Complexities of Contact Potential Difference Measurements on Metal Halide Perovskite Surfaces

Conference · Thu Dec 05 00:00:00 EST 2019

DOI:https://doi.org/10.2172/1659409· OSTI ID:1659409

Zhang, Fengyu; Ullrich, Florian; Silver, Scott; Kerner, Ross; Rand, Barry; Kahn, Antoine

Understanding the electronic structure of metal halide perovskite (MHP) surfaces and interfaces is of considerable interest for the development of MHP based devices. A number of techniques, including ultraviolet and X-ray photoemission spectroscopy (UPS, XPS), contact potential difference (CPD) measurements, and Kelvin probe force microscopy (KPFM), have been applied to gather information on these surfaces and interfaces.[1-2] However, standard photoemission spectroscopy, such as UPS and XPS, only probes the top few nm of the film and cannot distinguish between flat bands and band bending occurring over several tens of nanometers. Furthermore, photoemission can induce non-equilibrium conditions of surface photovoltage (SPV) and unwanted band-flattening, providing distorted information on the equilibrium position of the Fermi level. Therefore, contactless, non-invasive, and non-destructive, Kelvin probe (KP) based CPD measurements serve as an important complementary technique to sort out some of these issues.[3] CPD measurements in the dark and under illumination (SPV measurements) provide valuable information on work function changes caused by the generation of electron-hole pairs near the surface/interface and on charging and discharging of surface/interface defect states upon illumination. Yet, CPD measurements can also be skewed and lead to erroneous results owing to changes in chemical composition and electronic structure of the surface under illumination.[4-5] Obtaining accurate SPV signals can therefore be challenging when working on perovskite surfaces, which are sensitive to, and degrade under, irradiation. Care must be taken to distinguish SPV signal and reversible band flattening as a result of photo-excitation, which occurs over short time scales (μs), from the long-term changes in surface work function due to surface reorganization or changes in stoichiometry (irreversible, or slowly reversible over hours), e.g., when the perovskite film surface undergoes degradation and decomposition during the CPD or SPV measurement. We present here a vacuum-based study[6] of the surface potential and response to illumination of two different types of perovskite films, methylammonium lead bromide (MAPbBr₃) and the 2D Ruddlesden–Popper phase butylammonium lead iodide (BA₂PbI₄, n = 1), using KP-based CPD and SPV measurements. We show that supra-band gap illumination of both MAPbBr₃ and BA₂PbI₄ leads to halide loss from the surface of the material, accompanied by a contamination-induced modification of the KP work function.[6] If undetected, this can lead to misinterpretations of the MHP surface potential. Repetitive calibration of the tip work function with HOPG is necessary to ensure reliable results. In contrast to MAPbBr₃, BA₂PbI₄ exhibits a significant SPV corresponding to a partial flattening of an upward surface band bending. Our results illustrate the effectiveness of the Kelvin probe-based technique in providing complementary information on the energetics of perovskite surfaces and the necessity to monitor the work function of the probe to avoid erroneous conclusions when working on these materials. [1] Schulz, P. et al. Energy Environ. Sci. 2014, 7, 1377–1381. [2] Miller, E. M. et al. Phys. Chem. Chem. Phys. 2014, 16, 22122–22130. [3] Barnea-Nehoshtan, L.et al., J. Phys. Chem. Lett. 2014, 5, 2408–2413. [4] Zu, F. S. et al. Adv. Opt. Mater. 2017, 5, 1700139. [5] Li, Y. et al. J. Phys. Chem. C 2015, 119, 23996–24002. [6] Zhang, F. et al. J. Phys. Chem. Lett. 2019, 10, 890–896.

• Research Organization: Princeton Univ., NJ (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE), Renewable Power Office. Solar Energy Technologies Office
• DOE Contract Number: EE0008560
• OSTI ID: 1659409
• Resource Relation: Conference: Fall MRS, Boston, MA, USA, December 2-6, 2019
• Country of Publication: United States
• Language: English