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Efficient organic-inorganic hybrid flexible perovskite solar cells prepared by lamination of PTAA/CH3NH3PbI3/anodized Ti metal substrate and graphene/PDMS transparent electrode substrate Jin Hyuck Heo, Dong Hee Shin, Myung Lae Lee, Man Gu Kang, and Sang Hyuk Im ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11411 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Applied Materials & Interfaces

Efficient organic-inorganic hybrid flexible perovskite solar cells prepared by lamination of PTAA/CH3NH3PbI3/anodized Ti metal substrate and graphene/PDMS transparent electrode substrate

Jin Hyuck Heo,† Dong Hee Shin,† Myung Lae Lee,‡ Man Gu Kang,*,‡ and Sang Hyuk Im*,† †

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro,

Seongbuk-gu, Seoul 136-713, Republic of Korea ‡

ICT Materials and Components Laboratory, Electronics and Telecommunications Research

Institute, Daejeon 34129, Republic of Korea

Abstract Flexible Ti metal substrate-based efficient planar type CH3NH3PbI3 (MAPbI3) organic-inorganic hybrid perovskite solar cells are fabricated by lamination of the flexible Ti metal substrate/dense TiO2 electron transporting layer formed by anodization reaction/MAPbI3/polytriarylamine (PTAA) and the graphene/polydimethylsiloxane (PDMS) transparent electrode substrate. By adjusting the anodization reaction time of the polished Ti metal substrate and the number of graphene layer in graphene/PDMS electrode, we can demonstrate the planar type MAPbI3 flexible solar cells with 15.0 % (mask area = 1cm2) of power conversion efficiency at 1 sun condition.

Keywords: Ti metal substrate, flexible solar cells, perovskite, graphene, anodization, polydimethylsiloxane.

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1. Introduction

As society is developing, the electrical energy consumption of buildings, cars, and portable electronic devices such as smart phones, tablets, notebook PCs, and wearable PCs is continuously growing. Therefore, the solar cells are ideal devices because they can directly generate electricity from the sun light. To cope with above potential needs for independent and portable power generation system, we need to develop efficient solar cells with high flexibility and bendability. From the flexibility aspect, organic photovoltaics (OPVs) and thin-film solar cells based on metal chalcogenides such as CdTe, Sb2S(e)3, and CIGS have been considered as promising candidates because they can fully absorb the light in thin-thickness (below 2 µm) due to direct energy bandgap.1-5

Recently, organic-inorganic hybrid perovskite (OHP) light absorbing materials such as CH3NH3PbX3 (MAPbX3, X = Br or I) have considered as alternative promising candidate for flexible thin-film solar cells because of their unique properties such as strong absorptivity in visible region, small exciton binding energy, convenient bandgap tenability, long charge carrier’s diffusion length, and solution processability.6-10 Especially, the OHP light absorber is very attractive in flexible solar cells because it can sufficiently absorb the light even in very thin thickness of 300-500 nm and has good flexibility.11-16 Rather, it is known that the failure of the bending test in the flexible OHP solar cells is originated from the crack of brittle transparent conducting oxide (TCO) such as ITO (indium tin oxide). Therefore, it is important to use the conductive flexible substrate without brittle TCO.

For examples, Li et al. fabricated a TCO-free conductive flexible substrate by depositing conducting polymer (PH1000) on PET (polyethylene terephthalate) substrate with embedded Ag-


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mesh and demonstrated 14 % flexible OHP solar cells.17 Sung et al. fabricated a TCO-free conductive flexible substrate by forming MoO3-doped graphene layer on a PET substrate and demonstrated 17.1 % flexible OHP solar cells.18 Very recently we (Im et al.) also reported several flexible OHP solar cells including 17.9 % super flexible FAPbI3-xBrx (CH(NH2)2PbI3-xBrx) TCO-free AuCl3 doped single-layer graphene/PET OHP solar cells.19 Aforementioned flexible OHP solar cells are formed on the transparent flexible plastic substrate which only allows the low/mild temperature process below 150 oC and requires additional barrier layer to prevent degradation of OHP solar cells from the invasion of water and oxygen molecules. Accordingly, the low/mild temperature processable materials such as PEDOT:PSS (poly-3,4-ethylene dioxythiophene:polystyrenesulfonate), ZnO nano-sol, and ZnSnO3 nano-sol can be only applicable.

