Subject Number :S-17-MS-2010
Support Type : Common use (including technical support necessary for the training),
Proposal Title (English) : Nano-scale Chemical Mapping of Vapor Processed-(PEA)2(MA)n-1PbnI3n+1 Quasi-2D perovskite Solar Cells
Username (English) : H. W. Shiu1, M. S. Li2, L. C. Yu1, Y. L. Lai1, T. Ohigashi3, N. Kosugi3, P. Chen2, and Y. J. Hsu1
Affiliation (English) : 1National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
2 Department of Photonics, National Cheng Kung University 701, Tainan, Taiwan
3Institute for Molecular Science, 38 Nishigo-naka, Myodaiji, Okazaki, Aichi 444-8585, Japan

In the last couple years, a dimensionally tuned quasi-2D perovskite thin film was developed which is systematically synthesized by introducing a large organic cation, phenylethylammonium (PEA = C8H9NH3) at a judiciously-chosen stoichiometry [1]. The quasi-2D perovskite exhibits the capability of combination of enhanced stability of 2D perovskite and outstanding optoelectronic properties of 3D perovskite [2]. In this work, we systematically studied the layer number dependent behavior of (PEA)2(MA)n-1PbnI3n+1 Quasi-2D perovskite by X-ray photoelectron spectroscopy (XPS), Near-edge X-ray absorption fine structure (NEXAFS), and scanning transmission X-ray microscopy (STXM).
In our previous studies, we have successfully synthesized dimensional tunable quasi-2D perovskite by mixing stoichiometric quantities of lead iodide (PbI2), MAI (Ch3NH3I) and PEAI to yield a series of compounds, (PEA)2(MA)n-1PbnI3n+1 with different layer numbers of n. The MAI vapor-assisted method can control the morphology to achieve compact and highly stable crystalline layer. The PCEs up to 19.1% and 18.69% were achieved for n = 40 and n =60 layers compound, respectively. Both of them showed better performance than the pristine 3D perovskite of MAPbI3 (PCEs = 17.31%, n = ∞) and 2D perovskite of (PEA)2MAPb2I7 (PCEs < 15%). Besides, compare with the SEM results, it indicated that PEAI may play an important role for the photovoltaic performance, because the change of the morphology was correlated with the amount of large organic cation. To further confirm the correlation between the morphology, photovoltaic properties and the effect of MAI vapor, chemical mapping of carbon, nitrogen and titanium in the aggregation of SiN/TiO2/(PEA)2(MA)n-1PbnI3n+1 (n =1, 2, 20, 40 and ∞) are studied by NEXAFS and STXM. Figure 1 shows the STXM OD images of n = 2, quasi-2D perovskite with and without MAI vapor treatment at 293.5 eV, which is corresponding to the C-Cσ* resonances. As shown in Fig. 1 (a), most of the areas is covered by the plump caterpillar-like perovskite films without MAI treatment. In contrast, the perovskite becomes slim and aggregating on to TiO2 nano-structures as observed in Fig. 1 (b). Some bright spots with intense spherical shapes in carbon mapping is observed at low dose of the MAI treatments while it is not observed at higher dosage. The corresponding micro NEXAFS spectra at carbon K-edge with circular marks in Figure 1 are shown in Figure 2a. Figure 2b shows the reference NEXAFS spectra of n = 1, n = 20, n = 40 and MAPbI3 (n = ∞) for comparison. When the PEAI is doped into the PbI2 film for n = 2 without MAI vapor reaction, the absorption profile along with characteristic peaks are similar to the 2D (PEA)2PbI4 perovskite (n = 1). After MAI vapor treatment, the n = 2 film starts growing in the mixture phase containing CH3NH3+ (MA+) cation and aggregates onto TiO2 substrate. The spectrum is similar to the measurement of MAPbI3. For lower MAI dosage, an intermediate state is observed as shown in Fig. 2a that may explain the reaction process of MAI vapor treatment. (a) (b) Fig. 1. C K-edge optical density images of n = 2 quasi-2D perovskite (a) without and (b) with MAI vapor treatment onto SiN/TiO2 substrate. The carbon OD images are obtained at 293.5 eV. (a) (b) Fig. 2. (a) C K-edge spectra of n = 2 quasi-2D perovskite without (Fig.1a) and with low (Fig.1b) MAI vapor dosage. (b) C K-edge reference spectra for n =1, n = 20, n = 40 and MAPbI3 (n = ∞). [1] I. C. Smith, E. T. Hoke, D. S.-Ibarra, M. D. McGehee, H. I. Karunadasa, Angew. Chem. Int. Ed. 2014, 53, 1. [2] L. N. Quan, M. Yuan, R. Comin, O. Voznyy, E. M. Beauregard, S. Hoogland, A. Buin, A. R. Kirmani, K. Zhao, A. Amassian, D. H. Kim, E. H. Sargent, J. Am. Chem. Soc. 2016, 138, 2649.