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Solution-Processed, Silver-Doped NiOx as Hole Transporting Layer for High Efficiency Inverted Perovskite Solar Cells Jianghui Zheng, Long Hu, Jae Sung Yun, Meng Zhang, Cho-Fai Jonathan Lau, Jueming Bing, Xiaofan Deng, Qingshan Ma, Yongyoon Cho, Wei-Fei Fu, Chao Chen, Martin A. Green, Shujuan Huang, and Anita W. Y. Ho-Baillie ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00129 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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ACS Applied Energy Materials

Solution-Processed, Silver-Doped NiOx as Hole Transporting Layer for High Efficiency Inverted Perovskite Solar Cells Jianghui Zheng1,2, Long Hu1, Jae S. Yun1, Meng Zhang1, Cho Fai Jonathan Lau1, Jueming Bing1, Xiaofan Deng1, Qingshan Ma1, Yongyoon Cho1, Weifei Fu3, Chao Chen2,*, Martin A. Green1, Shujuan Huang1 and Anita W. Y. Ho-Baillie1,* 1

Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia 2 College of Energy, Xiamen University, Xiamen, 361005, China 3 State Key Laboratory of Silicon Materials, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China * Corresponding Author: Anita Ho-Baillie, Email: [email protected] Chao Chen, Email: [email protected]

ABSTRACT: NiOx is as a promising hole transporting layer (HTL) for perovskite solar cells (PSCs) due to its good stability, large bandgap, and deep valence band. The use of NiOx as a HTL for “inverted” PSC as part of a monolithic silicon/perovskite tandem solar cell is also suitable when the processing temperature is suitably low. Solution processed NiOx at low temperature for PSCs remains to be improved due to the relatively low short circuit current density (Jsc) and fill factor (FF) of reported devices. In this work, the use of Ag-doping is reported for solution processed NiOx film at 300°C for inverted planer PSCs. We have shown that Ag-doping has no negative effect on the optical transmittance and morphology of the NiOx film and the overlying perovskite film. In addition, Ag doping is effective in improving conductivity, improving carrier extraction,

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and enhancing p-type property of the NiOx film confirmed by electrical characterisation, photo-luminescence measurements and ultraviolet photoelectron spectroscopy. These improvement

result

in

better

devices

based

on

the

ITO/Ag:NiOx/CH3NH3PbI3/PCBM/BCP/Ag structure with improved average FF (from 69% to 75 %), enhanced average JSC (by 1.2 mA/cm2 absolute) and enhanced average VOC (by 29mV absolute). The average efficiency of these devices is 16.3% while the best device achieves a PCE of 17.3% with negligible hysteresis and a stabilized efficiency of 17.1%. In comparison, devices that use un-doped NiOx have an average efficiency of 13.5%. This work demonstrates that silver is a promising doping material for NiOx by a simple solution process for high performance inverted PSCs and perovskite tandems.

KEYWORDS: Hole Transport layer, NiOx, Ag-doped NiOx, perovskite solar cells, inverted structure.

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INTRODUCTION Rapid development of organic-inorganic hybrid perovskite solar cells (PSCs),1 has attracted huge interest due to its high-efficiency and low cost potential which can become an alternative to conventional photovoltaic devices such as Si, CdTe and CIGS solar cells if comparable lifetime can be achieved.2 Nevertheless, the certificated power conversion efficiency (PCE) of state of the art cells has risen from 14.1% 3 in May 2013 to 22.1% 4 in Mar 2016 showing the great performance potential of this cell technology. Most state of the art PSC devices use a conventional structure or a “n-i-p” structure which resembles this structure transparent conductive oxides (TCO)/compact TiO2/mesoporous TiO2/perovskite layer/hole transporting layer (HTL)/metal electrode where the TCO side of the cell is the light receiving side.5-9 This type of PSCs has some shortcomings such as the requirement of high temperature (500oC) treatment for the compact and mesoporous TiO2 electron transport layers and UV instability associated with TiO2 10, 11 and instability of the commonly used HTL with additives.12, 13 Recently, “inverted” or a “p-i-n” HTL/perovskite/electron transporting layer (ETL) device structure has emerged.14-16 This device structure has many advantages such as the fact that it is a planar structure and lower hysteresis compared to planar “n-i-p” device.16-18 Other advantages include low processing temperature while maintaining respectable efficiencies and promising stability.17-21 Moreover, this device structure is well matched to the polarity of commercial p-n junction crystalline silicon (c-Si) solar cells when the

