Characterization of Perovskite Obtained from Two-Step Deposition on

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Characterizations of Perovskite Obtained from Two-Step Deposition on Mesoporous Titania Shanshan Chen, Lei Lei, Songwang Yang, Yan Liu, and Zhong-Sheng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07511 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 2, 2015

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ACS Applied Materials & Interfaces

Characterizations of Perovskite Obtained from TwoStep Deposition on Mesoporous Titania ‡

‡

Shanshan Chen,a, Lei Lei,b,c, Songwang Yang,b Yan Liu*,b and Zhong-Sheng Wang*,a a

Department of Chemistry, Lab of Advanced Materials, Collaborative Innovation Center of

Chemistry for Energy Materials, Fudan University, 2205 Songhu Road, Shanghai 200438, P. R. China. b

Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, 1295

Dingxi Road, Shanghai, 200050, P. R. China. c

University of Chinese Academy of Sciences, Beijing 100039, P. R. China.

E-mail: [email protected] (Z.W.), [email protected] (Y.L.)

ABSTRACT: The properties of perovskite films are sensitive to the fabrication method, which plays a crucial role in the performance of perovskite solar cell. In this work, we fabricate organolead iodide perovskite on mesoporous TiO2 films through two different two-step deposition methods, respectively, for the purpose to study the crystal growth of perovskite film and its effect on light harvesting efficiency, defect density, charge extraction rate and energy levels. The crystal growth exerts a significant influence on the morphology and hence the film properties, which are found to correlate with the performance of solar cells. It is found that vapor deposition

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of methylammonium iodide in the PbI2 lattice gives a more complete coverage on mesoporous TiO2 with a flatter surface and Fermi level closer to the middle of the band gap, resulting in higher light absorption in the visible spectral region, lower defect density, and faster charge extraction, as compared to the sequential solution deposition. For this reason, the vaporprocessed perovskite film achieves higher short-circuit photocurrent and power conversion efficiency than the solution-processed film.

KEYWORDS: Crystal growth engineering, energy level, light harvesting, perovskite solar cell, time-resolved photoluminescence decay. 1. INTRODUCTION Organic-inorganic lead

halide perovskites

such

as

methylammonium

lead

iodide

(CH3NH3PbI3) and its family have been used as visible-light sensitizers due to their ambipolar charge transport characters.1,2 Aside from the role of visible light absorber,1 perovskites have been intensively investigated as competitive candidates to inorganic silicon solar cells due to their potential for comparable efficiency, cost-effectiveness, high charge carrier mobility, tunable optical properties, and flexible device fabrication.3-6 Within the past several years, power conversion efficiencies of over 20% have been achieved on both planar and mesoporous structures.7,8 Perovskite solar cells can be fabricated using either spin coating or thermalevaporation deposition method, making perovskites especially attractive for scalable manufacturing of thin film photovoltaic devices.9-12 Pioneering work demonstrates that these perovskite materials exhibit structure and/or composition dependent characteristics, which can be obtained with various processing technologies.13


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Several technologies have been developed to fabricate perovskite films, such as the simple one-step spin coating,14 two-step sequential solution deposition (SD),15 vapor-assisted solution process (VASP),12,16 and dual-source vapor deposition in a high-vacuum chamber.10 Additionally, a lot of device architectures, including planar architecture,10,17,18 mesoscopic TiO2 structure,11,19 and insulating Al2O3 scaffold11,19 have been developed to obtain high power conversion efficiency of perovskite solar cells. When the CH3NH3PbI3 perovskite devices are fabricated with different procedures, the properties of perovskite, such as film crystallization, morphology, carrier mobility, and carrier lifetime, are different, which thus brings about corresponding changes of solar cell performance. However, it is not clear that how the fabrication process of perovskite influences properties and solar cell performance. Therefore, investigation of the influence of different preparing methods upon the performance of perovskite devices is highly desired. Herein, we choose the typical SD and VASP approaches to prepare perovskite, CH3NH3PbI3 (MAPbI3), films on mesoporous TiO2 to form mesoporous device structures, and investigate their influence on the perovskite morphology, properties and solar cell performance. To the best of our knowledge, a systematic study on the correlation between film properties and solar cell performance has rarely been reported. The morphology of MAPbI3 films is found to depend critically on the crystal growth of perovskite, which is crucial in determining the film properties. The film fabricated via VASP is flat with a full surface coverage on TiO2 and Fermi level closer to the middle of the band gap, as compared to the rough SD perovskite film with a partial coverage on TiO2. As a consequence, the former yields higher light harvesting in the visible spectral range, faster electron extraction rate and lower defect density than the latter, resulting in


