The use of singlet fission to enhance device efficiency or enable new capabilities raises the need to investigate it on a slow operational timescale. Using a transistor-based measurement to examine temperature-dependent singlet fission in tetracene single crystals, we observe that it is activated to 210 K, at which point it undergoes an optoelectronic phase transition. We compare these results to those of pentacene and suggest that the phase change is due to a change in the singlet fission kinetics. We further examine the interplay between this readout and other extrinsic and intrinsic device properties, such as disorder and trap states. Our results give insight not only into the readout mechanism of this OFET-based measurement, but also strategies to manipulate and tune the response.
Unique exciton states are being explored for use in quantum information systems. Excitons formed in organic semiconductors are long lived at room temperature owing to their high binding energies, and they can form quantum relevant multiexciton states like the spin-entangled triplet pair which is an intermediate during singlet fission. Electrical measurements of excitons provide complementary measurement to common optical based techniques by probing low density exciton dynamics and dark states. Here we tune singlet fission rates by using varied molecular packing to relate optical and device measurements to aid development of transistor-based exciton measurements.
Dark-state triplet excitons lead to energy loss in organic light emitting diodes and photovoltaics; recently there has been considerable effort to convert this energy to improve device efficiency. While some observed improvement has been tied to control of the triplet state, alternate explanations cannot be ruled out as the influence of device architecture can mask underlying physics. In order to investigate triplet contribution to device output, we study single crystal 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF TES ADT) using orientation dependence of the magneto-photocurrent (MPC) in a transistor. MPC changes with magnetic field orientation in the crystal, showing triplet-controlled device output using the anisotropic spin character of the triplet.
Low temperature transport measurements of classical semiconductors are a well-defined method to determine the physics of transport behavior. These measurements are also used to evaluate organic semiconductors, though physical interpretation is not yet fully developed. The similar energy ranges of the various processes involved in charge transport in organic semiconductors, including excitonic coupling, charge-phonon coupling, and trap distributions, result in ambiguity in the interpretation of temperature dependent electrical measurements. The wide variety of organic semiconductors, ranging from well-ordered small molecule crystals to disordered polymers, manifest varying degrees of “ideal” device behavior and require intensive studies in order to capture the full range of physical mechanisms involved in electronic transport in this class of materials. In addition, the physics at electrical contacts and dielectric material interfaces strongly affect device characteristics and results in temperature dependent behavior that is unrelated to the semiconductor itself. In light of these complications, our group is working toward understanding the origins of temperature dependent transport in single crystal, small molecule organic semiconductors with ordered packing. In order to disentangle competing physical effects on device characterization at low temperature, we use TEM and Raman spectroscopy to track changes in the structure and thermal molecular motion, correlated with density functional theory calculations. We perform electrical characterization, including DC current-voltage, AC impedance, and displacement current measurements, on transistors built with a variety of contact and dielectric materials in order to fully understand the origin of the transport behavior. Results of tetracene on silicon dioxide and Cytop dielectrics will be discussed.
Conference Committee Involvement (2)
Physical Chemistry of Semiconductor Materials and Interfaces XXIV
3 August 2025 | San Diego, California, United States
Physical Chemistry of Semiconductor Materials and Interfaces XXIII
18 August 2024 | San Diego, California, United States
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