Triplet population dynamics of solution cast films of isolated polymorphs of 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐Pn) provide quantitative experimental evidence that triplet excitation ...energy transfer is the dominant mechanism for correlated triplet pair (CTP) separation during singlet fission. Variations in CTP separation rates are compared for polymorphs of TIPS‐Pn with their triplet diffusion characteristics that are controlled by their crystal structures. Since triplet energy transfer is a spin‐forbidden process requiring direct wavefunction overlap, simple calculations of electron and hole transfer integrals are used to predict how molecular packing arrangements would influence triplet transfer rates. The transfer integrals reveal how differences in the packing arrangements affect electronic interactions between pairs of TIPS‐Pn molecules, which are correlated with the relative rates of CTP separation in the polymorphs. These findings suggest that relatively simple computations in conjunction with measurements of molecular packing structures may be used as screening tools to predict a priori whether new types of singlet fission sensitizers have the potential to undergo fast separation of CTP states to form multiplied triplets.
Ultrafast spectroscopy of 6,13‐bis(triisopropylsilylethynyl)‐pentacene polymorphs reveals triplet transfer as the mechanism of correlated triplet pair separation in singlet fission. Crystal structures, solved through both X‐ray and computational methods, explain differences in their triplet separation characteristics. Charge transfer integrals form a metric for assessing triplet pair separation, codifying a new approach to a priori screening of emerging singlet fission materials.
Ultrafast vibrational spectroscopy in the mid-infrared spectral range provides the opportunity to probe the dynamics of electronic states involved in all stages of the singlet fission reaction ...through their unique vibrational frequencies. This capability is demonstrated using a model singlet fission chromophore, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn). The alkyne groups of the TIPS side chains are coupled to the conjugated framework of the pentacene cores, enabling direct examination of the dynamics of triplet excitons that have successfully separated from correlated triplet pair states in crystalline films of TIPS-Pn. Relaxation processes during the separation of triplet excitons and triplet–triplet annihilation after their separation result in the formation of hot ground state molecules that also exhibit unique vibrational frequencies. Because all organic molecules possess native vibrational modes, ultrafast vibrational spectroscopy offers a new approach to examine the dynamics of electronic intermediates that may inform ongoing efforts to utilize singlet fission to overcome thermalization losses in photovoltaic applications.
Blending of small‐molecule organic semiconductors (OSCs) with amorphous polymers is known to yield high performance organic thin film transistors (OTFTs). Vertical stratification of the OSC and ...polymer binder into well‐defined layers is crucial in such systems and their vertical order determines whether the coating is compatible with a top and/or a bottom gate OTFT configuration. Here, we investigate the formation of blends prepared via spin‐coating in conditions which yield bilayer and trilayer stratifications. We use a combination of in situ experimental and computational tools to study the competing effects of formulation thermodynamics and process kinetics in mediating the final vertical stratification. It is shown that trilayer stratification (OSC/polymer/OSC) is the thermodynamically favored configuration and that formation of the buried OSC layer can be kinetically inhibited in certain conditions of spin‐coating, resulting in a bilayer stack instead. The analysis reveals here that preferential loss of the OSC, combined with early aggregation of the polymer phase due to rapid drying, inhibit the formation of the buried OSC layer. The fluid dynamics and drying kinetics are then moderated during spin‐coating to promote trilayer stratification with a high quality buried OSC layer which yields unusually high mobility >2 cm2 V−1 s−1 in the bottom‐gate top‐contact configuration.
Trilayer stratification (organic semiconductor/polymer/organic semiconductor) is the thermodynamically favored configuration. However, it is shown—using a combination of in situ experiments and phase field simulations—that formation of the buried organic semiconductor layer can be kinetically controlled. Formation of a high quality buried semiconductor layer is induced using careful control. It yields unusually high mobility >2 cm2 V−1 s−1 in the bottom‐gate top‐contact configuration.
Solution-processable electronic devices are highly desirable due to their low cost and compatibility with flexible substrates. However, they are often challenging to fabricate due to the hydrophobic ...nature of the surfaces of the constituent layers. Here, we use a protein solution to modify the surface properties and to improve the wettability of the fluoropolymer dielectric Cytop. The engineered hydrophilic surface is successfully incorporated in bottom-gate solution-deposited organic field-effect transistors (OFETs) and hybrid organic–inorganic trihalide perovskite field-effect transistors (HTP-FETs) fabricated on flexible substrates. Our analysis of the density of trapping states at the semiconductor–dielectric interface suggests that the increase in the trap density as a result of the chemical treatment is minimal. As a result, the devices exhibit good charge carrier mobilities, near-zero threshold voltages, and low electrical hysteresis.
