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Highly Efficient Flexible Perovskite Solar Cells Using Solution-Derived NiOx Hole Contacts Xingtian Yin,*,† Peng Chen,† Meidan Que,† Yonglei Xing,† Wenxiu Que,*,† Chunming Niu,‡ and Jinyou Shao§ †

Electronic Materials Research Laboratory, International Center for Dielectric Research, Key Laboratory of the Ministry of Education, School of Electronic & Information Engineering, ‡Center of Nanomaterials for Renewable Energy (CNRE), State Key Lab of Electrical Insulation and Power Equipment, School of Electrical Engineering, and §State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi People’s Republic of China S Supporting Information *

ABSTRACT: A solution-derived NiOx film was employed as the hole contact of a flexible organic−inorganic hybrid perovskite solar cell. The NiOx film, which was spin coated from presynthesized NiOx nanoparticles solution, can extract holes and block electrons efficiently, without any other post-treatments. An optimal power conversion efficiency (PCE) of 16.47% was demonstrated in the NiOx-based perovskite solar cell on an ITO-glass substrate, which is much higher than that of the perovskite solar cells using high temperature-derived NiOx film contacts. The low-temperature deposition process made the NiOx films suitable for flexible devices. NiOx-based flexible perovskite solar cells were fabricated on ITO-PEN substrates, and a preliminary PCE of 13.43% was achieved. KEYWORDS: NiOx, hole contact, flexible, perovskite solar cell

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inorganic hole transport materials, have been employed to replace PEDOT:PSS layers in inverted perovskite devices, such as PbS quantum dots, 22 CuSCN films, 23,24 and NiO x films.23,25−29 Particularly, research on NiOx-based perovskite solar cells have achieved a great progress.25 However, the employed NiOx films were deposited by using an expensive pulse laser deposition method, which is not suitable for large scale fabrication. Furthermore, a postannealing treatment must be conducted at 200 °C to improve the quality of the NiOx films, making them incompatible with flexible substrates. Besides, efficient NiOx hole contacts can also be prepared by using low cost solution methods. Unfortunately, the indispensable annealing process at 300−500 °C23,26−30 prevents their application in flexible solar cells. Recently, Jen et. al employed a combustion method to prepare Cu-doped NiOx hole contact for perovskite solar cell, and improved the PCE to 17.74%.31 Although low-temperature sputtered NiOx films may be compatible with flexible devices, their low PCE even on rigid substrates (below 10%) makes them unattractive to the research society.32 Actually, most reported inverted flexible perovskite solar cells are based on organic hole transport materials, especially PEDOT:PSS.18,33 Thus, it is very mean-

rganic−inorganic hybrid perovskites have been demonstrated to be efficient light absorbers for solar cells with a power conversion efficiency (PCE) exceeding 20%.1−4 Perovskite solar cells with different structures have been explored and investigated in detail during the past several years, including perovskite sensitized solar cells,5−7 mesoscopic perovskite solar cells,8−10 and planar heterojunction perovskite solar cells.11−13 The planar heterojunction perovskite solar cells have the simplest structures among these devices due to the absence of high-temperaturederived mesoporous layers.14 As one important class of perovskite solar cells, inverted planar heterojunction perovskite solar cells with a p-i-n structure attracted considerable attentions. They not only have less serious hysteresis than the normal n-i-p structured planar heterojunction devices,15 but also can be fabricated through a low-temperature solution route using poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT:PSS) films as the hole contact layers, making them very suitable for flexible solar cells.16−19 For example, hysteresis-less inverted planar heterojunction perovskite solar cells with a PCE of 18.1% have been demonstrated with PEDOT:PSS hole contact films.20 However, unfortunately, PEDOT:PSS is not good for device long-term stability due to its high acidity and hygroscopicity, which has been already demonstrated in organic photovoltaics and light emitting diodes (LED).21 Therefore, different materials, especially the © XXXX American Chemical Society

Received: December 24, 2015 Accepted: March 9, 2016

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Figure 1. (a) XRD patterns of the powders annealed at different temperatures. TEM images of the NiOx nanoparticles annealed at (b) 270 °C and (c) 400 °C. HRTEM images of the NiOx nanoparticles annealed at (d) 270 °C and (e) 400 °C.

