Simultaneous Ligand Exchange Fabrication of Flexible Perovskite

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Simultaneous Ligand Exchange Fabrication of Flexible Perovskite Solar Cells Using Newly Synthesized Uniform Tin Oxide Quantum-Dots So Yeon Park, Mi Yeon Baek, Yeon Kyeong Ju, Dong Hoe Kim, Chan Su Moon, Jun Hong Noh, and Hyun Suk Jung J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02408 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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The Journal of Physical Chemistry Letters

Simultaneous Ligand Exchange Fabrication of Flexible Perovskite Solar Cells using Newly Synthesized Uniform Tin Oxide Quantum-Dots So Yeon Park† ,⊥, Mi Yeon Baek† ,⊥, Yeon Kyeong Ju †, Dong Hoe Kim||, Chan Su Moon‡, §, Jun Hong Noh‡, § *, Hyun Suk Jung †* †

School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon

16419, Republic of Korea. ‡

School of Civil, Environmental and Architectural Engineering, Korea University, 145 Anam-ro,

Seongbuk-gu, Seoul 02841, Republic of Korea §

Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT),

141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea ||

Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado

80401, USA AUTHOR INFORMATION Corresponding Author *(H. S. J.) E-mail: [email protected] *(J. H. N.) E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Halide perovskite solar cells (HPSCs) have a significant potential for future photovoltaic systems due to a high power conversion efficiency (PCE) exceeding 23 % using the solution process. A low-temperature processed oxide layer is a challenging issue towards large-scale manufacture of flexible and low-cost HPSCs. Here, we propose a simple reverse micelle-water injection method for a highly dispersed ligand capped ultrafine SnO2 quantum-dots (QDs). Interestingly, we observed that the ligands which help in the formation of a uniform SnO2 QDs thin film, spontaneously exchange for halide through a perovskite solution and finally we form a suitable SnO2 QDs-halide junction for high-performance HPSCs. The flexible HPSC with the SnO2 QDshalide junction formed via the ligand exchange exhibits a high PEC of 17.7% using a flexible substrate. It also shows an excellent flexibility where the initial PCE is maintained within 92% after 1000 bending cycles for a bending radius of 18 mm.

TOC GRAPHICS

KEYWORDS perovskite solar cells, tin oxide, quantum dot, ligand exchange, large-area


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Halide perovskite solar cells (HPSCs) are excellent as future photovoltaic systems because the simple wet-process has made a remarkable progress in power conversion efficiency (PCE) up to 23.3 % using halide perovskite solutions.1-7 The wet-process can fabricate the low-cost solar modules through high throughput coating system using flexible substrate at low temperature like a roll to roll printing system. However, the current high-performance HPSC bases on high temperature (> 400 oC) processed TiO2 layers as an n-type layer such as spray coated TiO2 blocking layer and mesoporous TiO2 layer.1-8 Therefore, the development of alternatives to the TiO2 layers has attracted significant interest to achieve a low-cost fabrication in HPSCs. Several alternative n-type oxides such as SnO2, ZnO, Zn2SnO4 and BaSnO3 have been reported.9-14 Among them, SnO2 has been reported in various forms because it has various advantages for the n-type layer in HPSCs such as a firm chemical stability, a wide band gap (~ 3.6 eV), high electron mobility, and a proper band alignment to the light absorbing halides.9, 15-18 One of the essential points to consider in the use of n-type oxide layer is low-temperature processability due to the high crystallization temperature of oxides. The typical strategy is to use a colloidal solution consisting of oxide nanocrystals (NCs) for fabricating the n-type oxide layer on a transparent conducting oxide (TCO) glass substrate at a low temperature.13-14,

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For

example, the HPSC based on the SnO2 NCs film was reported as an n-type layer that is fabricated using SnO2 NCs colloidal solution.18 However, because oxide NCs in a solution tend to aggregate, dispersibility and dispersion stability of the oxide colloidal solution are remarkably crucial for the formation of a uniform oxide layer in a large area and the reproducibility for large-scale manufacturing. Therefore, we need to control the dispersion to prevent NCs from forming the aggregates for large area HPSCs toward commercialization.


