The name "perovskite" designates a certain crystal structure with the structural formula ABX3, which exists in a multiplicity of most different compositions in nature. Thereby, the crystal form was first discovered in the mineral calcium titanate CaTiO3 by Gustav Rose in the Ural Mountains more than a hundred years ago and named in honor of the Russian mineralogist Count Lev Perovski. Depending on the material class, perovskite crystals have different properties, from ferroelectric to superconducting and finally photovoltaic. The latter, perovskite crystals suitable for photovoltaics, are composed of organic (cations) and inorganic components (anions). Currently, the most promising compounds are methylammonium-lead triX (CH3NH3PbX3), with X = I-, Br- or Cl-.
Perovskites can be deposited on any substrate from the liquid phase (Fig. 2) or by vacuum evaporation. The band gap of the organic-metallic halide perovskite crystals - crucial for efficient conversion of sunlight into electrical energy - can be specifically changed by varying the cationic and anionic components of the crystal. This property is very attractive for future applications, e.g. in buildings, architecture, etc., in terms of color diversity and spectral matching.
Not least because of this diversity of application possibilities, photoactive layers of perovskite semiconductors are already being successfully combined with other PV technologies. In this process, a thin layer of a few hundred nanometers is applied to conventional solar cells consisting of silicon, but also to thin-film solar cells, such as copper indium gallium diselenide (CIGS), in order to achieve a high overall efficiency.
Whether as an additional layer on inorganic solar cells or as the sole photovoltaic layer of a solar cell, perovskite semiconductors have led to worldwide attention and interest in the photovoltaic community in only a very short time. Already, the possibilities of large-scale production are being pursued with the goal of rapidly bringing this PV technology to market.
However, the fundamentals of perovskite solar cells and how they work have only been partially studied. Above all, fundamental investigations on the absorber materials and the layer structures are indispensable. At ZAE Bayern, extremely long relaxation times for the open-circuit voltage were recently observed over several seconds, indicating long charge carrier lifetimes, which finds no analogue in either organic solar cells or silicon PV.¹
In addition to the aforementioned issues, the potential toxicity of lead-containing perovskite absorbers (such as methylammonium lead iodide) and the durability of perovskite solar cells are of particular concern.
The research field is expected to attract far more attention in the coming years, if only because of the many research groups involved worldwide. Activities in the field of perovskite solar cells in Germany are unfortunately still very limited. This is due on the one hand to the scope of funding and on the other hand to the relatively high scientific and technical risk for the development of the technology. In order to ensure competitiveness in Germany in this very promising PV research field, research and development must also be promoted more strongly in Germany.
¹ A. Baumann et al. Persistent Photovoltage in Methylammonium Lead Iodide Perovskite Solar Cells, APL Materials 2, 081501 (2014).
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