Anharmonic lattice dynamics via the special displacement method

On the basis of the self-consistent phonon theory and the special displacement method, we develop a new approach for the treatment of anharmonicity in solids. We show that this approach enables the efficient calculation of temperature-dependent anharmonic phonon dispersions, requiring very few steps to achieve minimization of the system's free energy. We demonstrate this methodology in the regime of strongly anharmonic materials which exhibit a multi-well potential energy surface, like cubic SrTiO$_3$, CsPbBr$_3$, CsPbI$_3$, CsSnI$_3$, and Zr. Our results are in good agreement with experiments and previous first-principles studies relying on perturbative, stochastic nonperturbative, and molecular dynamics simulations. We achieve a very robust workflow by using harmonic phonons of the polymorphous ground state as the starting point and an iterative mixing scheme of the dynamical matrix. Given the simplicity, efficiency, and stability of the present treatment to anharmonicity, it is especially suitable for use with any electronic structure code and for investigating electron-phonon couplings in strongly anharmonic systems.Comment: 10 figure

Perovskite-perovskite tandem photovoltaics with optimized bandgaps

We demonstrate four and two-terminal perovskite-perovskite tandem solar cells with ideally matched bandgaps. We develop an infrared absorbing 1.2eV bandgap perovskite, $FA_{0.75}Cs_{0.25}Sn_{0.5}Pb_{0.5}I_3$, that can deliver 14.8 % efficiency. By combining this material with a wider bandgap $FA_{0.83}Cs_{0.17}Pb(I_{0.5}Br_{0.5})_3$ material, we reach monolithic two terminal tandem efficiencies of 17.0 % with over 1.65 volts open-circuit voltage. We also make mechanically stacked four terminal tandem cells and obtain 20.3 % efficiency. Crucially, we find that our infrared absorbing perovskite cells exhibit excellent thermal and atmospheric stability, unprecedented for Sn based perovskites. This device architecture and materials set will enable 'all perovskite' thin film solar cells to reach the highest efficiencies in the long term at the lowest costs

Surface and optical properties of phase-pure silver iodobismuthate nanocrystals

The study of surface defects is one of the forefronts of halide perovskite research. In the nanoscale regime, where the surface-to-volume ratio is high, the surface plays a key role in determining the electronic properties of perovskites. Perovskite-inspired silver iodobismuthates are promising photovoltaic absorbers. Herein, we demonstrate the colloidal synthesis of phase pure and highly crystalline AgBiI4 nanocrystals (NCs). Surface-sensitive spectroscopic techniques reveal the rich surface features of the NCs that enable their impressive long-term environmental and thermal stabilities. Notably, the surface termination and its passivation effects on the electronic properties of AgBiI4 are investigated. Our atomistic simulations suggest that a bismuth iodide-rich surface, as in the case of AgBiI4 NCs, does not introduce surface trap states within the band gap region of AgBiI4, unlike a silver iodide-rich surface. These findings may encourage the investigation of surfaces of other lead-free perovskite-inspired materials.publishedVersionPeer reviewe

Crystallographic, Optical, and Electronic Properties of the Cs2AgBi1–xInxBr6 Double Perovskite: Understanding the Fundamental Photovoltaic Efficiency Challenges

We present a crystallographic and optoelectronic study of the double perovskite Cs2AgBi1–xInxBr6. From structural characterization we determine that the indium cation shrinks the lattice and shifts the cubic-to-tetragonal phase transition point to lower temperatures. The absorption onset is shifted to shorter wavelengths upon increasing the indium content, leading to wider band gaps, which we rationalize through first-principles band structure calculations. Despite the unfavorable band gap shift, we observe an enhancement in the steady-state photoluminescence intensity, and n-i-p photovoltaic devices present short-circuit current greater than that of neat Cs2AgBiBr6 devices. In order to evaluate the prospects of this material as a solar absorber, we combine accurate absorption measurements with thermodynamic modeling and identify the fundamental limitations of this system. Provided radiative efficiency can be increased and the choice of charge extraction layers are specifically improved, this material could prove to be a useful wide band gap solar absorber

Theoretical study of organic semiconductors, interaction with inorganic material for the formation of heterostructures and properties

