Polymer‐ and small‐molecule‐based organic electronic devices are being developed for applications including electroluminescent displays, transistors, and solar cells due to the promise of low‐cost ...manufacturing. It has become clear that these materials exhibit nanoscale heterogeneities in their optical and electrical properties that affect device performance, and that this nanoscale structure varies as a function of film processing and device‐fabrication conditions. Thus, there is a need for high‐resolution measurements that directly correlate both electronic and optical properties with local film structure in organic semiconductor films. In this article, we highlight the use of electrical scanning probe microscopy techniques, such as conductive atomic force microscopy (c‐AFM), electrostatic force microscopy (EFM), scanning Kelvin probe microscopy (SKPM), and similar variants to elucidate charge injection/extraction, transport, trapping, and generation/recombination in organic devices. We discuss the use of these tools to probe device structures ranging from light‐emitting diodes (LEDs) and thin‐film transistors (TFT), to light‐emitting electrochemical cells (LECs) and organic photovoltaics.
Organic and conjugated‐polymer electronics are being developed for light‐emitting diodes (LEDs), thin‐film transistors (TFTs), and solar cells (organic photovoltaic/ OPV). We highlight how scanning‐probe techniques, such as electrostatic force microscopy (EFM), conductive atomic force microscopy (c‐AFM), and scanning Kelvin probe microscopy (SKPM), are applied to probe local electronic properties, and the effects of nanoscale film morphology in these devices.
The origin and yield of charges in neat conjugated polymers has long been controversial. In this paper, we review the body of literature that has been created over the past three decades of research ...in this field and provide insight from our own recent work highlighting the importance of polymer microstructure in understanding the photophysics of these materials. We focus primarily on polythiophene, poly(p-phenylene vinylene), and ladder-type poly(p-phenylene) derivatives, as these three prototypical polymer backbone structures have undergone the most complete study. We find compelling evidence that the primary photoexcitations in conjugated polymers include both intrachain excitons and excimers, that charges are produced in a secondary process, primarily from breakup of intrachain excitons, and that the locus of long-lived charge generation is at the interface between amorphous and crystalline domains of the polymer. Interestingly, the existence of interchromophore complexes that we refer to as excimers has largely been ignored in the development of organic photovoltaics based on conjugated polymers. We suggest that the prevalence of this species may help explain certain mysterious features in that body of work, in particular the excess energy offset required for efficient charge separation in donor/acceptor blends and the requirement for intimately mixed phases of the two materials for maximally efficient photocurrent generation.
Photoexcited triplet states are promising candidates for hybrid qubit systems, as they can be used as a controlling gate for nuclear spins. But microwave readout schemes do not generally offer the ...sensitivity needed to approach the single-molecule limit or the scope to integrate such systems into devices. Here, we demonstrate the possibility of electrical readout of triplet spins at room temperature through a specific mechanism of magnetoconductance (MC) in polycrystalline pentacene. We show that hole-only pentacene devices exhibit a positive photoinduced MC response that is consistent with a trap-filling mechanism. Spin and magnetic-field-dependent quenching of photogenerated triplets by holes quantitatively explains the MC response we observe. These results are distinct in both sign and proposed mechanism compared to previous reports on polyacene materials and provide clear design rules for future spintronic devices based on this spin-sensing mechanism.
Poor energy transport in disordered organic materials is one of the key problems that must be overcome to produce efficient organic solar cells. Usually, this is accomplished by blending the donor ...and acceptor molecules into a bulk heterojunction. In this article, we investigate an alternative approach to cell design: planar mulitilayer hetrojunctions with efficient energy transport to a central reaction center. We use an experimentally verified Monte Carlo model of energy transport to show that an appropriately engineered planar multilayer stack can achieve power conversion efficiencies comparable to those of the best bulk heterojunction devices. The key to this surprising performance is careful control of the optical properties and thicknesses of each layer to promote Förster resonance energy transfer from antenna/transport layers to a central reaction center. We provide detailed design rules for fabricating efficient planar heterojunction organic cells.
We use flash-photolysis time-resolved microwave conductivity experiments (FP-TRMC) and femtosecond–nanosecond pump–probe transient absorption spectroscopy to investigate photoinduced carrier ...generation and recombination dynamics of a trilayer cascade heterojunction composed of poly(3-hexylthiophene) (P3HT), titanyl phthalocyanine (TiOPc), and fullerene (C60). Carrier generation following selective photoexcitation of TiOPc is independently observed at both the P3HT/TiOPc and TiOPc/C60 interfaces. The transient absorption results indicate that following initial charge generation processes to produce P3HT•+/TiOPc•– and TiOPc•+/C60 •– at each interface from (P3HT/TiOPc*/C60), the final charge-separated product of (P3HT•+/TiOPc/C60 •–) is responsible for the long-lived photoconductance signals in FP-TRMC. At the P3HT/TiOPc interface in both P3HT/TiOPc and P3HT/TiOPc/C60 samples, the electron transfer appears to occur only with the crystalline (weakly coupled H-aggregate) phase of the P3HT.
