Achieving High Current Density of Perovskite Solar ... - ACS Publications

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Achieving High Current Density of Perovskite Solar Cells by Modulating the Dominated Facets of Room-Temperature DC Magnetron Sputtered TiO2 Electron Extraction Layer Aibin Huang,†,‡ Lei Lei,*,† Jingting Zhu,†,‡ Yu Yu,†,‡ Yan Liu,† Songwang Yang,§ Shanhu Bao,† Xun Cao,*,† and Ping Jin*,†,∥ †

State Key Laboratory of High Performance Ceramics and superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Dingxi, 1295, Changning, Shanghai 200050, China ‡ University of Chinese Academy of Sciences, Yuquan 19, Shijingshan, Beijing 100049, China § CAS Key Laboratory of Materials for Energy Conversion, Shanghai Instute of Ceramics, Chinese Academy of Sciences, Heshuo 588, Jiading, Shanghai 201899, China ∥ National Institute of Advanced Industrial Science and Technology (AIST), Moriyama, Nagoya 463-8560, Japan S Supporting Information *

ABSTRACT: The short circuit current density of perovskite solar cell (PSC) was boosted by modulating the dominated plane facets of TiO2 electron transport layer (ETL). Under optimized condition, TiO2 with dominant {001} facets showed (i) low incident light loss, (ii) highly smooth surface and excellent wettability for precursor solution, (iii) efficient electron extraction, and (iv) high conductivity in perovskite photovoltaic application. A current density of 24.19 mA cm−2 was achieved as a value near the maximum limit. The power conversion efficiency was improved to 17.25%, which was the record value of PSCs with DC magnetron sputtered carrier transport layer. What is more, the room-temperature process had a great significance for the cost reduction and flexible application of PSCs. KEYWORDS: DC magnetron sputtering, {001} facet, TiO2 electron transport layer, perovskite solar cells, excellent performance

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minimized the carrier transport loss in ETL.13 The light as well as carrier loss would be minimized and JSC could hence be notably improved by carefully selecting and preparing of ETL.14 Compact TiO2 film was widely employed as the ETL of PSCs for the low cost, simple preparation process, high chemical stability and excellent transparency. As TiO2 was not well-conductive, suitable fabrication approach was considerably important to ensure the small thickness. Besides, FTO, the transparent conductive electrode of PSCs generally has large fall head, preparing technique should ensure the homogeneity of thickness and full coverage of the substrate. The previous researches on PSCs were mostly adopted solution-processed methods, spin coating,15 spray coating7 and spray pyrolysis16 for instance. Although several problems existed in these synthesis processes, limitation of area, uniformity and involving high temperature treatment were concerned. The solutionbased under layer preparation techniques not only limited the

ver the past several years, the emergence of perovskite solar cell (PSC) changed the game in photovoltaic field. The solar to electricity conversion efficiency was boosted with unparalleled speed. Tremendous attentions were thus drawn to the research of carrier transport layer selection, material preparation and structure optimization.1−4 The outstanding cell performance was attributed to the advantages of perovskite structured organo-lead halide materials: suitable band gaps ranging between about 1.2 and 2.3 eV,5 high light absorption coefficients,6,7 long charge carrier lifetimes8,9 and widely tunable compositions.10,11 Typically, n-i-p structure was used in PSCs where the perovskite absorber was sandwiched between electron transport layer (ETL) and hole transport layer (HTL).12 As one of the vital components in PSCs, several requirements for ETL were of significance to the short circuit current density (JSC) enhancement: (i) highly transparent across the visible light region, (ii) low surface roughness, (iii) excellent electron-extraction ability, and (iv) reduced resistance. In perovskite photovoltaics, high transparency ensured low incident light loss, low surface roughness facilitated the perovskite crystallization, effective electron extraction decreased the carrier transfer loss at the interface and high conductivity © XXXX American Chemical Society

Received: November 3, 2016 Accepted: January 10, 2017 Published: January 10, 2017 A

DOI: 10.1021/acsami.6b14040 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (A) Schematic diagram of a dc reactive magnetron sputtering configuration. (B) Atomic structures of the (001) planes in anatase TiO2 (up) and projection of the (001) planes on the surface normal to the ion incident direction. The blue solid and hollow spheres represent titanium and oxygen elements, respectively. (C) Atomic structure of ion radiation damage on (101) plane. (D) Atomic structure of ion radiation damage on (101) plane(001) plane. (E) TEM and HRTEM (inset) images of TiO2 film. (E) XRD patterns of anatase TiO2 films with dominated facets of {101} (red) and {001} (black) prepared by DC magnetron sputtering at room temperature without heat treatment.

