Overview

Solution-processable organic semiconductors are poised to revolutionize the electronic and printing industries, but to achieve the ultimate desirable properties, new creative approaches and synthetic strategies are required.  Conventional approaches to organic semiconductor design typically rely on purely planar pi-conjugated systems whose opto-electronic properties are tuned with the nature of the chemical backbone, functional groups and side chains.  To access new functionalities, this project turns to hybrid organic/inorganic coordination compounds, where coordination with the main group or transition metals provides an additional powerful tool to tune properties of pi-conjugated systems, such as molecular shape, crystallinity and opto-electronic properties.  This research exploits azadipyrromethene complexes as a versatile platform for developing the next generation of solution-processable hybrid semiconductors. 

Dr. Sauve of the Department of Chemistry at Case Western Reserve University previously found that zinc(II) complexes of azadipyrromethene are promising molecular semiconductors for organic photovoltaics (OPV). These molecules have intense absorption in the visible region of the spectra and a high tendency to accept electrons, making them good electron acceptors. The coordination of azadipyrromethene with zinc(II) metal center creates a non-planar geometry that is beneficial for OPV by facilitating favorable phase separation from pi-conjugated polymer donors and providing charge transport path in all direction (isotropic). With this award funded by the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Dr. Sauve explored chemical substitutions to guide the self-assembly of these non-planar molecules in films, with the goal of optimizing device performance.

Intellectual Merit

The Sauve group found that using 2-naphthylethynyl pyrrolic groups in Zn(L2)₂ (Figure 1) improves crystallization and give a high PCE of 5.5% when blended with inexpensive poly(3-hexylthiophene) (P3HT) donor. More importantly, it has a much simpler synthesis than other state-of-the art acceptors.

• J. Mater. Chem. A 2019, 7, 24614-24625 https://doi.org/10.1039/C9TA08654D

• J. Phys. Chem. C 2020, 124, 8541-8549 https://pubs.acs.org/doi/10.1021/acs.jpcc.0c00401

One factor that may limit the performance of Zn(L2)₂ is its relatively low electron mobility. We were able to increase electron mobility by an order of magnitude using strategic fluorination (Figure 1). Crystal structures showed that the fluorine on the naphthyls encourages co-facial pi-pi stacking of the naphthylethynyl groups from adjacent molecules in films, which is beneficial for electron transport. Such co-facial pi-pi stacking had not been seen before for these types of non-planar hybrid coordination compounds. The fluorinated compounds are also more crystalline and showed improved performance in OPVs.

• J. Phys. Chem. C 2022, 126, 15, 6543-6555. https://doi.org/10.1021/acs.jpcc.1c09734

The Sauve group also explored optimizing charge carrier mobility with the use of side chains. Molecules with hexyl or hexyloxy groups, placed either on the proximal or distal phenyls of Zn(WS3)₂ were synthesized and studied (Figure 2). The complexes with hexyl groups tend to be amorphous in films with evidence of isotropic electron mobility. Adding the hexyl on the proximal phenyls increases electron mobility to a very high value of 9x10^-4 cm²V^-1s^-1 in diodes. On the other hand, hexyloxy groups promote crystallization and order in film, especially when placed on the distal phenyls, as observed from polarized microscopy. The complexes with hexyloxy groups have low electron mobility but very high hole mobility in diodes: 3-4x10^-3 cm²V^-1s^-1. The hole mobilities estimated in transistors are a couple orders of magnitude lower than in diodes, suggesting that self-assembly in film favors hole transport across the film, ideal for diode applications such as OPVs. These molecules thus have potential as electron donor in OPV and as hole transport layer for thin film photovoltaics.

• Dyes & Pigments 2023, 208, 110858. https://doi.org/10.1016/j.dyepig.2022.110858

• Paper in preparation for J. Mater. Chem. C.

Broader Impact

The knowledge gained in this work will impact the fields of pi-conjugated systems, dyes with visible NIR absorption and organic electronics. The materials prepared are expected to impact the printable electronic industry, providing portable devices and alternative ways to convert solar energy to high value electricity. Students associated with this project are exposed to interdisciplinary research and prepared to be next generation of scientists. This award enabled the training of four Ph.D. graduate students (including two women), of which three have graduated. Jayvic C. Jimerez graduated in 2021 and took a postdoctoral position with Prof. French in Materials Science and Engineering at CWRU. In 2023, he moved to Lawrence Livermore National Laboratory. Muyuan Zhao graduated in 2022 and is now an engineer at Zolix Instrument Company. Quynh Tran graduated in October of 2023 and will start a postdoctoral position with Prof. French in November 2023. This grant also enabled training of eight undergraduate students (five women) and three high school students (including one female minority student). Results were also disseminated in refereed journals, conferences, and invited seminars at universities.

Last Modified: 10/29/2023
Modified by: Genevieve Sauve