Figure 2. (a) 3D-AFM images of the ITO substrate and the NiOx film on ITO substrate with sizes of 2 μm × 2 μm. (b) Top-view and (c) crosssectional SEM images of the NiOx film on ITO substrate. XPS spectra of (d) Ni 2p and (e) O 1s core level.

ingful to explore low-temperature processed NiOx films with efficient hole extraction capabilities for flexible perovskite solar cells. In this communication, we report a solution-derived NiOxbased inverted planar heterojunction perovskite solar cell with a PCE of as high as 16.47% on an ITO-glass substrate. The efficient NiOx hole contact layer can be deposited at a temperature as low as 130 °C without any post-treatments, making it very suitable and attractive for flexible devices. A

preliminary efficiency of 13.43% was demonstrated with a NiOx-based flexible perovskite solar cell using an ITO-PEN (polyethylene naphthalate) substrate.

RESULTS AND DISCUSSION NiOx nanoparticles were prepared by using a chemical precipitation method.34 Figure 1a shows X-ray diffraction (XRD) patterns of the nanoparticles after being annealed at different temperatures. The unannealed powder shows B

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sequentially baked at 100 °C for 30 min. The SEM image shown in Figure 3a indicates a full coverage of the perovskite

diffraction peaks at 33.1°, 38.6°, 52.1°, 59.1°, and 62.7°, which can be assigned to the (100), (011), (012), (110), and (111) plane of Ni(OH)2, respectively. After being annealed at 250 °C for 2 h, these peaks are weakened dramatically and new peaks appeared due to the decomposition of Ni(OH)2 to NiOx. The peaks at 37.2°, 43.3°, and 62.7° can be assigned to the (111), (200), and (220) planes of NiO. As the annealing temperature was increased to 270 °C and above, all of the Ni(OH)2 peaks disappeared. The peak width was reduced with increasing annealing temperature, indicating the grain growth of NiO, as confirmed by the transmission electron microscope (TEM) images shown in Figure 1b,c. At low annealing temperature (270 °C, Figure 1b), the powders are composed of small nanoparticles with a size of about 10 nm. As the annealing temperature was increased to 400 °C (Figure 1c), most of the NiOx nanoparticles were larger than 40 nm, while some of them were as large as 100 nm. Figure 1d,e present high resolution TEM (HRTEM) images of the samples annealed at 270 and 400 °C, respectively. Both of them showed clear lattice fringes and the space between two adjacent fringes in both samples was 0.24 nm, corresponding to the (111) plane of NiOx. The as-prepared NiOx powders were dispersed into water to prepare a NiOx nanoparticle solution for spin coating. Figure 2a shows 3D atomic force microscope (AFM) images of the ITOglass substrate and the NiOx film. It can be seen that the ITO surface is very flat with a root-mean-square (RMS) roughness of 0.802 nm. However, the RMS roughness was increased to 6.650 nm after the deposition of the NiOx film. It can be also seen from the top-view scanning electron microscope (SEM) image (Figure 2b) that the NiOx film consists of many sheetlike aggregations of NiOx nanoparticles, thus resulting in a nonuniform film as demonstrated by the cross-sectional SEM image shown in Figure 2c. The NiOx film on the ITO−glass substrate has a high transmission in the visible region (Figure S1a). The optical band gap of the NiOx can be obtained by measuring the absorption spectrum of the film deposited on the quartz substrate. An optical band gap of 3.7 eV was obtained for the NiOx film by plotting (ABS·hυ)2 as a function of hυ (Figure S1b). X-ray photoelectron spectroscopy (XPS) was used to analyze element composition of the NiOx film. Figure 2 panels d and e show XPS spectra of Ni 2p and O 1s core level, respectively, in which the spectra were satisfactorily fitted by several peaks on top of a Shirley background. The peak at 860.8 eV in the Ni 2p spectrum is ascribed to the shake-up process of NiOx. The XPS spectra comprise several components, including NiO (Ni 2p at 853.5 eV and O 1s at 529.1 eV), Ni2O3 (Ni 2p at 855.0 eV and O 1s at 530.8 eV), and NiOOH (Ni 2p at 856.2 eV and O 1s at 532.1 eV).34 Therefore, the as-prepared NiO is nonstoichiometric and thus is noted as NiOx. It should be mentioned that increasing the annealing temperature of NiOx powders does not change the composition of the NiOx powders significantly (Figure S2), but it does affect the dispersibilty of the NiOx nanoparticle solution badly. As shown in Figure S3, when the annealing temperature was 270 °C, the NiOx nanoparticle solution was still homogeneous even after 9 days’ storage. However, as the annealing temperature increased to 400 °C, the NiOx nanoparticle solution stratified after 4 days’ storage, and this time decreased to less than 12 h for the powder annealed at 500 °C. Therefore, the NiOx powder annealed at 270 °C was employed for all perovskite devices. Perovskite film was deposited on top of the NiOx film in a glovebox by using a one-step spin-coating process and