JSC as well as PCE enhancement but somehow hindered the commercialization proceed and application fields of PSCs. Magnetron sputtering as a versatile film deposition approach was introduced in the fabrication of TiO2 ETL since 2014. Magnetron sputtering was employed to prepare TiO2 ETL that fulfills the requirements of high transparency, full coverage and homogeneity and these advantages were mentioned by several researchers.17−19 The carrier transport loss in magnetronsputtered TiO2, however, remains a problem. The PCE record of PSC with sputtered TiO2 compact layer was hence limited to 16.12%.11 The electrical properties of TiO2 film are aeolotropic, both theoretical and experimental studies showed that the {001} facets had much more reactive properties than the {101} facets. With a {001} facets exposed TiO2 film, the electron extraction at the TiO2/perovskite interface would be more efficient and the carrier transport loss in ETL would reduce. High short circuit density (JSC) could hence be achieved. In this report, the most exposed crystal facets set is defined as the dominated facets. According to the thermodynamic stability analysis, TiO2 crystals were usually dominated by {101} facets, because of their surface energy (0.44 J·m−2) is the lowest. The fabrication of TiO2 film with a {001} preference is difficult since the

surface energy of [001] facet sets is more than doubled (0.91 J m−2) compared to {101} facets. More often than not, {001} facets are replaced by {101} facets and disappear for high energy lattice planes have higher priority for free atoms or ions to grow.20 In this report, TiO2 films with both {101} and {001} preferences were fabricated by elaborately control the sputter operation parameters, substrate-target distance and atmosphere. The carrier transfer as well as transport loss in PSC could hence be further reduced. PSCs were fabricated with TiO2 ETLs dominated by different facets. The short circuit current density (JSC) and fill factor (FF) of PSCs were dramatically enhanced with {001} dominated TiO2 ETL compared with TiO2 {101} preference. The magnetron sputtering system used in this report to prepared TiO2 with different orientation was illustrated in Figure 1A. The preference of TiO2 film is determined by the relative stability of each lattice plane during deposition. Generally, (101) plane is the most stable because the atoms are density packed and this plane has the lowest free energy (Figure 1B). While for (001) plane who has sparse atomic arrangement, the stability is low. During the magnetron sputtering preparation process, however, the lattice plane B


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Figure 4. AFM images of (A) {001} and B {101} dominated TiO2 films prepared by DC magnetron sputtering at room temperature, the insets are corresponding 3D morphology pictures; record images of contact angle test with perovskite solution in touch of (C){001} and (D){101} dominated TiO2 films.

Figure 2. (A) Schematic view of the device structure, (B) J−V characteristics of perovskite solar cells.

(Figure 1D). As a result, the relative stability of (101) and (001) planes would change with the operation power. When the energy of injection ions is low, the TiO2 developed a {101} planes preference for the thermodynamic stability. While when the operation energy is high, the TiO2 crystallites are surrounded by {001} planes for the low ion radiation damage during the deposition. To examine the structure of the as-deposited film, highresolution transmission electron microscopy (HRTEM) (Figure 1E) and X-ray diffraction (XRD) (Figure 1F) were employed. The HRTEM images (Figure 1E) indicated that the film had a dense and continuous columnar structure oriented highly perpendicular to the surface of the substrate. The lattice fringes in Figure 1E was 0.238 nm, which was assigned to (004) plane of anatase TiO2 and was in accordance with our previous analysis that {001} facets were developed vertical to the substrate surface. The HRTEM results not only demonstrated the well crystallinity of sputtered TiO2, but also verified the orientation of the film.20 As crystallites in specific film are mainly surrounded by the dominated facet, X-ray is intensively diffracted by dominated facet yielding an acute peak in the XRD pattern with the highest intensity. XRD is, therefore, could be used as the technique to identify the domination plane. As the XRD patterns in Figure 1F showed, TiO2 films with both {101} and {001} preferences were available by tuning the ion energy during the sputtering process. In the XRD pattern of {001} dominated TiO2, the (004) diffraction peak was the most intense and sharp, confirming a preferential growth along the c axis of the anatase lattice. The XRD pattern (Figure S1) presented a slight shift toward low angle compared to the anatase phase (JCPDS No. 21−1272), implying defect-induced distortion in the film caused by persistent high-energy ions bombardment.21 Figure S2 was the cross-sectional SEM image

