Surface, Interface, and Bulk Electronic and Chemical Properties of Complete Perovskite Solar Cells: Tapered Cross-Section Photoelectron Spectroscopy, a Novel Solution

Abstract

The surface, interface, and bulk properties are a few of the most critical factors that influence the performance of perovskite solar cells. The photoelectron spectroscopy (PES) is used as a technique to analyze these properties. However, the information depth of PES is limited to 10–20 nm, which makes it not suitable to study the complete devices, which have a thickness of ∼1 μm. Here, we introduce a novel and simple technique of PES on a tapered cross section (TCS-PES). It provides both lateral and vertical resolutions compared to the conventional PES so that it is suitable to study a complete perovskite solar cell. It offers many benefits over conventional PES methods such as the chemical composition in the micrometer scale from the surface to the bulk and the electronic properties at the multiple interfaces. The chemical natures of different layers of the perovskite-based solar cells [(FAPbI3)0.85(MAPbBr3)0.15] can be identified precisely for the first time using the TCS-PES method. We found that the perovskite layer has higher iodine concentration at the Spiro/perovskite interface and higher bromine concentration at the TiO2/perovskite interface. UPS measurements on the tapered cross section revealed that the perovskite is n-type, and the solar cell studied here is a p-n-n structure type device. The unique possibilities to analyze the complete solar cell by XPS and UPS allow us to estimate the band bending in a working solar cell. Moreover, this technique can further be used to study the device under operating conditions, and it can be applied in other solid-state devices like solid electrolyte Li-ion batteries, LEDs, or photoelectrodes.

Top‐Down Approach to Study Chemical and Electronic Properties of Perovskite Solar Cells: Sputtered Depth Profiling Versus Tapered Cross‐Sectional Photoelectron Spectroscopies

“…These binding energies may correspond to spiro‐MeOTAD, perovskite, and/or to an adsorbed carbon on the sample. [ 27,36,48 ] The N1s, I3d 5/2 , and Pb4f 7/2 have main peak binding energies located at around 399.1, 619.5, and 138.4 eV(also see FigureS2, Supporting Information, for a pristine perovskite film). Similar binding energies on the TCS‐PES have been observed in previous reports.…”

“…Similar binding energies on the TCS‐PES have been observed in previous reports. [ 27,29 ] The N1s intensity starts to increase just after the first measurement point and maintains a constant intensity parallel to the intensity profile of C1s until point 25, which can be related to the adsorption of C and N on the TCS‐PSC surface and/or from the perovskite layer. I3d 5/2 and Pb4f 7/2 , which are elements of the investigated perovskite film, start to increase at point 5, which continues until point 17.…”

“…In our previous studies also, it was found that the mixed halide perovskite has a higher Br concentration in the perovskite. [ 27,29 ] The stoichiometric presence of perovskite elements at point 5 and onward, shows this point can be interpreted as the transition point from Ag to perovskite through the spiro‐MeOTAD. The Ti and O atomic concentrations start to increase after point 15 until point 20.…”

“…To overcome the substrate degradation caused by sputtering, recently the mechanical process of etching has been tried by various groups to study the chemistry of the entire PSCs. [ 27–29 ]…”

Mixed Cation Halide Perovskite under Environmental and Physical Stress

“…Moreover, along the whole study, we carefully considered the possible presence of unwanted X-ray photoinduced effects on the material chemistry. Considering the fragile chemical stability of perovskites, the awareness that in some conditions, the measured spectra might exhibit spurious information due to the interaction with the photon beam is of primary importance when dealing with a widely used characterization method as XPS [ 25 , 26 , 27 , 28 ], especially when high intensity synchrotron radiation is used as photon source.…”