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perovskite (with the HTL fabricated first followed by perovskite deposition and electron transport (n-) layer on the light receiving side) is directly fabricated on the Si cell to achieve a monolithic c-Si/perovskite tandem. It is been demonstrated that perovskite cells that employ a [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) heterojunction in an inverted structure shows relatively small hysteresis

and

respectable

performance.22,

23

For

this

structure,

Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) or its variants is the most commonly used p-type HTL material in inverted structure.16, 24-27 However, due to the mismatch between the work functions (WF) of perovskite and PEDOT:PSS, devices that use PEDOT:PSS exhibit limited open voltages (Voc; normally Voc< 1V) and therefore have lower PCE potentials than devices using other HTL materials.28 Some groups have reported the use of modified PEDOT:PSS with an additional organic HTL or interfacial material to achieve higher VOC.

23, 29, 30

For example, Lin et al. and

Malinkiewicz et al. has demonstrated cells that use poly(N,N′ -bis(4-butylphenyl)-N,N′ -bis(phenyl)benzidine)

(polyTPD)

or

(poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di(thien-2-yl)-2′,1′,3′-benzothiadia zole)) (PCDTBT) modified PEDOT:PSS achieving Voc of 1.05 V.

23, 31

However, for

these devices, perovskite layer needs to evaporated on the HTL to obtain good contact due to the non-wetting properties of organic surface; Moreover, PEDOT:PSS is not suitable for long-term stable devices due to its high acidity and hygroscopicity.16

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Alternatively, inorganic HTL materials, such as NiOx,32, 33 CuAlO2,34 V2Ox,35 WO3,36, 37 Cu2O,

38

GeO2,

39, 40

CuSCN,41 CuI.24, 42 , and doped copper phthalocyanine

43

have

emerged for inverted PSCs as they are solution-processable, they have higher WF, and better stability. Solution processed V2Ox has applied as HTL by Peng et al in PSC achieving a PCE of 14.8%.35 Li et al employed WO3 as HTL in PSC producing a PCE of 7.7%.37 When an electrodeposited CuSCN was used in p-i-n PSCs devices, a PCE of 16.6% could be achieved with VOC of 1.0V.41 Wang et al. demonstrated a PCE of 14.7% by employing CuI as a HTL prepared by solid–gas transformation in the inverted PSCs.42 Although the choices are plenty, not all inorganic HTL are suitable for p-i-n PSCs unless they are able to deliver cell performance close to those of the state of the art PSC devices.

Among the p-type candidates, NiOx is attractive due to its good stability, large bandgap, deep valence band,44, 45 higher Voc output and therefore higher PCE potential, and better stability for its application in PSCs. A few notable perovskite solar cells using NiOx as HTLs processed by different methods include work by Jin et al. who demonstrated a 12.6% efficient cell using cobalt-doped NiOx by sputtering.46 Seo et al. fabricated the NiOx film by atomic layer deposition (ALD) and achieved a PCE at 16.4%.47 Kim et al. reported a uniform NiOx layer prepared by electrochemical deposition process and achieved a 17.0% efficient cell with a device area of 1.084 cm2.48 Most recently, Han et

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al. demonstrated 19.6% device on 1.02 cm2 using Li, Mg co-doped NiOx film by spray pyrolysis. The 1cm2 cells is certified to be 19.2%.49 This is the highest PCE achieved on NiOx -based devices. Among the various deposition methods, solution process by spin coating nickel precursor is the simplest. In 2014, Hu et al. demonstrated a 7.6% efficient PSCs by sequential deposition of a CH3NH3PbI3 (MAPbI3) layer on solution processed planar NiO film.50 After that, many researcher increased the performance of solution processed NiOx-based PSCs device.19, 21, 32, 45, 51-55 Zhang et al, and Zhu et al, have demonstrated that room temperature processed NiOx can be fabricated for perovskite solar cells32, 54 and perovskite photodetectors55. With regards to doping NiOx, Jen et al. demonstrated a high-efficiency planar PSCs based on solution processed copper

