Field-induced Electron Spin Resonance Research Articles

Spin relaxation of electron and hole polarons in ambipolar conjugated polymers

The charge-transport properties of conjugated polymers have been studied extensively for opto-electronic device applications. Some polymer semiconductors not only support the ambipolar transport of electrons and holes, but do so with comparable carrier mobilities. This opens the possibility of gaining deeper insight into the charge-transport physics of these complex materials via comparison between electron and hole dynamics while keeping other factors, such as polymer microstructure, equal. Here, we use field-induced electron spin resonance spectroscopy to compare the spin relaxation behavior of electron and hole polarons in three ambipolar conjugated polymers. Our experiments show unique relaxation regimes as a function of temperature for electrons and holes, whereby at lower temperatures electrons relax slower than holes, but at higher temperatures, in the so-called spin-shuttling regime, the trend is reversed. On the basis of theoretical simulations, we attribute this to differences in the delocalization of electron and hole wavefunctions and show that spin relaxation in the spin shuttling regimes provides a sensitive probe of the intimate coupling between charge and structural dynamics.

Field-Induced Electron Spin Resonance of Site-Selective Carrier Accumulation in Field-Effect Transistors Composed of Organic Semiconductor Solid Solutions

The formation and characterization of solid solutions with guest (or impurity) molecules are difficult to realize in crystalline organic semiconductors. Here, we demonstrate that an operando field-induced electron spin resonance (FIESR) experiment allows the site-selective analysis of accumulated charge carriers in polycrystalline organic field-effect transistors (FETs), which comprise molecular solid solutions as channel semiconductor layers. We utilize vacuum-deposited dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) thin films with various amounts of dibenzotetrathiafulvalene (DBTTF) as guest molecules. Our measurements reveal that the field-induced carriers are first trapped at guest DBTTF sites at low gate voltages and then begin to accumulate at host DNTT sites at higher gate voltages. Furthermore, we perform dispersion-corrected density-functional theory calculations to investigate the stability of the solid-solution crystals. It is shown that the DBTTF molecules can be stably incorporated by aligning molecular long axes within the crystal lattices of DNTT, the results of which agree well with the angle-dependent FIESR measurements. We demonstrate that the mobility, threshold voltage, and subthreshold swing are controlled by the concentration of guest molecules in solid-solution organic FETs.

Microscopic observation of efficient charge transport processes across domain boundaries in donor-acceptor-type conjugated polymers

Backbone rigidity of conjugated polymers is suggested to play an essential role in realizing high-mobility transistors through the efficient interconnection of crystalline domains by tie molecules as discussed for the recently-developed donor-acceptor (DA)-type copolymers. However, no studies have directly observed interdomain hopping in these DA copolymers. Here, highly-efficient interdomain charge transport is observed in two typical high-mobility DA copolymers from the microscopic observation of charge carriers using field-induced electron spin resonance (ESR) spectroscopy. The in-plane ESR signal exhibits a clear motional narrowing effect associated with the carrier motion across the boundaries. The activation energy of the interdomain charge motion is as low as that of intradomain motion (~10 meV), both of which are clearly lower than those observed in the conventional semicrystalline polymer. The structural origin of this efficient interdomain electrical connection is the rigid, nearly torsion-free backbone conformation of the tie molecule, as demonstrated from density functional theory calculations.

Microscopic observation of highly mobile charge carriers in organic transistors of semicrystalline conducting polymers

Charge carrier dynamics in organic field-effect transistors (OFETs) of semicrystalline conducting polymers poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) and poly(3-hexylthiophene) (P3HT) have been investigated down to 4 K by field-induced electron spin resonance (FI-ESR) spectroscopy. The highly mobile nature of charge carriers within the ordered regions of the polymers has been clarified from the observation of the motional narrowing effect of the ESR spectra even below 30 K, where device operation cannot be observed presumably owing to the effect of domain boundaries. The activation energy of carrier motion observed by ESR has been determined as 17 meV for PBTTT and 13 meV for P3HT, which are an order of magnitude smaller than that of FET mobility (>110 meV) obtained for the same devices. These results demonstrate that the intrinsic carrier mobility within the ordered region is much higher than that expected from the macroscopic transport measurements in the semicrystalline polymers.