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Surface engineering of NCs is acceptable in improving dispersibility via steric ligand effects by introducing ligand molecules with long alkyl chains such as oleic acid and oleylamine. It is well established in chalcogenide quantum dots (QDs) such as PbS and CdSe due to their narrow band gap leading to a unique optoelectronic functionalities.20-22 However, oxide QDs with a wide band gap like SnO2 are relatively not noticeable. Although a few studies report on ligand caped SnO2 QDs, they are limited to facile scale up due to high synthetic temperature over 100 oC.23-25 Furthermore, because the capping ligands with long alkyl chain hinder charge carriers from transporting and transferring into QDs from a light absorbing halide,26-28 ligand capped oxide QDs as an n-type material are barely used despite having merits concerning layer formation due to an excellent dispersibility in colloidal solutions for HPLCs. In this paper, we propose the simple inverse micelle-water injection method that results in a highly dispersed SnO2 colloidal solution consisting of ligand-capped SnO2 QDs with an average size of 2.9 nm at a low-temperature under 100 oC. Additionally, we successfully fabricated compact and uniform ligand capped SnO2 QDs films using the SnO2 colloidal solution by a simple spin coating process. Interestingly, we observed that the capping ligands of SnO2 QDs were spontaneously exchanged during a coating process of perovskite halide solution. Moreover, SnO2 QD-perovskite halide junction whose intersection is suitable for charge transferring between halide to SnO2 QD layer, simultaneously formed. The HPSCs employing the SnO2 QDs film as a n-type layer and CH3NH3PbI3 halide did not only show high PCE of 18.8 % in small active area of 0.14 cm2 under 1 sun illumination (100 mW/cm2) but also a small drop of PCE to 17.2 % by enlarging active area to 1 cm2. Furthermore, we demonstrated the flexible HPSCs with a high PCE of 17.7 % employing the ligand capped SnO2 QDs film on a tin-doped indium oxide


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(ITO)-polyethylene naphthalate flexible substrate and maintained 92 % of the initial PCE after 1000 bending trials for a bending radius of 18 mm. We described the strategy of synthesizing the ligand capped SnO2 QDs at a low temperature below 100 oC and finally obtaining a highly dispersed SnO2 QDs colloidal solution in Scheme 1. It is hard to make a profoundly dispersed ultra-small SnO2 QDs under 100 oC because the synthesis reactions for ligand capped QDs usually need to be conducted over 300 oC to replace the tin oleate with SnO2 and the SnO2 QDs without ligands easily aggregate during the synthesis.23-25 We introduced the reverse micelle-water injection method to synthesize the SnO2 QDs which results in a well dispersed and uniform-sized ultrafine QDs under 100 oC. Firstly, we induced the tin oleate with reverse micelle structure by replacing the acetate ligands with oleate ligands by dissolving the tin metal acetate salt in a non-polar solvent xylene with oleic acid and oleylamine. During the formation of the tin-oleate phase, the polar head group of the surfactant and aqueous metal acetate pose the inside whereas the organic tails of the surfactant directed towards outside in non-polar solvent, resulting in reverse micelle structure.29-30 We further injected the water into the prepared tin oleate reverse micelle solution at 90 oC which quickly changed to an opaque solution. Through the reactions, tin ions in the core of the tin-oleate reverse micelle could form tin hydroxide Sn(OH)x as an intermediate compound in the presence of oleylamine because of proper pH.31 We refluxed the resulting solution at 90 oC until it became transparent to decompose the tin hydroxide to tin oxide. As shown in the picture of the solution in Scheme 1, the resultant colloidal solution was utterly transparent without any laser scattering with a wavelength of 660nm, revealing the SnO2 QDs were well dispersed in non-polar solvent. We conducted an X-ray diffraction (XRD) through a high-resolution transmission electron microscope (HRTEM) analyses to investigate the structural and morphological characteristics of


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the synthesized SnO2 QDs. Figure. 1a represents the XRD spectrum for the synthesized powder showing typical powder diffraction pattern of the rutile SnO2 crystal corresponding to JCPDS card no. 41-1445. The distinct and broad XRD peaks mean that the ultra-fine SnO2 crystals were successfully synthesized via the reverse micelle-water injection method. Figure. 1b shows HRTEM images of the synthesized SnO2 NCs and Figure. S1 represents their size distribution. The average size of the NCs is around 2.9 nm and it is relatively uniform. We know the Bohr radius of SnO2 to be 2.7 nm, revealing a quantum size effect can appear under 5.4 nm.32-33 Therefore, we can conclude that the synthesized SnO2 NCs with 2.9 nm are QDs. We can calculate the size of the QDs using the Brus equation on effective band gap based on an effective mass approximation.34 We evaluated the optical band-gap energy of the synthesized SnO2 QDs as 4.35 eV using Tauc’s plot, which we calculated from absorbance spectra in Figure. S2. The Eg are larger than the value of bulk SnO2 (3.6 eV) because of the quantum confinement effect that indicates the effective band gap energy of 4.35 eV for 2.9 nm-sized SnO2 QDs.33

