arXiv subjects

Davide Moia

Publications and source records attributed to Davide Moia.

10 recordsLinked to original sources

arXiv2025

Relating the dynamics of photo de-mixing in mixed bromide-iodide perovskites to ionic and electronic transport

The observation of reversible de-mixing phenomena in mixed-halide perovskites under illumination is one of the most challenging as well as intriguing aspects of this class of materials. On the one hand, it poses critical constraints to the compositional space that allows reliable design of absorbers for perovskite photovoltaics. On the other hand, it holds potential for the development of novel optoionic devices where an ionic response is triggered via optical stimuli. Funda-mental questions about the origin of such photo de-mixing process remain unanswered, both in terms of its mechanism as well as thermodynamic description. Here, we relate in-situ measurements of ionic and electronic transport of mixed bromide-iodide perovskite thin films performed during photo de-mixing with the evolution of their optical and morpho-logical properties. The results point to the definition of different stages of the de-mixing process which, based on micros-copy and spectroscopic measurements, we assign to regimes of spinodal decomposition and nucleation of quasi-equilibrium iodide- and bromide-rich phases. Combined with density functional theory calculations, we explore the role of dimensionality in the mechanism and reversibility of photo de-mixing and dark re-mixing processes, referring to elec-tronic and ionic contributions to the de-mixing driving force. Additionally, our data emphasizes the role of the surface, as significantly different de-mixing dynamics, in terms of extent and reversibility, are observed for films with or without encapsulation. Our comprehensive analysis of transport, phase and optical properties of mixed-halide perovskites pro-vides guidelines for future materials design as well as for the more general fundamental understanding of light-induced ionic phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Defect chemistry of mixed ionic-electronic conductors under light: halide perovskites as master example

Shining light on a mixed ionic-electronic conductor induces variations in both its electronic and ionic behaviors. While optoelectronic processes in semiconductors with negligible ionic conductivities are well understood, the role of mobile ions in photo-active mixed conductors, such as hybrid halide perovskites, is largely unexplored. Here, we propose a model addressing this problem, relating optoelectronics and optoionics. Using methylammonium lead iodide (MAPI) as model material, we discuss the expected influence of optical bias on the material's charge carrier chemistry under steady-state conditions. We show that changes in the concentration of ionic defects under light with respect to the dark case are a direct consequence of their coupling to electrons and holes through the component chemical potential (here iodine) and the electroneutrality condition. Based on the trend in the quasi-Fermi level splitting in MAPI, we emphasize implications of controlling point defect chemistry for the function and performance optimization of solar energy conversion devices based on halide perovskites. Lastly, we show that in the presence of multiple redox reactions mediating the iodine quasi-equilibrium, either positive or negative changes in the ionic defect pair chemical potential can be obtained. These findings indicate the intriguing possibility to increase or to reduce ionic defect concentrations in mixed conductors through exposure to light.

physics.chem-ph↗

arXiv2024

Equivalent circuit modeling of electron-hole recombination in semiconductor and mixed ionic-electronic conductor based devices

The understanding and optimization of solar energy conversion and light emitting devices can greatly benefit from equivalent circuit models describing their response. However, a general model of electron-hole recombination in semiconductors is currently missing. This study presents equivalent circuit models of radiative and non-radiative electron-hole recombination based on their linearized analytical treatment. These are integrated in a circuit model of complete devices that is equivalent to the linearized drift-diffusion equations in one dimension. The analysis of the model shows that, for most situations involving semiconductors without mobile ions, approximated models that do not account for local electrostatics are sufficient to describe the contribution of recombination to the device response. The influence of local electrostatics on non-radiative trap-mediated and Auger recombination becomes essential in mixed conducting devices, and it should be included explicitly in equivalent circuit models. Resistors used for traditional semiconductors are indeed a special case of the general model of non-radiative recombination, for which transistors gated by the local change in electrostatic potential implement a more accurate representation. For mixed conducting devices, such as halide perovskite solar cells, appropriate simplifications of the complete model provide analytical solutions describing the bulk and interfacial polarization effects that influence the small signal electrostatics, recombination currents and overall impedance. The resulting analysis is relevant to a wide range of materials and devices used for solar energy conversion as well as other optoelectronic and photo-electrochemical applications.

