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perovskite solar cells with graphene quantum dots-doped PCBM electron transport layer
Dyes and Pigments 170 (2019) 107630
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Highly-flexible graphene transparent conductive electrode/perovskite solar cells with graphene quantum dots-doped PCBM electron transport layer
T
Dong Hee Shin, Jong Min Kim, Seung Hyun Shin, Suk-Ho Choi∗ Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yongin, 17104, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite solar cell Graphene APTES Graphene quantum dot PCBM Transparent conductive electrode Flexible
In recent years, organic-inorganic hybrid perovskite solar cells (PSCs) have received strong attention due to their high power conversion efficiency (PCE) and low cost. Here, we first employ 3-aminopropyl triethoxysilane (APTES)-treated graphene (GR) as a transparent conductive electrode (TCE) and quantum dots (GQDs)-doped phenyl C61 butyric acid methyl ester (PCBM) as an electron transport layer (ETL) for flexible PSCs. By increasing the concentration of GQDs to 2.5 mg/L, the PCE of the PSCs with GR/APTES TCE increases up to 16.4 and 15.0% on rigid and flexible substrates, respectively due to the reduced charge recombination at the ETL/perovskite interface and improved conductivity of the ETL. The flexible PSCs exhibit excellent bending stability by maintaining ∼80% of the original PCE even after 3000 bending cycles at a curvature radius of 4 mm, resulting from the improved flexibility by the APTES interlayer.
1. Introduction In the last several years, organic-inorganic hybrid perovskite solar cells (PSCs) have received strong attention in photovoltaic sciences and industries due to their excellent properties such as high absorption coefficient in visible range, small exciton binding energy, long charge carrier diffusion length, and solution processability at low temperatures (< 150 °C) [1–5]. For these reasons, PSCs have also been intensively studied for flexible power generation systems [6]. PSCs are classified into n-i-p (normal) or p-i-n (inverted) architectures depending on the direction of the carrier transport [7]. To date, the PCE of PSCs has been dramatically improved through material morphology control, device architecture optimization, and interface engineering. Recently, high efficiency was achieved in n-i-p-type PSCs by using mesoporous (mp)TiO2 layer [8], graphene quantum dots (GQDs)/mp-TiO2 [9], and mpTiO2/ZIF-8-20 [10], or by improving the device fabrication process [11]. However, all of these approaches were not so useful for the application in flexible cells because of the need for high temperature processing [4]. For practical flexible PSCs, it is necessary to use flexible transparent conductive electrodes (TCEs) instead of conventional transparent conductive oxides such as indium tin oxide and fluorinedoped tin oxide and low-temperature solution-processed electron transport layers (ETLs). Among the well-known TCEs, graphene electrodes have already been employed for flexible optoelectronic devices due to their excellent properties such as high mobility, good
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transmittance, and chemical stability [12–14]. However, deterioration or delamination may occur in GR electrodes by repetitive bending due to the weak bonding with the transferred substrate. In previous studies, a GR/3-aminopropyl triethoxysilane (APTES)/polyethylene terephthalate (PET) stack was shown to be very effective for maintaining the device performance even under repeated bending tests [15]. Many groups have focused on the fabrication of planar perovskite devices using low-temperature solution-processed ETL such as TiO2 [16], ZnO [17], Zn1−xMgxO [18], SnO2 [19], and Zn2SnO4 films [20], but most of them exhibit hysteresis and low stability due to the undesirable charge accumulation/recombination at the ETL/perovskite interface. On the other hand, organic transport layers such as phenyl C61 butyric acid methyl ester (PCBM) [21], fullerene [22], and PCBDAN [23] were also very attractive as ETLs in PSCs because their fabrication was based on simple and low temperature processes. Recently, high stability of the PSCs in ultraviolet (UV) region was demonstrated by adding the derivatives to the PCBM ETL [24,25], but the inherently-low electrical conductivity/electron mobility of the PCBM still limited the cell performance [26]. Several studies have also been done on the electrical improvement of the ETLs by adding carbon-based materials to the ETLs [27–31]. Especially, GQDs were effectively added to PCBM to enhance the performance and durability of the PSCs on rigid substrates [31], but there is no report for employing this approach on flexible substrates. Here, we first report GR/APTES TCE-based n-i-p type PSCs on flexible PET substrates by using GQDs-doped PCBM
Corresponding author. E-mail address: [email protected] (S.-H. Choi).
https://doi.org/10.1016/j.dyepig.2019.107630 Received 14 April 2019; Received in revised form 6 June 2019; Accepted 6 June 2019 Available online 08 June 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.
Dyes and Pigments 170 (2019) 107630
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Fig. 1. (a) Schematic diagram describing the chemical interaction between the PET substrate and the GR sheet by the APTES interlayer. (b) XPS spectra showing C 1s and N 1s core levels of the GR sheet with/without APTES treatment. (c) Raman spectra, (d) sheet resistance, (e) work function, (f) ISD-VG characteristics, and (g) transmittance of the GR with/without APTES treatment.
L, and stirred at room temperature for 12 h. The ETL solutions of PCBM and PCBM:GQDs were spin-coated at 1500 rpm for 30 s on the substrates, and annealed at 100 °C for 10 min. CH3NH3PbI3 (MAPbI3) solution was prepared by dispersing 1:1-ratio CH3NH3I and PbI2 powder in N,N-dimethylformamide:dimethylsulfoxide mixed solvent (9:1, volume ratio) at 60 °C for 1 h. The MAPbI3 solution was then spin-coated on the ETL layer by consecutive spin-coatings at 1000 and 5000 rpm for 10 and 20 s, respectively. During the second spin-coating step, toluene was quickly dropped onto the rotating substrate and subsequently dried at 100 °C for 5 min. As a next step, the hole transport layer was prepared on the surface of the MAPbI3 film by spin coating with polytriarylamine (PTAA) in toluene solution with Li-bis(trifluoromethanesulfonyl) imide (Li-TFSI)/acetonitrile and TBP at 3000 rpm for 30 s, and then dried in the air. Au counter electrode was then deposited on top of the PTAA layer by thermal evaporation to finalize the solar cell structure.