-doped

NiOx

with

impressive

PCEs

up

to

15.4%

compared

to

un-doped-NiOx-based devices with the highest PCE being 8.9%.56 Most recently, Chen et al. demonstrated that Cs doped NiOx HTL exhibits better electron conductivity and higher work function, resulting in 19.4% efficiency.57 However, PCE of most PSC’s using solution processed NiOx is still limited due to relatively low values of short-circuit current density (JSC) and fill factor (FF) achieved. In this work, we report the use of Ag doping for solution processed NiOx film at 300°C as a HTL for PSCs. We report the effect of Ag-doping on NiOx films’ optical transmittance property, surface morphology (and its effect on the morphology of the overlying perovskite layer), conductivity, carrier dynamics, and energy levels using a

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suite of characterisations. We show that Ag doped NiOx has enhanced photovoltaic properties

resulting

in

better

devices

based

on

the

ITO/Ag:NiOx/CH3NH3PbI3/PCBM/BCP/Ag structure with improved average FF (from 69% to 75 %), enhanced average JSC (by 1.2 mA/cm2 absolute) and enhanced average VOC (by 29mV absolute). The average efficiency of demonstrated devices is 16.3% while the best device achieves a PCE of 17.3% with negligible hysteresis and a stabilized efficiency of 17.1%. In comparison, devices that use un-doped NiOx have an average efficiency of 13.5%. This work demonstrates that silver is a promising doping material for NiOx by a simple solution process for high performance inverted PSCs suitable for tandem solar cells.

EXPERIMENTAL SECTION Materials. All of the chemical and materials were purchased and used as received. Nickel nitrate hexahydrate (99.99%), silver nitrate (99%), ethylenediamine, anhydrous ethylene glycol, chlorobenzene, isopropyl alcohol (IPA) were all purchased from Sigma Aldrich. PbI2 (99.9985%) was purchased from Alfa Aesar. CH3NH3I (MAI) was purchased from Dyesol. PCBM (99%) was purchased from Solenne. Bathocuproine (99%) was purchased from LumTec. Device fabrication. Patterned ITO-coated glass (8 Ω−1, transmittance 86%) was cleaned by sonication in de-ionized water with 2% Hellmanex, de-ionized water,

7



acetone and isopropanol for 20 min, respectively. The ITO substrate was treated by UVO cleaner for 15 min. For undoped NiOx films, a 1M solution of nickel nitrate hexahydrate and ethylenediamine in anhydrous ethylene glycol was spun on ITO-coated glass at 5,000 r.p.m. for 50 s. For Ag doped NiOx films, a 1M solution of silver nitrate with different molar ratios (from 3% to 8%) was mixed with nickel nitrate hexahydrate and ethylenediamine in anhydrous ethylene glycol which was then spun on ITO-coated glass at 5,000 r.p.m. for 50 s. All the samples were then dried at 100 °C for 5 min followed by annealing at 300 °C for 1 h on a hotplate in air. After that, the as-prepared NiOx coated substrates were directly transferred to a N2 filled glovebox without any treatments. The PbI2 precursor was prepared by dissolving 461 mg PbI2 powder in 1 ml DMF and 71µl DMSO, and was then spin coated on the as-prepared NiOx coated substrate at 3000 rpm for 30 s. The perovskite films were then spin-coated at 3000 rpm for 30 s by dropping a solution of MAI in IPA (40 mg/ml). The samples were dried at 100 oC for 10 min after deposition to produce a dark brown dense MAPbI3 film. Subsequently, the electron transporting material PCBM in anhydrous chlorobenzene (20 mg/ml) was deposited on the perovskite films by spin coating at 2000 rpm for 45 s. The bathocuproine (BCP) in IPA (0.5mg/ml) was dropwise then added dropwise on top of the PCBM during 6000 rpm for 15s spin-coating. Finally, 100 nm silver electrodes were deposited by thermal evaporation.

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Characterisations. X-ray diffraction (XRD) patterns were measured using a PANalytical Xpert Materials Research diffractometer system with a Cu Kα radiation source ( λ = 0.1541 nm) at 45 kV and 40 mA. The optical reﬂection and transmission spectra were measured using a Perkin Elmer Lambda1050 UV/Vis/NIR spectrophotometer. Top view and cross-sectional scanning electron microscopy (SEM) images were obtained using a ﬁeld emission SEM (NanoSEM 230). Photoluminescence (PL) imaging, and the PL decay traces were measured by Microtime200 microscope (Picoquant) using time correlated single photon counting (TCSPC) technique with excitation of 470 nm laser at 5 MHz repetition rate and detection through 760/40 nm band-pass filter. A bi-exponential function was used to fit the time resolved PL decays to obtain the fast (τ2) and slow (τ1) components and their weightings (A1 and A2).

