In order to further improve the efficiencies and in particular the open-circuit voltages of perovskite solar cells it is important to understand non-radiative recombination and their relation to defect densities. Here, the charge-carrier lifetime is frequently measured using transient spectroscopic techniques to provide an assay of charge carrier recombination. Frequently, the expectation is that at sufficiently low excitation conditions the experimentally observed lifetime should be a constant value that scales inversely with the defect density. We show that in (our) triple-cation perovskite films, layer stacks and devices, recombination is actually dominated by shallow defects, implying that the lifetime becomes a quantity that is continuously changing with carrier density. We thereby show that (i) it is not the low density of deep defects that limits the performance in typically triple-cation perovskites but a quite high density of shallow defects. Furthermore, we show that lifetime cannot be used anymore as a figure of merit if it is not plotted vs. carrier density, Fermi level splitting or voltage [1].

For the continuous improvement of optoelectronic devices such as organic light-emitting diodes and solar cells in terms of e.g. stability and efficiency, a comprehensive model including all major physical processes is crucial. Further, the combined analysis of measurement and simulation paves the way for the understanding of novel devices such as perovskite solar cells. In this work, an up-scaled R&D solar cell is investigated. Once a model and material & device parameter set describing the cell is found, the optimization of the cell performance can start. The initial challenge, however, is to determine such a set of parameters since the availability of e.g. material parameters is limited and is usually obtained by tailored measurements or taken from literature. The values for certain material parameters can vary depending on the literature source or the sequence of layers in the experimental stack. A way to determine the parameters is fitting. In least-square algorithms the sum of the squared differences between the measurement and simulation is minimized by varying the material parameters until an optimal set is found. This task however can be cumbersome, especially with a large number of unknown parameters. Often the parameters are also correlated which again increases the complexity of the problem. In such situations, domain knowledge is required to facilitate the search for the minimal error.
In this contribution, we discuss traditional and machine-learning assisted approaches [1] to determine the model parameters for perovskite solar cells based on electroluminescence images and current-voltage measurements.

Organic semiconductor-based photovoltaic (OPV) devices have many advantageous properties which makes them attractive for future energy harvesting technologies [1]. Owing to the low charge carrier mobilities in OPVs, a substantial built-in voltage is generally required within the active layer to ensure efficient charge carrier extraction [2,3]. Nevertheless, reliable methods to determine the built-in voltage in thin film OPVs are currently lacking. Standard Mott-Schottky analysis of capacitance-voltage (C-V) characteristics has been frequently used in the past to determine the built-in voltage in organic semiconductor diode devices. However, since OPVs are typically dominated by contact-induced background carriers with highly non-uniform charge distributions [4], this technique is generally not applicable for thin film OPVs [5]. In this work, we present an alternative method based on C-V characteristics, which accounts for the influence of injected carriers. Guided by device simulations, we derive a theoretical framework which describes the relationship between the capacitance and the built-in voltage in thin diode devices. We further use numerical drift-diffusion simulations to validate the theoretical framework. Finally, we substantiate the method experimentally on organic solar cells. Based on these findings, we clarify the role of ohmic contacts and the meaning of the built-in voltage in OPVs and related devices.

Halide perovskite semiconductors have captured significant research attention for application in optoelectronic devices, in particular solar cells, due to their outstanding properties and straightforward material synthesis. Amongst these favourable properties, the seemingly “slow” cooling of hot (nonequilibrium, high energy) carriers—which distinguishes halide perovskites as candidate materials for the light-absorbing layer in hot carrier solar architectures—is the subject of an increasing share of the community’s focus. The dynamics of hot carriers are commonly studied experimentally via transient absorption spectroscopy (TA), which allows for observation of the cooling of hot carriers following photoexcitation by an ultrashort laser pulse, and the advanced two-dimensional electronic spectroscopy technique (2DES), which can resolve shorter timescales and, thus, additionally enables scrutiny of carrier thermalization. These techniques are powerful, but there is generally a limit to the strength of the conclusions that can be drawn concerning the physical processes underlying the carrier dynamics in the probed material by virtue of these experimental measurements alone. Consequently, the origin of observations of long hot carrier cooling times in halide perovskite materials remains debated, despite the increase in traction of the “hot phonon bottleneck” hypothesis. Here, we present semiclassical simulation of semiconductor charge carrier dynamics—using the ensemble Monte Carlo method to solve the Boltzmann transport equation for non-equilibrium carrier distributions—as a valuable partner to experiment in elucidating the transient carrier dynamics in halide perovskites and other semiconductors. We include results from such simulations and draw qualitative inferences about the carrier dynamics in halide perovskites, also highlighting the care that must be taken in analysing TA experiments. We discuss remaining challenges and opportunities to develop our approach to allow experimental measurements to be reproduced.