(PCBM:GQDs) as an ETL. We systematically characterize the PCBM:GQDs layers electrically and optically by varying the concentration of GQDs (nG). We also evaluate the bending stability of the flexible PSCs after 3000 bending cycles at a curvature radius of 4 mm. 2. Experimental section 2.1. Fabrication of GR/APTES TCEs Prior to coating with the APTES solution, the PET substrate was cleaned by Ar/O2 plasma treatment. 1-wt% APTES solution was prepared by dissolving APTES in methanol. The APTES solution was dropped on the entire PET substrate and then spin-coated at 3000 rpm for 60 s. The APTES/PET substrate was washed with methanol to remove the residual solvent. Then, the GR sheets grown by chemical vapor deposition were transferred onto the APTES/PET substrates using well-known poly (methyl methacrylate) supporting films.
2.3. Characterizations 2.2. Fabrication of perovskite solar cells The size and distribution of GQDs were analyzed using a high-resolution transmission electron microscope (TEM, JEOL JEM-2100F) with electronic energy loss spectroscopy (EELS) mapping capabilities. The topographic images of PCBM:GQDs were obtained in a noncontact
GQDs powder was dispersed in a mixed solvent (deionized water:isopropanol = 1:10) Subsequently, the GQDs solution was added to PCBM by adjusting nG of the resulting PCBM:GQDs to 1, 2.5, and 5 mg/ 2
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Fig. 2. (a) Low-/(b) high-magnification TEM images and (c) Carbon K-edge EELS spectrum of GQDs.
other hand, a FTIR peak corresponding to C–N–H bonding was detected only in graphite/APTES sample, indicating successful chemical bonding of single-layer GR and APTES. Fig. 1c shows Raman spectra of GR and GR/APTES. In the GR/ APTES, sp3 bond-related D peaks are observed, indicating more-active electron transfer in the doped GR, consistent with previous results [36]. The Rs of the GR and GR/APTES are 700 ± 33 and 486 ± 23 Ω/sq, respectively, as shown in Fig. 1d. It should be noted that the Rs of the GR is considerably reduced by the APTES treatment. The work function of the GR and GR/APTES are −4.60 ± 0.04 and −4.46 ± 0.02 eV, respectively, as shown in Fig. 1e, indicating n-type characteristics of the GR/APTES, consistent with previous reports [36]. To further confirm the doping effect by the APTES, the ISD-VG (source-drain current vs gate bias) characteristics were measured in a structure of field effect transistor (FET) containing pristine GR or GR/APTES on SiO2/Si substrates, as shown in Fig. 1f. The Dirac point of the GR FET appears at a positive Dirac voltage (VDirac = +20 V) whilst that of the GR/APTES FET is shifted towards the negative voltage direction, further indicating n-type characteristics of the GR/APTES, consistent with the work function behaviors. Fig. 1g shows similar transmission spectra of GR and GR/ APTES in the wavelength range of 300 nm–900 nm. Fig. 2a–b shows low- and high-magnification TEM images of the GQDs. The diameter of GQDs is in the range of 3.8–7.8 nm and has an average peak population at ∼5 nm. EELS was employed to further analyze the GQDs on the TEM images. The K-edge EELS spectrum is peaked at ∼285 and ∼291 eV, as shown in Fig. 2c, corresponding to the π* and σ* states, respectively, originating from the GQDs [37]. To improve the properties of the ETL layer of the PSCs, the GQDs were doped into PCBM by varying nG. N-i-p-type PSCs were then fabricated in a structure of APTES/GR PCBM:GQDs/MAPbI3/PTAA/Au, as shown in Fig. 3a. The root-mean-square (RMS) roughnesses of the PCBM and PCBM:GQDs films coated on the GR/APTES surface were as low as 0.85 nm and 0.89 nm, respectively (Supporting Information, Fig. S2), indicating good uniformity of both films. Fig. 3b shows a crosssectional FE-SEM image of a typical planar-type MAPbI3 PSC consisting of ∼50-nm Au, ∼50-nm PTAA, ∼330-nm MAPbI3, and ∼40 nm PCBM:GQDs layer on GR/APTES/glass substrate. Photovoltaic characteristics extracted from the nG-dependent J-V curves of the GR/APTES PSCs (Supporting Information, Fig. S3) are summarized in Table 1. All the photovoltaic parameters are maximized at nG = 2.5 mg/L. As nG increases from 0 to 2.5 mg/L, the PCE is enhanced by about 20/32% from 12.79/11.92 to 16.41/16.15% for forward/reverse scans, respectively. Statistical deviations of the nG-dependent photovoltaic parameters including maximum PCE of 14.97 ± 2.07% at nG = 2.5 mg/L (Supporting Information, Fig. S4) were also obtained from 16 PSCs. Fig. 3c–d shows dark and photo J-V curves of the GR and GR/APTES TCEs-based PSCs at nG = 2.5 mg/L, respectively. The comparison of their photovoltaic parameters is also summarized in Table 1. Especially, the PCE is enhanced from 12.82/12.03 to 16.41/16.15% for forward/ reverse scans, respectively due to the decrease of the energy barrier at the GR/PCBM:GQDs interface and the increase in the conductivity of
mode of atomic force microscope (AFM, Park System XE-100). Crosssectional structures of the PSCs were observed by field-emission scanning electron microscopy (FE-SEM) (Carl Zeiss, model LEO SUPRA 55). The atomic bonding states of the GR/APTES were studied by Fouriertransform infrared spectroscopy (FTIR, Thermo electron corp Nicolet 5700) and X-ray photoelectron spectroscopy (XPS) using Al ka line of 1486.6 eV. The absorbance/transmittance, sheet resistance (Rs), and work function of the samples were measured by UV–visible–near-infrared optical spectrometer (Agilent Varian, model cary 5000), 4 probe van der Pauw method (Dasol eng, model FPP-HS8-40K), and Kelvin probe force microscopy (Park systems, model XE 100), respectively. Here, the calibration of the work function was done by using Au as a reference. The optical properties of the samples were analyzed by steady-state photoluminescence (PL), time-resolved PL (TRPL), and Raman spectroscopy using 325 nm, 470 nm, and 532 nm laser line as the excitation sources, respectively, and the Raman peaks were corrected by the reference Si peaks. Electrochemical impedance spectra (EIS) of the devices were measured using Zive Lab (Wonatech). Current density-voltage (J-V) characteristics were measured at a scan rate of 200 ms/10 mV by a Keithley 2400 source meter. Photovoltaic parameters of the solar cells were measured by a solar simulator (McScinece K201) under illumination of 1 sun (100 mW-cm−2, AM 1.5 G). The active area of the cells was defined as 0.16 cm2, and the J-V curves were measured at a 0.01 cm2 active area. External quantum efficiency (EQE) was measured under monochromatic light generated by a Xenon arclamp (Oriel Apex Illuminator, Newport) in combination with a monochromator (Cornerstone 260, Newport). Repeated bending stabilities of flexible solar cells were analyzed at a 0.5 Hz bending frequency. 