Instead, Yun et al. used Ti metal foil, which allows the high temperature process so that the conventional materials such as TiO2 and NiOx can be usable without limitation of material selection, as conductive flexible substrate in dye-sensitized solar cells.20 In addition, the flexible metal substrate has excellent barrier properties so it does not require additional expansive barrier layer. From the aspect on the commercialization of flexible OHP solar cells, many factors such as flexibility of substrate, barrier properties, conductivity of electrode on a flexible substrate, processing temperature window, efficiency, and cost should be considered. Hence, the metal substrate is one of promising candidate as flexible substrate for flexible OHP solar cells and it is still challenging to fabricate high performance metal substrate based flexible OHP solar cells.

Recently, Lee et al. reported 6 % flexible mesoscopic type OHP solar cells comprised to Ti foil/blocking

TiO2/mesoporous

TiO2/OHP/spiro-OMeTAD

(2,2’,7,7’-tetrakis-(N,N-di-p-


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methoxy phenylamine)-9,9’-spirobifluorene)/thin Ag.21 Troughton et al. reported 10.3 % flexible meso-superstructure type OHP cells composed by Ti foil/TiO2/mesoscopic Al2O3/OHP/spiroOMeTAD/PEDOT:PSS/transparent conductive adhesive/PET with embedded Ni mesh.22 Although the metal substrate has many advantages as conductive flexible substrate, so far additional studies seem not to have been reported. Here, we used a polished Ti metal substrate as conductive flexible substrate and formed TiO2 electron transporting layer via electrochemical anodization. We fabricated the planar type flexible MAPbI3 OHP solar cells comprised to polished

Ti

metal

substrate/TiO2

electron

transporting

layer

formed

by

anodization/MAPbI3/PTAA (poly-triarylamine)/graphene/PDMS (polydimethylsiloxane) by lamination of Ti susbeatrate/TiO2/MAPbI3/PTAA and graphene/PDMS substrate and demonstrated flexible OHP solar cells with 15 % by systematic investigation.

2. Experimental section

2.1.

Preparation of graphene/PDMS counter electrode

To prepare graphene/PDMS counter electrode, we grew graphene sheets on 120 µm thick Cu-Ni foil (3 × 3 cm size, 120 µm-thick; weight percent, 88.00% Cu, 9.90% Ni, 0.44% Mn, 1.54% Fe, and 0.10% Zn) by conventional chemical-vapor-deposition (CVD) method with 10 sccm H2 and 20 sccm CH4 flowing at pressure of 3 Torr at a specific reaction temperature of 950, 980, 1000, and 1030 oC for mono-, bi-, tri-, and quad-layered graphene.23 After growing graphene sheet, we formed the PDMS substrate on the graphene/Cu-Ni alloys foil by spin coating at 3000 rpm for 60 s with the mixture of poly(dimethylsiloxane) and curing agent (10 : 1 wt/wt) and then cured it in an 100 oC oven for 30 min. The graphene layer formed on the opposite side of Cu-Ni foil was


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removed by O2 plasma at 100 W for 5 s in reactive ion etching. We then etched Cu-Ni foils by 1M ammonium persulfate for 10 h and subsequently washed graphene/PDMS substrate by ethanol. To make a graphene electrode pattern of 1 × 2.5 cm on the 3 × 3 cm graphene sheet, we attached 1 × 2.5 cm thermal release tape on graphene sheet and then etched by O2 plasma at 100 W for 5 s in reactive ion etching. Then, the thermal release tape was removed by heat treatment at 100 ° C for 30 s.

2.2.

Characterizations of number of graphene layers electrode

The Raman spectra were measured by a 1-mW 532-nm laser with a spot size of 1 µm using a home-built scanning confocal microscope with a charge coupled device (Andor model DU401ABU). Raman mapping could be created by moving the sample under the microscope through the use of correctly aligned step motors. Hall-effect measurements were performed by van der Pauw method (Ecopia model HEM-2000). UV-visible transmittance spectra were recorded on a Varian Cary-5000 spectrophotometer in the wavelength range of 300 to 900 nm. The sheet resistance and work function were measured by 4-probe method (Dasol eng, model FPP-HS8-40K) and Kelvin-probe force microscope (Park System, model XE-100), respectively. The transmittance of the films was measured using a UV-vis spectrophotometer.