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better photoelectric response in the range of 500-800 nm and higher power conversion efficiency than the latter. 2. EXPERIMENTAL SECTION 2.1. Fabrication of Solar Cells. Fabrications of perovskite films and solar cell devices are provided in the Supporting Information (SI). 2.2. Characterizations. Scanning electron microscopy (SEM) was investigated on a Field Emission Scanning Electron Microscopy (FEI, Magellan 400). X-ray diffraction (XRD) measurements were performed with a Ultima IV X-ray diffractometer using Cu Kα radiation under operation condition of 40 kV and 40 mA from 10° to 60°, with a scanning speed of 5° per minute. The UV-Vis absorption spectra of perovskite-sensitized TiO2 film were recorded on a Shimadzu UV-2550PC spectrometer. Steady photoluminescence (PL) measurements were conducted at room temperature on a Horiba-Ltd FluoroMax-4 device with an excitation wavelength of 470 nm. Time-resolved PL spectra were measured using a fluorescence lifetime spectrometers (Photo Technology International, Inc). The PL lifetime of the CH3NH3PbI3 film on a meso-TiO2/FTO glass substrate was calculated by fitting the experimental decay transient data to the bi-exponential decay model. Ultraviolet photoelectron spectroscopy (UPS) was performed on a Kratos Axis Ultra DLD system with He I (hν = 21.22 eV) excitation. Current-voltage characteristics of solar cells were measured under simulated AM1.5G illumination of 100 mW cm-2 with a Keithley-2420 source meter in combination with a Sol3A class AAA solar simulator IEC/JIS/ASTM equipped with an AM1.5G filter and a 450 W Xenon lamp. The light intensity was calibrated with a reference silicon solar cell (Oriel-91150). The J-V curves were respectively measured through backward (forward bias (1.1 V) to short circuit (0 V), BS) and forward (short circuit (0 V) to forward bias (1.1 V), FS) scan with a scan rate of 40 mV s-1. Incident photon-to-


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current conversion efficiency (IPCE) was measured on a SM-250 system (Bunkoh-keiki, Japan). The intensity of monochromatic light was measured with a Si photodiode (S1337-1010BQ). 3. RESULTS AND DISCUSSION 3.1. Characterizations of Perovskite. Fig. 1 shows the XRD patterns for the VASP and SD films. The observed XRD peak positions match well with the reported data.15,20 The presence of XRD peaks at 2Ө = 14.21°, 23.60°, 24.57°, 28.55°, 31.90°, and 50.24°, corresponding to the (110), (211), (202), (220), (310), and (404) planes respectively, confirms the formation of tetragonal perovskite structure. For both samples, the (110) plane yields the strongest XRD peak,15 indicating the highest exposure probability of (110) plane. In other words, (110) is the main preferred direction in crystal growth of perovskite. However, the intensity of some other XRD peaks for the VASP and SD films is quite different. The (202) plane is highly preferred in the SD perovskite. The preference of these crystal facets forms the cubic crystal of SD perovskite on TiO2. For VASP, the second preferred plane is (310), and these preferred planes form perovskite poly-prism on mesoporous scaffold. The imparity in preferred orientation of exposed crystal facets leads to different XRD peak intensity, which is influenced by the fabrication approaches.21-23 The difference of the preference plane not only shapes the crystal morphology but also influences the carrier transport. As the electrons are supposed to transport along [PbI6] octahedral chains,22 different perovskite contacting facet may have different charge separation rate.