= exp + exp

(1)

where I is PL intensity. The effective lifetimes (τeff ) can then be calculated using the lifetimes and weights using the following58: τ =

∑ ∑

(2)

The current density–voltage (J–V) measurements were performed using a solar cell I–V testing system from Abet Technologies, Inc. (using class AAA solar simulator) under an illumination power of 100 mW cm-2 with an 0.159 cm2 aperture and a scan rate of 30

9



mV s

-1

both from VOC to JSC direction (1.1 V to -0.1 V) or from JSC to VOC direction

(-0.1 V to 1.1 V). The bias voltage for the steady-state measurements was set at the maximum power point (MPP) voltage obtained from the J-V measurement. The external quantum efficiency (EQE) measurement was carried out using the PV Measurement QXE7 Spectral Response system with monochromatic light from a xenon arc lamp. X-ray photoelectron spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS) were carried out using ESCALAB250Xi, Thermo Scientific, UK. Test structures involving doped and un-doped NiOx films on silicon substrates are fabricated for this measurement. Atomic Force Microscopy (AFM) measurements were performed on implied perovskite films to evaluate surface roughness using Bruker Dimension ICON SPM. All measurements (except XPS, UPS and SEM which are under vacuum) were undertaken at room temperature in ambient condition. RESULTS AND DISCUSSION Figure 1a shows the X-ray diffraction (XRD) patterns of the un-doped NiOx and Ag-doped NiOx (5 mol% Ag in NiO, Ag:NiOx) films on glass. As displayed, the Bragg peaks of the un-doped sample matched well with that of the cubic structure of NiO PDF#[47-1049]. The XRD peaks of Ag:NiOx film shifted slightly towards smaller 2theta suggesting expansion of the lattice. This is because when the larger Ag+ (ionic

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radius r = 1.15 Å for 1+ oxidation state, 6-fold coordination) substituted the smaller Ni2+ ions (r = 0.69 Å for 2+ oxidation state, 6-fold coordination),59 a cubic structure is formed with larger lattice constant according to the Bragg's Law. This result indicated that Ag+ ions have partially displaced Ni2+ ions, and have therefore, doped NiO. X-ray photoelectron spectroscopy (XPS) measurements are also performed on un-doped NiOx and Ag doped NiOx films. Results are shown in Figure S1. Figure S1a. shows the XPS spectra for Ni 2p2/1 and Ni 2p2/3 at around 873 eV and 854 eV, respectively indicating the presence of both NiO (Ni 2p2/3 peak at 854.1 eV) and Ni2O3 (Ni 2p2/3 855.8 eV) phases in both films. In Figure S1b, the additional peaks for Ag 3d3/2 and Ag3d5/2 are also found in Ag doped NiOx films. These peaks are not present in un-doped NiOx film. The Ag content in the Ag:NiOx film is estimated to be 3.6%, which is close to the Ag molar ratio used in NiO precursor (5 mol%) indicating good incorporation of Ag in the doped NiOx film. Figure 1b shows the optical transmittance of the same films coated on glass. Both films show high transmittance (92%) at 400 to 800 nm. Figure 1c shows theirs Tauc plots to estimate optical band gaps. The estimated optical gaps of ~4.35 eV are well matched with the reported values.50, 60 The transmittance of un-doped NiOx and Ag-doped NiOx films on ITO glass were also measured (Figure 1d) showing the negligible effect on the optical properties of NiOx after Ag doping. The high average transmittances of NiOx and Ag:NiOx coated ITO from 400 to 800 nm region at 82% show their suitability for high performance PSCs and perovskite tandems.

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Figure 1. (a): XRD patterns, (b) transmittance, and (c) its Tauc plot for the un-doped and Ag-doped NiOx films on glass. (d) Transmittance spectra for the bare ITO; undoped NiOx and Ag-doped NiOx films on ITO glass.