Figure 3. Transmittance spectra of FTO substrate and TiO2 films with {001} and {101} orientation deposited on FTO.

stability is influenced by the ion impinging especially when high power density (8.9 W cm−2 in this work) and substrate bias (−80 V in this work) are imposed on the target and substrate. The attached atoms may detach from the film under high energy ions bombarding, which is defined as ion radiation damage. The regular atomic arrangement in (001) plane forms ion channels21 as illustrated in Figure 1B, which would help reduce the ion radiation damage of (001) planes. Although the (101) plane is the most stable under normal conditions, the ion radiation damage is high for this densely packed atom network, especially when the ion energy is extraordinarily high (Figure 1C). For (001) planes, if the ion injection direction is parallel to the ion channels (or normal to (001) lattice planes), the high-energy ions transmit through ion channels and hence the ion impinging damage for this plane would be much lower C


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Figure 5. (A) Steady-state PL spectra of FTO/perovskite and FTO/TiO2/perovskite films (blue: {101} oriented TiO2, red: {001} oriented TiO2). (B) TRPL profiles of FTO/perovskite and FTO/TiO2/perovskite (green, {101} oriented TiO2; black, {001} oriented TiO2).

different orientation in the range from ultraviolet (UV) to nearinfrared (NIR) was quantified with transmittance spectra. Figure 3 primarily depicted the optical transmission spectra of the FTO and FTO/TiO2 films. It was clearly presented in the picture that the transmittance of the as-deposited layers in visible region was up to 80%, revealing the high transparency of the films prepared at room temperature. The orientation of TiO2 films did not have significant influence on the transmittance, the {001} dominated film had a slightly higher transparency from 420 to 800 nm compared with {101} preferred one. Consequently, maximum photo influx or lowest incident light loss would be achieved with films with both {001} and {101} orientation. Second, the surface state of TiO2 film with both {001} and {101} orientation was characterized with atom force microscopy (AFM). The AFM images (shown in Figure 4A, B) illustrated that the TiO2 films with different dominant facets had smooth surface. The RMS (root-mean-square) roughness of the {001} dominate film was around 5.927 nm, which was slightly smaller than that of {101} domination (6.198 nm). The superior surface of {001} oriented TiO2 film originated from the exposure of high energy ions bombard would improve the electric contact between ETL and perovskite absorber material and reduce the interface resistance. Contact angles (CA) between perovskite solution and ETLs with different preference were tested (shown in Figure 4C, D). The {001} facets dominated TiO2 film showed significantly smaller CA than that of the {101} facets preference. Hence, it was easier for the precursor solution to wetting and spreading on the film surface during the spin-coating process.23 Third, the carrier-extraction ability of {001} and {101} facetdominated TiO2 films was evaluated with steady and transient photoluminescence (PL) spectroscopy (Figure 5). Figure 5A presented the PL spectra of perovskite on different substrates. The PL intensity dramatically decreased after the ETLs were introduced between FTO and perovskite absorber which was known as the quenching effect as the electrons were extracted by TiO2 and the carriers for direct band recombination decreased. The lower PL intensity of perovskite film on top of {001} oriented TiO2 compared to TiO2 with normal preference indicated a superior electron extract ability. Time-resolved photoluminescence (TRPL) spectroscopy was utilized to gain more insights to the extraction dynamics at the interfaces of the ETLs and perovskite. Figure 5B illustrated the TRPL spectra with biexponential function fit to estimate the PL intensity

Figure 6. I−V curves of TiO2 films on FTO with different dominated facets obtained with conductive-AFM.