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Figure 1. X-ray diffraction (XRD) patterns of VASP and SD perovskite films. In perovskite solar devices, it is of great significance to engineer the interface between CH3NH3PbI3 and TiO2.7,24 Scanning electron microscopy (SEM) was performed to characterize the perovskite film, as shown in Fig. 2. The top-view morphology reveals that ~100 nm perovskite crystals were formed on TiO2 scaffold with poor coverage (Fig. 2(a)) from the SD method whereas ~250 nm perovskite crystals were formed on TiO2 with full coverage (Fig. 2(b)) for the VASP case. The SD perovskite crystals were separated with TiO2 nanoparticles exposed, while the VASP crystals were interconnected together, covering the TiO2 nanoparticles completely. An incomplete coverage of TiO2 with perovskite obtained from the SD method would lead to a direct contact between TiO2 and hole-transporting material (HTM), resulting in serious charge recombination and thus deteriorating the electron extraction.7,25 Fig. 2 (c, d) displays the cross-section morphologies of the two films. The compact TiO2 layer is ~40 nm thick, the mesoporous TiO2 layer filled with perovskite is ~300 nm thick, and the perovskite capping layer on top of the mesoporous TiO2 is ~100 nm thick. The SD method gave a discontinuous capping layer with a rough surface, but the VASP approach yielded a continuous


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capping layer with a flat surface. In addition, the VASP approach exhibited better pore filling than the SD method. It is concluded that the VASP approach outperforms the SD method in the perovskite film quality.

Figure 2. Top-view SEM images of (a) SD and (b) VASP perovskite films and cross-section SEM images of (c) SD and (d) VASP perovskite films. Fig. 3 illustrates the film fabrication processes of perovskite. Since the deposition proceeds at RT for the SD method (Fig. 3a), it is difficult for the MA+ and I- ions in the solvent to diffuse in the PbI2 lattice. Thus, the perovskite film is likely MAI deficient.26 In addition, the dissolution and recrystallization of perovskite lead to mass migration due to Ostwald ripening, yielding a


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rough film with a poor coverage on TiO2 and a poor pore filling as well. By contrast, the VASP process is conducted at 110 °С under a reduced pressure (100 Pa), which facilitates the ion diffusion in the PbI2 lattice (Fig. 3b), causing more MAI in the perovskite film. As the mass migration is very difficult in the absence of solvent for the VASP case, a flat film with full coverage on TiO2 and excellent pore filling is achieved.10,16,24

Figure 3. Schematic illustration of (a) SD and (b) VASP processes. 3.2. Properties of Perovskite. The absorbance of CH3NH3PbI3 thin films prepared by VASP and SD techniques was identified by UV-Vis spectroscopy (Fig. 4a). It is evident that the VASP perovskite film harvests more visible light than the SD film. The different absorbance of films is attributed to their different coverage on TiO2, originating from their different kinetics of crystal growth. It is reasonable that a higher coverage gives higher absorbance at the same thickness. The higher absorbance of the VASP sample results in higher light harvesting efficiency (LHE) in the visible region, as shown in the inset of Fig. 4a. Owing to the higher LHE, it is expected for the VASP film to generate higher photocurrent in the perovskite solar cell device.


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Figure 4. (a) UV-Vis absorption and (b) (αhυ)2~hυ plots of perovskite films. The inset of (a) is the comparison of light harvesting efficiency (LHE). The band-gap of perovskite films can be obtained from the Tauc’s equation:22 αhν=B(hν-Eg)n

(1)

where α is the absorbance coefficient, calculated by the absorbance (A) of the film and the film thickness (d) with α = 2.303A/d, hυ is the photon energy, B is a constant, Eg is the band-gap of the film and n = 1/2 as perovskite is a direct band-gap semiconductor.22 Tauc plot is shown in Fig. 4b, where the band-gap Eg is determined by extrapolating the linear part of the curve to zero of vertical axis. The band-gaps of SD and VASP perovskite films are determined to be similar at ~1.6 eV, corresponding to an absorption onset at ~760 nm.27


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Figure 5. (a) Photoluminescence (PL) spectra and (b) time-resolved PL decays for perovskite (CH3NH3PbI3) on the mesoporous TiO2 film prepared with VASP and SD method, respectively, monitored at 765 nm with excitation at 457 nm. As the two kinds of perovskite films have different surface coverage, it is expected for them to show different defect density. To confirm this issue, photoluminescence (PL) spectra were recorded (Fig. 5a) as the PL intensity gives insights into the defect density of perovskite film. Compared to VASP perovskite, the SD perovskite on the TiO2 scaffold has higher integration PL intensity (Fig. 5a), indicating a higher defect density due to the more grain boundaries in the SD film (Fig. 2a, b).