In this dissertation first-principles calculations are applied to study organic semiconductors (OS) and their interaction with inorganic materials. At first, different state-of-the-art Density Functional Theory (DFT) based approaches were used to relax and calculate the electronic properties of crystal structures formed by OS, such as members of the oligo-acenes family, the polymer P3HT, C60 fullerene and PC60BM. Regarding the oligo-acenes, the relation between the length of the molecule’s long-axis and the electronic band gap (Eg) is found. It is well known that C60 fullerenes adopt a face centered cubic (FCC) lattice. In contrast, PC60BM’s crystal structure is still not known. The calculations revealed that PC60BM’s stable molecular crystal structure is the simple cubic (SC), just slightly more favorable than the body-centered cubic (BCC). The enhanced stability of the SC and the BCC lattice is associated with the presence of a hydrogen bond between neighboring molecules. P3HT’s structure was modeled and the optical and electronics properties were calculated. Strong absorption peaks are found along the polythiophene backbone, while along the direction of the alkyl side-chains the absorption begins for energies higher than 4 eV. Next, continuous transformations on the molecular crystals of C60 and PC60BM were applied. Regarding the crystal of C60, the details of polymerization are analyzed, and the ideal strength is obtained. For PC60BM, several local energy-minimum structures that differ significantly in terms of molecular conformations and electronic properties are identified. Another part of this dissertation deals with the electronic effects introduced by oxygen and water on the properties of OS. Several different types of configurations are shown including intact molecules in crystalline voids to oxidized P3HT and PC60BM crystals. Furthermore, a number of impurity configurations are found to create states within the energy band gaps of the materials, creating thus shallow and deep carrier traps on both semiconductors. H2O, on the other hand, is found to adsorb intact in both semiconductors and is associated with a shallow acceptor-like trap in PC60BM and minimal effects in P3HT. Finally, the vibrational properties of defective pentacene crystal configurations are studied. The impurity-related frequencies of the vibrations are calculated giving a way for the identification of the defects in experiments. The last part of this dissertation deals with the interactions between OS and inorganic metal surface of silver and gold. PC60BM can adsorb on silver with different orientations, but slightly prefers to adsorb with the C60 part towards the surface. At the same time, the work function (Wf) modification is found to depend strongly on the tail orientation. Specifically, the favorable configuration increases the work function, while for adsorption with the ‘tail’ towards the surface lowers it. Adsorption on silver with the ‘tail’ parallel to the surface, practically doesn’t change the Wf, while adsorption on gold with the similar structural details, lowers gold’s Wf. Finally, P3HT adsorption on either surface, leads to a decrease of the Wf, which is found larger the case of gold.