Projects

Ilja Akimov (TU Dortmund) & Vladimir Dyakonov (Uni Würzburg)

The ideal solution for future optoelectronic and photovoltaic applications based on perovskites is to find stable, inherently non-toxic, earth-abundant and at the same time efficiently light-absorbing, charge-transporting, and energetically tunable material systems. However, the search for competitive lead-free perovskite semiconductor systems for energy conversion and light emission proved more complex than initially anticipated. Lead-free double perovskites are regarded as an important platform for this, but their three-dimensional prototype Cs₂AgBiBr₆ has so far shown only moderate performance when used in solar cells. The physical reasons for this are currently the subject of controversial discussions. The aim of our proposal is to explore and understand the potential of lead-free double perovskites of different dimensionality (3D, 2D/3D, 2D) for photovoltaic and photonic applications and to provide a conceptual understanding of the details of the energy fine structure and excited states and their dependence on the symmetry and dimensionality of the crystalline host systems. The starting point of our project will be single crystals and films of the reference material Cs₂AgBiBr₆, whose synthesis was well established in the project in the first funding period of SPP2196. To investigate the effect of dimensionality, we will also fabricate and study 2D and multidimensional 2D/3D double perovskites. Finally, we not only want to avoid toxic elements (e.g., Pb), but rather use earth-abundant and cheap elements. Therefore, we will also grow double perovskites Cs₂AgFeCl₆ and Cs₂NaFeCl₆, which we recently found to have intriguing thermochromic and magnetic properties but are largely unexplored. We will investigate the properties of neutral (excitons) and charged (electrons/holes) excited states and their interactions in double perovskite materials of different dimensionality and composition using advanced optical spectroscopy. More specifically, we will use coherent nonlinear optical spectroscopy, in particular, time-resolved, polarization-dependent pump-probe and photon echo methods, which can provide rich information about the energy structure and relaxation processes despite strong inhomogeneous broadening of optical transitions in the systems under investigation. Most importantly, the exciton itself will serve as a probe for the interaction with the crystal lattice, local potential fluctuations, other excitons and charge carriers in our studies. We will study the exciton and photoexcited carrier lifetimes, their binding energy, the symmetry-induced optical anisotropy and the effects of localization (self-trapping) of excitons, charge carriers and more complex quasiparticles. The expected results will not only be of fundamental interest but will also help in the development of new non-toxic and stable perovskite materials for photovoltaics and photonics, but also beyond, for example in quantum applications where coherence effects are important.

Steve Albrecht (HZB), Thomas Riedl (Uni Wuppertal), Henry Snaith (Uni Oxford), Felix Lang / Martin Stolterfoht (Uni Potsdam)

The recent improvements of the photovoltaic performance of metal-halide perovskites give promise for the realization of all-perovskite tandem solar cells with power conversion efficiencies exceeding the Shockley-Queisser limit for single junctions. Until today, very promising monolithic all-perovskite tandem cells with experimental efficiency values of 26.4% have been demonstrated and various theoretical and semi-empirical estimates predict values well above 30%. In the proposed second period of the highly successful project HIPSTER, the same four partners from the first period: Helmholtz-Center (HZB), University of Oxford (UOx), University of Potsdam (UPo), and University of Wuppertal (UWu) will push the characterization and understanding of this tandem solar cell technology to the next level: Work in the second phase will be devoted to the understanding and suppression of compositional instabilities and overall degradation mechanisms as well as to the significant further reduction of non-radiative recombination losses. While the team has already proven its ability to highlight various sub-cell limitations by detailed analysis and to develop highly efficient tandem solar cells in the first phase, in the second phase the (photo)chemical stability of perovskite absorbers needs to be analysed and further increased. The work on stability will be complemented by innovative characterization methods such as sub-cell characterization directly for tandem solar cells to quantify recombination and ionic losses from different interfaces and contact materials within the full tandem stack. As such, we will be able to identify the best suited contact, interconnection, and interlayer materials as well as passivation strategies for highest tandem performance. Ultimately, we strive to demonstrate stable monolithic all-perovskite tandem devices by advanced analyses and consequent reduction of loss mechanisms to approach the performance of perovskite-silicon based tandem solar cells.

Denis Andrienko (MPI Mainz), Wolfgang Brütting (Uni Augsburg), Marcus Scheele (Uni Tuebingen)

Lead halide perovskites (LHP) are well-known materials that have recently gained considerable attention in optoelectronics. Particularly LHP nanocrystals of the composition CsPbX₃ (with X=Cl, Br, I) are very attractive as luminescent materials for potential applications in light-emitting diodes. They combine the advantages of bulk LHP – notably their defect tolerance, solution processability, and band-width tunability – with well-known features of colloidal quantum dots, like high photoluminescence quantum yield with narrow emission bandwidth as well as size and composition tunable colors.

A fundamental problem to be addressed before the commercialization of perovskite light-emitting diodes (PeLEDs) is the poor stability of LHPs under an electric current. In conventional PeLEDs, the injection of electrons into the LHP layer leads to the irreversible formation of elemental lead and the destruction of the LHP structure. In addition, the high ion mobility in LHPs poses the problem that a significant proportion of the current in a PeLED is provided by ions, which makes no contribution to electroluminescence and has an additional destabilizing effect on the LHP structure.

We address these inherent problems of PeLEDs by using organic pi-systems as surface ligands on the nanocrystals. As such, they fulfill three purposes: 1) By saturating surface defects, they increase the PLQY of the nanocrystals. 2) By inducing a surface dipole, they increase the work function of the LHP layer and thus the stability under negative charging. 3) The rigid structure in combination with the good charge carrier conductivity of the organic pi-systems suppresses ion diffusion and increases the electronic part of the total current. The project investigates the impact of this strategy on PeLEDs, covering the complete RGB color scheme and optimizing the devices in terms of external quantum efficiency, luminance and long-term stability. Computational modeling accompanies the material development and guides its optimization.

Niels Benson (Uni Duesseldorf) & Doru Lupascu (Uni Duisburg-Essen)

This project is designed to use dielectric studies to better understand the underlying transport and screening mechanisms of the hybrid “perovskite” photovoltaic absorber materials. This is also with the intention to oppose the current trend in literature to confuse and misuse terminology from dielectric science in the context of the perovskite solar cell absorbers.

We have previously shown how much a dielectric assessment of these material can tell about charge transport and the type of mobile charge species. Defect screening appears to be a major asset in this class of materials largely provided by local dielectric properties.

Different from true ferroelectricity hysteresis in the solar cell hybrid “perovskites” is most likely due to charge transport. The project intends to understand how much of this is due to space charge (electronic carriers and Schottky barrier effects) or due to ionic drift. We intend to deconvolute ionic drift from electronic effects. Typical device structures are thin so accessing the underlying mechanisms is difficult, because typical screening lengths extend over the entire device thickness. The effects of the individual interface on device performance is thus hard to determine.

In order to design “clean” systems, we will grow a series of single crystals, characterize them by dielectric spectroscopy (mHz to IR including Raman spectroscopy), ultrasound spectroscopy, Kelvin probe spectroscopy, and electrical hysteresis studies. Techniques comprise quantitative ToF-SIMS, Kelvin probe microscopy and µ-photoluminescence in order to understand how much ionic drift will alter the electrical structure of the device.

On the local scale, conductive AFM, Kelvin probe spectroscopy and scanning probe local elasticity studies will be provided amended by TEM EDX. The observed electrical profiles will be correlated to the ionic distributions. Conductive AFM will provide information of the electrical properties of domain and grain boundaries. Elastic as well as potentially ferroelectric domain walls will be thoroughly investigated plus the particular role of grain boundaries. Micro photo­luminescence yields information on the local optical properties near grain boundaries.

Classical parameters as charge carrier lifetime and work functions (from XPS and UPS) will be provided in collaboration with other groups from the SPP.

Large high quality single crystal hybrid perovskites will be synthesized analyzing the dynamics of crystallization, the morphology, and their impact on the optoelectronic properties. We will start on drosophila CH₃NH₃PbI₃ and CH₃NH₃PbBr₃, continue on lead-free double perovskite Cs₂AgBiBr₆, 2D systems: A₂BX₄ (CH₃NH₃)₂BX₄ (B= Cu, Mn; X= Cl, Br, I) and, likely in the second funding period, the 0D system: (CH₃NH₃)₃Bi₂I₉.

It is our intention to collaborate with as many projects within the Schwerpunktprogramm as possible and investigate our own crystals as well as the respective thin film systems from others.

