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IEK-5 Photovoltaics

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Materials Search for Photovoltaic Devices

Material Tornado From materials databases containing thousands of materials structures, we perform state-of-the-art first-principles calculations to select those with most promising properties for photovoltaic devices.
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While crystalline silicon is still the dominant technology in photovoltaics, metal-halide perovskites have become highly promising for low-cost, high-efficiency multijunction solar cells. The first breakthrough came with the use of methylammonium-lead-iodide. But the fairly volatile methylammonium leads to a low thermal stability of the lattice and the Pb cation is toxic. Thus, the research community initiated substantial efforts to find related compounds with improved stability and reduced toxicity. The computational efforts so far have mostly relied on density functional theory (DFT) and have focused on the band gap, stability, and effective masses of the candidates. However, one of the key shortcomings of first-principles methods to identify promising materials for photovoltaics lies in the correct translation of the computed material properties into real photovoltaic performance. The band gap and effective masses are indeed key parameters, but they, alone, are insufficient [1]. We thus want to explore the quantum properties of materials from the viewpoint of device simulations in order to assess the solar-cell performance beyond the properties of the "bare" material in equilibrium. For that, we first describe the materials properties with state-of-the-art methods beyond DFT. The theoretical approach used includes renormalization effects due to electron-electron scattering within the GW approximation. In IEK-5, we implement the search for efficient materials in high-throughput workflows that take as starting point crystal structures from well-known databases containing thousands of materials. We then base the first materials screening on our calculations of GW band gaps and GW-based optical properties obtained with the SPEX code [2].

The output of these calculations will then be used as an input for non-equilibrium Green's functions (NEGF) calculations. Within this formalism, optical excitations, recombination and transport processes are included to obtain a more realistic prediction of device efficiencies. At IEK-5, we also develop a quantum-kinetic approach based on the NEGF formalism that provides a powerful instrument for the treatment of mesoscopic photovoltaic properties [3-5].

[1] T. Kirchartz und U. Rau, Adv. Energy Mater. 8, 1703385 (2018)
[2] C. Friedrich, S. Blügel und A. Schindlmayr, Phys Rev. B 81, 125102 (2010); http://spex.readthedocs.io
[3] U. Aeberhard, J. Phys. D: Appl. Phys. 51, 323002 (2018)
[4] U. Aeberhard, Phys. Rev. B 99, 125302 (2019)
[5] U. Aeberhard, Phys. Status Solidi B 256, 1800500 (2019)

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