HyPerCell PhD Projects

All stipendiary positions of this school are occupied.

Layer formation from perovskite nanoparticles with tunable optical and electronic properties

Supervised by Yan Lu, Thomas Unold and Andreas Taubert

 

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.

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-stundent 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.

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

Supervised by Dieter Neher, Matias Bargheer and 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.

 

The possible applicant should be familiar with solid state spectroscopy, including the physics of phonons and electrons in semiconducting materials.

Energetics in in single layer and tandem devices

Supervised by Norbert Koch and 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.

 

The applicant should have a good background knowledge in semiconductor physics. Experience with surface science techniques is advantageous.