The exact molecular interactions at the interface of an organic heterojunction are key to the development of efficient optoelectronic devices. Due to the difficulty in characterizing these buried and (likely) disordered heterointerfaces the exact interfacial structure in most systems remains a mystery. Here, we demonstrate a novel synthetic strategy to design and control model interfaces, allowing for their detailed study in isolation from the bulk material. This is achieved and highlighted through the synthesis of a donor polymer – non-fullerene acceptor through-space linked system, where the exact position and orientation of the components is completely controlled. By placing the acceptor above different portions of the conjugated polymer, drastic effects on the CT state properties are observed. We observe that through synthetic control the oscillator strength of the CT state and the rate of its formation can be controlled by several orders of magnitude. These results have signficant implications for the design and operation of OPV devices

Implantable neurostimulation devices play a key role in treating many injuries and diseases by providing a direct therapeutic link to the nervous system. This enables brain stimulation for treatment of Parkinson’s disease and epilepsy, nerve guidance and regeneration to remedy spinal cord injury, and retinal prosthetic devices that could cure blindness.^[1] To address such issues, new bioelectronic systems that can deliver electrical stimulation to nerve cells are required. Although silicon microelectronics and metal electrodes have been the historic gold standard for bioelectronic interfaces, the main obstacles to further translation of these devices include a low biocompatibility that reduces in vivo lifetimes, a mechanical rigidity that is poorly matched with soft tissue, causing inflammation and ineffective electrical contact, and a requirement for costly external power supplies to deliver current.^[2] These issues result in indiscriminate tissue activation, with a consequent lack of spatial selectivity.^[3]

In this work, we report our recent efforts to simultaneously address these issues by combining soft carbon-based organic semiconductors and nanoscale science to build bioelectrodes that allow optical neurostimulation without external power. Our approach creates bioelectronic interfaces from organic semiconductors that can be formed into customized nanoparticles with established solution-based chemistry methodologies. This approach enables the stimulating electrodes to be combined with targeted pharmaceutical factors in the fabrication procedure, which subsequently optimise connections to the neural network when released in-vivo.^[4]

We will discuss how we tuned the optoelectronic properties of the organic nanoparticles to cover red, green, and blue wavelengths, allowing spectrally selective platforms for neurostimulation. These semiconductors are turned into electroactive inks, and subsequently fabricated into pixelated arrays using inkjet printing. This approach establishes a new low-cost manufacturing methodology that is applicable to other organic materials and can be used for a variety of bioelectronic devices, creating a new manufacturing paradigm for healthcare.

We demonstrate both the anatomical and functional biocompatibility of neural tissue with our organic bioelectronic systems using immunolabelling with neuronal marker MAP2 and visualisation with epifluorescence microscopy to detect neurons cultured on the organic conductors. We demonstrate the controlled release of drugs from the organic nanoparticles, aiding in precise spatial delivery of pharmaceutical factors. Finally, we employ whole-cell patch clamp electrophysiology recordings to demonstrate an exciting result; purely optical neurostimultation of dorsal root ganglion nerve cells. We demonstrate that the organic conductors can trigger changes in the nerve cell membrane potentials via a capacitive coupling mechanism, the efficacy of which can be improved by judicious selection of the device architecture.^[5]

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Detection of trace levels of hazardous chemicals is an important challenge for environmental monitoring, homeland security and humanitarian landmine clearance. Organic semiconductor sensors, which exploit a fluorescence quenching mechanism in the presence of certain target analyte molecules, offer an attractive approach for extremely sensitive detection. In this presentation we will describe how molecular design and thin-film structure influence both the microscopic physics, and the sensor performance including their sensitivity, speed of response and recovery, and potential for discrimination between target analytes. 

We will also present progress and potential in developing applications of these sensors in-field conditions for humanitarian demining and environmental monitoring. For example, the ability to detect the presence of explosives across an area of interest would be of particular use in technical surveys of suspected minefields. We show that these sensors can be used to detect nano-gram level quantities of nitroaromatic (TNT-like) molecules, and combine them for the first time with a novel preconcentration approach to detect buried explosives. Initial field trials on a test minefield will be presented, in which the sensors are used to detect trace explosives collected by colonies of foraging honeybees. We will also describe how these sensors can provide a new approach for early warning of environmental pollution events in water bodies.

The growing demands for smaller, faster, yet less expensive and more versatile electronics, have led to a surge of efforts focused on the development of new componentry technologies. Molecular-scale devices could address this challenge and are being touted to become a core technology of the 21st century, complementing the conventional silicon-based industry with applications currently not possible. One of the simplest configurations is the molecular diode, which consists of a self-assembled monolayer (SAM) sandwiched between two electrodes and its function is to introduce an asymmetry between the current measured under positive and negative bias regimes, respectively. The efficiency of current rectification, quantified as the rectification ratio, is dependent on many factors, including the molecular structure of the SAM, its density and degree of order, as well as coupling to the electrodes. In this talk I will address these topics by discussing first the dependence of the rectification on the strength of the dipole moment of the molecules, followed by an example of manipulating the properties by exploiting the charge-transfer that results from co-assembling molecules with strong electron donor and acceptor termini.¹ We further show that the molecule/electron coupling resulting from the delocalization of electrons lone pairs controls the position of the molecular orbitals participating in transport, making it efficient for one bias polarity and inefficient for the opposite polarity. In the case of a strong coupling, this led to rectification ratios greater than 2500.² This performance was obtained in molecular diodes fabricated on silicon, which has a very low natural roughness, and thus require no additional processing. Our molecules are obtained from a simple, robust, and high-yield synthetic procedure and may yield hybrid systems when integrated with other more mature silicon technologies that are currently included in consumer electronics to expand its use toward novel functionalities governed by the molecular species grafted onto its surface. One example is that of a water sensor, which transduces the ambient moisture absorbed into the device through the porous electrode to electrical signal. By monitoring these changes, we demonstrate clear and reproducible changes in rectification ratio that are reversible upon multiple cycles.

