arXiv subjects

Olivier J. J. Ronsin

Publications and source records attributed to Olivier J. J. Ronsin.

15 recordsLinked to original sources

arXiv2026

Predicting Organic Solar Cell Performance and Stability from Fast, Morphology-aware Current-Voltage Modeling

Understanding the relationship between morphology and performance in organic solar cells is essential for developing devices that are both high performing and resilient to aging. This work introduces a unique method capable of calculating the current-voltage (JV) curve of complex heterojunction morphologies containing up to five phases (donor amorphous, donor crystalline, acceptor amorphous, acceptor crystalline, mixed amorphous) with a very low computation time using morphology-aware descriptors of light absorption, exciton dissociation, non-geminate recombination and free charge carrier mobilities. The method is validated against Monte Carlo and 3D drift-diffusion simulations and applied to P3HT:PCBM and PM6:Y6 systems, shedding light on the physical compromises encountered to optimize device performance and lifetime. Finally, we show that the morphology-performance relationship is dependent on the materials system studied.

cond-mat.mtrl-sci↗

arXiv2025

Extending the Flory-Huggins Theory for Crystalline Multicomponent Mixtures

The Flory-Huggins theory is a well-established lattice model that is commonly used to study the mixing of distinct chemical species. It can successfully predict phase separation phenomena in blends of incompatible materials. However, it is limited to amorphous mixtures, excluding systems where the phase segregation is shaped by the concurrent crystallization of one or several blend components. A generalization of the Flory-Huggins formalism is thus necessary to capture the coupling and the interplay of crystallization with amorphous demixing mechanisms, such as spinodal decomposition. This work therefore revolves around the derivation of a free energy model for multicomponent mixtures that encompasses the physics of both processes. It is detailed which concepts from the original Flory-Huggins theory are required to apprehend the presented developments and how the current framework is built upon them. Furthermore, additional discussion points address chemical potential calculations and selected examples of binary and ternary phase diagrams, thereby highlighting the variety of blend behaviors that can be represented.

cond-mat.mtrl-sci↗

arXiv2025

Interplay of Crystallization and Amorphous Spinodal Decomposition during Thermal Annealing of Organic Photoactive Layers

Tailoring the nanomorphology of organic photoactive layers through a specialized chain of processing steps is an imperative challenge on the path towards reliable and performant organic electronic manufacturing. This hurdle generally proves delicate to be overcome, as organic materials can be subject to many different phase transformation phenomena that are able to interfere with each other and produce a wide variety of morphological configurations with distinct structural, mechanical, and optoelectronic properties. A typical combination of such mechanisms, which the present systems are often prone to, and which is complex to investigate experimentally at the nanoscale, is the phase separation resulting from the interplay between amorphous demixing and crystallization. In this work, an in-house Phase-Field modeling framework is employed to simulate and, consequently, explain the phenomenological behavior of a photoactive bulk heterojunction during a thermal annealing treatment. The model predictions are validated against available electron microscopy imaging of the nanostructural evolution during the process. It is demonstrated that the simulations can successfully provide a detailed comprehension of crystal nucleation and growth shaped by amorphous spinodal decomposition, so as to yield valuable insights for physically-based morphology control. In addition, this study shows the relevance of extensive thermodynamic and kinetic characterizations of organic semiconductor mixtures (e.g., phase diagram assessments, surface tension measurements, composition-dependent molecular diffusivity evaluations) for the associated field of research.

cond-mat.mtrl-sci↗

arXiv2025

Understanding the effect of drying time in process-structure-performance relationships for PM6-Y6 organic solar cells

Making solution-cast organic solar cells industrially available generally comes at the cost of significant performance losses compared to device prototypes manufactured under laboratory conditions. Adjusting solvent evaporation kinetics is postulated to recover efficiency. Yet, a comprehensive characterization of their effect, independently of other property-defining parameters, is lacking. Thus, the present objective is to isolate the influence of the solvent drying rate on solution-deposited organic active layer nanomorphologies and performances. To this end, a specially designed gas quenching technique is employed to fabricate PM6:Y6 donor-acceptor films under systematic variations of evaporation conditions. Using an extensive investigation protocol that combines insights from numerical simulations and experimental measurements, process-structure-performance relationships are unraveled. It is found that higher drying rates imply finer and more dispersed nanomorphologies with increased fractions of amorphous material. This enhances electric charge generation, thereby improving short-circuit current density and overall cell performance. The open-circuit voltage is also boosted under accelerated evaporation due to changes in the aggregation mode of the Y6 small molecule that induce higher effective bandgaps. The results demonstrate that the developed gas-quenching technique is a valuable tool for optimizing the performance of upscaled organic photovoltaics, as it is readily compatible with high-throughput equipment, such as roll-to-roll coating machines.