of the as-deposited {001} facets dominated TiO2 film and its thickness was 125 nm at a deposition time of 30 min which was consistent with the TEM image. Having conformed the success of fabricating {001} oriented TiO2, PSCs were fabricated with TiO2 films with different preferences. As the simplest architecture, the planar heterojunction structure was adopted. An illustration of this multilayer device was presented in Figure 2A, where FTO was used as electrode at illumination side, TiO2 served as the ETL, spiroOMeTAD acted as HTL, CH3NH3PbI3 performed as the absorber and Ag was employed as the back contact. After the deposition of active material, the phase and morphology of perovskite light absorber were examined. The XRD pattern (Figure S1) and top view SEM image (Figure S2) of CH3NH3PbI3 demonstrated that a high quality perovskite absorber film was prepared. The typical diffraction peaks in XRD pattern were all assigned to lattice planes of CH3NH3PbI3 and no extra peak was detected. The grains of perovskite on top of {001} facets dominated TiO2 were distinct and compact.22 The J−V curves and photovoltaic parameters of PSCs with {001} and {101} facets dominated TiO2 ETLs were shown in Figure 2 (B). The JSC of PSC was boosted to a value as high as 24.19 mA cm−2. Besides, the fill factor (FF) of PSC was dramatically enhanced from 64.12 to 68.13% with the {001} facets preferred ETL. The properties required for ETLs in high performance PSCs were checked. First, the transparency of TiO2 films with D


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Figure 7. J−V hysteresis curve of perovskite solar cells with (A) {001} and (B) {101} facets dominated TiO2 compact layer, (C) energy band diagram illustration of the PSC and (D) the IPCE evolution of perovskite solar cells with {001} and {101} facets dominated TiO2 compact layer and the corresponding integrated current densities.

layer. The hysteresis suppression effect was attributed to the superior interface and carrier transport ability which would decrease the charge accumulation. The enhanced quality of perovskite grains on smoother surface, besides, might contributed to this phenomenon.26 The VOC of PSC based on {001} TiO2 had a slight drop. As we reported previously, the Ti3+ developed during the deposition process were so abundant as to induce continuous vacancy bands of electronic states just below the conduction band (CB) edge of TiO2. The defect states below TiO2 CB thus substantially caused the band-blending (Figure 7C). Because of the relative large disruption in energy levels between CBs of perovskite material and {001}-dominated TiO2, the electrons would be easier to inject from the absorber layer to TiO2 ETL. However, the slightly narrower difference between ETL and HTL for the band-bending effect leaded to smaller VOC, which was expected when referring our previous report. With notably enhanced JSC and FF, a PCE of 17.25% was achieved in this report, which was the highest value of PSCs based on the DC magnetron sputtered ETLs with CH3NH3PbI3 as light absorber as far as we concerned. The incident phototo-electron conversion efficiencies (IPCE) of PSCs based on different crystal plane oriented TiO2 films were measured to verify the validity of current density values obtained with J−V characteristics as shown in Figure 2A. Broad plateaus were observed for both devices within the visible light region while the values of PSC based on {001} facets dominated TiO2 were far higher and a maximum value of 96.31% was observed (Figure 7D). Integrated current densities calculated from the relevant IPCE were 23.15 mA cm−2 and 21.11 mA cm−2 for PSCs based on {001} and {101} facets dominated TiO2 compact layers, respectively. The good accordance between calculated values and J−V measurement values justified the validity of our high JSC obtained by modulating the dominated facets of TiO2 ETL.