Semitransparent organic solar cells (STOSC) exhibit promising applications as power-generating windows in buildings and agricultural greenhouses. Due to their widely adjustable optical band gap, organic semiconducting materials have the potential to efficiently utilize near infrared light while maintaining semitransparency in the visible light range. One challenge in improving the overall performance of STOSCs is maximizing both power conversion efficiency (PCE) and average visible transmittance (AVT) simultaneously, since intrinsically they are often in a trade-off relationship.^[1] A key factor in optimizing both values, is choosing the right material for each intermediate layer of the cell. One well-established material used for electrodes in semitransparent cells is Indium tin oxide (ITO). Although known for its high transparency and good conductivity, it is considered brittle, and its production is cost- and energy-intensive. Furthermore, Indium is a metal that rarely occurs in nature.^[2] Lastly ITO exhibits a comparably low reflection in the infrared region.^[3] One alternative for these problems is using multiple interlayers of silver and Al-doped ZnO (AZO) as a distributed Bragg reflector on the backside of the solar cell. By varying the layer thicknesses, the optical properties of this electrode can be modified to maximize reflection in the infrared range. This way the current generation of the photoactive layer can be improved while simultaneously maintaining a high transparency for the cell.  

Herein we report recent progress in ITO-free semitransparent organic solar cells by implementing a multilayer silver back electrode as an infrared mirror, achieving a power conversion efficiency of 10.2% for a cell stack based on PV-X Plus with an average visible transmittance of 33.1%. Both optical modelling and experimental findings were used to maximize the photocurrent generation and the average visible transmission of the solar cell. The back electrode consists of alternating layers of Al-doped ZnO and thin silver. As the reflected part of the light passes the photoactive layer a second time, an increase in absorption is achieved. Optical simulations show that the distance between the two silver layers has a strong influence on the wavelength range of the reflected light and thus determines the generation of the photocurrent in the cell. By optimizing the spacing between the two silver layers, it was possible to increase the photocurrent for a solar cell based on PV-X Plus by 9.8% (from 19.3 mA/cm² to 21.2 mA/cm²). At the same time, it was found that the fill factor decreases with an increase in the thickness of the interlayer. When using a single silver layer of 14 nm, the fill factor could be kept reasonably high (71.3%) that despite a comparably lower current generation (17.0 mA/cm²) a LUE (PCE x AVT) of 3.38% was achieved. 

Thermally Activated Delayed Fluorescence (TADF) process has appeared as the most popular design strategy towards reaching 100% internal quantum efficiency for Organic Light-Emitting Diodes (OLEDs). TADF consists in promoting upconversion of triplet excited states into emissive singlet ones through Reverse InterSystem Crossing (RISC), a process driven by spin-orbit coupling (SOC) and requiring a small singlet-triplet gap dE_ST. The advancement of the TADF field occurred essentially through materials design, the first strategy, as proposed by C. Adachi and co, consisting in connecting electron donating and accepting units to decrease the dE_ST. However, in doing so, the lower-lying singlet and triplet excited states bear a dominant charge transfer character that translates into a broad emission spectrum.

In this contribution, we will discuss based on computational considerations how doped triangle-shaped molecules can lead to (i) concomitant narrow emission, high quantum yield of emission and small dE_ST resulting in a whole new generation of TADF emitters, the multi-resonant TADF emitters and to (ii) a new family of compounds with an inverted singlet-triplet gap and potentially, a downwards energy RISC. To do so, we rely on high level quantum chemical calculations and show that an accurate description of electron correlation effects is key to correctly predict the excited states ordering as well as the optical properties of these compounds.

