Tailoring the Molecular Structure to Suppress Extrinsic Disorder in Organic Transistors

Minder, Nikolas A.; Lu, Shaofeng; Fratini, Simone; Ciuchi, S.; Facchetti, Antonio; Morpurgo, Alberto F.

doi:10.1002/adma.201304130

Cited by 46 publications

(62 citation statements)

References 47 publications

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“…As was studied systematically both theoretically [52] and experimentally [25], extrinsic sources of disorder can affect carrier transport even in the purest samples available nowadays, especially when these are placed in FET geometries. Static disorder causes the mobility to degrade at low temperature in the best cases, and wash out the intrinsic transport regime completely in the worst cases, leading to a thermally activated behavior.…”

Section: Molecularmentioning

confidence: 99%

“…In the static limit, the polarization acts instead as an additional source of disorder, due to the random arrangement of charged dipoles in the substrate. [55,25] Both effects can convert the "bandlike" temperature dependent mobility intrinsic to organic semiconductors into a thermally activated behavior, and will not be considered here.…”

Section: Molecularmentioning

confidence: 99%

“…From the experimental point of view, several key experiments are in agreement with the transient localization scenario, and an increasing number of results is now discussed in terms of these ideas. [20,21,22,23,24,25] Despite a number of review articles and books published on the subject of charge transport in organic semiconductors, this important theoretical framework has not been comprehensively described in any review yet (although some aspects are mentioned in Refs. [21,26,27]).…”

Section: Introductionmentioning

confidence: 99%

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The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

Fratini

Mayou

Ciuchi

2016

Adv Funct Materials

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338

452

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Charge transport in crystalline organic semiconductors is intrinsically limited by the presence of large thermal molecular motions, which are a direct consequence of the weak van der Waals inter-molecular interactions. These lead to an original regime of transport called transient localization, sharing features of both localized and itinerant electron systems. After a brief review of experimental observations that pose a challenge to the theory, we concentrate on a commonly studied model which describes the interaction of the charge carriers with inter-molecular vibrations. We present different theoretical approaches that have been applied to the problem in the past, and then turn to more modern approaches that are able to capture the key microscopic phenomenon at the origin of the puzzling experimental observations, i.e. the quantum localization of the electronic wavefuntion at timescales shorter than the typical molecular motions. We describe in particular a relaxation time approximation which clarifies how the transient localization due to dynamical molecular motions relates to the Anderson localization realized for static disorder, and allows us to devise strategies to improve the mobility of actual compounds. The relevance of the transient localization scenario to other classes of systems is briefly discussed.

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Section: Molecularmentioning

confidence: 99%

Section: Molecularmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

Fratini

Mayou

Ciuchi

2016

Adv Funct Materials

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338

452

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“…Thus, synthesizing molecules with steric hindrance is crucial to achieving full coherence under ambient conditions, an ongoing topic in research. 13,[24][25][26][27] …”

Section: Quantifying Dynamic Disordermentioning

confidence: 99%

The emergence of charge coherence in soft molecular organic semiconductors via the suppression of thermal fluctuations

et al. 2016

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Organic semiconductors are already widely used in electronics. Nevertheless, their fundamental properties are still being debated. In particular, charge transport, which determines the performance of organic light-emitting diodes, solar cells and organic field-effect transistors, has been described by a wide range of complementary but incompatible theories. These theories involve either localized charge carriers hopping from molecule to molecule, leading to incoherent charge transport, or delocalized charge carriers moving around freely in the semiconductor, leading to coherent transport. In this communication, we reveal the first experimental evidence that charge coherence can be tuned from partially to fully coherent in one and the same material -pentacene-showing a continuous transition from one transport mechanism to the other. Microscopically, the transport mechanism depends on the overlap of adjacent molecular orbitals, which in turn is sensitive to molecular thermal fluctuations. We control these fluctuations through moderate variations of pressure and temperature, leading to a mobility increase of 75%. We quantify the influence of these thermal fluctuations by estimating the critical value below which fully coherent charge transport emerges. The ability to control thermal fluctuations and therefore to effectively tune the charge coherence is an important key to improving charge transport in soft molecular materials. INTRODUCTIONHistorically, the existence of coherent electronic states in weakly van der Waals bonded crystals has been a controversial topic. 1 The weak intermolecular interaction leads to a strong thermal vibration of each molecule, resulting in the dynamic disorder of the crystal lattice. Dynamic disorder subsequently leads to a complex interaction between the soft lattice and the charge carriers, which is unique to these systems. In classical semiconductors, the charge transport mechanism can be directly determined from the temperature dependence of the mobility. In an organic semiconductor, however, the temperature dependence of the mobility does not necessarily reveal the nature of the charge transport and therefore it cannot be determined whether a system is described by the short-range hopping of localized charge carriers or band transport due to extended electronic wavefunctions. 2 For this reason, a more direct method is required to study the nature of charge transport in these systems. The Hall effect is such a method, as it is directly connected to the coherence of the charge carrier wavefunction via interaction with the applied magnetic field. The quantum mechanical origin of this interaction lies in the coupling of the wave number k of the delocalized charge carrier wavefunction with the vector potential A of the magnetic field.

