Perovskite Thin Films

Why are we interested in perovskites?

Perovskites have been known for a long time as materials with many unique and fascinating properties, which strongly depend on the material composition. Over the last decade, hybrid perovskites have emerged as promising materials for building solar cells, which might help to meet our growing energy demand by harvesting solar energy. Within less than ten years of intensive research, the efficiency of perovskite solar cells increased dramatically from the first perovskite sensitized solar cells with merely 3.8% conversion efficiency to now striking 25% for lab-scale devices. This impressive efficiency boost within such a short time might predict that perovskite solar cells will soon compete with conventional solar cell materials. In comparison, silicon solar cells have an efficiency of about 27% depending on the construction type. Even higher efficiencies are achievable upon combining both perovskite and silicon in tandem cells (29.1% so far). The greatest advantages of perovskite materials against conventional solar cell materials such as silicon lie in the fabrication process: hybrid perovskite materials could be produced from abundant starting materials using simple processes. As a result, production costs could be significantly reduced and solar cells could be fabricated easily and more economically. In addition, perovskite production needs comparatively little energy input. As a result, the energy balance, from the production of a perovskite solar cell through its use to its end, has a higher output than conventional materials. Silicon solar cells, for instance, are usually fabricated with very energy-intensive processes. That is why it can take several years to regain the energy spent for production of silicon solar cells while for perovskite cells this can be achieved within a few months. In addition, perovskites exhibit further remarkable properties that open up new possibilities: they are suitable, for example, for curved, flexible or even transparent substrate materials. This enables them to be used on bent surfaces or in elastic components. The easy tunability of perovskite materials is also a strong benefit, as will be described below.

Figure 1: (right) Perovskite solar cell.

What are perovskites?

The perovskite structure, which was derived from the original mineral perovskite, can be formed by a variety of materials. They all have an analogous structure composed of three types of ions in order to obtain an ABX3 composition. Here, A and B are two different types of cations, and X is an anion. In the general perovskite structure, the ions always have the same arrangement which is shown in Figure 2. In order to form the structure, certain requirements for the size of the ions must be met so that everything fits together well, as shown here. In addition, the charge balance must also be guaranteed, so that the material is electrically neutral. Ion size and charge neutrality are thus the two most important criteria when selecting the ions for a perovskite material. What we mean when we speak of "perovskite" is a variety of materials (which are made by us) that form a perovskite structure. There are quite a number of these, as in principle many different ions can be mixed in order to obtain a material with a perovskite structure, taking into account the conditions mentioned above. We focus on a certain selection of materials, which we can, however, constantly change and expand, especially with regard to the cations. Generally speaking, we are investigating organic-inorganic hybrid metal-halide perovskites. The most frequently used ions that compose our perovskite materials are also listed in Figure 2. The great diversity and variability in the composition of perovskites is an important part of their appealc. It enables an enormous adaptability with simple means, as it did not exist with conventional solar cell or semiconductor materials so far.

Figure 2: (left) Perovskite structure.

Kinetics of perovskite thin film formation and its structural properties studied by GIWAXS

By tuning the composition of the perovskite films, not only the intrinsic material properties, but also the growth and formation of the crystal structure and morphology can be significantly improved. We are using grazing incidence wide angle x-ray scattering (GIWAXS) to determine relevant structural properties of perovskite thin films: precursor material conversion, intermediate phase formation and perovskite unit cell parameter changes during thin film crystallization. All this information can be extracted from the GIWAXS images. In addition to ex-situ GIWAXS to characterize finished films, we also perform in-situ measurements during film production. For that purpose, we employ a self-built spin coating chamber, which enables us to observe the film formation and crystallization in real time. Since this is likely to produce a massive amount of data, we also apply machine learning to accelerate the data analysis.

Figure 3: (right) In-situ real-time GIWAXS "X-ray movie" during MAPbI3 film growth

For more information on perovskites and perovskite applications please visit the funPero project.


[1] N. Arora et al., Science 358, 768 (2017)
[2] A. Greco et al., J. Phys. Chem. Lett. 9, 6750 (2018)
[3] Y. Liu et al., Sci. Adv. 5 (2019) eaaw2543

For our recent work on perovskites, see list of publications.