Figures 2a and 2b show the top view scanning electron microscopy (SEM) images of un-doped NiOx and Ag:NiOx films on ITO glass. Figures 2c and 2d show the corresponding 3-dimensional (3-D) atomic force microscopy (AFM) images. It is revealed that both films are uniform and crack-free. The Ag-doped NiOx film is slightly smoother with a root mean square roughness RRMS of 6.0 nm (Figure 2d) compared to an un-doped film (RRMS=7.1 nm) (Figure 2c). We then characterize the morphologies of

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perovskite layers for the un-doped and doped NiOx. The corresponding SEM images and 3-D AFM images are shown in Figures 2e to 2h. The morphologies of perovskite layers on both types of HTL layer are comparable with similar uniformity and grain size (200-500 nm). These results are consistent with the reports on perovskite layers fabricated on Co2+ and Cs+ doped NiOx films.46, 57

Figure 2 Morphology of the NiOx and Ag:NiOx and perovskite films fabricated on NiOx and Ag:NiOx: SEM images of (a) NiOx on ITO glass, (b) Ag:NiOx on ITO glass, (e) MAPbI3/NiOx/ITO glass and (f) MAPbI3/Ag:NiOx/ITO glass, the scale bar is 1 µm. 3D AFM images of (c) NiOx on ITO glass, (d) Ag:NiOx on ITO glass, (g) MAPbI3/NiOx/ITO glass and (h) MAPbI3/Ag:NiOx/ITO glass . The scan area is 2µm×2µm.

Ultraviolet Photoelectron Spectroscopy (UPS) was carried out to check the energy level of the NiOx films and the results are shown in Figure 3a. The photoemission cut-offs of both films shown on the left indicate that the work function (WF) (from vacuum level) has been shifted upwards from 5.02 eV to 5.13 eV after Ag-doping (the work function

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values were calculated according to the formula Φ = hν (21.22 eV) − Ecut-off). The increased WF has the potential to produce higher Voc in the corresponding PSC devices. On the other hand, as shown on the right of Figure 3a, the valence band shifted by about 0.12 eV closer to the Fermi energy level after Ag doping showing an enhanced p-type property. This shift can help improve the film conductivity.60 To determine the effect of Ag-doping on the electrical conductivity of the NiOx, the conductivity of the films was obtained by the following, 61

σ = d/(AR)

(3)

where " is the conductivity, A is the active area (0.5x0.5 cm2), d is the thickness of the films (~15 nm), R is the resistance determined from the current voltage (I-V) measured across an Au/NiOx/ITO test structure (as illustrated in Figure 3b). The conductivity of the different NiOx films with different Ag doping concentrations were estimated to be 6.6x10-7 S·cm-1 (undoped), 1.3x10-6 S·cm-1 (3 mol%), 2.2x10-6 S·cm-1 (5 mol%) and 9.6x10-7 S·cm-1 (8 mol%). It is found that Ag doped NiOx shows enhanced conductivity compared to the undoped NiOx film. The increased conductivity from Ag doping compared to un-doped film is likely to be due to an increase in the disorder of the NiOx structure, an increase in oxygen vacancy with Ag+, see Equations 4 and 5 below, resulting in an increased number of hole carriers and hence an increase in conductibility. However, as Ag+ doping is further increased, there can be an opposing effect where Ag+ substitutes Ni2+ reducing the number of hole carriers associated with the Ni vacancies

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(intrinsic defects).53, 62, 63 This explains the drop in conductivity as Ag doping equals or is greater than 8 mol%. 2NiO → V)* ′′ + 2h∙ +2O- . + Ni/0123 . 4.678

(4)

2NiO 9::::; Ag )* ′ + V)* + 3h∙ +2.5O- . + Ni/0123 . ′′

(5)

where VNi” is the Ni cation vacancy at intrinsic Ni site, h∙ is the electron defect (hole), Oox is the normal oxygen at intrinsic O site, Ni/0123 . is the normal Ni atom at surface site and Ag )* ′ is the Ag cation vacancy at intrinsic Ni site.

Figure 3 (a) UPS results of the NiOx and Ag:NiOx films coated silicon substrate, and (b) I-V curves for un-doped NiOx and Ag:NiOx films based on Ag/NiOx/ITO structure. (c) Steady state and PL spectra of the ITO/MAPbI3,

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ITO/NiOx/MAPbI3 and


Page 16 of 33

ITO/Ag:NiOx/MAPbI3 films. (d) Time resolved PL spectra of ITO/NiOx/MAPbI3 film and ITO/Ag:NiOx/MAPbI3 films.

