Synthesis, Characterization, and Stability Studies of Ge-Based Perovskites of Controllable Mixed Cation Composition, Produced with an Ambient Surfactant-Free Approach

Abstract

In this report, we have applied a facile, ligand-free, ambient synthesis protocol toward the fabrication of not only a series of lead-free Ge-based perovskites with the general formulation of MA1–xFAxGeI3 (where x was changed from 0, 0.25, 0.5, 0.75, to 1) but also CsGeI3. Specifically, our methodology for producing ABX3 systems is generalizable, regardless of the identity of either the A site cation or the X site halide ion. Moreover, it incorporates many advantages, including (i) the possibility of efficiently generating pure Ge-based perovskite particles of any desired chemical composition, (ii) the use of readily available, commercial precursors and comparatively lower toxicity solvents, (iii) the practicality of scale up, and (iv) the elimination of the need for any superfluous organic surface ligands or surfactants. In addition to providing mechanistic insights into their formation, we have examined the chemical composition, crystallite size, morphology, surface attributes, oxidation states, and optical properties of our as-prepared perovskites using a combination of diffraction, microscopy, and spectroscopy techniques. Specifically, we noted that the optical band gap could be reliably tuned as a function of chemical composition, via the identity of the A site cation. Moreover, we have probed their stability, not only under standard storage conditions but also, for the first time, when subjected to both e-beam- and X-ray-induced degradation, using cumulative data from sources such as synchrotron-based scanning hard X-ray microscopy. Importantly, of relevance for the potential practical incorporation of these Pb-free perovskites, our work has emphasized the possibility of controlling the chemical composition within Ge-based perovskites as a means of rationally tuning their observed band gaps and optical behavior.

Germanium‐Based Halide Perovskites: Materials, Properties, and Applications

“…5, the authors devised a simple synthetic procedure modifying and adapting traditional ligand-assisted reprecipitation method for Pb-based perovskite synthesis, thus avoiding the use of concentrated HI and H 3 PO 2 . [8] X-ray Diffraction (XRD) study of the prepared samples confirmed phase pure materials with a progressive increase of the trigonal unit cell by increasing the amount of bigger FA. Scanning Electron Microscopy (SEM) yielded composition-dependent morphologies in terms of size and shape and representative images for MAGeI3 and FAGeI3 are reported in Figure 1b and 1c, respectively.…”

“…Noteworthy, the recent emergence of chiral perovskites as suitable materials for nonlinear optical properties, among which SHG response, could be further explored by considering Ge-containing systems. [7] More recently, the MA1-xFAxGeI3 (x = 0, 0.25, 0.5, 0.75, 1) solid solution was systematically investigated by Yue et al [8] Differently from ref. 5, the authors devised a simple synthetic procedure modifying and adapting traditional ligand-assisted reprecipitation method for Pb-based perovskite synthesis, thus avoiding the use of concentrated HI and H 3 PO 2 .…”

“…In general, while a good stability under argon storage has been found, air-exposure leads to GeO 2 formation after 24 hours, thus confirming the instability of the Ge 2 + ion toward oxidation to Ge 4 + . [8] One of the first attempts to include Ge-based halide perovskites in PSCs dates back to 2015. [13] In particular, after computationally screening a series of possible candidates, Krishnamoorthy et al prepared solar cells with the germanium iodide perovskites (MAGeI 3 and CsGeI 3 ) using compact and mesoporous TiO 2 as electron transport layer (ETL) and Spiro-OMeTAD as hole transport layer (HTL), respectively.…”

“…[13] In ref. 8, for example, FAGeI 3 could be deposited by spin-coating only with poor film quality which led to the impossibility of recording any photocurrent. For the PCSs based on MAGeI 3 and CsGeI 3 device efficiencies (PCEs) of 0.11 % and 0.20 % were obtained, respectively.…”

“…Characteristic parameters (A, J SC ; B, V OC ; C, FF; D, PCE) of the prepared solar cells depending on the amount of Br as X-site anion in MAGeX 3(X=I, Br). Reproduced from reference[8] with permission from the American Chemical Society.…”