Letter pubs.acs.org/NanoLett
Fluorine Functionalized Graphene Nano Platelets for Highly Stable Inverted Perovskite Solar Cells Gi-Hwan Kim,† Hyungsu Jang,† Yung Jin Yoon,† Jaeki Jeong,† Song Yi Park,† Bright Walker,† In-Yup Jeon,‡ Yimhyun Jo,§ Hyun Yoon,§ Minjin Kim,§ Jong-Beom Baek,† Dong Suk Kim,*,§ and Jin Young Kim*,† †
Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, South Korea Department of Chemical Engineering, Wonkwang University, Iksan, 54538, South Korea § KIER-UNIST Advanced Center for Energy, Korea Institute of Energy Research (KIER), Ulsan 44919, South Korea ‡
S Supporting Information *
ABSTRACT: Edged-selectively fluorine (F) functionalized graphene nanoplatelets (EFGnPs-F) with a p−i−n structure of perovskite solar cells achieved 82% stability relative to initial performance over 30 days of air exposure without encapsulation. The enhanced stability stems from Fsubstitution on EFGnPs; fluorocarbons such as polytetrafluoroethylene are well-known for their superhydrophobic properties and being impervious to chemical degradation. These hydrophobic moieties tightly protect perovskite layers from air degradation. To directly compare the effect of similar hydrophilic graphene layers, edge-selectively hydrogen functionalized graphene nanoplatelet (EFGnPs-H) treated devices were tested under the same conditions. Like the pristine MAPbI3 perovskite devices, EFGnPs-H treated devices were completely degraded after 10 days. The hydrophobic properties of EFGnPs-F were characterized by contact angle measurement. The test results showed great water repellency compared to pristine perovskite films or EFGnPs-H coated films. This resulted in highly air-stable p−i−n perovskite solar cells. KEYWORDS: Perovskite solar cells, p−i−n, stability, graphene, functionalized graphene, hydrophobic
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compared to nonfunctionalized graphene.16−18 The recently discovered ball-milling method for EFGnPs synthesis constitutes an efficient and easy synthetic method and has been applied successfully in several devices.19,20 From graphite, first, the kinetic ball-milling method cracks the graphite sheets, and the newly formed edges become functionalized depending on the materials present in the ball mill. The edges are normally functionalized with hydrogen atoms or carboxylic groups, yielding hydrophilic properties. The edges of the graphene particles can also be easily substituted with halogen groups by adding halogen gas, which imparts hydrophobic properties to the graphene. These halogen-functionalized EFGnPs have the advantage of having good electrical characteristics and may be used as an electrode material to replace Pt in dye-sensitized solar cells.20 Over 20% PCE has been reported with perovskite solar cells and certified by several groups.5,21,22 However, the n−i−p architecture used in these devices has several disadvantages for commercialization such as being expensive, high work-function metal electrodes (Au), and high temperature processing of the
ncreasing energy consumption is an inevitable consequence of worldwide economic development, which requires dramatically increased amounts of useable energy every year. As limited energy sources such as fossil fuels face increasing use in every industry, the need for unlimited clean energy sources such as sunlight, wind, rain, waves, and geothermal has increased. Solar cells are a major candidate for renewable energy devices which actively develop day-by-day. Hybrid organic−inorganic lead halide-based perovskite solar cells have been actively studied, and the highest power conversion efficiency (PCE) reached with single junction cells is currently over 22%,1−6 which is similar to commercialized silicon based solar cells. To date, PCE development has been the primary focus of research and development efforts to compete in the commercial market. However, the stability issue is still an unsolved problem in realistic market needs of perovskite solar cells.7,8 Nano carbon materials can be easily functionalized to have unique optical and electrical properties and have found applications in high-performance electronic devices such as solar cells, field effect transistors, and light-emitting diodes, and so forth.9−15 Recently, edged-selectively functionalized graphene nanoplatelets (EFGnPs) have been intensively studied due to various characteristic changes of properties in EFGnPs © 2017 American Chemical Society
Received: July 28, 2017 Revised: September 5, 2017 Published: September 12, 2017 6385
DOI: 10.1021/acs.nanolett.7b03225 Nano Lett. 2017, 17, 6385−6390
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Nano Letters
Figure 1. Schematic procedure of mechanochemically driven reaction transforming graphite (a) to edge (b) hydrogen, or (c) fluorine functionalized graphene nanoplatelets. (d) Schematic device diagram showing MAPbI3 perovskite solar cells with the configuration: Glass/ITO/PEDOT:PSS/ MAPbI3/ PCBM/EFGnPs-F/Al. X-ray photoelectron spectra of EFGnPs-F showing (e) survey and (f) F 1s region.
