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Low temperature solution processable TiO2 nano-sol for electron transporting layer of flexible perovskite solar cells
Solar Energy Materials and Solar Cells 194 (2019) 1–6
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Low temperature solution processable TiO2 nano-sol for electron transporting layer of flexible perovskite solar cells
T ⁎
Myung Sang Youa, Jin Hyuck Heob, Jin Kyoung Parka,b, Sang Hwa Moona,b, Bum Jun Parka, , ⁎ Sang Hyuk Imb, a
Functional Crystallization Center (ERC), Department of Chemical Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446701, Republic of Korea b Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-713, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: TiO2 nano-sol Low temperature Solution process Electron transporting layer Perovskite solar cells
Low-temperature solution processable TiO2 nano-sol for electron transporting layer (ETL) of flexible perovskite solar cells is prepared by synthesizing TiO2 nano-sols by peptization in acidic aqueous solution and re-dispersing the TiO2 nanoparticles in H2O, ethanol (EtOH), dimethylsulfoxied (DMSO), and N,N-dimethylformamide (DMF). By considering dielectric constant, zeta potential, and surface tension of solvent media, we found that the DMF is the best to form uniform TiO2 ETL by spin-coating among them because it has low surface tension and high zeta potential. Accordingly, we could demonstrate 18.2% power conversion efficiency (PCE) for rigid glass substrate based perovskite solar cells and 15.8% PCE for flexible polymer substrate based devices by low-temperature solution process.
1. Introduction Solar light energy has been considered as a promising alternative energy source for substitution of conventional fossil fuel because it is abundant, clean, sustainable and renewable. Solar cells can generate direct current by converting the solar light into electricity, so many researchers have intensively studied them for several decades to develop cost-effective next-generation solar cells, which will have comparable power generation cost with fossil fuels. In parallel, to date needs for potable and independent power generators are gradually growing as mobile communication technologies are advanced. To deal with the electrical energy demands for mobile devices, it is necessary to develop highly efficient, lightweight, and flexible solar cells. So far, various flexible solar cells such as thin film, dye-sensitized, organic, and perovskite solar cells have been intensively studied [1–5]. Among them, metal halide perovskite solar cells (PeSCs)[6–15] have attracted great attention as a promising candidate for portable power generation devices because they have high efficiency, simple device structure, low-cost solution processability, and good flexibility. However, high performance PeSCs have been demonstrated by bi-layer type device architecture, which requires mesoscopic TiO2 electron transporting layer (ETL) on a rigid F-doped tin oxide (FTO)/glass substrate because the mesoscopic TiO2 is formed by high temperature sintering
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process of ~ 450 °C. The TiO2 ETL has important role in PeSCs because electrons generated in the perovskite are injected into the TiO2 ETL. High temperature process cannot be applicable to flexible PeSCs because the flexible polymer substrates such as poly-ethyleneterephthalate (PET) and poly-ethylenenaphthalate (PEN) are unstable. Accordingly, entire process of the flexible polymer substrate-based perovskite solar cells must be conducted at low temperature (below 150 °C). This means that the TiO2 ETL should be formed at low temperature because the other layers such as perovskite, hole transporting layer (HTL), and counter electrode can be formed at low temperature. Hence, it is still challenging to develop solution processable TiO2 ETL at low temperature. Recently, Heo et al. synthesized a ZnO nano-sol and fabricated a flexible PeSC composed of PEN/ZnO ETL/perovskite/poly-triarylamine (PTA) HTL/Au [16]. They obtained high efficiency and reduced current-voltage hysteresis with respect to scan direction and scan rate, but they could not guarantee long-term stability of perovskite solar cells because the ZnO can be reacted with perovskite due to its inherent poor chemical stability. Accordingly, to improve chemical stability of ZnO nanoparticles, Shin et al. synthesized Zn2SnO4 nano-sol by solution chemistry and succeeded to fabricated flexible PeSCs [17]. Giacomo et al. formed TiO2 ETL by atomic layer deposition (ALD) at low temperature (150 °C) and obtained 8.4% power conversion efficiency (PCE)
Corresponding authors. E-mail addresses: [email protected] (B.J. Park), [email protected] (S.H. Im).
https://doi.org/10.1016/j.solmat.2019.02.003 Received 27 October 2018; Received in revised form 20 January 2019; Accepted 2 February 2019 0927-0248/ © 2019 Published by Elsevier B.V.
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for a flexible PeSC [18]. Qiu et al. formed pinhole-free TiO2 ETL by electron beam deposition and demonstrated 13.5% PCE [19]. Recently, Mali et al. deposited TiO2 ETL by RF sputtering and obtained 15.88% PCE [20]. Very recently, Zhao et al. used TiO2 nanocrystals passivated by titanium diisopropoxide bis(acetylacetonate) as ETL and demonstrated 18.3% PCE [21]. Besides the low temperature processability of TiO2 ETL, the surface of TiO2 ETL should have hydrophilicity to deposit perovskite material on it because higher surface energy of TiO2 ETL is more desirable to wet perovskite solution. If not, the perovskite solution is not wetted on the TiO2 ETL and consequently the perovskite layer is not formed on it by spin-coating process. Hence, here we synthesized TiO2 nanoparticles in aqueous phase for hydrophilic TiO2 ETL and systematically studied to find proper solvent medium for the TiO2 nano-sol in order to deposit uniform TiO2 ETL on indium doped SnO2 (ITO) substrate. By using this spin-coatable TiO2 nano-sol, we fabricated rigid ITO/glass and flexible ITO/PET substrate-based perovskite solar cells.
ITO/glass substrate at 3000 rom for 30 s and dried it on a hot plate at 100 °C for 30 min. FAPbI3-xBrx perovskite film was then deposited on the TiO2/ITO/glass substrate. To make FAPbI3-xBrx perovskite films, we synthesized a PbI2(DMSO)2 complex by dissolving 50 g PbI2 in 150 ml DMSO at 60 °C for 30 min and slowly dropped 350 ml toluene into the PbI2 solution. Then, we filtered the white precipitate and annealed it in a vacuum oven at 60 °C for 5 h to make PbI2(DMSO). We then spincoated the 1 M PbI2(DMSO) in DMF at 3000 rpm for 30 s and consecutively spin-coated 0.5 M FAI: MABr (0.85:0.15 mol:mol) mixture solution in IPA at 5000 rpm for 30 s. The films were dried on a hot plate at 150 °C 20 min. PTAA hole conductor with additives was deposited on the MAPbI3/TiO2/ITO substrates by spin-coating a mixture of PTAA/ toluene (15 mg/1 ml) with 7.5 μl Li-TFSI/ACN (170 mg/1 ml) and 7.5 μl t-BP/ACN (1 ml/1 ml) additives at 3000 rpm for 30 s. Finally, an Au counter electrode was deposited by thermal evaporation. For flexible perovskite solar cells, we only changed the rigid glass substrate to flexible PET/ITO substrate and fabricated devices by following the same procedure. The active area was fixed at 0.16 cm2. All devices fabricated under relative humidity below 25%. The current densityvoltage 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 1.5 G), which is calibrated by Si-reference cell certificated from JIS (Japanase Industrial Standards). J-V curves of devices were measured by masking metal mask with aperture of 0.096 cm2. External quantum efficiency (EQE) was measured by a power source (ABET, 150 W Xenon lamp, 13014) with a monochromator (DONGWOO OPTRON photovoltage were measured by potentiostat (IVIUM, IviumStat).
