Ion Implantation-Modified Fluorine-Doped Tin Oxide by Zirconium with

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Ion Implantation Modified Fluorine-doped Tin Oxide by Zirconium with Continuously Tunable Work Function and Its Application in Perovskite Solar Cells Dong Han, Cuncun Wu, Yunbiao Zhao, Yi Chen, Lixin Xiao, and Ziqiang Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12476 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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ACS Applied Materials & Interfaces

Ion Implantation Modified Fluorine-doped Tin Oxide by Zirconium with Continuously Tunable Work Function and Its Application in Perovskite Solar Cells Dong Han,† Cuncun Wu,‡ Yunbiao Zhao,† Yi Chen,† Lixin Xiao,‡, * Ziqiang Zhao†,* †State Key Laboratory of Nuclear Physics and Technology, ‡State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing, 100871, PR China

KEYWORDS: perovskite, solar cell, ion implantation, work function, FTO

ABSTRACT In recent years, perovskite solar cells have drawn widespread attention. As an electrode material, fluorine-doped tin oxide (FTO) is widely used in various kinds of solar cells. However, the relatively low work function (WF) (~4.6 eV) limits its application. The potential barrier between the transparent conductive oxide (TCO) electrode and the hole transport layer (HTL) in inverted perovskite solar cells results in device performance decrease. In this paper, we propose a method to adjust WF of FTO by implanting zirconium ions into the FTO surface. The WF of FTO can be

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precisely and continuously tuned between 4.59 eV and 5.55 eV through different dopant concentration of zirconium. In the meantime, the modified FTO, which had WF of 5.1 eV to match well the HOMO energy level of poly (3, 4-ethylenedioxylenethiophene): polystyrene sulfonate (PEDOT:PSS), which was used as the HTL in inverted planar perovskite solar cells. Compared with the pristine FTO electrode based device, the open circuit voltage (Voc) increased from 0.82 V to 0.91 V and the power conversion efficiency (PCE) increased from 11.6 % to 14.0 %. INTRODUCTION Organic-inorganic lead halide perovskite solar cells have developed rapidly in recent years.1-5 Methylammonium lead halide (MAPbX3, X = halogen) is most widely used in perovskite solar cells due to its direct band gap, large absorption coefficient and high carrier mobility.6-8 The modifications of the electron/hole transport and perovskite materials have been extensively studied.9-12 In contrast, the modification about TCO electrodes is rarely reported. TCO electrodes such as ITO and FTO are widely used in perovskite solar cells and other photoelectric devices due to their excellent conductivity and high transmittance. However, the potential barrier between TCO electrode and other transport layers would lead to device performance decrease.13,14 In order to match the energy level of different transport materials, the WF of TCO electrode should be tunable. Chlorinated indium tin oxide electrodes with high WF were reported by Helander et al. in 2011, which provide a direct match to the energy-levels of the charge transporting materials in organic light-emitting diodes and thereby improves the device performance.15 TCO with tunable WF not only extends its range of applications, but also improves the performance of the device.


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Ion implantation is a commonly used technique for doping semiconductor materials, which is more flexible than the diffusion doping method, by which the dopant ions are generated by ion source and then accelerated to form ion beam.16-18 The ion beam with tunable fluence and energy, which correspond to doping concentration and doping depth, respectively, is directly implanted into the sample surface, e.g., metal ions or non-metallic ions are implanted into the target lattice and form substitutional doping after thermal annealing process, which improves the specific performance of semiconductor materials. In this paper, a method to adjust the FTO WF is proposed. Zirconium ions were implanted onto the FTO surface. It is suggested that the WF of the modified FTO is dependent on the doping concentration of zirconium. The higher the doping concentration, the higher the FTO WF. When the implantation fluence reaches 8 1015 ions/cm2, the FTO WF reaches 5.55 eV. Moreover, unlike the ITO treated by oxygen plasma, of which the increased WF will restore within about 20 hours, there is no obvious change of FTO WF after zirconium implantation in almost one year.19,20 Finally, FTO with continuously tunable WF is available, which is more flexible to match the energy level of different transport layers. In order to verify the effect of the modified FTO in perovskite solar cell, the FTO WF was modified to 5.1 eV, which is matched well with the highest occupied molecular orbital (HOMO) energy level of PEDOT:PSS. By using the modified FTO and the pristine FTO electrode to prepare perovskite solar cells, the device with Zr:FTO has a higher FF, Voc and PCE than the control device with the pristine FTO electrode. The favorable energy level matching of the Zr:FTO/ PEDOT:PSS interface helps to improve the performance of the device. These indicate that ion implantation modified FTO via zirconium is a promising way to continuously tune its WF to match energy level of charge transport layer of photo-electronic devices.


