ASM 2019

Poster (P44)


Organic-Inorganic Mixed Valenced Halides as Promising Photovoltaic Absorber : M2Au2X6; M=Cs, FA, MA; X=I, Br, Cl

Supriti Ghorui1, Jiban Kangsabanik1, Aftab Alam1
1Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India.


Promising power conversion efficiency (22%) along with flexible synthesis process and device fabrication technique make hybrid lead halide (MAPbX
3, X=I, Br, Cl) as one of the apt choice for solar absorber but Pb toxicity and instability in external environment limit its use in commercial solar device. In the meantime, double perovskite halide (A2BḂX6 , X=I, Br, Cl) came out as potential alternatives being environment friendly and having band gap in suitable range. However, it has either indirect or large direct optical band gap which is not ideal for the photovoltaic application. Since discovery of high-Tc super-conductors, mixed valence perovskites are of great interest where the superconductivity depends on the valence charge state of the element. Gold mixed valence semiconductors, Cs2Au2X6, (X=I, Br, Cl); forms in distorted perovskite structure possessing elongated (Au+3) and compressed (Au+1) AuI6 octahedra alternatively. This class of compounds show phase transition at high pressure where valence state of Au+1 and Au+3 closely approach to Au+2, and the whole compound show metallic behaviour. Conduction in such compounds containing Au+2 ions were of particular interest, to study potential superconductivity. In the other side, at room temperature and ambient pressure these compounds possess a very favourable band gap to be considered as photovoltaic absorber.

We show a detailed study of electronic structure and optical properties and simulated various thin-film device parameters for Cs2Au2X6, X=I, Br, Cl; compounds, from photovoltaics perspective. Careful analysis of the band structure reveals, the compounds have slightly indirect band gap, contrary to the previous reports. But interestingly, optically allowed direct band gap remains very close (within 15 meV) to the indirect gap. Sharp rise in absorption coefficient near band gap results in very high short-circuit current density, thus giving higher theoretical efficiency compared to the state of the art hybrid perovskite MAPbI3, for Br and I compounds. We confirm chemical stability against formation of binary halides, along with mechanical and dynamical stability for these compounds. Our investigation on possibility and effect of organic cation replacement at `Cs site, reveals increase in band gap, thus limiting suitability as solar absorber. Our detailed defect physics study shows that halide vacancy will form even in anion rich conditions in case of Cs2Au2I6, which is not the case for Cs2Au2Br6 indicating a better practical efficiency for the bromide counterpart. We suggest the necessary chemical environments while synthesizing these compounds, to reach their fullest potential as photovoltaic absorber. In conclusion, we see Cs2Au2X6 especially Cs2Au2Br6 can be considered as potential candidate for solar absorber for its high efficiency band gap value, stable and nontoxic nature.

 

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