Figure 3. (a) Top-view SEM image and 3D-AFM image of the perovskite film deposited on NiOx film. AFM image has a size of 5 μm × 5 μm. (b) PL spectra of the perovskite films deposited on different films.

film on the NiOx film. 3D-AFM image reveals a rough surface with a RMS roughness of 5.601 nm, which is slightly lower than that of the NiOx film. XRD patterns (Figure S4) suggest a good crystallinity of the perovskite film. Figure 3b shows photoluminescence (PL) spectra of the perovskite layers deposited on different films. Clearly, the PL spectra of the perovskite films can be effectively quenched by inserting a thin layer of NiOx or PEDOT:PSS. The quenching efficiencies for the PEDOT:PSS and NiOx films are 99.8% and 94.1%, respectively, indicating a fast hole extraction from the perovskite films to both films. Figure 4a shows a false color cross-sectional-view SEM image of the as-fabricated NiOx-based perovskite solar cell. Clearly, the NiOx film is not flat and has an average thickness of 50 nm. The perovskite film is pinhole free, with a thickness of about 330 nm, while the PCBM and the Ag films have a thickness of about 80 and 150 nm, respectively. The perovskite solar cells have been fabricated on NiOx film and PEDOT:PSS filmcoated ITO-glass substrates. Figure 4b shows device performance distribution for 30 devices measured under AM 1.5 simulated sun light. The voltage was scanned from the short circuit current density (Jsc) to the open circuit voltage (Voc) at a scan rate of 0.1 V/s. The average Jsc and Voc of the NiOx-based devices are much higher than those of the PEDOT:PSS-based devices (19.38 mA/cm2 vs 15.98 mA/cm2 and 1.05 V vs 0.95 V). Because the average filling factor (FF) of the NiOx-based device is also higher than that of the PEDOT: PSS-based device (74.1% vs 70.6%), the average PCE of NiOx-based device is C

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Figure 4. (a) False color cross-sectional SEM image of the NiOx-based perovskite solar cell. (b) Photovoltaic parameters of the perovskite devices based on different hole contacts. (c) Typical Nyquist plots of NiOx and PEDOT:PSS based perovskite solar cells. (d) J−V curves and (e) EQE spectra for the best devices based on PDEDOT:PSS and NiOx layers. (f) Steady state PCE. The bias voltage for PEDOT:PSS and NiOx-based devices are 0.75 and 0.89 V, respectively.

Table 1. Photovoltaic Parameters of the Two Groups of Devices Jsc (mA/cm2)

PCE (%)

hole contact

scan direction

Voc (V)

FF (%)

J−V

EQE

J−V

steady state

PEDOT:PSS

Jsc→Voc Voc→Jsc Jsc→Voc Voc→Jsc

0.95 0.95 1.07 1.07

72.1 69.6 74.8 71.7

16.47 16.48 20.58 20.69

15.9

11.26 10.89 16.47 15.87

11.13

NiOx

15.02%, which is ∼40% higher than that of the PEDOT:PSS device. The carrier transport and recombination behaviors in devices were further investigated by the electrochemical impedance spectroscopy. Figure 4c displays a Nyquist plot of typical devices measured over the frequency range between 4 MHz and 1 Hz under an illumination of 37 W/m2 at a forward bias of 0.7 V, in which two distinct frequency regions can be observed. The first arc at a high frequency region (4 MHz ∼ 10 kHz) is usually related to the carrier transport process at the interface in the device. While the second arc at lower frequency is usually attributed to the charge recombination within the perovskite film and the interface of charge transport layer.31,35 The recombination resistances for devices can be extracted by fitting the plots using the circuit shown in the inset of Figure 4c. NiOx shows an increased recombination resistance (2.39 kΩ) as compared to that of PEDOT:PSS-based device (1.43 kΩ), indicating a lower recombination rate in the NiOx device. This is mainly because the NiOx film has a higher electron blocking property than the PEDOT:PSS film, due to its high conduction band edge position, as shown in Figure S5a. This was further demonstrated by the current density−voltage (J−V) curves of the devices measured in darkness as shown in Figure S5b. The NiOx-based device shows a much better diode performance. Figure 4d shows J−V curves of the best devices based on PEDOT:PSS and NiOx hole contacts. Detail parameters are listed in Table 1. The NiOx-based device has a Voc of 1.07 V, Jsc of 20.6 mA/cm2, and FF of 74.8%, resulting in a PCE of

