Photodecomposition and Morphology Evolution of Organometal Halide Perovskite Solar Cells

We study the photoinduced degradation of hybrid organometal perovskite photovoltaics under illumination and ambient atmosphere using UV–vis absorption, atomic force microscopy, and device performance. We correlate the structural changes in the surface of the perovskite film with changes in the optical and electronic properties of the devices. The photodecomposition of the methylammonium lead triiodide perovskite layer itself proceeds much more slowly than the photodegradation of the performance of devices with fullerene/bathocuproine/aluminum top contacts, indicating that the active layer alone is more stable than the interface with the electrodes in this geometry. The evolution of the perovskite active layer performance proceeded through several phases: (1) an initial improvement in device characteristics, (2) a plateau with very slow degradation, and (3) a catastrophic decline in material performance accompanied by marked changes in film morphology. The rapid increase in surface roughness of the active perovskite semiconductor associated with sudden failure also correlates with decreased absorption at the perovskite band edge and growth of a lead iodide absorption feature. We find that degradation requires both light and moisture, is accelerated at increased humidity, and scales linearly with light intensity, depending primarily on total photon dose

Orientation of Ferroelectric Domains and Disappearance upon Heating Methylammonium Lead Triiodide Perovskite from Tetragonal to Cubic Phase

We study the spontaneous polarization of the archetypal semiconducting halide perovskite methylammonium lead triiodide (CH₃NH₃PbI₃) that is currently being investigated for use in thin film solar cells and light-emitting diodes. Using both lateral and vertical piezoresponse force microscopy (PFM) to image polycrystalline thin films, we observed domains in the piezoresponse that reversibly appear and disappear below and above the tetragonal-to-cubic phase transition temperature. Importantly, we observe these domains to exhibit a piezoresponse that is predominantly in-plane for films with the (110) plane oriented parallel to the substrate, providing a measure of the polarization associated with specific crystal planes. We characterize the polarization and its temporal response using both local switching spectroscopy and time-dependent PFM spectra. These data show hysteresis loops with the polarization switching with bias but relaxing back on time scales of several minutes. Our results suggest the existence of ferroelectric behavior due to off-center displacement of the Pb²⁺ cation, although the local polarization response is complicated by the presence of local ionic and electronic conductivity. Understanding the nature of these domains paves the way for further optimization of optoelectronic devices using CH₃NH₃PbI₃ perovskite material

Nanoscale Surface Potential Variation Correlates with Local S/Se Ratio in Solution-Processed CZTSSe Solar Cells

Thin film solar cells made from Cu, Zn, Sn, and S/Se can be processed from solution to yield high-performing kesterite (CZTS or CZTSSe) photovoltaics. We present a microstructural study of solution-deposited CZTSSe films prepared by nanocrystal-based ink approaches using scanning probe microscopy (SPM) and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). We correlate scanning Kelvin probe microscopy (SKPM) maps of local surface potential with SEM/EDS images of the exact same regions of the film, allowing us to relate observed variations in surface potential to local variations in stoichiometry. Specifically, we find a correlation between surface potential and the S/(S + Se) composition ratio. In particular, we find that regions with high S/(S + Se) ratios are often associated with regions of more negative surface potential and thus higher work function. The change in work function is larger than the expected change in the valence band position with these small changes in sulfur, and thus the data suggest an increase in acceptor-like defects with increasing sulfur. These findings provide new experimental insight into the microscopic relationships between composition, structure, and electronic properties in these promising photovoltaic materials