5b, some obvious XRD peaks at 14.6°, 20.5°, 28.9° and 32.4° can be observed for all of the patterns, which can be assigned to the (110), (112), (220) and (310) reflections of the MAPbI3 tetragonal phase.47 The full width at half maximum (FWHM) of the (110) peak of the MAPbI3 film on NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx is 0.167°, 0.166°, 0.161° and 0.152°, respectively. More interestingly, the intensity ratio of the (110) peak to (112) peak for the MAPbI3 film grown on Li,Ag:NiOx is largest among all samples. These results suggest that the MAPbI3 film on Li and Ag co-doped NiOx exhibits better crystallinity and crystalline orientation, which is consistent with the SEM observations. Such discrepancy between the crystalline quality of MAPbI3 on various HTLs is likely originated from the different surface nature of NiOx films. We further perform the contact angle measurements using dimethyl sulfoxide (DMSO) as the probing solvent for the pristine NiOx and Li,Ag:NiOx films. As revealed in Figure S8, the contact angle of the pristine NiOx and Li,Ag:NiOx films shows no significant difference, which is 30.5° and 35.1°, respectively. Therefore, the smoother surface morphology of the Li,Ag:NiOx film (Figure 1d) might be the main factor which influences the nucleation and growth manner of the above perovskite.7,16 In addition, the ultraviolet-visible (UV-vis) absorption spectra of the MAPbI3 films on various HTLs are shown in Figure S9. It can be observed that the absorption intensities of the perovskite films grown on the doped NiOx films shows an obvious improvement compared to that of the perovskite on the NiOx films. Moreover, a red-shift of absorption edge can be observed in the MAPbI3 film on Li,Ag:NiOx, which may be caused by the larger crystal grain size. The better crystallinity and higher absorbance of perovskite films induced by the Li and Ag co-doped NiOx will be beneficial for the improvement of device performance. Besides, to explore the impact of the doping on charge extraction efficiency at the interface of the NiOx HTLs and perovskite layers, steady-state and time-resolved photoluminescence (PL) measurements are conducted. Figure 5c presents the PL spectra of MAPbI3 films on different HTLs. As a reference, we also deposit MAPbI3 on top of glass. Relative to the intrinsic fluorescence emission of MAPbI3 on glass,



the PL emission is dramatically quenched by employing NiOx-based HTLs and the Li,Ag:NiOx hole selective contact exhibits the highest quenching rates among all NiOx-based HTLs. Time-resolved photoluminescence (TRPL) spectra in Figure S10 also demonstrates the same tendency. A gradual decrease in the PL lifetime from the MAPbI3/NiOx, MAPbI3/Li:NiOx, MAPbI3/Ag:NiOx to MAPbI3/Li,Ag:NiOx can be observed. The enhanced hole extraction and transport ability in the case of the Li,Ag:NiOx HTL is reasonably related to its increased p-type conductivity, faster hole mobility and higher WF.48

a

110

c

220 310 112

MAPbI3/glass MAPbI3/NiOx

MAPbI3/Li,Ag:NiOx

MAPbI3/Ag:NiOx

MAPbI3/Li:NiOx

MAPbI3/Li:NiOx

Intensity (a.u.)

b 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

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MAPbI3/Ag:NiOx MAPbI3/Li,Ag:NiOx

MAPbI3/NiOx

10

20

30

2 Theta (deg)

40

50

730

740

750

760

770

780

790

800

Wavelength (nm)

Figure 5. (a) SEM images and (b) XRD patterns of MAPbI3 films deposited on the NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx film, (c) steady-state PL spectra of the MAPbI3 films deposited on the glass, NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx film.


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Motivated by the improved morphology and crystallinity of perovskite films, and the enhanced charge transport at the HTLs/perovskite interface induced by Li and Ag co-doped NiOx HTLs, the inverted planar PSCs, consisting of glass/ITO/undoped or doped