2. Experimental section 2.1. Materials All chemicals were used as received without further purification. Titanium tetraisopropoxide (TTIP, 97%), lead iodide (PbI2), N,N-dimethylformamide (DMF), dimethylsulfoxied (DMSO), acetonitrile (ACN), toluene, Li-bis(trifluoromethanesulfonyl) imide (Li-TFSI), tertbutylpyridine (t-BP), and formamidinium iodide (FAI), and methylammonium bromide (MABr) were purchased from Sigma-aldrich. Ethanol (EtOH), iso-propanol (IPA), toluene and nitric acid (HNO3) were purchased from Samchun. Poly-triarylamine (PTAA) was purchased from EM index Co., Ltd.
3. Results and discussion To produce TiO2 nano-sol with below 10 nm in size by solution chemistry, sol-gel chemistry is generally adapted by using titanium alkoxide such as TTIP or TiCl4 aqueous solution. The TiCl4 makes stable TiOCl2 intermediate phase and it produces anatase or rutile crystalline TiO2 by chemical reaction. However, the small anatase TiO2 nanoparticles produced in pH controlled aqueous solution have trouble in redisperse of TiO2 nanoparticles in certain solution after fully drying of it to correctly control the weight fraction of TiO2 nano-sol possibly due to the aggregation of TiO2 primary nanoparticles by aging or the TiO2 nanoparticles are gradually precipitated by aging even storing the produced TiO2 nano-sol in ambient condition. Accordingly, we used the TTIP as TiO2 precursor to synthesize TiO2 nano-sol for spin-coating. To synthesize spin-coatable TiO2 nano-sol, we first poured 84 g of TTIP in the 525 g of distilled water charged in 1 L of water jacket reactor at room temperature under stirring. Then we elevated the reaction temperature to 80 °C under stirring and added 14 g of HNO3 for peptization reaction. After proceeding the reaction for 15 h, we obtained TiO2 nano-sol. Upon dropping TTIP precursor into water, white precipitate, of which chemical structure is Ti(OH)m(OH2)n (m + n = 6), is immediately formed by rapid hydrolysis reaction. By adding HNO3, the white precipitate is slowly peptizated by sol-gel reaction and subsequently results in milky transparent solution. Fig. 1(a) is a transmission electron microscopy (TEM) image of assynthesized TiO2 nano-sol indicating the formation of ~ 5 nm-sized crystalline TiO2 nanoparticles. The low magnified TEM image was shown in Fig. S1 for better recognition of the shape of the synthesized TiO2 nanoparticles. An X-ray diffraction (XRD) pattern of as-synthesized TiO2 nano-sol in Fig. 1(b) confirms that the crystalline phase of synthesized TiO2 nanoparticles is anatase phase. From the Scherrer equation (d = 0.9λ/Bcosθ, where d, λ, B, and θ stands for the size of TiO2 nanoparticle, the wavelength of X-ray source (0.154 nm), the line width at half maximum (rad), and the angle of XRD) [22,23], we obtain a calculated size of TiO2 nanocrystal ~ 5 nm by using the (101) peak of XRD positioning at 2θ = 25.3°. This is well matched with the result of TEM analysis.
2.2. Preparation of TiO2 nano-sol We first synthesized TiO2 nano-sol by sol-gel method. 84 g of TTIP was poured dropwise into 525 g of distilled water with stirring at 80 °C and the white precipitates were formed immediately. After these were dispersed in solution, 14 g of HNO3 was added in solution and proceeded the reaction for 15 h. The TiO2 nano-sol was then fully dried at 60 °C for 18 h in a convection oven. The TiO2 nano-powder was then redispersed in H2O, EtOH, DMSO, and DMF to prepare TiO2 nano-sol with specific wt% of concentration. 2.3. Characterization Size of TiO2 nanoparticles were characterized by transmission electron microscope (TEM, JEM-2100F, JEOL). Their crystal structures were characterized X-ray diffraction (XRD, D8 advance, Bruker) machine using Cu Kα radiation at 40 kV and 40 mA and scanning at 6°/ min. The topographic image and height profile of TiO2 films were obtained in a non-contact mode of AFM (Park system, model XE-100). The UV–Vis transmission spectra were obtained using a UV-2450PC spectrophotometer (SHIMADZU, Japan). 2.4. Device fabrication and characterization To check I-V characteristics of TiO2 ETL prepared by 5, 10, and 20 wt% nano-sol in DMF, each solution was spin-coated on an oxygen plasma treated ITO/glass substrate at 3000 rpm for 30 s and heattreated on a hot plate at 100 °C for 30 min. The heat-treatment ensures the following orthogonal processability of perovskite solution in DMF because the remained hydroxyl groups on the surface of TiO2 nanoparticles can be aged by condensation reaction so the TiO2 nanoparticles are tightly bonded by each other. For fabrication of rigid FAPbI3-xBrx perovskite solar cells, we deposited a TiO2 ETL by spincoating 10 wt% of TiO2 nano-sol in DMF on an oxygen plasma treated 2
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Fig. 1. (a) Transmission electron microscopy (TEM) image and (b) X-ray diffraction (XRD) pattern of as-synthesized TiO2 nano-sol. Blue solid line is JCPDF#211272. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
and TiO2/DMSO (TiO2-DMSO) films had slightly lower transmission than the bare glass. The TiO2-DMF thin-film sample only shows interference pattern, so it is expected that it has even-surface. To check the surface topology of each TiO2/glass thin-film, we compared the atomic force microscopy (AMF) topology images as shown in Fig. 2(d). Similar to the photograph and UV–visible transmission spectra, the root-meansquare (rms) roughness (Rq) of TiO2-H2O, TiO2-DMSO, and TiO2-DMF sample was 38.98, 7.24, and 2.92 nm, respectively. Therefore, we found that the DMF solvent is the best to form uniform TiO2 nano-sol based ETL in our experimental condition. Fig. 3(a) is current-voltage (I-V) graphs of glass/ITO/TiO2 thin-film deposited by spin-coating with 5 (DMF 5%), 10 (DMF 10%), and 20 wt % (DMF 20%) TiO2 nano-sol/DMF solutions/Au samples. The DMF 5% sample exhibited ohmic behavior because the 5 wt% TiO2 nano-sol could not fully cover ITO surface. On the other hand, the I-V graphs of DMF 10% and 20% samples had semi-conductor behavior. From the log-log plots of I-V, we could recognize that the 10 wt% TiO2 nano-sol based thin-film makes ohmic contact at TiO2/ITO interface, but the 20 wt% sample does not. The DMF 10% sample exhibited a conductivity of ~ 7 × 10-6 mS/cm, which is calculated from the equation: direct current conductivity (σ0) = (dI)/(AV), where d, I, A, and V stands for the thickness of TiO2 film, current, measured area (0.16 cm2), and voltage [16]. A representative scanning electron microscopy (SEM) cross-sectional image of the DMF 10% sample was shown in Fig. 3(b). This clearly confirms that the thin TiO2 film with ~ 50 nm in thickness