arXiv subjects

Thomas Kirchartz

Publications and source records attributed to Thomas Kirchartz.

At least 19 recordsLinked to original sources

arXiv2026

Unraveling the Roles of Shallow, Deep and Auger Trapping in Charge Carrier Recombination in Triple-Cation Perovskites

Understanding charge-carrier recombination in metal halide perovskites is essential for accurately identifying the factors limiting solar cell efficiency, yet it remains challenging due to the interplay of multiple competing processes. Here, we combine time-resolved photoluminescence and excitation dependent photoluminescence quantum yield measurements over a wide range of fluences and repetition rates to investigate recombination dynamics in triple-cation perovskite thin films. By jointly analyzing these multidimensional datasets, we develop a unified model that quantitatively reproduces both photoluminescence decays and absolute quantum yields across all excitation conditions. Our results reveal the coexistence of deep and shallow traps, as well as a second-order nonradiative recombination pathway attributed to Auger-assisted trapping. Importantly, this mechanism dominates under one-sun illumination, making it a critical limiting factor for photovoltaic performance. These findings provide a comprehensive framework for understanding recombination in perovskites and highlight the importance of higher-order defect-mediated processes in determining their efficiency.

cond-mat.mtrl-sci↗

arXiv2026

Characterizing Fill Factor Limitations in Perovskite-Silicon Tandem Solar Cells

Perovskite-silicon tandem technology has exceeded the single junction theoretical efficiency limit. However, there is still distance to the thermodynamic limit mainly caused by the fill factor. This work presents a methodology to illustrate the mechanisms of FF loss in perovskite-Si monolithic tandem solar cells. Apart from the series resistance related loss characterized by electroluminescence, another loss factor is from the photoshunt, a phenomenon in which the parallel resistance apparently reduces under illumination in perovskite solar cells due to the moderate charge transport layer mobility. In addoition, the two-diode property of the Si cell can also influence the FF of tandem devices. The photoshunt can be hidden when the bottom cell is over illuminated, which explains highly efficient tandem solar cells are usually bottom cell limited. This work outlines strategies that overcoming the photoshunt issue can move the perovskite top cell closer to low FF losses in tandem solar cells.

cond-mat.mtrl-sci↗

arXiv2026

Bayesian Parameter Estimation for Predictive Modeling of Illumination-Dependent Current-Voltage Curves

Machine learning enables rapid estimation of material parameters in solar cells via neural-network-based surrogate models. However, the reliability of extracted parameters depends on underlying assumptions such as the choice of one-dimensional drift-diffusion model and selection of free material parameters. To validate the inferred parameters, we perform predictive modeling of light-intensity-dependent current-voltage (JV) characteristics. Well-known physical effects, including the influence of external resistance and recombination dynamics on illumination-dependent device performance, are reflected in parameter estimation and prediction workflow. We show that correct treatment of dark shunt resistance and emphasizing shifted current (J + Jsc) during fitting enhances prediction accuracy at low to intermediate illumination level. Additionally, we analyze the information content of various input JV curve combinations, demonstrating that including at least one illuminated JV, preferably not under high illumination due to series resistance effects, is critical for reliable parameter estimation and device performance prediction.

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

Understanding the fill-factor limit of organic solar cells

Although the power conversion efficiencies of organic solar cells (OSCs) have surpassed 20%, they still lag behind commercial inorganic solar cells and emerging perovskite solar cells. To bridge this efficiency gap, improving the fill factor (FF) is critical, provided other photovoltaic parameters are not compromised. However, the fundamental understanding of the FF in OSCs remains incomplete. In this work, we systematically investigate a wide range of OSCs with the FF values spanning 0.27 to 0.80, and analyse the effect of free charge generation and recombination on the FF in OSCs. To explain our observations, we developed an analytical model that quantitatively correlates the applied electric field with the energetics of excited states in donor-acceptor blends. By combining device characterisation, spectroscopy, and theoretical modelling, we reveal that the Stark effect and the field-dependent charge transfer significantly impact the FF in state-of-the-art OSCs with low voltage losses. Our findings highlight that suppressing geminate decay by increasing exciton lifetime is a promising strategy for boosting the FF and achieving future efficiency gains in OSCs.

cond-mat.mtrl-sci↗

arXiv2025

Inferring Material Parameters from Current-Voltage Curves in Organic Solar Cells via Neural-Network-Based Surrogate Models

