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Solar cells based on hybrid organic-inorganic lead-halide perovskites are among the most promising emerging photovoltaic materials of the past decade. Within only a very short research time span, record efficiencies were achieved with solar cells based on methylammonium lead iodide (CH3NH3PbI3).
Despite these notable achievements, many of the basic properties of hybrid perovskite materials are not yet fully understood. In addition, the presence of Pb in the currently most efficient perovskite solar cells has raised questions over the possible toxicity of these devices and the extent of their environmental impact. Therefore, a lot of research has been devoted to finding alternative perovskite materials with similar or even better optoelectronic properties. A flipside strategy to improve the efficiency of thin-film solar cells is to build efficient tandem cells by combining materials with specifically tailored band-gaps.
In this project, we aim to find alternative, lead-free hybrid perovskite materials specifically tailored for applications in tandem solar cells. To this end, we substitute the Pb by other homovalent metals, the I by all other halogens (F, Cl and Br), and the methyl ammonium with alternative organic cations. Since we're interested in tandem solar cell applications, we screen for materials that satisfy a specific range for the band gap (1-2 eV). To this end, we employ a cascade of DFT and GW calculations, ranging from low-accuracy/high-efficiency to high-accuracy/low-efficiency approaches, thus, identifying the most promising candidate structures and their band structure.
The materials satisfying the band gap criteria are then further tested for phase stability at room temperature. This is achieved by calculating the phonon contribution to the Gibb's free energy using quasi-harmonic approximation for the different crystal phases. The overall goal of this project is to identify the most promising hybrid perovskite materials to be used in tandem cells. These materials will then be further studied by our experimental collaborators.
Funding: This project is part of HyPercells, a joint graduate school of the University of Potsdam and the Helmholtz-Zentrum Berlin, which involves theoretical and experimental groups performing research on novel hybrid perovskite materials for solar cell applications. For more details, please visit www.perovskites.de.
Project Members: Manaswita Kar