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Oxygen Intercalation Induced by Photocatalysis on the Surface of Hybrid Lead-Halide Perovskites Weiguang Kong, Arash Rahimi-Iman, Gang Bi, Xusheng Dai, and Huizhen Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00496 • Publication Date (Web): 21 Mar 2016 Downloaded from http://pubs.acs.org on March 21, 2016
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The Journal of Physical Chemistry
Oxygen Intercalation Induced by Photocatalysis on the Surface of Hybrid Lead-halide Perovskites Weiguang Konga, Arash Rahimi-Imanb, Gang Bic, Xusheng Daia, Huizhen Wua* a
Department of Physics and State Key Laboratory of Silicon Materials, Zhejiang
University, Hangzhou, Zhejiang 310027, P.R. China b
Faculty of Physics & Materials Science Center, Philipps-Universität Marburg,
D-35032 Marburg, Germany c
School of Information and Electrical Engineering, Zhejiang University, City
College, Hangzhou, Zhejiang 310015, P.R. China Abstract: Methylammonium lead-iodide (MAPbI3) perovskite has emerged as a dazzling nova in the solar cell realm. However, the robustness or stability of the material exposed to different ambiences is a key issue. In this paper, resonance Raman spectroscopy is combined with surface and bulk crystal characterizations to interpret the oxygen intercalation phenomenon on the surface of MAPbI3. We observe that oxygen can intercalate into the frameworks of MAPbI3 with the assistance of laser excitation. By lowering down the pressure in the experimental chamber, the intercalated oxygen can be readily removed. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) characterizations suggest that Pb-O bonds are mainly formed on the surface of MAPbI3, but are constrained to avoid the formation of PbO compound. The quantum chemical calculation based on density functional theory (DFT) supports the above conclusions. The understanding of oxygen intercalation in 1
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MAPbI3 shall benefit the improvement of stability of the important solar cell materials. Keywords: CH3NH3PbI3; perovskite; Raman spectroscopy; intercalation; DFT calculations.
Introduction Hybrid lead-halide perovskites claimed as “the next big thing in photovoltaics” are revolutionizing the photovoltaic landscape.1 From the first application in solar cells as sensitizers by Kojima et al. in 2009,2 photovoltaic devices based on these materials have
showed
a
fast
and
continuous
increase
in
their
performance.3-9
Methylammonium lead-iodide (CH3NH3PbI3, short: MAPbI3), representing a prototype system for photovoltaic applications, has recently demonstrated a top efficiency of 22.1% (KRICT, South Korea).10 However, the stability of MAPbI3 based devices is still unsatisfactory and has become serious bottleneck impeding their commercialization. Some stimuli, e.g.，electric field, temperature, and pressure and Lewis based chemicals can induce phase transition of MAPbI3.11-13 Besides, it has been reported that MAPbI3 in a suitable ambient environment would degrade to PbI2 and organic components.14 The whole degradation process can be accelerated by O2, or UV irradiation, leading to the degradation of perovskite entirely to PbI2.11, 14, 15 However, the oxygen intercalation into the frameworks of MAPbI3 induced by photocatalysis on the surface of MAPbI3 has rarely been reported. It is well known that Raman-scattering spectroscopy is widely used as a powerful technique for the 2
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The Journal of Physical Chemistry
identification of the molecular structure and interactions, as well as the symmetry and crystal structure of compound materials. Though characterizations of the hybrid structure of MAPbI3 and the interpretation of optical properties along with the exploration of electronic structures have been of wide interest, Raman spectroscopy studies of MAPbI3 are still in an early stage.16-18 In this paper we report on the use of Raman-scattering spectroscopy for the exploration of stability of MAPbI3 in different atmospheric ambiences. The Raman and PL spectra are measured on a confocal laser scanning microscope system, where the incident excitation light is normal to the MAPbI3 film. This system can help us in situ monitoring the PL and Raman activity variations as the excitation power changes. We found that under weak laser excitation, MAPbI3 in oxygen-rich ambience is as stable as in a nitrogen atmosphere. The Raman features of MAPbI3 in oxygen-poor atmosphere are very similar to those of 4H-PbI2. However, under high excitation power, Raman activities of MAPbI3 in oxygen-rich atmosphere can transform into those similar to orthorhombic PbO. The transformation is due to the intercalation of O into the frameworks of MAPbI3. Lowering down the pressure in the experimental chamber can expel the intercalated oxygen. The measurement of X-ray photoelectron spectra
(XPS),
excitation-power
dependent
Raman
scattering
spectra
and
photoluminescence (PL) of MAPbI3 under different atmosphere pressures were carried out. The quantum-chemical calculation based on density function theory (DFT) is also utilized to support our experimental observations. The understanding of the long-term stabilities of the perovskite-based material related to ambient forces may 3
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not only benefit solar cell applications, but also open up a new route to the development of novel optoelectronic devices.
Normalized Raman Intensity (a.u.)
Results and Discussion
B
(a)
C MAPbI3 PbI2 A
Excitation density: 20W/cm
2
Environment: N2 D E
100
200
300
400
-1
Raman Shift (cm )
Raman Intensity (a.u.)
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B'
2