n-type TiO2 layer.5,23 In addition, TiO2 layers are photochemicaly reactive.24,25 These issues constitute a critical weak point for n−i−p structure perovskite solar cell commercialization. The PCE of p−i−n structure perovskite devices has so far reached the n−i−p structure;23,26 additionally, low stability issue is a momentous weak point due to the easy oxidation of low work function metal electrodes (Al).7,26 To solve these issues and make progress toward the commercialization of perovskite solar cells, we explore, for the first time, the development of a highly stable new p−i−n structure for perovskite solar cells using fluorine functionalized EFGnPs to fully cover the perovskite active layer and protect against the ingress of water for high-stability perovskite solar cells. A long-term stability study of these perovskite solar cells was carried out in ambient air conditions, and 82% stability was demonstrated over 30 days, a dramatic improvement compared to reference devices. The enhanced stability stems from Fsubstitution on EFGnPs; fluorocarbons such as polytetrafluoroethylene (Teflon) are well-known for their superhydrophobic properties and being impervious to chemical degradation.27,28 These, resisting on water moieties that tightly protect the perovskite layer from air conditions, characterize the EFGnPs. EFGnPs were synthesized following previous reports using a mechanochemical ball-milling method.29 The ball-milling method relies on kinetic energy to crack the graphite and dissociate some of the carbon−carbon bonds of graphite, as shown in Figure 1. This induces reactions on the newly formed edges of delaminated graphite layers. A reactant gas (H2 or XeF2) is introduced to functionalize graphene to introduce functional groups (−OH, −CO, −COOH) at the edges of the graphene sheets. Schematic diagrams illustrating hydrogen or oxygen functionalization EFGnPs (EFGnPs-H) and fluorine functionalization EFGnPs (EFGnPs-F) are shown in Figure 1b and c, respectively. EFGnPs were dispersed in isopropyl alcohol to fabricate devices. The device structure of high stable perovskite p−i−n photovoltaic cell is shown in Figure 1d.
Briefly, a PEDOT:PSS layer was deposited by spin-casting on top of an indium tin oxide (ITO) substrate. The photoactive MAPbI3 perovskite film was then deposited on top in one step with an antisolvent treatment method. After antisolvent treatment, the perovskite film was shiny and smooth. Then a phenyl-C61-butyric acid (PCBM) solution was deposited by spin-casting on MAPbI3 perovskite film. On top of the PCBM layer, EFGnPs were deposited by dropping the solution for 3 s then spin-casting at 2000 rpm for 30 s. Finally, aluminum electrodes were deposited by thermal evaporator. Detailed fabrication conditions are included in the Experimental Section. The material composition was analyzed by X-ray photoelectron spectroscopy (XPS); survey spectra of MAPbI3 perovskites are shown in Figure 1e, and the binding energy region 675−700 eV (F 1s) was measured to confirm the presence of fluorine in EFGnPs-F deposited on MAPbI3 perovskites, as shown in Figure 1f. XPS data clearly show fluorine functionalized EFGnPs existed on the MAPbI3 surface. The carbon fluorine bond generally has highly hydrophobic chemical properties which impart a repulsive property to water molecules. For example, many commercial applications exist where C−F chemical bonding is exploited to produce products with super hydrophobic properties, such as polytetrafluoroethylene (Teflon). The EFGnPs-F also comprise C−F bonding which imparts hydrophobic properties. As shown in Figure 1d, the EFGnPs were deposited on top of the PCBM layer. The hydrophobic EFGnPs act as a protecting layer for the perovskite film, which is easily decomposed by water or hydrophilic molecules. Therefore, the introduction of a fluorocarbon barrier crucially improves the device stability by protecting perovskite layer from hydrophilic segments. Contact angle measurements with a 5 mg/mL solution concentration of the same condition of solar cells device were used to directly show the hydrophobic properties. EFGnPs-F layers on MAPbI3 perovskite/PCBM films are shown in Figure 2c, and compared to a pristine MAPbI3 perovskite/PCBM layer 6386
DOI: 10.1021/acs.nanolett.7b03225 Nano Lett. 2017, 17, 6385−6390
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Nano Letters
Figure 2. In situ contact angle measurements taken 5, 30, and 60 s after dropping water onto MAPbI3/PCBM (a, d, and g), MAPbI3/PCBM/ EFGnPs-H (b, e, and h), and MAPbI3/PCBM/EFGnPs-F (c, f, and i).
Figure 3. Absorption spectra of perovskite (a) MAPbI3 films with (b) EFGnPs-H and (c) EFGnPs-F surface coatings storage in air at ∼50% humidity depending on the time flow 0 day, 10 days, and 30 days.
