Graphene Solar Cells without a Hole-Transport Layer

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Letter

Perovskite/Graphene Solar Cells without a Hole-Transport Layer Ryousuke Ishikawa, Sho Watanabe, Sohei Yamazaki, Tomoya Oya, and Nozomu Tsuboi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01606 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 3, 2019

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ACS Applied Energy Materials

Perovskite/Graphene Solar Cells without a HoleTransport Layer Ryousuke Ishikawa*, Sho Watanabe, Sohei Yamazaki, Tomoya Oya, and Nozomu Tsuboi

Niigata University, 8050 Ikarashi 2-nocho, Nishi-ku, Niigata 950-2181, Japan

Corresponding Author *E-mail:

[email protected]

ACS Paragon Plus Environment

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ABSTRACT

This study was conducted with the objective of improving the stability of perovskite solar cells by using the unique characteristics of graphene in order to facilitate the widespread application of such solar cells, for example, in multi-junction devices. We consequently developed a new transfer method for graphene using vacuum lamination and, using graphene, successfully fabricated a perovskite solar cell that does not require a holetransport layer. Initial stability tests indicated that the new device has better stability than a control perovskite solar cell using spiro-OMeTAD. Although the new solar cell design exhibited poorer cell performance than the control, we determined via modeling that its performance can be improved by modifying the interface state between perovskite and graphene or by modulating the work function of graphene.

KEYWORDS: graphene transfer method, vacuum lamination, organometallic halide perovskite solar cell, hole transport layer, stability


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TOC GRAPHICS


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Perovskite solar cells based on organometallic halides, such as CH3NH3PbI3 (MAPbI3), are a new class of solar cell first demonstrated to have high potential in 20091 and have made remarkable progress in recent years.2-5 Currently, various groups around the world are conducting research for early commercialization, but challenges related to stability and durability remain.5-6 A major cause of perovskite solar cell degradation is decomposition of the perovskite crystal due to reactions with atmospheric water and oxygen.6 This problem can be suppressed to a certain extent by using the same sealing technology as used for organic devices; however, to improve the stability and reduce the cost by eliminating the sealing process, the solar cell structure should be designed to limit water and oxygen diffusion. Currently, the most common perovskite solar cell structure consists of glass/SnO2:F (FTO)/TiO2 (electron transport layer)/perovskite/spiro-OMeTAD (hole transport layer: HTL)/Au electrode. A very stable perovskite solar cell was demonstrated that maintained its conversion efficiency for over a year, which had a structure of glass/FTO/TiO2/ZrO2/perovskite/carbon electrode.7-8 The main reason for the dramatically improved stability was that the carbon electrode blocked diffusion of water


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and oxygen and suppressed degradation of the perovskite layer; in addition, a highly stable 2D/3D hybrid perovskite was used.

Graphene is a nanocarbon materials with a 2D sheet structure (single atom thickness) in which carbon atoms are bonded in a honeycomb lattice. This structure imparts unique properties9-11 that our research group have been exploiting for solar cell applications.12-14 Whereas several research groups have reported success in using graphene derivatives to improve the characteristics and stability,15 only a few have reported using chemical vapor deposition (CVD) graphene, which is considered to have a high barrier capability. In this study, we aimed to improve the stability of perovskite solar cells for a wide range of applications (e.g., multi-junctions) by utilizing the unique characteristics of graphene.

Although a high efficiency can be obtained using the anti-solvent method,3,16 sufficient reproducibility has not yet been demonstrated in our research environment due to the narrow processing window. We fabricated perovskite layers with a modified solventsolvent extraction method where the perovskite precursor film is immersed in an antisolvent bath after spin-coating.17-19


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Graphene was synthesized using thermal CVD.20-21 The number of graphene layers can be controlled by the film thickness of the Ni catalyst, gas flow rate, pressure, and temperature profile.22 Here, about seven layers of multilayer graphene were used. The basic physical properties of multilayer graphene are shown in Fig. S2. The most common method for transferring graphene uses a polymethylmethacrylate (PMMA) protective film.23 However, this method is not appropriate for perovskite solar cells as the perovskite thin film needs to be immersed in a solvent, such as water; the perovskite layer can be easily dissolved in the solvent. Therefore, we developed a new dry process for transferring graphene using a vacuum laminating apparatus, where this process bonds the layers under pressure and vacuum. This method can also be used to bond silicon and other rigid substrates by activating the surface by plasma irradiation.24


