Singlet fission and triplet-triplet annihilation represent two highly promising ways of increasing the efficiency of photovoltaic devices. Both processes are believed to be mediated by a biexcitonic triplet-pair state, 1 (TT). Recently however, controversy has arisen over the role of 1 (TT) in triplet-triplet annihilation.Here we use intensity-dependent, low-temperature photoluminescence measurements, combined with kinetic modelling, to show that distinct 1 (TT) emission arises directly from triplet-triplet annihilation in high-quality pentacene single crystals and anthradithiophene (diF-TES-ADT) thin films. This work demonstrates that a real, emissive triplet-pair state acts as an intermediate in both singlet fission and triplet-triplet annihilation and that this is true for both endo-and exo-thermic singlet fission materials.

Optoelectronic properties of anisotropic crystals vary with direction requiring that the orientation of molecular organic semiconductor crystals is controlled in optoelectronic device active layers to achieve optimal performance. Here, a generalizable strategy to introduce periodic variations in the out‐of‐plane orientations of 5,11‐bis(triisopropylsilylethynyl)anthradithiophene (TIPS ADT) crystals is presented. TIPS ADT crystallized from the melt in the presence of 16 wt.% polyethylene (PE) forms banded spherulites of crystalline fibrils that twist in concert about the radial growth direction. These spherulites exhibit band‐dependent light absorption, photoluminescence, and Raman scattering depending on the local orientation of crystals. Mueller matrix imaging reveals strong circular extinction (CE), with TIPS ADT banded spherulites exhibiting domains of positive or negative CE signal depending on the crystal twisting sense. Furthermore, orientation‐dependent enhancement in charge injection and extraction in films of twisted TIPS ADT crystals compared to films of straight crystals is visualized in local conductive atomic force microscopy maps. This enhancement leads to 3.3‐ and 6.2‐times larger photocurrents and external quantum efficiencies, respectively, in photodetectors comprising twisted crystals than those comprising straight crystals.

Radiation therapy is one of the most prevalent procedures for cancer treatment, but the risks of malignancies induced by peripheral beam in healthy tissues surrounding the target is high. Therefore, being able to accurately measure the exposure dose is a critical aspect of patient care. Here a radiation detector based on an organic field‐effect transistor (RAD‐OFET) is introduced, an in vivo dosimeter that can be placed directly on a patient's skin to validate in real time the dose being delivered and ensure that for nearby regions an acceptable level of low dose is being received. This device reduces the errors faced by current technologies in approximating the dose profile in a patient's body, is sensitive for doses relevant to radiation treatment procedures, and robust when incorporated into conformal large‐area electronics. A model is proposed to describe the operation of RAD‐OFETs, based on the interplay between charge photogeneration and trapping.

Access to the dynamics of trap annihilation/generation resulting from isomer rearrangement identifies the performance-limiting processes in organic thin-film transistors.

Hybrid organic−inorganic metal-halide perovskites have emerged as versatile materials for enabling low-cost, mechanically flexible optoelectronic applications. The progress has been commendable; however, technological breakthroughs have outgrown the basic understanding of processes occurring in bulk and at device interfaces. Here, we investigated the photocurrent at perovskite/organic semiconductor interfaces in relation to the microstructure of electronically active layers. We found that the photocurrent response is significantly enhanced in the bilayer structure as a result of a more efficient dissociation of the photogenerated excitons and trions in the perovskite layer. The increase in the grain size within the organic semiconductor layer results in reduced trapping and further enhances the photocurrent by extending the photocarriers' lifetime. The photodetector responsivity and detectivity have improved by 1 order of magnitude in the optimized samples, reaching values of 6.1 ± 1.1 A W −1 , and 1.5 × 10 11 ± 4.7 × 10 10 Jones, respectively, and the current−voltage hysteresis has been eliminated. Our results highlight the importance of fine-tuning film microstructure in reducing the loss processes in thin-film optoelectronics based on metal-halide semiconductors and provide a powerful interfacial design method to consistently achieve high-performance photodetectors.

We explore the photodegradation mechanisms in functionalized tetracene (TIPS-Tc) films and how they are influenced by strong exciton−photon coupling in planar microcavities. We demonstrate that degradation of TIPS-Tc films exposed to air proceeds mainly through an oxygen-mediated pathway, assigned to endoperoxide (EPO) formation, whereas degradation in microcavities proceeds through oxygen-independent photodimerization. The aerobic and anaerobic decay mechanisms were found to differ in rate by more than two orders of magnitude. Both the EPO formation and photodimerization proceeded more efficiently in molecules in configurations favorable for the correlated triplet pair (TT) state formation (precursor to the singlet fission) and their immediate surroundings. For the photodimerization, an alkyne dimer is reported as one of the photoproducts, and its optical properties are presented. Strong coupling of TIPS-Tc to resonant microcavities enhanced the photodimerization quantum yield by a factor of 4.2, with the enhancement robust with respect to cavity detuning.

Approaches to control the self‐assembly of aromatic structures to enhance intermolecular electronic coupling are the key to the development of new electronic and photonic materials. Acenes in particular have proven simple to functionalize to induce strong π‐stacking interactions, although finer control of intermolecular π‐overlap has proven more difficult to accomplish. In this report, we describe how very weak hydrogen bonding interactions can exert profound impact on solid‐state order in solubilized pentacenes, inducing self‐assembly in either head‐to‐tail motifs with strong 2‐D π‐stacking, or head‐to‐head orientations with much weaker, 1‐D π‐stacking arrangements. In order to achieve 3‐D π‐stacking useful for photovoltaic applications, we elaborated a series of diethynyl pentacenes to their trimeric dehydro[18]annulene forms. These large, strongly interacting structures did indeed behave as acceptors in polymer photovoltaic devices.

Dehydroannulenes are alkyne‐rich macrocycles possessing rigid, planar, π‐conjugated backbones. Octadehydro[12]annulenes in particular are formally antiaromatic, and have been shown to possess stable reduced states with exploitable LUMO energies. However, very few examples of this type of annulene have been structurally characterized, and there is little information on the stability of these antiaromatic molecules in solution or in the solid state. We have synthesized a range of substituted octadehydro[12]annulenes, and characterized their optical and redox properties. Contrary to prior reports, dehydro[12]annulenes with overlapping π‐surfaces are reasonably stable both in solution and thin films, suggesting potential use in practical applications.