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The introduction of mesoporous TiO2 as an electron collector forms a hetero-junction between perovskite and TiO2, where the carrier separation can be described with the simple diffusion controlled model. According to this model, the charge extraction rate at the interface is far faster than the diffusion of carrier in the bulk, and the charge separation rate depends on the diffusion rate. To compare the electron transfer rate from perovskite to TiO2, transient PL was recorded on the two films, as shown in Fig. 5b. The PL lifetime (τ), defined as the time taken from the PL intensity falling to 1/e of its initial intensity, reflects the total carrier-consuming rate. The PL lifetime of VASP perovskite (~17.63 ± 0.38 ns) on TiO2 scaffold is far shorter than that of SD film (~74.22 ± 2.23 ns).28 This indicates that charge separation and electron transfer from perovskite to TiO2 are more efficient for the VASP film as compared to the SD film.29-31 For the SD film, a lot of grain boundaries present in the discontinuous film slow down the diffusion of carriers, and the partial surface coverage on TiO2 is disadvantageous to interfacial electron transfer. For the VASP film, however, no grain boundaries in the continuous film favors fast diffusion of carriers, and the full surface coverage is advantageous to interfacial electron transfer. Thus, the different PL lifetime is the result of different film morphologies. Fig. 6a illustrates the device structure studied in the work. To identify the energy levels of perovskite on mesoporous TiO2 scaffold, ultraviolet photoelectron spectroscopy (UPS) was performed with excitation energy of 21.22 eV. Fig. 6 (b, c) shows the UPS spectra in different energy ranges. The work function of SD and VASP perovkite films on TiO2 is determined to be 4.28 eV (= 21.22 eV – 16.94 eV) and 4.50 eV (= 21.22 eV – 16.72 eV) respectively from the cutoff regions (Fig. 6b).32-35 The Femi levels are thus – 4.28 eV and -4.50 eV for the SD and VASP perovkite films, respectively. The valance band (VB) levels, estimated from Fig. 6c, are 1.37 eV and 1.05 eV below the Fermi levels for the SD and VASP perovskite films, respectively. Thus,


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the VB levels are -5.65 eV and -5.55 eV for the SD and VASP perovkite films, respectively. According to the relationship of gap = CB – VB, the conduction band (CB) levels of SD and VASP perovskite are calculated of -4.05 eV and -3.95 eV respectively. As a result, the CB and VB of the VASP film are respectively higher than those of the SD film by 0.1 eV.

Figure 6. (a) Schematic illustration of the solar cell device structure, (b, c) UPS spectra in different energy ranges, and (d) energy diagram of each component in the solar cell device. As compared to the SD film, the Fermi level of the VASP film is closer to the middle of bandgap. Stoichiometric offset is likely responsible for the different position of Fermi level according to a previous work about the dependence of self-doping type on composition.26 The SD perovskite is likely MAI deficient as discussed previously, which moves the Fermi level farther


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away from the middle of band-gap.26 On the contrary, MAI deficiency can be easily overcome by changing the vapor deposition time for the VASP case. With a Fermi position closer to the middle of band-gap, VASP perovskite has a better balance between electron and hole diffusion, which is considered as a virtue of perovskite materials. The observed faster PL decay (Fig. 5(b)) is likely the result of balanced diffusions of electron and hole. The driving force of charge separation on the TiO2/VASP-perovskite interface is slightly higher than that on the TiO2/SDperovskite interface, which may be a latent reason of higher electron extraction rate revealed by the transient PL spectra. The energy levels of each component in the device are depicted in Fig. 6d using VASP perovskite as an example. Table 1. Summary of photovoltaic performance parameters for ten parallel devices Device Voc (mV) Jsc (mA cm-2) FF (%) PCE (%) VASP