is conformally coated on the ~ 150 nm-thick ITO transparent conducting oxide. The SEM surface image of the DMF 10% sample in Fig. S2 confirms that its surface is even and smooth similar to the AFM image in Fig. 2(f). To check if the TiO2 thin-film prepared by spin-coating 10 wt% TiO2 nano-sol/DMF solution can be used as ETL for perovskite solar cells, we fabricated rigid and flexible perovskite solar cells composed of glass (or PET)/ITO/TiO2/FAPbI3-xBrx/PTAA/Au. Fig. 4(a) is a schematic energy band diagram of the perovskite solar cells. Upon illumination of light, the FAPbI3-xBrx perovskite generates electron-hole pairs. The electrons (holes) are then promptly transported to TiO2 ETL (PTAA HTL). Fig. 4(b) is a SEM cross-sectional image of glass/ITO/TiO2/FAPbI3-xBrx/ PTAA/Au indicating that the thickness of ITO, TiO2, FAPbI3-xBrx, PTAA, and Au is ~ 150, ~ 50, ~ 500, ~ 50, and ~ 70 nm, respectively. Photovoltaic properties of perovskite solar cells were summarized in Table 2. A current density-voltage (J-V) curves and corresponding external quantum efficiency (EQE) spectrum of rigid perovskite solar cell were shown in Fig. 4(c) and (d). The rigid cell exhibited 22.6 mA/cm2 of short-circuit current density (Jsc), 1.03 V of open-circuit voltage (Voc), 75.4% of fill factor (FF), and 17.6% of power conversion
To check conformal coatability of TiO2 nano-sols with respect to kind of solvent medium via spin-coating process, we re-dispersed the fully dried TiO2 nano-sol in H2O, ethanol (EtOH), DMF, and DMSO solvent medium as shown in Fig. 2(a). Apparently, the dried TiO2 nanoparticles are well re-dispersed in H2O, DMF, and DMSO solvent, but they are not in EtOH. Hereafter, the EtOH solvent was disregarded as candidate for spin-coatable TiO2 nano-sol. The physical properties of used solvent medium were listed in Table 1. The dielectric constant of H2O, EtOH, DMF, and DMSO solvent is 80.1, 24.5, 36.7, and 46.7, respectively. Accordingly, the polarity of solvent is in the order to EtOH < DMF < DMSO < H2O. The TiO2 nanoparticles are synthesized in acidic condition so their surface should have positive charges. Therefore, it is expected that the re-dispersity of TiO2 nanoparticles in the solvent is in the order to EtOH < DMF < DMSO < H2O. To check the re-dispersity of TiO2 nanoparticles in the solvent, we measured zeta potential and mean size of re-dispersed TiO2 nanoparticles in each solvent as summarized in Table 1. The Zeta potential of TiO2 nano-sol is 24.2, 3.6, and 2.4 mV for H2O, DMF, and DMSO solvent, respectively. The zeta potential of all TiO2 nano-sols had positive value due to the synthesis in acid condition. The highest zeta potential of TiO2 nano-sol in H2O is attributed to that the H2O is polar protic solvent, whereas the DMF and DMSO is polar aprotic solvent. Although the DMSO has higher dielectric constant than the DMF, it had lower Zeta potential than the DMF. This might be associated with the fact that the zeta potential is related to the electric double layer. Namely, the DMSO with higher dipole moment screens the positive surface of TiO2 nanoparticles more effectively than the DMF and results in lower zeta potential. Therefore, the mean size of secondary TiO2 nanoparticles was 19.5, 22.4, and 30.3 nm for H2O, DMF, and DMSO solvent, respectively because the zeta potential value of TiO2 nanoparticles was in the order to DMSO < DMF ≪ H2O solvent. To find proper solvent of spin-coatable TiO2 nano-sol for TiO2 blocking ETL, we spin-coated the 10 wt% of TiO2/H2O, TiO2/DMF, and TiO2/DMSO nano-sol on a glass substrate at 3000 rpm for 30 s and subsequently heat-treated on hot plate pre-set to 100 °C for 30 min. Fig. 2(b) is the deposited TiO2 films indicating that the TiO2/DMF, and TiO2/DMSO nano-sols form transparent thin-film, but the TiO2/H2O nano-sol forms hazy thin-film. This might be attributed to the much higher surface tension of H2O (71.99 mN/m) than that of DMF (37.10 mN/m) and DMSO (43.54 mN/m). Accordingly, the TiO2/H2O nano-sol is more easily de-wetted than the others during spin-coating process. Specular transmission spectra of bare glass and corresponding TiO2/glass films were shown in Fig. 2(c). The TiO2/glass film formed by TiO2/H2O nano-sol (TiO2-H2O) exhibited low transmission due to scattering and the TiO2/glass films formed by TiO2/DMF (TiO2-DMF) 3
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Fig. 2. (a) Photograph of 10 wt% TiO2 nano-sols re-dispersed in H2O, EtOH, DMF, and DMSO solvent, (b) photograph of spin-coated TiO2 thin-film on a glass substrate by using the 10 wt% TiO2/H2O, TiO2/DMF, and TiO2/DMSO solution, (c) UV–visible transmission spectra of corresponding TiO2/glass films and (d-f) atomic force microscopy (AFM) topology of the spin-coated TiO2 thin-film with (d) TiO2/H2O, (e) TiO2/DMSO, and (f) TiO2/DMF solution.
efficiency (PCE) for forward scan condition and 22.9 mA/cm2 Jsc, 1.04 V Voc, 76.6% FF, and 18.2% PCE for reverse scan condition. The calculated Jsc from the integration of EQE spectrum in Fig. 4(d) was 22.7 mA/cm2, which is well matched with the Jsc of perovskite device. The box plots of mean and standard deviation of 30 rigid perovskite devices for Jsc, Voc, FF, and PCE were shown in Fig. 4(e). The average Jsc, Voc, FF, and PCE for 30 samples were 22.11 ± 0.49 mA/cm2, 0.98 ± 0.04 V, 74.6 ± 1.47%, and 16.17 ± 1.12%, respectively. The J-V curve of rigid perovskite solar cell prepared with spin-coating 5 wt % TiO2 nano-sol/DMF solution for ETL was shown in Fig. S3. Due to the imperfect coverage of TiO2 nanoparticles on the surface of ITO, it had poor device performance. To compare the robustness and effectiveness of the spin-coated TiO2 film with 10 wt% TiO2 nano-sol/DMF solution as ETL for perovskite solar cells, we compared transient
Table 1 Physical properties of each solvent and surface properties of TiO2 nan-sols redispersed in H2O, DMF, and DMSO solvent. Solvent
Dielectric constant
Dipole moment (D)
B.P (°C)
Vapor pressure (KPa)
Surface tension (mN/m)
Viscosity (cP at 20 °C)
H2O EtOH DMF DMSO
80.1 24.5 36.7 46.7
1.85 1.69 3.82 3.96
100 78 152 189
2.30 5.95 0.52 0.06
71.99 21.55 37.10 43.54
1.00 1.20 0.92 2.00
Sample TiO2 in H2O TiO2 in DMF TiO2 in DMSO
Zeta potential (mV) 24.2 3.6 2.4
Mean size by Zeta sizer (nm) 19.5 22.4 30.3
4
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Fig. 3. (a) Current-voltage (I-V) graphs of glass/ITO/spin-coated TiO2 thin-films with 5 (DMF 5%), 10 (DMF 10%), and 20 wt% (DMF 20%) TiO2 nano-sol/DMF solutions/Au samples and (b) scanning electron microscopy (SEM) cross-sectional image of spin-coated TiO2 thin-film with 10 wt% TiO2 nano-sol/DMF solution.