34

Thus, the

calculated size of QDs using Brus equation matches well to that TEM results. The reduced fast Fourier transform (FFT) pattern of SnO2 QDs shown in the inset of Figure. 1b also gives the (110) and (101) lattice plane of rutile SnO2. We further investigated the band position and band gap of the synthesized SnO2 QDs using a UV photoelectron spectrometer (UPS) and a UV-vis spectrophotometer to confirm whether they are appropriate for n-type layer in HPSCs or not. Figure. 1c represents the UPS spectrum of SnO2 QDs. The positions of the valance band edge and conduction band edge can be calculated by obtaining the values of work function and the valance band maximum from the UPS spectrum and obtaining the optical band gap from UV-VIS absorption spectrum as -8.45 eV and -4.10 eV, respectively. Figure. 1d shows the possible band alignment of synthesized SnO2 presents in an


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energy level diagram with other layers of PSCs compared to the value of bulk SnO2. The bulk SnO2 has an issue as a n-type layer in HPSCs due to its deeper conduction band edge (-4.5 eV) compared to that (-4.0 eV) of TiO2 which might lead to a significant potential loss during charge transfer from halide to SnO2 by wider conduction band edge gap between SnO2 and perovskite halide, resulting in a low open circuit voltage.35-38 Meanwhile, the synthesized ultrafine SnO2 QDs have a broadened Eg with upshifting conduction band edge that can reduce the potential loss of the charge transfer and improve charge blocking property.35-38 Before fabrication of HPSCs, we characterized the SnO2 QD thin films, which are deposited by spin coating using the prepared SnO2 QD colloidal solution. Figure. 2a shows scanned electron microscopy (SEM) images in the plane and cross-sectional viewpoints, and 3dimensional image of atomic force microscopy (AFM) of the formed SnO2 QD thin film on glass/ITO substrate. As we can see, the above SEM image from a plane viewpoint indicates that a uniformly 2-3nm sized SnO2 QDs completely covered ITO surface without any pin-holes. Despite a simple spin-coating process and low drying process at 80 oC, the cross-sectional SEM image notes that the SnO2 QDs form a compact thin layer of about 30 nm thickness. Fourier transform infrared (FT-IR) spectroscopy analysis was performed to evaluate ligand state on the surface of SnO2 QDs in thin film form, and Figure. 2b represents the results. The C-H stretching signals of oleic acid and oleylamine reveal firm peaks of 2851-2853 cm-1 and 2922-2925 cm-1. We observed the peaks at 1462 cm-1, 1546 cm-1, and 1624 cm-1 which associated with the bounded carboxylate group, while the band of the carboxyl group of free-standing oleic acid at 1707 cm-1 is not shown in the spectra of the SnO2 QD film. Additionally, we observed the evidence of the presence of oleylamine on the surface of SnO2 QDs by the peak of 1385 cm-1 and 1562 cm-1 due to the C-N stretching and NH2 band respectively.32, 39 The FT-IR results mean that


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the films deposited from the prepared colloidal solution naturally consist of oleic acid- and oleylamine-capped SnO2 QDs. Moreover, the AFM analysis (Figure. 2a and Figure. S3) elucidates that SnO2 thin layers with SnO2 QDs capped with ligand are morphologically outstanding with 1.879 nm root-mean-square (RMS) values within 50 × 50 µm for next perovskite deposition steps. For comparative analysis, we purchased the SnO2 colloidal solution based on water from Alfa-aesar (We denote this SnO2 by A-SnO2). The A-SnO2 NCs film showed no C-H stretching signal in FT-IR result in Figure. S5, revealing that they are presumably not capped by ligands having long alkyl chains. The A-SnO2 thin film by colloids which are not capped with ligand and dispersed in H2O shows a higher RMS values over 10 nm (Figure. S4). Perhaps the severe roughness and non-uniformity of the surface are due to aggregates of SnO2 NCs. These results signify that the ligand capping SnO2 QDs may play a pivotal role in enhancing not only dispersibility but also of coating uniformity. By this time, the metal oxide QD films are rarely used as an n-type layer in HPSCs, because the surface ligands with long alkyl chain such as oleic acid and oleylamine are well known for an obstacle to transfer charge carriers.40 Thus, before forming perovskite layer, we conventionally utilized the high-temperature annealing process or chemical treatment to remove the surface ligands. Here, interestingly, we observed that perovskite solution coating process voluntarily removes the ligands. We used FT-IR spectroscopy as exhibited in the inset of Figure. 2b to verify this phenomenon. The annealing process was similar to the process of forming perovskite layer, therefore, we retained the marked peaks associated with the C-H stretching of ligands on the annealed SnO2 QD film. In addition, the SnO2 film after UV-ozone treatment for 20 min, which is conducted to coat perovskite film, had still C-H stretching of oleate ligand as shown in Figure. S6. In contrast, we observed the absence of these strong peaks distinctively when the perovskite