Solution processability of organic semiconductors allows high‐throughput fabrication on arbitrary substrates at low‐cost, but the films often exhibit low performance. Here, we report on a new method ...for device fabrication, vibration assisted crystallization (VAC) that produces superior films, which approach the fundamental performance limits shown in corresponding single‐crystal measurements.
A new way to investigate and control the growth of solution‐cast thin films is presented. The combination of in situ quartz crystal microbalance measurements with dissipation capabilities (QCM‐D) and ...in situ grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) in an environmental chamber provides unique quantitative insights into the time‐evolution of the concentration of the solution, the onset of nucleation, and the mode of growth of the organic semiconductor under varied drying conditions. It is demonstrated that careful control over the kinetics of solution drying enhances carrier transport significantly by promoting phase transformation predominantly via heterogeneous nucleation and sustained surface growth of a highly lamellar structure at the solid‐liquid interface at the expense of homogeneous nucleation.
A new way to investigate and control the growth of drop‐cast thin films is presented. The solution‐processing of small‐molecule thin films of TIPS‐pentacene is investigated using time‐resolved techniques to reveal the mechanisms of nucleation and growth leading to solid film formation. By tuning the drying speed of the solution, the balance between surface and bulk growth modes is altered, thereby controlling the lamellar formation and tuning the carrier mobility in organic field‐effect transistors
Fluorination has been demonstrated to improve stability and processing in thiophene‐containing small‐molecule semiconductors. Here, the impact of partial fluorination on these parameters in a ...pentacene derivative is examined. Although the improvement in photostability is not as dramatic, there is a clear improvement in the stability of the chromophore upon fluorination. The improvement in processability is more dramatic; devices formed by spin‐coating with the fluorinated derivative perform substantially better than those formed from the nonfluorinated compound.
Crystal clear: Fluorination of 6,13‐bis(triisopropylsilylethynyl)pentacene leads to a stable organic semiconducting material. Field‐effect transistors are fabricated by solution‐phase deposition, and the hole‐transporting channels show contact‐induced self‐assembly with highly ordered edge‐on crystallites. The analysis of a transfer characteristic reveals nonidealities related to the gate‐bias‐dependent mobility and the contact resistance.
Soluble small molecule organic semiconductors combine the high-performance of small molecule organic semiconductors with the versatile processability of polymeric materials, but the control of device ...performance and uniformity is challenged by the complex film microstructure formed in these materials, and its strong dependence on processing conditions. These films crystallize via a nucleation and growth mechanism that can be difficult to control. In this study we used highly fluorinated self-assembled monolayers (SAMs) to modify the surface of the source and drain contacts and improve the performance of organic thin-film transistors (OTFTs) through controlling film microstructure and lowering the contact resistance. We reached charge carrier mobilities as high as 5.7 cm2/V in 2,8-Difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT), one order of magnitude greater than what we obtained in devices on untreated substrates, and on par with the value reported for single crystal devices. Kelvin probe measurements distinguished an increase in the work function between 0.28 eV and 0.5 eV, depending on the molecular structure of the SAM. Selected area electron diffraction (SAED) confirmed the preferential “edge-on” molecular orientation of the semiconductor. We discuss the device performance in relation to the film morphology and contact resistance.
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•We controlled film crystallization in solution deposited organic semiconductors with the use of highly fluorinated SAM treatment.•SAM treatment minimizes the contribution of the inter-spherulite boundaries and lowers the contact resistance.•We obtained OTFT performance on par with single crystal devices by controlling film crystallization and improving injection.
X‐ray microbeam scattering is used to map the microstructure of the organic semiconductor along the channel length of solution‐processed bottom‐contact OFET devices. Contact‐induced nucleation is ...known to influence the crystallization behavior within the channel. We find that microstructural inhomogeneities in the center of the channel act as a bottleneck to charge transport. This problem can be overcome by controlling crystallization of the preferable texture, thus favoring more efficient charge transport throughout the channel.
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