Cesium Doped NiOx as an Efficient Hole Extraction ...

FULL PAPER Perovskite Solar Cells

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Cesium Doped NiOx as an Efficient Hole Extraction Layer for Inverted Planar Perovskite Solar Cells Wei Chen, Fang-Zhou Liu, Xi-Yuan Feng, Aleksandra B. Djurišic´,* Wai Kin Chan, and Zhu-Bing He* transport layers in inverted planar PSCs, including both organic and inorganic p-type semiconductors.[9,10,16–18] Among these, inorganic p-type semiconductors are particularly attractive due to their high transmittance, high hole mobility, and high chemical stability.[9,10] Nickel oxide (NiO) is a particularly attractive inorganic p-type semiconductor since it can be readily deposited by a variety of methods. In addition, it has a wide band gap and consequently good transmittance in the visible spectral range, good chemical stability, and convenient energy level alignment with the perovskite facilitating hole collection and electron blocking.[9,10,19] Consequently, it has been investigated as a hole extraction layer (HEL) in perovskite solar cells in different device architectures.[20–39] Since the device stability is a major concern for PSCs,[6] NiO is particularly promising since its excellent chemical stability could yield improved device stability. The stability of devices with NiO was significantly improved compared to commonly used organic hole transport material in inverted devices, poly(3,4-eth ylenedioxythiophene):polystyrene sulfonate.[20,37,40,41] Cells with NiO demonstrated good stability under light soaking as well.[8] Despite the promising results obtained using NiO as HEL to date, further improvement in terms of efficiency is needed for NiO-based inverted PSCs. One of the key issues in improving the performance is increasing the conductivity of NiO. Low conductivity results in increased recombination and reduced hole extraction.[39] The conductivity can be increased by adjusting the stoichiometry of the films or by doping. Stoichiometric NiO is insulating, while the commonly observed p-type conductivity in undoped NiO is typically attributed to the nickel vacancies VNi.[42,43] However, the hole density in undoped NiO is limited due to large ionization energy of the Ni vacancies, but it can be increased by extrinsic dopants with more shallow acceptor levels.[43] Common dopant used for NiO is Li,[42–44] although other dopants such as Cu,[45–47] Mg,[41] Co,[48] Cs,[49,50] as well as codoping approach with Li, Cu[51] and Li, and Mg[41] have been reported. Efficient perovskite solar cells have been demonstrated for Cu:NiOx (best power conversion efficiency of 17.30%),[45] Li, Cu codoped NiOx (14.53%),[51] Mg, Li:NiO (18.3%),[41] while Co-doping was demonstrated in dye-sensitized solar cells[48] and Cs doping was not applied to photovoltaic devices so far.

Organic–inorganic hybrid perovskite solar cells have resulted in tremendous interest in developing next generation photovoltaics due to high record efficiency exceeding 22%. For inverted structure perovskite solar cells, the hole extraction layers play a significant role in achieving efficient and stable perovskite solar cell by modifying charge extraction, interfacial recombination losses, and band alignment. Here, cesium doped NiOx is selected as a hole extraction layer to study the impact of Cs dopant on the optoelectronic properties of NiOx and the photovoltaic performance. Cs doped NiOx films are prepared by a simple solution-based method. Both doped and undoped NiOx films are smooth and highly transparent, while the Cs doped NiOx exhibits better electron conductivity and higher work function. Therefore, Cs doping results in a significant improvement in the performance of NiOxbased inverted planar perovskite solar cells. The best efficiency of Cs doped NiOx devices is 19.35%, and those devices show high stability as well. The improved efficiency in devices with Cs:NiOx is attributed to a significant improvement in the hole extraction and better band alignment compared to undoped NiOx. This work reveals that Cs doped NiOx is very promising hole extraction material for high and stable inverted perovskite solar cells.

1. Introduction Organometallic halide perovskite solar cells have been attracting increasing attention in recent years due to their rapidly increasing efficiency.[1–8] Among different device structures for perovskite solar cells (PSCs), inverted planar structure is attracting increasing attention due to its simplicity, possibility of low temperature processing, and low hysteresis.[9–15] Different materials have been used as hole extraction/hole W. Chen, Dr. F.-Z. Liu, Prof. A. B. Djurišic´ Department of Physics The University of Hong Kong Pokfulam, Hong Kong SAR E-mail: [email protected] W. Chen, X.-Y. Feng, Prof. Z.-B. He Department of Materials Science and Engineering Shenzhen Key Laboratory of Full Spectral Solar Electricity Generation (FSSEG) Southern University of Science and Technology No. 1088, Xueyuan Rd., Shenzhen 518055, Guangdong, P. R. China E-mail: [email protected] Prof. W. K. Chan Department of Chemistry The University of Hong Kong Pokfulam, Hong Kong SAR

DOI: 10.1002/aenm.201700722

Adv. Energy Mater. 2017, 1700722

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In this work, we demonstrate that Cs doping results in highly transparent Cs:NiOx films which exhibit significantly improved hole extraction compared to NiOx. The doped films are prepared by a simple solution-based method, and they are uniform and highly reproducible. Solution-based methods are of interest since they are compatible with low cost production of large-area devices, although scaling-up the device size would likely require process optimization and interface modifications to achieve efficient large area devices.[52] Inverted planar perov skite device with Cs:NiOx exhibit an optimal performance over 19%. Thus, Cs:NiOx is an excellent candidate for a hole extraction layer to achieving high efficiency perovskite solar cells.