The morphological origin of anisotropic charge transport in uniaxially strain aligned poly(3‐hexylthiophene) (P3HT) films is investigated. The macroscale field effect mobility anisotropy is measured ...in an organic thin film transistor (OTFT) configuration and compared to the local aggregate P3HT mobility anisotropy determined using time‐resolved microwave conductivity (TRMC) measurements. The field effect mobility anisotropy in highly aligned P3HT films is substantially higher than the local mobility anisotropy in the aggregate P3HT. This difference is attributed to preferentially aligned polymer tie‐chains at grain boundaries that contribute to macroscale charge transport anisotropy but not the local anisotropy. The formation of sharp grains between oriented crystalline P3HT, through tie chain removal by thermal annealing the strained aligned films, results in an order of magnitude drop in the measured field effect mobility for charge transport parallel to the strain direction. The field effect mobility anisotropy is cut in half while the local mobility anisotropy remains relatively constant. The local mobility anisotropy is found to be surprisingly low in the aligned films, suggesting that the π−π stacking direction supports charge carrier mobility on the same order of magnitude as that in the intrachain direction, possibly due to poor intrachain mobility through chain torsion.
Macroscale charge transport anisotropy is compared to the local transport anisotropy in highly aligned P3HT films. The macroscale charge mobility anisotropy is substantially higher than the local anisotropy, which is attributed to preferentially aligned tie‐chains. The local mobility anisotropy is found to be surprisingly low, suggesting that P3HT has relative low intrachain mobility due to local disorder associated with the flexible backbone.
Although molecular charge-transfer doping is widely used to manipulate carrier density in organic semiconductors, only a small fraction of charge carriers typically escape the Coulomb potential of ...dopant counterions to contribute to electrical conductivity. Here, we utilize microwave and direct-current (DC) measurements of electrical conductivity to demonstrate that a high percentage of charge carriers in redox-doped semiconducting single-walled carbon nanotube (s-SWCNT) networks is delocalized as a free carrier density in the π-electron system (estimated as >46% at high doping densities). The microwave and four-point probe conductivities of hole-doped s-SWCNT films quantitatively match over almost 4 orders of magnitude in conductance, indicating that both measurements are dominated by the same population of delocalized carriers. We address the relevance of this surprising one-to-one correspondence by discussing the degree to which local environmental parameters (e.g., tube–tube junctions, Coulombic stabilization, and local bonding environment) may impact the relative magnitudes of each transport measurement.
Conspectus Preparing and manipulating pure magnetic states in molecular systems are the key initial requirements for harnessing the power of synthetic chemistry to drive practical quantum sensing and ...computing technologies. One route for achieving the requisite higher spin states in organic systems exploits the phenomenon of singlet fission, which produces pairs of triplet excited states from initially photoexcited singlets in molecular assemblies with multiple chromophores. The resulting spin states are characterized by total spin (quintet, triplet, or singlet) and its projection onto a specified molecular or magnetic field axis. These excited states are typically highly polarized but exhibit an impure spin population pattern. Herein, we report the prediction and experimental verification of molecular design rules that drive the population of a single pure magnetic state and describe the progress toward its experimental realization. A vital feature of this work is the close partnership among theory, chemical synthesis, and spectroscopy. We begin by presenting our theoretical framework for understanding spin manifold interconversion in singlet fission systems. This theory makes specific testable predictions about the intermolecular structure and orientation relative to an external magnetic field that should lead to pure magnetic state preparation and provides a powerful tool for interpreting magnetic spectra. We then test these predictions through detailed magnetic spectroscopy experiments on a series of new molecular architectures that meet one or more of the identified structural criteria. Many of these architectures rely on the synthesis of molecules with features unique to this effort: rigid bridges between chromophores in dimers, heteroacenes with tailored singlet/triplet-pair energy level matching, or side-group engineering to produce specific crystal structures. The spin evolution of these systems is revealed through our application and development of several magnetic resonance methods, each of which has different sensitivities and relevance in environments relevant to quantum applications. Our theoretical predictions prove to be remarkably consistent with our experimental results, though experimentally meeting all the structural prescriptions demanded by theory for true pure-state preparation remains a challenge. Our magnetic spectra agree with our model of triplet-pair behavior, including funneling of the population to the m s = 0 magnetic sublevel of the quintet under specified conditions in dimers and crystals, showing that this phenomenon is subject to control through molecular design. Moreover, our demonstration of novel and/or highly sensitive detection mechanisms of spin states in singlet fission systems, including photoluminescence (PL), photoinduced absorption (PA), and magnetoconductance (MC), points the way toward both a deeper understanding of how these systems evolve and technologically feasible routes toward experiments at the single-molecule quantum limit that are desirable for computational applications.
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