962 ± 49

21.26 ± 1.28

66 ± 2

13.50 ± 1.09

SD

934 ± 44

18.19 ± 0.84

55 ± 3

9.34 ± 0.60

3.3. Solar Cell Performance. The photovoltaic performance of SD and VASP perovskite films were assessed by the current-voltage characteristics under simulated AM1.5G sunlight (100 mW cm-2). Table 1 summarizes the average photovoltaic performance parameters for ten parallel devices. Averagely, the VASP solar cell produces much higher power conversion efficiency (PCE) than the SD cell by 45%, due to the much higher short circuit current density (Jsc) and fill factor (FF). The two devices show comparable open-circuit voltage (Voc). Fig. 7 shows the J-V curves for the best VASP and SD solar cells under our conditions. The VASP cell produces a Jsc of 22.04 mA·cm-2, a Voc of 960 mV, an FF of 0.68, corresponding a PCE of 14.39%, while the SD cell generates a Jsc of 18.76 mA·cm-2, a Voc of 939 mV, an FF of 0.57, and yielding a PCE of


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10.04%. Owing to the higher LHE and more efficient charge extraction, perovskite solar cell with VASP film yields higher Jsc. Furthermore, the VASP cell produces a higher FF than the SD cell, attributed to the lower defect density and faster charge extraction.

Figure 7. J-V curves of mesostructured perovskite solar cells, fabricated from the SD (violet) and VASP (orange) method, respectively, under illumination of simulated AM 1.5G solar light (100 mW cm-2). The scan rate is 40 mV s-1. The SD method is a frequently used approach for fabrication of highly efficient perovskite solar cells. However, the rough morphology due to the partial dissolution of perovskite during immersion in MAI solution and film washing limits the photovoltaic performance. To overcome the shortcomings of SD method, various modifications were reported.36-40 Washing the perovkite film with dichloromethane instead of isopropanol could affect the morphology, and efficiency of 13.5% was achieved.36 Modification of the immersion solution was found to enhance photocurrent, giving average efficiency of 10.5%.37 Controllable morphology of perovskite could be achieved by using a volume-expansion-adjustable precursor38 or an intermolecular selfassembly strategy,39 which yields efficiency greater than 17%. It was also reported that the


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morphology of perovskite could be easily controlled using a mixture precursor containing PbL2 and PbI2.40 The validity of photocurrents in this work was justified by incident photon-to-electron conversion efficiency (IPCE). Fig. 8a shows the IPCE spectra along with the integrated photocurrents. As light soaking is required to get high photocurrent, it is reasonable that the integrated photocurrent is lower than the Jsc obtained under one-sun illumination because light soaking is not applied for the IPCE measurement but applied for the current-voltage scan. For the VASP cell, the IPCE value increases with increasing wavelength from 300 to 400 nm, remains almost constant in the range of 400-740 nm, and then decreases sharply with further increasing the wavelength. On the contrary, the IPCE value increases with increasing wavelength up to 500 nm and then decreases gradually in the range of 500-740 nm followed by a sharp decrease with further increasing the wavelength. Interestingly, the IPCE response in the range of 500-740 nm is quite different for the two devices. This is at least attributed to the higher LHE for the VASP film as compared to the SD film. However, the IPCE/LHE ratio for the VASP device is larger than that for the SD device in the similar wavelength range (Fig. 8b). This indicates that charge extraction efficiency for the former is higher as compared to the latter, attributed to its lower defect density and faster PL decay. The Fermi Level closer to the middle of the band-gap for the VASP film is favorable for efficient charge separation and diffusion, which is responsible for the higher charge collection efficiency and hence IPCE.


