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Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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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 S. Yun,† Meng Zhang,† Cho Fai Jonathan Lau,† Jueming Bing,† Xiaofan Deng,† Qingshan Ma,† Yongyoon Cho,† Weifei Fu,§ Chao Chen,*,‡ Martin A. Green,† Shujuan Huang,† and Anita W. Y. Ho-Baillie*,† †

Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia ‡ College of Energy, Xiamen University, Xiamen, 361005, China § State Key Laboratory of Silicon Materials, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

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 shortcircuit 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 planar 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, and enhancing the p-type property of the NiOx film confirmed by electrical characterization, photoluminescence measurements, and ultraviolet photoelectron spectroscopy. These improvements 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 29 mV 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 undoped 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



transport layers and UV instability associated with TiO210,11 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 has lower hysteresis compared to a 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 perovskite (with the HTL fabricated first followed by perovskite deposition and electron transport (n-) layer on the light

INTRODUCTION Rapid development of organic−inorganic hybrid perovskite solar cells (PSCs)1 has attracted huge interest due to their highefficiency 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 March 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 (500 °C) treatment for the compact and mesoporous TiO2 electron © XXXX American Chemical Society

Received: November 10, 2017 Accepted: January 12, 2018 Published: January 12, 2018 A

DOI: 10.1021/acsaem.7b00129 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

perovskite photodetectors.55 With regard to doping NiOx, Jen et al. demonstrated high-efficiency planar PSCs based on solution-processed copper-doped NiOx with impressive PCEs up to 15.4% compared to undoped 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, PCEs of most PSCs using solution-processed NiOx are still limited due to the relatively low values achieved of short-circuit current density (JSC) and fill factor (FF). In this work, we report the use of Ag-doping for solutionprocessed 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 suite of characterizations. 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 V OC (by 29 mV 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 undoped 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.

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 are the most commonly used p-type HTL material in inverted structure.16,24−27 However, due to the mismatch between the work functions (WFs) of perovskite and PEDOT:PSS, devices that use PEDOT:PSS exhibit limited open voltages (Voc; normally Voc < 1 V) 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. have demonstrated cells that use poly(N,NÂ -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′-benzothiadiazole)) (PCDTBT) modified PEDOT:PSS achieving Voc = 1.05 V.23,31 However, for these devices, the perovskite layer needs to evaporate on the HTL to obtain good contact due to the nonwetting properties of the organic surface; moreover, PEDOT:PSS is not suitable for long-term stable devices due to its high acidity and hygroscopicity.16 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 and they have higher WF and better stability. Solution-processed V2Ox has been applied as HTL by Peng et al. in PSC achieving a PCE of 14.8%.35 Li 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 = 1.0 V.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 HTLs 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 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 al. demonstrated a 19.6% device on 1.02 cm2 using Li, Mg-co-doped NiOx film by spray pyrolysis. The 1 cm2 cells are 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 7.6% efficient PSCs by sequential deposition of a CH3NH3PbI3 (MAPbI3) layer on solution-processed planar NiO film.50 After that, many researchers increased the performance of solution-processed NiOx-based PSCs devices.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



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, and 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 deionized water with 2% Hellmanex, deionized water, acetone, and isopropanol for 20 min, respectively. The ITO substrate was treated by UVO cleaner for 15 min. For undoped NiOx films, a 1 M solution of nickel nitrate hexahydrate and ethylenediamine in anhydrous ethylene glycol was spun on ITO-coated glass at 5000 rpm for 50 s. For Ag-doped NiOx films, a 1 M 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 5000 rpm for 50 s. All of the samples were then dried at 100 °C for 5 min followed by annealing at 300 °C for 1 h on a hot plate 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 of PbI2 powder in 1 mL of DMF and 71 μL of DMSO, and was then spincoated 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 °C 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 (0.5 mg/mL) was then added dropwise B

DOI: 10.1021/acsaem.7b00129 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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

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 bars are each 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 2 μm × 2 μm. A biexponential function was used to fit the time-resolved PL decays to obtain the slow (τ1) and fast (τ2) components and their weightings (A1 and A2).

on top of the PCBM during 6000 rpm for 15 s spin-coating. Finally, 100 nm silver electrodes were deposited by thermal evaporation. 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 PerkinElmer Lambda1050 UV/vis/near-IR 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 a 760/ 40 nm band-pass filter.