2.3.

Device fabrication

To fabricate the flexible planar type MAPbI3 OHP solar cells, the 40 wt% MAPbI3/N,Ndimethylformamide (DMF) solution containing hydriodic acid (100 µL per 1mL MAPbI3/DMF solution) was spin-coated on the anodized TiO2/Ti metal substrate at 3000 rpm for 200 s, and then dried on a 100 oC hot plate for 2 min. Subsequently, poly(triaryl amine) (PTAA, EM index)


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hole conductorwas deposited on the each MAPbI3 perovskite/Ti electrode substrate by spin coating PTAA/toluene (15 mg/mL) solution with Li-bis(trifluoromethanesulfonyl)imide (LiTFSI)/acetonitrile (7.5 mL, 170 mg/mL) and tBP/acetonitrile (7.5 mL, 1:1) additives at 2000 rpm for 30 s. Finally, was covered on MAPbI3 perovskite/Ti electrode substrate. The active area was fixed as 1 × 2.5 cm by the patterned area of graphene/PDMS electrode. All device fabrication was conducted below relative humidity of 25%.

2.4.

Device characterization

The external quantum efficiency (EQE) was measured by a power source (ABET, 150W Xenon lamp, 13014) with a monochromator (DONGWOO OPTRON Co., Ltd., MonoRa-500i) and potentiostat (IVIUM, IviumStat). The current density-voltage (J-V) curves were measured by a solar simulator (Peccell, PEC-L01) with a potentiostat (IVIUM, IviumStat) at under illumination of 1 sun (100 mW/cm2 AM 1.5G) and a calibrated Si-reference cell certificated by JIS (Japanese Industrial Standards). To measure the hysteresis of J-V curves, the forward and reverse scan rate was set to 10 mV·200 ms-1 as a standard condition. The J-V curves of all devices were measured by masking the active area with mask of 1 cm2. Intensity-modulated photocurrent and photovoltage were measured by potentiostat (IVIUM, IviumStat) with light emitting diode (IVIUM, IM1225).

3. Results and discussion

3.1. Formation of TiO2 electron transporting layer by anodization of a flexible Ti metal substrate


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A flexible Ti metal substrate is very useful because it has both function of flexible substrate and conductive electrode. In addition, its oxide form (TiO2) is generally used as electron transporting layer for OHP solar cells. Therefore, we converted the surface of polished flexible Ti substrate into TiO2 by simple electrochemical anodization. Figure 1 is AFM (atomic force microscopy) topologies of polished flexible Ti substrate with anodizing reaction time. For the anodization of flexible Ti substrate, we used a polished 100 µm-thick Ti substrate with rms (root mean square) roughness of ~2.5 nm as shown in Figure 1a. The anodizing reaction was conducted by applying bias voltage of 50 V between the polished Ti working electrode and the platinum counter electrode for 30 min - 60 min, of which we used the ethylene glycol electrolyte containing 0.06 wt% NH4F and 2 vol% H2O. Apparently, the surface of flexible Ti substrate was gradually roughened as the anodizing reaction proceeded as shown in Figure 1b-e. The anodized Ti substrates were then thermally annealed at 450 oC for 30 min in air atmosphere in order to fully convert the formed amorphous-like TiO2 into dense crystalline TiO2 electron transporting layer. Figure 1f is the histogram for the rms roughness of annealed TiO2/Ti substrates indicating that the 30, 40, 50, and 60 min-anodized sample after heat-treatment has ~18.6, ~52.0, ~89.1, and ~100.6 nm of rms roughness, respectively. This clearly indicates that the thickness of TiO2 dense electron transporting layer is controllable with the anodizing reaction time.

To check if the annealed TiO2/Ti substrates form crystalline phase, we examined the XRD (Xray diffraction) patterns of them as shown in Figure 2. The pristine polished Ti flexible substrate (0 min sample) exhibited only metallic crystalline α-Ti phase. On the other hand, the annealed TiO2/Ti substrates had a mixture crystalline phase of anatase and rutile TiO2. The gradual increase of intensity ratio of (101) plane of antase TiO2 and metallic Ti peak also confirms that


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the thickness of TiO2 layer on the Ti flexible substrate is gradually increased with the anodizing reaction time.

