17 Oct Ivan Caño Prades has completed an internship at Purdue University under the direction of Dr. Rakesh Agrawal, to develop solution-based methodologies to synthesize chalcohalide thin films

Ivan Caño has done an internship at Purdue University during summer 2022, working with Dr. Rakesh Agrawal’s group in the development of solution-based methodologies to synthesize novel
chalcohalide materials for photovoltaic applications.

Purdue University is a public research university located in West Lafayette, Indiana (US). Founded in 1869, it is one of the leading American institutions in technology and engineering. Among its alumni there are several Nobel prize winners such as Edward M. Purcell (1952, discovery of nuclear magnetic resonance in liquids and solids), Ben Roy Mottelson (1975, development of a unified
theory of the structure and dynamics of the atomic nuclei), and Akira Suzuki (2010, discovery of a cross-coupling reaction of an organic boron compound using Pd as a catalyst). Likewise, other
prominent pioneers of the 20th century studied at Purdue University, including NASA astronauts Neill Armstrong and Gus Grissom, Deng Jiaxian (founding father of China’s A-bomb), and
Mohammed M. Atalla (inventor of the MOS transistor).

Dr. Rakesh Agrawal received an Sc.D in chemical engineering from the Massachusetts Institute of Technology in 1977, and joined Purdue University in 2004, where he leads a research group at the Davidson School of Chemical Engineering. His research interests include novel processes for the fabrication of low-cost thin-film solar cells, energy systems analysis, biomass to liquid fuel

conversion, synthesis of efficient multicomponent separation processes, and applied research in gas separations and liquefaction. Agrawal has published over 246 technical papers and holds 128 US and more than 500 foreign patents. He is the recipient of numerous awards, including the National Medal of Technology and Innovation in 2011 from President Barack Obama (the highest honor given by the US government for technology and innovation).

The Solar Energy group at Purdue University focuses on the fabrication of high efficiency solution processed solar cells using colloidal nanoparticle inks or molecular precursors. These solution-based methodologies forgo the typical high vacuum processing needed for semiconductor production, potentially leading to significantly reduced manufacturing costs, and opening the door to roll-to-roll processing for the fabrication of solar cells. The group has achieved among the highest efficiencies for Cu(In,Ga)(S,Se) 2 using the ink-based route (doi: 10.1002/pip.2588) and were the first to synthesize Cu 2 ZnSnS 4 solar cells (doi: 10.1021/ja904981r) by chemical methods. Currently, they are involved in the development of high efficiency Cu(In,Ga)(S,Se) 2 solar cells by versatile and easily scalable roll-to-roll solution-based processing, and state-of-the-art research of novel materials for photovoltaics, including chalcogenide perovskites.

During his stay at Dr. Agrawal’s group, Ivan worked on the development of molecular ink methodologies to synthesize novel chalcogenide and chalcohalide materials, aiming at their
implementation for photovoltaics. First, he successfully replicated the process developed by David H. Webber and Richard L. Brutchey (doi: 10.1021/ja4084336) to prepare Sb 2 Se 3 thin films using an amine-thiol solvent mixture. Thin films were prepared by blade-coating, demonstrating the viability of a future implementation of roll-to-roll manufacturing for Sb 2 Se 3 -based devices.

Secondly, Ivan explored different solvent mixtures to prepare SbSeI and SbSI. Antimony chalcohalides are highly attractive due to their wide bandgaps in the 1.6-2.0 eV range (ideal for
tandem and semi-transparent applications), customizable optical properties, defect-tolerance, and ferroelectric properties. Despite its success as a solvent for Sb 2 Se 3 and chalcogenide perovskite, the amine-thiol mixture is not suitable for SbSeI synthesis, invariably resulting in the loss of halide during processing, and the formation of Sb 2 Se 3 . Multiple variations and concentrations of the amine-thiol solution did not result in clear improvements. Alternatively, it was shown that a DMF solution, mixing stoichiometric amounts of selenourea and SbI 3 resulted in the successful formation of SbSeI. Unlike previous results reported elsewhere, the process developed at Purdue University allows to prepare the chalcohalide layer in a single step, with a sole solution containing all the precursors. Likewise, different blade-coating conditions were investigated to develop thin films.

Finally, Ivan developed a methodology based on molecular ink precursors to synthesize Ag 3 SBr, demonstrating the viability of solution-based methodologies to manufacture anti-perovskite thin films. Silver chalcohalides stand out for their crystalline structure analogous to perovskites, suggesting that they could possess great tolerance to defects. Also, theoretical calculations have reported that Ag chalcohalides have bandgaps in the 0.9-2.0 eV range, making them ideal for single-junction or tandem PV devices. Despite these promising properties, there is no published information on their implementation in solar cells, and so far, they have only been synthesized by solid-state reactions (powder) and laser ablation at high temperatures (ultra-thin films), which require long processing times and high costs. Therefore, this new process developed together with Dr. Agrawal’s group constitutes a promising step towards the fabrication of cost-effective anti-
perovskite thin films for photovoltaics, and will be the subject of further research at Dr. Saucedo’s facilities at UPC.

Overall, this internship at Purdue University has been a great scientific opportunity for Ivan, allowing him to learn new materials synthesis techniques, characterization strategies and laboratory practices; as well as the possibility of discovering other ways of approaching science and engineering, broadening his horizons to new possibilities and paradigms to understanding science. Furthermore, it has also been an enriching experience from a human point of view, by offering the chance to plunge into a different culture, become familiar with its customs and idiosyncrasies, and interact with its people. In his free time, Ivan has explored the state of Indiana and the Midwest region, visiting the cities of Chicago, Saint Louis and Indianapolis, and hiking in several national and state parks (such as Indiana Dunes, Mammoth Cave and Turkey Run).