Many-body interactions in two-dimensional halide perovskites: exciton-electron complexes & electron-phonon coupling

Alexey Chermikov (TU Dresden), Claudia Draxl (HU Berlin), David Egger (TU Muenchen), Henry Snaith (Uni Oxford)

Carsten Deibel (TU Chemnitz) & Yana Vaynzof (TU Dresden & IFW Dresden)

Defects in metal halide perovskites for photovoltaics are a topic of intense interest and scrutiny. While metal halide perovskites are generally categorized as ‘defect tolerant’, significant efforts are dedicated to suppressing defects in perovskite materials in order to improve the photovoltaic efficiency and stability of perovskite solar cells. This, seemingly contradicting, practice highlights the gaps in the fundamental understanding of the nature of defects in perovskite materials. In the first funding period of the project , we focused on identifying the spectroscopic signature of ionic defects by purposefully and systematically introducing them into the perovskite structure and investigating them by different types of defect spectroscopy. In the second period, we aim at deepening our knowledge of the thermodynamic properties of ionic defects in perovskite semiconductors, by transferring the developed methodology to the study of the impact of microstructure, in particular grain size in 3D perovskites, and dimensionality, considering 2D and quasi-2D perovskites, which are becoming increasingly relevant for photovoltaics and light-emitting applications. Furthermore, we will investigate the relationship between the properties of the ionic defects to the solar cell parameters and hysteresis behavior in high-efficiency perovskite solar cells by identifying the impact of ionic defects on device physics and energetics. Finally, we will investigate how defect properties are related to device degradation upon exposure to environmental factors, light and elevated temperatures. To summarize, in the second phase of , we will apply a complementary combination of methods, including time-resolved, spatially-resolved and temperature dependent electronic and optical spectroscopies to obtain a clear physical picture of the role of defects in perovskite solar cells for the photovoltaic performance and stability.

Perowskit Defekte: Physik, Evolution und Stabilität

Defekte in Metallhalogenid-Perowskite sind Gegenstand großen wissenschaftlichen Interesses und intensiver Untersuchungen im Bereich der Photovoltaik. Obwohl Metallhalogenid Perowskite häufig als ‚tolerant‘ gegenüber Defekten beschrieben werden, gibt es erhebliche Anstregungen, um Defekte in Perowskiten zu unterdrücken, um ihre photovoltaische Effizienz und Stabilität in Solarzellen zu verbessern. Diesee scheinbare Widerspruch unterstreicht das unzureichende Verständnis der Natur von Defekten in diesen Materialien. In der ersten Förderperiode des Projekts lag der Fokus auf der Identifikation der spektroskopischen Signatur von ionischen Defekten, indem diese gezielt und systematisch in die Kristallstruktur der Perowskiten eingebracht und durch verschiedene Arten der Defektspektroskopie untersucht wurden. Im Fokus der zweiten Förderperiode steht die Vertiefung des Wissens über die thermodynamischen Eigenschaften ionischer Defekte in Perowskit-Halbleitern. Dies wollen wir durch eine Übertragung der von uns zuvor entwickelten Methodik auf die Untersuchung des Einflusses der Mikrostruktur, insbesondere der Korngröße in 3D-Perowskiten, und der Dimensionalität, unter Berücksichtigung der für Photovoltaik und optoelektronischen Anwendung wichtigen 2D und Quasi-2D-Perowskite, erreichen. Darüber hinaus werden wir die Beziehung zwischen den Eigenschaften der ionischen Defekte und den Solarzellenparametern und der Strom-Spannungs-Hysterese in hocheffizienten Perowskit-Solarzellen untersuchen, indem wir den Einfluss ionischer Defekte auf deren Funktion und energetische Eigenschaften identifizieren. Schließlich werden wir untersuchen, wie die Defekteigenschaften mit der Degradation des Bauelements bei Einwirkung von Licht und erhöhten Temperaturen zusammenhängen. Zusammenfassend werden wir in der zweiten Phase von eine komplementäre Kombination an Methoden anwenden, einschließlich zeitaufgelöster, ortsaufgelöster und temperaturabhängiger elektronischer und optischer Spektroskopie, um ein tiefgehendes physikalisches Verständnis der Defekten in Perowskit-Solarzellen zu erhalten und ihre Rolle bei der Limitierung der Leistungsfähigkeit und Stabilität zu bestimmen.

Tian Du (FAU) & Jens Harting (FZ Juelich)

Hybrid organic-inorganic perovskite materials have rapidly emerged as a highly promising alternative to silicon solar cells. However, the fundamental properties of the film formation mechanisms during solution-processing of the photoactive layers still remain poorly understood, which hinders the development of thick, vertically monolithic perovskite films with superior semiconducting quality. In this project, the main goal is to develop a precise model predicting film formation of solution-processed perovskite films, gain control over the fundamental governing mechanisms, and subsequently improve the film morphology. We propose a coupled approach where experimental measurements and simulation results complement each other. We will develop and use an in-situ measurement chamber to monitor the film formation under controlled conditions and in parallel we will perform phase field simulations of the nucleation and growth of perovskite crystals upon drying and post-processing. This will give insight into the morphology formation process, as well as into structural features of the dried film such as surface coverage, roughness, crystal sizes and stacking. We aim at establishing mechanistic rules regarding the impact of process parameters such as solvent choice, quenching temperature, rate and method, and post-drying steps on the final film quality. We will also investigate how nucleation agents introduced in the solution or at the substrate can be used to monitor nucleation and improve the film quality. The established physics-based design rules for ink formulation and process parameters will be used to demonstrate experimentally defect-free polycrystalline film structures, even for thick layers and for different precursor solution formulations. Selected perovskite films will be completed to solar cells in order to validate the impact of the morphology improvement on the photovoltaic performance.

Vladimir Dyakonov (Uni Wuerzburg) & Zhaofu Fei (EPFL)

Aiming to find out how to improve the stability of lead halide perovskite solar cells, our interdisciplinary team investigated several stabilising additives developed at EPFL during the first funding period of SPP2196/1. We studied their effect on solar cells with Cs0.05MA0.05FA0.90PbI3 absorbers with a PCEs higher than 21% under thermal stress, while controlling the film morphology, crystallographic properties, optical properties, charge carrier mobility and open circuit voltage. The results obtained so far clearly indicate an improved stability of the films and devices prepared with functionalised additives, especially without compromising the performance efficiency. In addition, we have made some further interesting observations with regard to ion migration and photoluminescence quenching efficiency and found a strong influence of the mesoporous electron transport layer on all these effects. We therefore propose a targeted research project to definitively clarify the interaction mechanisms and the origin of the effect of pre-selected ionic liquids as well as the microscopic mechanism behind them, but also to prove their applicability to other perovskite compositions. On the experimental side, we will apply two transient techniques to study, on the one hand, relatively fast processes of free carrier recombination using the microwave-based TRMC method and on the other hand relatively slow processes related to ion migration using transient open circuit voltage (OCVD). The outcome of the research project will be the understanding of the mechanisms of interaction between ionic liquids and perovskite bulk as well as interfaces, and finally the development of concepts to improve the intrinsic stability of perovskite solar cells in general. Particular attention is paid to ion migration during device operation, which contributes to destabilisation of the perovskite layer. At present, it is not clear which exact mechanisms lead to this ion migration and how it can be minimised, e.g. to avoid possible reactions with the metal oxide or other internal interfaces, but also to ensure a defined electric field distribution within the solar cell so that its electrical behaviour can be reliably described and predicted. Last but not least, when using stabilising additives, which are also ions, it is also very important to distinguish between intrinsic and extrinsic ions in the perovskite absorber, which has not been possible experimentally so far.

Stefan Glunz (Uni Freiburg), Jan Christopher Goldschmidt (Uni Marburg), Stefan Weber (Uni Mainz)

All-perovskite tandem solar cells promise high efficiencies and low costs. Hybrid perovskites stand out because it is possible to produce films of only a few hundred nanometers and low defect density with relatively simple processes. The tandem geometry enables efficiencies beyond the radiative limit of single junction solar cells. Despite rapid progress, perovskite-perovskite tandem solar cells still face several challenges: (i) The stability of the absorbers. The high bromide content necessary for high bandgaps of top cell absorber favors phase transitions, while Sn2+ in the low-bandgap absorber might oxidize to Sn4+. (ii) Interface recombination at the interface between absorber and the electron transport layers (ETL) or at one of the various interconnection layers, e.g. caused by lateral heterogeneity and leakage. Furthermore, the loss of volatile components, ion migration, or chemical reaction might lead to recombination-active trap states or extraction barriers. (iii) Optical losses, such as parasitic absorption losses in charge transport and interconnection layers, reflection losses and incomplete absorption, which limit the current especially of the bottom low-bandgap solar cell. The key to solving these challenges lies in the many interfaces in a tandem solar cell. Strategies include additives, interface passivation, new contact materials and thinning of charge transport and interconnection layers as well as structuring for increased absorption. For a target-oriented optimization of tandem cells, a thorough understanding of the associated interfaces is of crucial importance for the further technology development. Therefore, the aim of this project is to investigate the physical and chemical effects that lead to performance losses at the interfaces of perovskite-perovskite tandem solar cells. To this end, we will manufacture perovskite-perovskite tandem device, and will develop advanced spatially resolved characterization tools complemented.

Perovskite solar cells with graphite electrodes: Advanced interfaces for highest performance and stability (PeroGAIN)

Jan Christopher Goldschmidt (Uni Marburg), Henry Snaith (Uni Oxford), Uli Würfel (Uni Freiburg)

Helen Grüninger (Uni Bayreuth) & Anna Köhler (Uni Bayreuth)

Lead halide perovskites are currently considered one of the most promising semiconductor materials for the next generation of solar cells. Due to their structural flexibility to incorporate mixtures of different halides, the band gap energy of mixed perovskite compositions is easily adaptable so that highly efficient tandem solar cells can be produced. However, when exposed to light, mixed halide perovskites exhibit a severe demixing behavior of their ions, which leads to deterioration of the optoelectronic and device performance of the perovskite and corresponding solar cells, currently hindering their successful commercialization. This project thus aims to comprehensively understand the complex structural dynamics that occur during the (de)mixing of halide perovskites, to improve their phase stability, while preserving their excellent optoelectronic properties.

