Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe influence of impurities on the charge carrier mobility of small molecule organic semiconductors
116
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Amorphous organic semiconductors based on small molecules and polymers are used in many applications, most prominently organic light emitting diodes (OLEDs) and organic solar cells. Impurities and charge traps are omnipresent in most currently available organic semiconductors and limit charge transport and thus device efficiency. The microscopic cause as well as the chemical nature of these traps are presently not well understood. Using a multiscale model we characterize the influence of impurities on the density of states and charge transport in small-molecule amorphous organic semiconductors. We use the model to quantitatively describe the influence of water molecules and water-oxygen complexes on the electron and hole mobilities. These species are seen to impact the shape of the density of states and to act as explicit charge traps within the energy gap. Our results show that trap states introduced by molecular oxygen can be deep enough to limit the electron mobility in widely used materials.
rate research
Read More
It is widely assumed that the dominant source of scattering in graphene is charged impurities in a substrate. We have tested this conjecture by studying graphene placed on various substrates and in high-k media. Unexpectedly, we have found no significant changes in carrier mobility either for different substrates or by using glycerol, ethanol and water as a top dielectric layer. This suggests that Coulomb impurities are not the scattering mechanism that limits the mean free path currently attainable for graphene on a substrate.
We explore the possibility that hyperfine interaction causes the recently discovered organic magnetoresistance (OMAR) effect. Our study employs both experiment and theoretical modelling. An excitonic pair mechanism model based on hyperfine interaction, previously suggested by others to explain magnetic field effects in organics, is examined. Whereas this model can explain a few key aspects of the experimental data, we, however, uncover several fundamental contradictions as well. By varying the injection efficiency for minority carriers in the devices, we show experimentally that OMAR is only weakly dependent on the ratio between excitons formed and carriers injected, likely excluding any excitonic effect as the origin of OMAR.
The processability and optoelectronic properties of organic semiconductors can be tuned and manipulated via chemical design. The substitution of the alkyl side chains by oligoethers has recently been successful for applications such as bioelectronic sensors and photocatalytic water-splitting. The carbon-oxygen bond in oligoethers is likely to render the system softer and more prone to dynamical disorder that can be detrimental to charge transport for example. We use neutron spectroscopy, X-Ray diffraction (XRD), differential scanning calorimetry (DSC) and polarized optical microscopy to study the effect of the substitution of n-hexyl (Hex) by triethylene glycol (TEG) on the structural dynamics of two organic semiconductors: a phenylene-bithiophene-phenylene (PTTP) molecule and a fluorene-co-dibenzothiophene (FS) polymer. Counterintuitively, inelastic neutron scattering (INS) reveals a softening of the modes of PTTP and FS with Hex side chains, pointing towards an increased dynamical disorder in these systems. However, T-dependent X-Ray and neutron diffraction, INS and DSC evidence an extra reversible transition close to room temperature (RT) for PTTP with TEG side chains. The observed transition, not accompanied by a change in birefringence, can also be observed by quasi-elastic neutron scattering. A fastening of the TEG side chains dynamics is observed in the case of PTTP and not FS. We therefore assign this transition to the melt of the TEG side chains which are promoting dynamical order at RT, but if crystallising, may introduce an extra reversible structural transition above RT leading to thermal instabilities. A deeper understanding of side chain polarity and structural dynamics can help guide materials design and navigate the intricate balance between electronic charge transport and aqueous swelling, sought for a number of emerging organic electronic and bioelectronic applications.
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 mono- to tetra-layer 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 band-like in subsequent layers. Such 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 ~3nm. 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.
The magneto-electronic field effects in organic semiconductors at high magnetic fields are described by field-dependent mixing between singlet and triplet states of weakly bound charge carrier pairs due to small differences in their Lande g-factors that arise from the weak spin-orbit coupling in the material. In this work, we corroborate theoretical models for the high-field magnetoresistance of organic semiconductors, in particular of diodes made of the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) at low temperatures, by conducting magnetoresistance measurements along with multi-frequency continuous-wave electrically detected magnetic resonance experiments. The measurements were performed on identical devices under similar conditions in order to independently assess the magnetic field-dependent spin-mixing mechanism, the so-called {Delta}g mechanism, which originates from differences in the charge-carrier g-factors induced by spin-orbit coupling.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