2. Results and Discussion The undoped NiOx films were prepared by a simple solution based process.[53] Cs:NiOx films were prepared by adding Cs precursors at different molar ratios (0.5–3 mol%) (see more details in the Experimental Section). The device architecture of our inverted planar PSCs on fluorine doped tin oxide (FTO) is presented at Figure 1a, where the NiOx or Cs doped NiOx and phenyl-C61-butyric acid methyl ester (PC61BM) are selected as HEL and electronic extraction layer (EEL), respectively. ZrAcac serves as an electrode interface layer to reduce energy barrier for electron extraction.[21] The MAPbI3 perovskite films were fabricated by one-step solvent engineering method with some modification (see detailed synthesis route in the Experimental Section).[54] As shown in Figure 1b, the cross-section scanning electron microscopy (SEM) image demonstrates that a

dense, compact, and uniform perovskite films with thickness of ≈310 nm can be obtained, and clearly distinguishes the Cs:NiOx HEL and PC61BM EEL in the device. The energy level arrangements of various functional layers in the PSCs are shown at Figure 1c, where the energy levels of NiOx, MAPbI3, PC61BM, and ZrAcac/Ag are cited from literature,[21,53] and the work function of the NiOx and Cs doped NiOx are calculated from the ultraviolet photoelectron spectroscopy (UPS) measurement, which will be discussed in detail later. Obtained photovoltaic performances for devices with different Cs content are summarized in Table 1, and the corresponding J–V curves and the dependence of photovoltaic parameters on Cs content are shown in Figure 1d–f. A clear improvement in the device performance is obtained compared to undoped films. The improvement in device performance for Cs:NiOx compared to NiOx is also observed on tin doped indium oxide (ITO) substrates (see Figure S1 in the Supporting Information). From Table 1, it can be concluded that the best performance is obtained for Cs:NiOx prepared with 1 mol% Cs. Thus, this concentration was selected for in-depth study and the corresponding 1 mol% Cs doped NiOx was labeled as Cs:NiOx in the following. Both NiOx and Cs:NiOx films on quartz exhibited high transmittance in the visible spectral range, with average transmittance over 90% in the spectral range from 400 to 800 nm, as illustrated in Figure 2a. The extremely high transparency of the HELs minimizes the optical losses of the photovoltaic device, ensuring the higher photocurrent generation that is critical for achieving excellent power output of solar cells. The incorporation of Cs is confirmed from the X-ray photoelectron spectroscopy (XPS) measurements of NiOx and Cs:NiOx films, as

Figure 1. a) Device architecture of the inverted planar perovskite solar cell (PSCs). b) Cross-section SEM image of a typical Cs:NiOx perovskite device. c) Energy level diagram of the various layers in the PSCs. d) Current density–voltage (J–V) characteristics of the PSCs with different Cs content in Cs:NiOx measured under standard test conditions (AM1.5G, 100 mW cm−2), all devices here were not encapsulated and all the measurements were performed in ambient air with a relative humidity of ≈50%–65%. e,f) Dependence of photovoltaic parameters of the NiOx and Cs:NiOx perovskite devices on Cs contents.


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Table 1. Summary of the photovoltaic parameters of the inverted planar PSCs with NiOx and Cs-doped NiOx hole transport interlayers. Jsc [mA cm−2]

Voc [V]

FF [%]

PCE [%]

0

20.56 ± 0.18a) (20.68)b)

1.036 ± 0.023 (1.061)

69.9 ± 2.4 (73.1)

14.91 ± 0.61 (16.04)

0.5

20.66 ± 0.18 (20.89)

1.065 ± 0.019 (1.08)

71.5 ± 3.0 (75.3)

15.73 ± 0.67 (16.99)

1

21.62 ± 0.14 (21.77)

1.083 ± 0.024 (1.120)

74.4 ± 3.4 (79.3)

17.44 ± 0.95 (19.35)

2

21.43 ± 0.19 (21.51)

1.074 ± 0.026 (1.100)

72.2 ± 3.0 (74.7)

16.36 ± 0.87 (17.67)

3

21.03 ± 0.22 (21.08)

1.063 ± 0.021 (1.086)

70.2 ± 2.7 (73.1)

15.62 ± 0.76 (16.74)

Cs cons. [mol%]

a)The

averaged values with a standard deviation that were calculated from 16 devices from different batches; b)In the parenthesis are the values corresponding to the best devices.