To this end, we will produce, physical MAPbI₃ – MAPbBr₃ mixtures with well-defined morphological, as well as bulk and surface defect properties, e.g., using additives, and systematically expose them to temperature and/or illumination to initiate and drive mixing and subsequent demixing processes. These processes will be investigated in-situ using powerful and complementary characterization methods, i.e., a combination of XRD, NMR and PL spectroscopy. The multi-modal in-situ measurements and corresponding analysis will allow us to extract detailed insights about the time evolution of relevant parameters such as stoichiometry, defect densities, excited state properties and formation of intermediate phases.

We will evaluate which role the morphology, different types of defects (at the surface and in the bulk) and possible intermediate compositions with increased thermodynamic stability play for ion diffusion lengths and pathways. This will enable the development of a clear understanding of the kinetic and thermodynamic laws of ion migration and thus (de)mixing processes in mixed halide perovskites. With that, relevant material properties determining reversibility and irreversibility of (de)mixing processes will become clear.

Thus, this proposed project significantly will contribute to developing a fundamental understanding about the impact of morphology, defect properties and additives on ion migration and phase stability of mixed halide perovskites.

Correlating Defect Densities with Recombination Losses in Halide-Perovskite Solar Cells (CRE-ATIVE)

Thomas Kirchartz (FZ Juelich) & Michael Saliba (Uni Stuttgart)

Norbert Koch (HU Berlin), Dieter Neher (Uni Potsdam), Felix Lang / Martin Stolterfoth (Uni Potsdam), Thomas Unold (HZB)

During the first funding period, the SURPRISE project aimed at unravelling the processes that govern interfacial recombination in high performance pin-type cells with organic charge transport layers, with particular focus on understanding the interfacial recombination pathways, the energy level alignment at the interface, as well as the operation mechanism of interlayers and back-surface fields due to doping. The project has also opened up new research directions, by developing advanced opto-electronic characterization methodologies, by identifying a novel operando energy level alignment phenomenon, and by highlighting the role of mobile ions and of the built-in field on device performance. In the proposed follow-up project SURPRISE II, the same team from the first funding period will address demanding research questions of highest topicality with respect to interfacial recombination
and defect states. In particular, we will focus on the impact of the grain size, surface and interface chemistry, doping, and crystal strain on the interfacial recombination losses. New research objectives aim at unraveling the impact of interfaces on device ageing and degradation, and the development of advanced experimental methodologies to reliably determine the built-in voltage and the entire solar cells’ band diagram – in the ground state and under operation conditions. Furthermore, we target providing comprehensive understanding of the impact of mobile ions on interfacial recombination, especially with respect to device degradation. Finally, based on our findings in the first funding period with regard to the location of the dominant interfacial recombination loss, point contacts with electron TLs in combination with newly designed interlayers will be developed to reduce the
contact area between the perovskite and the TL. Overall, the SURPRISE II project will establish novel principles to systematically mitigate interfacial recombination for efficient and more stable pin-type cells approaching 25% power conversion efficiency.

Sabine Ludwigs (Uni Stuttgart) & Lukas Schmidt-Mende (Uni Konstanz)

The project “Perocryst” is a highly interdisciplinary project between physics and materials science / physical chemistry coupling the expertises of Prof. Lukas Schmidt-Mende (device physics, nanoscience) and Prof. Sabine Ludwigs (controlled crystallization, morphology tuning, materials science). The main objective of the project is to gain a deeper understanding and control of nucleation and crystal growth of high quality perovskite films to ultimately improve solar cell performance in terms of reproducibility, efficiency and stability. The crystallization shall be manipulated and guided towards single crystalline thin perovskite films with ideally no defects and grain boundaries. Systematic correlations of the number of grain boundaries and size of domains with respect to their role as trap states shall be established.

In the project we will focus on material processing and post-annealing, in depth investigation of the parameters influencing the film crystallization, such as topographically & chemically patterned substrates and electric field guided growth, and finally the opto-electronic characterization, including (anisotropic) charge transport, solar cell performance and dynamic characterization of transport and recombination.

The key to success of our strategy is that our preparation methods allow a reproducible perovskite film formation with tailored number of grain boundaries. Our post-annealing approaches allow a direct means to tune nucleation and guide crystal growth. Single crystals in films are ultimately targeted as perfect candidates for solar cells. The combination of in-situ characterization tools for the crystal growth and in depth-characterization on films allows a systematic approach to improve the film quality and understand the growth mechanism and its governing parameters.

With our project, we aim to achieve:

• single crystalline films or films with low number of grain boundaries / low nucleation densities
• establishing design rules for best quality crystallized films
• conclude on structure-function relationships between film quality, stability and device performance

Selina Olthof (Uni Köln) & Thomas Riedl (Uni Wuppertal)

Recently, two dimensional (2D) perovskites have gained more and more attention as a strategy to tailor the interfaces in devices. They have turned out to be a key to unlock high efficiencies and improved stability in perovskite solar cells, in particular when employed at the interface between the photoactive 3D perovskite and the charge transport layers. The reasons for these improvements are a subject of a vigorous ongoing debate and a clear understanding of the nature of these 2D/3D interfaces is still in its infancy.

In this project, we will investigate pure 2D perovskites to look for changes in their electronic structure, optical properties, and their stability depending of the choice and size of the bulky A-site cation. In comparison to their 3D analogues, their ability to form stable interfaces with charge transport materials will be studied in detail. This is particularly important, as no research on the chemical interaction between 2D perovskites and metal-oxides has been published so far. Next, 2D perovskites will be studied as thin layers on top and/or below an optimized 3D perovskite absorber material. The most promising combinations of 2D and 3D perovskites will be integrated in solar cells where we are in particular interested in comparing the open circuit voltage with the quasi Fermi level splitting to understand the contribution of parasitic recombination and limited charge extraction to the overall losses in device performance. This will help to clarify whether the presence of 2D interfacial layers can suppress recombination that would otherwise occur if the 3D material is in direct contact with other charge transport layers, such as fullerenes, metal-oxides, etc. Aside from shelf-life under various conditions (inert, ambient, heat), the operational stability of pure and mixed-halide systems is of paramount interest. The fundamental understanding that will be gained in this project will be indispensable for further substantial improvements of efficiency and long-term stability of perovskite solar cells.

Sebastian Polarz (Uni Hannover), Lukas Schmidt-Mende (Uni Konstanz), Stefan Weber (Uni Mainz)

The main task of the project is to create novel microcrystals composed of hybrid lead chalkogenide perovskites as model systems for spatially resolved measurements on how the occurrence of defects will influence optoelectronic and electrical properties. The challenge for the second phase of the project is to create systems in which a defect can be turned on (and ideally off again). This should be achieved by the incorporation of special, reactive compounds into the perovskite microcrystals. These special constituents will change their state if radiated by light or if exposed to a current or to a field, thus giving rise to a special type of point-defect in the microcrystals. The number and density of such point-defects will alter the physical properties of the microcrystals. A highly interesting case is, if the point-defect has a vectorial property such as a dipole moment, because this will generate an anisotropy of physical properties in the hybrid perovskite microcrystals. Potentially, the defects may also interact with each other resulting in collective defect features.

The optoelectronic properties and electrical properties will be investigated on a single-particle basis using spatially resolved techniques such as µ-photoluminescence, transient absorption microscopy and temperature-dependent measurements and dielectric spectroscopy using Kelvin-probe force microscopy measurements. This will allow to define the effect of defects and intentionally introduce defects to design the optoelectronic properties of perovskites crystals.

Roland Scheer (Martin-Luther- University, Halle) & Paul Pistor (Universidad Pablo de Olavide, Sevilla)

The goal of this project is the development and kinetic quantification of multi-stage all-vacuum growth processes of multi-component ABX₃ perovskite thin films for solar cells. With multi-component perovskite films we mean multi-cations and multi-anions in an alloy film. In contrast to simultaneous co-evaporation, multi-stage processes are preferable for industrial fabrication. The processes shall be all-dry. Different film growth stages can be
(a) Precursor vapor reaction where a solid state precursor film reacts with vapor forming the perovskite film.
(b) Diffusion couple reaction where two or more solid state precursor films react by chemical diffusion.
(c) Exchange reaction where one component (anion or cation) is fully or partly exchanged by another component during annealing processes.
State-of-the-art Pb-based perovskites but also prospective Sn-based compounds shall be studied. Using reaction processes performed under controlled temperature and fluxes, reaction constants, diffusion constants, and activation energies will be quantified. The main experimental method employed in this project is in-situ X-ray diffraction in real-time which gives information on the phase formation processes during thin film growth.

Selina Olthof (Uni Köln) & Thomas Riedl (Uni Wuppertal)

Tandem solar cells based on a serial connection of wide-gap and narrow-gap sub-cells allow to minimize thermalization losses and thereby unlock improved efficiencies compared to single junctions.