Machine learning has emerged as a promising approach for estimating material parameters in solar cells. Traditional methods for parameter extraction often rely on time-consuming numerical simulations that fail to capture the full complexity of the parameter space and discard valuable information from suboptimal simulations. In this study, we introduce a novel workflow for parameter estimation in organic solar cells based on a combination of numerical simulations and neural networks. The workflow begins with the selection of an appropriate experimental dataset, followed by the definition of a device model that accurately describes the experiment. To reduce computational complexity, the number of variable parameters is carefully selected, and reasonable ranges are set for each parameter. Instead of directly fitting the experimental data using a numerical model, a neural network was trained on a large dataset of simulated results, allowing for efficient exploration of the high-dimensional parameter space. This approach not only accelerates the parameter estimation process but also provides valuable insights into the likelihood and uncertainty of the estimated parameters. We demonstrate the effectiveness of this method on organic solar cells based on the PBDB-TF-T1:BTP-4F-12 material system, demonstrating the potential of machine learning for rapid and comprehensive characterization of emerging photovoltaic materials.

cond-mat.mtrl-sci↗

arXiv2025

How nanotextured interfaces influence the electronics in perovskite solar cells

Perovskite solar cells have reached power conversion efficiencies that rival those of established silicon photovoltaics. Nanotextures in perovskite solar cells scatter the incident light, thereby improving optical absorption. In addition, experiments show that nanotextures impact electronic performance, although the underlying mechanisms remain unclear. This study investigates the underlying theoretical reasons by combining multi-dimensional optical and charge-transport simulations for a single-junction perovskite solar cell. Our numerical results reveal that texturing redistributes the electric field, influencing carrier accumulation and recombination dynamics. We find that moderate texturing heights ($\leq 300$ nm) always increase the power conversion efficiency, regardless of surface recombination velocities. Our study also clarifies why experiments have reported that texturing both increased and reduced open-circuit voltages in perovskite solar cells: this behaviour originates from variations in surface recombination at the untextured electron transport layer. In contrast, surface recombination at the textured hole transport layer strongly affects the short-circuit current density, with lower recombination rates keeping it closer to the optical ideal. These findings provide new insights into the opto-electronic advantages of texturing and offer guidance for the design of next-generation textured perovskite-based solar cells, light emitting diodes, and photodetectors.

physics.comp-ph↗

arXiv2025

Determination of Electron Extraction in Semiconductor Photoanodes: Steady-state and Small-perturbation Response

This work develops an analytical model to consistently interpret the steady-state and small-perturbation response (both in the time and frequency domain) of photoanodes for solar water-splitting. In addition to accounting for the fundamental mechanisms of charge-carrier generation, recombination and slow hole transfer at the photoanode/electrolyte interface, the model overcomes the key shortcomings of existing models in the literature. These include consistency across measurements/bias conditions and the non-consideration of imperfect electron extraction at the collecting contact and its corresponding effect on the recombination rate in the bulk. We applied the model to analyse the time constants obtained from intensity-modulated photocurrent (IMPS) and photovoltage (IMVS) measurements of a hematite photoanode, obtaining an electron extraction velocity of 100 cm/s close to the 1 sun open-circuit potential, that corresponds to an electron mobility of 0.022 cm2V-1s-1. The model further predicts a linear dependence of the photocurrent versus anodic voltage, an observation whose origin has been strongly debated in the literature in the case of hematite photoanodes. The generality of the model allows its extension to other photoanodes and photovoltaic systems, by the addition or removal of specific physical mechanisms.

cond-mat.mtrl-sci↗

arXiv2024

Transport resistance dominates the fill factor losses in record organic solar cells

Organic photovoltaics are a promising solar cell technology well-suited to mass production using roll-to-roll processes. The efficiency of lab-scale solar cells has exceeded 20% and considerable attention is currently being given to understanding and minimising the remaining loss mechanisms preventing higher efficiencies. While recent efficiency improvements are partly owed to reducing non-radiative recombination losses at open-circuit, the low fill factor due to a significant transport resistance is becoming the Achilles heel of organic photovoltaics. The term transport resistance refers to a voltage and light intensity dependent charge collection loss in low-mobility materials. In this Perspective, we demonstrate that even the highest efficiency organic solar cells reported to-date have significant performance losses that can be attributed to transport resistance and that lead to high fill factor losses. We provide a closer look at the transport resistance and the material properties influencing it. We describe how to experimentally characterise and quantify the transport resistance by providing easy to follow instructions. Furthermore, the causes and theory behind transport resistance are detailed. In particular, we integrate the relevant figures of merit and different viewpoints on the transport resistance. Finally, we outline strategies that can be followed to minimise these charge collection losses in future solar cells.

cond-mat.mtrl-sci↗

arXiv2024

Deriving mobility-lifetime products in halide perovskite films from spectrally- and time-resolved photoluminescence