Excited state dynamics play key roles in numerous molecular and nanoscale materials designed for energy conversion. Controlling these far-from-equilibrium processes and steering them in desired directions require understanding of material’s dynamical response on the nanometer scale and with fine time resolution. We couple real-time time-dependent density functional theory for the evolution of electrons with non-adiabatic molecular dynamics for atomic motions to model such non-equilibrium response in the time-domain and at the atomistic level. The talk will introduce the simulation methodology [1] and discuss several exciting applications among the broad variety of systems and processes studied in our group [2,3], including metal halide perovskites, transition metal dichalcogenides, semiconducting and metallic quantum dots, metallic and semiconducting films, polymers, molecular crystals, graphene, carbon nanotubes, etc. Photo-induced charge and energy transfer, plasmonic excitations, Auger-type processes, energy losses and charge recombination create many challenges due to qualitative differences between molecular and periodic, and organic and inorganic matter. Our simulations provide a unifying description of quantum dynamics on the nanoscale, characterize the timescales and branching ratios of competing processes, resolve debated issues, and generate theoretical guidelines for development of novel systems.

Perovskites is a class of materials with notable crystal structure of ABX3, exceptionally wide tunability in chemical compositions and dimensionality in crystal-structures. Due to their exceptional optoelectronic properties, they are widely used for converting and storing (solar) energy, e.g. oxide perovskites as photocatalysts, halide perovskites as absorbers in solar cells, nitride perovskites as

mechanical energy harvesters. In these applications, the understanding of optoelectronic properties, chemical stability and their changes upon external stimuli (light excitation, mechanical, thermal and chemical stress) are paramount.

In this talk, I will show how our research group investigate these properties using atomistic multiscale modelling by combing electronic structure calculations with reactive molecular dynamics simulations. Her focus is on halide perovskites and the impact of defects on the efficiency and the stability of perovskite solar cells. We identify harmful defects which lead to either recombination losses and/or chemical degradations and show several strategies to mitigate and passivate these defects. These include engineering the composition of perovskite absorbers, optimizing interfaces with contact materials, and finetuning growth conditions. The atomistic insights provide a basis for further improving the efficiency and stability of perovskite materials and devices. The multiscale computational framework can be enhanced with The emerging data-driven approach and straightforwardly applied to other materials and applications.

In this talk, I will discuss the employment of state-of-the-art electronic structure theory for the modelling of novel perovskite-like materials within the Ag/Bi double salt and the vacancy ordered double perovskite (VODP) lattices. I will first discuss the case of halide double perovskites, and explore the link between the type and position of the atomic orbitals in the crystal and their electronic structure, and connect this established structure with the so-called rudorffites or Ag/Bi double salts. I will demonstrate the necessity of proper modelling of these, and present a symmetry-based method we developed [1], which correctly describes the exhibited electronic and optical properties of AgBiI₄, a prototypical material. I will employ this to probe other materials within the same family of double-salts, and further use the model to investigate the ideal (i.e., maximum limits) photovoltaic properties that these materials could achieve. In the second part, I will thoroughly analyze the properties of the VODP family of materials by employing state-of-art ab initio calculations and unveil key details of the electronic structure and the effects of electron-hole coupling on the optical properties [2]. I will discuss prototypical VODP structures by selecting members based on the electronic configuration of the tetravalent metal at the B-site (e.g. Sn, Te, Zr). I will go through their structural properties, and show that the size of the vacant site, can be tuned solely by the halogens. The electronic structures are investigated within the GW many-body green’s function method, and we obtain band-gaps spanning a range of 1.5-5.0 eV at G₀W₀. More importantly, the optical and excitonic properties of these compounds are probed within the independent particle approach, and by solving the Bethe-Salpeter equation (BSE). The exciton binding energies and dark-bright exciton exchange splitting are calculated for each type of VODP. Finally, the excitonic fine structure is unveiled by performing a complete symmetry analysis of the compound’s band structure and excitonic wavefunctions, on which a direct link between these and the metal site species is established. Overall, here I comment on the suitability and the prospect of the application in opto-electronics of Ag/Bi double salts, like AgBiI₄, and VODP, like Cs₂TeBr₆, might exhibit. I will identify the most and least promising materials that could act as photo-active materials or for selective charge transport layers, for light-emitting and solar-cell applications.