After a very successful first 2-year phase of this project we will continue our effort in this extension project to explore wide-gap perovskites, such as the all inorganic compounds CsPb_xSn_1-xBr₃ or CsPb(I_xBr_1-x)₃, in order to further improve on the long-term stability of the perovskite sub-cell. We will identify optimal charge extraction layers that afford minimal losses in V_oc and improved stability. The overreaching goal is to unravel the interactions taking place at these interfaces in order to understand their effects on phase segregation and quasi Fermi level splitting observed during the first funding phase. Complementing the device characteristics, the careful analysis of the electronic structure by photoelectron spectroscopy will be of paramount importance to gain these key insights. Regarding the organic small-gap sub cell, we will investigate the origin of the unexpected operational stability of our non-fullerene cells and explore if this is a general property that likewise applies for other NFAs, ideally with even further reduced energy gap. At the end of the second phase we aim to realize perovskite/organic multi-junction cells that outperform the best current perovskite/perovskite tandem cells. 

Dmitri R. Yakovlev (TU Dortmund Universtity, Germany) and Maksym V. Kovalenko (ETH Zürich, Switzerland)

The project goal is to study energy and spin structure of exciton complexes in low dimensional metal halide perovskites nanostructures (nanocrystals and 2D layers) and to clarify the role of quantum confinement on the spin-related parameters and on the spin-dependent phenomena. We plan technological and experimental studies being supported by collaboration with theorists. Synthesized samples will be all-inorganic and organic-inorganic lead halide perovskite nanocrystals of various sizes and chemical compositions, and 2D layered materials with thickness ranged from 1 to 5 monolayers. Also 2D samples will be fabricated with Pb substituted by Sn for lead-free materials or doped with magnetic Mn-ions to prove the concepts of diluted magnetic semiconductors. For experimental studies optical techniques, involving low temperatures, high magnetic fields up to 60 T, polarization analysis, time-resolution in temporal range from 200 fs to seconds and high spectral resolution will be used. We will focus on spin-dependent phenomena and spin dynamics of interacting spin systems of carriers (electrons and holes), excitons (neutral and charged) and nuclei spins by measuring g-factors, spin relaxation and spin coherence times and identifying mechanisms of spin-spin interactions. In Mn-doped samples interaction with localized Mn-spins will be additionally involved, which typically results in giant magneto-optical effects. We will search for new physical phenomena and new regimes of the known phenomena provided by the unique properties of the perovskites. The gained knowledge will be used for optimizing technology, parameters of light-emitting devices and for testing the feasibility of the perovskite nanostructures for spintronics applications.

Juliane Borchert (University of Freiburg, ISE)

There are significant gaps in our understanding of the vapor phase deposition processes of perovskites thin films, in particular on film formation, crystallization and optimization of perovskite layers. In order to close these gaps it requires measurements during deposition in the vacuum chamber to be able to follow the growth of the layers in-situ. In order to enable such in-situ measurements, we will combine two powerful methods: Vapor deposition and photoluminescence.

The layers are produced by physical vapor deposition. This method is not only ideal for the production of high-efficiency solar cells, but also enables the integration of perovskite layers in tandem solar cells. In tandem solar cells combine two different semiconductor materials, which in combination are more efficient than a standard solar cell with only one semiconductor. standard solar cell with only one semiconductor. Vapor phase deposition is ideal for this, as the method is additive and therefore there is no risk of damaging the semiconductor layer that has already been produced, when the second layer is applied.

• Steve Albrecht, Berlin
• Martin Stolterfoht, Dieter Neher, Potsdam
• Thomas Riedl, Wuppertal
• Henry Snaith, Oxford (UK)

The recent improvements of the photovoltaic performance of metal-halide perovskites give promise for the realization of thin-film multijunction solar cells with power conversion efficiencies exceeding the Shockley-Queisser limit for single junctions. However, by now, monolithic all-perovskite tandem devices perform well below their band gap-optimized single junction counterparts. In the proposed project, four partners from Helmholtz-Center Berlin, University of Oxford, University of Potsdam and University of Wuppertal join forces to analyse, understand, and develop perovskite solar cells with optical band gap of around 1.2 eV and 1.8 eV, and combine them into efficient monolithic all-perovskite tandem solar cells. Work will be devoted to the understanding and suppression of compositional instabilities from halide segregation in the high band gap perovskite absorbers, and the reduction of non-radiative recombination losses. In addition, the chemical stability and the charge carrier lifetime of low band gap perovskite absorbers will be analysed and increased to enable stable and efficient photon harvesting. Another focus of the joint work is evaluating the mechanisms of non-radiative recombination in layers of neat perovskites, and whether they differ for low and high band gap perovskite films. Also, inorganic and organic materials will be tested as charge selective layers and improved towards reduced contact-mediated open circuit voltage losses. Furthermore, the optical and electrical properties of the monolithic multilayer design will be investigated, including the development of highly transparent and chemically benign charge-transporting layers as well as suitable recombination interconnects. Expertise gained from this collaboration will finally be combined to demonstrate stable monolithic all-perovskite tandem devices with efficiencies that outperform the best perovskite single junctions."

• Denis Andrienko, Mainz
• Wolfgang Brütting, Augsburg
• Marcus Scheele, Tübingen

Lead halide perovskites (LHP) are well-known materials that have recently gained considerable attention in optoelectronics. Particularly LHP nanocrystals of the composition CsPbX3 (with X=Cl, Br, I) are very attractive as luminescent materials for potential applications in light-emitting diodes. They combine the advantages of bulk LHP – notably their defect tolerance, solution processability, and band-width tunability – with well-known features of colloidal quantum dots, like high photoluminescence quantum yield with narrow emission bandwidth as well as size and composition tunable colors. However, electroluminescent quantum dots often have the dilemma that high photoluminescence quantum yield and efficient charge injection and transport, which are the prerequisites for electroluminescence, are counteracting each other. For LHP nanocrystals similar problems may occur, but this has not been explored in detail yet. Thus the main goal of this project is to develop a novel ligand exchange strategy that simultaneously provides the required surface passivation of nanocrystals to yield high photoluminescence and at the same time induces sufficient electronic coupling between nanocrystals to enable good charge injection and transport in electroluminescent applications. Our unique approach will consist in using charge-neutral organic pi-conjugated ligand systems which induce covalent coupling of nanocrystals. In the course of the project we will address a number of scientific questions relevant to the fundamental understanding of LHP semiconductors.

• Thomas Bein, München
• Vladimir Dyakonov, Würzburg
• Carsten Reuter, München

Lead halide perovskite materials have created enormous excitement in the context of energy conversion as they offer promising properties for photovoltaic (PV) and light emitting applications. However, lead-halide perovskites suffer from drawbacks such as instability toward moisture, unreliable device performance and the inherent toxicity of lead. Although the encapsulation of solar panels can prevent the release of lead, lead will have to be collected at the end of the operational lifetime of those solar cells. The ideal solution for future perovskite solar cells therefore is to produce efficient and stable lead-free solar cells. The goal of our proposal is to explore and understand the potential of lead-free double perovskites for photovoltaic applications, and to provide conceptual tools for the design of new nontoxic and stable perovskite materials for PV applications. First single crystals and films of the reference material Cs2AgBiBr6 will be studied theoretically and experimentally regarding their optoelectronic properties and potential undesirable loss mechanisms, followed by a systematic exploration of the structural phase space of nontoxic lead-free double perovskites. To this end, we will systematically substitute the constituents of the reference material with the most promising elements, which will be suggested by high level theoretical calculations. The optical and electronic properties of those newly synthesized lead-free perovskite crystals and films will then again be studied experimentally and theoretically. Finally, the most promising double perovskite materials will be implemented into solar cells, followed by a detailed electronic characterization to provide guidance for further optimization of the newly synthesized lead-free double perovskite materials.

• Niels Benson, Duisburg
• Doru C. Lupascu, Essen

This project is designed to use ferroelectric and dielectric studies to better understand the underlying transport and screening mechanisms of the hybrid “perovskite” photovoltaic absorber materials. This is also with the intention to oppose the current trend in literature to confuse and misuse terminology from dielectric science in the context of the perovskite solar cell absorbers.

We have previously shown how much a dielectric assessment of these material can tell about charge transport and the type of mobile charge species [1]. Defect screening appears to be a major asset in this class of materials which is largely provided by local dielectric properties.

Different from true ferroelectricity, which follows a structural phase transition, hysteresis in the solar cell hybrid “perovskites” is mostly due to charge transport. It is thus fundamental to understand how much of this is due to space charge (electronic carriers and Schottky barrier effects) or due to ionic drift. We intend to deconvolute ionic drift from electronic effects. Typical device structures are thin so accessing the underlying mechanisms is difficult, because typical screening lengths extend over the entire device thickness. The effects of the individual interface on device performance is thus hard to disentangle from the influences of other interfaces.

In order to design “clean” systems, we will grow a series of single crystals, characterize them by dielectric spectroscopy (mHz to IR including Raman spectroscopy), ultrasound spectroscopy, Kelvin probe spectroscopy, and electrical hysteresis studies. One technique will be quantitative ToF-SIMS after different times of unipolar load combined with Kelvin probe microscopy and µ- hotoluminescence in order to understand how much ionic drift will alter the electrical structure of the device. Furthermore, temperature dependent Hall effect studies will provide effective masses of the carriers involved.