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solution reacted with SnO2 QD film. For this FT-IR analysis, we pre-formed the perovskite layer on the SnO2 QD film and then removed it by dissolving in dimethylformamide (DMF) to exclude effect of perovskite layer on FT-IR result. It means that a perovskite solution causes a surface ligand exchange on SnO2 QD from oleic acid and oleylamine into resultant materials during the coating process. In other word, the ligand exchange can be explained by that anion halide ligands in DMF react with the Sn4+ cation of SnO2 and organic ligands on SnO2 are dissociated and washed away with solvent like the previous mechanism of inorganic ligand exchange in organic ligand capped QDs.41-42 A previous report by E. H. Sargent et al. that surface ligands on PbS QDs can be exchanged into methylammonium lead iodide (MAPbI3) in DMF solution containing MAI and PbI2 also supports the voluntary ligand exchange in this study.28 We analyzed the steady-state photoluminescence (PL) in three samples: glass/perovskite, glass/SnO2-QDs/perovskite, and glass/perovskite/SnO2-QDs (Figure. 2c) to further investigate whether or not the surface ligands of SnO2 QD film could influence the charge transfer inside HPSC where we used MAPbI3 as perovskite. Identically, all samples exhibited a band to band emission of perovskite (MAPbI3) peak at around 770nm. The sample based on SnO2 QDs under the perovskite layer shows lower PL intensity values rather than that based on the perovskite layer, indicating high quenching effects derived from SnO2 QDs-MAPbI3 junction due to the ligand exchange reaction. On the other hand, as expected, the sample based on SnO2 QDs containing oleate ligands above the perovskite layer exhibits no PL quenching behavior that has a similar PL intensity to the single perovskite layer. It may result from the capping ligands which are retained on the surface of QD film and hinder charge carriers transfer as depicted in the schematic image (Figure. 2c). These phenomena elucidate that the voluntary ligand exchange occurs when the perovskite solution coated on ligand capped SnO2 QDs films as shown Figure.


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2d and the resultant SnO2 QDs-MAPbI3 junction is facilitating charge transfer. Additionally, it reveals that ligand molecules for QDs are not problematic for charge transfer if one induces a voluntary ligand exchange for deposition of the perovskite halide layer on a ligand capped QDs. We fabricated n-i-p typed HPSC with the architecture of ITO/SnO2-QDs/MAPbI3/Li-doped spiro-OMeTAD/Au to verify the enhanced photovoltaic performance of prepared SnO2 QDs film serving as an electron transfer layer. We worked out the entire fabrication process below 100 oC that is feasible for solvent engineering process except for the deposition process of the counter electrode. Notably, the perovskite films incorporate additional amounts of PbI2 to reduce hysteresis behaviors of photocurrent density-voltage (J-V) characteristics, detrimental ion migrations, and to improve the photovoltaic performances.43-44 Figure. 3a shows cross-sectional SEM image of fabricated perovskite solar cells of a SnO2 QDs electron transporting layer, and the densely grown perovskite layer with a thickness of approximately 600nm. We evaluated the photovoltaic properties as a function of thickness for SnO2 QD films as exhibited in Figure. S7 and S8 to figure out optimal fabrication condition. Although the thickness of SnO2 QD film reached about 200 nm, the photovoltaic device was working typically with a PCE of over 16%, which could be ascribed that the ligand exchange between the SnO2 QD film surface and perovskite occurred effectively on a 200 nm-thick film. When the thickness of QD film is 30 nm, we obtained highest values of the photovoltaic parameters as shown in the Table S1. Also, there was a little mismatch of J-V curves according to the scan directions indicating negligible hysteresis for the 30 nm-thick SnO2 QDs layer in Figure. 3b. The external quantum efficiency (EQE) spectrum for SnO2 QD device is shown in Figure. 3c. The maximum peak of the EQE spectrum was 89.24 % at 510 nm for the device based on SnO2 QD ETL, and the calculated short circuit current density (JSC) was 20.71 mA/cm2 almost corresponding to the values obtained from


