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.
Organic Photodetectors and Organic Photovoltaics
Work of our group aims to develop a deeper understanding of the molecular origin of factors such as high device efficiency and photostability of organic photovoltaics (OPVs) and organic photodetectors (OPDs). This is achieved by using a range of complementary experimental structural and electronic characterisation techniques in conjunction with computational DFT techniques to elucidate the link between molecular properties and device photophysics and performance. A systematic understanding of this relationship can be harnessed to formulate valuable molecular design rules in addition to opening novel routes for device design and functionality.
For example, recent work of our group together with Samsung Advanced Institute of Technology has investigated the role of molecular planarity on co-evaporated bulk heterojunction OPD light intensity dependent photoresponse times. While planar MPTA:C60 blends show faster response times at relatively light intensities, twisted NP-SA:C60 blends are favoured at relative low light intensities. Using ambient photoelectron spectroscopy (APS), the origins of the intensity dependent behaviour in blends with planar MPTA donor are attributed the higher density of relatively deep trap states in planar MPTA donor blends which become increasingly detrimental under low-light operation. In contrast, the twisted NP-SA donor maintain light independent photoresponse times due to their comparatively low deep trap state density .
Our work has also been fundamental in understanding the molecular origin of photodegradation in non-fullerene acceptors (NFAs) for OPV applications. For example, by using resonant Raman spectroscopy in conjunction with DFT simulations it is possible to identify which conformational changes of the molecule are responsible for degradation in addition to the degradation product itself. In a recent systematic study of ITIC derivatives, a strong correlation between molecular charge distribution (characterised by the molecular quadrupole moment) and NFA photostability was demonstrated due to differing intermolecular interaction strengths. In this study we were also able to identify factors that influence the magnitude of the quadrupole moment .
 C. Labanti, et al. Nat. Commun. 2022, 13, 3745
 J. Luke, et al. Adv. Energy Mat. 2022, 12, 30
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.