To study the effect of Ag doping on NiOx film on carrier dynamics, steady state photoluminescence (PL) and time resolved PL measurements were carried out on ITO/MAPbI3, ITO/NiOx/MAPbI3 and ITO/Ag:NiOx/MAPbI3 test structures. Results are shown in Figure 3c and 3d, the corresponding fitted lifetimes are summarized in Table 1. It can be seen from steady state PL results (Figure 3c) that PL quenching is strongest for MAPbI3 film on Ag:NiOx film. The weighting for the 2nd order decay (A2 in Table 1) in the time resolved PL (Figure 3c) increases and the associated lifetime ( τ2) for this decay decreases after Ag doping. These results suggest enhanced hole extraction ability and transport efficiency in the NiOx film after Ag doping.49, 56

Table 1. Bi-exponential fitting results of PL decay traces for ITO/NiOx/MAPbI3 film and ITO/Ag:NiOx/MAPbI3 films. Sample ITO/NiOx/MAPbI3 ITO/Ag:NiOx/MAPbI3

A1

A2

66.0% 34.0% 47.0% 53.0%

[@A] [@A] [@A] 48.26 31.17

10.05 8.81

44.52 25.82

For the demonstration of perovskite solar cells using Ag:NiOx and as part of optimisations, different Ag+ doping concentrations (from 0 to 8 mol %) were trialled on and with the configuration ITO/NiOx/MAPbI3/PCBM/BCP/Ag devices as illustrated in Figure 4a. The thicknesses of the layers are 200 nm (ITO), 15 nm (NiOx), 300 nm 16


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(MAPbI3), 80 nm (PCBM/BCP) and 100 nm (Ag) according to the cross sectional SEM, see Figure 4b. Based on the above measured band gap, work function and valence band edge for the un-doped NiOx and Ag:NiOx films, the energy level diagrams of PSCs with un-doped NiOx and Ag:NiOx HTLs are illustrated in Figure 4c. The smaller gap between the VB and the WF in the Ag:NiOx film indicates enhanced p-type property. Moreover, the WF for the Ag doped NiOx are slightly shifted allowing higher VOC in the associated PSC’s to be reached.

Figure 4. (a) Schematic illustration, and (b) cross-sectional SEM of the inverted PSCs with the cell structure ITO/Ag:NiOx/MAPbI3/PCBM/BCP/Ag. (c) Energy level diagram of perovskite solar cells with NiOx and Ag:NiOx HTLs. Electrical performances of devices that use NiOx doped by different concentrations of Ag are listed in Table 2. The distributions of PCE, JSC, FF and VOC are also summarized

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in Figure 5. Voc’s of cells with Ag doped HTL’s increase slightly (1.03 to 1.06 V) due to the modified WF (Figure 4c) as discussed above. The major improvement in PCE comes from the increased Jsc (19.2 to 20.4 mA/cm2) and FF (0.69 to 0.75) at the optimum Ag doping concentration at 5% due to enhanced electrical conductivity (Figure 3b), enhanced p-type property (Figure 4c) and better charge extraction (Table 1) as discussed above. At this optimum concentration, the Voc and Jsc have smaller spreads, see Figure 5 because the 5mol% Ag doped NiOx films have better quality in terms of conductivity and higher WF, which result in better repeatability. However, for Ag doping ≥5%, FF distribution widens possible due to increased defects in NiOx film reducing film quality and device repeatability. Table 2. Device parameters of the PSCs based on Ag-doping concentration. NiOx type Voc(mV) Jsc(mA/cm2) 1033±40 19.3±0.7 NiOx 1042±49 20.1±0.9 3%Ag:NiOx 1062±33 20.4±0.7 5%Ag:NiOx 1048±19 19.1±0.8 8%Ag:NiOx

Ag:NiOx HTL with different FF(%)

PCE (best) (%)

68.7±7.3 71.1±3.9 74.7±3.7 71.7±3.5

13.5±2.2 (15.7) 14.2±1.8 (16.0) 16.3±1.0 (17.3) 13.9±1.2 (15.1)

