Periodic Reporting for period 1 - OPTOCHARGE (Optical probing of charge traps in organic field-effect transistors)
Reporting period: 2023-05-01 to 2025-04-30
Organic electronics holds considerable promise for the development of large-area, cost-effective, and energy-efficient devices. The use of solution-based processing techniques, combined with access to a vast library of organic semiconducting materials, allows for precise tailoring of material properties to meet specific application requirements. These methods also minimise material waste and enable low-temperature fabrication, thereby reducing overall energy consumption.
While organic electronic technologies have already achieved commercial success—most notably in the form of large-area, flexible displays—their broader potential in emerging fields such as biomedical applications, chemical and biological sensing, and sustainable electronics has yet to be fully realised. A key limitation lies in the performance of fundamental device components, such as organic field-effect transistors (OFETs), which continue to suffer from restricted charge carrier mobility and inconsistent behaviour when novel material formulations are employed.
A major impediment to performance enhancement in these devices is the presence of charge traps—localized defects within the organic semiconductor bulk or at interfaces—that capture charge carriers and thereby hinder efficient transport through the device. Controlling these charge traps is essential for improving device reliability and functionality; however, their identification, characterization, and rational engineering remain significant challenges due to their complex and elusive nature.
The OPTOCHARGE project introduces a novel optical approach to investigate charge trapping phenomena with high spatial, temporal, and spectral resolution. By selectively stimulating traps using focused light and monitoring the resultant device response, the project seeks to uncover the key parameters that govern charge trapping behavior. A systematic investigation of crystallinity, morphology, polymer blending, dopant incorporation, and electrolyte gating is undertaken to elucidate the origins of trap formation and to develop practical strategies for enhancing the performance of organic electronic devices.
While organic electronic technologies have already achieved commercial success—most notably in the form of large-area, flexible displays—their broader potential in emerging fields such as biomedical applications, chemical and biological sensing, and sustainable electronics has yet to be fully realised. A key limitation lies in the performance of fundamental device components, such as organic field-effect transistors (OFETs), which continue to suffer from restricted charge carrier mobility and inconsistent behaviour when novel material formulations are employed.
A major impediment to performance enhancement in these devices is the presence of charge traps—localized defects within the organic semiconductor bulk or at interfaces—that capture charge carriers and thereby hinder efficient transport through the device. Controlling these charge traps is essential for improving device reliability and functionality; however, their identification, characterization, and rational engineering remain significant challenges due to their complex and elusive nature.
The OPTOCHARGE project introduces a novel optical approach to investigate charge trapping phenomena with high spatial, temporal, and spectral resolution. By selectively stimulating traps using focused light and monitoring the resultant device response, the project seeks to uncover the key parameters that govern charge trapping behavior. A systematic investigation of crystallinity, morphology, polymer blending, dopant incorporation, and electrolyte gating is undertaken to elucidate the origins of trap formation and to develop practical strategies for enhancing the performance of organic electronic devices.
The project made substantial progress in developing and applying advanced optical techniques to investigate charge trapping phenomena in organic field-effect transistors (OFETs). The core aim—to enable spatially resolved identification and characterization of charge traps using laser-based optical probing—was addressed through a structured programme of experimental and instrumentation work, encompassing optical system development, material optimization, and device fabrication.
A major achievement of the project was the successful design, construction, and commissioning of a custom optical setup for charge trap mapping. The system incorporates high-precision translation stages, microscope optics, and a multi-wavelength laser source, all integrated via custom-built mechanical components and software for automated control and data acquisition. This setup enabled localized optical excitation of OFETs with spatial resolution sufficient to detect position-dependent electrical responses, marking a significant step forward in the study of localized trapping phenomena.
Many OFET devices were fabricated using various organic semiconductor (OSC) materials and geometries. Among the tested materials, TMTES-pentacene, particularly when blended with polystyrene, exhibited the most robust and stable photoresponse. This material was ultimately selected as the model system for systematic studies. A reproducible device architecture was established, enabling consistent measurements of photoinduced effects under controlled conditions.
The optical studies revealed a rich and reproducible photoresponse localized near the source electrode of the OFETs. These effects were found to be persistent and dependent on the wavelength and dose of light exposure, consistent with the presence of long-lived trapped charge carriers. These observations provided new insight into minority carrier trapping mechanisms and their influence on OFET performance, and form the basis of an upcoming scientific publication.
In parallel, progress was made in understanding how fabrication parameters influence the photoresponse and trap characteristics of OFET devices. Systematic testing of different OSCs, blend ratios, and deposition techniques facilitated optimization of device structures suitable for optical probing. While some fabrication degrees of freedom were constrained by the need for high spatial resolution, critical insights into material selection and device stability were obtained.
In support of future work on electrolyte-gated OFETs (EG-OFETs), a novel electrical measurement geometry was developed and successfully integrated into the optical setup environment. Although full optical characterization of EG-OFETs was not conducted within the reporting period, the preparatory work, including the design and fabrication of a spring-loaded contact system, lays the foundation for subsequent optical studies on these complex devices.
In summary, the project successfully delivered a fully functional and highly specialized optical probing setup, demonstrated its applicability to OFET devices through extensive experimental measurements, and identified materials and fabrication protocols enabling reproducible studies of charge traps.