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Figure 1. Schematic diagram of the fabrication process for an HTL-free perovskite solar cell with graphene using vacuum lamination. Figure 1 shows the fabrication process for a HTL-free perovskite solar cell with graphene using a vacuum lamination process. A silicone (polydimethylsiloxane: PDMS) film was coated on multilayer graphene synthesized on a Ni thin film on a glass substrate. This structure was immersed in an etching solution to remove the Ni, allowing the graphene/PDMS film to be peeled from the glass substrate. In addition, a TiO2 layer was deposited on a FTO glass substrate by spin-coating, followed by deposition of a perovskite (CH3NH3PbI3) layer using the modified solvent-solvent extraction method to prepare a multilayer FTO/TiO2/perovskite film. The two multilayer films were bonded using the vacuum laminating apparatus shown in Fig. S3. Two stacked films were stacked and set on the lower stage in the chamber. After vacuuming the chamber to approximately 10-2 Pa, the stages were pressed against each other for 15 minutes with a force of 1.4 kN (without heating). Surprisingly, when the PDMS film was peeled off after lamination, all graphene layers were completely transferred to the perovskite layer. Finally, a Au back electrode was vacuum deposited to obtain an HTL-free perovskite solar cell with the


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structure of glass/FTO/TiO2/perovskite/graphene/Au (perovskite/graphene sample). A cross-sectional SEM image of this structure is shown in Fig. S5 (a).

The control samples used to verify the effect of graphene were solar cells using spiroOMeTAD as the HTL (spiro-OMeTAD; Fig. S4 (a)), and solar cells where the perovskite layer was directly coated with Au without any HTL (perovskite/Au; Fig. S4 (b)). The solar cell performances were measured using a solar simulator (AM1.5G) at room temperature in air without encapsulation. The J–V characteristics of the fabricated perovskite solar cells are shown in Fig. 2. The detailed photovoltaic parameters are summarized in Table 1. Both the short circuit current density (Jsc) and the open circuit voltage (Voc) of the perovskite/Au samples were markedly lower than those for the spiro-OMeTAD sample. The Jsc values of the perovskite/graphene sample were higher than those of the spiroOMeTAD samples, indicating the potential of graphene; however, due to a low Voc and fill factor (FF), the conversion efficiency () did not exceed that of the control sample. Although the influence of the pressing process on the solar cell characteristics was verified by processing the spiro-OMeTAD control sample under the same conditions as


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the graphene-containing sample, deterioration of solar cell performance was not observed. Hence, we attributed the low Voc and FF in the perovskite/graphene sample mainly to the interface between the perovskite and graphene. Further optimization of the graphene layer and transfer conditions is expected to improve solar cell performance.

The operating mechanism of a HTL-free perovskite solar cell with graphene was analyzed using a 1D device simulator and SCAPS simulation software (ver. 3.3. 00).25 The experimental values determined in this study, such as bandgap and thickness, were used in the model, while other simulation parameters were taken from previous reports.26 Table S1 summarizes the material properties and device parameters used in the simulation. The interface defective layers were assumed to include interface recombination. The experimental performance of the spiro-OMeTAD samples was reproduced well by the model using measured series and shunt resistances, as shown in Fig. 2. For perovskite/Au samples, the experimental solar cell characteristics were poorer than the simulated results where the work function (WF) of Au was assumed to be 5.4 eV, as shown in Fig. 3 (b). This was attributed to the perovskite layer being damaged


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during vacuum evaporation of the Au layer, deteriorating the interface. In the case of the perovskite/graphene samples, we assumed that the multilayer graphene was semimetallic; using a WF of 4.9 eV in the model, it was possible to reproduce the experimental solar cell performance. The low Voc may be due to the large energy difference (E = 0.55 eV) between the top of the valence band of the perovskite and the Fermi level of graphene, as shown in Fig. 3 (c). Hence, increasing the WF of graphene by surface modification or doping, as shown in Fig. S6a, could be an effective method for further improving Voc and .

