S T R U C T U R E O F M A T E R I A L S

S C I E N T I F I C H I G H L I G H T S

1 3 2 H I G H L I G H T S 2 0 2 1 I

Fig. 112: a) Shift of the {100} X-ray diffraction peak position of MAPbBr3 with increasing amount of Lys in solution. b) Increase in lattice contraction as a function of the amount of lattice-incorporated Lys. c) Change in thermal expansion coefficient versus lattice distortion, induced by Lys incorporation.

Stability enhancement and band gap engineering of a hybrid perovskite using a bio-inspired route

Hybrid perovskites demonstrate excellent opto- electronic properties but poor stability against humidity. X-ray diffraction studies showed that MAPbBr3 can incorporate Lysine into its crystal structure, inducing a shrinkage of the perovskite unit cell and an increase in its band gap, and resulting in improved perovskite stability under humid conditions.

Hybrid perovskites are a family of materials that includes dozens of different structures, all assembled of both organic and inorganic ions. These materials have been studied extensively over the past few decades due to their intriguing structural and functional characteristics. Some hybrid perovskites demonstrate exceptionally high conversion efficiencies, thereby making them promising candidates for opto-electronic applications, such as photovoltaics, photodetectors and light emitting diodes (LEDs). Nevertheless, one of their main disadvantages is their poor stability under humid conditions [1].

Biominerals are inorganic structures formed by living organisms in nature. These structures are mechanically superior when compared to their synthetic equivalents. Among the reasons for this enhancement in properties is the incorporation of organic macromolecules (such as proteins and carbohydrates) into the inorganic structure of the mineral host [2]. Inspired by this natural phenomenon, this work investigated the ability of methylammonium lead bromide (MAPbBr3), a classic hybrid perovskite, to incorporate single amino acids the building blocks of proteins.

MAPbBr3 was grown in the presence of all 20 common amino acids via two growth methods: fast growth at room

temperature; and slow growth at elevated temperature followed by slow cooling. Among all the amino acids, Lysine (Lys), which possesses two amino groups, demonstrated the highest incorporation level. Using high-resolution powder X-ray diffraction (HR-PXRD) at beamline ID22, it was shown that the degree of lattice shrinkage increases as the Lys concentration in the crystallisation solution increases (i.e., the lattice parameter decreases as shown in Figure 112a-b). Moreover, performing HR-PXRD at several different temperatures revealed that the thermal expansion coefficient of the hybrid crystal also changes, in a trend consistent with the extent of lattice distortions (Figure 112c).

The effect of Lys incorporation on the optical properties of MAPbBr3 was further investigated. Utilising diffuse reflectance, it was shown that the optical band gap of the host crystal slightly increases upon Lys incorporation (Figure 113d), corroborating phenomena observed in previous studies [3,4]. These structural and functional changes made it possible to develop a model that describes the incorporation mechanism: it was assumed that the two amino groups of the Lys molecule substitute two MA+ cations, as presented in Figures 113a-c. Moreover, it was hypothesised that the super-saturation of Br- anions in the growth solution, which is dictated by temperature, determines the specific crystallographic direction along which the Lys molecule is incorporated.

Finally, the effect of Lys incorporation on the stability of MAPbBr3 was studied. To this end, a simple electrochemical experiment was designed, in which the impedance of an aqueous solution was tracked over time, while dissolving the Lys-incorporated crystals. As seen in Figure 113e, slow-grown Lys-incorporated MAPbBr3 dissolved 40% slower as compared to that of pure MAPbBr3. In addition, the slow-grown samples were exposed to the controlled humidity conditions in a sealed desiccator. XRD collected from these samples after one week of exposure revealed