On the local scale, conductive AFM, piezoforce microscopy, Kelvin probe spectroscopy and scanning probe elasticity studies will be provided amended by TEM EDX. The observed electrical profiles will be correlated to the ionic distributions. Conductive AFM will provide information of the electrical properties of domain and grain boundaries. Elastic as well as potentially ferroelectric domain walls will be thoroughly investigated plus the particular role of grain boundaries. Micro photoluminescence yields information on the local optical properties near grain boundaries. Classical parameters as charge carrier lifetime and work functions (from XPS and UPS) will be provided in collaboration with other groups from the SPP.

Large high quality single crystal hybrid perovskites will be synthesized analyzing the dynamics of crystallization, the morphology and their impact on the optoelectronic properties. We will start again on drosophila CH3NH3PbI3 and CH3NH3PbBr3, continue on the lead-free double perovskite Cs2AgBiBr6, 2D systems: A2BX4 (CH3NH3)2BX4 (B= Cu, Mn; X= Cl, Br, I) and, likely in the second funding period, the 0D system: (CH3NH3)3Bi2I9. For most anisotropic systems crystal growth is highly anisotropic.

• Jan Behrends, Berlin
• Norbert Nickel, Berlin

As record efficiencies of hybrid perovskite-based single and perovskite/silicon tandem solar cells approach those of conventional silicon solar cells, their stability becomes the most important issue. Although research on the stabilization of hybrid perovskite-based devices progresses, radiation-induced defect states remain under-investigated. Under operation, however, any solar cell is exposed to the UV/VIS and NIR part of the electromagnetic spectrum. Especially the high energetic part of the spectrum evidently forms trap states that act as non-radiative recombination centers. The same phenomena render spectroscopic methods that require UV, electron, or X-Ray radiation unreliable.

This project proposal aims at a fundamental understanding of radiation-induced localized defects. Hence, defects will be generated in a controlled way employing light as well as high energetic proton irradiation. High energetic protons are an ideal choice for defect generation experiments, since they are capable of creating defects by dissociation of bonds in the organic cations and by displacing individual nuclei of the inorganic lattice. Defects generated by light and proton irradiation will be characterized with the aim to identify their electronic, structural, and optical properties and their impact on charge transport. For this purpose, a unique combination of electron paramagnetic resonance (EPR), vibrational spectroscopies, and surface photovoltage spectroscopy will be employed. To identify the nature of the defect states, we will not follow the concept of compositional engineering, which is widely used for high-efficiency solar cells. Instead, analysis will begin with one of the simplest hybrid perovskites: CH₃NH₃PbI₃. Then the halide anions, as well as the organic cations, will be interchanged one by one. Additionally, lead, carbon, nitrogen and hydrogen isotopes will be incorporated into the perovskites to unambiguously identify the microscopy structure of localized defects and defect complexes. With this approach we will establish a sound understanding of the formation and the nature of radiation-induced defect states. This is an important step that will lay the foundation for developing stable hybrid perovskites.

• Alexey Chernikov, Regensburg
• Claudia Draxl, Berlin
• David Egger, München
• Henry Snaith, Oxford

The recent intense scientific efforts related to halide perovskites (HaPs) have revolutionized and invigorated the solid-state research community. The tremendous interest was fueled by rapid advances in the performances of HaP-based photovoltaic and other optoelectronic devices. Several of the breakthroughs in the field were highly promising, considering today’s challenges in developing alternative energy sources and cost-efficient lighting applications. However, a wide array of fundamental questions remains to be addressed, especially regarding key issues associated with the interplay of optical and structural excitations. Complemented by the ongoing search for new HaPs, this motivates our scientific questions: We are interested in the microscopic nature of optical transport and how it is impacted by Coulomb interactions of the charge carriers and non-trivial lattice dynamics, towards fully exploiting the structural, electronic, and chemical tunability of HaPs.

The goal of our research is to provide answers to these critical questions by studying low-dimensional HaPs from combined experimental and theoretical perspectives. We aim for obtaining a comprehensive picture of propagation and scattering of interacting charge carriers coupled to lattice vibrations across a broad range of technologically relevant scenarios in two-dimensional (2D) HaPs. The motivation underlying our focus on 2D HaPs is the possibility of an efficient tuning of the Coulomb physics and a control over structural dynamics and exciton-phonon couplings through the exceptional material flexibility of this material platform. To this end, we will explore optical transport in 2D HaPs and develop a fundamental understanding of the interplay of Coulomb effects with structural dynamics. In our project, these insights will be directly transferred to 2D HaP device development and the synthesis of novel compounds.

To achieve these goals, a highly efficient feedback loop between theory, spectroscopy, synthesis, and device physics will be established. We will perform microscopic calculations of structural dynamics, study electronic and excitonic effects, conduct frontier optical spectroscopy allowing for direct monitoring of electron-hole propagation, and go all the way from addressing advanced materials synthesis to their applications in devices. The research efforts of our consortium are embedded in a broader collaborative network involving excellent theoretical and experimental partners. Our agenda addresses timely questions on the forefront of the ongoing research efforts in the field. We expect that achieving our research objectives will allow for establishing a fundamental basis to control optical excitations in HaP-based systems via exploring the interplay of optical and structural dynamics across a wide range of technologically relevant compounds. With this, we hope to develop a general framework guiding future advances in regard to hybrid semiconductors.

• Caterina Cocchi, Berlin
• Eva Unger, Berlin

The overall goal of this project is to gain fundamental understanding on the structural, electronic, and optical properties of metal-halide complexes in solutions and to follow their evolution towards the formation of crystalline semiconductors through low-dimensional intermediates. We will tackle these research questions in an interdisciplinary framework at the boundary between chemistry and physics adopting strongly intertwined experimental and theoretical approaches. The coordination chemistry of halido-plumbate complexes in different types of solvents will be investigated systematically with a combination of state-of-the-art X-ray spectroscopy techniques, including extended X-ray absorption fine structure, X-ray absorption near-edge structure, and X-ray diffraction (XRD). Geometries, structures, and binding strengths of these systems will be disclosed in combination with density-functional theory (DFT) calculations. In this way, we will be able to rationalize how solvents influence the formation of intermediate solvate phases. We will further identify and rationalize the spectral fingerprints of metal-halide complexes in solution by means of (time-resolved) optical spectroscopy complemented by first-principles calculations based on many-body perturbation theory (MBPT). Finally, the evolution of solution complexes as 0D building blocks of hybrid metal-halide perovskites towards low-dimensional crystalline intermediates and solid-state semiconductors will be followed by a combination of in-situ optical monitoring, based on XRD and X-ray fluorescence, and novel correlative optical and core-level techniques that will be developed within the project. These studies will be complemented by DFT and MBPT calculations on the crystalline intermediates, giving access to electronic, optical, and core-level excitations with state-of-the-art accuracy. The outcomes of this project will provide unprecedented insight into the fundamental processes leading to the formation of hybrid metal-halide perovskite semiconductors from solution complexes.

• Carsten Deibel, Chemnitz
• Sven Hüttner, Bayreuth
• Yana Vaynzof, Dresden

The advent of hybrid perovskite solar cells has given rise to extraordinary photovoltaic performance. However, the new physical characteristics of these materials are not yet completely understood, including the role of ionic and electronic defects on the solar cell performance, hysteresis and stability. In this proposal we will explore and characterize the nature of defects in lead halide perovskite thin films and photovoltaic devices, investigate how they influence the long-term stability and explore mitigation strategies for their passivation. We will fabricate both vertical photovoltaic devices and lateral devices from methylammonium lead triiodide and triple cation based perovskites, and tune the defect density in the perovskite active layer by varying the stoichiometry of the precursor solution with high precision in a systematic fashion. The relation of defect states and their properties — type, activation energy, concentration, distribution, surface or bulk, etc. — to the solar cell parameters and degradation will be investigated in view of the fractional changes in stoichiometry. We will apply a complementary combination of experiments, including time-resolved, spatially-resolved and temperature dependent electronic and optical methods to obtain a clear physical picture of the role of defects in perovskite materials. Beyond this fundamental understanding of defect physics, it is our goal to identify the nature of the most prominent defect states in lead halide perovskite solar cells and to pursue strategies for their passivation in order to improve both the performance and the long-term stability of perovskite photovoltaic devices.