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J-V curve measurement. These results reveal that the crystalline and transparent SnO2 QD films are enough to serve as electron transporting layers in a highly efficient HPSCs. To examine the impact of enhanced coating uniformity using ligands on device processability, comparison of photovoltaic performances of the SnO2 QD layer-device and ASnO2 layer-device was conducted. Figure. 3d displays J-V curves of the two types SnO2-based devices under AM 1.5G illumination at 100 mW/cm2 and the photovoltaic parameters are summarized in Table 1. The J-V curves denote the gap according to the increase of active area in HPSCs. The device based on synthesized SnO2 QDs exhibits a superior PCE (η) of 18.84 % compared to η of 17.53 % for the optimized HPSCs with A-SnO2 in case of 0.14 cm2 active. In addition, the gap of PCE between two devices became larger in case of 1.0 cm2 active area. The PCEs are 17.19 % and 14.70 % for the SnO2 QDs and A-SnO2 based devices with an active area of 1.0 cm2, respectively. We also observed a remarkable difference regarding the shunt resistance in J-V curves between the SnO2 QD device and the A-SnO2 device. In the case of the SnO2 QD device, it describes a high shunt resistance resulting in high fill factors regardless of the active area. Given previous AFM and UPS analysis, it may be attributed to an excellent coverage ability and slightly wide energy band gap of SnO2 QDs leading to prevention from direct contact between the ITO and perovskite layer, and efficient hole blocking.16 Additionally, we observed from the statistical data for photovoltaic parameters under 1.0 cm2 active area in Figure. S9 that the SnO2 QD devices show the narrower distribution for photovoltaic performance than the ASnO2 device. To show the great potential of SnO2 QDs for large-scale devices, we fabricated SnO2 QDs on ITO substrate with area of 64 cm2 by using spin coating method. The morphology and uniformity of SnO2 QDs film is further evaluated by SEM and AFM. As shown in Figure S10,


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the ligand capped SnO2 film on ITO substrates have around 20 nm and the average of RMS surface roughness of SnO2 QDs films were assessed AFM images was to be 2.19 nm in the area of 50 µm × 50 µm with good surface coverage and low deviation despite lower thickness than SnO2 QDs film in the area of 2 cm × 2 cm. These results imply that the well dispersed SnO2 QDs solution can be coated on substrate and SnO2 QDs film is formed with uniform and wellcoverage surface. These experimental findings intimate that the synthesized SnO2 QDs capped with ligands with long alkyl chains are more optimized for large-scale PSC fabrication owing to their high reproducibility and enhanced coating uniformity rather than the SnO2 colloids without ligands. We fabricated a flexible device based on a SnO2 QDs layer and evaluated various device properties to investigate the suitability for flexible HPSCs. As mentioned earlier, we demonstrated the efficient operation of a SnO2 QDs layer onto rigid substrates (ITO/glass) and applied the same architecture and fabrication process controlled under 100 oC onto flexible substrates (ITO/PEN). Figure. 4a exhibits the cross-sectional SEM image of flexible HPSC based on the SnO2 QDs layer. It shows that the compact electron transporting layer was not only entirely deposited onto flexible substrates with enhanced coating uniformity but also the MAPbI3 perovskite film was well constructed being similar to that on the rigid substrates. The J-V curves in Figure. 4b present a JSC of 19.7 mA /cm2, a VOC of 1.13 V and a FF of 0.79, resulting in the PCE of 17.7 %, which is nearly in accordance with the performance of HPSC we prepared onto the rigid substrate (Figure. 3b). Although we conducted the fabrication process via the solution process at low temperature, the device performance surpassed that of being fabricated in ultrahigh vacuum equipment.15 We further studied the average EQE of flexible HPSC which shows over 80 % values between 430 nm and 750 nm in Figure. 4c, and the integrated JSC is 19.68 mA