A major achievement of the project was the successful design, construction, and commissioning of a custom optical setup for charge trap mapping. The system incorporates high-precision translation stages, microscope optics, and a multi-wavelength laser source, all integrated via custom-built mechanical components and software for automated control and data acquisition. This setup enabled localized optical excitation of OFETs with spatial resolution sufficient to detect position-dependent electrical responses, marking a significant step forward in the study of localized trapping phenomena.
Many OFET devices were fabricated using various organic semiconductor (OSC) materials and geometries. Among the tested materials, TMTES-pentacene, particularly when blended with polystyrene, exhibited the most robust and stable photoresponse. This material was ultimately selected as the model system for systematic studies. A reproducible device architecture was established, enabling consistent measurements of photoinduced effects under controlled conditions.
The optical studies revealed a rich and reproducible photoresponse localized near the source electrode of the OFETs. These effects were found to be persistent and dependent on the wavelength and dose of light exposure, consistent with the presence of long-lived trapped charge carriers. These observations provided new insight into minority carrier trapping mechanisms and their influence on OFET performance, and form the basis of an upcoming scientific publication.
In parallel, progress was made in understanding how fabrication parameters influence the photoresponse and trap characteristics of OFET devices. Systematic testing of different OSCs, blend ratios, and deposition techniques facilitated optimization of device structures suitable for optical probing. While some fabrication degrees of freedom were constrained by the need for high spatial resolution, critical insights into material selection and device stability were obtained.
In support of future work on electrolyte-gated OFETs (EG-OFETs), a novel electrical measurement geometry was developed and successfully integrated into the optical setup environment. Although full optical characterization of EG-OFETs was not conducted within the reporting period, the preparatory work, including the design and fabrication of a spring-loaded contact system, lays the foundation for subsequent optical studies on these complex devices.
In summary, the project successfully delivered a fully functional and highly specialized optical probing setup, demonstrated its applicability to OFET devices through extensive experimental measurements, and identified materials and fabrication protocols enabling reproducible studies of charge traps.
The OPTOCHARGE project has delivered several key scientific and technological results that address longstanding challenges in the field of organic electronics, particularly in the identification, characterisation, and control of charge traps in organic field-effect transistors (OFETs). These traps are known to significantly limit device performance, yet remain difficult to detect and engineer due to their complex and localized nature.
Overview of Results
- Optical Probing Platform:
A novel, fully operational optical probing setup was designed, constructed, and commissioned. This system enables high-resolution spatial mapping of photo-induced effects in organic semiconductors. It combines multi-wavelength laser excitation, precision translation stages, and computer-controlled data acquisition. Importantly, it facilitates the identification of charge traps by observing persistent changes in device conductivity under focused light exposure.
- Material and Device Optimisation:
Systematic testing of various organic semiconductors led to the identification of TMTES-pentacene blended with polystyrene as a particularly responsive material system. Over 100 OFET devices were fabricated and tested under different geometries and conditions to optimise the experimental protocol for reliable optical trapping detection.
- Experimental Innovation:
Two key hardware components were developed with potential for further exploitation:
1) A hermetic enclosure enabling optoelectronic characterisation of devices under controlled atmospheres (e.g. vacuum or inert gas).
2) A low-cost optical shutter compatible with various optical setups, which is already being replicated within the host institution.
- Scientific Insights:
A reproducible, long-lived photoresponse was observed in organic devices, localised near the source electrode. This provides a novel method to investigate trap dynamics and their interaction with charge carriers, contributing new understanding to the physics of organic semiconductors.
- Software and Knowledge Transfer:
Custom software for experimental control and data analysis was developed and made openly available via GitHub. This is now routinely used by researchers in the host group, significantly improving experimental throughput and reproducibility.
Overview of Results
- Optical Probing Platform:
A novel, fully operational optical probing setup was designed, constructed, and commissioned. This system enables high-resolution spatial mapping of photo-induced effects in organic semiconductors. It combines multi-wavelength laser excitation, precision translation stages, and computer-controlled data acquisition. Importantly, it facilitates the identification of charge traps by observing persistent changes in device conductivity under focused light exposure.
- Material and Device Optimisation:
Systematic testing of various organic semiconductors led to the identification of TMTES-pentacene blended with polystyrene as a particularly responsive material system. Over 100 OFET devices were fabricated and tested under different geometries and conditions to optimise the experimental protocol for reliable optical trapping detection.
- Experimental Innovation:
Two key hardware components were developed with potential for further exploitation:
1) A hermetic enclosure enabling optoelectronic characterisation of devices under controlled atmospheres (e.g. vacuum or inert gas).
2) A low-cost optical shutter compatible with various optical setups, which is already being replicated within the host institution.
- Scientific Insights:
A reproducible, long-lived photoresponse was observed in organic devices, localised near the source electrode. This provides a novel method to investigate trap dynamics and their interaction with charge carriers, contributing new understanding to the physics of organic semiconductors.
- Software and Knowledge Transfer:
Custom software for experimental control and data analysis was developed and made openly available via GitHub. This is now routinely used by researchers in the host group, significantly improving experimental throughput and reproducibility.