physics.app-ph↗

arXiv2022

The dynamics of internal electric field screening in hybrid perovskite solar cells probed using electroabsorption

Electric fields arising from the distribution of charge in metal halide perovskite solar cells are critical for understanding the many weird and wonderful optoelectronic properties displayed by these devices. Mobile ionic defects are thought to accumulate at interfaces to screen electric fields within the bulk of the perovskite semiconductor on application of external bias, but tools are needed to directly probe the dynamics of the electric field in this process. Here we show that electroabsorption measurements allow the electric field within the active layer to be tracked as a function of frequency or time. The magnitude of the electroabsorption signal, corresponding to the strength of the electric field in the perovskite layer, falls off for externally applied low frequency voltages or at long times following voltage steps. Our observations are consistent with drift-diffusion simulations, impedance spectroscopy, and transient photocurrent measurements. They indicate charge screening/redistribution on time-scales ranging from 10 ms to 100 s depending on the device interlayer material, perovskite composition, dominant charged defect, and illumination conditions. The method can be performed on typical solar cell structures and has potential to become a routine characterization tool for optimizing hybrid perovskite devices.

physics.app-ph↗

arXiv2021

Photo de-mixing in Dion-Jacobson two-dimensional mixed halide perovskites

Two-dimensional (2D) halide perovskites feature a versatile structure, which not only enables the fine-tuning of their optoelectronic properties but also makes them appealing as model systems to investigate the fundamental properties of hybrid perovskites. In this study, we analyzed the changes in the optical absorption of 2D Dion-Jacobson mixed halide perovskite thin films (encapsulated) based on (PDMA)Pb(I0.5Br0.5)4 (PDMA: 1,4-phenylenedimethanammonium spacer) exposed to a constant illumination. We demonstrate that these 2D mixed-halide perovskites undergo photo de-mixing with direct transformation from the pristine phase to the de-mixed phases. Almost complete re-mixing of these phases occurs when the sample is left in the dark, showing that the process is reversible in terms of optical properties. On the other hand, exposure to light appears to induce structural changes in the thin film that are not reversible in the dark. We have further investigated temperature-dependent absorption measurements under light to extract the photo de-mixed compositions and to map the photo-miscibility-gap. This work thereby reveals that photo de-mixing occurs in Dion-Jacobson two-dimensional hybrid perovskites and provides strategies to address the role of light in the thermodynamic properties of these materials.

physics.chem-ph↗

arXiv2020

Design and evaluation of conjugated polymers with polar side chains as electrode materials for electrochemical energy storage in aqueous electrolytes

We report the development of redox-active conjugated polymers with potential application to electrochemical energy storage. Side chain engineering enables processing of the polymer electrodes from solution, stability in aqueous electrolytes and efficient transport of ionic and electronic charge carriers. We synthesized a 3,3' dialkoxybithiophene homo polymer (p type polymer) with glycol side chains and prepared naphthalene 1,4,5,8-tetracarboxylic-diimide-dialkoxybithiophene (NDI gT2) copolymers (n type polymer) with either a glycol or zwitterionic side chain on the NDI unit. For the latter, we developed a post-functionalization synthesis to attach the polar zwitterion side chains to the polymer backbone to avoid challenges of purifying polar intermediates. We demonstrate fast and reversible charging of solution processed electrodes for both the p- and n type polymers in aqueous electrolytes, without using additives or porous scaffolds and for films up to micrometers thick. We apply spectroelectrochemistry as an in operando technique to probe the state of charge of the electrodes. This reveals that thin films of the p-type polymer and zwitterion n-type polymer can be charged reversibly with up to two electronic charges per repeat unit (bipolaron formation). We combine thin films of these polymers in a two-electrode cell and demonstrate output voltages of up to 1.4 V with high redox stability. Our findings demonstrate the potential of functionalizing conjugated polymers with appropriate polar side chains to improve specific capacity, reversibility and rate capabilities of polymer electrodes in aqueous electrolytes.

physics.app-ph↗

arXiv2019

Photo-effect on ion transport in mixed cation and halide perovskites and implications for photo de-mixing