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“…DOI: 10.1103/PhysRevLett.116.016602 Organic field-effect transistors (OFETs) offer unique advantages of low cost, light weight, and flexibility and are widely used in the electronics and display industry. While the mobility of bulk organic semiconductors has increased dramatically [1][2][3], an outstanding issue is to directly examine the structure-property relationship at the semiconductor-dielectric interface [4], where charge transport actually occurs [5][6][7]. Ultrathin organic semiconductors a few nanometers thick are often dominated by traps and disorders and far away from the intrinsic transport regime [8][9][10].…”

mentioning

confidence: 99%

Precise Layering of Organic Semiconductors

Natelson

2016

Physics

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One of the basic assumptions in organic field-effect transistors, the most fundamental device unit in organic electronics, is that charge transport occurs two dimensionally in the first few molecular layers near the dielectric interface. Although the mobility of bulk organic semiconductors has increased dramatically, direct probing of intrinsic charge transport in the two-dimensional limit has not been possible due to excessive disorders and traps in ultrathin organic thin films. Here, highly ordered single-crystalline monoto tetralayer pentacene crystals are realized by van der Waals (vdW) epitaxy on hexagonal BN. We find that the charge transport is dominated by hopping in the first conductive layer, but transforms to bandlike in subsequent layers. Such an abrupt phase transition is attributed to strong modulation of the molecular packing by interfacial vdW interactions, as corroborated by quantitative structural characterization and density functional theory calculations. The structural modulation becomes negligible beyond the second conductive layer, leading to a mobility saturation thickness of only ∼3 nm. Highly ordered organic ultrathin films provide a platform for new physics and device structures (such as heterostructures and quantum wells) that are not possible in conventional bulk crystals. DOI: 10.1103/PhysRevLett.116.016602 Organic field-effect transistors (OFETs) offer unique advantages of low cost, light weight, and flexibility and are widely used in the electronics and display industry. While the mobility of bulk organic semiconductors has increased dramatically [1][2][3], an outstanding issue is to directly examine the structure-property relationship at the semiconductor-dielectric interface [4], where charge transport actually occurs [5][6][7]. Ultrathin organic semiconductors a few nanometers thick are often dominated by traps and disorders and far away from the intrinsic transport regime [8][9][10]. Another challenge in organic electronics is the development of layer-by-layer epitaxy with precision similar to molecular beam epitaxy in their inorganic counterparts [11]. These challenges may be alleviated if molecular crystals are processed into large-area, highly crystalline monolayers. Such a 2D form factor will also bring about new applications such as nanoporous membranes and insulating dielectrics [12,13]. Several recent breakthroughs in various types of 2D organic materials such as polymers [14,15], oligomers [16], and covalent organic frameworks [17] have already shown great promise in this direction. However, one of the most fundamental questions regarding the nature of charge transport at the 2D limit has not been addressed. In this work, we study the benchmark molecule pentacene, epitaxially crystallized on a BN substrate because of its high mobility and simple structure to model. The highly clean system allows us to provide the first definitive scenario of how molecular packing and charge transport are modulated near the interface, without being dominated by extrinsic factors. Our...

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Tailoring the Molecular Structure to Suppress Extrinsic Disorder in Organic Transistors

Cited by 46 publications

References 47 publications

The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

The emergence of charge coherence in soft molecular organic semiconductors via the suppression of thermal fluctuations

Precise Layering of Organic Semiconductors

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