Doctoral Researchers

Amram Al-Ashouri

Optimization of Perovskite Single Junctions for Hybrid Tandem Solar Cells

Supervised by: 

Prof. Dr. Bernd Rech and Dr. Steve Albrecht

Pascal Becker

Insitu-Röntgenabsorption an Perovskit-Dünnschichten

Supervised by:

Prof. Dr. Susan Schorr and Dr. Thomas Unold

Lucas Bodenstein-Dresler

X-ray and electron spectroscopy - based insights into perovskite solar cells layer stacks

Supervised by:

Prof. Dr. Marcus Bär

Sebastian Caicedo-Davila

Advanced Halide perovskite materials and device characterization

Supervised by:

PD Dr. Daniel Abou-Ras

Laura Canil

Self-assembling at the interface in perovskite solar cells

Supervised by:

Dr. Antonio Abate

Pietro Caproiglio

Deposition of Perovskite Films on Textured Surfaces

Supervised by:

Prof. Dr. Bernd Rech,  Dr. Steve Albrecht and Prof. Dr. Dieter Neher

The Ph.D. project aims in the realization of monolithic perovskite/silicon tandem structures on textured wafer surfaces that are utilized for optimum light management. The doctoral candidate will develop strategies to process inorganic-organic perovskite films and the selective contacts on top of various textured surfaces that are used for silicon photovoltaics. To realize this project, new film-processing techniques will be developed by the student In addition, fundamental physical understanding of the film formation will be realized by utilizing various analytical methods as basis for optimization of the tandem solar cell device performance.

David Diering

Defect spectroscopy and device analysis of perovskite thin films and solar cells

Supervised by:

Prof. Dr. Dieter Neher und Dr. Thomas Unold

Hybrid organometal-halide perovskite solar cells recently have shown very high conversion efficiencies exceeding 17%. This new class of solar cell materials can be synthesized either by solution chemistry or by co-evaporation in a vacuum environment.  Although the best performing perovskite solar cells so far have been obtained via the wet chemistry approach, the vacuum evaporation promises a much more homogeneous layer morphology and in principle allows for more control of the deposition and crystallization processes, in particular the substitution of elements, such as the  lead halides which are currently used in the highest efficiency devices.  As the role of bulk and interface defects in hybrid perovskite solar cells is still unclear at the moment, a fundamental understanding of recombination processes will become increasingly important as the device efficiencies reported start to approach 20%. The aim of this Ph.D. project is to prepare homogeneous hybrid perovskite absorber layers for solar cells by co-evaporation, and investigate the defect physics of these materials in dependence of the material composition and preparation conditions.  This will include the investigation of bulk defects as well as surface defects by various optoelectronic characterization techniques, and determining their influence on the performance of highly efficient perovskite-based solar cells.

Applicants interested in this project should have a good knowledge of solid state physics and be  interested in hands-on experimental work employing different state-of-the-art spectroscopic and solar cell characterization techniques.

Marion Flatken

Stable and efficient lead-free halide perovskite solar cells

Supervised by:

Dr. Antonio Abate

Hannah Funk

Correlative microscopy and spectroscopy on halide perovskite solar cells

Supervised by:

PD Dr. Daniel Abou-Ras and Prof. Christoph T. Koch, PhD

Katrin Hirselandt

Skalierbaren Schlitzdüsen- und Tintenstrahldruckprozessen für die großflächige Abscheidung hybrider Perowskit-Halbleiter und selektiver Kontaktschichten

Supervised by:

Prof. Dr. Bernd Rech and Dr. Eva Unger

Martin Kärgell

Layer Formation from Perovskite Nanoparticles with Tunable Optical and Electronic Properties

Supervised by:

Prof. Dr. Andreas Taubert and Dr. Thomas Unold

Recently, a new class of highly efficient thin film solar cells based on metal-organic hybrid perovskites has attracted much attention, with demonstrated power conversion efficiencies above 17%. These solar cells promise to be fabricated at low cost, as the perovskite absorber layer consists of abundant materials and can be easily synthesized by wet chemistry. Thin films prepared with APbX3 perovskite nanoparticles could be a good candidate for this purpose. The deliberate tuning of the optical and electrical properties of the perovskite absorber layer via the control of size and assembly of such nanoparticles is highly promising for the realization of highly efficient tandem cells. However, only limited work has been published regarding the fabrication of such nanoparticles, especially with sizes as small as several nanometers.