20.01

16.21

16.47%. In comparison, the PEDOT:PSS-based device only outputs a PCE of 11.39%, due to the lower Voc and Jsc. It is interesting to notice that both devices show slight hysteresis behavior, which is common in inverted perovskite solar cells. External quantum efficiency (EQE) spectra of the best device are shown in Figure 4e, where integral current densities as a function of wavelength are also presented. Obviously, the EQE of the NiOx-based device overweighs that of the PEDOT:PSSbased device in the whole visible region. The integral Jsc values from the EQE spectra (shown in Table 1) are roughly comparable with those obtained from the J−V curves. It has been reported that the PCE of perovskite solar cells adversely depended on the measurement conditions because of the wellknown hysteresis effect, which usually led to an over- or underestimate of the real PCE. Hence, a steady state PCE measured by prebiasing the device at its maximum power point is very useful to estimate the reliability of the J−V scans.25 The steady state PCEs of our best PEDOT:PSS and NiOx-based devices were shown in Figure 4f, and a detail record of current density at the maximum power point can be found in the Supporting Information (Figure S6). The steady state PCEs were measured to be 11.13% and 16.21% for the PEDOT:PSS and NiOx-based devices, which are indicated by the circle and square spots in Figure 4d, respectively. The two spots are very close to their corresponding J−V curves, suggesting a good reliability of the J−V scans. It should be mentioned that, as D

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state measurement. Figure 5b presents EQE spectrum and integral current density as a function of wavelength of the flexible device. The integral current density over the full spectrum is calculated to be 19.23 mA/cm2, which is in agreement with that obtained from the J−V curve roughly.

demonstrated in Table S1, this record is as high as those for the best-in-class NiOx-based perovskite solar cells. With the above methods, we deposited NiOx films onto ITOPEN substrate to fabricate flexible perovskite solar cells. As shown in Figure S7, the employed ITO-PEN substrate has a high transmission in the visible region. After depositing the NiOx film, the ITO-PEN/NiOx almost retained the same transmission spectrum except for some change of fluctuations. Figure 5a shows J−V curves with different scanning directions

CONCLUSIONS NiOx films can be deposited on ITO-coated glass and PEN substrates by spin coating a presynthesized high quality NiOx nanoparticle solution. The as-prepared NiOx films had excellent hole extraction and electron blocking property, thus providing a good hole contact for perovskite solar cells. A maximal PCE of 16.47% (steady state PCE of 16.21%) was achieved for the NiOx-based perovskite solar cell on the ITO−glass substrate. With ITO-PEN substrates, the NiOx-based flexible perovskite solar cell was demonstrated with a preliminary PCE of 13.43%. EXPERIMENTAL SECTION Materials. All the chemicals were used as received without any further purification. PbI2 and CH3NH3I were purchased from WeihuaSolar. Dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) were supplied by Alfa-Aesar. Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT:PSS) and phenyl-C61butyric acid methylester (PCBM) were provided by Clevios and American Dye Sources, respectively. The remainder of the chemicals were purchased from Sinopharm Group Chemical Reagent Co. Ltd. China. Preparation of NiOx Nanoparticles and NiOx Films. First, 12.885 g of NiCl2·6H2O was dissolved in 100 mL of deionized water under magnetic stirring. Then, 10 M NaOH solution was added into the solution drop by drop until the pH value reached 10. The obtained turbid green solution was centrifuged, and the precipitation was washed with deionized water for two times, after which the powder was dried at 80 °C overnight and then annealed at different temperatures for 2 h. NiOx nanoparticle solution was prepared by dispersing the NiOx nanoparticles in deionized water with sonication. Briefly, 150 mg of NiOx nanoparticles was added into 5 mL of deionized water, and then this mixture was ultrasonicated in an ultrasonic cleaner at a power of 100 W. The total time for the ultrasonication is about 8 h. The resulted solution was filtered through a polytetrafluoroethylene (TPFE) filter (0.45 μm). NiOx films were deposited on ITO-coated glass substrates and PEN substrates by using a spin coating method, at 2000 rpm for 30 s. The NiOx coated substrates were then baked at 130 °C for 20 min in air. Preparation of PEDOT:PSS Films. PEDOT:PSS (1 mL) and isopropyl alcohol (2 mL) were mixed, and the mixture was ultrasonicated for 10 min and then filtered through polytetrafluoroethylene (TPFE) filter (0.45 μm). PEDOT:PSS film was deposited by using spin-coating at 3000 rpm for 60 s and then dried at 130 °C for 20 min. Deposition of Perovskite Films on NiOx and PEDOT:PSS Coated Substrates. The deposition of the perovskite films was similar to that in our previous report.30,37 Briefly, 2.3 g of PbI2 and 0.8 g of CH3NH3I were dissolved in 1.5 mL of DMSO and 3.5 mL of DMF at 70 °C with vigorous stirring for 12 h. The prepared precursor was filtered through TPFE filters (0.45 μm). Perovskite films were deposited on the NiOx or PEDOT:PSS-coated substrates by using a spin coating method in a glovebox, which includes a low speed of 1000 rpm for 5 s to spread the solution and a high speed of 3000 rpm for 35 s to evaporate the solvent. At the end of the spin coating, 0.13 mL of chlorobenzene was dripped onto the film while spinning. Finally, the films were baked at 100 °C for 30 min. Device Fabrication. To fabricate the inverted planar heterojunction solar cells, a thin layer of PCBM was deposited onto the perovskite film from a 20 mg/mL chlorobenzene solution at 1500 rpm for 45 s. Then, Ag film with a thickness of about 150 nm was deposited onto the PCBM layer by using a thermal evaporation process. The