3. Results and discussion Fig. 1a shows a schematic diagram describing the strong covalent bonds of GR and PET substrate by the presence of APTES between them. It is well known that silanol groups are covalently bonded to hydroxyl groups upon insertion between GR and PET [15]. To confirm the formation of the chemical bonds between APTES and single-layer GR sheet, the XPS spectra were analyzed, as shown in Fig. 1b. Here, a SiO2/ Si substrate was used instead of a PET because the C1s peaks of a GR sheet is overlapped with those of a PET substrate. The C1s spectra of GR and GR/APTES were resolved into four and five components, respectively. Unlike GR, a peak related to C–N was additionally observed for GR/APTES. Furthermore, the C–N–H bond was found only in the APTES-treated GR. These results suggest that the GR and PET are chemically well bonded by the APTES treatment. To check further the chemical bonding between GR and APTES, we employed Fourier transform infrared (FTIR) spectroscopy. However, it was impossible to obtain the data for the single-layer GR because it is too thin. Therefore, we used graphite, a homogeneous material, for the FTIR analysis (Supporting Information, Fig. S1). In the FTIR spectra of separate graphite and APTES, bending vibrations related to their intrinsic properties were detected, consistent with the previous reports [32–35]. On the 3
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Fig. 3. (a) Schematic device architecture of a typical planar n-i-p-type PCS. (b) Cross-sectional FE-SEM image of the PCS in a structure of glass/APTES-GR/ PCBM:GQDs/MAPbI3/PTAA/Au. The scale bar indicates 200 nm. (c)–(d) J-V curves of the best cells without/with APTES at nG = 2.5 mg/L, respectively under forward bias and reverse scans. (e) EQE spectra and corresponding integrated Jsc curves of the GR and GR/APTES PSCs.
behaviors can be explained by measuring the Hall mobility as a function of nG (Supporting Information, Fig. S5 & Table S1). The reduction of the mobility at nG > 2.5 mg/L is considered to be due to the increase of the defects in the PCBM layer, caused by the excessive amount of GQDs, resulting in less efficient transport of the carriers. Consequently, the carrier recombination is enhanced at the perovskite/ETL (PCBM:GQDs) interface, thereby increasing the PL at nG = 5 mg/L. The PL behaviors are also well matched with the photovoltaic ones, as shown in Table 1. These results suggest that the extraction/transport of the photo-induced carriers to the GR electrode is more efficient than their recombination by the use of PCBM:GQDs ETL layer. To electrically characterize the effect of the GQDs in the PCBM layer, a sandwich structure of GR/APTES/PCBM:GQDs/Au was fabricated, as shown in the inset of Fig. 4c. Fig. 4c shows I–V characteristics of the sandwich device, which was used for calculating the conductivity (σ0) based on the formula: I = σ0Ad−1V, where d (40 nm) and A (0.16 cm2) are the thickness and area of the sample, respectively [28]. Resulting σ0 are 0.33, 0.58, 0.79, and 0.71 μS-cm−1 for nG = 0, 1, 2.5, and 5 mg/L, respectively (Supporting Information, Table S1), also well consistent with the PL and photovoltaic results. The difference of the PCE between the forward and reverse scans can be understood by comparing the conductivity and mobility between the ETL and HTL. For this, we measured the conductivity and Hall mobility of an Au/PTAA/ GR-APTES structure in the same manner as for the PCBM layer (Supporting Information, Fig. S6). The thickness and area of the PTAA are 50 nm and 0.16 cm2. The resulting conductivity and Hall mobility are 0.33 μS-cm−1 and 2.2 × 10−3 cm2/V-s, smaller than those of the ETL (PCBM:GQDs) (Supporting Information, Table S1). Therefore, the PCE is larger in the forward scan than in the reverse scan. Dark J-V curves can be used for evaluating the loss of the charge carriers through the recombination and leakage paths. Fig. 4d shows nG-dependent dark J-V curves, from which the leakage and recombination currents are extracted in the low and medium voltage ranges, respectively. Based on the diode equation, the ideality factor (n) is calculated to be 4.05, 3.57, 3.21, and 3.49 at nG = 0, 1, 2.5, and 5 mg/L, respectively, indicating best diode quality at nG = 2.5 mg/L. These results further suggest that the inclusion of the GQDs into the PCBM can effectively reduce the charge recombination loss for nG ≤ 2.5 mg/L. However, it is believed that the impurities are formed
Table 1 Photovoltaic parameters of perovskite solar cells with GR and APTES-GR for nG = 0–5 mg/L, measured in the forward and reverse scans. Electrode
PCBM:GQDs (mg/L)
Scan Direction
VOC (V)
JSC (mA/ cm2)
FF (%)
PCE (%)
GR
2.5
GR/APTES
0
Forward Reverse Forward Reverse Forward Reverse Forward Reverse Forward Reverse
0.99 0.98 1.03 1.02 1.05 1.03 1.07 1.06 1.07 1.06
18.58 18.16 18.97 18.44 19.71 19.50 20.75 20.69 20.71 20.69
69.68 67.56 65.44 63.32 71.45 70.65 73.89 73.62 73.35 72.85
12.82 12.03 12.79 11.92 14.78 14.19 16.41 16.15 16.25 15.98
1 2.5 5
the TCE by the APTES treatment, as proved by the work function and sheet resistance measurements. Fig. 3e shows EQE spectra of the GR and GR/APTES PSCs. In particular, the GR/APTES PSC exhibits a maximum EQE exceeding 80% in the 400–550 nm range. The EQE is enhanced over a broad band ranging from 300 to 900 nm by the APTES treatment, consistent with the Jsc behaviour, as shown in Table 1, resulting from the strong correlation of Jsc with integrated EQE [38]. This correlation is also maintained at other nG (Supporting Information, Fig, S2d). The Jsc obtained by integrating the EQE spectra is 17.56 and 19.52 mA cm−2 for the GR and GR/APTES cells, respectively, only about 6% different from the measured Jsc values, as shown in Table 1. To understand why the best PCE was obtained at nG = 0.25 mg/L, we measured PL and TRPL of the perovskite films on different substrates. Fig. 4a shows PL spectra of the perovskite films on glass, PCBM, and PCBM:GQDs. Compared to the film on glass, the PL is quenched on PCBM:GQDs by about 72, 80, 92, and 90% for nG = 0, 1, 2.5, and 5 mg/ L, respectively. Larger quenching of the PL at higher nG indicates more efficient transport of the charge carriers from the perovskite layer to the ETL. The decay time of the charge carriers was measured by TRPL for the same 4 samples. The PL decay curve is well expressed by an exponential function, as shown in Fig. 4b. The nG-dependent behaviors of the PL lifetime are consistent with those of the PL intensity. These PL 4
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Fig. 4. (a) PL and (b) TRPL of the MAPbI3 contacted on different materials: glass, PCBM, PCBM:GQDs for nG = 0, 1, 2.5, and 5 mg/L (c) I–V characteristics of the APTES-GR/PCBM:GQDs/Au sandwich structures for various nG. (d) Dark J-V curves of the PSCs for various nG.
