Periodic Reporting for period 3 - ConTROL (Charge-TRansfer states for high-performance Organic eLectronics)
Reporting period: 2023-07-01 to 2024-12-31
Thin films comprising a blend of electron donating and electron accepting molecules are ubiquitous in organic opto-electronic devices. At the donor-acceptor interfaces, intermolecular charge-transfer (CT) states form, in which an electron is transferred from donor to acceptor. Electrical doping involves CT in the ground-state, from dopant to host, and results in increased conductivities of the host organic semiconductor. Furthermore, efficient photo-induced charge generation in organic materials depends crucially on donor-acceptor interfaces where the CT state is an excited state. Organic electronics will in the future enable new applications for a healthy, green and connected society, such as for example low-cost, non-toxic and building integrated photovoltaics, efficient light sources, sensors for healthcare or artificial synapses. However, current progress is hampered by a lack of understanding of the fundamental properties of intermolecular CT states and their dissociation and decay mechanisms. Improving performance of devices is nowadays often based on time consuming rail-and-error methods. The objective of ConTROL is to fill the knowledge gap and link device performance to CT state properties and molecular parameters of donor and acceptor. Besides the established way of tuning electro-optical properties by molecular design and appropriate donor-acceptor selection, innovation in ConTROL lies in the use of weak and strong interactions with the opto-electronic device’s optical cavity. The newly developed models and simulation code taking these effects into account, from the molecular to the device level. The project will demonstrate rational design of new organic semiconductors resulting in improved performance of organic electronic applications, as well as novel device concepts for sensing and energy conversion based intermolecular CT states.
The objectives of ConTROL require state-of-the-art organic thin film and device fabrication and characterization facilities, which have been built up in this first half of the project. Together with the existing synthetic chemistry expertise at the institute, the lab now combines high-throughput solution and vacuum deposition tools and expertise for the fabrication of new donor-acceptor systems and devices. Optical and electrical models have been implemented and further aid in the design and synthesis of new donor-acceptor materials as well as device optimization. These facilities have enable the ConTROL team to generate fundamental knowledge in the field of organic electronics, applicable to current and future donor-acceptor systems and devices based hereupon. Specifically:
- We have revealed how the non-radiative decay mechanisms at the donor-acceptor interface set an upper limit for the achievable detectivity of organic photodetectors.
- We have elucidated how the molecular factors which affect the radiative and non-radiative decay properties of CT states also affect the lineshape of the emission spectrum. Our experimental data suggests that the donor-acceptors systems with the narrowest lineshape will have the lowest non-radiative decay rates.
- We have further experimentally verified and rationalized a relation between the distance of the donor and acceptor molecule, the non-radiative decay rates and the voltage of photovoltaic devices based on donor-acceptor blends.
The project team has also demonstrated new device concepts: Our understanding achieved on ground-state CT (doping) has enabled efficient narrowband near-infrared photo-detection by doping a donor-acceptor blend with an amine interlayer. The optical model developed within ConTROL has further been used to design resonant cavity device architectures in which we can manipulate CT absorption, increasing its absorption strength by more than a factor of 10 at specific wavelengths. This allows us to demonstrate organic narrowband photodetectors with extended detection wavelengths.
- We have revealed how the non-radiative decay mechanisms at the donor-acceptor interface set an upper limit for the achievable detectivity of organic photodetectors.
- We have elucidated how the molecular factors which affect the radiative and non-radiative decay properties of CT states also affect the lineshape of the emission spectrum. Our experimental data suggests that the donor-acceptors systems with the narrowest lineshape will have the lowest non-radiative decay rates.
- We have further experimentally verified and rationalized a relation between the distance of the donor and acceptor molecule, the non-radiative decay rates and the voltage of photovoltaic devices based on donor-acceptor blends.
The project team has also demonstrated new device concepts: Our understanding achieved on ground-state CT (doping) has enabled efficient narrowband near-infrared photo-detection by doping a donor-acceptor blend with an amine interlayer. The optical model developed within ConTROL has further been used to design resonant cavity device architectures in which we can manipulate CT absorption, increasing its absorption strength by more than a factor of 10 at specific wavelengths. This allows us to demonstrate organic narrowband photodetectors with extended detection wavelengths.
New material and device concepts for organic photodetectors, improving upon the current state-of-the-art have been demonstrated experimentally and described in publications. The improvement lies mostly in the extension of the wavelength range, deeper into the near infrared, while keeping a high performance. New device concepts for thermal and photon up-conversion based on CT states are currently under investigation. In order to make these devices economically interesting, there is also further work on device stability and finding appropriate applications.
The ConTROL team has developed a new method for an accurate measurement of the photoluminescence quantum efficiency improving the measurement accuracy by a factor of 10 as compared to classical measurement techniques. Our experience in photo-thermal deflection spectroscopy, built up in the first half of the project, was crucial in this development.
Further important fundamental findings have been published and are expected to advance the field significantly: We have identified non-radiative decay as CT states as the limiting factor when using organic semiconductors for photovoltaic and photo-detecting applications. The remainder of the project will now focus on identifying the molecular factors responsible for this non-radiative decay as well as for the dissociation of CT states into free carriers.
The ConTROL team has developed a new method for an accurate measurement of the photoluminescence quantum efficiency improving the measurement accuracy by a factor of 10 as compared to classical measurement techniques. Our experience in photo-thermal deflection spectroscopy, built up in the first half of the project, was crucial in this development.
Further important fundamental findings have been published and are expected to advance the field significantly: We have identified non-radiative decay as CT states as the limiting factor when using organic semiconductors for photovoltaic and photo-detecting applications. The remainder of the project will now focus on identifying the molecular factors responsible for this non-radiative decay as well as for the dissociation of CT states into free carriers.