Figure 4. (a) J−V characteristics of MAPbI3 devices with EFGnPs-H and EFGnPs-F layers. (b) Power conversion efficiency distribution of MAPbI3 devices with EFGnPs-H and EFGnPs-F layers. (c) Stability of MAPbI3 devices with EFGnPs-H and EFGnPs-F layers over 30 days. (d) J−V characteristics of MAPbI3 devices with variable concentrations of EFGnPs-F.
The immediate contact angle of MAPbI3 perovskite/PCBM is 30.2°. After 30 s, the droplet water infiltrates the MAPbI3 perovskite/PCBM layer which causes a decrease in contact
as shown in Figure 2a and EFGnPs-H layers on MAPbI3 perovskite/PCBM, as shown in Figure 2b as a function of time 0, 30, and 60 s after adding a water droplet to the top surface. 6387
DOI: 10.1021/acs.nanolett.7b03225 Nano Lett. 2017, 17, 6385−6390
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Nano Letters angle to 25.6°. After a further 30 s, the contact angle is decreased to 22.4°. The immediate contact angle of a EFGnPsH layer (38.1°) on an MAPbI3 perovskite/PCBM film is similar of MAPbI3 perovskite/PCBM. However, after 30 s, the contact angle is dramatically decreased to 18.5° and 16.5° in 60 s due to the hydrophilic property of EFGnPs-H. The EFGnPs-F layer contact angle indicates a robust repellent effect on the water droplet as shown in Figure 2c. When a water droplet was introduced on an EFGnPs-F layer on top of an MAPbI3 perovskite/PCBM film, the contact angle was 65.5°, which indicates that the EFGnPs-F of C−F chemical bonding has highly hydrophobic character as expected. Furthermore, after 60 s, the contact angle of EFGnPs-F remains relatively high at 58.6°. These results indicate that the hydrophobic property of EFGnPs-F can protect the perovskite layer from water ingress, to improve device stability. To observe the decomposition rate of perovskite films as a function of time when stored at room temperature with an ambient humidity of about 50%, optical properties of the perovskite films were measured with 5 mg/mL solution concentration of EFGnPs-H and EFGnPs-F, respectively, as shown in Figure 3. The absorption spectra of MAPbI3 perovskite films with EFGnPs-H, and EFGnPs-F are similar to typical MAPbI3 perovskite films. After 10 days of storage at room temperature and 50% humidity, the near-infrared (IR) absorption shoulder characteristic of the MAPbI3 perovskite phase disappears for pristine MAPbI3 perovskite films and MAPbI 3 films coated with EFGnPs-H. The observed degradation of absorption spectra is typical of perovskite films decomposed by moisture. In contrast, the EFGnPs-F coated MAPbI3 perovskite film absorption is stable even if the perovskite film is exposed to air over several days. After 30 days, the absorption onsets (at ∼780 nm) of pristine of MAPbI3 perovskite films and EFGnPs-H coated MAPbI3 perovskite films have become indistinguishable. The disappearance of this onset point indicates that the perovskite structured has bleached. Visually, the films take on a pale yellow-whitish appearance as shown in Figure S1. The changed color and absorption spectra directly confirm the decomposition of the perovskite structure after prolonged air exposure.30 However, even after 30 days stored in air at 50% humidity, the EFGnPs-F coated MAPbI3 perovskite films retain characteristic perovskite absorption spectra and appearance, showing robust stability due to the protective fluorocarbon coating. The EFGnPs-F coating dramatically increases the perovskite film humidity stability. Changes in color and film absorption as a function of air exposure time and humidity are shown in Figure 3 and Figure S1. To characterize the effect of EFGnPs layers on the properties of photovoltaic devices, we examined current density−voltage (J−V) curves of perovskite devices with EFGnPs-H and EFGnPs-F coatings under 100 mW/cm2 AM1.5G irradiation, as shown in Figure 4a. Corresponding performance parameters are summarized in Table 1. The inverted p−i−n structure of perovskite solar cells was fabricated to evaluate the effect of the EFGnPs layer. The power conversion efficiency (PCE) of solar cells with EFGnPs-F layers is similar to pristine perovskite solar cells. To confirm the reproducibility of this approach, we characterized the performance of over 50 cells with and without EFGnPs layers. A box-plot of the PCE distributions is shown in Figure 4b. To characterize the influence of EFGnPs layers, device performance was measured continuously up to 30 days with air exposure at room temperature with ambient humidity
Table 1. Device Characteristics of MAPbI3 Devices with EFGnPs-H and EFGnPs-F Layers structure MAPbI3 MAPbI3/ EFGnPs-H MAPbI3/ EFGnPs-F
EFGnPs concentration