Fig. 4. (a) Schematic energy band diagram of FAPbI3-xBrx perovskite solar cell, (b) SEM cross-sectional image of rigid device, and (c-i) photovoltaic properties of perovskite solar cells: (c-e) rigid glass based perovskite solar cells ((c) current density-voltage (J-V) curves, (d) external quantum efficiency (EQE) spectrum and calculated Jsc, and (e) box plots for distribution of Jsc, Voc, FF, and PCE for 30 rigid samples) and (f-i) flexible PET based perovskite solar cells ((f) J-V curves: inset = a photograph of bent flexible perovskite solar cell, (g) corresponding EQE spectrum and transmission spectrum of PET/ITO substrate, (h) normalized PCE degradation with bending radius of curvature, and (i) normalized PCE degradation with repeated bending cycles). 5
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TiO2/perovskite/PTAA/Au had 20.4 mA/cm2 Jsc, 1.01 V Voc, 76.7% FF, and 15.8% PCE at 1 sun condition. Furthermore, the flexible perovskite solar cells exhibited good mechanical stability against repeated bending, so it maintained ~95% of its initial PCE after 1000 repeated bending cycles under a bending radius of curvature of 12 mm.
Table 2 Photovoltaic properties of rigid and flexible perovskite solar cells prepared by TiO2 nano-sol based ETL. Device
Scan direction
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
Rigid
Forward Reverse Forward Reverse
22.6 22.9 20.2 20.4
1.03 1.04 1.00 1.01
75.4 76.6 75.3 76.7
17.6 18.2 15.2 15.8
Flexible
Acknowledgments This study was supported by the National Research Foundation of Korea (NRF) under the Ministry of Science, ICT & Future Planning (Basic Science Research Program (No. 2014R1A5A1009799), the Technology Development Program to Solve Climate Change (No. 2015M1A2A2055631), and Nano-Material Technology Development Program (No. 2017M3A7B4041696)) and the Ministry of Trade, Industry and Energy, Republic of Korea (New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20183010013820).
photoluminescent (TRPL) decay spectra of spin-coated TiO2 nano-sol/ perovskite film and the blocking TiO2 (bl-TiO2) deposited by spray pyrolysis deposition/perovskite sample as shown in Fig. S4. The quick TRPL quenching of both samples indicate that the electrons in the perovskite are quickly transferred into the spin-coated TiO2 and the spray pyrolysis deposited bl-TiO2 ETL. Accordingly, the spin-coated TiO2 ETL can be considered as an efficient ETL for perovskite solar cells. We also fabricated a flexible perovskite solar cell to confirm that the TiO2 nano-sol can be used as ETL by low temperature solution process. Fig. 4(f) is J-V curves of flexible perovskite solar cell exhibiting 20.2 mA/cm2 Jsc, 1.00 V Voc, 75.3% FF, and 15.2% PCE for forward scan condition and 20.4 mA/cm2 Jsc, 1.01 V Voc, 76.7% FF, and 15.8% PCE for reverse scan condition. A typical photograph of bent flexible perovskite solar cell was shown in inset in Fig. 4(f). The corresponding EQE spectrum and transmission spectrum of PET/ITO substrate were shown in Fig. 4(g). The PET/ITO substrate has ~ 10% lower transmittance than the glass/ITO substrate, so the flexible perovskite solar cell has lower EQE values than the rigid device because the EQE is product of light harvesting efficiency, charge transfer efficiency, and charge collection efficiency. Accordingly, the Jsc of flexible device had ~ 10% lower value than the rigid device. To investigate mechanical bending stability of the flexible perovskite solar cell, we checked degradation of PCE with bending radius of curvature (Fig. 4(h)) and repeated bending cycles (Fig. 4(i)). The flexible perovskite solar cells maintained ~ 95%, ~ 82%, and ~ 57% of its initial PCE after 1000 repeated bending cycles with a bending radius of curvature of 12, 8, and 4 mm, respectively.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2019.02.003. References [1] M. Pagliaro, R. Ciriminna, G. Palmisano, ChemSusChem 1 (2008) 880–891. [2] Y. Li, D.-K. Lee, J.Y. Kim, B.S. Kim, N.-G. Park, K. Kim, J.-H. Shin, I.-S. Choi, M.J. Ko, Energy Environ. Sci. 5 (2012) 8950–8957. [3] M. Kaltenbrunner, M.S. White, E.D. Głowacki, T. Sekitani, T. Someya, N.S. Sariciftci, S. Bauer, Nat. Commun. 3 (2012) 770. [4] D.H. Shin, J.H. Heo, S.H. Im, J. Korean Phys. Soc. 71 (2017) 593–607. [5] J.H. Heo, D.S. Lee, D.H. Shin, S.H. Im, J. Mater. Chem. A 7 (2019) 888–900. [6] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050–6051. [7] J.H. Heo, S.H. Im, J.H. Noh, T.N. Mandal, C.S. Lim, J.A. Chang, Y.H. Lee, H.J. Kim, A. Sarkar, M.K. Nazeeruddin, M. Grätzel, S.I. Seok, Nat. Photonics 7 (2013) 486–491. [8] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Grätzel, S. Mhaisalkar, T.C. Sum, Science 342 (2013) 344–347. [9] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J.P. Alcocer, T. Leijtens, L.M. Herz, A. Petrozza, H.J. Snaith, Science 342 (2013) 341–344. [10] J.H. Heo, H.J. Han, D. Kim, T.K. Ahn, S.H. Im, Energy Environ. Sci. 8 (2015) 1602–1608. [11] J.H. Heo, D.H. Song, H.J. Han, S.Y. Kim, J.H. Kim, D. Kim, H.W. Shin, T.K. Ahn, C. Wolf, T.-W. Lee, S.H. Im, Adv. Mater. 27 (2015) 3424–3430. [12] W.S. Yang, B.-W. Park, E.H. Jung, N.J. Jeon, Y.C. Kim, D.U. Lee, S.S. Shin, J. Seo, E.K. Kim, J.H. Noh, S.I. Seok, Science 356 (2017) 1376–1379. [13] J.H. Heo, M.H. Lee, M.H. Jang, S.H. Im, J. Mater. Chem. A 4 (2016) 17636–17642. [14] J.H. Heo, S.H. Im, Nanoscale 8 (2016) 2554–2560. [15] N.J. Jeon, H. Na, E.H. Jung, T. –Y. Yang, Y.G. Lee, G. Kim, H. –W. Shin, S.I. Seok, J. Lee, J. Seo, Nat. Energy 3 (2018) 682–689. [16] J.H. Heo, M.H. Lee, H.J. Han, B.R. Patil, S.H. Im, J. Mater. Chem. A 4 (2016) 1572–1578. [17] S.S. Shin, W.S. Yang, J.H. Noh, J.H. Suk, N.J. Jeon, J.H. Park, J.S. Kim, W.M. Seong, S.I. Seok, Nat. Commun. 6 (2015) 7410. [18] F. Di Giacomo, V. Zardetto, A. D'Epifanio, S. Pescetelli, F. Matteocci, S. Razza, A. Di Carlo, S. Licoccia, W.M.M. Kessels, M. Creatore, T.M. Brown, Adv. Energy Mater. 5 (2015) 1401808. [19] L. Qiu, J. Deng, X. Lu, Z. Yang, H. Peng, Angew. Chem. Int. Ed. 53 (2014) 10425–10428. [20] S.S. Mali, C.K. Hong, A.I. Inamdar, H.S. Im, S.E. Shim, Nanoscale 9 (2017) 3095–3104. [21] Y. Zhao, Y.-C. Zhao, W.-K. Zhou, R. Fu, Q. Li, D.-P. Yu, Q. Zhao, Chin. Phys. B 27 (2018) 018401. [22] B.D. Cullity, S.R. Stock, Pearson (2001). [23] M. Won Suh, S.J. Lee, M.S. You, S.B. Park, S.H. Im, RSC Adv. 5 (2015) 56954–56958.