decay lifetime. Compared with FTO/perovskite sample (8.2 ns), the decay lifetimes of FTO/TiO2/perovskite films were defined as 2.0 and 3.0 ns for TiO2 film with {001} and {101} facets preference, respectively. The data demonstrated that the photogenerated carriers were effectively extracted by TiO2 ETLs from the perovskite absorber before recombination, which was in accordance with the PL quenching. There were two origins of higher electron extraction or transfer efficiency: (i) the smoother surface and better wettability of the {001} facets dominated film; (ii) the vertical columnar crystallites grown on the substrates were beneficial to improve electron injection because of the reduced transfer route.24 At last, the conductivity of {001} facets dominated and normal TiO2 films was compared with conductive-AFM characterization. The I−V curves measured with AFM probe indicated an enhanced conductivity of TiO2 film with abundant {001} facets exposed (Figure 6). The origin was ascribed to the abundant Ti3+ ions in {001} facets dominated film and the superior morphology of pillar arrays. According to the transmittance analysis, surface state analysis, defect and carrier extraction analysis and conductivity analysis. We clarified that in virtue of the excellent properties of {001} facets dominated TiO2 film of (i) low incident light loss, (ii) enhanced wettability for perovskite solution, (iii) faster carrierextraction dynamic, and (iv) low carrier-transport loss, the JSC of PSC was boosted and the overall PCE of PSC was increased by 8.1%. The hysteresis of PSCs were a common concern which would affect the accuracy of J−V curve measurement especially for the n-i-p structured PSCs without mesoporous layer. To investigate the influence of ETL difference on the hysteresis extent, the J−V curves were measured in both forward (from JSC to VOC) and reverse (from VOC to JSC) directions (Figure 7A, B).25 As the picture demonstrated, the J−V hysteresis was notably suppressed with the {001} facets preferred TiO2 under E


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(3) Zhou, Y.; Guan, X.; Zhou, H.; Ramadoss, K.; Adam, S.; Liu, H.; Lee, S.; Shi, J.; Tsuchiya, M.; Fong, D. D.; Ramanathan, S. Strongly Correlated Perovskite Fuel Cells. Nature 2016, 534, 231−234. (4) Li, X.; Bi, D.; Yi, C.; Décoppet, J.-D.; Luo, J.; Zakeeruddin, S. M.; Hagfeldt, A.; Grätzel, M. A Vacuum Flash−assisted Solution Process for High-efficiency Large-area Perovskite Solar Cells. Science 2016, 353, 58−62. (5) Zong, Y.; Zhou, Y.; Ju, M.; Garces, H. F.; Krause, A. R.; Ji, F.; Cui, G.; Zeng, X. C.; Padture, N. P.; Pang, S. Thin-Film Transformation of NH4PbI3 to CH3NH3PbI3 Perovskite: A Methylamine-Induced Conversion−Healing Process. Angew. Chem., Int. Ed. 2016, 55 (47), 14723−14727. (6) Huang, A.; Zhu, J.; Zheng, J.; Yu, Y.; Liu, Y.; Yang, S.; Bao, S.; Lei, L.; Jin, P. Achieving High-performance Planar Perovskite Solar Cells with Co-sputtered Co-doping NiOx Hole Transport Layers by Efficient Extraction and Enhanced Mobility. J. Mater. Chem. C 2016, 4, 10839−10846. (7) Huang, A.; Zhu, J.; Zhou, Y.; Yu, Y.; Liu, Y.; Yang, S.; Ji, S.; Lei, L.; Jin, P. One Step Spray-coated TiO2 Electron-transport Layers for Decent Perovskite Solar Cells on Llarge and Flexible Substrates. Nanotechnology 2017, 28 (1), 01LT02. (8) Gao, Q.; Yang, S.; Lei, L.; Zhang, S.; Cao, Q.; Xie, J.; Li, J.; Liu, Y. An Effective TiO2 Blocking Layer for Perovskite Solar Cells with Enhanced Performance. Chem. Lett. 2015, 44 (5), 624−626. (9) Chen, S.; Lei, L.; Yang, S.; Liu, Y.; Wang, Z.-S. Characterization of Perovskite Obtained from Two-Step Deposition on Mesoporous Titania. ACS Appl. Mater. Interfaces 2015, 7 (46), 25770−25776. (10) Yi, C.; Li, X.; Luo, J.; Zakeeruddin, S. M.; Gratzel, M. Perovskite Photovoltaics with Outstanding Performance Produced by Chemical Conversion of Bilayer Mesostructured Lead Halide/TiO2 Films. Adv. Mater. 2016, 28 (15), 2964−2970. (11) Zhang, Sh.; Lei, L.; Yang, S.; Li, X.; Liu, Y.; Gao, Q.; Gao, X.; Cao, Q.; Yu, Y. Influence of TiO2 Blocking Layer Morphology on Planar Heterojunction Perovskite Solar Cells. Chem. Lett. 2016, 45 (6), 592−594. (12) Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Kim, H.-j.; Sarkar, A.; Nazeeruddin, M. K.; Gratzel, M.; Seok, S. I. Efficient Inorganic-organic Hybrid Heterojunction Solar Cells Containing Perovskite Compound and Polymeric Hole Conductors. Nat. Photonics 2013, 7 (6), 486−491. (13) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential Deposition as A Route to High-performance Perovskite-sensitized Solar Cells. Nature 2013, 499 (7458), 316−319. (14) Kaltenbrunner, M.; Adam, G.; Głowacki, E. D.; Drack, M.; Schwödiauer, R.; Leonat, L.; Apaydin, D. H.; Groiss, H.; Scharber, M. C.; White, M. S.; Sariciftci, N. S.; Bauer, S. Flexible High Power-perweight Perovskite Solar Cells with Chromium Oxide-metal Contacts for Improved Stability in Air. Nat. Mater. 2015, 14 (10), 1032−1039. (15) Zhang, W.; Saliba, M.; Moore, D. T.; Pathak, S. K.; Horantner, M. T.; Stergiopoulos, T.; Stranks, S. D.; Eperon, G. E.; AlexanderWebber, J. A.; Abate, A.; Sadhanala, A.; Yao, S.; Chen, Y.; Friend, R. H.; Estroff, L. A.; Wiesner, U.; Snaith, H. J. Ultrasmooth Organicinorganic Perovskite Thin-film Formation and Crystallization for Efficient Planar Heterojunction Solar Cells. Nat. Commun. 2015, 6, 6142−6152. (16) Wu, Y.; Yang, X.; Chen, H.; Zhang, K.; Qin, C.; Liu, J.; Peng, W.; Islam, A.; Bi, E.; Ye, F.; et al. Highly Compact TiO2 Layer for Efficient Hole-blocking in Perovskite Solar Cells. Appl. Phys. Express 2014, 7 (5), 052301. (17) Wang, J. T.-W.; Ball, J. M.; Barea, E. M.; Abate, A.; AlexanderWebber, J. A.; Huang, J.; Saliba, M.; Mora-Sero, I. n.; Bisquert, J.; Snaith, H. J.; Nicholas, R. J. Low-temperature Processed Electron Collection Layers of Graphene/TiO2 Nanocomposites in Thin Film Perovskite Solar Cells. Nano Lett. 2014, 14 (2), 724−730. (18) Yang, D.; Yang, R.; Zhang, J.; Yang, Z.; Liu, S. F.; Li, C. High Efficiency Flexible Perovskite Solar Cells Using Superior Low Temperature TiO2. Energy Environ. Sci. 2015, 8 (11), 3208−3214.