Fig. 5. Flexible PSCs employing GR/APTES TCE and PCBM:GQDs ETL. (a) Dark and photo J-V curves under forward and reverse scans for nG = 2.5 mg/L. (b) Statistical photovoltaic parameters. (c) EQE and integrated Jsc. The inset shows good flexibility of the PSC. (d) Normalized PCE degradation under repeated bending during 1000 bending cycles at R = 4 mm.
in the PCBM layer at nG > 2.5 mg/L due to the excessive GQDs, which could explain why all the electrical and optical properties of the cells are degraded above nG = 2.5 mg/L. To further study the carrier interfacial recombination of the PSCs, we performed EIS measurements of the PSCs for various nG under dark condition. The EIS results were fitted using an equivalent circuit model (Supporting Information, Fig. S7). The charge transfer resistance (RCT) associated with the arc was calculated by EIS modeling. Resulting RCT values are 951, 670, 500, and
Table 2 Photovoltaic parameters of flexible perovskite solar cells for nG = 2.5 mg/L. PCBM:GQDs
Scan Direction
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
0.25 mg/L
Forward Reverse
1.06 1.05
19.66 19.45
72.11 71.48
15.03 14.60
5
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562 Ω at nG = 0, 1, 2.5, and 5 mg/L, respectively. The smallest RCT value also support the best electron transport performance at nG = 2.5 mg/L. Based on the above results, we fabricated flexible PSCs containing both GR/APTES and PCBM:GQDs at nG = 2.5 mg/L. Fig. 5a shows J-V characteristics of the flexible PSCs, and their photovoltaic parameters (Voc, Jsc, FF, and PCE) are summarized in Table 2. The flexible PSC exhibits 1.06/1.05 V Voc, 19.66/19.45 mA/cm2 Jsc, 72.11/71.48% FF, and 15.03/14.60% PCE for the forward/reverse scans, respectively. Fig. 5b shows statistical photovoltaic parameters averaged for the 16 cells. Fig. 5c shows the EQE spectrum and integrated Jsc value (18.51 mA/cm2) of the flexible PSC. To check the practical availability of a flexible PSC, the stability of the operation was evaluated under repeated bending at a curvature radius (R) of 4 mm, as shown in Fig. 5d. The PCE of the PSC was maintained at ∼80% of its original value even after 3000 bending tests. These results clearly confirm that the inclusion of the APTES interlayer between the PET substrate and the GR TCE can significantly improve the mechanical stability.
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4. Conclusion We proposed an effective method to improve the performance of planar n-i-p-type PSCs by using GR/APTES TCE and PCBM:GQDs ETL at low process temperatures (< 100 °C). We observed largest PL quenching and its fastest decay in the perovskite films coated on the PCBM:GQDs ETL layer with the highest σ0 (0.79 μS-cm-1) and the lowest n (3.21) at nG = 2.5 mg/L, indicating significant reduction of the carrier recombination at the ETL/perovskite interface and subsequent efficient charge extraction to the electrodes, resulting in maximum/ average PCEs of 16.41%/14.97 ± 2.07%. The flexible PSCs exhibited maximum/average PCE of 15.03%/13.52 ± 3.03% at nG = 2.5 mg/L and excellent bending stability maintaining ∼80% of their original PCE values even after 3000 bending tests at R = 4 mm. Acknowledgments This work was supported by the National Research Foundation of Korea grant funded by the Ministry of Science, ICT and Future Planning (NRF-2017R1A2B3006054). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.dyepig.2019.107630. References [1] Kojima A, Teshima K, Shirai Y, Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J Am Chem Soc 2009;131:6050–1. [2] Im J-H, Lee C-R, Lee J-W, Park S-W, Park N-G. 6.5% efficient perovskite quantumdot-sensitized solar cell. Nanoscale 2011;3:4088–93. [3] Lee MM, Teuscher J, Miyasaka T, Murakami TN, Snaith HJ. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 2012;338:643–7. [4] Yang WS, Noh JH, Jeon NJ, Kim YC, Ryu S, Seo J, et al. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015;348:1234–7. [5] Green MA, Ho-Baillie A, Snaith HJ. The emergence of perovskite solar cells. Nat Photon 2014;8:506–14. [6] Li Y, Meng L, Yang YM, Xu G, Hong Z, Chen Q, et al. High-efficiency robust perovskite solar cells on ultrathin flexible substrates. Nat Commun 2016;7:10124. [7] Yang S, Fu W, Zhang Z, Chen H, Li C-Z. Recent advances in perovskite solar cells: efficiency, stability and lead-free perovskite. J Mater Chem A 2017;5:11462–82. [8] Yang WS, Park B-W, Jung EH, Jeon NJ, Kim YC, Lee DU, et al. Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells. Science 2017;356:1376–9. [9] Shen D, Zhang W, Xie F, Li Y, Abate A, Wei M. Graphene quantum dots decorated TiO2 mesoporous film as an efficient electron transport layer for high-performance perovskite solar cells. J Power Sources 2018;402:320–6.