Die Wiederentdeckung von hybriden Perowskithalbleitern als Solarzellenabsorber hat zu außerordentlichen photovoltaischen Wirkungsgraden geführt. Die physikalischen Charakteristika dieser Materialklasse sind allerdings noch nicht vollständig verstanden; insbesondere die Rolle von ionischen und elektronischen Defekten hinsichtlich Leistungsfähigkeit, Hysterese und Stabilität ist noch nicht abschließend geklärt. In diesem Forschungsvorhaben wollen wir die Natur der Defekte in Dünnschichten und Solarzellen aus Bleihalogenid-Perowskiten ergründen, wie sie die Langzeitstabilität beeinflussen, und mit welchen Strategien sie passiviert werden können. Wir werden sowohl vertikale Solarzellenarchitekturen verwenden, als auch laterale Bauteile; als aktive Materialien haben wir Perowskite auf der Basis von Methylammoniak-Bleitriiodid und Dreifach-Kationen vorgesehen. Die Defektdichte in der aktiven Perowskitschicht werden wir über eine systematische Variation der Stöchiometrie der Precursor-Lösung mit hoher Genauigkeit ändern und charakterisieren. Der Zusammenhang zwischen Defektzuständen und ihren Kenngrößen — Typ, Aktivierungsenergie, Konzentration, Verteilung, im Volumen oder an der Oberfläche vorkommend — zu den Solarzellparametern und Degradation werden wir in Abhängigkeit kleiner Stöchiometrieänderungen untersuchen. Die drei Projektpartner benutzen sich sehr gut ergänzende Experimente, welche transiente, ortsaufgelöste und temperaturabhängige elektrische und optische Methoden einschliessen, um ein genaues physikalisches Verständnis der Rolle von Defekten in Perowskithalbleitern zu erlangen. Ein über das Erlangen dieses grundlegenden Verständnisses hinausgehendes Ziel ist die Identifikation des Ursprungs der dominanten Defektzustände in Bleihalogenid-Perowskiten. Ferner erwarten wir, Strategien zur Passivierung dieser Defekte zu entwickeln, um die Leistungsfähigkeit und Langzeitstabilität von Perowskitphotovoltaik nachhaltig zu verbessern.

• Vladimir Dyakonov, Würzburg
• Mohammad Khaja Nazeeruddin, Lausanne
• Cristina Roldán Carmona, Sion

Reaching power conversion efficiencies of up to 23%, hybrid organic inorganic leadtri-halide perovskites have been established as the most promising class of materials for next generation thin film photovoltaics. Although improved processing protocols enabled an unprecedented increase in the device performance in less than 9 years, this type of absorber material has inherent instability resulting in undesirable degradation, ultimately resulting in short lifetimes of the solar cells. So far, most approaches to enhance device stability focused on device encapsulation in order to avoid extrinsic degradation via exposure to oxygen and/or moisture. In this collaborative interdisciplinary project combining inorganic chemistry, materials science and physics, we aim to understand the exact mechanisms behind the degradation of the perovskite layers with the ultimate goal of overcoming them with the use of certain functionalized ionic liquids as additives. Our preliminary works revealed an improved stability of CH3NH3PbI3 (MAPI) perovskite solar cells employing1-(4-ethenylbenzyl)-3-(tridecafluorooctyl)-imidazolium iodide (ETI) as a polymerizable additive without compromising power conversion efficiency. However, the physical nature of this effect is not yet known and therefore its transferability to other perovskites absorbers is not straightforward. The project aims to answer both questions. We will apply highly sensitive electrical as well as optical measurement techniques, especially thermally stimulated current (TSC), time resolved photoluminescence (TRPL),microwave conductivity (TRMC) and electron spin resonance (ESR) in order to detect microscopical compositional degradation of perovskite due to heat, vacuum and other stress factors and correlate it to changes of the optoelectronic properties, e.g. emerging of trap states. Particularly important is the formation of a PbI2 side-phase via break down of methylammonium cations, which will be probed by means of X-ray diffraction (XRD), mass spectrometry (MS), TSC, Raman spectroscopy, ESR andcurrent-voltage measurements. Finally, we intend to transfer our findings to other perovskite compositions with the goal of obtaining highly efficient mixed cation halide perovskite solar cells with exceptional long-term stability.

• Norbert Koch
• Dieter Neher, Potsdam
• Martin Stolterfoht, Potsdam
• Thomas Unold, Berlin

Halide perovskite semiconductors continue to surprise the community by their many extraordinary physical properties. A key for solar cell applications is their highly fluorescent nature likely due to low defect densities despite their simple processability from solution. In combination with long charge carrier diffusion lengths, perovskite solar cells have the potential to reach the power conversion efficiencies of monocrystalline silicon or even of GaAs cells. However, present perovskite devices usually reach significantly lower open-circuit voltages than allowed by the perovskite absorber, which implies that major non-radiative recombination losses occur elsewhere in the multilayer solar cell stack. In particular, for typical pin-type perovskite cells it has been shown that the recombination loss at hybrid perovskite/transport layer (TL) interfaces is usually 1-2 orders of magnitudes larger than the defect recombination in the absorber layer. Consequently, a comprehensive understanding of this key phenomenon is urgently required to further advance the field.  In this project, we aim at unravelling the most important mechanisms that govern interfacial recombination in order to devise systematic means to supress this recombination loss. Fundamental questions that will be addressed include the role of energy level alignment between the perovskite and the TLs, or whether interfacial recombination proceeds primarily through traps at the perovskite surface, across the interface or within the TL. To identify the dominant recombination pathway at the interface we will use a range of complimentary techniques with high time resolution (ps-to-ms) such as transient photoluminescence, transient absorption and THz spectroscopy. In combination with numerical simulations, we aim at establishing a comprehensive kinetic model to describe charge transfer and recombination at the interface and to address the working mechanism of wide-gap interlayers. With respect to the role of the interface energetics, an important step will be the determination of all relevant energy levels throughout the multilayer stack using photoelectron spectroscopy in dark and under realistic solar cell conditions. Knowledge of the actual device energetics will allow a deep understanding of solar cell operation through detailed drift-diffusion simulations and also provide essential design rules for efficient TLs. Furthermore, we aim to demonstrate improved pin-type perovskite solar cells (>23% efficiency) by minimizing the impact of interfacial recombination via perovskite (and TL) doping and creation of a back-surface field that repels minority carriers from the interface. Overall, our concerted fundamental approach is expected to greatly improve our understanding of interfacial recombination and will contribute to further approaching the thermodynamic efficiency limit in perovskite solar cells.

• Anna Köhler, Bayreuth
• Mukundan Thelakkat, Bayreuth

The joint project Köhler-Thelakkat focusses on the control and manipulation of charge transport and extraction in layered hybrid perovskites by modification of the internal as well as external interfaces of a 2D perovskite.

In the first part, we address the issue of anisotropy of charge transport in 2D perovskites, since these highly stable layered materials suffer from poor vertical charge transport due to isolating organic interlayers. To overcome the lack of charge percolation through the organic layers, we will synthesize and incorporate organic semiconductor ammonium cations that fit within the 2D layered perovskite crystalline structure and thus contribute to charge transport and absorption. Particularly the molecular energy levels and HOMO-LUMO gap of these ammonium cations will be tailored relative to the band gap of the inorganic layer. With this, we can address the fundamental question how the electronically active organic layer modifies the quantum well structure, and thus absorption and subsequent energy or charge transfer.

In the second part, we address the issues concerning the external interface between perovskite and the p-type extraction layer. Here we envisage the synthesis of novel doped p-type extraction layers by co-evaporation of diverse direct redox dopants and hole conductors in order to control the degree of doping, to avoid uncertain air-oxidation and to guarantee uniform distribution of dopants in hole conductor. We investigate the influence of dopants on the width of the DOS, trap-filling, Fermi-energy formation, the nature of charge transport (pseudo-percolation) and the resulting charge carrier mobility. Both the novel 2D perovskites and p-type layers will be incorporated in a p-i-n structure of solar cell to evaluate and understand the implications of our innovative approach. Charge injection and recombination at interfaces will be extensively studied in a joint work with Laura Herz, Oxford University.

• Karl Leo, Dresden
• Frank Schreiber, Tübingen

Perovskite photovoltaics have developed rapidly in recent years, reaching photovoltaic efficiencies well above 20% - close to the thermodynamic (Shockley-Queisser) limit. Furthermore, hybrid perovskites can be directly grown in 2D layered configuration by clever choice of the organic cation, thus creating a prestructured layer stack. For these types of perovskites, stability is increased immensely, even if only a thin layer of 2D perovskite is used as a barrier on top of a thicker “3D” thin-film. In this project, we plan to address the astonishing physical properties of these materials and the influence of dimensionality using vacuum evaporation. Though 2D-perovskites have been demonstrated from solution in a self-layering manner, 2D-layering using highly precise vacuum evaporation techniques has not been shown. Within PHIVE-X, we will use the variability in crystal structure and electric properties of the perovskites to follow two main paths: Realization of vacuum-deposited, self-structured 2D perovskites and manufacturing of highly-precise thin-film stacks of alternating perovskites with different stoichiometry – forced 2D perovskites. The latter is exclusively viable using the unique film control of vacuum deposition. We thus widen the possible material choices and open up a completely new field of perovskite research. By precisely controlling stoichiometry during growth, variations in the materials alter the band gap and refractive index in the ultra-thin films. In particular, we intend to exploit the opportunities of this method in a comprehensive manner, like tuning the band gap by almost 0.8 eV via interchanging iodide and bromine, as well as methyl ammonium and formamidinium. Both paths will be applied to device concepts: Self-structured and forced 2D perovskites will be implemented in solar cells as well as light-emitting devices and investigated towards their performance and stability. With forced 2D perovskites, we will also effectively form double heterostructures and superlattices. If correctly tuned, carrier and light confinement create a range of opportunities for optoelectronic applications like perovskite lasers. The novel structures created with this technique will be extensively studied with regard to the structural and electronic properties, combining the extensive experience of the Dresden and Tübingen groups.