Στην διατριβή αυτή χρησιμοποιήθηκαν κβαντομηχανικοί υπολογισμοί πρώτων αρχών για την μελέτη πρότυπων οργανικών ημιαγώγιμων υλικών. Αρχικά, εξετάστηκαν οι δομικές λεπτομέρειες των μοριακών κρυσταλλικών δομών που σχηματίζουν οργανικοί ημιαγωγοί διαφόρων ειδών. Για όλα αυτά τα οργανικά συστήματα υπολογίστηκαν οι σταθερές, ενεργειακά προτιμητέες κρυσταλλικές τους δομές. Συμπερασματικά, βρέθηκε η σχέση μεταξύ του μήκους των ακενίων και του ενεργειακού τους χάσματος καθώς επίσης εντοπίστηκαν διαφορές στην ηλεκτρονική απόκριση των πολυμόρφων του πεντακενίου. Για το PC60BM η ενεργειακά προτιμητέα κρυσταλλική κυβική δομή είναι η απλή κυβική (SC), λίγο χαμηλότερα ενεργειακά από την ενδοκεντρωμένη (BCC). Αυτή η ενεργειακή προτίμηση των SC και BCC δομών συσχετίστηκε με την ύπαρξη δεσμού υδρογόνου μεταξύ των γειτονικών μορίων, δεσμός που απουσιάζει στην FCC δομή. Για το πολυμερές P3HT, υπολογίστηκε η ηλεκτρονική toy απόκριση. Τα αποτελέσματα δείχνουν έντονη ανισοτροπία στην διηλεκτρική συνάρτηση ε(ω), με δύο έντονες απορροφήσεις στην ε(ω) κατά μήκος των αλυσίδων. Kατά μήκος των πλευρικών αλκυλομάδων, εμφανίζεται χαμηλότερη συνεισφορά στην ε(ω) μόνο σε ενέργειες υψηλότερες από 4 eV. Στη συνέχεια, υπολογίστηκαν οι συνεχείς μετασχηματισμοί των μοριακών κρυστάλλων του C60 φουλερενίου και του παράγωγου τους PC60BM. Για το C60 κατά μήκος των μετασχηματισμών εντοπίστηκε ένα νέο πολύμορφο με BCC πλέγμα και επιπλέον βρέθηκαν πολυμερικές δομές του C60, όπου τα μόρια έχουν συνδεθεί μεταξύ τους με ομοιοπολικούς δεσμούς. Για το PC60BM, σύμφωνα με τους υπολογισμούς, αποδείχθηκε η ύπαρξη πολλών διαφορετικών διαδρομών κατά μήκος των μετασχηματισμών ανάλογα με την θέση της επιπρόσθετης χημικής ομάδας (‘ουράς’). To αποτέλεσμα αυτό καταδεικνύει τον σημαντικό ρόλο της ‘ουράς’ του μορίου κατά τον σχηματισμό των μοριακών κρυστάλλων. Επιπλέον, εντοπίστηκαν οι διαφορές στις ηλεκτρονικές ιδιότητες που εμφανίζουν οι δομές κατά μήκος των μετασχηματισμών. Ακολούθως, εξετάστηκε η επίδραση πρότυπων ατελειών (H2O και O2) στους οργανικούς ημιαγωγούς PC60BM και P3HT. Βρέθηκε ότι το O2 στο PC60BM και στο P3HT σχηματίζει διάφορες σταθερές δομές, είτε με την χημική σύνδεση του με τα μόρια (chemisorption) είτε χωρίς αυτή (physisorption). Αναλόγως της σύνδεσης του μπορεί να δημιουργούνται στάθμες στο εσωτερικό του ενεργειακού χάσματος. Αυτές οι στάθμες λειτουργούν ως παγίδες των φορέων αγωγιμότητας του κάθε ημιαγωγού. Λιγότερη επίδραση φαίνεται να έχει το H2O, το οποίο εισέρχεται στο εσωτερικό των κρυστάλλων, δεν σχηματίζει δεσμούς με τα υλικά και έχει πρακτικά μηδενική επίδραση στις ιδιότητες του P3HT, ενώ στο PC60BM σχηματίζει μια στάθμη κοντά στη ταινία σθένους. Για την περίπτωση ατελειών στο πεντακένιο τα αποτελέσματα περιλαμβάνουν τις χαρακτηριστικές συχνότητες δόνησης των ατελειών οι οποίες σε συνδυασμό με κατάλληλα πειραματικά δεδομένα και τεχνικές, μπορούν να χρησιμοποιηθούν για τον έλεγχο ύπαρξης ατελειών. Τέλος, μελετήθηκε η αλληλεπίδραση οργανικών ημιαγωγών με τις ανόργανες μεταλλικές επιφάνειες του αργύρου και του χρυσού. Εξετάστηκε η πιθανότητα προσρόφησης του PC60BM στην επιφάνεια του αργύρου με διαφορετική τοποθέτηση της ‘ουράς’ και βρέθηκε ότι αναλόγως με τον τρόπο σύνδεσης με την επιφάνεια, αλλάζει και η επίδραση στο έργο εξόδου του συστήματος. Επίσης, μελετήθηκε η προσρόφηση μιας αλυσίδας P3HT στις δύο μεταλλικές επιφάνειες και βρέθηκε η προτιμητέα σύνδεση. Ισχυρότερη ενέργεια σύνδεσης εμφανίζει η σύνδεση με την επιφάνεια του χρυσού, ενώ μετά την προσρόφηση και στις δύο επιφάνειες το έργο εξόδου βρέθηκε μικρότερο

