Fabrication of methylammonium lead iodide thin films via sequential closed space sublimation

Abstract

In this work, the characterization of methylammonium lead iodide (MAPI) layers, which were fabricated from PbI2 and PbCl2 via a sequential (2step) closed space sublimation (CSS) route under high vacuum, is presented. The characterization was carried out by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), UV/VIS absorption spectroscopy, photoluminescence (PL) spectroscopy and in vacuo photoelectron spectroscopy (PES). In addition, the 2step CSS MAPI layers were incorporated into planar solar cells and which were subsequently analyzed. For the transformation of the lead salt layers in the CSS, four substrate temperatures (75 °C, 90 °C, 130 °C, 150 °C) were chosen. The crucible temperatures and transformation times were adjusted to obtain most complete transformations. A high phase purity for the 2step CSS MAPI fabricated from PbI2 and from PbCl2 can be derived from the XRD measurements in the whole substrate temperature range. The SEM measurements show that the morphology of the MAPI layers undergoes significant changes which become more pronounced with increasing substrate temperature and can be separated into three distinct processes taking place simultaneously: the formation of the perovskite by incorporation of MAI into the lead salt grains, the recrystallization of the perovskite grains and an Ostwald ripening like growth of the recrystallized grains. From UV/VIS spectroscopy experiments a band gap of MAPI around 1.58 eV could be derived. The in vacuo PES experiments show the Fermi level pinned to the conduction band minimum. The UV/VIS and PE spectroscopy results appear to be independent on the substrate temperature. Combining the UV/VIS and the PES results, band energy diagrams for PbI2, PbCl2, MAI and MAPI could be created. Those band energy diagrams highlight the importance of a complete reaction of the lead salt on one hand and the avoidance of a MAI capping layer on top of the MAPI absorber, on the other hand. Working solar cells incorporating MAPI layers derived form PbI2 and from PbCl2 could be fabricated for all examined substrate temperatures. However, the efficiencies of the fabricated solar cells were mostly limited to the range of 2-3 %, with few solar cells exceeding 4 %. The reason for this is probably the combination of a hindered charge extraction due to a thin PbI2 interface layer between the MAPI absorber and the FTO/TiO2 electrode with the recombination of photoexcited charge carriers in the MAPI layers. Nevertheless, the relatively large size and the narrow efficiency distribution of the \nsolar cells on one substrate indicate the potential of the 2step CSS process to fabricate solar cells with active areas in the square centimeter regime. Thereby, the unique property of the CSS, the combination of high processing temperatures and a high vacuum environment, is expected to open promising opportunities, especially for research on inorganic perovskite absorber materials.