cond-mat.mtrl-sci↗

arXiv2025

Towards a fully differentiable digital twin for solar cells

Maximizing energy yield (EY) - the total electric energy generated by a solar cell within a year at a specific location - is crucial in photovoltaics (PV), especially for emerging technologies. Computational methods provide the necessary insights and guidance for future research. However, existing simulations typically focus on only isolated aspects of solar cells. This lack of consistency highlights the need for a framework unifying all computational levels, from material to cell properties, for accurate prediction and optimization of EY prediction. To address this challenge, a differentiable digital twin, Sol(Di)$^2$T, is introduced to enable comprehensive end-to-end optimization of solar cells. The workflow starts with material properties and morphological processing parameters, followed by optical and electrical simulations. Finally, climatic conditions and geographic location are incorporated to predict the EY. Each step is either intrinsically differentiable or replaced with a machine-learned surrogate model, enabling not only accurate EY prediction but also gradient-based optimization with respect to input parameters. Consequently, Sol(Di)$^2$T extends EY predictions to previously unexplored conditions. Demonstrated for an organic solar cell, the proposed framework marks a significant step towards tailoring solar cells for specific applications while ensuring maximal performance.

physics.comp-ph↗

arXiv2025

Controlled nucleation in methylamine-treated perovskite films by artificial seeding and phase-field simulations

Large perovskite crystals with reduced defect density enable superior charge transport and stability. Therefore, controlling their nucleation and growth is key to advancing high-performance optoelectronic devices based on perovskite semiconductors. Millimeter-scale perovskite crystals can be synthesized as a continuous film through methylamine treatment, with nucleation sites directed by pre-patterned seeds. Nonetheless, certain configurations may lead to unwanted parasitic nucleation. To predict and mitigate this effect, we employ phase-field simulations alongside an analytical model. Their predictive capability is demonstrated across three distinct material-substrate systems, enabling precise control over nucleation and subsequent crystal growth. Notably, the only material-specific input required is the nucleation density (i.e., the number of crystals nucleated per unit area on an unpatterned substrate). This generality makes the models broadly applicable to diverse material systems for achieving controlled two-dimensional crystallization for improved optoelectronic device performance.

cond-mat.mtrl-sci↗

arXiv2025

Taxonomy of amorphous ternary phase diagrams: the importance of interaction parameters

Understanding phase diagrams is essential for material selection and design, as they provide a comprehensive representation of the thermodynamics of mixtures. This work delivers a broad and systematic overview of possible ternary phase diagrams for amorphous systems representative of polymers, small organic molecules, and solvents. Thanks to computationally efficient methods, an unprecedented library of $>$80,000 ternary phase diagrams is generated based on a systematic screening of interaction parameters. Twenty-one phase diagram types, including unreported ones, are identified. They are classified according to simple rules related to the number of immiscible material pairs, of miscibility gaps, and of three-phase regions. They are mapped onto the three-dimensional interaction parameters space, providing a clear picture of their likelihood and existence conditions. Four well-known phase-diagram types with 0, 1, 2, or 3 immiscible pairs are found to be the most likely. The numerous uncommon phase diagrams are mostly observed within a small parameter window around the critical interaction parameter values. For the most common phase diagram types, we show that the size of the processability window becomes sensitive to interaction parameter variations close to critical values. The sensitivity decreases for materials with increasing molar size. Finally, successful comparisons of simulated and experimental phase diagrams showcase the real-world relevance of this theoretical analysis. The presented results lay a robust foundation for rational design of solution processing conditions and for blend morphology control. Immediate applications include organic thin films and the identification of green solvents for sustainable processing.