Lead halide perovskites have revolutionized the scenario of photovoltaics due to their excellent and tunable optoelectronic properties leading to solar cell efficiencies comparable to traditional high-quality silicon materials.[1] Issues concerning the presence of lead, however, have led researchers to find less toxic candidates possibly replacing lead in the perovskite, particularly tin. The performance of tin halide perovskites, however, is strongly limited by an enhanced self p-doping and the easy oxidation of Sn(II) to Sn(IV).[2] To guide the development of lead-free materials withenhanced efficiency and stability, a deep understanding of the defect chemistry and photophysics of these materials is needed. Ab-initio simulations represent a powerful tool to investigate defect properties and processes at the atomistic level. In this presentation a theoretical tour of the defect activity in lead- and tin-based perovskites is provided. We will show how DFT calculations may be applied to study defects in materials, the useful quantities obtained by simulations and the best practices in the case of metal halide perovskites. Hence, the trend in defect activity by moving from lead to tin will be discussed.[3] The nature of most abundant defects, their trapping activity and the impact on the doping of the materials will be illustrated. Defect driven mechanisms possibly activating the degradation of these perovskites will be discussed based on the defect analysis.[4] Furthermore, the effects of quantum confinement on the optoelectronic features and defect properties moving to low dimensional 2D perovskites will be showed, also in relation to emission features emerging in these quantum confined systems, such as broad emission, whose origin is hotly debated.[5] This work aims to provide a unified picture of defect chemistry of metal halide perovskites with emphasis on the factors governing the stability and the efficiency of these materials. It also aims to stimulate the community to a synergistic use of computer simulations to support the design of more
efficient and long-term stable devices.

Device modeling and energy yield (EY) calculations are essential tools to optimize solar cell architectures. Device modeling through opto-electrical simulations allows to analyze device performance under standard conditions. In perovskite solar cells (PSC); the energy alignment at the interfaces, ion migration and potential distribution along the device play a critical role on device performance. To investigate them, we have characterized horizontal PSC microstructures[1]. We were able to draw the potential distribution along the solar cell structure by coupling X‐ray photoelectron spectroscopy (XPS) and drift-diffusion modeling[1]. Furthermore, in this work we considered the role of ion migration for the analysis of the band device structure. On the other hand, EY calculations estimate the total output generated energy of a solar cell after one year in a specific place. EY calculations also allow the analysis of device stability which is highly affected by device temperature[2]. Therefore, it is crucial to determine accurately the device temperature. To estimate cell temperature, we propose a thermal model which is a function of device parameters, environmental variables, and is strongly linked with the experimental optical-electrical-thermal performance[2]. We studied the effect of realistic temperature conditions on the performance of PSCs and their transient response to environmental external changes using a theoretical-experimental combined approach. Linking the experimental results and our model, we were able to evaluate the most sensible device layers that increment device temperature affecting device stability.

The dynamic response of metal halide perovskite devices shows a variety of physical responses that need to be understood and classified for enhancing the performance and stability and for identifying physical behaviours that may lead to developing new applications. Beyond the well-established characteristics of regular impedance arcs, we address the appearance of inductor effect at high voltage in perovskite solar cell. We present a physical model in terms of delayed recombination current that explains the evolution of impedance spectra and the evolution of current-voltage curves. A multitude of chemical, biological, and material systems present an inductive behavior that is not electromagnetic in origin. Here, it is termed a chemical inductor. We show that the structure of the chemical inductor consists of a two-dimensional system that couples a fast conduction mode and a slowing down element. Therefore, it is generally defined in dynamical terms rather than by a specific physicochemical mechanism. The impedance spectra announce the type of hysteresis, either regular for capacitive response or inverted hysteresis for inductive response. We apply the dynamic picture based on a few neuron-like equations to the characterization of halide perovskite memristors. Memristors show an intense hysteresis that can be characterized in terms of the emergence of inductive components. We address the characterization of electron diffusion and radiative emission in halide perovskites using a range of light stimulated techniques as IMPS, IMVS, and voltage controlled light emission technique (LEVS).

