Friday Materials Science Colloquia (#7)


Friday, March 31st  2023, 12.15 p.m., Seminar room, U5 Building – via Roberto Cozzi 55, Milano.

Lecturer: Dr. Vanira Trifiletti

Title: Materials and devices for renewable energy production

Abstract: Renewable energy production requires a range of materials and devices that are designed to harness energy from natural resources, sustainably and efficiently. These technologies are constantly evolving and improving, as researchers in the field seek to develop new materials and devices that are more efficient, durable, and cost-effective. Photovoltaic (PV) has the leading role in the production of green energy, being so versatile that it can be used to power a wide range of systems (from small objects to entire communities). Even if silicon is the market leader, the new PV generations are evolving rapidly, allowing PV to be integrated into most systems that require electricity. Dye-sensitized solar cells (DSCs), kesterite-based solar cells (KSCs), and perovskite solar cells (PSCs) are some examples of the latest PV technologies. By appropriate materials and device design and engineering, some of these technologies can be coupled to work together, or matched with silicon, in a tandem configuration. However, if DSCs and PSCs have found their slice of a potential market, the KSCs are not yet competitive. KSCs’ key advantage is their potential for low-cost energy production since the light-absorber material is made from abundant and non-toxic elements, which makes them an attractive alternative to other thin-film solar cells, that rely on scarce or toxic materials. Besides, the kesterite bandgap can be tuned for the total or partial substitution of the components, always showing high light absorption coefficients in a layer of a few microns.[1] Some challenges, such as the difficulty of controlling the composition and quality of the kesterite layer, as well as the low open-circuit voltage limiting their efficiency, can be faced by taking advantage of the design of the precursor solution to be employed to synthesise the light-absorber material.[2] Moreover, lately, we have also proven that device components that were inherited from the conventional thin-film PV architecture, and which are not ideal for kesterite, can be successfully replaced with components designed on purpose.[3] Kesterite and related compounds have been also successfully employed in thermoelectrics,[4] both in applications exploiting a temperature gradient for energy production and in devices using little power to cool a system. Perovskite compounds also have shown great potential for a wide range of applications beyond just PV (e.g., thermoelectrics, memristors, sensing and detection), and the ongoing research is focused on exploring their full potential.

[1] V. Trifiletti et al., Sol. Energy Mater. Sol. Cells 254,112247 (2023).
[2] V. Trifiletti, ChemistrySelect 4, 4905-4912 (2019).
[3] G. Tseberlidis et al., ACS Mater. Lett. 5, 219-224 (2023).
[4] U. Syafiq et al., ACS Appl. Energy Mater. 5, 5909-5918 (2022).


Relatore: Dr. Stefano Cecchi

Titolo:  Van der Waals epitaxy and characterization of quasi two-dimensional Ge-Sb-Te materials and heterostructures


The advent of two-dimensional materials redefined the material science in the last decade, promising disrupting advances in many technological fields. Among the available synthesis techniques, van der Waals (vdW) epitaxy [1] ensures high quality, purity and scalability, all crucial for the integration with microelectronic technology.
Beyond the well-known phase change functionality used in non-volatile memories, the Ge-Sb-Te chalcogenide family possesses a generous variety of functional properties. As an example, the binary compound GeTe is the father of a new class of materials, namely ferroelectric Rashba semiconductors, in which ferroelectricity is used to control the spin texture at room temperature [2]. A key element for the exploitation of this rich playground is the high crystal quality achieved for the material deposited by molecular beam epitaxy (MBE) on Sb-passivated Si(111) substrates.
In this presentation, I will first give an overview on the fabrication by MBE of Ge-Sb-Te layered materials and heterostructures [3–5]. Next, I will present recent results on the vdW epitaxy and characterization of (1) GeTe-rich GST films, providing breakthrough evidence of their composition-dependent ferroelectric behavior, and (2) the 1D chiral crystal tellurene, which has been integrated for the first time on silicon. Finally, I will discuss strategies to implement the vdW epitaxy concept in different material systems, e.g. for the integration of novel nitrides on Si.

[1] A. Koma, Thin Solid Films, 216(1), 72–76 (1992).
[2] S. Varotto et al., Nat Electron 4(10), 740–747 (2021).
[3] S. Cecchi et al., APL Materials 5(2), 026107 (2017).
[4] R. Wang et al., Adv. Funct. Mater. 28(14), 1705901 (2018).
[5] S. Cecchi et al., Adv. Funct. Mater. 29(2), 1805184 (2019).