Lattice Dynamics and Charge Transport in Metastable Ternary Nitrides for Solar Energy Conversion

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In view of dramatically rising global temperatures and increasing energy demand worldwide, it becomes more urgent than ever to find renewable and clean energy sources. Out of those, solar power is paramount; even in the coldest regions on Earth, there is more than enough to meet society’s energy needs. The problem lies in harvesting this abundant solar energy and making it accessible to the people. The proposed project “Lattice Dynamics and Charge Transport in Metastable Nitrides for Solar Energy Conversion” tackles this challenge by achieving fundamental understanding of a new class of materials, metastable nitrides, that will have a large impact on next-generation solar energy devices. By means of computational and experimental methods, this project addresses their mechanisms of dynamical stabilisation and charge transport as well as their mutual interplay. This is not only crucial for photovoltaic applications but will also lead to new insights into the underlying physics of metastable materials. Metastable materials present a relatively unexplored phase space that provides new design opportunities as well as an ideal platform for exciting fundamental research.The representative copper tantalum nitride CuTaN2 will be first investigated as it shows large promises for applications in solar cells. Apart from its extremely efficient light absorption, it is experimentally stable up to 250°C, which is ideal for the proposed study and real device implementation. From the gained knowledge of lattice dynamics and charge transport in CuTaN2, relationships between those fundamental mechanisms and the metastability of the material will be derived. These concepts will then be generalised to a much broader pool of metastable nitrides, specifically those that do not contain rare elements. Particularly promising are (Sn1-xTix)3N4 and CaxZn2-xN2, which were recently synthesised for the first time. They exhibit desirable optoelectronic properties while being composed of Earth-abundant elements. With the methodology established for CuTaN2, we will investigate their basic properties, which will allow us to draw comparisons between the different nitride materials and to assess the universality of the underlying mechanisms.Taken together, the specific research objectives are: (i) to provide a detailed theoretical and experimental account on the structural, electronic, and optical properties of CuTaN2, (ii) to comprehensively investigate its lattice dynamics and anharmonic effects, (iii) to study charge carrier transport and how it is influenced by dynamical nuclear motions, and (iv) to apply the established protocol and to assess the relevance of our models to other metastable nitride materials.All in all, the proposed approach will pave the path towards a new generation of environmentally benign, abundant, and inexpensive light-absorbing semiconductor materials and, thus, towards finding better and more efficient solutions for harvesting solar light.