The review colloquium has taken place from 31/01/2019 – 01/02/2019 in the DPG House of physics in Bad Honnef. 58 projects have been presented. Unfortunately only 18 of them will be funded. The funded projects have generally cut down to allow to include more projects to be funded.

We thank the reviewers for their efforts to select the best proposals to have an exciting SPP.

Projects

Band Gap Tailored Perovskites with Reduced Losses and Improved Stability: Towards Highly Efficient All-Perovskite Tandem Solar Cells (HIPSTER)

  • Steve Albrecht, Berlin
  • 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."

Electroluminescent perovskite nanocrystals – From tailor-made assemblies to opto-electronic properties

  • 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.

Lead-free Double Perovskite Materials for Photovoltaic Applications

  • 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.

Dielectric Effects in Hybrid Perovskites: Impacht on Charge Carriers and Anisotropy

  • 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.

Nature of Proton and Light induced Defects in Lead Halide Perovskites

  • 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.

Transport of optical excitations in low-dimensional halide perovskites: Coulomb effects and structural dynamics

  • 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.

Illuminating Building Block Evolution of Metal-Halide Perovskite Semiconductors from Solutions to Thin Films (GLIMPSE)

  • 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.

PERFECT PVs: Perovskite defects: Physics, Evolution and Stability

  • 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.

Improving intrinsic stability of perovskite solar cells by additives

  • 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.

Understanding and suppressing interfacial charge recombination for high performance perovskite solar cells (SURPRISE)

  • 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.

Kontrolle interner und externer Grenzflächen in 2D Perowskiten zur Überwindung der intrinsischen Anisotropie des Ladungstransports in Solarzellen

  • Anna Köhler, Bayreuth
  • Mukundan Thelakkat, Bayreuth

Perovskite Heterostructure Investigations using Vacuum Evaporation and X-ray diffraction

  • 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.

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 Modeling

  • 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.

A-site Modified Hybrid Perovskites: Compositional Engineering and Role of Grain Boundaries on Optoelectronic Properties

  • 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).

Epitaxially Grown tin Perovskites

  • 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.

Point-Defect Design and Facet-Selective Optoelectronic Properties in Doped Hybrid Perovskite Microcrystals

  • 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.

Control over grain size and crystallinity: Role of trap states in perovskites

  • 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.

Spin and recombination dynamics of excitons and carries in metal halide perovskites

  • 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.

To increase the strength of our SPP network we have associated PIs with running DFG projects which fit very well into the theme of our SPP. The associated project are listed below.

Associated projects

Long-lived hot carriers, coherent spin transport, and the role of surfaces in lead halide perovskites

- 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.

Control of excitation recombination and transfer with tailored material design

(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.