Lead-halide perovskites are semiconductor materials with attractive properties for photovoltaic and other optoelectronic applications. However, determining crucial electronic material parameters, such as charge-carrier mobility and lifetime, is plagued by a wide range of reported values and inconsistencies caused by interpreting and reporting data originating from different measurement techniques. In this paper, we propose a method for the simultaneous determination of mobility and lifetime using only one technique: transient photoluminescence spectroscopy. By measuring and simulating the decay of the photoluminescence intensity and the redshift of the photoluminescence peak as a function of time after the laser pulse, we extract the mobility, lifetime, and diffusion length of halide perovskite films. With a voltage-dependent steady-state photoluminescence measurement on a cell, we relate the diffusion length to the external voltage and quantify its value at the maximum power point.

cond-mat.mtrl-sci↗

arXiv2023

Resolving the Hydrophobicity of Me-4PACz Hole Transport Layer for High-Efficiency Inverted Perovskite Solar Cells

[4-(3,6-dimethyl-9H-carbazole-9-yl)butyl]phosphonic acid (Me-4PACz) self-assembled monolayer (SAM) has been employed in perovskite single junction and tandem devices demonstrating high efficiencies. However, a uniform perovskite layer does not form due to the hydrophobicity of Me-4PACz. Here, we tackle this challenge by adding a conjugated polyelectrolyte poly(9,9-bis(3'-(N,N-dimethyl)-N-ethylammonium-propyl-2,7-fluorene)-alt-2,7-(9,9dioctylfluorene)dibromide (PFN-Br) to the Me-4PACz in a specific ratio, defines as Pz:PFN. With this mixing engineering strategy of Pz:PFN, the PFN-Br interacts with the A-site cation and is confirmed via solution-state nuclear magnetic resonance studies. The narrow full width at half maximum (FWHM) of diffraction peaks of perovskite film revealed improved crystallization on the optimal mixing ratio of Pz:PFN. Interestingly, the mixing of PFN-Br additionally tunes the work function of the Me-4PACz as revealed by the Kelvin probe force microscopy and built-in-voltage estimation in solar cells. Devices employing optimized Pz:PFN mixing ratio deliver open-circuit voltage (Voc)of 1.16 V and efficiency >20% for perovskites with a bandgap of 1.6 eV with high reproducibility and concomitant stability. Considering significant research on Me-4PACz SAM, our work highlights the importance of obtaining a uniform perovskite layer with improved yield and performance.

cond-mat.mtrl-sci↗

arXiv2023

How Charge Carrier Exchange between Absorber and Contact influences Time Constants in the Frequency Domain Response of Perovskite Solar Cells

A model is derived for the frequency- and time-domain opto-electronic response of perovskite solar cells (PSCs) that emphasizes the role of charge carrier exchange, .i.e. extraction and injection, from (to) the perovskite through the transport layer to (from) the collecting electrode. This process is described by a charge carrier exchange velocity that depends on the mobility and electric field inside the transport layer. The losses implied by this process are modelled in an equivalent circuit model in the form of a voltage-dependent transport layer resistance. The analysis of the model predicts that the voltage dependence of the measured time constants allows discriminating situations where the transport layer properties dominate the experimental response. Application of this method to experimental impedance spectroscopy data identifies charge extraction velocities between 1-100 cm/s at 1 sun open-circuit conditions for p-i-n PSCs with PTAA as the hole transport layer, that corresponds to transport layer mobilities between 10^-4 - 3 x 10^-3 cm^2V^-1s^-1. The model paves the way for accurate estimation of photocurrent and fill factor losses in PSCs caused by the low mobilities in the transport layers, using small perturbation measurements in the time and frequency domain.

cond-mat.mtrl-sci↗

arXiv2023

Characterizing the Influence of Charge Extraction Layers on the Performance of Triple-Cation Perovskite Solar Cells

Selecting suitable charge transport layers and suppressing non-radiative recombination at interfaces to the absorber layer are vital to maximize the efficiency of halide perovskite solar cells. In this work, high-quality perovskite thin films and devices are fabricated with different fullerene-based electron transport layers and different self-assembled monolayers as hole transport layers. We then perform a comparative study of a significant variety of different electrical, optical and photoemission-based characterization techniques to quantify the properties of the solar cells, the individual layers and importantly the interfaces between them. In addition, we highlight the limitations and problems of the different measurements, the insights gained by combining different methods and the different strategies to extract information from the experimental raw data.

cond-mat.mtrl-sci↗

arXiv2023

A Universal Strategy of Perovskite Ink-Substrate Interaction to Overcome the Poor Wettability of a Self-Assembled Monolayer for Reproducible Perovskite Solar Cells

