Forthcoming Events

26.10.2020 - 28.10.2020, Paul Scherrer Institut (PSI), Villigen,Switzerland

Vapor-assisted deposition of highly efficient, stable black-phase FAPbI3 perovskite solar cells

October 2, 2020

Perovskites are a class of materials with ABX3 structure, where A and B are cations and X is an anion. In recent years, halide perovskites have been at the forefront of the materials’ research and are used to make solar cells, light emitting diode, lights, lasers, photocatalyst, transistor and photodetectors. Over the past decade, the power-conversion efficiency of perovskite solar cells (PSCs) have increased from 3.8% to 25.5%, thereby surpassing other thin-film technologies of CdTe, CIGS and including the market-leader, polycrystalline silicon.

Perovskites electronics are usually manufactured by mixing and layering various materials together on a transparent conducting substrate. Various sustainable and relatively cost-effective “chemical deposition” processing techniques have opened the possibility to develop these high efficiency materials for optoelectronic applications.

The vast majority of high-performance PSCs are based on a formamidinium lead iodide (FAPbI3) composition. But a major problem with this material is that the thermodynamically stable form at room temperature is a photoinactive δ-phase while the photovoltaically suitable perovskite phase becomes only the most stable form at high temperature. Solar cells made with metastable photoactive cubic α-FAPbI3 have long-term stability issues as this phase can get converted to photoinactive δ-FAPbI3.

In this study, a collaborative research efforts at EPFL led by Michael Grätzel and Anders Hafgeldt, an innovative chemical deposition method has been developed that overcomes these issues while maintaining more than 23% power-conversion efficiency and long-term operational and thermal stability. The fabricated solar cells also featured low (330 mV) open-circuit voltage loss and a low (0.75 V) turn-on voltage for electroluminescence.

In the new method, the photoinactive δ-FAPbI3 perovskite films are converted to the desired photosensitive cubic α-FAPbI3 by treating them with a vapor of methylammonium thiocyanate (MASCN) or formamidinium thiocyanate (FASCN). Molecular simulations have played a pivotal role in understanding the atomic level mechanism of the phase transition between δ-FAPbI3 and α-FAPbI3. The simulations have identified the role of SCN- anions in triggering the phase transition at lower temperatures. In particular, we have found that the SCNanions can induce intermediate phases which favor the formation of α-FAPbI3. Our simulations also indicate that the stabilization of the α-perovskite phase can also be induced by other similar synthesis processes that can be experimentally designed based on theory presented in this study.

Movie S5
The transformation of face-sharing octahedra to corner-sharing octahedra on the top surface layer of δ-FAPbI3. Here we show a part of the simulation box, mainly to clearly visualize the transformation by the strongly coordinated SCN- ions on the top surface layer. Pb-I octahedra are represented in golden color with iodide in corners as pink balls. SCN- ions are shown with balls and sticks configuration: sulfur with yellow color, carbon with light blue color, and nitrogen atoms with blue balls. SCN- coordinating octahedra are shown with red color for better view.

See also:
 
Reference: In a collaborative research efforts at EPFL led by Michael Grätzel and Anders Hafgeldt, with the group of Ursula Röthlisberger an innovative chemical deposition method has been developed that overcomes these issues while maintaining more than 23% power-conversion efficiency and long-term operational and thermal stability. The fabricated solar cells also featured low (330 mV) open-circuit voltage loss and a low (0.75 V) turn-on voltage for electroluminescence.

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