21pAC-7 大面積塗布有機単結晶の電界誘起電子スピン共鳴測定

21pAC-7 大面積塗布有機単結晶の電界誘起電子スピン共鳴測定

Microscopic signature of insulator-to-metal transition in highly doped semicrystalline conducting polymers in ionic-liquid-gated transistors

Electronic state of charge carriers, in particular, in highly doped regions, in thin-film transistors of a semicrystalline conducting polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene), has been studied by using field-induced electron spin resonance (ESR) spectroscopy. By adopting an ionic-liquid gate insulator, a gate-controlled reversible electrochemical hole-doping of the polymer backbone is achieved, as confirmed from the change of the optical absorption spectra. The edge-on molecular orientation in the pristine film is maintained even after the electrochemical doping, which is clarified from the angular dependence of the g value. As the doping level increases, spin 1/2 polarons transform into spinless bipolarons, which is demonstrated from the spin-charge relation showing a spin concentration peak around 1%, contrasting to the monotonic increase in the charge concentration. At high doping levels, a drastic change in the linewidth anisotropy due to the generation of conduction electrons is observed, indicating the onset of metallic state, which is also supported by the temperature dependence of the spin susceptibility and the ESR linewidth. Our results suggest that semicrystalline conducting polymers become metallic with retaining their molecular orientational order, when appropriate doping methods are chosen.

Electrically induced ambipolar spin vanishments in carbon nanotubes.

Carbon nanotubes (CNTs) exhibit various excellent properties, such as ballistic transport. However, their electrically induced charge carriers and the relation between their spin states and the ballistic transport have not yet been microscopically investigated because of experimental difficulties. Here we show an electron spin resonance (ESR) study of semiconducting single-walled CNT thin films to investigate their spin states and electrically induced charge carriers using transistor structures under device operation. The field-induced ESR technique is suitable for microscopic investigation because it can directly observe spins in the CNTs. We observed a clear correlation between the ESR decrease and the current increase under high charge density conditions, which directly demonstrated electrically induced ambipolar spin vanishments in the CNTs. The result provides a first clear evidence of antimagnetic interactions between spins of electrically induced charge carriers and vacancies in the CNTs. The ambipolar spin vanishments would contribute the improvement of transport properties of CNTs because of greatly reduced carrier scatterings.

Low-temperature carrier dynamics in high-mobility organic transistors of alkylated dinaphtho-thienothiophene as investigated by electron spin resonance

Charge carriers in high-mobility organic thin-film transistors of alkylated dinaphtho-thienothiophene (C10-DNTT) have been directly observed by field-induced electron spin resonance (FI-ESR) down to 4 K. FI-ESR spectra of π-electron hole carriers of C10-DNTT exhibited clear anisotropy, indicating a highly organized end-on molecular orientation at the device interface. The intra-grain and inter-grain carrier motion were probed by the motional narrowing effect of the ESR spectra. The intra-grain motion was clearly observed even at 4 K, showing intrinsically high mobility of C10-DNTT crystallites. On the other hand, significantly low activation energy of ∼10 meV for inter-grain carrier hopping, compared with pristine DNTT, was observed, which shows that the alkyl substitution drastically enhances the carrier mobility of DNTT system.

Microscopic Characterization of Printable Low-Voltage Electrolyte-Gated Transistors by Electron Spin Resonance

We have investigated the microscopic properties of printable low-voltage electrolyte-gated transistors using electron spin resonance (ESR). The utilized devices were ion gel-gated regioregular poly(3-hexylthiophene) (RR-P3HT) thin-film transistors. We performed simultaneous mesurements of field-induced ESR (FI-ESR) and device characteristics using the same device. Clear FI-ESR signals due to hole carriers (positive polarons) were observed by applying a negative gate voltage. The anisotropy of the ESR linewidth indicated two-dimensional magnetic interactions between high density charges. The magnetism of the majority of hole carriers at a high charge density was found to be nonmagnetic from the simultaneous measurements of FI-ESR and transfer characteristics. The anisotropy of the g value of the ESR signal at a high gate voltage indicated the disordered molecular orientation of RR-P3HT where paramagnetic holes exist. These results provide insight into the charge transport mechanism of RR-P3HT polymer semiconductors with high charge densities.