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Highly-flexible graphene transparent conductive electrode/perovskite solar cells with graphene quantum dots-doped PCBM electron transport layer
T
Dong Hee Shin, Jong Min Kim, Seung Hyun Shin, Suk-Ho Choi∗ Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yongin, 17104, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite solar cell Graphene APTES Graphene quantum dot PCBM Transparent conductive electrode Flexible
In recent years, organic-inorganic hybrid perovskite solar cells (PSCs) have received strong attention due to their high power conversion efficiency (PCE) and low cost. Here, we first employ 3-aminopropyl triethoxysilane (APTES)-treated graphene (GR) as a transparent conductive electrode (TCE) and quantum dots (GQDs)-doped phenyl C61 butyric acid methyl ester (PCBM) as an electron transport layer (ETL) for flexible PSCs. By increasing the concentration of GQDs to 2.5 mg/L, the PCE of the PSCs with GR/APTES TCE increases up to 16.4 and 15.0% on rigid and flexible substrates, respectively due to the reduced charge recombination at the ETL/perovskite interface and improved conductivity of the ETL. The flexible PSCs exhibit excellent bending stability by maintaining ∼80% of the original PCE even after 3000 bending cycles at a curvature radius of 4 mm, resulting from the improved flexibility by the APTES interlayer.
1. Introduction In the last several years, organic-inorganic hybrid perovskite solar cells (PSCs) have received strong attention in photovoltaic sciences and industries due to their excellent properties such as high absorption coefficient in visible range, small exciton binding energy, long charge carrier diffusion length, and solution processability at low temperatures (< 150 °C) [1–5]. For these reasons, PSCs have also been intensively studied for flexible power generation systems [6]. PSCs are classified into n-i-p (normal) or p-i-n (inverted) architectures depending on the direction of the carrier transport [7]. To date, the PCE of PSCs has been dramatically improved through material morphology control, device architecture optimization, and interface engineering. Recently, high efficiency was achieved in n-i-p-type PSCs by using mesoporous (mp)TiO2 layer [8], graphene quantum dots (GQDs)/mp-TiO2 [9], and mpTiO2/ZIF-8-20 [10], or by improving the device fabrication process [11]. However, all of these approaches were not so useful for the application in flexible cells because of the need for high temperature processing [4]. For practical flexible PSCs, it is necessary to use flexible transparent conductive electrodes (TCEs) instead of conventional transparent conductive oxides such as indium tin oxide and fluorinedoped tin oxide and low-temperature solution-processed electron transport layers (ETLs). Among the well-known TCEs, graphene electrodes have already been employed for flexible optoelectronic devices due to their excellent properties such as high mobility, good
∗
transmittance, and chemical stability [12–14]. However, deterioration or delamination may occur in GR electrodes by repetitive bending due to the weak bonding with the transferred substrate. In previous studies, a GR/3-aminopropyl triethoxysilane (APTES)/polyethylene terephthalate (PET) stack was shown to be very effective for maintaining the device performance even under repeated bending tests [15]. Many groups have focused on the fabrication of planar perovskite devices using low-temperature solution-processed ETL such as TiO2 [16], ZnO [17], Zn1−xMgxO [18], SnO2 [19], and Zn2SnO4 films [20], but most of them exhibit hysteresis and low stability due to the undesirable charge accumulation/recombination at the ETL/perovskite interface. On the other hand, organic transport layers such as phenyl C61 butyric acid methyl ester (PCBM) [21], fullerene [22], and PCBDAN [23] were also very attractive as ETLs in PSCs because their fabrication was based on simple and low temperature processes. Recently, high stability of the PSCs in ultraviolet (UV) region was demonstrated by adding the derivatives to the PCBM ETL [24,25], but the inherently-low electrical conductivity/electron mobility of the PCBM still limited the cell performance [26]. Several studies have also been done on the electrical improvement of the ETLs by adding carbon-based materials to the ETLs [27–31]. Especially, GQDs were effectively added to PCBM to enhance the performance and durability of the PSCs on rigid substrates [31], but there is no report for employing this approach on flexible substrates. Here, we first report GR/APTES TCE-based n-i-p type PSCs on flexible PET substrates by using GQDs-doped PCBM
Corresponding author. E-mail address: [email protected] (S.-H. Choi).
https://doi.org/10.1016/j.dyepig.2019.107630 Received 14 April 2019; Received in revised form 6 June 2019; Accepted 6 June 2019 Available online 08 June 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. (a) Schematic diagram describing the chemical interaction between the PET substrate and the GR sheet by the APTES interlayer. (b) XPS spectra showing C 1s and N 1s core levels of the GR sheet with/without APTES treatment. (c) Raman spectra, (d) sheet resistance, (e) work function, (f) ISD-VG characteristics, and (g) transmittance of the GR with/without APTES treatment.
L, and stirred at room temperature for 12 h. The ETL solutions of PCBM and PCBM:GQDs were spin-coated at 1500 rpm for 30 s on the substrates, and annealed at 100 °C for 10 min. CH3NH3PbI3 (MAPbI3) solution was prepared by dispersing 1:1-ratio CH3NH3I and PbI2 powder in N,N-dimethylformamide:dimethylsulfoxide mixed solvent (9:1, volume ratio) at 60 °C for 1 h. The MAPbI3 solution was then spin-coated on the ETL layer by consecutive spin-coatings at 1000 and 5000 rpm for 10 and 20 s, respectively. During the second spin-coating step, toluene was quickly dropped onto the rotating substrate and subsequently dried at 100 °C for 5 min. As a next step, the hole transport layer was prepared on the surface of the MAPbI3 film by spin coating with polytriarylamine (PTAA) in toluene solution with Li-bis(trifluoromethanesulfonyl) imide (Li-TFSI)/acetonitrile and TBP at 3000 rpm for 30 s, and then dried in the air. Au counter electrode was then deposited on top of the PTAA layer by thermal evaporation to finalize the solar cell structure.