shown in Figure 1b,c. The obtained Cs content for the films prepared with 1 mol% Cs precursor is ≈2.73% indicating that Cs is readily incorporated into NiOx.[55] According to XPS spectra, both doped and undoped films are nonstoichiometric, with the typical peaks corresponding to Ni2+ (853.8 eV) as well as Ni3+ (855.6 eV) observed in the films.[27,31,36–38] The obtained Ni3+/Ni2+ ratios were calculated to be 1.76 and 1.98 for NiOx and Cs:NiOx samples, respectively. The Ni3+ state (occurring in Ni2O3) is an indication of oxygen deficiency in NiO and thus the presence of Ni vacancy.[31] Therefore, the slightly higher Ni3+ ratio in the doped film may contribute to the increase of hole conductivity.[43] To further confirm our hypothesis, we performed conductive atomic force microscopy (AFM) scans of the undoped and doped NiOx films, as shown in Figure 1d,e, respectively. It is clearly observed that in the case of Cs:NiOx significantly higher current is obtained, indicating higher conductivity of the doped films. We propose that the Cs dopant can either substitute the Ni atom or go into the interstitial site as Cs+ state, the former one appears to dominate due to the larger atomic radius of Cs (298 pm) than Ni (149 pm).[56] This might cause lattice distortion, resulting in more amorphous phase in Cs:NiOx compared to NiOx, which would increase the Ni3+ acceptor ratio, and consequently the hole conductivity.[49] Our findings are consistent with reports about Li+, Co2+, and Cu2+-doped NiO films.[41,45,48,57]

To investigate further the reasons for the improved performance of the devices with Cs:NiOx hole extraction layers, their morphology and optoelectronic properties were comprehensively characterized. SEM images of both NiOx and Cs:NiOx demonstrate very uniform and smooth surface morphologies, which facilitate the formation and full coverage of the perov skite films (Figure 3a,b).[35] Furthermore, it can be observed from the 3D AFM images that cesium-doping results in a small reduction of Root mean square (Rms) surface roughness compared to undoped NiOx (Figure 3c,d), while resulting perovskite film morphologies on both hole extraction layers are similar (Figure 3e,f). Surface potential variations measured by scanning Kelvin probe microscopy (SKPM) reveal a lower surface potential profile of the Cs:NiOx film compared to that of undoped films (Figure 4a,b), indicating that the Cs doping results in a higher work function of the NiOx films.[58] Obtained work function values by UPS measurements (Figure 4c) further demonstrate that Cs doping results in more favorable work function for hole extraction, with an increase from 4.89 eV for undoped films to 5.11 eV for Cs:NiOx films (Figure 1c). It is known that the work function of NiOx depends significantly on its processing conditions including postdeposition treatments,[19,26,34] and it can vary from ≈4.7 to ≈5.5 eV.[19,26,41,59] The doping of NiOx can also cause energy band shifts and alter its work function.[41] Unlike Li doping which causes unfavorable valence band shift and a decrease in the work function,[41]

Figure 2. a) Transmittance spectra of the NiOx and Cs:NiOx films coated on quartz after thermal annealing. The inset shows the corresponding photographs of both films. b,c) High resolution XPS profiles for the Ni and Cs elements in the NiOx and Cs:NiOx films, respectively. Conductive-AFM characteristics of the NiOx d) and Cs:NiOx e) films coated on FTO glass. The scan size is 5 µm × 5 µm.


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Figure 3. Morphology of the NiOx, Cs:NiOx, and perovskite films: SEM images of a) the NiOx and b) Cs:NiOx on FTO glass, the scale bar is 2 µm. 3D AFM images of c) the corresponding NiOx and d) Cs:NiOx films. The scan size is 5 µm × 5 µm. SEM images of perovskite films on e) NiOx and f) Cs:NiOx. The scale bar is 500 nm.

Cs-doping does not shift the energy band, but increases the work function as well as the conductivity, facilitating improved hole extraction and better band alignment between NiOx HEL and perovskite valance band (Figure 1c). To verify this, the hole only devices with structure FTO/ NiOx or Cs:NiOx/MoO3/Ag were fabricated to test their hole extraction behaviors. The J–V curves show that the Cs-doped

NiOx film exhibits higher current density at the same forward bias than the undoped one, indicating the better capability of hole extraction of the Cs:NiOx (Figure 4d).[41] Among different doping concentrations, the highest current density is obtained for 1% of Cs precursor, and slightly lower current density is obtained for 2% of Cs precursor, followed by comparable values for 0.5% and 3% of Cs precursor. For all precursor

Figure 4. SKPM characteristics of a) the NiOx and b) Cs:NiOx films on FTO glass. The scan size is 5 µm × 5 µm. c) The UPS spectra of the NiOx and Cs:NiOx film coated on silicon wafer, measured under −10 eV bias. The work function values are calculated according to the formula Φ = hν – (E0−EF). d) J–V curves of hole only devices with NiOx or Cs:NiOx hole extraction layers, the device structure is FTO/NiOx or Cs:NiOx/MoO3/Ag. e) Steady state photoluminescence (PL) and f) time resolved PL spectra for perovskite films on different substrates: FTO (green), NiOx (blue), and Cs:NiOx (yellow).


