Abstract
Understanding the surface properties of hybrid perovskite materials is a key aspect to improve not only the interface properties in photovoltaic cells but also the stability against moisture degradation. In this work, we study the local electronic properties of two series of CH 3 NH 3 PbI 3 perovskite films by atomic force microscopybased methods. We correlate nanoscale features such as the local surface potential (as measured by Kelvin probe force microscopy) to the current response (as measured by conductive atomic force microscopy). CH 3 NH 3 PbI 3 perovskites made using lead acetate as a precursor result in films with high purity and crystallinity and also result in heterogeneous local electrical properties, attributed to variations in the density of surface states. In contrast, when using lead iodide as a precursor, the perovskite surface exhibits a uniform distribution of surface states. This work also aims to understand the early stages of water-induced degradation at the surface of those films. Through high-precision exposure to small amounts of water vapor, we observe a higher stability for surfaces prepared with lead iodide precursors. More importantly, each precursor-based fabrication route is associated with either nor p-type behavior of the films. These characteristics are determined by the type of surface states, which also and eventually preside over stability. This work should help discriminate between perovskite synthesis routes and improve their stability in photovoltaic cell applications.
Highlights
Several methods are proposed for reducing these detrimental effects for photovoltaic performances, involving, among others, tuning of the perovskite composition and interfacial engineering.[19,20,21] For instance, reducing the recombination at the interface can be achieved by introducing a chemical linker between the perovskite and the electron transport layer.[22]
Local heterogeneity at the surface of MAPbI3 perovskites The first set of perovskite studied consisted of MAPbI3 films deposited on top of a glass/indium tin oxide (ITO)/PEDOT:PSS substrate, using PbAc2 as a precursor and following the fabrication procedure described in the Methods section
Freshly-prepared MAPI-PbAc2 thin films were analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to determine their chemical composition and crystallinity, respectively, in order to establish reference profiles prior to any degradation process, and to confirm standards reported in literature for pristine MAPbI3
Summary
Several methods are proposed for reducing these detrimental effects for photovoltaic performances, involving, among others, tuning of the perovskite composition and interfacial engineering.[19,20,21] For instance, reducing the recombination at the interface can be achieved by introducing a chemical linker between the perovskite and the electron transport layer.[22]. To face such a major issue, the electronic properties of the surface of perovskite thin films ought to be further investigated where variations can be observed, i.e. at the nanoscale In this regard, Scanning Probe Microscopy (SPM) provides several characterization methods to locally map the electronic properties of materials with nanometer resolution.[25,26,27,28,29,30,31] Kim et al recently used Kelvin probe force microscopy (KPFM) and bias-dependent atomic force microscopy (AFM) to show that the properties of the (112) surface of a perovskite crystal can be affected upon external bias while the (100) surface remains unchanged.[32] KPFM has been used by many other groups to study the surface potential of perovskite thin films: Bergmann et al used cross-section KPFM on a MAPbI3 based solar cell to show the unbalanced charge-carrier extraction between electrons and holes.[27] Harwell et al exposed the MAPbI3 perovskite repeatedly to light and found that the Fermi level eventually returns with time to its initial value with slow decay, this effect being attributed to trapped or slow charges within the device.[33] Stecker et al used SPM to identify vacancy-assisted transport and, combined with density functional theory (DFT) calculations, they predicted an increase of the work function when increasing the number of vacancies in MAPbBr3.34 Gallet et al found electronic heterogeneities at the MAPbI3 perovskite surface and attributed them to differences in surface terminations.[35]. As a current measurement method, C-AFM can be sensitive to contributions from surface properties, such as defects, as they are expected to impact the charge transport mechanism via carrier injection.[23,39,40] Combining FM-KPFM and C-AFM allows us to locally correlate carrier injection to work function (WF) in view of understanding the electronic properties of the sample surface
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