Organic semiconductors might enable new applications in low-cost, light-weight, flexible electronics. To build a solid foundation for these technologies, more fundamental studies of ...electro-mechanical properties of various types of organic semiconductors are necessary. Here we perform basic studies of charge transport in highly crystalline solution-processed organic semiconductors as a function of applied mechanical strain. As a test bed, we use small molecules crystallized on thin plastic sheets, resulting in high-performance flexible field-effect transistors. These devices can be bent multiple times without degradation to a radius as small as ~200 μm, demonstrating that crystalline solution-processed organic semiconductors are intrinsically highly flexible. This study of electro-mechanical properties suggests that solution-processable organic semiconductors are suitable for applications in flexible electronics, provided that integration with other important technological advances, such as device scalability and low-voltage operation, is achieved in the future.
A combination of surface energy‐guided blade coating and inkjet printing is used to fabricate an all‐printed high performance, high yield, and low variability organic thin film transistor (OTFT) ...array on a plastic substrate. Functional inks and printing processes were optimized to yield self‐assembled homogenous thin films in every layer of the OTFT stack. Specifically, we investigated the effect of capillary number, semiconductor ink composition (small molecule‐polymer ratio), and additive high boiling point solvent concentrations on film fidelity, pattern design, device performance and yields.
We compare the singlet fission dynamics of five pentacene derivatives precipitated to form nanoparticles. Two nanoparticle types were distinguished by differences in their solid-state order and ...kinetics of triplet formation. Nanoparticles that comprise primarily weakly coupled chromophores lack the bulk structural order of the single crystal and exhibit nonexponential triplet formation kinetics (Type I), while nanoparticles that comprise primarily more strongly coupled chromophores exhibit order resembling that of the bulk crystal and triplet formation kinetics associated with the intrinsic singlet fission rates (Type II). In the highly ordered nanoparticles, singlet fission occurs most rapidly. We relate the molecular packing arrangement derived from the crystal structure of the pentacene derivatives to their singlet fission dynamics and find that slip stacking leads to rapid, subpicosecond singlet fission. We present evidence that exciton delocalization, coincident with an increased relative admixture of charge-transfer configurations in the description of the exciton wave function, facilitates rapid triplet pair formation in the case of single-step singlet fission. We extend the study to include two hexacene derivatives and find that these conclusions are generally applicable. This work highlights acene derivatives as versatile singlet fission chromophores and shows how chemical functionalization affects both solid-state order and exciton interactions and how these attributes in turn affect the rate of singlet fission.
The multiplication of excitons in organic semiconductors via singlet fission offers the potential for photovoltaic cells that exceed the Shockley–Quiesser limit for single-junction devices. To fully ...utilize the potential of singlet fission sensitizers in devices, it is necessary to understand and control the diffusion of the resultant triplet excitons. In this work, a new processing method is reported to systematically tune the intermolecular order and crystalline structure in films of a model singlet fission chromophore, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn), without the need for chemical modifications. A combination of transient absorption spectroscopy and quantitative materials characterization enabled a detailed examination of the distance- and time-dependence of triplet exciton diffusion following singlet fission in these nanocrystalline TIPS-Pn films. Triplet–triplet annihilation rate constants were found to be representative of the weighted average of crystalline and amorphous phases in TIPS-Pn films comprising a mixture of phases. Adopting a diffusion model used to describe triplet–triplet annihilation, the triplet diffusion lengths for nanocrystalline and amorphous films of TIPS-Pn were estimated to be ∼75 and ∼14 nm, respectively. Importantly, the presence of even a small fraction (<10%) of the amorphous phase in the TIPS-Pn films greatly decreased the ultimate triplet diffusion length, suggesting that pure crystalline materials may be essential to efficiently harvest multiplied triplets even when singlet fission occurs on ultrafast time scales.