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Table 2. Summary of the PL lifetime parameters from fitting curves of the PL decay measurements. Samples

τ1 [ns]

B1 [%]

B2 [%]

τ2 [ns]

Weighted average τ [ns]

FTO/MAPbI3

5.1

3.13

94.9

84.23

84.07

FTO/NiOx/MAPbI3

30.7

1.50

69.3

55.61

54.97

FTO/Cs:NiOx /MAPbI3

55.2

3.05

44.8

32.12

29.08

concentrations, obtained current density is higher compared to undoped films, and the observed trends are consistent with the trends in the performance in solar cells. A decrease in the current density with the increase of Cs precursor concentration above 1% indicates a reduction in the conductivity compared to optimal concentration. In addition, photoluminescence (PL) and time-resolved PL spectra were measured and the obtained results are shown in Figure 3e,f, and summarized in Table 2. A pheno menological bi-exponential fitting function as follows:[20]  t − t0   t − t0  was used for calculating I(t ) = I 0 + B1 exp  − + B2 exp  −  τ 1   τ 2 

the decay times. The average decay times were calculated by the equation τ avg. = ∑ Biτ i . We can clearly observe a significant 2

∑B τ

decrease in PL intensity as well as PL lifetime for Cs:NiOx. A decrease in the PL lifetime for the Cs:NiOx/MAPbI3 (29.08 ns) as compared to the NiOx/MAPbI3 (54.97 ns) indicates more effective charge extraction by Cs:NiOx HEL from perovskite active layer,[37] as expected from increased work function of Cs:NiOx. Furthermore, we have performed a detailed comparison of our champion devices basing on NiOx and Cs:NiOx HELs under different scanning conditions and examined the steady state photocurrent output under illumination over time as well, as shown in Figure 5. The detailed comparison of photovoltaic parameters is summarized in Table 3. It can be observed that both devices exhibit negligible hysteresis (Figure 5a) and stable steady state photocurrent output at the maximum power point over 600 s (Figure 5b), and the photovoltaic curves for both devices behave identically when changing the scanning rates (Figure 5c,d). External quantum efficiency (EQE) measurements demonstrate that Cs:NiOx devices have higher photo response capability compared to NiOx devices, consistent with improved hole extraction and increased Jsc values (Figure 5e). It can be observed that all photovoltaic performance para meters improve by replacing undoped NiOx with Cs:NiOx. Since the short circuit current density is dependent on the absorption in the active layer and the charge collection and the two films have comparable transmittance (and thus comparable optical losses), the improvement in Jsc likely occurs due

i i

Figure 5. a) Current density–voltage (J–V) characteristics of the NiOx and Cs:NiOx devices in forward ( −0.2 to 1.2 V) and reverse scan (1.2 to −0.2 V) with 10 mV interval and scanning rate of 125 mV s−1. b) Steady-state photocurrent output at the maximum power point of the corresponding cells in (a) under continuous simulated AM1.5G illumination for 600 s. J–V curves in the reverse scan direction with different scan rates (from 125 mV to 19 mV s−1) for c) NiOx and d) Cs:NiOx, respectively. External quantum efficiency (EQE) spectra e) of the corresponding devices. The Jsc calculated from the EQE curves are 19.64 and 20.72 mA cm−2 for NiOx and Cs:NiOx devices, respectively. All devices here were not encapsulated and all the measurements were performed in ambient air with a relative humidity of ≈50%–65%.


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www.advancedsciencenews.com Table 3. Photovoltaic parameters of the optimal NiOx and Cs:NiOx devices measured under different scanning direction. Devices NiOx

Cs:NiOx

Scan direction

Jsc [mA cm−2]

Voc [mV]

FF [%]

PCE [%]

Average PCE [%]

Rs [Ω cm−2]

Rsh [kΩ cm−2]