3.2. Fabrication of a conformal flexible transparent graphene/PDMS electrode

The schematic procedure of preparation of PDMS/graphene conformal flexible transparent electrode was shown in Figure 3. Graphene layers were grown on polycrystalline Cu-Ni foils (3 × 3 cm size) in a halogen-lamp-based CVD quartz tube furnace by flowing 10 sccm H2 and 20 sccm CH4 under a pressure of 3 Torr at a specific growth temperature of 950, 980, 1000, and 1030 oC for the formation of mono-, bi-, tri-, and quad-layered graphene sheets. The reactant gases are contacted on both front and back surface of Cu-Ni foil so the graphene is formed on both surfaces. To form conformal flexible transparent substrate on the conducting graphene electrode, the PDMS resin with 10 weight fraction curing agent was spin-coated on the graphene/Cu-Ni foil at 3000 rpm for 60 s in ambient condition and it was cured on a hot plate at 100 oC for 30 min. Then the graphene layer formed on the opposite side was removed by reactive ion etching using O2 plasma because the graphene layer reduces the transmission of incident light. The Cu-Ni foil was then removed by immersing the Cu-Ni foil/graphene/PDMS substrate in 1 M ammonium persulfate solution for 10 h. The graphene/PDMS flexible transparent conducting electrode was washed by ethanol and then was dried. To make an electrode pattern (1 × 2.5 cm), we masked it with thermal release tape (1 × 2.5 cm) and subsequently etched the unmasked graphene by reactive ion etching. By subsequent heat-treatment of the patterned thermal release film/graphene/PDMS substrate at 100 oC for 30 s, we removed the thermal release film and finally made the conformal flexible transparent conducting graphene/PDMS substrates.


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The optical and electrical properties of the prepared conformal flexible graphene/PDMS transparent electrode were shown in Figure 4. The transmission spectra of graphene/PDMS electrode in Figure 4a indicate that the transmittance of graphene/PMDS electrode is gradually decreased as the growth temperature of CVD process is elevated from 950 to 980, 1000, and 1030 oC. The transmittance of each sample was ~ 97.5, ~95, ~92.5, and ~90 %, respectively, so we can conclude that mono-, bi-, tri-, and quad-layered graphene is formed at 950, 980, 1000, and 1030 oC reaction temperature, respectively. The transmittance at wavelength (λ) = 550 nm for the four films are shown in inset of Figure 4a. By fitting the data according to Beer’s law, the attenuation coefficient (α = ~2.5 %) per layer is calculated which is very close to the theoretical attenuation coefficient (2.3 %) of graphene per layer.24,25 Figure 4b shows the sheet resistances of graphene/PDMS transparent electrodes measured by the van der Pauw method. The sheet resistances of mono-, bi-, tri-, and quad-layered graphene/PDMS transparent electrodes are 722 ± 18, 494 ± 17, 396 ± 23, and 322 ± 18 Ω/cm2, respectively. Although the sheet resistance and the transmittance of graphene/PDMS transparent electrode has trade-off relationship, it is notable that the quad-layered graphene/PDMS electrode still maintains a relatively high transmittance of about 90 %.

In the case of graphene/PDMS electrode, there was a difficulty in Raman measurement due to the overlapping peaks of PDMS and graphene. Therefore, we measured Raman spectra by transferring the graphene films onto 285 nm-thick SiO2/Si substrate as shown in Figure 4c. The Raman spectra of the graphene films exhibited three intense D, G, and 2D peaks at ∼1350, ∼1580, and ∼2700 cm−1, respectively, which correspond to unique characteristic peaks of graphene.26,27 As the number of graphene layers increases, both the G and 2D bands are red-shifted due to the change of the electronic band structure to graphitic structure.28,29 The Raman intensity ratios of


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the 2D to G peaks (I ) and D to G peaks (I ) are related to the number of stacked graphene layer and disorder, respectively. Figure 4d indicates that the I and I of mono-, bi-, tri-, and quad-layered graphene is ~2.17/~0.055, ~1.50/~0.055, ~1.32/~0.060, and ~0.94/~0.092 (I

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