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Figure 5. Distribution of (a) PCE, (b) JSC, (c) FF, and (d) and VOC for 12 devices with different Ag-doping concentration. The highest value is a maximum value. The highest bar is the 75th percentile value. The middle bar is the median value. The square mark is for average. The lowest bar is the 25th percentile value. The lowest value is the minimum. Figure 6a and 6c show the J-V curves (scanned at 30 mV/s) of the champion device of un-doped NiOx, Ag:NiOx based PSC devices, respectively. Figure 6b and 6d show the steady-state current densities and efficiencies of the same devices. Both devices have negligible hysteresis at the scan rate of 30 mV/s. In addition, the stabilized PCE’s at 15.5% and 17.1% are very close to the scanned IV efficiencies at 15.6%-15.7% and 17.2%-17.3%, for cells that use un-doped and Ag doped NiOx, respectively. Figure 7 shows the EQE spectra of NiOx and Ag:NiOx based PSC devices were also measured

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from 300 to 850 nm. The integrated current densities calculated form EQE spectra agree well with the values measured from J-V measurements. The improvement in current of Ag:NiOx based device comes from the improved short wavelength response which is due to the better carrier extraction by the Ag:NiOx compared to the un-doped NiOx. This agrees with results of PL measurements as discussed above. Table S1 summaries the high performance and corresponding preparation methods of the MAPbI3 based PSCs using NiOx as HTL and PCBM as electron transport layer. Results in the top section of Table S1 shows the results of PSCs using high temperature processed NiOx, with the highest PCE at 19% when the NiOx is deposited by spray pyrolysis at 500°C. This is not suitable for perovskite/Si tandem as the high processing temperature causes performance degradation in the under-lying Si device. The second half of Table S1 lists the PSCs reported using low temperature processed and solution processed NiOx. While the most recently demonstrated device that uses Cs doped NiOx as the HTL and PCBM/ZrAcac as the electron transport stack achieves a PCE of 19%,

57

our champion

cell that uses Ag doped NiOx achieves a respectable efficiency of 17.3% which is the highest amongst devices that use the most commonly used PCBM/BCP electron transport stack.

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Figure 6. (a)-(d) (a) J-V curve and (b) steady-state current density and efficiency of the champion device of NiOx based PSCs. (c) J-V curve and (d) steady-state current density and efficiency of the champion device of Ag:NiOx based PSCs.

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Figure 7. EQE spectra and integrated current densities of NiOx and Ag:NiOx based PSCs. We have also carried out a preliminary stability study on the shelf life of our PSCs as silver can be an issue if there is a possible reaction with perovskite.53 The performance of the cells with un-doped NiOx and Ag:NiOx are measured again after 30 days of storage in N2 box in the dark. Results are shown in Figure 8. Both devices retained 93% of their initial performance after storage. Results indicate that the small amount silver present in the NiOx does not degrade the device more so than the device without Ag doping. This is because the amount of Ag-doping is very small (5mol %) and the Ag+ ions have been mostly incorporated within the NiOx unit cell, inferred by XRD and XPS results presented above.

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Figure 8 Normalized power conversion efficiency of PSCs based on NiOx and Ag:NiOx HTLs as a function of storage (N2 box in the dark) time. Inset images show the photo the Ag:NiOx based PSC as fabricated (left) and after 30 day of storage (right).

In summary, we have shown that Ag-doping is effective in producing NiOx film with enhanced conductivity, more efficient charge extraction and more favourable energy level alignment resulting in enhanced p-type property confirmed by electrical characterisation, photo-luminescence measurements and ultraviolet photoelectron spectroscopy.

Capitalising

on

these

improvements,

the

champion

ITO/

Ag:NiOx/CH3NH3PbI3/PCBM/BCP/Ag device achieved a forward scan PCE at 17.3% with negligible hysteresis (at a scan rate of 30 mV.S) and a stabilized efficiency at 17.1%. Compared with un-doped NiOx based devices, the performance of Ag-doped NiOx based devices improves by 21% (from 13.5% to 16.3% average PCE) with remarkable enhanced Jsc (from 19.23 to 20.37 mA/cm2) and FF (from 68.7 to 74.7 %). This work provides pathway for high performance inverted PSCs using low temperature solution process method suitable for future tandem device demonstrations.

SUPPORTING INFORMATION Experimental details and supplementary characterizations of materials and devices.

ACKNOWLEDGMENT

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The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australian-based activities of the Australia-US Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). This project is also supported by ARENA via the project 2014 RND075. J. Zheng wishes to acknowledge the support from Chinese Scholarship Council (CSC). We thank the Electron Microscopy Unit and the BioMedical Imaging Facility at UNSW for the SEM and fluorescence imaging supports. REFERENCES

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