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Figure 8. (a) Monochromatic incident photon-to-electron conversion efficiencies along with the corresponding integration current densities and (b) IPCE/LHE ratios as a function of wavelength. Finally, we discuss the hysteresis effect in the J-V measurement of the devices. Recently, unexpected photocurrent hysteresis has been observed in the J-V curves of perovskite solar cells.41-45 Therefore, the J-V curves of the devices with different scanning directions were recorded, and a scanning speed of 40 mV s-1 was adopted, as shown in Fig. 9. The PCE values obtained from the forward scan (short circuit to forward bias, FS) and the backward scan (forward bias to short circuit, BS) of meso-TiO2 based device were 11.68% and 13.76% for VASP, and 8.59% and 9.14% for SD, respectively. Evidently, the VASP-based perovskite solar cell produced higher efficiency than the SD-based perovskite solar cell for both FS and BS scans.


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The difference of PCE values between forward and backward scans for the SD meso-TiO2 based device was smaller than that of the VASP meso-TiO2 based one. The SD perovskite film with an incomplete coverage on TiO2 was found to exhibit smaller hysteresis than the VASP perovskite film with a full coverage on TiO2. The remanent polarization in the capping layer is mainly responsible for the hysteresis.43 Therefore, the SD perovskite film with a lower capping coverage on the TiO2 surface gave weaker hysteresis than the VASP film. If the hysteresis of VASP solar cells could be minimized, the performance of VASP perovskite solar cell could be enhanced significantly in view of its uniform and flat morphology.

Figure 9. J-V characteristics of forward scan (FS) and backward scan (BS) for mesostructured perovskite solar cells under illumination of simulated AM1.5G light (100 mW cm-2). 4. CONCLUSIONS In summary, the perovskite films obtained from the SD and VASP approaches have been characterized in detail, and the influence of crystal growth on film morphology, properties and the solar cell performance has been discussed. There are two basic processes in two-step deposition of perovskite: ion diffusion and mass migration. The ion diffusion in the PbI2 lattice is


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slow but mass migration is fast for the SD method. By contrast, the VASP approach is in the opposite way of SD method. The quick Ostwald ripening renders the SD film poor coverage on the mesoporous scaffold with a rough and discontinuous surface, resulting in several disadvantages, such as lower LHE in the spectral range of 500-800 nm, higher PL intensity (or a higher defect density), and longer PL lifetime (an inefficient charge separation). The above shortcomings can be overcome when the VASP approach is employed. For this reason, the power conversion efficiency is improved remarkably due to the significant increase in Jsc and FF.

ASSOCIATED CONTENT Supporting Information. The preparation of perovskite films, fabrication of perovskite solar cells and the photovoltaic performance for the VASP perovskite obtained from different reaction time. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel/Fax: (+86)21-5163-0345. Author Contributions ‡

S.C. and L.L. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work was financially supported by STCSM (12JC1401500), the National Basic Research Program of China (2011CB933302), National Natural Science Foundation of China (50972157), National High Technology Research and Development Program of China (2014AA052002), the Shanghai Municipal Sciences and Technology Commission (12dz1203900), and the Shanghai High-&-New Technology's Industrialization Major Program (2013-2). REFERENCES (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskite as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Baikie, T.; Fang, Y. N.; Kadro, J. M. Schreyer, M.; Wei, F. X.; Mhaisalkar, S. G.; Graetzel, M.; White, T. J. Synthesis and Crystal Chemistry of the Hybrid Perovskite (CH3NH3)PbI3 for Solid-State Sensitised Solar Cell Applications. J. Mater. Chem. A 2013, 1, 5628-5641. (3) Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J.; A Roll-To-Roll Process to Flexible Polymer Solar Cells: Model Studies, Manufacture and Operational Stability Studies. J. Mater. Chem. 2009, 19, 5442-5451. (4) Na, S.-I.; Kim, S.-S.; Jo, J.; Kim, D.-Y. Efficient and Flexible ITO-Free Organic Solar Cells using Highly Conductive Polymer Anodes. Adv. Mater. 2008, 20, 4061-4067. (5) Hou, S. C.; Lv, Z. B.; Wu, H. W.; Cai, X.; Chu Z. Z. Yiliguma; Zou, D. C. Flexible Conductive Threads for Wearable Dye-Sensitized Solar Cells. J. Mater. Chem. 2012, 22, 6549-6552.


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Characterization of Perovskite Obtained from Two-Step Deposition on

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