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RESULTS AND DISCUSSION The preparation process of zirconium doped FTO was shown in Figure 1a. Five keV zirconium ion beam with different fluence were implanted into FTO surface, followed by heat treatment at 500 °C. Finally, the modified FTO was obtained. Since WF is a surface-sensitive parameter, a low energy ion beam is used to achieve surface doping. In order to elucidate the doping depth of zirconium ions, the stopping and range of ions in matter (SRIM) software was used to simulate the concentration distribution of 5 keV zirconium ions implanted into FTO. As shown in Figure 1b, it is suggested that zirconium are mainly distributed within the depth of 10 nm after ion implantation process and the highest doping concentration is about 4 nm in depth. This is the advantage of ion implantation doping: the dopant concentration and depth can be precisely controlled by the fluence and energy of the ion beam, respectively. The results of photoelectron spectra were shown in Figure S1 and Figure S2. The measurements of WF are shown in Figure 1c. The WF of pristine FTO is 4.59 eV and it increases with the doping concentration of zirconium, which can reach 5.55 eV at fluence of 8 1015 ions/cm-2. Ion implantation is a common technology in the production of semiconductor material, which can be applied in largescale production. In the meantime, zirconium is an inexpensive material, which does not require high cost. Unlike other surface modification methods such as spin coating a thin layer on the substrate, the FTO modified by ion implantation can hold its modified properties for a very long time since the ions with energy have been implanted into the material and formed a stable structure. Therefore, this modification is considered to be long-term effective and suitable for large-scale production and preservation, which does not require expensive fabrication processing. The results indicate that the WF of FTO can be tunable by doping zirconium, which


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broaden the range of applications of FTO. This is of great significance for the preparation of various photovoltaic devices.

Figure 1. The modification process and WF measurement of FTO electrode: (a) Schematics of the zirconium ion implantation process. (b) The concentration distribution of 5 keV zirconium ions implanted into FTO surface simulated by SRIM software. (c) The WF of modified FTO implanted with different fluence.

To figure out the interaction between zirconium and FTO, XRD and XPS were conducted to analyze the states of FTO before and after ion implantation process. According to Figure 2a,


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three more peaks appeared after zirconium implantation with the fluence of 2 1015 ions/cm2. The three peaks correspond to Zr (3d), Zr (3p3/2) and Zr (3p1/2), respectively, which indicates that zirconium was successfully doped into the FTO surface. Figure 2b is a magnified image of Zr (3d) peak in Figure 2a. The corrected binding energies for Zr (3p3/2) and Zr (3p1/2) components are in good accordance with literature data for ZrO2.21-23 It indicates that the doped zirconium forms ZrO2 on the FTO surface after thermal annealing in air atmosphere. Figure 2c compares the XRD pattern of pristine FTO and modified FTO. As we can see, the peaks of modified FTO tend to move towards a small angle and it is more obvious when the doping concentration increases with zirconium. According to the Bragg's formula: d sinθ = nλ, zirconium has a larger atomic radius than tin, which means the value of d increase. Since the value of nλ is constant, the value of θ is supposed to decrease, which means the peaks of modified FTO tend to move towards left in XRD pattern. It is considered that the doped zirconium take the replacement of tin to form ZrO2, which can play a role of an interface dipole layer on the surface of FTO and lower the Femi level, thereby increase the WF of FTO.24-27


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Figure 2. Chemical composition and structure characterization of pristine and modified FTO: (a) XPS analysis of the surface chemical composition of FTO before and after zirconium ion


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implantation with the fluence of 2 1015 ions/cm2. (b) A magnified image of Zr (3d) peak in Figure 2a. (c) XRD patterns of pristine and modified FTO implanted with different fluence.

FTO glass has excellent conductivity and high transmittance, which is expected to be retained in the modified FTO. Sheet resistance and transmittance of FTO were characterized by UV-Vis spectrophotometer and four-probe, respectively, as shown in Figure 3a and 3b. The sheet resistance of the modified FTO with different doping concentrations is about 15 Ω, which is almost the same as pristine FTO. Figure 3b displays transmission spectra of FTO before and after zirconium doping, through which the transmittance of wavelengths from 300 nm to 900 nm can be clearly seen. The Figure 3c-f compares surface morphology of pristine FTO and modified FTO with different doping concentration. The AFM images show surface morphology in 10 10 µm2 and the roughness of four different samples are 10.38 nm, 10.11 nm, 9.39 nm and 10.10 nm, respectively. The results of AFM indicate that the roughness of modified FTO is almost the same as the pristine one. According to the characterization about sheet resistance, transmittance and roughness, it is suggested that the modified FTO not only make WF tunable but also retain the excellent performance of the original sample. This greatly broadens the range of FTO electrodes applications. In the meantime, FTO electrodes with tunable WF would be more flexible to match the energy levels of different transport layers, which can also help to improve the performance of the devices.