• Thomas Mayer, Darmstadt
• Uli Würfel, Freiburg

Interrogate: Interfaces in perovskite solar cells investigated with photoelectron spectroscopy and modelling: Feed Back Loop of Full Device Fabrication, Full Device Photoelectron spectroscopy Operando Characterization and Full Device Device Modeling.

The project Interrogate aims at achieving a deeper understanding of the impact of interfaces between perovskite absorber and electron and hole transport materials in perovskite solar cells. To this end, we bring together our expertise in characterization and numerical device simulations. Advanced tapered cross sections photoelectron spectroscopy (TCS-PES) on full devices (Technical University of Darmstadt, Thomas Mayer) will be combined with different characterization techniques such as current-voltage characteristics, photo- and electroluminescence spectroscopy, transient photoluminescence and Suns-VOC (University of Freiburg, Uli Würfel). With the newly developed method tapered cross section photoelectron spectroscopy (TCS-PES, Thomas Mayer) we transfer the nm scale of the depth profile normal to the material layers to the mm scale of the tapered cross section by using a small angle of (0.02°). XPS and UPS line scans on these tapered cross sections are performed with a step width of 50µm allowing to map the potential profile in the space charge regions and throughout the device. These measurements will be performed on a number of devices fabricated and characterized at the University of Freiburg with different electron and hole transport layers, respectively. The experimental work will be complemented by numerical device simulations (University of Freiburg, Uli Würfel) in order to identify an appropriate quantitative description of all experimental data from the view-point of a full device model. Particular emphasis will be placed on the impact of the above mentioned interfaces. This shall enable to set up a meaningful hypothesis which will be verified or falsified in additional experiments based on rational and systematic parameter variation. This continuous feedback loop between the characterizations carried out at the labs of both partners and the refinement of the device model will enable the successful implementation of the work programme and to realize an improved understanding of how interfaces limit device performance and stability and identify ways to overcome these challenges. Thus, we will obtain most valuable information about chemical composition and the potential distribution in the working device. This will be implemented in the numerical device simulations by adjusting parameters such as band-gap, recombination coefficient and the density and energetic distribution of trap states and mobile ion/ion vacancy concentrations.

• Sanjay Mathur, Köln
• Klaus Meerholz, Köln

Hybrid lead iodide perovskites based on a mixture of A-site cations have attracted significant attention due to their higher stability when compared to the parent compound methyl ammonium lead triiodide ((H₃C-NH₃)PbI₃).  The objectives of this project center around fundamental understanding of formability, crystallization and grain boundary engineering in solution-processed A-site modified hybrid alkyl ammonium lead iodide perovskites. Through an integrated synthesis, processing and computational effort, functional perovskite inks with optimal crystal size and solvent chemistry will be developed (RG Mathur) to examine the optoelectronic properties of solid-state absorber films in device structures (RG Meerholz). In order to understand the synergistic effect of multi-cation hybrid perovskite structures on the electronic properties, detailed information on the role and position of the different A-site cations (both organic and inorganic) and crystal symmetry (stability) is critically important. Owing to the chemical and structural degrees of freedom available in the design of hybrid perovskites (ABX₃  à (A₁,A₂,A₃,..)(B₁/B₂/B₃)(X₁, X₂, X₃,…)₃), we propose to systematically vary the substitution of A-site cations, which are majorly responsible for the structural parameters (tolerance factor) and modulations in optical band gap (e.g., through rotational disorder). The research tasks in this project will primarily focus on (i) formulation of perovskite inks based on A-site modified chemical compositions ((A₁,A₂,A₃,A₄)PbI₃) in conjunction with solution studies (²⁰⁷Pb Nuclear Magnetic Resonance, Electrokinetic Sonic Amplitude (concentrated solutions) and Dynamic Light Scattering (dilute solutions) to develop synthetic protocols for controlled crystal growth and test new solvent systems for precise crystal engineering (ii) control of on-surface grain growth and tailoring of grain boundaries in single and tandem application methods (spin-coating and electrospraying) (iii) fabrication of perovskite solid-state absorber films to investigate the transport properties as function of the size of perovskite domains (iv) detailed understanding of perovskite materials, their composition, nucleation and layer growth and effects of intergranular and intragranular transport mechanisms in predevice structures  (v) demonstrate the impact of A-site engineering in mixed-cation perovskites by validating the experimental data through DFT calculations and (vi) fabricate thin film solar cell device structures and evaluation of the solvent influence, deposition technique and processing conditions on the device performance and stability. Preliminary studies on degradation of pre-device structures through light or heat-stimulated processes will be initiated as well.

In summary, this interdisciplinary effort will pursue an integrated synthesis – application – modelling approach to gather new insights in the nucleation behavior of perovskite crystals with A-site variations and in-situ studies on solvent effects to obtain stable and processable inks (RG Mathur), film formation and understanding of pre-device structure towards optical and transport properties (RG Meerholz) that will be validated by DFT calculations on defect chemistry of A-site mixed systems and modelling of grain boundary transport (RG Choi).

• Robin Ohmann, Siegen
• Alex Redinger, Luxembourg
• Michael Saliba, Darmstadt

This project explores epitaxially grown tin (Sn) perovskites as alternatives to lead (Pb) based materials. Importantly, Sn perovskites have a narrow bandgap that is in the ideal range for a single-junction solar cell, which will also enable all-perovskite multijunction solar cells. In addition, they are more environmentally friendly. However, so far power conversion efficiencies of Sn perovskites are falling short to their lead counterparts. One of the most critical obstacles to overcome is the tendency of Sn2+ to oxidize to Sn4+. Here, we will employ epitaxial growth methods to avoid solvents that promote chemical reactions leading to oxidation of Sn and we will characterize these Sn perovskites down to the atomic level. Specifically, we will experimentally evidence the atomic structure of Sn perovskite surfaces such as CH3NH3SnI3 to localize Sn4+ defects and understand interface phenomena that readily occur in perovskite solar cells. Furthermore, we will add dopants and adsorbates, such as environmental gas molecules to the surfaces, to fundamentally study the specific interactions that occur on the atomic scale with respect to performance enhancements, degradation, and oxidation. To bridge the size gap to the application, we will use surface techniques on the micrometer scale probing large-scale inhomogeneities, grain boundaries, workfunctions, contact potential differences and surface photovoltages. A full understanding on a device level necessitates the fabrication of Sn-based perovskite solar cells using state of the art solution-processing as a benchmark. Then we will apply the optimized architectures and use the knowledge from the nanoscopic and microscopic characterizations as well as epitaxially grown Sn perovskite absorbers to fabricate novel Sn based solar cells that are highly efficient and long-term stable. We believe that the correlation between atomic, micro- and macroscale on the same type of samples will be particularly fruitful to gain a thorough understanding of Sn perovskites.

• Sebastian Polarz, Konstanz
• Lukas Schmidt-Mende, Konstanz
• Stefan Weber, Mainz

Type and density of defects significantly affect the properties and function of a semiconductor leading to enhanced or reduced performance. The application in photovoltaics or in optoelectronics requires in-depth knowledge of defect-property relationships. This aspect is more prevailing for hybrid lead-halide perovskites, in which a high defect density exists due to relatively low cohesion energy of these solution-processed solids. The resulting films exhibit a broad range of defects such as various point-defects (vacancies, interstitials or heteroatoms), but also interfaces between phases within the perovskite or with charge injection/extraction materials (2-D defects) are relevant. Metal-halide perovskites show surprising tolerance to defects, nevertheless, non-radiative recombination is still the major loss channel and needs to be minimized to optimize device operation. Systematic investigations are aggravated by the complexity of hybrid perovskites and the technological need for sustainable, solution-based fabrication. Model systems are needed to answer questions about the limits and placing of defects in the lattice, and how their interplay determine associated electrical and photophysical properties. The generation of a specific defect architecture has not been achieved to enable, for example, controlled doping. The project brings together expertise in chemical synthesis (Polarz), physics of semiconductor nanostructures (Schmidt-Mende) and spatially resolved electronic and optoelectronic measurements (Weber, Deschler) to tackle following tasks: (i) aerosol synthesis of MAPX (CH3NH3PbX3); (ii) role of crystal direction and facets on optical and electronic properties; (iii) role of controlled dopant concentration (stoichiometry), (iv) advanced functionalization of MAPX by inclusion of new dopants. We apply a gas-phase based method, which generates highly crystalline and differently faceted microcrystals from special liquid single-source precursors. The precursor acts also as solvent, which allows us to dissolve heteroelements in the aerosol droplets for the controlled introduction of point-defects. The advantage of micropartices deposited on any desired substrate is, one can probe individual crystals, orientation and facets. We will apply techniques like conductive atomic force microscopy (C-AFM) and time-resolved Kelvin probe spectroscopy for clarification how the stoichiometry, presence of dopants and defects influence the local electrical and ionic conductivity. The effect of ferroelastic domains will be investigated on a single-particle level as well using piezoresponse force microscopy (PFM). Our understanding of electronic properties of defect-property correlations will be complemented by spectroscopic measurements on the photophysics of the defect-doped systems, with high-resolution photoluminescence microscopy, which is a unique method to resolve dynamical processes and radiative recombination on the relevant ultrafast time-scales.