Phonon-Limited Mobility and Electron-Phonon Coupling in Lead-Free Halide Double Perovskites

International audienceLead-free halide double perovskites have attracted considerable attention as complements to lead-based halide perovskites in a range of optoelectronic applications. Experiments on Cs(2)AgBiBr(6) indicate carrier mobilities in the range of 0.3-11 cm(2)/(V s) at room temperature, considerably lower than in lead-based perovskites. The origin of low mobilities is currently unclear, calling for an atomic-scale investigation. We report state-of-the-art ab initio calculations of the phonon-limited mobility of charge carriers in lead-free halide double perovskites Cs(2)AgBiX(6) (X = Br, Cl). For Cs(2)AgBiBr(6), we obtain room-temperature electron and hole mobilities of 17 and 14 cm(2)/(V s), respectively, in line with experiments. We demonstrate that the cause for the lower mobility of this compound, compared to CH(3)NH(3)PbI(3), resides in the heavier carrier effective masses. A mode-resolved analysis of scattering rates reveals the predominance of Fröhlich electron-phonon scattering, similar to lead-based perovskites. Our results indicate that, to increase the mobility of lead-free perovskites, it is necessary to reduce the effective masses, for example by cation engineering

Electronic structure and electron-transport properties of three metal hexacyanoferrates

Metal hexacyanometallates, or Prussian blue analogs (PBAs), are active materials in important electrochemical technologies, including next-generation sodium- and potassium- ion batteries. They have tunable properties including reduction potential, ionic conductivity, and color. However, little is known about their electronic conductivities. In this work, we use density-functional theory to model electronic structure and to explore the likely electron-conduction mechanism in three promising cathodes (manganese, iron, and cobalt hexacyanoferrate) in each of three oxidation states. First, we demonstrate that hybrid functionals reliably reproduce experimentally observed spin configurations and geometric phase changes. We confirm these materials are semiconductors or insu- lators with band gaps ranging from 1.90 eV up to 4.94 eV. We further identify that for most of the compounds the electronic band edges originate from carbon-coordinated- iron orbitals, suggesting that doping at the carbon-coordinated site may strongly affect carrier conductivity. Finally, we calculate charge-carrier effective masses, which we find are very heavy. This study is an important foundation for making electronic conductivity a tunable PBA material property

Origin of the High Specific Capacity in Sodium Manganese Hexacyanomanganate br

International audienceSodium manganese hexacyanomanganate, NaxMn[Mn(CN)6], is anelectrochemically active Prussian blue analogue (PBA) that has been studiedexperimentally as an electrode material in rechargeable sodium-ion batteries. It hasa reversible specific capacity of 209 mA h g-1, which is substantially higher than thetheoretical specific capacity of 172 mA h g-1expected for two reduction eventsconventional in PBAs. It has been suggested that the high specific capacityoriginates from this compound's unique ability to insert a third sodium ion performula unit. However, the plausibility of this mechanism has remainedambiguous. Here, we use density functional theory (DFT) with a hybridfunctional to calculate the formation energies of various oxidation states andmagnetic phases of the NaxMn[Mn(CN)6] system. We confirm that the compound Na3MnII[MnI(CN)6]is,indeed,thermodynamically stable. It contains manganese(I), and the sodium ions occupy the interfacial position of the lattice subcubes.We also provide strong evidence that the phase of the fully oxidized Mn[Mn(CN)6] compound is charge-disproportionated,containing manganese(II) and manganese(IV). We proceed to show that the presence of crystalline water increases the reductionpotential of the system and that the hydrated compounds have theoretical crystal geometries and reduction potentials that closelymatch the experiment. This work clarifies the charge-storage mechanism in a well-known but less-understood PB

Anharmonic electron-phonon coupling in ultrasoft and locally disordered perovskites

Abstract Anharmonicity and local disorder (polymorphism) are ubiquitous in perovskite physics, inducing various phenomena observed in scattering and spectroscopy experiments. Several of these phenomena still lack interpretation from first principles since, hitherto, no approach is available to account for anharmonicity and disorder in electron–phonon couplings. Here, relying on the special displacement method, we develop a unified treatment of both and demonstrate that electron–phonon coupling is strongly influenced when we employ polymorphous perovskite networks. We uncover that polymorphism in halide perovskites leads to vibrational dynamics far from the ideal noninteracting phonon picture and drives the gradual change in their band gap around phase transition temperatures. We also clarify that combined band gap corrections arising from disorder, spin-orbit coupling, exchange–correlation functionals of high accuracy, and electron–phonon coupling are all essential. Our findings agree with experiments, suggesting that polymorphism is the key to address pending questions on perovskites’ technological applications