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Figure 3. Characterization of electrical, optical properties and roughness of pristine and modified FTO: (a) Sheet resistance measured by four-probe and (b) transmittance measured by UV-Vis spectrophotometer. (c- f) The morphology of pristine and modified FTO implanted with different fluence characterized by AFM.


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To confirm the effect of the modified FTO electrode in perovskite solar cells, inverted planar perovskite solar cells with the configuration of Zr:FTO/ PEDOT:PSS/ CH3NH3PbI3/ PCBM/ BCP/ Ag were fabricated as shown in Figure 4a. Zr:FTO with WF of 5.1 eV, which can match well with the HOMO of PEDOT:PSS, was used as electrode. As a comparison, an FTO device with the same structure was also fabricated.28 The SEM images of the perovskite layer prepared by the low pressure assisted method are shown in Figure 4c. A non-pinhole perovskite film can be clearly observed. Figure 4d displays the J-V curves of the solar cells under simulated AM 1.5G solar irradiation at 100 mW/cm2. The results show that the solar cell with FTO gives Voc of 0.83 V, Jsc of ~20mA/cm2 and PCE of 11.6% as listed in Table 1. As for the solar cell with Zr:FTO, the Voc increases to 0.91 V and the PCE increases to 14.0%. The carriers are injected into the hole transport layer, when the HOMO of perovskite layer is lower than that of hole transport layer, and then transport to electrode, as shown in figure 4b. The potential barrier is not conducive to the carrier injection process, which blocks carrier from reaching the positive electrode and leads to recombination. That means the unmatched energy level between the different layers in perovskite solar cell would lead to device performance decrease. This indicates that the favorable energy level matching of Zr:FTO/ PEDOT:PSS interface helps to improve the FF and Voc, thereby improve the PCE of the solar cells. The modified FTO was proved to be effective in perovskite solar cells. Furthermore, we believe the WF-tunable FTO electrodes could also be applied in a variety of other photovoltaic devices.


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Figure 4. The application of modified FTO in inverted planar perovskite solar cell: (a) The configuration of the solar cell in this work. (b) Energy level diagram of the discussed solar cells. (c) SEM image of the perovskite film prepared by a low-pressure-assisted method. (d) The J-V curves of the solar cells based on Zr:FTO and FTO as electrode respectively under simulated AM 1.5G solar irradiation at 100 mW/cm2.


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Table 1. Performance parameters of solar cells with pristine FTO and Zr:FTO.

a

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

Ref.

FTO

19.7

0.82

0.72

11.6

This work a

Zr:FTO

20.2

0.91

0.76

14.0

This work a

Each parameter of solar cells is an average from 20 individual devices.

CONCLUSIONS In summary, we propose a method to make the WF of FTO continuously tunable by zirconium ion implantation doping. It is suggested that the formed ZrO2 layer on the surface of FTO is contributed to increase the WF. The Zr:FTO was applied to fabricate perovskite solar cells. Finally, it achieved the improvement of Voc (from 0.82 V to 0.91 V) and PCE (from 11.6% to 14.0%) with Zr:FTO as the electrode than that of the traditional FTO electrode, which mainly results from the favorable energy level matching of Zr:FTO/ PEDOT:PSS interface. Since the WF is continuously tunable, the modified FTO is flexible to match the energy level of different transport layers. Thus, it is considered that the FTO with tunable WF can be widely applied not only in perovskite solar cells, but also in a variety of other photoelectric devices.

EXPERIMENTAL SECTION Perovskite precursor preparation. The perovskite precursor was prepared by dissolved lead (II) iodide (PbI2, Polymor, 99.99%), methylamine iodide and dimethyl sulfoxide (DMSO, Kermel) at molar ratio 1:1:1 in anhydrous N, N-dimethylformamide (DMF, Alfa Aesar) with


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concentration of 1 mol/L. The solution should be stirred for 30 minutes before use. The perovskite CH3NH3PbI3 film was fabricated by a home-made low-pressure-assisted method as reported in literature.29,30