cond-mat.mtrl-sci↗

arXiv2024

Phase field simulations of thermal annealing for all-small molecule organic solar cells

Interest in organic solar cells (OSCs) is constantly rising in the field of photovoltaic devices. The device performance relies on the bulk heterojunction (BHJ) nanomorphology, which develops during the drying process and additional post-treatment. This work studies the effect of thermal annealing (TA) on an all-small molecule DRCN5T: PC71 BM blend with phase field simulations. The objective is to determine the physical phenomena driving the evolution of the BHJ morphology for a better understanding of the posttreatment/morphology relationship. Phase-field simulation results are used to investigate the impact on the final BHJ morphology of the DRCN5T crystallization-related mechanisms, including nucleation, growth, crystal stability, impingement, grain coarsening, and Ostwald ripening, of the amorphous-amorphous phase separation (AAPS), and of diffusion limitations. The comparison of simulation results with experimental data shows that the morphological evolution of the BHJ under TA is dominated by dissolution of the smallest, unstable DRCN5T crystals and anisotropic growth of the largest crystals.

cond-mat.mtrl-sci↗

arXiv2023

Crystalline Morphology Formation in Phase-Field Simulations of Binary Mixtures

Understanding the morphology formation process of solution-cast photoactive layers (PALs) is crucial to derive design rules for optimized and reliable third generation solar cell fabrication. For this purpose, a Phase-Field (PF) computational framework dedicated to the simulation of PAL processing has recently been developed. In this study focused on non-evaporating, crystallizing binary mixtures, distinct crystalline morphology formation pathways are characterized by a systematic exploration of the model's parameter space. It is identified how, depending on material properties, regular, dilution-enhanced, diffusion-limited and demixing-assisted crystallization can take place, and which associated structures then arise. A comprehensive description of the thermodynamic and kinetic mechanisms that respectively drive these separate crystallization modes is provided. Finally, comparisons with experimental results reported in the literature highlight the promising potential of PF simulations to support the determination of process-structure relationships for improved PAL production.

cond-mat.mtrl-sci↗

arXiv2022

Phase-field simulations of the morphology formation in evaporating crystalline multicomponent films

In numerous solution-processed thin films, a complex morphology resulting from liquid-liquid phase separation (LLPS) or from polycrystallization arises during the drying or subsequent processing steps. The morphology has a strong influence on the performance of the final device but unfortunately the process-structure relationship is often poorly and only qualitatively understood. This is because many different physical mechanisms (miscibility, evaporation, crystallization, diffusion, advection) are active at potentially different time scales, and because the kinetics plays a crucial role: the morphology develops until it is kinetically quenched far from equilibrium. In order to unravel the various possible structure formation pathways, we propose a unified theoretical framework that takes into account all these physical phenomena. This phase-field simulation tool is based on the Cahn-Hilliard equations for diffusion and the Allen-Cahn equation for crystallization and evaporation, which are coupled to the equations for the dynamics of the fluid. We discuss and verify the behavior of the coupled model based on simple test cases. Furthermore, we illustrate how this framework allows to investigate the morphology formation in a drying film undergoing evaporation-induced LLPS and crystallization, which is typically a situation encountered, e.g., in organic photovoltaics applications.

physics.app-ph↗

arXiv2021

Phase-field simulation of liquid-vapor equilibrium and evaporation of fluid mixtures

In solution-processing of thin films, the material layer is deposited from a solution composed of several solutes and solvents. The final morphology and hence the properties of the film often depend on the time needed for the evaporation of the solvents. This is typically the case for organic photoactive or electronic layers. Therefore, it is important to be able to predict the evaporation kinetics of such mixtures. We propose here a new phase-field model for the simulation of evaporating fluid mixtures and simulate their evaporation kinetics. Similar to the Hertz-Knudsen theory, the local liquid-vapor equilibrium is assumed to be reached at the film surface and evaporation is driven by diffusion away from this gas layer. In the situation where the evaporation is purely driven by the liquid-vapor equilibrium, the simulations match the behavior expected theoretically from the free energy: for evaporation of pure solvents, the evaporation rate is constant and proportional to the vaporpressure. For mixtures, the evaporation rate is in general strongly time-dependent because of the changing composition of the film. Nevertheless, for highly non-ideal mixtures, such as poorly compatible fluids or polymer solutions, the evaporation rate becomes almost constant in the limit of low Biot numbers. The results of the simulation have been successfully compared to experiments on a polystyrene-toluene mixture. The model allows to take into account deformations of the liquid-vapor interface and therefore to simulate film roughness or dewetting.