JSC (mA cm−2)
VOC (V)
FF
PCE (%)
X 5 mg/mL
19.1 18.1
0.98 0.98
0.78 0.78
14.7 13.0
5 mg/mL
18.5
0.98
0.78
14.3
of about 50%. As expected, devices showed improved stability with EFGnPs-F layers. 82% of the original PCE was retained after 30 days due to the water-repellent effect of the EFGnPs-F layer. In contrast, the performance of perovskite-only and perovskite/EFGnPs-H devices decreased rapidly to less than 50% within 10 days. To investigate the effect of EFGnPs-F layer thickness on device performance, we fabricated devices with various concentrations of EFGnPs-F solutions to modulate the thickness and characterized the solar cell parameters as shown in Figure 4d. Detailed performance parameters are listed in Table S1. When the concentration of EFGnPs-F solution was higher than 5 mg/mL, the device performance decreased slightly. Highly concentrated dispersions of EFGnPsF yield thicker layers of EFGnPs-F on perovskite films (see Figure S2) which protect the perovskite more effectively, for high stability devices as shown in Figure S3. To characterize the intrinsic electrical properties of EFGnPs materials, the field effect transistor, with a bottom gate, top source, and drain configuration, were measured. As shown in Figure S4, with the applied drain bias from 5 to 40 V, the drain current was steadily increased in the case of EFGnPs-H, 10−9 to 10−7 A, and EFGnPs-F, 10−6 to 10−4 A, respectively. This high current of EFGnPs-F compared to EFGnPs-H is correlated to charge transfer ability in solar cell devices as an electron transfer to electrode to achieve high efficiency and stability of the inverted p−i−n perovskite solar cells. These high electrical properties of EFGnPs-F have been demonstrated in previous reports.29 Conclusion. In summary, we have demonstrated, for the first time, the successful achievement of highly stable p−i−n perovskite devices using EFGnPs-F. The chemical inertness and hydrophobic properties imparted by C−F chemical bonding in EFGnPs-F helps to prevent perovskite degradation. With EFGnPs-F, after 30 days exposure to ambient humidity of ∼50% at room temperature, p−i−n perovskite solar cells were able to maintain 82% of their initial PCE. To compare the effect of similar hydrophilic graphene layer, EFGnPs-H treated devices were tested under the same conditions. Like the pristine MAPbI3 perovskite devices, EFGnPs-H treated devices were completely degraded after 30 days. The hydrophobic properties of EFGnPs-F were characterized by contact angle measurement. The test results show great water repellency compared to pristine perovskite films or EFGnPs-H coated films. Furthermore, the devices with EFGnPs-F layers exhibit similar device performance compared to pristine perovskite devices. This work paves the way toward resolving the main problem in p−i−n perovskite devices in ambient conditions. Experimental Section. Materials and Preparation of Perovskite. Lead iodide (PbI2) and anhydrous N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich. Methylammonium iodide (MAI) was synthesized as following previous method31 by mixing 30 mL of CH3NH2 (40 wt % in 6388
DOI: 10.1021/acs.nanolett.7b03225 Nano Lett. 2017, 17, 6385−6390
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water, Sigma-Aldrich) and 30 mL of HI (57 wt % in water, Sigma-Aldrich) in a 250 mL, three-neck flask at 0 °C for 2 h stirring. The precipitate was recovered by evaporation under vacuum at 60 °C. To purify, the crude MAI was redissolved in ethanol and recrystallized with diethyl either. Finally, the MAI product was dried at 60 °C under vacuum oven for 24 h. To prepare MAPbI3 precursor solutions, MAI and PbI2 were dissolved at a 1:1 molar ratio in DMF solution with a concentration of 40 wt %. Device Fabrication. Indium tin oxide (ITO)-coated glass substrates were cleaned by ultrasonication in deionized water, acetone, and isopropyl alcohol for 10 min each. A poly(3,4ethylenedioxythiophene)−polystyrene sulfonic acid (PEDOT:PSS) layer was deposited on cleaned ITO substrates by spin-casting at 4000 rpm for 40 s, followed by annealing at 150 °C for 15 min. On top of the PEDOT:PSS (Al4083) layer, MAPbI3 perovskite precursor solutions were spin-cast at 2500 rpm for 30 s and dried on a hot-plate at 100 °C for 10 min in air. On top of the perovskite layer, a 25 mg/mL solution of PCBM in 1:1 mixture of chlorobenzene and chloroform was spin-cast at 2000 rpm for 30 s. Then, edged-selectively functionalized graphene nanoplatelets dissolved in isopropyl alcohol were spin-cast at 2000 rpm for 30 s. Subsequently, Al electrodes with a thickness of 100 nm were deposited on the PCBM layer under vacuum (
