Study of Synthesis Parameters on the Formation and Physical Properties of 2D/3D Hybrid Halide Perovskite Heterostructures for Solar Cells

Perovskite based solar cells, using three-dimensional (3D) hybrid halide perovskites as the active layer, have reached power conversion efficiencies (PCE) of 25.5%^[1] as a single cell, getting closer to the silicon technology. However, this technology is still limited for commercialisation, one of its remaining drawbacks being the intrinsic and extrinsic instabilities of the 3D perovskites when exposed to light, temperature, oxygen and moisture.<br/>In parallel, two-dimensional (2D) layered hybrid halide perovskites have attracted a considerable amount of research and shown higher stability and versatility than their 3D counterparts, with a current PCE record of 17,8% ^[2]. Merging perovskites of 2D and 3D dimensions, in order to create a so-called 2D/3D heterostructure, taking advantage of both 2D and 3D perovskites, seems a promising way to get more stable and efficient perovskite solar cells ^[3] and silicon-perovskite tandem cells.^[4]<br/>In this work, we focus on the synthesis of a 2D/3D perovskite heterostructure by depositing a solution containing a PhenylEthylAmmonium (PEA) cation salt - or its fluorinated variation (para-FPEA) – on the annealed 3D perovskite layer, followed by a subsequent annealing. This leads to the formation of a thin layer of 2D perovskite crystallized on top of the 3D bulk perovskite layer.<br/>In order to optimize the 2D layer, we tested different synthesis parameters, such as the concentration of 2D cation solution, as well as the time soaking between the 2D cation solution and the 3D perovskite, or even the lead iodide content of the 3D perovskite. Furthermore, it is crucial to understand the formation mechanisms of the 2D perovskite and the chemical and physical phenomena occurring at this interface. For this aim, we applied a large panel of characterization, ranging from the analysis of extreme surface (XPS) and morphology (SEM) to the characterization of the bulk (PL, XRD). We showed that the concentration of the 2D cation solution is the most efficient parameter to control in order to affect the 2D perovskite layer properties. Forming a pure 2D phase layer under 10nm thick, by using a low solution concentration, seems the best way to have a smooth 2D/3D interface morphology without deteriorating the 3D bulk, with better optical properties and improved performances on small-scale (0.09 square cm aperture) solar cell devices, leading to best PCE over 18%.<br/>These studies pave the route to a better understanding of the chemical and optoelectronic properties of the 2D/3D perovskites stack, very crucial to improve the efficiency and stability of perovskite based solar cells.<br/><br/>REFERENCES:<br/><br/>[1] Green, M. <i>et al.</i> Progress in Photovoltaics: Research and Applications <b>29</b>, 657–667 (2021), DOI : 10.1002/pip.3444.<br/>[2] Ren, H. <i>et al.</i> Nat. Photonics <b>14, </b>154–163 (2020), DOI : 10.1038/s41566-019-0572-6.<br/>[3] Grancini, G. <i>et al.</i> Nature Communications <b>8</b>, 15684 (2017), DOI : 10.1038/ncomms15684.<br/>[4] Kim, D. <i>et al</i>. Science, 368(6487) : 155-160 (2020), DOI : 10.1126/science.aba3433.