The aim of the PhD thesis is to develop highly controllable and reproducible synthesis techniques to fabricate novel perovskite layers via solution chemistry approaches at low temperatures, by careful consideration of the interactions between organic and inorganic elements. One important task will be to tune the optical properties by the size, organic ligand and composition of the perovskite nanoparticles. In particular, the influence of the perovskite layer structure and composition on the optical and electronic properties will be studied, ideally yielding homogeneous and stable absorber layers for high efficiency hybrid perovskite solar cells.

Manaswita Kar

Electronic structure and band gap tuning of perovskites from first principles

Supervised by:

Thomas Körzdörfer and Norbert Koch

Organometallic perovskite solar cells have revolutionized the field of emerging photovoltaics, rapidly surpassing the performance of the conventional dye-sensitized and organic technologies. In addition to their favorable optoelectronic properties, perovskites are structurally and compositionally very flexible, which makes them ideal candidates for the application in tandem solar cells. Efficient tandem architectures, however, require the combination of materials with precisely tuned optoelectronic properties. Despite the extremely fast progress in recent years, relatively little is understood about the key electronic properties of perovskite-based solar cells and how they can  be specifically tuned, e.g., by changing the compositional or structural properties.

The PhD-student working on this project will perform in-depth theoretical investigations of the electronic structure of perovskite materials using state-of-the-art electronic structure methods. The aim is to clarify how the key electronic properties of perovskites, such as the optical band gap, can be tuned by changing the composition and/or structures. It will be of central importance to gain a profound understanding of the key ingredients that affect the electrical, optical, and transport properties of these perovskite materials, thus, opening the way to the design of new and improved materials.

In terms of the methods, the student will mainly use density functional theory (DFT), time-dependent density functional theory (TD-DFT), and many-body perturbation theory calculations in the GW approximation (and beyond). Experience with any of these methods or, more generally, with electronic structure calculations for periodic systems would be helpful, yet not strictly required. While the focus of this work is theoretical, the student will be working closely together with the experimental projects associated with the graduate school. Not only will the insights from this theoretical work lead to a better understanding of the key electronic properties of perovskites, but its outcome will also yield important input towards the design of novel perovskite materials and help to identify ideal candidates for tandem cell applications.  In summary, this is a very challenging but also very rewarding project, requiring an enthusiastic PhD candidate with a sound training in theoretical/computational physics or chemistry and interest in theoretical materials science.

Lukas Kegelmann

Photon and current management in hybrid perovskite tandem cells

Supervised by:

Prof. Dr. Dieter Neher and Prof. Dr. Bernd Rech

The combination of two solar cells with a suitable combination of two different band gaps allows for a more efficient harvesting of the solar spectrum. Therefore, such tandem solar cells are perceived as the photovoltaic devices of the future. One realization of this device concept uses a high efficiency amorphous/crystalline (a-Si:H/c-Si) heterojunction solar cell as bottom cell, while the top cell consists of a cell based on a perovskite absorber, as sketched below.

The a-Si:H/c-Si heterojunction cell is a well-established technology with a demonstrated world record single-junction power conversion efficiency η > 25%, and η > 20% for cells routinely processed at HZB. The perovskite cells, on the other hand, constitute the cutting edge of current thin-film PV research, due to their recently demonstrated high efficiencies above 20% and ease of fabrication.

The PhD thesis will explore this perovskite/silicon tandem cell concept. The student will work in close collaboration with members from both the a-Si:H/c-Si and the organic PV/perovskite groups to design, implement and characterize thin films with the desired properties into solar cell architectures, using state of the art deposition and characterization tools. The experimental work will be complemented by optical and electrical modelling of the cell's layer stack, which will serve to understand the experimental results as well as to find optimum cell designs and to calculate the ultimate efficiency limits of devices with realistic materials properties. This work is also expected to provide key contributions to HZB's efforts in the European project MESO, and the PhD candidate will collaborate with other members of the international MESO project team, such as the Snaith group at the University of Oxford/UK.

Hans Köbler

Investigation and improvement of the long-term stability of solution-processed metal halide perovskite-based solar cells

Supervised by:

Prof. Dr. Bernd Rech and Dr. Antonio Abate

Eike Köhnen

Development of transparent contacts for hybrid tandem solar cells

Supervised by:

Prof. Dr. Bernd Rech and Dr. Steve Albrecht

Frederike Lehmann

Synthese und Charakterisierung von Hybrid-Perowskiten: Variation der Komponenten

Supervised by:

Prof. Dr. Susan Schorr and Prof. Dr. Andreas Taubert

Metal-organic hybrid perovskites based on lead halides have attracted tremendous interest for use in solar cells. One key advantage of these materials is their high flexibility as far as synthesis is concerned and the resulting tuneability of their physical properties. As of now, however, only a fairly small number of variants of these perovskites has been described in the literature. There is thus a need to evaluate the variability, the chemical adaptability and tuning of these perovskites in much more detail to enable the fabrication of advanced solar cells and other optoelectronic devices.
 
