Two-dimensional perovskites: from fundamental understanding to their application at interfaces in perovskite solar cells

Joint project between University of Cologne (Dr. Selina Olthof) und University Wuppertal (Prof. Thomas Riedl) as part oof the SPP2196

Funding agency: DFG

Halide perovskites have revolutionized the field of thin film solar cells with a remarkable increase in performance yielding efficiencies that are on par with single crystalline silicon. Aside from the perovskite material itself, interfaces to charge transport layers critically influence not only the overall performance but also the device stability. 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 start off with a systematic investigation of pure 2D perovskites and study 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 materialswill 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 integrated as thin layers on top and/or below an optimized 3D perovskite absorber material. Detailed analysis of these heterostructures will allow us to unravel how these bulky cations and the specific processing parameters affect the formation, electronic structure, and dimensionality of the respective interlayer. The insights gained from these fundamental studies will be correlated to the electrical characteristics of unipolar (hole-only; electron only) devices based on 2D/3D perovskite interfaces. Ultimately, the most promising combinations of 2D and 3D perovskites will be integrated in solar cells. 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. In the latter systems, we will directly study the impact of 2D/3D interfaces to potentially mitigate the notorious halide segregation.

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.

Hybrid multi-junction solar cells based on a monolithic integration of a wide-bandgap organo-metal-halide perovskite and low-gap organic polymer sub-cells

Joint project between University of Cologne (Dr. Selina Olthof) und University Wuppertal (Prof. Thomas Riedl)

Funding agency: DFG

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 Voc and high Jsc, currently states the main limitation. Even in the best organic wide-gap devices the voltage loss, i.e. 1/q*Eg - 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-xCsxPb(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)Bry)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%.

Background: Halide perovskites have emerged as one of the most studied semiconductors due to their excellent optoelectronic properties. This is evidenced by the rapid development of perovskite solar cells with a record certified photoconversion efficiency of 24.2%, similar to those of silicon cells. Nonetheless, industrial application of is critically hampered by instability issues, including intrinsic, environmental, and operational factors. Instability is attributed to several chemical and dynamical processes that occur at very distinct time scales, like slow ionic rearrangements and physical and chemical interactions in the bulk and at interfaces with contact layers.

ScaleUp Project: To overcome these issues we formed a consortium of experimental physicists, theoreticians, device engineers, and an industrial partner. Based on results from fundamental material investigation and device characteristics, we will develop versatile numerical models for large scale molecular dynamics, capable to capture the physics and chemistry that trigger processes causing instability issues. The availability of such numerical tools and a resulting user-friendly software, with the potential to describe with reasonable accuracy complex halide perovskite alloys for large sizes and in the long-time scale, will make it possible to accomplish key advances to extend the durability of perovskite based solar cells and to provide software and testing benchmarks to enable researchers to achieve this goal.

In Cologne, we will contribute by measuring relevant halide perovskite properties that are needed as input data for the numerical simulation. Such properties include changes in density of states, crystal structure, phase segregation, chemical interactions, or the appearance of degradation species. Various device-relevant perovskite composition and contact materials (transport layers) will be tested for this.

TIDE is a DFG-funded Research Training Group providing a comprehensive doctoral education program in the field of Organic Electronics. Our team of more than 13 expert researchers strengthens TIDE in terms of breath, complexity and interdisciplinarity coming from different disciplines like Physical, Organic, and Theoretical Chemistry, as well as Physics and Materials Science, and is located at at the two universities University of Cologne, and University Bonn.

Our group provides detailed insight on the formation of inetrfaces between ordered substrates and small moelcuel. In particular we are using UPS to look at changes in the electronic structucture and XPS to investigate the substrate-moelcule interactions. LEED helps us to identify the presence of ordered layers and helps to determine the growth mode.