title: Modeling Compositional Disorder and Interfaces in Perovskite Materials
Creators: name: Spreafico, Samuele
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2025
Subjects: subject: DFT
subject: Perovskites
subject: Material Modeling
subject: Ferroelectric material

description: Perovskite ceramics have emerged as new-generation materials across various technological domains, owing to their multifaceted properties, such as ferroelectricity, piezoelectricity, or photoluminescence. Taking the example of the halide perovskites, the intensive experimentation around perovskite materials in the last decade, mostly in the form of thin films or Quantum Dot (QD) structures, has demonstrated their extensive employability as enabling components in photovoltaic and optoelectronic applications, such as multilayer perovskite-based solar cells, light-emitting devices, and photodetectors. Notably, with respect to the predecessor enabling materials, halide perovskites’ technological advantage lies in using Earth-abundant raw materials processed from solution at low temperatures, offering a cost-reduced technology also available to developing world countries. Nevertheless, the widespread adoption of perovskite technology faces challenging hurdles. It’s well-documented that in photovoltaic applications devices degrade in the presence of oxygen, moisture, light, and heat. Their low photo- and thermal stability inevitably leads to a loss in device efficiency and operational lifetime, especially for room temperature and outdoor applications. Consequently, various strategies for hindering material property degradation were developed. Among others, embedding perovskite QD crystals in solid matrices has shown promise in mitigating thermal instability and defect migration, thus extending device longevity. However, challenges persist, particularly regarding the resulting planar defect, i.e., the interface between the QDs and the matrix, according to the synthesis conditions whose structure can induce undesired defect states and compromise bulk photophysical properties. Moreover, environmental concerns about the high toxicity of lead, which is included in the highest performing currently employed perovskite compositions (such as Pb(ZrTi)O3 (PZT) and MAPbX3), majorly hinder the green light for extensive employment in civil applications. In response, the search for lead-free perovskite alternatives has intensified. One promising avenue involves composition-engineering approaches applied to BaTiO3 (BT), which have demonstrated efficacy in enhancing and tailoring its photoferroelectric properties. Notably, BT-based polycrystalline Ba1-xCaxZryTi1-yO3 (BCZT) has been found to display an exceptional piezoelectric coefficient (d33 ~ 620 pC/N), positioning it as a viable lead-free competitor for PZT in piezoelectric applications. Another promising doped BT composition was found in BaSnxTi1-xO3 (BST), which was reported to manifest both a large photovoltaic response and an enhanced piezoelectric coefficient at room temperature for a range of Sn concentrations between 3 and 12%. In particular, the highest piezoelectric coefficient (d33 ~ 700 pC/N) was reported at a polymorphic phase boundary region (x = 11%), whereas the co-existence of various phases and the resulting nano-scale domain morphology is argued to trigger an enhanced piezoelectric response. However, the clear origin of the enhancement of piezoelectric response observed in BCZT and BST, whether arising from intrinsic or extrinsic piezoelectric effects, is not yet clear. In this regard, a detailed understanding of the microscopic influence of substitutional defects in terms of local interaction and electronic contributions on perovskite photoferroelectric properties is currently missing. Alongside the role of chemical substitution in altering BT piezoelectricity, defects at the mesoscale level can also play an equivalently effective role. In this regard, the motion of the polarization domain boundary, the so-called Domain Wall (DW), is reported to deliver a major contribution to maximizing the macroscopic ferroelectric response. However, there is currently an absence of a comprehensive understanding regarding the intricate mechanisms driving the evolution of domain wall motion. Hence, there is a compelling demand to obtain both qualitative and quantitative insights into the factors operating at the mesoscale level, which can influence the domain walls’ motion. In this thesis, we have developed computational models tailored to various perovskite materials, customizing each to match specific experimental samples and to feature the unique chemical compositions and interfaces they showcase. The central objective of this research is to delve into the role of substituents and defects within perovskite crystals, particularly their influence on material photoferroelectric properties, using a first-principles Density-Functional Theory (DFT) approach. Two main research projects from this thesis were dedicated to designing computational models reflecting modified chemical compositions of BT lead-free perovskites, namely BCZT and BST. Both these works were carried out in close cooperation with Prof. Kyle G. Webber’s experimental group in the framework of the Graduate School IGK2495. In Chapter 3, we reproduced the experimental composition of a BCZT single crystal sample (namely, Ba0.798Ca0.202Zr0.006Ti0.994O3) with our DFT model. This was done by introducing Ca and Zr as substitutional defects on the A- and B-sublattices of BT. As a preliminary step, we started by substituting only one Ti ion with a Zr to discern the contribution of the dopants. Despite the larger radius of the Zr ion with respect to Ti, the substitution did not cause significant changes in the lattice dimension. Further, we replaced six Ba ions with Ca on the A-lattice. For that, we screened multiple Ca arrangements of various shapes, seeking to determine the most favorable configuration of Ca and Zr substitutional elements. Ca ions prefer to sit on neighboring A-sites, forming a planar arrangement oriented along the polar axis. This ion arrangement is more stable when it is displaced away from the Zr ion site. From a set of optimized structures featuring this optimal arrangement of Ca ions, we investigated the local atomic environment of Zr and Ca dopants in terms of atomic distances, atomic displacements, and the resulting spontaneous polarization (Ps). The