Modified FTO preparation. The schematic illustration for metal vapor vacuum arc (MEVVA) and instrument photograph are shown in figure S3. The working mechanism of MEVVA ion source is to trigger the arc discharge between the cathode and anode, so that the cathode material can evaporate into the discharge chamber. The evaporated atoms are ionized to positive ions during the plasma discharge process. Finally, an ion beam is obtained through the lead-out electrodes and then accelerates to the surface of the target. In our experiment, zirconium is used as cathode material. Since the average charge state of ionized zirconium is 1.9 e, the acceleration voltage applied in our experiment is 2.6 kV. A zirconium ion beam with energy of 5 keV is obtained, which was implanted into the surface of cleaned FTO. In order to control doping concentration, the fluence of the ion beam was adjusted from 1 1015 to 8 1015 ions/cm2 controlled by the counter. After ion implantation process, the FTO were annealed at 300 °C in air atmosphere for 30 minutes.

Perovskite solar cell fabrication. Pristine FTO glass and Zr:FTO glass were used as substrates for inverted planar perovskite solar cells, respectively. First, the FTO and Zr:FTO glass substrates were cleaned by ultrasonic treatment in deionized water, acetone and ethanol subsequently for 30 minutes each. The cleaned glass substrates were then dried with a nitrogen stream and treated by oxygen plasma cleaning for 10 minutes immediately. PEDOT:PSS was


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used as HTL and spin-coated on the glass substrate at 4000 rpm for 30 seconds, followed by annealing at 140 °C for 10 minutes. The perovskite precursor was spin-coated onto PEDOT:PSS at 3000 rpm for 8 seconds and then the film was treated by a low-pressure-assisted method for 40 seconds immediately, followed by annealing at 100 °C for 10 minutes. [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) dissolving in chlorobenzene with concentration of 20 mg/mL, was then spin-coated onto the perovskite film at 1500 rpm for 50 s. 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP) and silver layers were both deposited by high-vacuum thermal evaporation. BCP powder and silver particle were placed in quartz evaporator and tantalum flake, respectively, which were fixed at the electrodes in the vacuum chamber. By heating the evaporator, a 5nm BCP layer was deposited onto the sample surface under vacuum condition of 8 10-4 Pa. Finally, a 100 nm layer of Ag was deposited as the cathode. The current required is related to the size of the evaporator and tantalum flake. Finally, perovskite solar cell with the configuration of Zr:FTO (or FTO)/ PEDOT:PSS/ CH3NH3PbI3/ PCBM/ BCP/ Ag was obtained. All devices fabrication processes were carried out in ambient environment at room temperature.

Characterization. The photoelectron spectrometer (Riken Keiki, AC-2) was used to measure the WF of the pristine and modified FTO (Light source: deuterium lamp, energy scanning range: 3.4 - 6.2 eV, accuracy: 0.02eV). X-ray photoelectron spectroscopy (XPS, Kratos, Axis Ultra) analysis was performed at RT, equipped with a monochromatic Al Kα X-ray source (1486.7 eV). The X-ray diffraction (XRD) patterns were measured using X-ray diffraction system (PANalytical Inc.) with monochromatic Cu Kα irradiation (λ= 1.5418 Å). The sheet resistance and transmittance of FTO were characterized by UV-visible spectrophotometer (Agilent 8453) and four-probe, respectively. Atomic force microscopy (AFM, Agilent 5500) was used to


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measure the surface morphology and the roughness of FTO. The morphology of the perovskite film was characterized by scanning electron microscopy (SEM, FEI Nano 430) at an acceleration voltage of 5 kV. The illuminated current density-voltage (J-V) curves were characterized by using a Keithley 2611 source meter. Current density was measured under simulated AM 1.5G solar irradiation at 100 mW/cm2 from a Newport solar simulator. The system was calibrated against a certified reference solar cell. All the measurements of the solar cells were performed under ambient atmosphere at room temperature without encapsulation. ASSOCIATED CONTENT Supporting Information. Photoelectron spectra of FTO. AUTHOR INFORMATION Corresponding Author Corresponding Authors *E-mail: [email protected]. (L. X. Xiao) *E-mail: [email protected]. (Z. Q. Zhao) ACKNOWLEDGMENT This study was financially supported by the National Key R&D Program of China (No. 2016YFB0401000), National Natural Science Foundation of China (Grant Nos. 61575005, 61775004, 91426304 and 91226202) and ITER special funding from Ministry of Science and Technology (Award Number 2015GB121004). REFERENCES


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The J-V curves and energy level diagram of the solar cells based on Zr:FTO and FTO as electrode respectively 44x25mm (300 x 300 DPI)


Ion Implantation-Modified Fluorine-Doped Tin Oxide by Zirconium with

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