(PCBM:GQDs) as an ETL. We systematically characterize the PCBM:GQDs layers electrically and optically by varying the concentration of GQDs (nG). We also evaluate the bending stability of the flexible PSCs after 3000 bending cycles at a curvature radius of 4 mm. 2. Experimental section 2.1. Fabrication of GR/APTES TCEs Prior to coating with the APTES solution, the PET substrate was cleaned by Ar/O2 plasma treatment. 1-wt% APTES solution was prepared by dissolving APTES in methanol. The APTES solution was dropped on the entire PET substrate and then spin-coated at 3000 rpm for 60 s. The APTES/PET substrate was washed with methanol to remove the residual solvent. Then, the GR sheets grown by chemical vapor deposition were transferred onto the APTES/PET substrates using well-known poly (methyl methacrylate) supporting films.
2.3. Characterizations 2.2. Fabrication of perovskite solar cells The size and distribution of GQDs were analyzed using a high-resolution transmission electron microscope (TEM, JEOL JEM-2100F) with electronic energy loss spectroscopy (EELS) mapping capabilities. The topographic images of PCBM:GQDs were obtained in a noncontact
GQDs powder was dispersed in a mixed solvent (deionized water:isopropanol = 1:10) Subsequently, the GQDs solution was added to PCBM by adjusting nG of the resulting PCBM:GQDs to 1, 2.5, and 5 mg/ 2
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Fig. 2. (a) Low-/(b) high-magnification TEM images and (c) Carbon K-edge EELS spectrum of GQDs.
other hand, a FTIR peak corresponding to C–N–H bonding was detected only in graphite/APTES sample, indicating successful chemical bonding of single-layer GR and APTES. Fig. 1c shows Raman spectra of GR and GR/APTES. In the GR/ APTES, sp3 bond-related D peaks are observed, indicating more-active electron transfer in the doped GR, consistent with previous results [36]. The Rs of the GR and GR/APTES are 700 ± 33 and 486 ± 23 Ω/sq, respectively, as shown in Fig. 1d. It should be noted that the Rs of the GR is considerably reduced by the APTES treatment. The work function of the GR and GR/APTES are −4.60 ± 0.04 and −4.46 ± 0.02 eV, respectively, as shown in Fig. 1e, indicating n-type characteristics of the GR/APTES, consistent with previous reports [36]. To further confirm the doping effect by the APTES, the ISD-VG (source-drain current vs gate bias) characteristics were measured in a structure of field effect transistor (FET) containing pristine GR or GR/APTES on SiO2/Si substrates, as shown in Fig. 1f. The Dirac point of the GR FET appears at a positive Dirac voltage (VDirac = +20 V) whilst that of the GR/APTES FET is shifted towards the negative voltage direction, further indicating n-type characteristics of the GR/APTES, consistent with the work function behaviors. Fig. 1g shows similar transmission spectra of GR and GR/ APTES in the wavelength range of 300 nm–900 nm. Fig. 2a–b shows low- and high-magnification TEM images of the GQDs. The diameter of GQDs is in the range of 3.8–7.8 nm and has an average peak population at ∼5 nm. EELS was employed to further analyze the GQDs on the TEM images. The K-edge EELS spectrum is peaked at ∼285 and ∼291 eV, as shown in Fig. 2c, corresponding to the π* and σ* states, respectively, originating from the GQDs [37]. To improve the properties of the ETL layer of the PSCs, the GQDs were doped into PCBM by varying nG. N-i-p-type PSCs were then fabricated in a structure of APTES/GR PCBM:GQDs/MAPbI3/PTAA/Au, as shown in Fig. 3a. The root-mean-square (RMS) roughnesses of the PCBM and PCBM:GQDs films coated on the GR/APTES surface were as low as 0.85 nm and 0.89 nm, respectively (Supporting Information, Fig. S2), indicating good uniformity of both films. Fig. 3b shows a crosssectional FE-SEM image of a typical planar-type MAPbI3 PSC consisting of ∼50-nm Au, ∼50-nm PTAA, ∼330-nm MAPbI3, and ∼40 nm PCBM:GQDs layer on GR/APTES/glass substrate. Photovoltaic characteristics extracted from the nG-dependent J-V curves of the GR/APTES PSCs (Supporting Information, Fig. S3) are summarized in Table 1. All the photovoltaic parameters are maximized at nG = 2.5 mg/L. As nG increases from 0 to 2.5 mg/L, the PCE is enhanced by about 20/32% from 12.79/11.92 to 16.41/16.15% for forward/reverse scans, respectively. Statistical deviations of the nG-dependent photovoltaic parameters including maximum PCE of 14.97 ± 2.07% at nG = 2.5 mg/L (Supporting Information, Fig. S4) were also obtained from 16 PSCs. Fig. 3c–d shows dark and photo J-V curves of the GR and GR/APTES TCEs-based PSCs at nG = 2.5 mg/L, respectively. The comparison of their photovoltaic parameters is also summarized in Table 1. Especially, the PCE is enhanced from 12.82/12.03 to 16.41/16.15% for forward/ reverse scans, respectively due to the decrease of the energy barrier at the GR/PCBM:GQDs interface and the increase in the conductivity of
mode of atomic force microscope (AFM, Park System XE-100). Crosssectional structures of the PSCs were observed by field-emission scanning electron microscopy (FE-SEM) (Carl Zeiss, model LEO SUPRA 55). The atomic bonding states of the GR/APTES were studied by Fouriertransform infrared spectroscopy (FTIR, Thermo electron corp Nicolet 5700) and X-ray photoelectron spectroscopy (XPS) using Al ka line of 1486.6 eV. The absorbance/transmittance, sheet resistance (Rs), and work function of the samples were measured by UV–visible–near-infrared optical spectrometer (Agilent Varian, model cary 5000), 4 probe van der Pauw method (Dasol eng, model FPP-HS8-40K), and Kelvin probe force microscopy (Park systems, model XE 100), respectively. Here, the calibration of the work function was done by using Au as a reference. The optical properties of the samples were analyzed by steady-state photoluminescence (PL), time-resolved PL (TRPL), and Raman spectroscopy using 325 nm, 470 nm, and 532 nm laser line as the excitation sources, respectively, and the Raman peaks were corrected by the reference Si peaks. Electrochemical impedance spectra (EIS) of the devices were measured using Zive Lab (Wonatech). Current density-voltage (J-V) characteristics were measured at a scan rate of 200 ms/10 mV by a Keithley 2400 source meter. Photovoltaic parameters of the solar cells were measured by a solar simulator (McScinece K201) under illumination of 1 sun (100 mW-cm−2, AM 1.5 G). The active area of the cells was defined as 0.16 cm2, and the J-V curves were measured at a 0.01 cm2 active area. External quantum efficiency (EQE) was measured under monochromatic light generated by a Xenon arclamp (Oriel Apex Illuminator, Newport) in combination with a monochromator (Cornerstone 260, Newport). Repeated bending stabilities of flexible solar cells were analyzed at a 0.5 Hz bending frequency. 