Forward

20.76

1.060

72.3

15.91

15.98

6.1

3.5

Reverse

20.68

1.061

73.1

16.04

Forward

21.79

1.110

79.5

19.23

19.29

3.7

4.8

Reverse

21.77

1.120

79.3

19.35

to improved hole collection. As we discussed before, increase in Voc of the Cs:NiOx devices should originate from increased work function of Cs:NiOx HEL,[41] while the improvement in the fill factor likely occurs due to improved Cs:NiOx conductivity as well as improved hole extraction and lower interfacial recombination losses (lower Rs and higher Rsh of the Cs:NiOx devices) (Table 3). Since both Voc and FF are dependent on the recombination losses (evident from Rsh values), we also examined recombination by measuring photovoltage transients (see Figure S2 in the Supporting Information). A clear increase in photovoltage decay time is observed for Cs:NiOx devices, which is consistent with reduced recombination at the HEL/ perovskite interface.[60,61] The reduced recombination would be expected from the improved charge extraction in Cs:NiOx devices. Finally, we monitored the stability of the encapsulated Cs:NiOx devices stored at argon glove box (H2O < 0.1 ppm, O2 < 30 ppm), and the devices were taken out and tested at ambient environment. The dependence of the photovoltaic performance parameters on the storage time for a period over two months is illustrated in Figure 6a,b. Excellent stability can be observed in the first 30 d, followed by a slow decay. After almost 80 d, the devices retain ≈90% of the initial efficiency. Note that we need to take out the device for J–V curves measurements, and the devices can degrade relatively fast due to air and moisture exposure during the testing period. Therefore, our devices would be even more stable if the devices were kept at inert environment all the time. To verify our hypothesis, comparison experiments have been performed on a device which was measured as-prepared and then stored in a glove box for 70 d before taking it out for repeated measurement. As shown in the J–V curves (Figure 6c), the device demonstrated excellent stability, maintaining 98% of the initial efficiency after 70 d. The high

stability of the Cs:NiOx devices should originate from the excellent chemical stability of the inorganic hole extraction layer and stable cathode interfacial layer.[21,37,41]

3. Conclusion In summary, we have demonstrated that cesium is an effective dopant for NiOx to achieve high performance inverted planar perovskite solar cells. The devices with Cs:NiOx hole extraction layer demonstrate improved hole extraction due to higher work function and higher conductivity of cesium doped NiOx. Obtained PL lifetimes demonstrate significant reduction from ≈55 to ≈29 ns, while the solar cell efficiency is increased from ≈16% to ≈19%. The devices also exhibited stable steady-state current output and low hysteresis. Finally, inert device stability tests have proved the Cs:NiOx-based inverted perovskite solar cell exhibit good stability over long time. Hence, our work demonstrates cesium doped nickel oxide is an excellent candidate to obtain high performance and stable perovskite solar cells.

4. Experimental Section Materials: Materials including solvents (N,N-Dimethylformamide (DMF), Dimethyl sulfoxide (DMSO), Isopropanol), PbI2 (99%), ethanolamine (>99%), nickel (II) acetate tetrahydrate (99.998%), and cesium acetate (99.9%) were purchased from Sigma-Aldrich and used as received. CH3NH3I (LT-S9126) and PC61BM (LT-S905) were purchased from Taiwan Lumtec Corp. Undoped and Cs-Doped NiOx Precursor Solution: Undoped NiOx films were prepared according to a previously reported procedure.[53] Briefly, 0.1 mmol Ni(Ac)2·4H2O was dissolved in 1 mL of isopropanol with

Figure 6. a,b) Photovoltaic performance parameters of Cs:NiOx perovskite devices (five cells) as a function of storage time under inert environment almost 80 d. c) J–V curves of the as-prepared device and after 70 d storage at inert environments. All devices were encapsulated with cover glass where the edge areas were sealed with epoxy. The devices were measured outside glove box and stored at glove box during other periods.


