A. Conjugated Polymers/Organic Semiconductors

Conjugated polymers, with their extended pi conjugation, are typically organic semiconductors. Similar to inorganic semiconductors (such as Si, GaAs), conjugated polymers generally have a rather narrow band gap (2.0 eV or less) and can transport charge carriers (electrons or holes). Thus, conjugated polymers have been actively explored for most (inorganic) semiconductors-based applications (such as solar cells, light-emitting diodes, transistors, sensors, to name a few).

Importantly, given the ‘soft’ nature of these conjugated polymers, they offer significant advantages over traditional inorganic semiconductors, such as flexibility, lightweight, and potentially lost-cost production. Furthermore, new applications specifically utilize the unique features of conjugated polymers have been under intensive research, such as bioelectronics.

1. Solar Cells

Our lab has done significant work in conjugated polymers based solar cells, particularly in elucidating the structure-property relationships in a systematic manner. Many of the design rationales and concepts have been adopted by the research community to create better materials to achieve higher efficiency of polymer solar cells.

As the record-high efficiency of polymer solar cells has reached almost 20%, currently we are focusing on (a) new methods to improve the stability of polymer solar cells, and (b) new synthetic approaches to lower the cost of materials for polymer solar cells.

Selected Publications:

• Macromolecules 2012, 45, 607-632.  Link  (general review)

• Adv. Mater. 2017, 29, 1601391.  Link  (general review)

• J. Phys. Chem. Lett. 2013, 4, 1802-1810.  Link  (review on ternary blends)

• Nature Photonics 2015, 9, 491-500.  Link  (review on ternary blends)

• Acc. Chem. Res. 2017, 50, 2401−2409.  Link  (review on fluorine impact)

• Nature Materials 2021, 20, 525–532. Link  (general framework for stability of OPV)

• ChemSusChem 2021, 14, 3561–3568. Link (low cost synthesis of PTQ10)

• Chem. Mater. 2021, 33, 4745–4756. Link (thermoclevable side chains for better stability of P3HT)

• Chem. Mater. 2023, 35, 10139–10149. Link (thermoclevable side chain for PCE11)

• ACS Appl. Polym. Mater. 2023, 5, 1937-1944. Link (new synthetic approach for D18 series)

2. Green/Sustainable Electronics

If organic electronics were going to enter the commercial market (we certainly hope so!), no one would want to repeat the same mistake we have made with commodity polymers (i.e., plastics pollution) and commercial electronics (i.e., electronic waste). Ideally, such organic polymers would be sourced from renewable stocks and recyclable, and the manufacturing process should use green solvents. Achieving all these requires intensive research, which naturally has become our research focus.

Selected Publications:

• ACS Appl. Polym. Mater., 2019, 1, 804–814. Link (OEG side chains)

• Macromolecules 2023, 56, 2092–2103. Link (OEG side chains for green solvent processability

3. Multifunctional Sensors

When exposed to certain stimuli (light, water, etc.), conjugated polymers can change their conductivity and other physical properties, often in a quantitative manner. This intriguing behavior has been utilized to design sensors to detect these stimuli. For example, in collaboration with colleagues, our lab has designed a specifically designed conjugated polymer that demonstrated rather unique combination of high mobility, excellent stability and impressive stretchability with its wrinkled film. Given the vast design space of conjugated polymers, we are actively working with other labs to explore novel sensors and integrated devices for unique applications, particularly towards human health.

Selected Publications:

• Nature Communications 2022, 13, 2739. Link (stretchable sensor)

• Advanced Materials Technologies 2014, 56, 2092–2103. Link (breath sensor)

4. Photodetectors

In organic solar cells, conjugated polymers and small molecules absorb light and convert it into electricity. Similar operating principles can be used to achieve light-detecting (i.e., photodetector or photosensor); however, to achieve high figures-of-merit for photodetectors, including photoresponsivity, quantum efficiency and gain, response time, specific detectivity, etc., conjugated polymers and small organic molecules-based photodetectors require different materials and device configurations.

We are particularly interested in developing photodetectors targeting short-wave infrared (SWIR) range (1.0 to 2.5 μm). While it is not visible to human eyes, SWIR light has several significant advantages over visible light for imaging applications. For example, SWIR light can penetrate fog or smoke thus SWIR photodetectors (imaging sensors) are extremely useful for automobiles.

Selected Publications:

• coming soon…

5. Doping

Doping of organic semiconductors (conjugated polymers and small molecules) plays an important and growing role in organic electronics, ranging from organic light emitting diodes and organic solar cells to thermoelectrics and bioelectronics. However, unlike Si, where parts per million dopant concentrations can significantly alter conductivity, doping organic semiconductors with molecular dopants typically requires a much higher concentration. Complex interplay between the microstructure, electronic structure and molecular interactions of the polymer and dopants has prevented systematic understanding.

Coordinating the efforts of a large collaborative team (7 research groups across 4 institutions), the DOPE Center (supported by the DoD MURI program) aims to transform the field of solution-processed, doped conducting polymers by:

1. Co-designing optimized p- and n -doped conducting polymer systems and achieving facile n-doping.
2. Extending air-stable photoredox doping in conjugate polymers.
3. Accelerating the pace of data generation by collaborative workflow integration of robotic experimentation and machine learning (ML) and artificial intelligence (AI) approaches.
4. Creating fundamental knowledge that informs novel materials synthesis and system optimization.
5. Creating new technologies enabled by these advances including 3D printing and photopatterning of complementary circuits.

Selected Publications:

• J. Phys. Chem. C. 2024, 128, 2748-2758. Link (quantify doping efficiency)

6. Chirality-Induced Spin Selectivity

The Chirality-Induced Spin Selectivity (CISS) effect is an intriguing quantum effect that can convert between two different properties of the electron, its charge and spin. This effect stems from the chirality of certain materials, where their structural nature represents a unique “handedness” which cannot be superimposed onto similar materials with the opposite handedness. CISS provides a strong coupling between spin and charge transport properties in chiral systems without the need for magnetic materials, thereby launching a variety of CISS-related spintronic, chemical, biological, and quantum information applications using different chiral assemblies. However, much is still unknow about CISS, particularly the fundamental mechanism.

Supported by the DoD MURI program, our collaborative team (8 research groups across 5 institutions) aims to overcome this long-standing challenge and elucidate the interplay of chirality and spin phenomena in chiral assemblies by:

1. Exploiting chirality-driven rich interactions between spins, electrons, photons, and phonons in conjugated polymer-based chiral semiconductors and insulators.
2. Developing a qualitatively comprehensive CISS model.
3. Establishing fundamental structure-property relationships in a variety of synthetically tunable chiral conjugated polymers and their assemblies.
4. Extending from fundamental science to demonstration of novel spintronic and bio-inspired device concepts.

Selected Publications:

• Nature Materials 2023, 22, 322–328. Link (chiral phonon spin Seebeck)