4. Conclusions In summary, we successfully prepared spin-coatable TiO2 nano-sol for low-temperature solution processable ETL of flexible perovskite solar cells. The anatase phased TiO2 nanoparticles could be synthesized by peptization in acidic aqueous solution and the TiO2 nano-sols were prepared by re-dispersing the fully dried TiO2 nano-powders in H2O, EtOH, DMSO, and DMF. Due to the low dielectric constant of EtOH, the negatively charged TiO2 nanoparticles was aggregated in EtOH, whereas they are well re-dispersed in the others. The H2O has the highest dielectric constant and zeta potential among the solvents, so it has the best re-dispersity of TiO2 nano-powder, but it makes hazy TiO2 thin-film after spin-coating due to its high surface tension. The DMF solvent has higher zeta potential and lower surface tension than the DMSO solvent, so it formed better quality of TiO2 thin-film on ITO/glass substrate than the DMSO. By using 10 wt% of TiO2 nano-sol in DMF, we could deposit uniform ~ 50 nm-thick TiO2 ETL on ITO/glass by lowtemperature spin-coating process. The rigid perovskite solar cells composed of glass/ITO/TiO2/perovskite/PTAA/Au exhibited 22.9 mA/ cm2 Jsc, 1.04 V Voc, 76.6% FF, and 18.2% PCE at 1 sun condition (AM1.5 G 100 mW/cm2) and the flexible device composed of PET/ITO/
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Low temperature solution processable TiO2 nano-sol for electron transporting layer of flexible perovskite solar cells
T ⁎
Myung Sang Youa, Jin Hyuck Heob, Jin Kyoung Parka,b, Sang Hwa Moona,b, Bum Jun Parka, , ⁎ Sang Hyuk Imb, a
Functional Crystallization Center (ERC), Department of Chemical Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446701, Republic of Korea b Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-713, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: TiO2 nano-sol Low temperature Solution process Electron transporting layer Perovskite solar cells
Low-temperature solution processable TiO2 nano-sol for electron transporting layer (ETL) of flexible perovskite solar cells is prepared by synthesizing TiO2 nano-sols by peptization in acidic aqueous solution and re-dispersing the TiO2 nanoparticles in H2O, ethanol (EtOH), dimethylsulfoxied (DMSO), and N,N-dimethylformamide (DMF). By considering dielectric constant, zeta potential, and surface tension of solvent media, we found that the DMF is the best to form uniform TiO2 ETL by spin-coating among them because it has low surface tension and high zeta potential. Accordingly, we could demonstrate 18.2% power conversion efficiency (PCE) for rigid glass substrate based perovskite solar cells and 15.8% PCE for flexible polymer substrate based devices by low-temperature solution process.
1. Introduction Solar light energy has been considered as a promising alternative energy source for substitution of conventional fossil fuel because it is abundant, clean, sustainable and renewable. Solar cells can generate direct current by converting the solar light into electricity, so many researchers have intensively studied them for several decades to develop cost-effective next-generation solar cells, which will have comparable power generation cost with fossil fuels. In parallel, to date needs for potable and independent power generators are gradually growing as mobile communication technologies are advanced. To deal with the electrical energy demands for mobile devices, it is necessary to develop highly efficient, lightweight, and flexible solar cells. So far, various flexible solar cells such as thin film, dye-sensitized, organic, and perovskite solar cells have been intensively studied [1–5]. Among them, metal halide perovskite solar cells (PeSCs)[6–15] have attracted great attention as a promising candidate for portable power generation devices because they have high efficiency, simple device structure, low-cost solution processability, and good flexibility. However, high performance PeSCs have been demonstrated by bi-layer type device architecture, which requires mesoscopic TiO2 electron transporting layer (ETL) on a rigid F-doped tin oxide (FTO)/glass substrate because the mesoscopic TiO2 is formed by high temperature sintering
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process of ~ 450 °C. The TiO2 ETL has important role in PeSCs because electrons generated in the perovskite are injected into the TiO2 ETL. High temperature process cannot be applicable to flexible PeSCs because the flexible polymer substrates such as poly-ethyleneterephthalate (PET) and poly-ethylenenaphthalate (PEN) are unstable. Accordingly, entire process of the flexible polymer substrate-based perovskite solar cells must be conducted at low temperature (below 150 °C). This means that the TiO2 ETL should be formed at low temperature because the other layers such as perovskite, hole transporting layer (HTL), and counter electrode can be formed at low temperature. Hence, it is still challenging to develop solution processable TiO2 ETL at low temperature. Recently, Heo et al. synthesized a ZnO nano-sol and fabricated a flexible PeSC composed of PEN/ZnO ETL/perovskite/poly-triarylamine (PTA) HTL/Au [16]. They obtained high efficiency and reduced current-voltage hysteresis with respect to scan direction and scan rate, but they could not guarantee long-term stability of perovskite solar cells because the ZnO can be reacted with perovskite due to its inherent poor chemical stability. Accordingly, to improve chemical stability of ZnO nanoparticles, Shin et al. synthesized Zn2SnO4 nano-sol by solution chemistry and succeeded to fabricated flexible PeSCs [17]. Giacomo et al. formed TiO2 ETL by atomic layer deposition (ALD) at low temperature (150 °C) and obtained 8.4% power conversion efficiency (PCE)
Corresponding authors. E-mail addresses: [email protected] (B.J. Park), [email protected] (S.H. Im).
https://doi.org/10.1016/j.solmat.2019.02.003 Received 27 October 2018; Received in revised form 20 January 2019; Accepted 2 February 2019 0927-0248/ © 2019 Published by Elsevier B.V.
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for a flexible PeSC [18]. Qiu et al. formed pinhole-free TiO2 ETL by electron beam deposition and demonstrated 13.5% PCE [19]. Recently, Mali et al. deposited TiO2 ETL by RF sputtering and obtained 15.88% PCE [20]. Very recently, Zhao et al. used TiO2 nanocrystals passivated by titanium diisopropoxide bis(acetylacetonate) as ETL and demonstrated 18.3% PCE [21]. Besides the low temperature processability of TiO2 ETL, the surface of TiO2 ETL should have hydrophilicity to deposit perovskite material on it because higher surface energy of TiO2 ETL is more desirable to wet perovskite solution. If not, the perovskite solution is not wetted on the TiO2 ETL and consequently the perovskite layer is not formed on it by spin-coating process. Hence, here we synthesized TiO2 nanoparticles in aqueous phase for hydrophilic TiO2 ETL and systematically studied to find proper solvent medium for the TiO2 nano-sol in order to deposit uniform TiO2 ETL on indium doped SnO2 (ITO) substrate. By using this spin-coatable TiO2 nano-sol, we fabricated rigid ITO/glass and flexible ITO/PET substrate-based perovskite solar cells.