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ethanolamine (NH2CH2CH2OH). The mole ratio of Ni2+/EA in solution was maintained at 1:1. The solution was kept in a sealed glass vial and stirred overnight in air at 70 °C and homogeneous green solution was obtained. For the Cs-doped NiOx, cesium acetate with different molar ratios (from 0.5% to 3%) was mixed with Ni(Ac)2·4H2O solution and stirred under the same conditions. Device Fabrication: First, the patterned FTO substrates (Nippon Sheet Glass, Japan) were cleaned ultrasonically with toluene, acetone, and isopropanol for 10 min, respectively. Then, undoped and Cs-doped NiOx films were deposited by spin casting the precursor solution at 1500 rpm for 30 s on cleaned FTO substrates at room temperature. The NiOx precursor films were then annealed at 275 °C for 1 h in air. The thickness values of the annealed NiOx and Cs:NiOx were determined to be ≈23 nm from the cross-section SEM measurements (Figure 1b). The as-prepared perovskite precursor solution (PbI2 and CH3NH3I with mole ratio of 1:1 with concentration of 1.1 m in a mixture of DMF and DMSO (7:3 v/v)) was filtered using 0.45 µm PTFE syringe filter and coated onto the FTO/NiOx or FTO/Cs:NiOx substrates with speed of 1000 rpm for 10 s and 5000 rpm for 25 s. During the last 10 s of the spinning process, the substrate was treated by drop-casting chlorobenzene solvent. The substrates were then annealed on a hot plate at 100 °C for 10 min, followed by spin-coating a layer of PC61BM (2 wt% in chlorobenzene, 1500 rpm, 45 s) on top and annealed at 100 °C for 40 min. Then, zirconium acetylacetonate (ZrAcac) with concentration of 0.1 wt% in isopropanol was coated on top of PC61BM films (5000 rpm, 30 s).[21] Finally, a 100 nm thick Ag electrode was deposited through a shadow mask by thermal evaporation to define the cell area 10 mm2. Device Characterizations: The J–V curves were recorded using a Keithley 2400 sourcemeter. Simulated solar illumination was provided by an Oriel Sol3A solar simulator with AM1.5G spectrum and light intensity of 100 mW cm−2, which was determined by a calibrated crystalline Si-cell. During device characterization, a shadow mask with an opening of 10 mm−2 was used. The devices were measured in reverse scan (1.2 V → −0.2 V, step 10 mV, scan rates: 125 → 19 mV s−1) and forward scan (−0.2 V → 1.2 V, step 10 m V, scan rates: 125 mV s−1). The EQE spectra were recorded with by an Enli Technology (Taiwan) EQE measurement system (QE-R), and the light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. Absorption and transmission spectra were recorded by Cary 50 Bio UV– vis spectrophotometer. Top-view and cross-section SEM images were characterized by JEOL JMS-7001F, LEO 1530 FEG scanning electron microscope, and Hitachi S-4800 FEG scanning electron microscope. AFM (MFP-3D-Stand Alone, Asylum Research) was employed to investigate the film surface morphology, surface potentials, and conductivity. Room temperature PL and time resolved PL spectra were recorded by spectrofluorometer (FS5, Edinburgh instruments). 405 nm pulsed laser was used as excitation source for the time resolved PL measurement. The XPS and UPS measurements of the undoped and Cs-doped NiOx films coated on silicon wafer were performed in an ESCALAB 250Xi, Thermo Fisher (by using Al Kα X-ray source) under high vacuum (10−9 mbar). The XPS spectra were calibrated by the binding energy of 284.8 eV for C 1s. Transient photovoltage measurement was performed with controlled intensity modulated photo spectroscopy system provided by Zennium workstation (ZAHNER-elektrik GmbH).

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors acknowledge financial support from the Strategic Research Theme, the University Development Fund, and the Seed Fund for Basic Research of the University of Hong Kong, the Natural Science Foundation


of Shenzhen Innovation Committee (No. JCYJ20150529152146471), and the Shenzhen Key Laboratory Project (No. ZDSYS201602261933302).

Conflict of Interest The authors declare no conflict of interest.

Keywords cesium doping, nickel oxide, organometallic halide perovskite, solar cells Received: March 17, 2017 Revised: April 21, 2017 Published online:

[1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050. [2] B. Saparov, D. B. Mitzi, Chem. Rev. 2016, 116, 4558. [3] S. Kazim, M. K. Nazeeruddin, M. Gratzel, S. Ahmad, Angew. Chem., Int. Ed. Engl. 2014, 53, 2812. [4] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643. [5] G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Gratzel, S. Mhaisalkar, T. C. Sum, Science 2013, 342, 344. [6] J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Gratzel, Nature 2013, 499, 316. [7] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395. [8] Y. Rong, L. Liu, A. Mei, X. Li, H. Han, Adv. Energy Mater. 2015, 5, 1501066. [9] W. B. Yan, S. Y. Ye, Y. L. Li, W. H. Sun, H. X. Rao, Z. W. Liu, Z. Q. Bian, C. Huang, Adv. Energy Mater. 2016, 6, 1600474. [10] M.-H. Li, P.-S. Shen, K.-C. Wang, T.-F. Guo, P. Chen, J. Mater. Chem. A 2015, 3, 9011. [11] H. Azimi, T. Ameri, H. Zhang, Y. Hou, C. O. R. Quiroz, J. Min, M. Y. Hu, Z. G. Zhang, T. Przybilla, G. J. Matt, E. Spiecker, Y. F. Li, C. J. Brabec, Adv. Energy Mater. 2015, 5, 1401692. [12] G. Kakavelakis, T. Maksudov, D. Konios, I. Paradisanos, G. Kioseoglou, E. Stratakis, E. Kymakis, Adv. Energy Mater. 2017, 7, 1602120. [13] A. E. Labban, H. Chen, M. Kirkus, J. Barbe, S. Del Gobbo, M. Neophytou, I. McCulloch, J. Eid, Adv. Energy Mater. 2016, 6, 1502101. [14] H.-C. Liao, P. Guo, C.-P. Hsu, M. Lin, B. Wang, L. Zeng, W. Huang, C. M. M. Soe, W.-F. Su, M. J. Bedzyk, M. R. Wasielewski, A. Facchetti, R. P. H. Chang, M. G. Kanatzidis, T. J. Marks, Adv. Energy Mater. 2017, 7, 1601660. [15] Q. Xue, Y. Bai, M. Liu, R. Xia, Z. Hu, Z. Chen, X.-F. Jiang, F. Huang, S. Yang, Y. Matsuo, H.-L. Yip, Y. Cao, Adv. Energy Mater. 2017, 7, 1602333. [16] Y. Liu, L. A. Renna, Z. A. Page, H. B. Thompson, P. Y. Kim, M. D. Barnes, T. Emrick, D. Venkataraman, T. P. Russell, Adv. Energy Mater. 2016, 6, 1600664. [17] D. Zhao, M. Sexton, H.-Y. Park, G. Baure, J. C. Nino, F. So, Adv. Energy Mater. 2015, 5, 1401855. [18] C. Zuo, L. Ding, Small 2015, 11, 5528. [19] B. Mustafa, J. Griffin, A. S. Alsulami, D. G. Lidzey, A. R. Buckley, Appl. Phys. Lett. 2014, 104, 063302. [20] W. Chen, G. N. Zhang, L. M. Xu, R. Gu, Z. H. Xu, H. J. Wang, Z. B. He, Mater. Today Energy 2016, 1–2, 1. [21] W. Chen, L. Xu, X. Feng, J. Jie, Z. He, Adv. Mater. 2017, 29, 1603923.

