Low Temperature Optical Properties of Novel Lead‐Free Cs₂NaFeCl₆ Perovskite Single Crystals

Abstract

Lead‐free double perovskites have attracted much attention as possible alternatives to lead halide based perovskites in photovoltaic applications. However, to date only few double perovskites have been successfully employed in optoelectronic device prototypes. Therefore, the search for stable and lead‐free materials is ongoing. Here, we present the successful growth of high‐quality Cs2NaFeCl6 single crystals and their temperature‐dependent structural and optical properties. By combining electron paramagnetic resonance (EPR), crystal structure analysis and density functional theory (DFT) we could determine a cubic crystal structure with a spin of 5/2 for this material, showing strongly spin polarized character. Furthermore, combining photoluminescence (PL) and optical absorption measurements we find a bandgap of approximately 2.1 eV at room temperature as well as the presence of excitonic states. Using Elliot's formula, we are able to extract the temperature‐dependent behavior of the bandgap as well as an estimated exciton binding energy of only 20 meV at 80 K.

Investigations on the Carrier Mobility of Cs2NaFeCl6 Double Perovskites

“…Recently, a new double perovskite, Cs 2 NaFeCl 6 , was successfully synthesized, displaying environmental stability, thermal stability, and thermochromic cyclic stability [29,30]. The exciton-binding energy of Cs 2 NaFeCl 6 is only 20 meV at 80 K, facilitating the separation of light-induced electron/hole pairs and leading to high optoelectronic efficiency [31]. However, the experimentally measured carrier mobility of Cs 2 NaFeCl 6 at room temperature is initially low.…”

Air-Stable Lead-Free Antimony-Based Perovskite Inspired Solar Cells and Modules

“…Over the past decade, lead-halide perovskite solar cells (PSCs) have made tremendous progress with power conversion efficiency (PCE) reaching 26.1%. , Although the matter is complex and must be analyzed from different angles (e.g., toxicity of elements and derivative compounds according to different tests, environmental footprint, life cycle analysis, which depends also on efficiency and performance etc. ), − and despite the excellent properties of Pb-based perovskites and low amounts found in PV cells and electronic products in which they will be integrated, the toxicity of heavy metal Pb may remain a concern for some of its practical applications. ,, Therefore, in recent years, scientists have initiated actions on developing potentially low-toxicity eco-friendly perovskite alternatives to produce the next-generation lead-free PSCs. − Major alternative classes to lead–halide perovskite comprise tin (Sn)-based − and germanium-based perovskites and derivatives, ,, double perovskites, , and bismuth (Bi)-based − and antimony (Sb)-based perovskite derivatives. − In this race to lead-free compounds, Sn-based perovskites have been more prominent with their PCE exceeding 14%. − Historically, these perovskites suffer from severe instability in the air; e.g., MASnI 3 undergoes chemical decomposition within seconds, due to the undesired oxidation of Sn 2+ into Sn 4+ . − Such poor stability together with unfortunate film morphology are the main reasons why fully laser-patterned (P1, P2, P3) working lead-free large-area PV modules had not been reported so far, since the large lasers used for module manufacturing are placed outside the glovebox and the laser scribing process often takes over tens of minutes or even hours …”