3. Results and discussion Fig. 1a shows a schematic diagram describing the strong covalent bonds of GR and PET substrate by the presence of APTES between them. It is well known that silanol groups are covalently bonded to hydroxyl groups upon insertion between GR and PET [15]. To confirm the formation of the chemical bonds between APTES and single-layer GR sheet, the XPS spectra were analyzed, as shown in Fig. 1b. Here, a SiO2/ Si substrate was used instead of a PET because the C1s peaks of a GR sheet is overlapped with those of a PET substrate. The C1s spectra of GR and GR/APTES were resolved into four and five components, respectively. Unlike GR, a peak related to C–N was additionally observed for GR/APTES. Furthermore, the C–N–H bond was found only in the APTES-treated GR. These results suggest that the GR and PET are chemically well bonded by the APTES treatment. To check further the chemical bonding between GR and APTES, we employed Fourier transform infrared (FTIR) spectroscopy. However, it was impossible to obtain the data for the single-layer GR because it is too thin. Therefore, we used graphite, a homogeneous material, for the FTIR analysis (Supporting Information, Fig. S1). In the FTIR spectra of separate graphite and APTES, bending vibrations related to their intrinsic properties were detected, consistent with the previous reports [32–35]. On the 3
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Fig. 3. (a) Schematic device architecture of a typical planar n-i-p-type PCS. (b) Cross-sectional FE-SEM image of the PCS in a structure of glass/APTES-GR/ PCBM:GQDs/MAPbI3/PTAA/Au. The scale bar indicates 200 nm. (c)–(d) J-V curves of the best cells without/with APTES at nG = 2.5 mg/L, respectively under forward bias and reverse scans. (e) EQE spectra and corresponding integrated Jsc curves of the GR and GR/APTES PSCs.
behaviors can be explained by measuring the Hall mobility as a function of nG (Supporting Information, Fig. S5 & Table S1). The reduction of the mobility at nG > 2.5 mg/L is considered to be due to the increase of the defects in the PCBM layer, caused by the excessive amount of GQDs, resulting in less efficient transport of the carriers. Consequently, the carrier recombination is enhanced at the perovskite/ETL (PCBM:GQDs) interface, thereby increasing the PL at nG = 5 mg/L. The PL behaviors are also well matched with the photovoltaic ones, as shown in Table 1. These results suggest that the extraction/transport of the photo-induced carriers to the GR electrode is more efficient than their recombination by the use of PCBM:GQDs ETL layer. To electrically characterize the effect of the GQDs in the PCBM layer, a sandwich structure of GR/APTES/PCBM:GQDs/Au was fabricated, as shown in the inset of Fig. 4c. Fig. 4c shows I–V characteristics of the sandwich device, which was used for calculating the conductivity (σ0) based on the formula: I = σ0Ad−1V, where d (40 nm) and A (0.16 cm2) are the thickness and area of the sample, respectively [28]. Resulting σ0 are 0.33, 0.58, 0.79, and 0.71 μS-cm−1 for nG = 0, 1, 2.5, and 5 mg/L, respectively (Supporting Information, Table S1), also well consistent with the PL and photovoltaic results. The difference of the PCE between the forward and reverse scans can be understood by comparing the conductivity and mobility between the ETL and HTL. For this, we measured the conductivity and Hall mobility of an Au/PTAA/ GR-APTES structure in the same manner as for the PCBM layer (Supporting Information, Fig. S6). The thickness and area of the PTAA are 50 nm and 0.16 cm2. The resulting conductivity and Hall mobility are 0.33 μS-cm−1 and 2.2 × 10−3 cm2/V-s, smaller than those of the ETL (PCBM:GQDs) (Supporting Information, Table S1). Therefore, the PCE is larger in the forward scan than in the reverse scan. Dark J-V curves can be used for evaluating the loss of the charge carriers through the recombination and leakage paths. Fig. 4d shows nG-dependent dark J-V curves, from which the leakage and recombination currents are extracted in the low and medium voltage ranges, respectively. Based on the diode equation, the ideality factor (n) is calculated to be 4.05, 3.57, 3.21, and 3.49 at nG = 0, 1, 2.5, and 5 mg/L, respectively, indicating best diode quality at nG = 2.5 mg/L. These results further suggest that the inclusion of the GQDs into the PCBM can effectively reduce the charge recombination loss for nG ≤ 2.5 mg/L. However, it is believed that the impurities are formed
Table 1 Photovoltaic parameters of perovskite solar cells with GR and APTES-GR for nG = 0–5 mg/L, measured in the forward and reverse scans. Electrode
PCBM:GQDs (mg/L)
Scan Direction
VOC (V)
JSC (mA/ cm2)
FF (%)
PCE (%)
GR
2.5
GR/APTES
0
Forward Reverse Forward Reverse Forward Reverse Forward Reverse Forward Reverse
0.99 0.98 1.03 1.02 1.05 1.03 1.07 1.06 1.07 1.06
18.58 18.16 18.97 18.44 19.71 19.50 20.75 20.69 20.71 20.69
69.68 67.56 65.44 63.32 71.45 70.65 73.89 73.62 73.35 72.85
12.82 12.03 12.79 11.92 14.78 14.19 16.41 16.15 16.25 15.98
1 2.5 5
the TCE by the APTES treatment, as proved by the work function and sheet resistance measurements. Fig. 3e shows EQE spectra of the GR and GR/APTES PSCs. In particular, the GR/APTES PSC exhibits a maximum EQE exceeding 80% in the 400–550 nm range. The EQE is enhanced over a broad band ranging from 300 to 900 nm by the APTES treatment, consistent with the Jsc behaviour, as shown in Table 1, resulting from the strong correlation of Jsc with integrated EQE [38]. This correlation is also maintained at other nG (Supporting Information, Fig, S2d). The Jsc obtained by integrating the EQE spectra is 17.56 and 19.52 mA cm−2 for the GR and GR/APTES cells, respectively, only about 6% different from the measured Jsc values, as shown in Table 1. To understand why the best PCE was obtained at nG = 0.25 mg/L, we measured PL and TRPL of the perovskite films on different substrates. Fig. 4a shows PL spectra of the perovskite films on glass, PCBM, and PCBM:GQDs. Compared to the film on glass, the PL is quenched on PCBM:GQDs by about 72, 80, 92, and 90% for nG = 0, 1, 2.5, and 5 mg/ L, respectively. Larger quenching of the PL at higher nG indicates more efficient transport of the charge carriers from the perovskite layer to the ETL. The decay time of the charge carriers was measured by TRPL for the same 4 samples. The PL decay curve is well expressed by an exponential function, as shown in Fig. 4b. The nG-dependent behaviors of the PL lifetime are consistent with those of the PL intensity. These PL 4
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Fig. 4. (a) PL and (b) TRPL of the MAPbI3 contacted on different materials: glass, PCBM, PCBM:GQDs for nG = 0, 1, 2.5, and 5 mg/L (c) I–V characteristics of the APTES-GR/PCBM:GQDs/Au sandwich structures for various nG. (d) Dark J-V curves of the PSCs for various nG.