Perovskite solar cells employing self assembled monolayers such as Me-4PACz as hole transport layer has been reported to demonstrate high device efficiency. However, the poor perovskite wetting on the Me-4PACz caused by poor perovskite ink interaction with the underlying Me-4PACz presents significant challenges for fabricating efficient perovskite devices. A triple co-solvent system comprising of dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP) is employed to improve the perovskite ink-substrate interaction and obtain a uniform perovskite layer. In comparison to DMF, DMSO-based inks, the inclusion of NMP shows considerably higher binding energies of the perovskite ink with Me-4PACz as revealed by density-functional theory calculations. With the optimized triple co-solvent ratio, the perovskite devices deliver high power conversion efficiencies of >20%, 19.5% and ~18.5% for active areas of 0.16 cm2, 0.72 cm2 and 1.08 cm2 respectively. Importantly, this perovskite ink-substrate interaction approach is universal and helps in obtaining a uniform layer and high photovoltaic device performance for other perovskite compositions such as MAPbI3, FAMAPbI3-xBrx, and MA-free FACsPbI3-xBrx.

physics.app-ph↗

arXiv2023

Quantifying Charge Extraction and Recombination Using the Rise and Decay of the Transient Photovoltage of Perovskite Solar Cells

The extraction of photogenerated charge carriers and the generation of a photovoltage belong to the fundamental functionalities of any solar cell. These processes happen not instantaneously but rather come with finite time constants, e.g., a time constant related to the rise of the externally measured open circuit voltage following a short light pulse. The present paper provides a new method to analyze transient photovoltage measurements at different bias light intensities combining rise and decay times of the photovoltage. The approach uses a linearized version of a system of two coupled differential equations that is solved analytically be determining the eigenvalues of a 2 x 2 matrix. By comparison between the eigenvalues and the measured rise and decay times during a transient photovoltage measurement, we determine the rates of carrier recombination and extraction as a function of bias voltage and establish a simple link between their ratio and the efficiency losses in the perovskite solar cell.

cond-mat.mtrl-sci↗

arXiv2023

Comparing Methods of Characterizing Energetic Disorder in Organic Solar Cells

Energetic disorder has been known for decades to limit the performance of structurally disordered semiconductors such as amorphous silicon and organic semiconductors. However, in the past years, high performance organic solar cells have emerged showing a continuously reduced amount of energetic disorder. While searching for future high efficiency material systems, it is therefore important to correctly characterize this energetic disorder. While there are several techniques in literature, the most common approaches to probe the density of defect states are using optical excitation as in external quantum efficiency measurements or sequential filling of the tail states by applying an external voltage as in admittance spectroscopy. A metanalysis of available literature as well as our experiments using four characterization techniques on two material systems reveal that electrical, voltage-dependent measurements frequently yield higher values of energetic disorder than optical measurements. With drift-diffusion simulations, we demonstrate that the approaches probe different energy ranges of the subband-gap density of states. We further explore the limitations of the techniques and find that extraction of information from a capacitance-voltage curve can be inhibited by an internal series resistance. Thereby, we explain the discrepancies between measurements techniques with sensitivity to different energy ranges and electronic parameters.

cond-mat.mtrl-sci↗

arXiv2022

Fill Factor Losses and Deviations from the Superposition Principle in Lead-Halide Perovskite Solar Cells

The enhancement of the fill factor in the current generation of perovskite solar cells is the key for further efficiency improvement. Thus, methods to quantify the fill factor losses are urgently needed. A classical method to quantify Ohmic and non-Ohmic resistive losses in solar cells is based on the comparison between the voltage in the dark and under illumination analysed at equal recombination current density. Applied to perovskite solar cells, we observe a combination of an Ohmic series resistance with a voltage-dependent resistance that is most prominent at short circuit and low forward bias. The latter is most likely caused by the poor transport properties of the electron and/or hole transport layers. By measuring the photoluminescence of perovskite solar cells as a function of applied voltage, we provide direct evidence for a high quasi-Fermi level splitting at low and moderate forward bias that substantially exceeds the externally applied voltage. This quasi-Fermi level splitting causes recombination losses and, thus, reduces both the short-circuit current and the fill factor of the solar cell.

physics.app-ph↗

arXiv2022

Interpretation of Mott-Schottky Plots of Photoanodes for Water Splitting

A large body of literature reports that both bismuth vanadate and haematite photoanodes are semiconductors with an extremely high doping density between 10^18-10^21 cm^-3. Such values are obtained from Mott-Schottky plots by assuming that the measured capacitance is dominated by the capacitance of the depletion layer formed by the doping density within the photoanode. In this work, we show that such an assumption is erroneous in many cases because the injection of electrons from the collecting contact creates a ubiquitous capacitance step that is very difficult to distinguish from that of the depletion layer. Based on this reasoning, we derive an analytical resolution limit that is independent of the assumed active area and surface roughness of the photoanode, below which doping densities cannot be measured in a capacitance measurement. We find that the reported doping densities in literature lie very close to this value and therefore conclude that there is no credible evidence from capacitance measurements that confirms that bismuth vanadate and haematite photoanodes contain high doping densities.

cond-mat.mtrl-sci↗