1700722 (7 of 8)


www.advenergymat.de

www.advancedsciencenews.com [22] H. Zhang, J. Cheng, F. Lin, H. He, J. Mao, K. S. Wong, A. K. Jen, W. C. Choy, ACS Nano 2016, 10, 1503. [23] V. Trifiletti, V. Roiati, S. Colella, R. Giannuzzi, L. De Marco, A. Rizzo, M. Manca, A. Listorti, G. Gigli, ACS Appl. Mater. Interfaces 2015, 7, 4283. [24] K. Cao, Z. Zuo, J. Cui, Y. Shen, T. Moehl, S. M. Zakeeruddin, M. Grätzel, M. Wang, Nano Energy 2015, 17, 171. [25] J. H. Park, J. Seo, S. Park, S. S. Shin, Y. C. Kim, N. J. Jeon, H. W. Shin, T. K. Ahn, J. H. Noh, S. C. Yoon, C. S. Hwang, S. I. Seok, Adv. Mater. 2015, 27, 4013. [26] J. Cui, F. Meng, H. Zhang, K. Cao, H. Yuan, Y. Cheng, F. Huang, M. Wang, ACS Appl. Mater. Interfaces 2014, 6, 22862. [27] K.-C. Wang, P.-S. Shen, M.-H. Li, S. Chen, M.-W. Lin, P. Chen, T.-F. Guo, ACS Appl. Mater. Interfaces 2014, 6, 11851. [28] Z. Liu, M. Zhang, X. Xu, F. Cai, H. Yuan, L. Bu, W. Li, A. Zhu, Z. Zhao, M. Wang, Y.-B. Cheng, H. He, J. Mater. Chem. A 2015, 3, 24121. [29] X. Yin, J. Liu, J. Ma, C. Zhang, P. Chen, M. Que, Y. Yang, W. Que, C. Niu, J. Shao, J. Power Sources 2016, 329, 398. [30] J. Cui, P. Li, Z. Chen, K. Cao, D. Li, J. Han, Y. Shen, M. Peng, Y. Q. Fu, M. Wang, Appl. Phys. Lett. 2016, 109, 171103. [31] U. Kwon, B.-G. Kim, D. C. Nguyen, J.-H. Park, N. Y. Ha, S.-J. Kim, S. H. Ko, S. Lee, D. Lee, H. J. Park, Sci. Rep. 2016, 6, 30759. [32] A. S. Subbiah, A. Halder, S. Ghosh, N. Mahuli, G. Hodes, S. K. Sarkar, J. Phys. Chem. Lett. 2014, 5, 1748. [33] J. Y. Jeng, K. C. Chen, T. Y. Chiang, P. Y. Lin, T. D. Tsai, Y. C. Chang, T. F. Guo, P. Chen, T. C. Wen, Y. J. Hsu, Adv. Mater. 2014, 26, 4107. [34] L. Hu, J. Peng, W. Wang, Z. Xia, J. Yuan, J. Lu, X. Huang, W. Ma, H. Song, W. Chen, Y.-B. Cheng, J. Tang, ACS Photonics 2014, 1, 547. [35] Y. Hou, W. Chen, D. Baran, T. Stubhan, N. A. Luechinger, B. Hartmeier, M. Richter, J. Min, S. Chen, C. O. Quiroz, N. Li, H. Zhang, T. Heumueller, G. J. Matt, A. Osvet, K. Forberich, Z. G. Zhang, Y. Li, B. Winter, P. Schweizer, E. Spiecker, C. J. Brabec, Adv. Mater. 2016, 28, 5112. [36] X. Yin, P. Chen, M. Que, Y. Xing, W. Que, C. Niu, J. Shao, ACS Nano 2016, 10, 3630. [37] J. You, L. Meng, T. B. Song, T. F. Guo, Y. M. Yang, W. H. Chang, Z. Hong, H. Chen, H. Zhou, Q. Chen, Y. Liu, N. De Marco, Y. Yang, Nat. Nanotechnol. 2016, 11, 75. [38] S. Seo, I. J. Park, M. Kim, S. Lee, C. Bae, H. S. Jung, N.-G. Park, J. Y. Kim, H. Shin, Nanoscale 2016, 8, 11403. [39] A. Corani, M.-H. Li, P.-S. Shen, P. Chen, T.-F. Guo, A. El Nahhas, K. Zheng, A. Yartsev, V. Sundström, C. S. Ponseca, J. Phys. Chem. Lett. 2016, 7, 1096.