Most advanced optoelectronic devices rely on the spatial patterning of the relevant properties of the active components. An example of this would be the RGB pixels sitting side by side in LED based screens. Interestingly, between a homogeneous thin film, and a highly complex pixelated board, there is a whole parameter space that can be navigated: that of samples with gradual changes in the properties of interest.

In this talk, I will first describe a number of methods that we have developed to produce organic semiconductors based gradients in film thickness, composition, doping level, molecular orientation and microstructure. Then, I will briefly mention how these samples, containing a wealth of information, can be employed for fundamental studies (such as deducing the phase diagram) or combinatorial optimization of solar cells. And then, in the last part of the talk, I will describe new device concepts based on samples with gradients, such as polarimeters without moving parts based on spherullitic macroscopic alignment, position sensitive photodetectors, and monolithic miniature spectrometers based on microcavities with thickness wedge active layers.

The rapid development of charge transport and light‐emission in organic materials in the last decades has advanced the field of organic optoelectronics, highlighting the high potential of light‐emitting devices for industrial applications. Demonstrated for the first time over 15 years ago, light-emitting field-effect transistors (LEFETs) have transformed from an optoelectronic curiosity to a serious competitor in the race for cheaper and more efficient displays, also showing promise for injection lasers. Thus, what is a LEFET, how does it work, and what are the current challenges for its integration into mainstream technologies? The talk will shed some light on these questions. The fundamental working principle of LEFETs, materials that have been used, and device physics and architectures involved in the progression of LEFET technology for displays will also be discussed. Finally, the state‐of‐the‐art development of LEFETs will be presented as prospect avenues for the future of research and applications in this area.

Inherently narrowband near-infrared organic photodetectors (NIR-OPD) are highly desired for many applications, including biological imaging and surveillance. To obtain a narrowband response, one of the most used approaches is to integrate a photodiode in an optical cavity configuration. With manipulating the cavity length, tuneable wavelength-selective NIR-OPDs were realized^[1].  However, they suffer from a low external quantum efficiency (EQE) and strongly rely on the used material or on a nano-photonic device architecture. Here, we demonstrate a general and facile approach towards wavelength-selective NIR-detection through intentionally n-doping 500-600 nm-thick nonfullerene blends. We show that a electron-donating amine-interlayer can induce n-doping, resulting in a localized electric field near the anode and selective collection of photo-generated carriers in this region. As only weakly absorbed photons reach this region, the devices have a narrowband response at wavelengths close to the absorption onset of the blends with a high spectral rejection ratio. These spectrally selective NIR-OPDs exhibit EQEs of ~20-30% at NIR wavelengths of 900-1100 nm, with a full-width-at-half- maximum of ≤50 nm, as well as detectivities of >10¹² Jones (to be published in Nature communications soon).

Efficient, sensitive and wavelength-selective light detection has become central to modern consumer electronics, and also in science and technology. Photodetectors based on crystalline inorganic elemental materials such as silicon and compound semiconductors are the core of today’s photodetectors. However, a new trend has begun: next generation semiconductors such as organics, perovskites, and nanocrystals are now becoming increasingly interesting candidates for low noise, color-selective, efficient photodetection. As a promising candidate for next-generation photodetectors, solution-processed organic-based photodetectors (OPDs) provide the opportunity to develop innovative, low cost, and large-area imaging technologies for industrial applications. Thanks to their advantages as a large range of available materials, a tunable spectral response range, compatibility with lightweight flexible substrates and device architectures, they have gained a continuous increasing interest.

OPDs have drawn extensive research efforts due to their tailorable spectral response but to detect specific signals under low illumination intensities, OPD devices should deliver a low dark current and a relatively high sensitivity. The spectral response can be either broadband or narrowband. Benefiting from advanced material synthesizing techniques, narrowband OPDs with many improved properties have been emerging in recent years, with promising features like high selectivity of the target wavelength compared to broadband counterparts. In addition and to avoid specific synthesis of the photo-absorbing materials, the charge collection narrowing (CCN) can be applied.^[1] Starting from a broadband absorbers often used in organic solar cells (OSCs), narrowband OPDs are obtained by the realization of thick layers (700 nm), which flattened the spectral response, reduced the dark current and decreased performance variations.

In the present study, broadband and narrowband OPDs were realized with an active layer composed of PM6/ITIC-4F bulk heterojunction blend. During the past years, the development of non-fullerene acceptors (NFA) led to an impressive improvement of the OSC power conversion efficiency to above 18%.^[2] This new family of electron accepting materials with absorption edges up to 1100 nm has great potential for both broadband and narrowband OPDs. Depending on the structure of the OPD device, normal or inverted, the switch between broadband and narrowband OPDs has been controlled by the donor:acceptor mass ratio, the final blend concentration, the film thickness or most notably by the post-annealing temperature. The impact and role of these parameters were studied by absorption and water contact angle measurements. The morphology and microstructure of thin films made by liquid deposition techniques were analyzed by AFM, Raman and X-ray diffraction.