Soft Electronic Materials for Energy, Display and Healthcare
Organic Gas Sensors
With ever-developing and readily accessible portable technology, there are growing demands for cost-effective sensors featuring easy scalability, portability, and quick and reliable detection. Solution-processable organic semiconductors offer huge advantages such as low cost (including material and device fabrication costs), scalability (large-area printing), mechanical flexibility (including flexible substrates), synthetic tuneability (fine-tune sensitivity and selectivity), easy integration with other electronic devices, and room temperature operations. However, organic gas sensors are still in the early stages of applied research.
One exciting scientific research area concerning organic gas sensors is early, readily accessible, reliable, and non-invasive diagnosis of certain diseases by detection of specific volatile organic compounds (VOCs) present in an exhaled human breath. In this respect, organic semiconductors can transduce local dielectric environment changes induced by VOCs (external stimuli) into distinct electrical signals. At present, the major challenges lie in device performances such as low sensitivity (several hundred parts-per-million, ppm), limited VOC selectivity (limited understanding of the required synthetic tuning for given VOCs), and slow response times (several minutes).
Some of our research areas within organic gas sensors include:
- Sensor device conception and optimization .
- Fundamental concepts of charge carrier dynamics and polaron formation .
- Mechanism of sensing .
1. Kwon, S.; Pak, Y.; Kim, B.; Park, B.; Kim, J.; Kim, G.; Jo, Y.-R.; Limbu, S.; Stewart, K.; Kim, H., Molecular-level electrochemical doping for fine discrimination of volatile organic compounds in organic chemiresistors. Journal of Materials Chemistry A 2020, 8 (33), 16884-16891.
2. Nightingale, J.; Wade, J.; Moia, D.; Nelson, J.; Kim, J.-S., Impact of Molecular Order on Polaron Formation in Conjugated Polymers. The Journal of Physical Chemistry C 2018, 122 (51), 29129-29140.
3. Stewart, K.; Limbu, S.; Nightingale, J.; Pagano, K.; Park, B.; Hong, S.; Lee, K.; Kwon, S.; Kim, J.-S., Molecular understanding of a π-conjugated polymer/solid-state ionic liquid complex as a highly sensitive and selective gas sensor. Journal of Materials Chemistry C 2020, 8 (43), 15268-15276.
4. Limbu, S.; Stewart, K.; Nightingale, J.; Yan, H.; Balamurugan, C.; Hong, S.; Kim, J.; Lee, K.; Kwon, S.; Kim, J.-S., Solid-State Ionic Liquid: Key to Efficient Detection and Discrimination in Organic Semiconductor Gas Sensors. ACS Applied Electronic Materials 2021.
Together with sunlight conversion to electricity in photovoltaic applications, organic semiconductors are under the spotlight for being a highly promising alternative to silicon-based photodetector technologies. The aim of organic photodetectors (OPDs) is to probe photons of a specific wavelength range, maximising the output of generated electrons . Together with high detectivity, wavelength selectivity, fast response speed and low dark current, organic materials offer the advantage of being light, flexible and transparent, for possibilities of revolutionary design in wearable and biomedical sensors or the Internet of things .
In our group, we contribute to the fast-growing OPD field with our expertise in both device characterisation and microstructural analysis of materials, using tools such as vibrational spectroscopy (Raman) combined with atomic force microscopy, energetics measurements and transient extraction techniques. In particular, we collaborate with Samsung Advanced Institute of Technology (SAIT) to elucidate the requirements for molecular design to optimise OPD performances and understand the impact of donor small molecule structure on blend morphology and device operation. For example, we recently demonstrated the importance of nanoaggregation by thermal treatments to improve charge transport in OPD materials .
Other topics of ongoing research are focused on the comparison of bulk heterojunction and bilayer OPD structures as well as on fullerene and nonfullerene-based OPD blends. A primary aim is to understand the detecting performances when typical organic photovoltaic materials are employed in OPDs with different architectures for clarifying and controlling the main loss mechanisms.
 K.-J. Baeg et al. Adv. Mater. 25 (2013), 4267-4295.
 M.G. Han et al. ACS Appl. Mater. Interfaces 8 (2016), 26143-26151.
 S. Limbu, J.-S. Kim et al. ACS Nano 15, 1 (2021), 1217-1228.
Organic photovoltaic (OPV) devices have attracted great attention recently due to their high power conversion efficiencies (PCEs) of over 18%, which is comparably high with thin-film inorganic solar cells. The development of non-fullerene acceptors with minimised energy losses and proper band gaps through careful molecular design is what enabled these exceptionally high PCEs to be achieved. To further develop OPVs, in addition to high PCEs, their practical applications, such as use in low-light conditions, should be focused on.
Detailed and systematic study of the highly efficient organic photoactive materials can give insight into the factors that influence their electrical and optical properties. In our group, we have contributed to the field of OPV research by investigating:
- How the energetics and tail states of organic semiconductors affect OPV device performance by correlating the molecular structures and their orientation.
- What factors are related to the operational photo-stability.
- How OPV devices behave differently under low-light (indoor) light conditions compared to 1 Sun.
Alongside the above research topics, we are also interested in bilayer OPV devices, all-polymer solar cells and detailed energetic studies using high-efficiency non-fullerene acceptors.
With the growing demands on clean energy, researchers are looking for alternative materials to silicon for next generation solar cells. Mixed organic-inorganic Perovskite materials show great optoelectronic properties, which make them one of the most promising materials for future solar cell markets. The high charge mobility, low fabrication cost and facile tenability allows perovskite solar cells (PSCs) to reach power conversion efficiencies (PCE) of 25.5% . Despite the high PCE, PSCs are still falling behind the theoretical Shockley-Queisser limit. This is due to the interfacial recombination, the energetic alignment between charge transport materials and the energy bandgap of perovskite itself. In our group, we aim to understand the mechanism behind these three non-ideal properties by advanced energetic and photonic measurements.
Current areas of research include:
- Interfacial Recombination: we study different transport materials and their interfacial properties between perovskite layers .
- Organic transport materials for higher performing PSCs: we compare different organic transport materials to investigate the impact of energetic alignment on device performance: .
- Fine-tuning perovskite composition: we are investigating the impact of blending different perovskite dimensionalitoies together on energetic properties, such as density of states.
 https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200104.pdf. Best research cell efficiency NREL. (Accessed Jun 25, 2020).
 Daboczi, M.; Hamilton, I.; Xu, S.; Luke, J.; Limbu, S.; Lee, J.; McLachlan, M. A.; Lee, K.; Durrant, J. R.; Baikie, I. D.; Kim, J. S.Origin of Open-Circuit Voltage Losses in Perovskite Solar Cells Investigated by Surface Photovoltage Measurement. ACS Appl. Mater. Interfaces 2019, 11 (50), 46808–46817. https://doi.org/10.1021/acsami.9b16394
 Du, T.; Xu, W.; Daboczi, M.; Kim, J.; Xu, S.; Lin, C. T.; Kang, H.; Lee, K.; Heeney, M. J.; Kim, J. S.; Durrant, J. R.; McLachlan, M. A.P-Doping of Organic Hole Transport Layers in p-i-n Perovskite Solar Cells: Correlating Open-Circuit Voltage and Photoluminescence Quenching. J. Mater. Chem. A 2019, 7 (32), 18971–18979. https://doi.org/10.1039/c9ta03896e.