Figure 2. Experimental and simulated J–V characteristics of the perovskite-based solar cells.


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Table 1 Photovoltaic parameters of the fabricated and simulated perovskite solar cells.

Jsc (A/cm2) Voc (V) (a) spiro-OMeTAD

 (%) Rs () Rsh ()

18.3

1.02

0.67

12.6

6.1

536.7

7.9

0.72

0.49

2.8

4.0

65.1

(c) graphene

21.3

0.71

0.47

7.1

4.2

53.4

(a) spiro-OMeTAD

19.9

1.01

0.64

12.5

6.1

536.7

(b) gold

19.3

0.99

0.45

8.5

4.0

65.1

(c) graphene

19.2

0.68

0.52

6.8

4.2

53.4

experimental (b) gold

simulation

FF


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Figure 3. Simulated band diagrams of (a) spiro-OMeTAD, (b) perovskite/Au, and (c) perovskite/graphene samples.

Finally, the stability of the perovskite/graphene sample was evaluated. We measured the performance of the solar cells after continuous irradiation with AM1.5G simulated sunlight for 1 h at 25°C and a humidity of 50% without any encapsulation. The spiroOMeTAD sample showed a decrease in Jsc and Voc values to 20% and 34% of the initial values, respectively, while the conversion efficiency deteriorated to less than 1% (hence, longer measurements were not possible). However, for the perovskite/graphene sample, both Jsc and Voc retained about 80% of the initial value, and the conversion efficiency was 3.9%, which was about half of the initial value. Although only an initial short-term test, these results suggest that the use of graphene resulted in superior stability compared with spiro-OMeTAD. In order to investigate the origin of this enhanced stability, X-ray diffraction (XRD) measurements of the solar cells before and after holding at 25°C and 70% humidity for 48 h in the dark were performed, as shown in Fig. 4. In the spiroOMeTAD sample, the formation of PbI2 after humidification treatment was observed,


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while the perovskite/graphene sample did not show any significant differences in the XRD spectra. As shown in Eq. (1) and (2), when the CH3NH3PbI3 perovskite crystal reacts with water or oxygen, it is decomposed to produce PbI2.6

CH3NH3PbI3 + H2O  PbI2 + CH3NH2 + 1/2 I2 + 1/2 H2 + H2O

(1)

CH3NH3PbI3 + 1/4 O2  PbI2 + CH3NH2 + 1/2 I2 + 1/2 H2O

(2)

The spiro-OMeTAD layer allowed atmospheric water and oxygen to penetrate the cell, while graphene blocks penetration of water and oxygen, preventing decomposition of the perovskite and increasing solar cell stability. Ideal graphene, with no grain boundaries and defects, does not transmit any molecules or atoms at room temperature,27 while multilayered CVD-grown graphene has also shown very high gas-barrier performance.28 This demonstrates the possibility of producing graphene-containing solar cells with stability equal to or higher than that the very stable perovskite solar cells using carbon electrodes, which showed stable conversion efficiency for more than one year.7-8


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Figure 4. XRD spectra of the solar cells using (a) spiro-OMeTAD and (b) graphene before and after humidification treatment.

In conclusion, we demonstrated the successful fabrication of HTL-free perovskite solar cells, facilitated by a new transfer method for graphene using vacuum lamination. In order to reduce the cost and increase the size of the cells for mass production, we propose transferring graphene using a roll-to-roll method in the future. Although the cell performance of the new solar cell was not as good as state-of-the-art HTL-based perovskite solar cells, our device simulations predicted that the performance could be improved by modifying the interface state between perovskite and graphene or by


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modulating the work function of graphene. Initial short-term stability tests showed that the HTL-free cell had better stability than the HTL-based cells due to graphene effectively blocking water and oxygen diffusion (similar to a carbon electrode). Considering the very high gas barrier properties of graphene, we expect it to be superior to carbon electrodes using carbon paste. It is also interesting that a Voc of 1.57 V (close to the maximum reported value), was achieved with a carbon electrode and a wide-gap perovskite (CH3NH3PbBr3) solar cell, which is promising as a top cell of a multi-junction solar cell.29 We are also aiming to apply graphene to ultra-high efficiency multi-junction solar cells as an intermediate electrode and tunnel junction layer by taking advantage of its high optical transparency (which is not a characteristic of carbon electrodes).