ITO/glass substrate at 3000 rom for 30 s and dried it on a hot plate at 100 °C for 30 min. FAPbI3-xBrx perovskite film was then deposited on the TiO2/ITO/glass substrate. To make FAPbI3-xBrx perovskite films, we synthesized a PbI2(DMSO)2 complex by dissolving 50 g PbI2 in 150 ml DMSO at 60 °C for 30 min and slowly dropped 350 ml toluene into the PbI2 solution. Then, we filtered the white precipitate and annealed it in a vacuum oven at 60 °C for 5 h to make PbI2(DMSO). We then spincoated the 1 M PbI2(DMSO) in DMF at 3000 rpm for 30 s and consecutively spin-coated 0.5 M FAI: MABr (0.85:0.15 mol:mol) mixture solution in IPA at 5000 rpm for 30 s. The films were dried on a hot plate at 150 °C 20 min. PTAA hole conductor with additives was deposited on the MAPbI3/TiO2/ITO substrates by spin-coating a mixture of PTAA/ toluene (15 mg/1 ml) with 7.5 μl Li-TFSI/ACN (170 mg/1 ml) and 7.5 μl t-BP/ACN (1 ml/1 ml) additives at 3000 rpm for 30 s. Finally, an Au counter electrode was deposited by thermal evaporation. For flexible perovskite solar cells, we only changed the rigid glass substrate to flexible PET/ITO substrate and fabricated devices by following the same procedure. The active area was fixed at 0.16 cm2. All devices fabricated under relative humidity below 25%. The current densityvoltage 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 1.5 G), which is calibrated by Si-reference cell certificated from JIS (Japanase Industrial Standards). J-V curves of devices were measured by masking metal mask with aperture of 0.096 cm2. External quantum efficiency (EQE) was measured by a power source (ABET, 150 W Xenon lamp, 13014) with a monochromator (DONGWOO OPTRON photovoltage were measured by potentiostat (IVIUM, IviumStat).
2. Experimental section 2.1. Materials All chemicals were used as received without further purification. Titanium tetraisopropoxide (TTIP, 97%), lead iodide (PbI2), N,N-dimethylformamide (DMF), dimethylsulfoxied (DMSO), acetonitrile (ACN), toluene, Li-bis(trifluoromethanesulfonyl) imide (Li-TFSI), tertbutylpyridine (t-BP), and formamidinium iodide (FAI), and methylammonium bromide (MABr) were purchased from Sigma-aldrich. Ethanol (EtOH), iso-propanol (IPA), toluene and nitric acid (HNO3) were purchased from Samchun. Poly-triarylamine (PTAA) was purchased from EM index Co., Ltd.
3. Results and discussion To produce TiO2 nano-sol with below 10 nm in size by solution chemistry, sol-gel chemistry is generally adapted by using titanium alkoxide such as TTIP or TiCl4 aqueous solution. The TiCl4 makes stable TiOCl2 intermediate phase and it produces anatase or rutile crystalline TiO2 by chemical reaction. However, the small anatase TiO2 nanoparticles produced in pH controlled aqueous solution have trouble in redisperse of TiO2 nanoparticles in certain solution after fully drying of it to correctly control the weight fraction of TiO2 nano-sol possibly due to the aggregation of TiO2 primary nanoparticles by aging or the TiO2 nanoparticles are gradually precipitated by aging even storing the produced TiO2 nano-sol in ambient condition. Accordingly, we used the TTIP as TiO2 precursor to synthesize TiO2 nano-sol for spin-coating. To synthesize spin-coatable TiO2 nano-sol, we first poured 84 g of TTIP in the 525 g of distilled water charged in 1 L of water jacket reactor at room temperature under stirring. Then we elevated the reaction temperature to 80 °C under stirring and added 14 g of HNO3 for peptization reaction. After proceeding the reaction for 15 h, we obtained TiO2 nano-sol. Upon dropping TTIP precursor into water, white precipitate, of which chemical structure is Ti(OH)m(OH2)n (m + n = 6), is immediately formed by rapid hydrolysis reaction. By adding HNO3, the white precipitate is slowly peptizated by sol-gel reaction and subsequently results in milky transparent solution. Fig. 1(a) is a transmission electron microscopy (TEM) image of assynthesized TiO2 nano-sol indicating the formation of ~ 5 nm-sized crystalline TiO2 nanoparticles. The low magnified TEM image was shown in Fig. S1 for better recognition of the shape of the synthesized TiO2 nanoparticles. An X-ray diffraction (XRD) pattern of as-synthesized TiO2 nano-sol in Fig. 1(b) confirms that the crystalline phase of synthesized TiO2 nanoparticles is anatase phase. From the Scherrer equation (d = 0.9λ/Bcosθ, where d, λ, B, and θ stands for the size of TiO2 nanoparticle, the wavelength of X-ray source (0.154 nm), the line width at half maximum (rad), and the angle of XRD) [22,23], we obtain a calculated size of TiO2 nanocrystal ~ 5 nm by using the (101) peak of XRD positioning at 2θ = 25.3°. This is well matched with the result of TEM analysis.
