Gold and iodine diffusion in large area perovskite solar cells under illumination

S. Cacovich, L. Ciná, F. Matteocci, G. Divitini, P. A. Midgley, A. Di Carlo, C. Ducati
2017 Nanoscale  
Operational stability is the main issue hindering the commercialisation of perovskite solar cells. Here, a long term light soaking test was performed on large area hybrid halide perovskite solar cells to investigate the morphological and chemical changes associated with the degradation of photovoltaic performance occurring within the devices. Using Scanning Transmission Electron Microscopy (STEM) in conjunction with EDX analysis on device cross sections, we observe the formation of gold
more » ... ion of gold clusters in the perovskite active layer as well as in the TiO2 mesoporous layer, and a severe degradation of the perovskite due to iodine migration into the hole transporter. All these phenomena are associated with a drastic drop of all the photovoltaic parameters. The use of advanced electron microscopy techniques and data processing provides new insights on the degradation pathways, directly correlating the nanoscale structure and chemistry to the macroscopic properties of hybrid perovskite devices. Over the last few years, organic-inorganic perovskite based solar cells have attracted strong interest due to their excellent photovoltaic properties, resulting in an unprecedented rise in power conversion efficiency 1 . One of the fundamental challenges the community is facing today concerns the stability of the cells 2-5 . Intrinsic and extrinsic degradation processes underlying the drop in device performance have been widely investigated using macroscopic characterization tools 6-8 , and several studies reported humidity 9, 10 , temperature 11, 12 and light exposure [13] [14] [15] as responsible factors. In particular, two recently published papers highlight the presence of elemental migration within a perovskite thin film and in a state-of-the-art complete solar cell due to light exposure. De Quillettes et al. investigated the changes in the local PL (photoluminescence) intensity and PL lifetime in a methylammonium lead iodide thin film after 1 hour of light soaking 16 . The redistribution of the iodine content, studied via ToF-SIMS, was correlated to a local improvement in the optical properties. Elemental migration under light exposure was reported by Domansky et al. in a combined thermal and light soaking test over 15 hours, showing evidence of gold migration into the mesoporous and compact perovskite layer using a ToF-SIMS approach 17 . In order to establish a direct connection between illumination and irreversible degradation of photovoltaic performance it is essential to investigate how the morphology and chemical composition of the nanostructured active layers are affected by light soaking under electrical bias. Imaging and analytical tools with high spatial resolution are necessary in order to measure changes that happen at nanometer scale. In this work we report a study of the effect of light soaking on large area hybrid perovskite solar cells, using Scanning Transmission Electron Microscopy in conjunction with Energy Dispersive X-ray spectroscopy (STEM-EDX), and characterizing, at the nanoscale, the chemical changes that 4 occur within devices illuminated for 200 hours under dynamical bias. The large area solar cells (1.05 cm 2 ) are fabricated using solvent engineering 18 , in an established architecture: fluorine doped tin oxide (FTO) coated glass / compact TiO2 / mesoporous TiO2 / perovskite / Spiro-OMeTAD/ Au. In order to improve hole transport, FK209 -a cobalt based compound -was employed as Spiro-OMeTAD dopant 19 . The devices were sealed according to a previously published method 20 , to protect the cells from contamination and moisture, thus preventing cell degradation due to extrinsic factors. In this work we investigated the behavior of an "early failed" cell, to evaluate the phenomena leading to the cell breakdown. We compared an as-produced device aged in dark, in open circuit configuration, with an identical device that was exposed to a light soaking stability test for 200 hours, under 1 sun of illumination, at room temperature and dynamically biased at Maximum Power Point (MPP). A white light LED source was chosen in order to avoid possible degradation due to UV and IR radiation, limiting the optical excitation to the visible spectrum. The main photovoltaic parameters were recorded every 20 minutes with an I-V scan. The overall trends are reported in Figure 1a . The stability test induced a dramatic fall of the short circuit current (Jsc) during the first 50 hours, together with a linear reduction of the open circuit voltage (Voc). This results in an obvious drop of the fill factor (FF) and the Power Conversion Efficiency (PCE), which decreased from the initial value of 15.89% to 0.37% after 100 hours of light exposure, showing a strong correlation with the trend of the Jsc. Interestingly, Jsc and Rs show a change in their slope at about 45 hours. At this time the cell reached a 90% of drop in efficiency and we hypothesize the activation of a new degradation phenomenon which increases the relative loss of resistance. The effect of the light soaking stability test over the main physical processes of the device was investigated with a set of steady state and transient measurements before and after the test, as same time more stable perovskite formulations should be investigated, such as the multi-cation materials that have been recently proposed 49 . ASSOCIATED CONTENT Supporting Information. Experimental methods: device fabrication, sealing procedure, photovoltaic characterization, electron microscopy characterization, EDX quantitative maps, cross sectional STEM. "This material is available free of charge via the Internet at
doi:10.1039/c7nr00784a pmid:28345699 fatcat:k3lebktobjcqhnet6xb4fv6woe