NiOx/MAPbI3/phenyl-C61-butyric

acid

methyl

ester

(PC61BM)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Ag, are fabricated to evaluate their optoelectronic properties. Here, the thickness of the pristine or doped NiOx film is optimized to be about 80 nm (inset of Figure 6a),39 capped by ≈ 310 nm of MAPbI3 active layer. The measured J-V curves of the best-performance devices using an AM 1.5G solar simulator (100 mW cm-2) is presented in Figure 6a, and the corresponding photovoltaic parameters, including short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) are summarized in Table 1. As expected, taking the doped NiOx as HTLs can significantly improve the device performance, specifically for the Li,Ag:NiOx case. For the best-performing Li,Ag:NiOx device, a Voc ~ 1.13 V with a FF of 80% and a Jsc of 21.29 mA cm-2 is obtained, resulting in a peak PCE of 19.24%. External quantum efficiency (EQE) spectra of the best-performing devices with the NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx as HTLs are shown in Figure 6b. Likewise, the EQE value of the device with Li and Ag co-doped NiOx is the highest among all devices across the wavelength ranging from 300-800 nm. The integrated Jsc values from the EQE spectra are consistent with the results of J-V curves for all samples. More intriguingly, the Li,Ag:NiOx-based device shows a wide HTL thickness-processing window, i.e. the PCE of Li,Ag:NiOx PSCs exhibits a weak dependency on the HTL thickness (inset of Figure 6a). Whereas, the device performance of PSCs with the pristine NiOx HTL shows a sharp drop when the HTL thickness increases to 120 nm. Such wide thickness-processing window of the Li,Ag:NiOx HTL will be in favor of industrial production. Further analysis of the relevant photovoltaic parameters reveals that the improvement of device performance primarily lies in the dramatic increase of Jsc. As discussed earlier, the significant improvement in Jsc of the Li,Ag:NiOx-based device may originate from the improved electrical properties of the NiOx film co-doped with Li and Ag, and the enhanced crystal quality of above perovskite, which enables more



efficient charge extraction/transport at the HTL/perovskite interface. To acquire in-depth insights into the origin of the device performance enhancement resulting from the employment of the Li,Ag:NiOx HTL, the Mott-Schottky analysis (Figure 6c) and electrochemical impedance spectroscopy (EIS, Figure 6d) are measured to study the internal charge extraction and carrier recombination behaviors. In principle, the trap-assisted charge recombination occurring at the perovskite photoactive layer and the device interfaces can lower the energetic offset between EFn and Ep (EFn and Ep refer to the quasi Fermi level of charge transport layer/electrode contact and perovskite film, respectively), eventually resulting in the decrease of the flat-band potential.49 Actually, C-2-V characteristics based on Mott-Schottky model can reveal the influence of interfacial charge accumulation on the potential barrier according to the equation: 50-51

= Where NA is the density of excited states, Voc is the applied bias (i.e. open-circuit voltage), q is elementary charge, A is the active area, ε is the static permittivity and ε0 is the permittivity of free space. Figure 6c shows that the build-in potential (Vbi) values of the pristine NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx-based devices are 0.79 V, 0.84 V, 0.95 V and 0.98 V in the dark condition, respectively. Apparently, the Vbi in Li,Ag:NiOx device is higher compared to other devices. The calculated NA value of the Li,Ag:NiOx-based device is 8.75 × 1016 cm−3, whereas the NA value is 3.21 × 1016 cm−3 for the pristine NiOx-based device. Evidently, Li and Ag co-doping can reduce the charge accumulation at interface. Correspondingly, the EIS is used to investigate the interfacial charge transport and recombination behaviors in the solar cells. The Nyquist plots are measured in a frequency range of 1 Hz to 106 Hz at an applied voltage close to the Voc of each PSC and an AC amplitude of 10 mV in the dark condition at 25 °C. A clear semicircle can be distinguished in the low-frequency region for all devices. As we know, in the Nyquist plot, the recombination resistance and transport resistance can be obtained by equivalent circuit at low frequency and high frequency, respectively. Therefore, the Li,Ag:NiOx device exhibits higher


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Page 15 of 28

recombination resistance as compared with other devices (Figure 6d). Hence, the application of the NiOx HTL co-doped with lithium and silver can effectively optimize the charge extraction/transport and diminish charge accumulation at the interfaces, thus, boosting the device performance, including reducing the hysteresis

0

NiOx

Li:NiOx

Ag:NiOx

b100

Li,Ag:NiOx

20

-5

-15

NiOx

15 14 13

20

40

60

80

100

120

Thinkness (nm)

20

60 10

40

5

20

0.2

0.4

0.6

0.8

1.0

0 300

1.2

Voltage (V)

c

Li,Ag:NiOx

15

16

-20 -25 0.0

Li:NiOx

Ag:NiOx

17

EQE (%)

-10

Li,Ag:NiOx

18

NiOx

80

19

PCE Value (%)

400

500

600

700

800

0

Wavelength (nm)

d

4

NiOx

-800

Li:NiOx Ag:NiOx Li,Ag:NiOx

3 2

-600

NiOx

-Zim (Ω)

a

Integrated Jsc (mA cm-2)

behavior. Current Density (mA cm-2)

a

C-2 (1018 F-2 cm-2)