Figure 5. (a) J−V curves and (b) EQE spectrum of the flexible perovskite solar cell with NiOx hole contact. Integral current density calculated from EQE spectra is also presented in panel b.

of a typical flexible perovskite solar cell with NiOx contact. For the forward scanning (from Jsc to Voc), the Voc and FF had relative high values of 1.04 V and 68.9%, respectively. Together with a current density of 18.74 mA/cm2, a PCE of 13.43% was achieved, which are comparable to those of the best flexible perovskite solar cells with either inverted or normal structures.18,36 It should be mentioned that the device showed some hysteresis behaviors, because the PCE from the reverse scanning was slightly lower due to its smaller FF. Detail parameters for the device are listed in Table 2. The black square spot indicates the maximum power point obtained from the steady state measurement, which is almost on the J−V curve, indicating a good consistence between the J−V scan and steady Table 2. Photovoltaic Parameters of the NiOx-Based Flexible Device Jsc (mA/cm2)

PCE(%)

scan direction

Voc (V)

FF (%)

J−V

EQE

J−V

steady state

Jsc→Voc Voc→Jsc

1.04 1.03

68.9 63.3

18.74 19.37

19.23

13.43 12.63

13.29

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ACS Nano active area of the devices was 0.07 cm2 determined by the size of Ag pads. Characterization. A transmission electron microscope (TEM, JEM-2010, JEOL Inc., Japan) was employed to observe the microstructural properties of the prepared nanoparticles. X-ray diffraction (XRD) analysis was employed to characterize crystalline properties of the films (SMARTLAB, Rigaku, Japan). A JASCO V-570 UV/vis/NIR spectrometer was employed to test UV−vis absorption spectra of the films. Morphological properties of the films were observed by using scanning electron microscopy (SEM, JSM-6390, JEOL Inc., Japan) and atomic force microscopy (AFM, Cypher S, Asylum Research). Element composition of the NiOx film was characterized by using X-ray photoelectron spectroscopy (XPS, AXIS Ultrabld, Kratos). Photoluminecence spectra were measured by using a fluorescence spectrometer (FLS980, Edinburgh Instruments). J−V curves of the as-fabricated devices were measured in air by using a PVIV-201 V I-V Station (Newport Oriel) at a scanning rate of 0.1 V/s, while the illumination source was calibrated by using a Newport 91150 V reference cell system. To measure the steady state power conversion efficiency, current density of the devices was measured by biasing the device at maximum power point for 100 s. External quantum efficiency (EQE) spectra of the devices were tested in air without bias light by using a Qtest Station 1000ADX system (Growntech, Inc.). The electrochemical impedance spectroscopy was carried out on ZAHNER ENNIUM Electrochemical Workstation in the frequency range between 4 MHz and 1 Hz under illumination of 37 W/m2 at a forward bias of 0.7 V.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b08135. Transmission sprectra, additional XPS spectra, XRD patterns, schematic band diagram and dark J−V curves of the solar cells devices, and performances of the reported NiOx-based organic−inorganic hybrid perovskite solar cells (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China under Grant No. 51502239, China Postdoctoral Science Foundation under Grant 2015M582659, the Research Fund for the Doctoral Program of Higher Education of China under Grant 20120201130004, partially the National Natural Science Foundation of China Major Research Plan on Nanomanufacturing under Grant No. 91323303, the Science and Technology Developing Project of Shaanxi Province under Grant No. 2015KW-001, and the 111 Project of China (B14040). The SEM and TEM work were done at International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an, P. R. China. REFERENCES (1) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341− 344. F

DOI: 10.1021/acsnano.5b08135 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.5b08135 ACS Nano XXXX, XXX, XXX−XXX