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/cm2 corresponding to the obtained value from the J-V measurement. We carried out the bending test depending on the bending radius (R) and cycle variables (Figure. 4d) to verify the stability against mechanical deformation and the practicability as a flexible device. We consequentially conducted over 1000 times bending according to three bending radii: 18mm, 10mm, and 4mm. The PCE values maintained at approximately 92% from the initial PCE value despite 1000 bending trials in the case of 18 mm bending radius, revealing that developed SnO2 QDs and the ligand exchanging fabrication are significantly useful for large-area flexible HPSC compared to reported values summarized in Table S2. Nevertheless, we observed the abrupt decrease of PCE value at 4 mm bending radius after 100 bending cycles, which might be induced from the deformation of plastic substrates and the breaking of brittle ITO layers.45-46 In conclusion, we synthesized a highly-dispersed SnO2 QD colloidal solution due to ligand capping via the reverse micelle-water injection method based on non-polar solvent at a low temperature of 90 oC for consideration of mass production. The surface ligands on SnO2 QDs enables to fabricate uniform and sizeable reproducible area SnO2 QD thin films on ITO substrates under 100 oC and then successfully exchanged for perovskite halides during the perovskite solution coating process without any additional ligand removal process. The formed SnO2 QDs-perovskite halide junction showed excellent charge transferring property due to the absence of ligand interference, resulting in high-performance flexible HPSCs. This observation of the simultaneous ligand exchange opens new avenues for applying various functionalized QDs with capping ligands including oxides and chalcogenides in perovskite halide electric devices.


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Scheme 1. Schematic design of the water injection method for synthesis of SnO2 quantum dots (QDs) and synthesized SnO2 QD colloidal solution image (inset)


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Figure. 1. (a) X-ray diffraction (XRD) patterns of synthesized SnO2 quantum dots (QDs), (b) Transmission electron microscope (TEM) image (inset images: high resolution TEM and reduced FFT) of the SnO2 QDs (c) Ultraviolet photoelectron spectroscopy (UPS) spectra showing the Fermi edge and cut-off energy (d) The corresponding energy level diagram of the HPSC.


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Figure. 2. (a) Plane and cross-sectional SEM image of deposited ligand-capped SnO2 QDs on ITO/glass substrate and 3-dimensional AFM topographical image of the deposited film surface (inset) (b) FT-IR spectra of deposited ligand capped-SnO2 QDs film on Si wafer (black), SnO2 QDs film annealed at 150 oC (blue), and perovskite solution reacted SnO2 QDs film (red) (c) steady-state PL spectra of perovskite films as depicted in schematic of sample structure: glass/MAPbI3, glass/SnO2-QDs/MAPbI3, and glass/MAPbI3/ligand-capped SnO2-QDs (d) schematic design of the spontaneous ligand exchange reaction from ligand-capped SnO2 QDs to SnO2 QDs-MAPbI3 during perovskite layer deposition


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Figure. 3. (a) Cross-sectional SEM image of the HPSC with SnO2 QD film on ITO/glass substrate (Thickness of perovskite film is 560 nm.) (b) J-V curves and photovoltaic values measured with forward and reverse scan directions under AM 1.5 G illumination (c) EQE and integrated Jsc of SnO2 QDs HPSC (d) J-V curves for SnO2 QDs and A-SnO2 HPSC with 0.14cm2 and 1.0cm2 active area, respectively.


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Figure. 4. (a) Cross-sectional SEM image of flexible HPSC employing SnO2 QDs on PEN substrate (Thickness of perovskite film is about 460 nm.) (b) J-V curve of best performing SnO2 QDs flexible HPSC and device image (inset) (c) EQE spectra and integrated JSC of SnO2 QDs flexible HPSC (d) bending durability test for SnO2 QDs flexible HPSC for 1000 bending cycles depending on respective bending radius


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Active area

0.14 cm

1.0 cm

2

2

Device

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

SnO2 QDs

21.6

1.13

0.77

18.8

A-SnO2

21.1

1.12

0.74

17.5

SnO2 QDs

21.5

1.14

0.70

17.2

A-SnO2

20.9

1.07

0.66

14.7

Table 1. Summary of photovoltaic parameters obtained from J-V curves in Figure. 3d for SnO2 QDs and A-SnO2 HPSC according to different active area with 0.14cm2 and 1.0cm2 under one sun illumination (AM 1.5G, 100 mWcm-2)


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental Section and Supporting data (PDF)

AUTHOR INFORMATION Corresponding Author *H. S. J.: E-mail: [email protected] *J. H. N.: E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT S.Y.P. and M.Y.B. contributed equally to this work. This research was supported by Basic Research Lab Program through the National Research Foundation of Korea (NRF) funded the Ministry of Science, ICT & Future Planning (2014R1A4A1008474). This work was also supported from the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation (under contract No. 2012M3A6A7054855). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2017R1A2B2009676, NRF-2017M3A6A7051089, NRF2017R1A4A1015022)


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Simultaneous Ligand Exchange Fabrication of Flexible Perovskite

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