[40] W. Chen, Y. D. Zhu, Y. Z. Yu, L. M. Xu, G. N. Zhang, Z. B. He, Chem. Mater. 2016, 28, 4879. [41] W. Chen, Y. Wu, Y. Yue, J. Liu, W. Zhang, X. Yang, H. Chen, E. Bi, I. Ashraful, M. Gratzel, L. Han, Science 2015, 350, 944. [42] S. Lany, J. Osorio-Guillén, A. Zunger, Phys. Rev. B 2007, 75, 241203. [43] H. L. Z. Kelvin, X. Kai, G. B. Mark, G. E. Russell, J. Phys.: Condens. Matter 2016, 28, 383002. [44] S. Mutsumi, N. Hiroshi, S. Gaku, Y. Aika, F. C. Shigefusa, Jpn. J. Appl. Phys. 2016, 55, 088003. [45] J. W. Jung, C. C. Chueh, A. K. Y. Jen, Adv. Mater. 2015, 27, 7874. [46] Z. Yang, C.-C. Chueh, P.-W. Liang, M. Crump, F. Lin, Z. Zhu, A. K. Y. Jen, Nano Energy 2016, 22, 328. [47] A. Rajagopal, S. T. Williams, C.-C. Chueh, A. K. Y. Jen, J. Phys. Chem. Lett. 2016, 7, 995. [48] G. Natu, P. Hasin, Z. Huang, Z. Ji, M. He, Y. Wu, ACS Appl. Mater. Interfaces 2012, 4, 5922. [49] F. A. Al-Agel, Mater. Lett. 2013, 100, 115. [50] R. Fernández, J. Estelle, Y. Cesteros, P. Salagre, F. Medina, J.-E. Sueiras, J.-L. G. Fierro, J. Mol. Catal. A: Chem. 1997, 119, 77. [51] M.-H. Liu, Z.-J. Zhou, P.-P. Zhang, Q.-W. Tian, W.-H. Zhou, D.-X. Kou, S.-X. Wu, Opt. Express 2016, 24, A1349. [52] A. Agresti, S. Pescetelli, A. L. Palma, A. E. Del Rio Castillo, D. Konios, G. Kakavelakis, S. Razza, L. Cinà, E. Kymakis, F. Bonaccorso, A. Di Carlo, ACS Energy Lett. 2017, 2, 279. [53] J. R. Manders, S.-W. Tsang, M. J. Hartel, T.-H. Lai, S. Chen, C. M. Amb, J. R. Reynolds, F. So, Adv. Funct. Mater. 2013, 23, 2993. [54] N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu, S. I. Seok, Nat. Mater. 2014, 13, 897. [55] D. Sarma, C. D. Malliakas, K. S. Subrahmanyam, S. M. Islam, M. G. Kanatzidis, Chem. Sci. 2016, 7, 1121. [56] E. Clementi, D. L. Raimondi, W. P. Reinhardt, J. Chem. Phys. 1967, 47, 1300. [57] C.-C. Wu, C.-F. Yang, Sol. Energy Mater. Sol. Cells 2015, 132, 492. [58] Y. Zhang, W. Cui, Y. Zhu, F. Zu, L. Liao, S.-T. Lee, B. Sun, Energy Environ. Sci. 2015, 8, 297. [59] C. J. Flynn, S. M. McCullough, L. Li, C. L. Donley, Y. Kanai, J. F. Cahoon, J. Phys. Chem. C 2016, 120, 16568. [60] H. R. Tan, A. Jain, O. Voznyy, H. Z. Lan, F. Pelayo García de Arquer, J. Z. Fan, R. Quintero-Bermudez, M. J. Yuan, B. Zhang, Y. C. Zhao, F. J. Fan, P. C. Li, L. N. Quan, Y. B. Zhao, Z. H. Lu, Z. Y. Yang, S. Hoogland, E. H. Sargent, Science 2017, 355, 722. [61] Q. Jiang, L. Q. Zhang, H. L. Wang, X. L. Yang, J. H. Meng, H. Liu, Z. G. Yin, J. L. Wu, X. W. Zhang, J. B. You, Nat. Energy 2016, 1, 16177.

1700722 (8 of 8)