Laser-induced transient grating (LITG) spectroscopy has been used to measure the carrier transport properties of perovskite materials and devices, including the diffusion coefficient and carrier lifetime [1]. These measurements are important for understanding the electronic properties of materials and how they can be optimized for different applications. LITG has the advantage of being a non-destructive method, which can be important for protecting the integrity of the material during measurement.

LITG method itself was realized several decades ago [2, 3]. The principle scheme of a LITG measurement is depicted in Fig.1. A pair of ultrashort pulses are overlapped both spatially and temporally to create an interference pattern on the sample – a transient grating with a spatial period Λ that depends on the pump wavelength and the beam intersection angle. Excitation by periodic pattern excites a spatially-modulated carrier distribution and, effectively, a periodic modulation of the refractive index, thus, diffracting the delayed probe beam.

Over time, the laser-induced grating decays due to carrier recombination (electronic decay with the rate of τ_R) and carrier diffusion (spatial decay with the rate of τ_D). The diffusion term depends on the transient grating period: fine gratings diffuse faster than coarser ones. Accordingly, if we measure the temporal behavior of the diffracted signal over a series of different periods Λ, we can determine the carrier diffusion coefficient D [2].

The device presented in this paper allows automated continuous tuning of the excitation grating period Λ and selection of excitation wavelength from 340 to 560 nm, which allows measuring carrier diffusion in the range from 0.1 to 50 cm²/s and carrier lifetime in the range from 1 ps to ca. 80 ns.

Exemplary carrier diffusion and lifetime measurements were performed with a thin film of metal-halide perovskite Cs_0.05(FA_0.85MA_0.15)Pb(I_0.85Br_0.15)₃ with Et2NH additive in a wide range of excitation densities. The results showed that the charge carriers of the material are in a trap-filling and de-trapping diffusion regime, from which carriers easily escape with increasing excitation density.

The extensive experience in optomechanical engineering and high-level automation enabled the all-optical measurement for carrier diffusion and carrier lifetime. It was extensively tested with different samples, including but not limited to the aforementioned perovskites. The results will be presented in detail during the conference.

The photovoltaic (PV) effect provides the most efficient means of converting the overwhelming amount of freely available energy from the sun (ca. 1000 times greater than the total energy consumption worldwide) into electricity. Emerging PV technologies based on materials such as organic, transparent oxides, kesterites, quantum dots and hybrid halide perovskites are increasingly gaining in importance nowadays.[1] Within the emerging PV technologies, many of the breakthroughs achieved in the power conversion efficiency and in device lifetime have been obtained through intensive materials research.[1, 2, 3] Current trends in PV research focus on the search for higher efficiencies through multi-junction concept and the expansion of the range of applications beyond standard solar farms. In this context, material screening and characterization requires many different pieces of equipment (one solar simulator, another light source for indoor, another set for EQE measurements, etc.). In cases like lateral tandem devices, the required setups are simply not commercially available.

We report a multi-purpose spectrum-on-demand light source (SOLS), conceived, primarily, but not exclusively, for the multiple and advanced characterization of photovoltaic (PV) materials and devices. The apparatus is a spectral shaper illumination device, providing a tunable, spectrally shaped and focused light beam, modulated in intensity and/or in a wavelength range with respect to a primary light source. SOLS stands out from the state of the art because it produces almost any spectrum on demand and delivers two types of output: a spectrally shaped and spatially homogeneous beam over its cross section for areal illumination; or a spatially and spectrally split beam into its wavelength components, a unique capability suited to characterize lateral-tandem (Rainbow) solar cells. The tuneability from broadband to narrow band illumination enables two characterization devices into one, namely, a solar simulator for the determination of the power conversion efficiency and an external quantum efficiency measuring system. We expect the SOLS setup to accelerate material screening, enabling the discovery and optimization of novel multi-component materials and devices, in particular, for emergent PV technologies like organic or metal halide perovskites PV, indoors and building integrated PV, agrovoltaics, multi-junction, etc.