Fig. 5. Flexible PSCs employing GR/APTES TCE and PCBM:GQDs ETL. (a) Dark and photo J-V curves under forward and reverse scans for nG = 2.5 mg/L. (b) Statistical photovoltaic parameters. (c) EQE and integrated Jsc. The inset shows good flexibility of the PSC. (d) Normalized PCE degradation under repeated bending during 1000 bending cycles at R = 4 mm.
in the PCBM layer at nG > 2.5 mg/L due to the excessive GQDs, which could explain why all the electrical and optical properties of the cells are degraded above nG = 2.5 mg/L. To further study the carrier interfacial recombination of the PSCs, we performed EIS measurements of the PSCs for various nG under dark condition. The EIS results were fitted using an equivalent circuit model (Supporting Information, Fig. S7). The charge transfer resistance (RCT) associated with the arc was calculated by EIS modeling. Resulting RCT values are 951, 670, 500, and
Table 2 Photovoltaic parameters of flexible perovskite solar cells for nG = 2.5 mg/L. PCBM:GQDs
Scan Direction
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
0.25 mg/L
Forward Reverse
1.06 1.05
19.66 19.45
72.11 71.48
15.03 14.60
5
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562 Ω at nG = 0, 1, 2.5, and 5 mg/L, respectively. The smallest RCT value also support the best electron transport performance at nG = 2.5 mg/L. Based on the above results, we fabricated flexible PSCs containing both GR/APTES and PCBM:GQDs at nG = 2.5 mg/L. Fig. 5a shows J-V characteristics of the flexible PSCs, and their photovoltaic parameters (Voc, Jsc, FF, and PCE) are summarized in Table 2. The flexible PSC exhibits 1.06/1.05 V Voc, 19.66/19.45 mA/cm2 Jsc, 72.11/71.48% FF, and 15.03/14.60% PCE for the forward/reverse scans, respectively. Fig. 5b shows statistical photovoltaic parameters averaged for the 16 cells. Fig. 5c shows the EQE spectrum and integrated Jsc value (18.51 mA/cm2) of the flexible PSC. To check the practical availability of a flexible PSC, the stability of the operation was evaluated under repeated bending at a curvature radius (R) of 4 mm, as shown in Fig. 5d. The PCE of the PSC was maintained at ∼80% of its original value even after 3000 bending tests. These results clearly confirm that the inclusion of the APTES interlayer between the PET substrate and the GR TCE can significantly improve the mechanical stability.
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4. Conclusion We proposed an effective method to improve the performance of planar n-i-p-type PSCs by using GR/APTES TCE and PCBM:GQDs ETL at low process temperatures (< 100 °C). We observed largest PL quenching and its fastest decay in the perovskite films coated on the PCBM:GQDs ETL layer with the highest σ0 (0.79 μS-cm-1) and the lowest n (3.21) at nG = 2.5 mg/L, indicating significant reduction of the carrier recombination at the ETL/perovskite interface and subsequent efficient charge extraction to the electrodes, resulting in maximum/ average PCEs of 16.41%/14.97 ± 2.07%. The flexible PSCs exhibited maximum/average PCE of 15.03%/13.52 ± 3.03% at nG = 2.5 mg/L and excellent bending stability maintaining ∼80% of their original PCE values even after 3000 bending tests at R = 4 mm. Acknowledgments This work was supported by the National Research Foundation of Korea grant funded by the Ministry of Science, ICT and Future Planning (NRF-2017R1A2B3006054). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.dyepig.2019.107630. References [1] Kojima A, Teshima K, Shirai Y, Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J Am Chem Soc 2009;131:6050–1. [2] Im J-H, Lee C-R, Lee J-W, Park S-W, Park N-G. 6.5% efficient perovskite quantumdot-sensitized solar cell. Nanoscale 2011;3:4088–93. [3] Lee MM, Teuscher J, Miyasaka T, Murakami TN, Snaith HJ. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 2012;338:643–7. [4] Yang WS, Noh JH, Jeon NJ, Kim YC, Ryu S, Seo J, et al. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015;348:1234–7. [5] Green MA, Ho-Baillie A, Snaith HJ. The emergence of perovskite solar cells. Nat Photon 2014;8:506–14. [6] Li Y, Meng L, Yang YM, Xu G, Hong Z, Chen Q, et al. High-efficiency robust perovskite solar cells on ultrathin flexible substrates. Nat Commun 2016;7:10124. [7] Yang S, Fu W, Zhang Z, Chen H, Li C-Z. Recent advances in perovskite solar cells: efficiency, stability and lead-free perovskite. J Mater Chem A 2017;5:11462–82. [8] Yang WS, Park B-W, Jung EH, Jeon NJ, Kim YC, Lee DU, et al. Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells. Science 2017;356:1376–9. [9] Shen D, Zhang W, Xie F, Li Y, Abate A, Wei M. Graphene quantum dots decorated TiO2 mesoporous film as an efficient electron transport layer for high-performance perovskite solar cells. J Power Sources 2018;402:320–6.
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