CONCLUSIONS In summary, we proposed the ETL used in PSCs should fulfill the following requirements besides full coverage of transparent conductive electrode and energy level match: (i) high transparency within the light absorption range of active material, (ii) low surface roughness, (iii) excellent electronextraction ability, and (iv) good conductivity. According to these principles, the dominating facets of compact TiO2 film were first modulated for perovskite photovoltaic application by DC magnetron sputtering. The {001} facet-dominated TiO2 film well-meet the requirements of ETL in PSC as abundant characterization demonstrated. With {001} facets preferred TiO2 film, the short circuit current density was boosted to a maximum value reasonably achievable and a PCE of 17.25% was achieved, which to the best of our knowledge, was the record value based on DC magnetron sputtering. What is more, the sputtered films were used without calcination, which had the capability for flexible application. Our research not only has significance in enhancing the PCE of PSCs and broaden the application territory of perovskite photovoltaics but also offers a general guide for the deposition and optimization of carriertransport layers in thin-film solar devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14040. XRD pattern and SEM image of the fabricated CH3NH3PbI3; cross-sectional and top view SEM images and optical performance of amorphous and partly crystallized TiO2 layers; Photovoltaic performance of the TiO2-based devices with different crystallinity degrees; contact angle of the different TiO2 layers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail, [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Aibin Huang: 0000-0002-6126-0207 Songwang Yang: 0000-0001-6304-5941 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the high-tech project of MOST (2014AA032802), the Key Research Program of the Chinese Academy of Sciences (KFZD-SW-403), the Science and Technology Commission of Shanghai Municipality (STCSM, 13NM1402200), and Shanghai Municipal Natural Science Foundation (Grant 6ZR1441000).

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REFERENCES

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