latter was measured via both the Berry Phase (BP) and Born Effective Charges (BEC) method. The first method provides us with a reference value for the overall material polarization, while the other allows us to quantify the polarization contribution derived from each atom type. Interestingly, the Zr ion does not induce a major distortion on the local environment around it and shows negligible displacement from the centrosymmetric position. On the other hand, Ca ions show much larger displacements compared to the larger Ba. With this atomic insight, we were able to identify the Ca substitution as being the main cause for enhancing the Ps with respect to BT. In Chapter 4, we systematically modeled compositions of BST with increasing Sn content ranging between 0 and 12.5%. The research focused on the influence of the Sn cation concentration and ordering at the B-site of BT on both the spontaneous polarization Ps and the bandgap Eg. We chose to model the rhombohedral phase because, according to the BST phase diagram, it is the phase that spans the whole range of tin concentrations under examination. In this way, we could coherently reproduce four compositions, namely 0, 4.17, 8.33, and 12.5%, within the same computational setup with one supercell size by substituting 0, 1, 2, and 3 Sn cations on the BT B-lattice, respectively. As we endeavored to calculate the crystal Ps, we adopted a hexagonal lattice representation for the rhombohedral phase to align the polarization orientation [111] along an easier-to-handle z-axis. We performed an extensive screening of Sn distribution on the B-site for the structures featuring 2 and 3 Sn cations to determine the most stable arrangements. For both structures, this screening revealed that the Sn ions prefer to sit as 2nd neighbors with respect to each other along the Ti-O-Ti bond orientation. Given the larger radius of the Sn ion with respect to Ti, the lattice expanded monotonously with increasing Sn content, as also confirmed by the XRD experimental results. Band structure calculations revealed that the dopant minimally impacts the band gap for the whole range of concentration. We refined the electronic investigation, getting insight into the defect state energy in the electronic structure via pDOS analysis. It turns out that Sn generates only states deep in the valence band region at -6 eV and, therefore, does not affect the band edges. Furthermore, we calculated the polarization for each structure, coupling the BEC and BP methods in a similar manner as we did for BCZT. As a result of both methods, the increasing Sn content leads to an increasing drop of Ps, losing up to 25% of the initial BT value at 12.5% Sn concentration. In Chapter 5, we build an advanced model for reproducing a composite system constituted by a photoactive QD of CsPbBr3 (CPB), embedded in a CsBr (CB) matrix. The focus of this work is modeling a section of the atomic interface between a CsPbBr3 and CsBr crystal plane and investigating how the chemical composition of planar defect perturbs the quality and nature of the embedded QDs photoresponse at room temperature. The design process of a realistic computational model for the interface structure passed through several refinement steps and challenges. Firstly, given the framework of periodic boundary condition calculations, we sought the best compromise between the crystal lattice dimensions and the size of the supercell, which minimizes the artificial lattice mismatch resulting from simulating two different crystal planes within the same simulation box. We found that the (001) surface of both CPB and CB crystals in the cubic phase were compatible for being modeled within the same supercell area dimension. Secondly, we performed a qualitative screening of the thermodynamic landscape of potential surface matches among the PbBr2 and CsBr terminations constituting the crystals’ surfaces, gaining useful insight for further refinement. Interestingly, among all the matching cases, the PbBr2-terminated surface resulted in the most stable interface structures, showing a higher chemical reactivity. Based on the insight from this preliminary screening, we generated additional interface structures, each characterized by non-trivial atom combination within the interface, broadening the set of interface structure samples. At this point, we refined the accuracy of the model by employing the room-temperature orthorhombic phase of CPB. Furthermore, we evaluated the thermodynamic stability of each interface structure. From the acquired phase diagram, we detected the most stable interfaces resulting from different synthesis conditions and evaluated the electronic properties at the interface. The band edge alignment at the CPB/CB interfaces allows us to forecast two scenarios for the composite system’s photoresponse: when the interface is synthesized under PbBr2-rich conditions, it is expected to maintain optimal conditions for photoluminescence and related optoelectronic applications. Conversely, under CsBr-rich synthesizing conditions, the interfaces are expected to show a low Schottky barrier for holes, making it employable for photovoltaic applications. Chapter 6 presents a research work carried out in collaboration with the research group of Dr. Frank Wendler on the topic of domain walls (DWs) motion evolution. With the use of the Phase-Field Model (PFM), they simulate the polarization domain growth in BT bulk crystals according to the thermodynamic driving forces, featuring the material at the mesoscale level. Given the ferroelectric nature of BT perovskites, it is expected that the photostrictive effect can play a critical role in driving the DW motion, and therefore, they aim to implement this effect in their PFM. In this regard, they are modeling the effect of light on the system’s thermodynamics in terms of the model order parameters Ps and strain. However, the coefficients required to describe the photo-mechanical response of BT, i.e., the photo-induced stress and the photo-induced electric field, are derivable only from the material electronic structure and, thus, only via the first-principles calculations. In this context, we performed electronic structure calculations on distorted BT geometries, either strained or sheared. From the dependencies of the resulting bandgap values with respect to Ps and strain, we derived and provided these system-specific coefficients as an input for the PFM.