The central goal of the PhD thesis therefore is the synthesis of novel perovskites derived from existing prototypes and model compounds with the special aim to improve and scale up the synthesis to larger amounts of material, once suitable candidate materials have been identified. The project will focus on adapting the optical and electronic properties such as the band gap by exploring different strategies like doping, replacement of specific elements of the metal-organic perovskites by other chemical elements or building blocks. The resulting materials will be investigated for their structures and physical properties along with their potential for use in solar cells.

Dongyang Liu

Operational stability of perovskite solar cells

Supervised by:

Dr. Antonio Abate

Nga Phung

Active materials and interfaces for stable perovskite solar cells

Supervised by:

Dr. Antonio Abate and Prof. Dr. Bernd Rech

Perovskite solar cells have been attracting great attentions of solar energy researchers in recent years. Its efficiency development progress is remarkable reaching more than 22% efficiency until now. However, its application in large industrial scale is still questionable because of its instability. Therefore, in the new young investigator group at Helmholtz-Zentrum Berlin: ‘Active materials and interfaces for stable perovskite solar cells’ (INTER-ACTIVE), we focus on the stability of perovskite solar cells by various approaches.

Within the goal of the project’s big picture, I will work on the perovskite layer’s stability as my PhD project. In particular, using different characterisation techniques, I want to understand the dynamic behaviour of electronic components in perovskite solar cells during operational process. Along with this objective, I want to use chemical doping to improve the stability of perovskite.

Carolon Rehermann

Materials synthesis and device optimization for higher bandgap halide perovskites

Supervised by:

Dr. Eva Unger and Prof. Dr. Marcus Bär

Marcel Roß

Development and optimization of vapor deposited perovskite solar cells

Supervised by:

Dr. Steve Albrecht and Prof. Dr. Bernd Rech

Ibrahim Simsek

Chalcogenide Perovskites

Supervised by:

Dr. Thomas Unold

Philipp Tockhorn

Optimierung von Perowskit-Einzelsolarzellen für die Anwendung in hybriden Tandem-Solarzellen

Supervised by:

Prof. Dr. Bernd Rech and Dr. Lars Korte

Christian Wolf

Excitation and charge carrier dynamics in solution and vacuum processed perovskite cells

Supervised by:

Prof. Dr. Dieter Neher and Dr. Thomas Unold

The realization of efficient tandem cells necessitates the combination of subcells with well adapted optical and optoelectronic properties. Even though high efficiencies have now been realized with perovskite-based solar cells, we are far from a detailed understanding of how photovoltaic parameters are related to the chemical structure and the morphology of the active material. Moreover, the picture on the charge carrier dynamics and the electron-photon coupling in these systems is far from being complete.

 The aim of the thesis work is to perform an in-depth experimental investigation of the electron and photon dynamics in perovskite-based solar cells, utilizing state of the art pump-probe techniques. These studies will yield valuable information about the mechanisms which dictate the efficiency of charge carrier generation and extraction. With samples of well-defined composition and structure being supplied by the collaborators, we aim at establishing conclusive structure-property relationships. The outcome of these studies will allow for a knowledge-guided fine-tuning of the photovoltaic properties of these cells.

Fengshuo Zu

Energetics in single layer and tandem devices

Supervised by:

Prof. Dr. Norbert Koch and PD Dr. Thomas Körzdörfer

The realization of efficient single and tandem cells based on perovskites relies significantly on matching the electronic energy levels in the entire device stack. Proper electrical contact between the electrodes must be established, which will require the development of methods to optimize interface energy levels. For tandem cells the center-connecting electrode should possess proper charge generation / charge recombination functionality, which will also require energy level tuning.

In this project, the energy levels at all interfaces within single and tandem solar cells will be determined, mainly using ultraviolet photoelectron spectroscopy (UPS), with complementary methods such as Kelvin-probe and photoelectron yield spectroscopy. Once the mechanisms that govern the level alignment are understood, means to tune the interface electronic states will be evaluated, mainly based on the introduction of strong molecular donors and acceptors.