EXPERIMENTAL METHODS

Materials. All reagent-grade chemicals were purchased from Wako Pure Chemical Industries unless noted otherwise. PbI2 as the perovskite precursor30 and TiO2 paste for the mesoporous-TiO2 layer (PST-18NR) were purchased from Tokyo Chemical Industry


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and JGC Catalysts & Chemicals, respectively. FTO-coated glass substrates and gold wire (99.95%) for back contacts were purchased from Furuuchi Chemical and Nilaco, respectively. PDMS (Sylgard 184) was purchased from Dow Corning.

Perovskite Film Fabrication. An equimolar mixture of PbI2 and methylammonium iodide (MAI) was dissolved in N-methylpyrrolidone (NMP) at room temperature. A dense TiO2 layer (a few nm) and a mesoporous TiO2 layer (200 nm thick) were formed on the FTOcoated glass following our previous work 19 The TiO2-coated FTO glass substrates were preheated on a hotplate at 80°C. The prepared perovskite precursor solution was spincoated onto the preheated substrates at 5500 rpm for 15 s. Immediately after spincoating, the substrates were immersed into a 30 ml bath of anhydrous diethyl ether for 2 min until a brown film formed. The fabricated perovskite films were dried at 25 °C. The hole-transporting layer was deposited on the perovskite layer by spin-coating a solution of spiro-OMeTAD in chlorobenzene containing 4-tert-butylpyridine and lithium bis(trifluoromethylsulfonyl)imide as dopants. The entire perovskite film fabrication process was performed in an inert glovebox.


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Graphene Growth. Graphene was synthesized using a thermal CVD system, as shown in Fig. S1 (a). An 800 nm Ni thin film sputtered on a quartz substrate was used as the catalyst metal. A multilayered graphene film was synthesized using H2 and CH4 gas under the conditions shown in Fig. S1 (b).

Characterization. The J–V measurements were performed under simulated AM1.5G illumination at room temperature in air without any encapsulation using a metal mask with a square hole with a side of 3 mm opened to determine the active area. The J–V curves presented here were obtained with a reverse scan from +1.1 to -0.1 V with a scan rate of 50 mV/s. XRD patterns were obtained using Cu K ( = 1.5406 Å) radiation in a reflection geometry with a Rigaku RINT-2100 Ultima system. Transmittance spectra were obtained using ultraviolet-visible near-infrared spectrophotometry (JASCO ARSN-733). The electrical properties were determined using Hall-effect measurements and the van der Pauw method (ECOPIA HMS-5000). Cross-sectional images were observed using a fieldemission scanning electron microscopy (FE-SEM; JEOL JSM-6500F).


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ASSOCIATED CONTENT

Supporting Information.

Thermal CVD system for graphene growth; basic properties of multilayer graphene; photographs and schematic image of the vacuum laminating apparatus; cross-sectional SEM images of fabricated perovskite solar cells; materials properties and device parameters used for device simulation; simulated J–V curves of solar cells as a function of the work function of graphene.

AUTHOR INFORMATION

Corresponding Author *E-mail:

[email protected]

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by Japan Society for the Promotion of Science KAKENHI (grant numbers JP16K18356 and JP17H03532).

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Figure 1. Schematic diagram of the fabrication process for an HTL-free perovskite solar cell with graphene using vacuum lamination. 488x163mm (150 x 150 DPI)


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Figure 2. Experimental and simulated J–V characteristics of the perovskite-based solar cells. 180x138mm (150 x 150 DPI)



Figure 3. Simulated band diagrams of (a) spiro-OMeTAD, (b) perovskite/Au, and (c) perovskite/graphene samples. 324x82mm (150 x 150 DPI)


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Figure 4. XRD spectra of the solar cells using (a) spiro-OMeTAD and (b) graphene before and after humidification treatment. 315x120mm (150 x 150 DPI)


Graphene Solar Cells without a Hole-Transport Layer

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