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


Li:NiOx Ag:NiOx

1

Li,Ag:NiOx

0 0.2

0.4

-400 -200

0.6

0.8

1.0

Voltage (V)

0

0

500

1000

1500

2000

Zre (Ω)

Figure 6. (a) Current-voltage (J-V) curves, (b) EQE spectra and the corresponding integrated Jsc of the best performance PSCs based on the NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx HTLs, (c) Mott-Schottky plots of capacitance-voltage measurements of the PSCs based on the NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx HTLs and (d) Nyquist plots obtained from electrochemical impedance spectra (EIS) of the PSCs

cbased on the NiO , Li:NiO , Ag:NiO x

x

x

and Li,Ag:NiOx HTLs measured in a frequency

range of 1 Hz to 106 Hz at an applied voltage close to the Voc of each PSC and an AC amplitude of 10 mV in the dark condition at 25 °C. Inset of (a): the PCE variations of the PSCs based on NiOx and Li,Ag:NiOx HTLs with different thicknesses.



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Table 1. Summary of the photovoltaic parameters of the best-performing PSCs based on the NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx HTLs. HTL

Voc (V)

Jsc (mA cm-2)

FF

PCE (%)

NiOx

1.08

19.20

0.78

16.19

Li:NiOx

1.11

19.78

0.79

17.34

Ag:NiOx

1.10

20.25

0.79

17.59

Li,Ag:NiOx

1.13

21.29

0.80

19.24

It has been reported that the hysteresis behavior stems from the defect-assisted trap, ionic migration in the electric field and the charge accumulate at the interface52, which will influence the photovoltage performance of devices. Figure 7a and 7b present the J − V curves in reverse and forward scan direction for the best-performing devices based on the pristine NiOx and Li,Ag:NiOx HTLs, respectively. It is obvious that the NiOx cell shows more severe hysteresis with the PCE of only 13.99% under forward scan condition, degrades by 13.58% compared with reverse scan direction. On the contrary, the Li,Ag:NiOx cell shows reduced hysteresis with the PCE as high as 18.75% under forward scan condition, decreased by 2.49% compared to the reverse scan direction. It is not surprising that the Li,Ag:NiOx cell shows less hysteresis effect compared with the NiOx cell, which is benefited from the excellent charge transport/collection ability endowed by the Li,Ag:NiOx HTL. At the same time, the device reproducibility is also improved by introducing the Li,Ag:NiOx HTL, as displayed in Figure 7c. Finally, the stability of PSCs is evaluated by measuring the photovoltaic performance at regular intervals in 30 days. The unencapsulated devices are stored in a glass container at 25 °C with the relative humidity (RH) of 30 ± 2% and tested at ambient environment. As a comparison, the PEDOT:PSS-based device is also fabricated because PEDOT:PSS is the most commonly used HTLs in photovoltaic devices. The device performance as a function of storage is recorded in Figure 7d. We discover that the stability of NiOx-based devices is better than that of PEDOT:PSS-based devices due to the inherent acidity and hygroscopicity of PEDOT:PSS. More importantly, compared with the pristine NiOx cell, the stability of


Page 17 of 28

PSC can be further improved via the Li,Ag:NiOx HTL. The Li,Ag:NiOx cell can retain 95% of its initial PCE after 30 days while the PCE of the NiOx cell has decayed to about only 85% of its initial value within the same time period. The better long-term stability of the Li,Ag:NiOx cell may be ascribed to the excellent chemical and physical stability of inorganic HTLs, stable HTL/perovskite interface, better crystallinity and larger grain size of the above perovskite grown on the Li,Ag:NiOx

0

Scanning

Jsc (mA cm-2)

directions

Voc (V)

FF (%)

PCE (%)

Reverse

19.20

1.08

0.78

16.19

Forward

18.85

1.08

0.70

13.99

Li,Ag:NiOxForward Li,Ag:NiOxBackward

-5 Scanning

Jsc (mA cm-2)

Voc (V)

FF (%)

PCE (%)

Reverse

21.29

1.13

0.80

19.24

Forward

21.20

1.12

0.79

18.75

directions

-15 -20

-25 0.0

0.2

0.4

0.6

0.8

Voltage (V) 22

d

1.16 1.14

21

19

1.10 1.08 1.06 1.04

18

NiOx

Li,Ag:NiOx

0.6

0.8

Voltage (V)