Organic-inorganic hybrid perovskites are considered to be most promising photovoltaic materials. Highest efficiencies of perovskite solar cells have been achieved by using appropriate cation and anion mixtures. Mixed perovskite solar cells also show an improved stability. For both performance as well as stability, experimental information on electronic and ionic charge carriers is key, an information that so far has only been provided for methylammonium lead iodide; there we also found that light can enhance not only electronic but also ionic conductivities by more than one order of magnitude. We also proposed a mechanism for this surprising photo-ionic effect and explained its impact on photo-decomposition. Here we quantitatively deconvolute ionic and electronic transport properties for the practically relevant substitutions and mixtures. Specifically, we investigate various cation and anion substitutions (Cs; FA; Br) with a special eye on their photo-ionic effect. The results are not only of importance for light-induced degradation but also for light-induced demixing. As far as the photo-ionic effect is concerned, we find that the choice of the halide is of crucial importance, while the cationic substitutions are less relevant. The huge ionic conductivity enhancement found for iodide perovskites, is weakened by bromide substitution and eventually becomes insignificant for the pure bromide. Based on these experimental results, we provide a rationale for the experimentally observed photo-demixing.

physics.chem-ph↗

arXiv2019

Ionically generated built in equilibrium space charge zones -- a paradigm change for lead halide perovskite interfaces

Methylammonium lead iodide (MAPI) is the archetype of the intensively researched class of perovskites for photovoltaics. Nonetheless, even equilibrium aspects are far from being fully understood. Here we discuss equilibrium space charge effects at the MAPI/TiO2 and MAPI/Al2O3 interfaces, which are of paramount significance for solar cells. Different from the photovoltaic literature in which such built-in potentials are considered as being generated solely by electronic charge carriers, we will apply a generalized picture that considers the equilibrium distribution of both ionic and electronic carriers. We give experimental evidences that it is the ions that are responsible for the equilibrium space charge potential in MAPI, the reason being a pronounced ion adsorption at the contacts. The occurrence of equilibrium space charge effects generated by ionic redistribution has not been considered for photovoltaic materials and as such provides a novel path for modifying charge-selective interfaces in solar cells, as well as a better understanding of the behavior in mesoporous systems.

physics.chem-ph↗

arXiv2019

Ionic-to-electronic current amplification in hybrid perovskite solar cells: ionically gated transistor-interface circuit model explains hysteresis and impedance of mixed conducting devices

Mobile ions in hybrid perovskite semiconductors introduce a new degree of freedom to electronic devices suggesting applications beyond photovoltaics. An intuitive device model describing the interplay between ionic and electronic charge transfer is needed to unlock the full potential of the technology. We describe the perovskite-contact interfaces as transistors which couple ionic charge redistribution to energetic barriers controlling electronic injection and recombination. This reveals an amplification factor between the out of phase electronic current and the ionic current. Our findings suggest a strategy to design thin film electronic components with large, tuneable, capacitor-like and inductor-like characteristics. The resulting simple equivalent circuit model, which we verified with time-dependent drift-diffusion simulations of measured impedance spectra, allows a general description and interpretation of perovskite solar cell behaviour.

physics.app-ph↗

arXiv2017

A salt water battery with high stability and charging rates made from solution processed conjugated polymers with polar side chains

We report a neutral salt water based battery which uses p-type and n-type solution processed polymer films as the cathode and the anode of the cell. The specific capacity of the electrodes (approximately 30 mAh cm-3) is achieved via formation of bipolarons in both the p-type and n-type polymers. By engineering ethylene glycol and zwitterion based side chains attached to the polymer backbone we facilitate rapid ion transport through the non-porous polymer films. This, combined with efficient transport of electronic charge via the conjugated polymer backbones, allowed the films to maintain constant capacity at high charge and discharge rates (>1000 C-rate). The electrodes also show good stability during electrochemical cycling (less than 30% decrease in capacity over >1000 cycles) and an output voltage up to 1.4 V. The performance of these semiconducting polymers with polar side-chains demonstrates the potential of this material class for fast-charging, water based electrochemical energy storage devices.

physics.app-ph↗