b. Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei 230026, China
Perovskite solar cells are considered as one of the most promising candidates for the next generation photovoltaic technology due to the high power conversion efficiency (over 22.7%) and low cost fabrication [1, 2]. However, despite these advantages, the long-term stability issues inhibit the further commercial development [3, 4]. In order to address these issues, numerous research efforts have been made to understand the degradation mechanisms caused by the halide perovskite itself and charge transport materials as well as interfaces of the devices [5-8]. The inorganic-organic hybrid halide perovskite materials are extremely sensitive to environmental conditions, such as moisture, temperature, ultraviolet irradiation, and oxygen atmosphere [9-11]. However, considerable degradation of perovskite solar cells was still observed even when the encapsulation techniques were applied to perovskite solar cells to avoid these environmental factors [12-14]. These results indicate that the environmental effects are not the only crucial factors determining device stability, but the intrinsic factors also facilitate the degradation. Therefore, it is necessary to understand the role of interfaces in stability of perovskite solar cells to achieve highly stable devices.
Typically, perovskite solar cells have a sandwich structure. The top electrode, as one of the most essential components, has a direct impact on the device performance and long-term stability [15-17]. Silver (Ag) is one of the most commonly used electrode materials in p-i-n perovskite solar cells due to the low-cost and favorable energy level alignment compared to Au electrode. Recent studies have demonstrated that the reactivity between the metal electrode and perovskite is the main degradation route, which directly influences the chemical stability of the devices [18-20]. Han et al., for example, studied degradation of encapsulated planar CH
In this work, we focus on the effects of the interfacial structures of Ag electrode with the perovskite layers on the degradation of perovskite layers. The corrosion of Ag electrode and the decomposition of perovskite layers were in situ investigated by high resolution synchrotron radiation photoemission spectroscopy (SRPES) and XPS. The PbI
The SRPES and XPS experiments were performed at the Catalysis and Surface Science Endstation at the BL11U beamline of National Synchrotron Radiation Laboratory (NSRL), Hefei, China. The detailed description of the beamline and endstation can be found elsewhere . The core levels of Ag 3d, N 1s, C 1s, and Pb 4f spectra were measured in situ immediately after each metal deposition with photon energies of 480 eV, 400 eV, and 200 eV, respectively, which were calibrated using the bulk Au 4f
FIG. 1(a) presents the XPS survey spectrum of the pristine CH
In order to get further insights into the interfacial interaction, the evolution of the Pb 4f spectra as a function of Ag thickness was monitored, as presented in FIG. 1(b). For the pristine perovskite layers, the Pb 4f
The XRD experiments were also performed for the samples before and after Ag deposition. The results are shown in FIG. 2. For the pristine sample, the XRD pattern shows peaks at
FIG. 3 (a) and (b) show the evolution of Ag 3d and I 3d core-level spectra for different thickness of Ag onto the CH
In order to illuminate the detailed interfacial interactions, the deconvolution of spectra was carried out for Ag 3d
We further analyzed the evolution of the integrated intensities of C 1s, N 1s, and I 3d spectra as a function of Ag thickness on the CH
To better understand the driving force of the iodide ions migration, the valence band and secondary electron cut-off measurements were carried out to investigate the energy level alignment at the CH
According to the above experimental observations, it is concluded that the chemical reaction occurs between the Ag electrode and perovskite layers, which is an important factor that influences the perovskite stability. In particular, the direct evidence has been provided to confirm the iodide ions migration and the interfacial dipole would facilitate this migration process. The migration and accumulation of iodide ions induced two effects: the decomposition of perovskite layers and the corrosion of the Ag electrode. Nevertheless, compared with Au electrode, Ag is still a favorable electrode material with the advantages of low cost, low work function, and the appropriate energy level alignment. Therefore, in order to improve the device stability, charge transport layers should be applied between the top electrode and perovskite layers to prevent the interfacial degradation [41, 42]. Because of the pinholes in spiro-MeOTAD layer and high "solubility" of iodide ions in PCBM layer, the blocking effect of these organic materials was not obvious. In contrast, the inorganic charge transport layers, such as MoO
In summary, in situ XPS and SRPES have been carried out to study the interfacial properties and the ions migration at the CH
Supplementary materials: The results of parallel controlled experiments and C 1s, N 1s spectra are shown.Ⅴ. ACKNOWLEDGEMENTS
This work was supported by the National Natural Science Foundation of China (No.21473178, No.21773222, No.21503203), the National Key R & D program of China (2017YFA0403403), the Key Program of Research and Development of Hefei Science Center of CAS (2017HSC-KPRD001) and the Collaborative Innovation Center of Suzhou Nano Science and Technology.Supplementary Material
FIG. S4 shows the evolution of C 1s and N 1s XP spectra as a function of the Ag thickness. For C 1s spectra, the peak at 286.4 eV is attributed to C-N bonds in CH3NH3+ units, and another peak at 284.5 eV is assigned to the C-C bonds, which is in good agreement with that reported in the literature [1, 2]. For N 1s spectra, the peak at 402.6 eV is attributed to N-C bonds in CH3NH3+ units. Obviously, with increasing Ag thickness, the intensities of C 1s and N 1s spectra gradually decrease, but the peak position of C 1s and N 1s spectra remains almost unchanged. These results indicate that CH3NH3+ ions in CH3NH3PbI3 layers don't react with Ag electrode.
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b. 中国科学技术大学材料科学与工程系，合肥微尺度物质科学国家研究中心，中国科学院能量转换材料重点实验室，量子信息与量子物理协同创新中心，合肥 230026