⎛t⎞ ⎛t ⎞ I = A1 exp⎜ ⎟ + A 2 exp⎜ ⎟ ⎝ τ1 ⎠ ⎝ τ2 ⎠

(1)

where I is the PL intensity. The effective lifetimes (τeff) can then be calculated using the τi lifetimes and Ai weights using the following:58

τeff =

∑ Ai τi 2 ∑ Ai τi

(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 C

DOI: 10.1021/acsaem.7b00129 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 3. (a) UPS results of the NiOx and Ag:NiOx films coated silicon substrate, and (b) I−V curves for undoped NiOx and Ag:NiOx films based on Ag/NiOx/ITO structure. (c) Steady-state and PL spectra of the ITO/MAPbI3, ITO/NiOx/MAPbI3, and ITO/Ag:NiOx/MAPbI3 films. (d) Timeresolved PL spectra of ITO/NiOx/MAPbI3 film and ITO/Ag:NiOx/MAPbI3 films. mW cm−2 with an 0.159 cm2 aperture and a scan rate of 30 mV s −1 both from VOC to JSC direction (1.1 to −0.1 V) or from JSC to VOC direction (−0.1 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, U.K. Test structures involving doped and undoped 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.

shifted slightly toward smaller 2θ, suggesting expansion of the lattice. This is because when the larger Ag+ (ionic 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 was formed with larger lattice constant according to Bragg’s law. This result indicated that Ag+ ions have partially displaced Ni2+ ions and have, therefore, doped NiO. X-ray photoelectron spectroscopy measurements are also performed on undoped NiOx and Ag-doped NiOx films. Results are shown in Supporting Information Figure S1. Figure S1a shows the XPS spectra for Ni 2p2/1 and Ni 2p2/3 at around 873 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 Ag 3d5/2 are also found in Ag-doped NiOx films. These peaks are not present in undoped 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 400−800 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 undoped 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 Agdoping. The high average transmittances of NiOx and Ag:NiOxcoated ITO from 400 to 800 nm region at 82% show their suitability for high-performance PSCs and perovskite tandems.



RESULTS AND DISCUSSION Figure 1a shows the X-ray diffraction (XRD) patterns of the undoped NiOx and Ag-doped NiOx (5 mol % Ag in NiO, Ag:NiOx) films on glass. As displayed, the Bragg peaks of the undoped sample matched well with that of the cubic structure of NiO PDF No. 47-1049. The XRD peaks of Ag:NiOx film Table 1. Biexponential Fitting Results of PL Decay Traces for ITO/NiOx/MAPbI3 Film and ITO/Ag:NiOx/MAPbI3 Film sample

A1 (%)

A2 (%)

τ1 (ns)

τ2 (ns)

τeff (ns)

ITO/NiOx/MAPbI3 ITO/Ag:NiOx/MAPbI3

66.0 47.0

34.0 53.0

48.26 31.17

10.05 8.81

44.52 25.82 D

DOI: 10.1021/acsaem.7b00129 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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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.

Table 2. Device Parameters of the PSCs Based on Ag:NiOx HTL with Different Silver-Doping Concentrations NiOx type

Voc (mV)

NiOx 3%Ag:NiOx 5%Ag:NiOx 8%Ag:NiOx

1033 1042 1062 1048

± ± ± ±

40 49 33 19

Jsc (mA/cm2) 19.3 20.1 20.4 19.1

± ± ± ±

Panels a and b of Figure 2 show the top view scanning electron microscopy images of undoped NiOx and Ag:NiOx films on ITO glass. Panels c and d of Figure 2 show the corresponding three-dimensional (3-D) atomic force microscopy 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 undoped film (RRMS = 7.1 nm; Figure 2c). We then characterize the morphologies of perovskite layers for the undoped and doped NiOx. The corresponding SEM images and 3-D AFM images are shown in Figure 2e−h. The morphologies of perovskite layers on both types of the 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 Ultraviolet photoelectron spectroscopy was carried out to check the energy level of the NiOx films, and the results are shown in Figure 3a. The photoemission cutoffs of both films shown on the left indicate that the work function (WF; from vacuum level) has been shifted upward from 5.02 to 5.13 eV after Ag-doping (the work function values were calculated according to the formula Φ = hν (21.22 eV) − Ecutoff). 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

FF (%)

0.7 0.9 0.7 0.8

68.7 71.1 74.7 71.7

± ± ± ±

7.3 3.9 3.7 3.5

PCE (best) (%) 13.5 14.2 16.3 13.9

± ± ± ±

2.2 1.8 1.0 1.2

(15.7) (16.0) (17.3) (15.1)

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 σ is the conductivity, A is the active area (0.5 × 0.5 cm2), d is the thickness of the films (∼15 nm), and 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 conductivities of the different NiOx films with different Agdoping concentrations were estimated to be 6.6 × 10−7 S·cm−1 (undoped), 1.3 × 10−6 S·cm−1 (3 mol %), 2.2 × 10−6 S·cm−1 (5 mol %), and 9.6 × 10−7 S·cm−1 (8 mol %). It is found that Agdoped NiOx shows enhanced conductivity compared to the undoped NiOx film. The increased conductivity from Agdoping compared to undoped film is likely to be due to an increase in the disorder of the NiOx structure, an increase in oxygen vacancy with Ag+ (see eqs 4 and 5), 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 (intrinsic defects).53,62,63 This explains the drop in conductivity as Ag-doping equals or is greater than 8 mol %. 2NiO → VNi″ + 2h• + 2Oo x + Ni surface x

(4)

0.5Ag 2O

2NiO ⎯⎯⎯⎯⎯⎯⎯→ Ag Ni′ + VNi″ + 3h• + 2.5Oo x + Ni surface x (5) E

DOI: 10.1021/acsaem.7b00129 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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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 the average. The lowest bar is the 25th percentile value. The lowest value is the minimum.