• Ulli Steiner, Fribourg
• Lukas Schmidt-Mende, Konstanz

The focus of this project lies on controlled growth from polycrystalline towards single crystalline thin perovskite films. We will investigate the structure of the films in detail, which allows to conclude on the structure-performance relationship. Defining the role of trap states in the film formed due to grain boundaries will one major concern. We will gain the control over the crystallinity by the recrystallization of perovskite films and flash infrared annealing. By the controlled introduction of methylamine gas, which then intercalates into the MAPbI3 crystal structure, the film will liquefy. The recrystallization can
now be induced by removing the methylamine gas atmosphere, but also by heating the film e.g. by flash infrared annealing. Introducing seed layers and controlling the parameters, such as temperature and atmosphere, will allow defined crystallization conditions, which in ideal case will give us the possibility to control the crystal size from small grains to large single crystalline films. Using in-situ measurement tools observing in detail the crystallization process enable us to learn systematically about the importance of the different crystallization parameters and their manipulation to form the desired film quality. Such films are then ideal system to investigate the role of the trap states, such as grain boundaries, by spectroscopic methods. This will allow us to define the ideal film morphology and help to improve the device performance and stability of perovskite solar cells.

• Maksym Kovalenko, Zürich
• Dmitri Yakovlev, Dortmund

The project goal is to receive new basic knowledge on the electronic states and exciton complexes in metal halide perovskites of various chemical compositions, structures (single crystals, 3D, 2D, 0D) and in their nanocrystals (quantum dots and nanoplatelets). We plan technological, experimental and theoretical study of excitons, their energy and spin structure, recombination dynamics and coupling with light in exciton-polaritons. We will also focus on spin-dependent phenomena and spin dynamics of interacting spin systems of carriers (electrons and holes), excitons (neutral and charged) and nuclei spins, by measuring g-factors, spin relaxation and spin coherence times and identifying mechanisms of spin-spin interactions. We will search for new physical phenomena and new regimes of the known phenomena provided by the unique properties of the perovskites. The gained knowledge will be used for optimizing technology, parameters of photovoltaic and light-emitting devices and for testing the feasibility of the perovskites for spintronics applications. For experimental studies optical techniques, involving low temperatures, high magnetic fields up to 60 T, polarization analysis, time-resolution in wide temporal range from 200 fs to seconds and high spectral resolution will be used. The project goals cover several focus areas of the SPP2196 call: (i) spin effects/spin-orbit coupling; (ii) charge carrier dynamics, recombination, and transport; (iii) effects of dimensionality (single crystals, 3D, 2D, 1D, 0D) and composition tuning; and (iv) role of lead and possible alternatives.

- Dr. Daniel Niesner, Erlangen-Nürnberg

The development of high-efficiency lead-halide perovskite solar cells in recent years poses a major breakthrough in the field of second generation photovoltaics and a step towards a third generation. First generation solar cells are made from crystalline silicon. They are available as commercial products with lifetimes lasting decades and efficiencies around 26%. Their fabrication, however, requires a significant investment of energy and capital, and their maximum efficiency is limited to 32%. Second generation photovoltaics are produced as thin films at a potentially reduced cost using smaller amounts of material with less strict requirements to crystallinity. Mechanical flexibility opens up additional fields of application. Thin-film solar cells made from lead-halide perovskite use abundant materials and are processed at low temperature to reduce their financial and energetic cost. They have reached efficiencies around 22% at the laboratory scale. Research for commercial production is already underway. Moreover, initial research demonstrated the potential of lead halide perovskites for third generation photovoltaics. In this future generation of photovoltaics, novel concepts will be used to achieve efficiencies beyond the fundamental limit of 32% which applies to the first two generations. Large-scale applications of lead halide perovskite solar cells are limited by the contained toxic lead and problems with long-term stability. This project hence follows a complementary approach: The unique properties of lead halide perovskites on the atomic scale will be investigated to derive general design principles for high-efficiency solar cell materials. Despite intensifying basic research since 2013, it remains unclear to date what physical mechanism make lead halide perovskite solar cells so exceptional. Together with collaboration partners I found several relevant effects that can contribute to a high solar-cell efficiency: Due to the Rashba effect, electrons of opposite spin propagate differently through lead halide perovskites. Spin is a property of electrons that is not yet made use of in todays conventional devices. In addition, lead halide perovskites preserve the high energy of the blue spectral components in sunlight a thousand times longer than more traditional materials like silicon. This may also be a result of the Rashba effect. Alternatively, the formation of large polarons was proposed. Polarons are distortions of the crystal structure which might stabilize the electronic energy. Within this project, these effects will be quantified using state-of-the-art time-resolved spectroscopies. Their influence on device performance will be investigated by transport measurements performed in parallel. On the one hand, the results may open up new fields of application for lead halide perovskites. On the other, they will help to identify new materials for high-efficiency solar cells based on similar working principles.

(Emmy Noether Project)

- Felix Deschler, Munich

There is a constant search for new semiconducting materials with tailored properties for optimized, highly efficient energy conversion and use. Functional materials are omnipresent in technology, and specifically semiconductors have promoted high-tech electronics and sustainable energy generation. However, guided exploration of the immense materials space has been limited, since a fundamental paradigm on how structure and composition control electronic and optical properties of high-quality semiconductors is missing. This project will address and overcome this limitation by a combined spectroscopic and structural investigation on how the dimensionality of excitations effects their photo-physical properties, with particular focus on radiative recombination and excitation transfer at interfaces. Here, materials are required, in which the photo-physical properties of excited states can be tailored by facile, controlled variations of material parameters. This is now possible in the recent material class of hybrid metal-halide perovskites - so far mostly investigated for their use in photovoltaic applications - which provides an exciting scientific opportunity. I propose a broader fundamental scientific scope for hybrid perovskites, which aims to take the field beyond its current device-optimization driven research. Changes in crystal structure, dimensionality and composition in these solution-processed materials, give now unprecedented control over excited states and band structure. Further, efficient excited state conversion requires clarification of the energy and charge transfer processes between materials, which exhibit excited states with distinct electronic properties. Multilayer hetero-junctions with perovskite monolayers will provide unique van der Waals structures, likely with novel physical features. These will give the opportunity to study transfer processes at single-atom interfaces between nanosheets without the interference from bulk effects and disorder. The results from this project will generate impact in two directions: (1) Insights on fundamental connections between material structure and excited state properties answer central physical questions on the material control of electronic states. Feedback of these criteria to informatics-based approaches will change the current perspective on material exploration. (2) Identification of the criteria for high-quality semiconductors will lead to the discovery of unexpected, novel materials for optoelectronic applications. The tailored development of optical electronic properties will produce solar cells and light-emitting diodes with optimized efficiency.

- Selina Olthof, Cologne

Tandem solar cells based on a serial connection of wide-gap and low-gap sub-cells allow to minimize losses due to thermalization and thereby unlock elevated efficiencies. In organic multi-junctions the wide-gap cell (energy-gap about 1.8 eV), which should simultaneously provide a high V_oc and high J_sc, currently states the main limitation. Even in the best organic wide-gap devices the voltage loss, i.e. 1/q ´ E_g - V_oc, is unsatisfactorily high (about 0.8-1 V).

In this project we intend to design and realize hybrid multi-junction solar cells where the wide-gap sub-cell is based on an organo-metal halide perovskite absorber, which allows for a voltage loss as low as 0.3-0.4 V.

Reports of single junction perovskite cells with an efficiency >20% are accompanied by serious concerns about the stability of established perovskites like methyl ammonium lead iodide (MAPbI3). Perovskites based on mixed cations (e.g. MA and Cs) and mixed halides (e.g. I and Br), such as MA_1-xCs_xPb(I_(1-y)Br_y)₃, bear the potential of enhanced stability. In general, the addition of Cs cations, which are smaller than MA, as well as the addition of Br, both lead to a widening of the bandgap of the perovskite, which is favorable for their use in a tandem cell. Regarding the sub-cell with low energy gap (1.2-1.3 eV), organic photo-active materials are available and some systems will be provided by the group of Prof. Janssen (TU Eindhoven) for this project. As of yet, no multi-junction devices of wide-gap perovskite cells based on MA_1-xCs_xPb(I_(1-y)Br_y)₃ and low-gap organic cells have been reported.

In this project we will first identify an optimum wide-gap perovksite material along with a robust preparation protocol. Alongside, the careful analysis of its electronic structure by photoelectron spectroscopy (PES) will be of paramount importance. Until now these studies are lacking for perovskites like MA_1-xCs_xPb(I_(1-y)Br_y)₃. The outcome of this research states the prerequisite for the selection of optimum interfacial materials that not only improve charge extraction but at the same time enable enhanced stability of the entire cell. As an example, microporous TiO2 is an established electron extraction material, that has to be prepared at high temperatures (>400°C) and its photocatalytic nature is frequently associated with reliability issues in perovskite cells. Opposed to that, we aim to use cross-linkable organic semiconductors or metal-oxides that can be prepared at temperatures below 100°C. In a combined approach of PES with dedicated device testing (e.g. unipolar electron/hole-only), we aim to identify optimum charge extraction layers for the selected wide-gap perovskite. These interfacial materials will also be the platform for the design of an interconnect, which must allow the loss-free monolithic integration of the sub-cells. We expect to achieve long-term stable hybrid tandem cells prepared at low temperatures (<100°C) with an efficiency > 20%.