Scalable manufacturing of small‐molecule organic thin film transistors (OTFTs) with performance approaching single crystals requires extraordinary control over microstructures and morphologies of ...organic semiconductors (OSCs). Here, contact‐induced nucleation in the context of small‐molecule OSCs and OSC:polymer blends prepared by blade coating, a printing process capable of mimicking large area batch and roll‐to‐roll manufacturing, is investigated. Using polarized optical microscopy, microbeam grazing incidence wide angle X‐ray scattering, and energy‐filtered transmission electron microscopy, it is revealed that previous design rules drawn from spin coating of OSCs and contact‐induced nucleation may have to be revisited in the context of blade coating. It is shown that blade coating achieves texture purity in case of 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl) anthradithiophene (diF‐TES‐ADT), irrespective of whether the contact is chemically treated with a halogenated self‐assembled monolayer (SAM) or not, in contrast to spin coating which requires an SAM. Here, it is demonstrated that OSC–contact interactions increase the nucleation density and can disrupt the vertical stratification in polymer:OSC blends with great detrimental effects on carrier transport. Using these lessons, we demonstrate bottom‐contact bottom‐gate OTFTs without chemical surface modification achieving hole mobilities of 4.6 and 3.6 cm2 V−1 s−1, using 6,13‐bis(triisopropylsilylethynyl)pentacene and diF‐TES‐ADT, respectively, blended with an insulating polymer.
Contact‐induced nucleation, normally a desirable effect when spin‐coating organic semiconductors (OSC), is shown to disrupt the crystallization of large OSC domains and to alter the vertical stratification of otherwise high‐performance polymer:OSC blends.
Solution-printed organic semiconductors have emerged in recent years as promising contenders for roll-to-roll manufacturing of electronic and optoelectronic circuits. The stringent performance ...requirements for organic thin-film transistors (OTFTs) in terms of carrier mobility, switching speed, turn-on voltage and uniformity over large areas require performance currently achieved by organic single-crystal devices, but these suffer from scale-up challenges. Here we present a new method based on blade coating of a blend of conjugated small molecules and amorphous insulating polymers to produce OTFTs with consistently excellent performance characteristics (carrier mobility as high as 6.7 cm(2) V(-1) s(-1), low threshold voltages of<1 V and low subthreshold swings <0.5 V dec(-1)). Our findings demonstrate that careful control over phase separation and crystallization can yield solution-printed polycrystalline organic semiconductor films with transport properties and other figures of merit on par with their single-crystal counterparts.
We have extended our functionalization strategy for pentacene to the higher acenes hexacene and heptacene. Provided a large enough alkyne substituent is used, these large aromatic rods are both ...stable and soluble and can be characterized spectroscopically as well as by single-crystal X-ray diffraction.
Singlet fission is an exciton multiplication mechanism in organic materials whereby high energy singlet excitons can be converted into two triplet excitons with near unity quantum yields. As new ...singlet fission sensitizers are developed with properties tailored to specific applications, there is an increasing need for design rules to understand how the molecular structure and crystal packing arrangements influence the rate and yield with which spin-correlated intermediates known as correlated triplet pairs can be successfully separateda prerequisite for harvesting the multiplied triplets. Toward this end, we identify new electronic transitions in the mid-infrared spectral range that are distinct for both initially excited singlet states and correlated triplet pair intermediate states using ultrafast mid-infrared transient absorption spectroscopy of crystalline films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn). We show that the dissociation dynamics of the intermediates can be measured through the time evolution of the mid-infrared transitions. Combining the mid-infrared with visible transient absorption and photoluminescence methods, we track the dynamics of the relevant electronic states through their unique electronic signatures and find that complete dissociation of the intermediate states to form independent triplet excitons occurs on time scales ranging from 100 ps to 1 ns. Our findings reveal that relaxation processes competing with triplet harvesting or charge transfer may need to be controlled on time scales that are orders of magnitude longer than previously believed even in systems like TIPS-Pn where the primary singlet fission events occur on the sub-picosecond time scale.
Single-crystal semiconductors have been at the forefront of scientific interest for more than 70 years, serving as the backbone of electronic devices. Inorganic single crystals are typically grown ...from a melt using time-consuming and energy-intensive processes. Organic semiconductor single crystals, however, can be grown using solution-based methods at room temperature in air, opening up the possibility of large-scale production of inexpensive electronics targeting applications ranging from field-effect transistors and light-emitting diodes to medical X-ray detectors. Here we demonstrate a low-cost, scalable spray-printing process to fabricate high-quality organic single crystals, based on various semiconducting small molecules on virtually any substrate by combining the advantages of antisolvent crystallization and solution shearing. The crystals' size, shape and orientation are controlled by the sheer force generated by the spray droplets' impact onto the antisolvent's surface. This method demonstrates the feasibility of a spray-on single-crystal organic electronics.
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