1.0

1.2

1.0

0.6

PEDOT:PSS NiOx

0.4

Li,Ag:NiOx

0.2 0.0

Li,Ag:NiOx

0

5

21

0.82

0.4

0.8

1.02

NiOx

0.2

1.0

Normalized PCE

Voltage (V)

1.12

20

-25 0.0

1.0

Normalized Voc

-20

10 15 20 Time (days)

25

30

0.9 0.8

PEDOT:PSS NiOx

0.7

Li,Ag:NiOx

0.6 0.5

0

5


25

30

20 18

0.76 0.74

15

0.70

13

Li,Ag:NiOx

0.8

16 14

NiOx

0.9

17

0.72

1.0

1.0 Normalized FF

19

0.78

Normalized Jsc

0.80

PCE

FF

0

-10

-15

Current desity (mA cm-2)

c

NiOx Backward

-5 -10

b

NiOx Forward

-2

a

Current Density (mA cm-2)

film. Curruent Density (mA cm )

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


PEDOT:PSS NiOx

0.7

Li,Ag:NiOx

0.6

0.9 0.8 0.7

PEDOT:PSS NiOx

0.6

Li,Ag:NiOx

12

NiOx

Li,Ag:NiOx

0.5

0

5


25

30

0.5

0

5


25

30

Figure 7. J − V curves in reverse and forward scan direction for the best-performing devices based on (a) the pristine NiOx HTL and (b) the Li,Ag:NiOx HTL; (c) Photovoltaic metrics of devices based on NiOx and Li,Ag:NiOx HTLs obtained from 30 devices, (d) The stability test of the devices based on NiOx and Li,Ag:NiOx HTLs as a function of storage under the ambient environment (30 ± 2% humidity, T = 25 °C).



3. Conclusions In summary, the solution-processed lithium and silver co-doped NiOx (Li,Ag:NiOx) film is successfully introduced as efficient HTLs in inverted planar PSCs. It has been found that the Li and Ag co-doping can significantly enhance the electrical conductivity and hole mobility of the NiOx film, as well as adjust the work function to provide better interface energy band alignment between HTLs and perovskite films. Meanwhile, the crystal quality of MAPbI3 film grown on the Li,Ag:NiOx films is dramatically improved. As a result, the performance of the Li,Ag:NiOx-based device exhibits a champion PCE of 19.24% along with less hysteresis effect compared to the devices based on the pristine NiOx or single-doped NiOx. In addition, the devices based on Li,Ag:NiOx HTLs show better stability with an 95% PCE retention after 30 days storage at the relative humidity of 30 ± 2% without encapsulation. This work reveals that Li and Ag co-doped NiOx is very promising hole extraction material for efficient and stable inverted planar PSCs.

ASSOCIATED CONTENT Supporting Information The materials, film preparation, devices fabrication, characterization and computation method were described in detail. X-ray diffraction spectra, laser scanning confocal microscopy images and scanning Kelvin probe microscopy (SKPM) characteristics of the NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx film; Schematic diagram of energy levels at the inverted PSC devices; SEM images, wide survey XPS spectra and contact angle measurements of the NiOx and Li,Ag:NiOx film and the energy-dispersive X-ray spectroscopy (EDS) element mapping of the corresponding elements; formation energy of related defects in Li,Ag-codoped NiOx; UV-vis absorption spectra and time-resolved PL spectra of the MAPbI3 films deposited on the NiOx, Li:NiOx, Ag:NiOx and Li,Ag:NiOx film were provided.


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Author Information Corresponding Author Fan Li, E-mail: [email protected] ORCID Fan Li: 0000-0001-8972-4715 #Xuefeng

Xia and Yihua Jiang contributed equally to this work.

NOTES These authors declare no competing financial interest.

Acknowledgments This work was supported by the National Natural Science Foundation of China (61664006 and 61464006) and Natural Science Foundation of Jiangxi Province, China (20171ACB21010). F. L. acknowledges the support from Young Scientist Project of Jiangxi Province (20142BCB23002). Xuefeng Xia thanks the support from Graduate Innovation Foundation of Jiangxi Province (YC2017-S001).

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Lithium and Silver Co-Doped Nickel Oxide Hole-Transporting Layer Boosting the Efficiency and Stability of Inverted Planar Perovskite Solar Cells

A lithium and silver co-doping strategy has been successfully implied to prepare NiOx films with excellent electrical properties, which can act as highly efficient HTLs for improving the efficiency and stability of inverted planar PSCs.


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Lithium and silver co-doped nickel oxide hole-transporting layer

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