2.2. Preparation of TiO2 nano-sol We first synthesized TiO2 nano-sol by sol-gel method. 84 g of TTIP was poured dropwise into 525 g of distilled water with stirring at 80 °C and the white precipitates were formed immediately. After these were dispersed in solution, 14 g of HNO3 was added in solution and proceeded the reaction for 15 h. The TiO2 nano-sol was then fully dried at 60 °C for 18 h in a convection oven. The TiO2 nano-powder was then redispersed in H2O, EtOH, DMSO, and DMF to prepare TiO2 nano-sol with specific wt% of concentration. 2.3. Characterization Size of TiO2 nanoparticles were characterized by transmission electron microscope (TEM, JEM-2100F, JEOL). Their crystal structures were characterized X-ray diffraction (XRD, D8 advance, Bruker) machine using Cu Kα radiation at 40 kV and 40 mA and scanning at 6°/ min. The topographic image and height profile of TiO2 films were obtained in a non-contact mode of AFM (Park system, model XE-100). The UV–Vis transmission spectra were obtained using a UV-2450PC spectrophotometer (SHIMADZU, Japan). 2.4. Device fabrication and characterization To check I-V characteristics of TiO2 ETL prepared by 5, 10, and 20 wt% nano-sol in DMF, each solution was spin-coated on an oxygen plasma treated ITO/glass substrate at 3000 rpm for 30 s and heattreated on a hot plate at 100 °C for 30 min. The heat-treatment ensures the following orthogonal processability of perovskite solution in DMF because the remained hydroxyl groups on the surface of TiO2 nanoparticles can be aged by condensation reaction so the TiO2 nanoparticles are tightly bonded by each other. For fabrication of rigid FAPbI3-xBrx perovskite solar cells, we deposited a TiO2 ETL by spincoating 10 wt% of TiO2 nano-sol in DMF on an oxygen plasma treated 2
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Fig. 1. (a) Transmission electron microscopy (TEM) image and (b) X-ray diffraction (XRD) pattern of as-synthesized TiO2 nano-sol. Blue solid line is JCPDF#211272. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
and TiO2/DMSO (TiO2-DMSO) films had slightly lower transmission than the bare glass. The TiO2-DMF thin-film sample only shows interference pattern, so it is expected that it has even-surface. To check the surface topology of each TiO2/glass thin-film, we compared the atomic force microscopy (AMF) topology images as shown in Fig. 2(d). Similar to the photograph and UV–visible transmission spectra, the root-meansquare (rms) roughness (Rq) of TiO2-H2O, TiO2-DMSO, and TiO2-DMF sample was 38.98, 7.24, and 2.92 nm, respectively. Therefore, we found that the DMF solvent is the best to form uniform TiO2 nano-sol based ETL in our experimental condition. Fig. 3(a) is current-voltage (I-V) graphs of glass/ITO/TiO2 thin-film deposited by spin-coating with 5 (DMF 5%), 10 (DMF 10%), and 20 wt % (DMF 20%) TiO2 nano-sol/DMF solutions/Au samples. The DMF 5% sample exhibited ohmic behavior because the 5 wt% TiO2 nano-sol could not fully cover ITO surface. On the other hand, the I-V graphs of DMF 10% and 20% samples had semi-conductor behavior. From the log-log plots of I-V, we could recognize that the 10 wt% TiO2 nano-sol based thin-film makes ohmic contact at TiO2/ITO interface, but the 20 wt% sample does not. The DMF 10% sample exhibited a conductivity of ~ 7 × 10-6 mS/cm, which is calculated from the equation: direct current conductivity (σ0) = (dI)/(AV), where d, I, A, and V stands for the thickness of TiO2 film, current, measured area (0.16 cm2), and voltage [16]. A representative scanning electron microscopy (SEM) cross-sectional image of the DMF 10% sample was shown in Fig. 3(b). This clearly confirms that the thin TiO2 film with ~ 50 nm in thickness is conformally coated on the ~ 150 nm-thick ITO transparent conducting oxide. The SEM surface image of the DMF 10% sample in Fig. S2 confirms that its surface is even and smooth similar to the AFM image in Fig. 2(f). To check if the TiO2 thin-film prepared by spin-coating 10 wt% TiO2 nano-sol/DMF solution can be used as ETL for perovskite solar cells, we fabricated rigid and flexible perovskite solar cells composed of glass (or PET)/ITO/TiO2/FAPbI3-xBrx/PTAA/Au. Fig. 4(a) is a schematic energy band diagram of the perovskite solar cells. Upon illumination of light, the FAPbI3-xBrx perovskite generates electron-hole pairs. The electrons (holes) are then promptly transported to TiO2 ETL (PTAA HTL). Fig. 4(b) is a SEM cross-sectional image of glass/ITO/TiO2/FAPbI3-xBrx/ PTAA/Au indicating that the thickness of ITO, TiO2, FAPbI3-xBrx, PTAA, and Au is ~ 150, ~ 50, ~ 500, ~ 50, and ~ 70 nm, respectively. Photovoltaic properties of perovskite solar cells were summarized in Table 2. A current density-voltage (J-V) curves and corresponding external quantum efficiency (EQE) spectrum of rigid perovskite solar cell were shown in Fig. 4(c) and (d). The rigid cell exhibited 22.6 mA/cm2 of short-circuit current density (Jsc), 1.03 V of open-circuit voltage (Voc), 75.4% of fill factor (FF), and 17.6% of power conversion
To check conformal coatability of TiO2 nano-sols with respect to kind of solvent medium via spin-coating process, we re-dispersed the fully dried TiO2 nano-sol in H2O, ethanol (EtOH), DMF, and DMSO solvent medium as shown in Fig. 2(a). Apparently, the dried TiO2 nanoparticles are well re-dispersed in H2O, DMF, and DMSO solvent, but they are not in EtOH. Hereafter, the EtOH solvent was disregarded as candidate for spin-coatable TiO2 nano-sol. The physical properties of used solvent medium were listed in Table 1. The dielectric constant of H2O, EtOH, DMF, and DMSO solvent is 80.1, 24.5, 36.7, and 46.7, respectively. Accordingly, the polarity of solvent is in the order to EtOH < DMF < DMSO < H2O. The TiO2 nanoparticles are synthesized in acidic condition so their surface should have positive charges. Therefore, it is expected that the re-dispersity of TiO2 nanoparticles in the solvent is in the order to EtOH < DMF < DMSO < H2O. To check the re-dispersity of TiO2 nanoparticles in the solvent, we measured zeta potential and mean size of re-dispersed TiO2 nanoparticles in each solvent as summarized in Table 1. The Zeta potential of TiO2 nano-sol is 24.2, 3.6, and 2.4 mV for H2O, DMF, and DMSO solvent, respectively. The zeta potential of all TiO2 nano-sols had positive value due to the synthesis in acid condition. The highest zeta potential of TiO2 nano-sol in H2O is attributed to that the H2O is polar protic solvent, whereas the DMF and DMSO is polar aprotic solvent. Although the DMSO has higher dielectric constant than the DMF, it had lower Zeta potential than the DMF. This might be associated with the fact that the zeta potential is related to the electric double layer. Namely, the DMSO with higher dipole moment screens the positive surface of TiO2 nanoparticles more effectively than the DMF and results in lower zeta potential. Therefore, the mean size of secondary TiO2 nanoparticles was 19.5, 22.4, and 30.3 nm for H2O, DMF, and DMSO solvent, respectively because the zeta potential value of TiO2 nanoparticles was in the order to DMSO < DMF ≪ H2O solvent. To find proper solvent of spin-coatable TiO2 nano-sol for TiO2 blocking ETL, we spin-coated the 10 wt% of TiO2/H2O, TiO2/DMF, and TiO2/DMSO nano-sol on a glass substrate at 3000 rpm for 30 s and subsequently heat-treated on hot plate pre-set to 100 °C for 30 min. Fig. 2(b) is the deposited TiO2 films indicating that the TiO2/DMF, and TiO2/DMSO nano-sols form transparent thin-film, but the TiO2/H2O nano-sol forms hazy thin-film. This might be attributed to the much higher surface tension of H2O (71.99 mN/m) than that of DMF (37.10 mN/m) and DMSO (43.54 mN/m). Accordingly, the TiO2/H2O nano-sol is more easily de-wetted than the others during spin-coating process. Specular transmission spectra of bare glass and corresponding TiO2/glass films were shown in Fig. 2(c). The TiO2/glass film formed by TiO2/H2O nano-sol (TiO2-H2O) exhibited low transmission due to scattering and the TiO2/glass films formed by TiO2/DMF (TiO2-DMF) 3
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Fig. 2. (a) Photograph of 10 wt% TiO2 nano-sols re-dispersed in H2O, EtOH, DMF, and DMSO solvent, (b) photograph of spin-coated TiO2 thin-film on a glass substrate by using the 10 wt% TiO2/H2O, TiO2/DMF, and TiO2/DMSO solution, (c) UV–visible transmission spectra of corresponding TiO2/glass films and (d-f) atomic force microscopy (AFM) topology of the spin-coated TiO2 thin-film with (d) TiO2/H2O, (e) TiO2/DMSO, and (f) TiO2/DMF solution.