physics.app-ph↗

arXiv2021

Two-dimensional Cahn-Hilliard simulations for coarsening kinetics of spinodal decomposition in binary mixtures

The evolution of the microstructure due to spinodal decomposition in phase separated mixtures has a strong impact on the final material properties. In the late stage of coarsening, the system is characterized by the growth of a single characteristic length scale $L\sim C t^α$. To understand the structure-property relationship, the knowledge of the coarsening exponent $α$ and the coarsening rate constant $C$ is mandatory. Since the existing literature is not entirely consistent, we perform phase field simulations based on the Cahn-Hilliard equation. We restrict ourselves to binary mixtures using a symmetric Flory-Huggins free energy and a constant mobility term and show that the coarsening for off-critical mixtures is slower than the expected $t^{1/3}$-growth. Instead, we find $α$ to be dependent on the mixture composition and thus from the morphology. Finally, we propose a model to describe the complete coarsening kinetics including the rate constant $C$.

cond-mat.soft↗

arXiv2020

A phase-field model for the evaporation of thin film mixtures

The performance of solution-processed solar cells strongly depends on the geometrical structure and roughness of the photovoltaic layers formed during film drying. During the drying process, the interplay of crystallization and liquid-liquid demixing leads to the structure formation on the nano- and microscale and to the final rough film. In order to better understand how the film structure can be improved by process engineering, we aim at theoretically investigating these systems by means of phase-field simulations. We introduce an evaporation model based on the Cahn-Hilliard equation for the evolution of the fluid concentrations coupled to the Allen-Cahn equation for the liquid-vapour phase transformation. We demonstrate its ability to match the experimentally measured drying kinetics and study the impact of the parameters of our model. Furthermore, the evaporation of solvent blends and solvent-vapour annealing are investigated. The dry film roughness emerges naturally from our set of equations, as illustrated through preliminary simulations of spinodal decomposition and film drying on structured substrates.

physics.app-ph↗

arXiv2020

Role of the interplay between spinodal decomposition and crystal growth in the morphological evolution of crystalline bulk heterojunctions

The stability of organic solar cells is strongly affected by the morphology of the photoactive layers, whose separated crystalline and/or amorphous phases are kinetically quenched far from their thermodynamic equilibrium during the production process. The evolution of these structures during the lifetime of the cell remains poorly understood. In this paper, a phase-field simulation framework is proposed, handling liquid-liquid demixing and polycrystalline growth at the same time in order to investigate the evolution of crystalline immiscible binary systems. We find that initially, the nuclei trigger the spinodal decomposition, while the growing crystals quench the phase coarsening in the amorphous mixture. Conversely, the separated liquid phases guide the crystal growth along the domains of high concentration. It is also demonstrated that with a higher crystallization rate, in the final morphology, single crystals are more structured and form percolating pathways for each material with smaller lateral dimensions.

cond-mat.mtrl-sci↗

arXiv2019

Strict equivalence between Maxwell-Stefan and fast-mode theory for multicomponent polymer mixtures

The applicability of theories describing the kinetic evolution of fluid mixtures depends on the underlying physical assumptions. The Maxwell-Stefan equations, widely used for miscible fluids, express forces depending on coupled fluxes. They need to be inverted to recover a Fickian form which is generally impossible analytically. Moreover, the concentration dependence of the diffusivities has to be modelled, e.g. by the multicomponent Darken equation. Cahn-Hilliard type equations are preferred for immiscible mixtures, whereby different assumptions on the coupling of fluxes lead to the slow-mode and fast-mode theories. For two components, these were derived from the Maxwell-Stefan theory in the past. Here, we prove that the fast-mode theory and the generalized Maxwell-Stefan theory together with the multicomponent Darken equation are strictly equivalent even for multicomponent systems with very different molecular sizes. Our findings allow to reduce the choice of a suitable theory to the most efficient algorithm for solving the underlying equations.

physics.chem-ph↗