Soft X-ray characterization of halide perovskite film by scanning transmission X-ray microscopy

Abstract

Organic–inorganic metal halide perovskites (MHPs) have recently been receiving a lot of attention due to their newfound application in optoelectronic devices, including perovskite solar cells (PSCs) which have reached power conversion efficiencies as high as 25.5%. However, the fundamental mechanisms in PSCs, including the correlation of degradation with the excellent optoelectrical properties of the perovskite absorbers, are poorly understood. In this paper, we have explored synchrotron-based soft X-ray characterization as an effective technique for the compositional analysis of MHP thin films. Most synchrotron-based studies used for investigating MHPs so far are based on hard X-rays (5–10 keV) which include various absorption edges (Pb L-edge, I L-edge, Br K-edge, etc.) but are not suited for the analysis of the organic component in these materials. In order to be sensitive to a maximum number of elements, we have employed soft X-ray-based scanning transmission X-ray microscopy (STXM) as a spectro-microscopy technique for the characterization of MHPs. We examined its sensitivity to iodine and organic components, aging, or oxidation by-products in MHPs to make sure that our suggested method is suitable for studying MHPs. Furthermore, methylammonium triiodide with different deposition ratios of PbI2 and CH3NH3I (MAI), and different thicknesses, were characterized for chemical inhomogeneity at the nanoscale by STXM. Through these measurements, we demonstrate that STXM is very sensitive to chemical composition and homogeneity in MHPs. Thus, we highlight the utility of STXM for an in-depth analysis of physical and chemical phenomena in PSCs.

On the use of Soft X-ray STXM for Organic-Inorganic Halide Perovskite Photovoltaic Materials

“…As mentioned earlier, it is worth noting that the C K-edge and N K-edge could be especially interesting as these belong to the inner shell transitions and could have NEXAFS features (fine structure) that could be very sensitive to chemical functionalities. NEXAFS has been widely used for a variety of materials and has been recently applied to PSC materials to disentangle degradation mechanisms [28,30]. Experimental spectra of various PSC materials were also obtained.…”

“…However, this could be impractical as spin-coating, often employed in PSC device preparation, will not allow for an X-ray transparent substrate that is necessary for STXM measurements. For bulk information specifically a thin film of appropriate thickness (not a full device), spin-coated or evaporated, can be easily investigated [30,35]. It can be prepared on commercially available Silicon Nitride (Si3N4) windows.…”

“…Experimental spectra of various PSC materials were also obtained. Figure 3 In order to demonstrate the effectiveness of XAS and to disentangle the detection of one element over another we have previously presented [30] several materials that were carefully chosen (KI, PbCl2). This demonstration also included degradation products PbO and PbI2.…”

“…shown in [30,35,42]. A reliable chemical quantification can be attempted if a full spectrum can be obtained.…”

Metal halide perovskite layers studied by scanning transmission X-ray microscopy

“…30–40 Organic–inorganic metal halide perovskites are relatively new. Apart from a paper from our group, 41 very few reports of STXM studies were found in the literature. In Lin et al , 42 the material formation was investigated following the exposure of PbI 2 films to methylammonium iodide and in Li et al 43 the integration of a phenyl ethylammonium cation to the perovskite network was studied.…”