efficiency (PCE) for forward scan condition and 22.9 mA/cm2 Jsc, 1.04 V Voc, 76.6% FF, and 18.2% PCE for reverse scan condition. The calculated Jsc from the integration of EQE spectrum in Fig. 4(d) was 22.7 mA/cm2, which is well matched with the Jsc of perovskite device. The box plots of mean and standard deviation of 30 rigid perovskite devices for Jsc, Voc, FF, and PCE were shown in Fig. 4(e). The average Jsc, Voc, FF, and PCE for 30 samples were 22.11 ± 0.49 mA/cm2, 0.98 ± 0.04 V, 74.6 ± 1.47%, and 16.17 ± 1.12%, respectively. The J-V curve of rigid perovskite solar cell prepared with spin-coating 5 wt % TiO2 nano-sol/DMF solution for ETL was shown in Fig. S3. Due to the imperfect coverage of TiO2 nanoparticles on the surface of ITO, it had poor device performance. To compare the robustness and effectiveness of the spin-coated TiO2 film with 10 wt% TiO2 nano-sol/DMF solution as ETL for perovskite solar cells, we compared transient
Table 1 Physical properties of each solvent and surface properties of TiO2 nan-sols redispersed in H2O, DMF, and DMSO solvent. Solvent
Dielectric constant
Dipole moment (D)
B.P (°C)
Vapor pressure (KPa)
Surface tension (mN/m)
Viscosity (cP at 20 °C)
H2O EtOH DMF DMSO
80.1 24.5 36.7 46.7
1.85 1.69 3.82 3.96
100 78 152 189
2.30 5.95 0.52 0.06
71.99 21.55 37.10 43.54
1.00 1.20 0.92 2.00
Sample TiO2 in H2O TiO2 in DMF TiO2 in DMSO
Zeta potential (mV) 24.2 3.6 2.4
Mean size by Zeta sizer (nm) 19.5 22.4 30.3
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Fig. 3. (a) Current-voltage (I-V) graphs of glass/ITO/spin-coated TiO2 thin-films with 5 (DMF 5%), 10 (DMF 10%), and 20 wt% (DMF 20%) TiO2 nano-sol/DMF solutions/Au samples and (b) scanning electron microscopy (SEM) cross-sectional image of spin-coated TiO2 thin-film with 10 wt% TiO2 nano-sol/DMF solution.
Fig. 4. (a) Schematic energy band diagram of FAPbI3-xBrx perovskite solar cell, (b) SEM cross-sectional image of rigid device, and (c-i) photovoltaic properties of perovskite solar cells: (c-e) rigid glass based perovskite solar cells ((c) current density-voltage (J-V) curves, (d) external quantum efficiency (EQE) spectrum and calculated Jsc, and (e) box plots for distribution of Jsc, Voc, FF, and PCE for 30 rigid samples) and (f-i) flexible PET based perovskite solar cells ((f) J-V curves: inset = a photograph of bent flexible perovskite solar cell, (g) corresponding EQE spectrum and transmission spectrum of PET/ITO substrate, (h) normalized PCE degradation with bending radius of curvature, and (i) normalized PCE degradation with repeated bending cycles). 5
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TiO2/perovskite/PTAA/Au had 20.4 mA/cm2 Jsc, 1.01 V Voc, 76.7% FF, and 15.8% PCE at 1 sun condition. Furthermore, the flexible perovskite solar cells exhibited good mechanical stability against repeated bending, so it maintained ~95% of its initial PCE after 1000 repeated bending cycles under a bending radius of curvature of 12 mm.
Table 2 Photovoltaic properties of rigid and flexible perovskite solar cells prepared by TiO2 nano-sol based ETL. Device
Scan direction
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
Rigid
Forward Reverse Forward Reverse
22.6 22.9 20.2 20.4
1.03 1.04 1.00 1.01
75.4 76.6 75.3 76.7
17.6 18.2 15.2 15.8
Flexible
Acknowledgments This study was supported by the National Research Foundation of Korea (NRF) under the Ministry of Science, ICT & Future Planning (Basic Science Research Program (No. 2014R1A5A1009799), the Technology Development Program to Solve Climate Change (No. 2015M1A2A2055631), and Nano-Material Technology Development Program (No. 2017M3A7B4041696)) and the Ministry of Trade, Industry and Energy, Republic of Korea (New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20183010013820).
photoluminescent (TRPL) decay spectra of spin-coated TiO2 nano-sol/ perovskite film and the blocking TiO2 (bl-TiO2) deposited by spray pyrolysis deposition/perovskite sample as shown in Fig. S4. The quick TRPL quenching of both samples indicate that the electrons in the perovskite are quickly transferred into the spin-coated TiO2 and the spray pyrolysis deposited bl-TiO2 ETL. Accordingly, the spin-coated TiO2 ETL can be considered as an efficient ETL for perovskite solar cells. We also fabricated a flexible perovskite solar cell to confirm that the TiO2 nano-sol can be used as ETL by low temperature solution process. Fig. 4(f) is J-V curves of flexible perovskite solar cell exhibiting 20.2 mA/cm2 Jsc, 1.00 V Voc, 75.3% FF, and 15.2% PCE for forward scan condition and 20.4 mA/cm2 Jsc, 1.01 V Voc, 76.7% FF, and 15.8% PCE for reverse scan condition. A typical photograph of bent flexible perovskite solar cell was shown in inset in Fig. 4(f). The corresponding EQE spectrum and transmission spectrum of PET/ITO substrate were shown in Fig. 4(g). The PET/ITO substrate has ~ 10% lower transmittance than the glass/ITO substrate, so the flexible perovskite solar cell has lower EQE values than the rigid device because the EQE is product of light harvesting efficiency, charge transfer efficiency, and charge collection efficiency. Accordingly, the Jsc of flexible device had ~ 10% lower value than the rigid device. To investigate mechanical bending stability of the flexible perovskite solar cell, we checked degradation of PCE with bending radius of curvature (Fig. 4(h)) and repeated bending cycles (Fig. 4(i)). The flexible perovskite solar cells maintained ~ 95%, ~ 82%, and ~ 57% of its initial PCE after 1000 repeated bending cycles with a bending radius of curvature of 12, 8, and 4 mm, respectively.
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4. Conclusions In summary, we successfully prepared spin-coatable TiO2 nano-sol for low-temperature solution processable ETL of flexible perovskite solar cells. The anatase phased TiO2 nanoparticles could be synthesized by peptization in acidic aqueous solution and the TiO2 nano-sols were prepared by re-dispersing the fully dried TiO2 nano-powders in H2O, EtOH, DMSO, and DMF. Due to the low dielectric constant of EtOH, the negatively charged TiO2 nanoparticles was aggregated in EtOH, whereas they are well re-dispersed in the others. The H2O has the highest dielectric constant and zeta potential among the solvents, so it has the best re-dispersity of TiO2 nano-powder, but it makes hazy TiO2 thin-film after spin-coating due to its high surface tension. The DMF solvent has higher zeta potential and lower surface tension than the DMSO solvent, so it formed better quality of TiO2 thin-film on ITO/glass substrate than the DMSO. By using 10 wt% of TiO2 nano-sol in DMF, we could deposit uniform ~ 50 nm-thick TiO2 ETL on ITO/glass by lowtemperature spin-coating process. The rigid perovskite solar cells composed of glass/ITO/TiO2/perovskite/PTAA/Au exhibited 22.9 mA/ cm2 Jsc, 1.04 V Voc, 76.6% FF, and 18.2% PCE at 1 sun condition (AM1.5 G 100 mW/cm2) and the flexible device composed of PET/ITO/
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