Figure 6. (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.

F

DOI: 10.1021/acsaem.7b00129 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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measured band gap, work function, and valence band edge for the undoped NiOx and Ag:NiOx films, the energy level diagrams of PSCs with undoped 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 WFs for the Ag-doped NiOx are slightly shifted allowing higher VOC in the associated PSC’s to be reached. 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 in Figure 5. Vocs of cells with Ag-doped HTLs increase slightly (from 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 (from 19.2 to 20.4 mA/cm2) and FF (from 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 5 mol % 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 possibly due to increased defects in the NiOx film reducing film quality and device repeatability. Panels a and c of Figure 6 show the J−V curves (scanned at 30 mV/s) of the champion device of undoped NiOx and Ag:NiOx-based PSC devices, respectively. Panels b and d of Figure 6 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 PCEs 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 undoped and Ag-doped NiOx, respectively. Figure 7 shows the EQE spectra of NiOx- and Ag:NiOx-based PSC devices that were also measured 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 Ag:NiOx compared to the undoped 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. The top section of Table S1 show the results of PSCs using high-temperatureprocessed 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 underlying Si device. The bottom section of Table S1 lists the PSCs reported using low-temperature-processed and solution-processed NiOx. While the most recently demonstrated device that uses Csdoped 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 among devices that use the most commonly used PCBM/BCP electron transport stack. 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 undoped NiOx and Ag:NiOx are measured again after 30 days of storage in a N2 box in the dark. Results are shown in

Figure 7. EQE spectra and integrated current densities of NiOx and Ag:NiOx-based PSCs.

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 photographs of the Ag:NiOx-based PSC as fabricated (left) and after 30 days of storage (right).

where VNi″ is the Ni cation vacancy at the intrinsic Ni site, h• is the electron defect (hole), Oox is the normal oxygen at intrinsic O site, Nisurfacex is the normal Ni atom at the surface site, and AgNi′ is the Ag cation vacancy at the intrinsic Ni site. To study the effect of Ag-doping on NiOx film on carrier dynamics, steady-state photoluminescence 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,d; the corresponding fitted lifetimes are summarized in Table 1. It can be seen from steadystate PL results (Figure 3c) that PL quenching is strongest for MAPbI3 film on Ag:NiOx film. The weighting for the second 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 For the demonstration of perovskite solar cells using Ag:NiOx and as part of the optimizations, trials were conducted on devices with different Ag+-doping concentrations (from 0 to 8 mol %) and with the configuration ITO/NiOx/MAPbI3/ PCBM/BCP/Ag as illustrated in Figure 4a. The thicknesses of the layers are 200 nm (ITO), 15 nm (NiOx), 300 nm (MAPbI3), 80 nm (PCBM/BCP), and 100 nm (Ag) according to the cross-sectional SEM; see Figure 4b. Based on the above G

DOI: 10.1021/acsaem.7b00129 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Energy Materials

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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 (5 mol %) and the Ag+ ions have been mostly incorporated within the NiOx unit cell, inferred by XRD and XPS results presented above. In summary, we have shown that Ag-doping is effective in producing NiOx film with enhanced conductivity, more efficient charge extraction, and more favorable energy level alignment resulting in enhanced p-type property confirmed by electrical characterization, photoluminescence measurements, and ultraviolet photoelectron spectroscopy. Capitalizing 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 undoped NiOxbased devices, the performance of Ag-doped NiOx-based devices improves by 21% (from 13.5% to 16.3% average PCE) with remarkably enhanced Jsc (from 19.23 to 20.37 mA/ cm2) and FF (from 68.7 to 74.7%). This work provides a pathway for high-performance inverted PSCs using a lowtemperature solution-processed method suitable for future tandem device demonstrations.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00129. XPS spectra of pristine NiOx and Ag:NiOx films, and the summary table of the high performance CH3NH3PbI3 based PSC devices using NiOx as HTL (PDF)



AUTHOR INFORMATION

Corresponding Authors

*(A.W.Y.H.-B.) E-mail: [email protected] *(C.C.) E-mail: [email protected] ORCID

Jianghui Zheng: 0000-0001-5123-7364 Meng Zhang: 0000-0002-1004-5662 Qingshan Ma: 0000-0002-7178-1081 Martin A. Green: 0000-0002-8860-396X Anita W. Y. Ho-Baillie: 0000-0001-9849-4755 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS 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.Z. acknowledges support from the Chinese Scholarship Council (CSC). We thank the Electron Microscopy Unit and the BioMedical Imaging Facility at UNSW for SEM and fluorescence imaging support.



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DOI: 10.1021/acsaem.7b00129 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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