Electron spin resonance observation of charge carrier concentration in organic field-effect transistors during device operation

Charge carrier concentration in operating organic field-effect transistors (OFETs) reflects the electric potential within the channel, acting as a key quantity to clarify the operation mechanism of the device. Here, we demonstrate a direct determination of charge carrier concentration in the operating devices of pentacene and poly(3-hexylthiophene) (P3HT) by field-induced electron spin resonance (FI-ESR) spectroscopy. This method sensitively detects polarons induced by applying gate voltage, giving a clear FI-ESR signal around $g=2.003$ in both devices. Upon applying drain-source voltage, carrier concentration decreases monotonically in the FET linear region, reaching about 70$%$ of the initial value at the pinch-off point, and stayed constant in the saturation region. The observed results are reproduced well from the theoretical potential profile based on the gradual channel model. In particular, the carrier concentration at the pinch-off point is calculated to be $\ensuremath{\beta}/(\ensuremath{\beta}+1)$ of the initial value, where \ensuremath{\beta} is the power exponent in the gate voltage (${V}_{\mathrm{gs}}$) dependence of the mobility (\ensuremath{\mu}), expressed as $\ensuremath{\mu}\ensuremath{\propto}{V}_{\mathrm{gs}}^{\ensuremath{\beta}\ensuremath{-}2}$, providing detailed information of charge transport. The present devices show $\ensuremath{\beta}=2.6$ for the pentacene and $\ensuremath{\beta}=2.3$ for the P3HT cases, consistent with those determined by transfer characteristics. The gate voltage dependence of the mobility, originating from the charge trapping at the device interface, is confirmed microscopically by the motional narrowing of the FI-ESR spectra.

Theoretical ESR g Values in Rubrene and Oligoacenes: Implication to Molecular Orientation at Interfaces in Organic FETs

Using density functional theory calculations, molecular g tensors are theoretically investigated for rubrene and oligoacenes. The anisotropy in the calculated g shifts of rubrene well explains experimental results observed by field-induced electron spin resonance (FI-ESR) measurements. We demonstrate that the combination of the FI-ESR technique and the calculation of molecular g tensor is a quite useful approach to determine the molecular orientation at interfaces in organic field-effect transistors.

Correlation between interdomain carrier hopping and apparent mobility in polycrystalline organic transistors as investigated by electron spin resonance

Microscopic charge dynamics inside microcrystal domains and across domain boundaries in polycrystalline organic thin-film transistors (OTFTs) were resolved by angle- and temperature-dependent field-induced electron spin resonance (FI-ESR) spectroscopy. We utilized small-molecule and polymer-based polycrystalline OTFTs, both of which are composed of planar microdomains oriented uniaxially on the substrates. Two types of motional narrowing effect are observed at magnetic fields perpendicular and parallel to the substrate: In the former, we observed the motional narrowing effect due to intradomain transport, while in the latter, we observed another motional narrowing effect due to the carrier hopping across the domain boundaries, which averaged out the variation in $g$ factors between respective microdomains. The analyses revealed that the activation energies for the intradomain transport are between 5 and 21 meV, while those for the interdomain carrier hopping are between 45 and 90 meV. The latter ones coincide with those for the apparent mobility for the respective OTFTs. These results demonstrate that the charge transport is limited by the domain boundaries, which form rate-determining barrier potential both in small-molecule and polymer-based polycrystalline OTFTs.

Direct determination of interfacial molecular orientations in field-effect devices of P3HT/PCBM composites by electron spin resonance

Field-induced electron spin resonance (FI-ESR) measurements have been performed on metal–insulator-semiconductor diode structures using regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) composites with various mixing ratios on a parylene gate insulator. For lower PCBM mixing ratios, clear FI-ESR signals of hole carriers were observed around g = 2.0031, with the external magnetic field normal to the substrate plane, indicating the edge-on orientation of P3HT lamellar structures at the device interface. On the other hand, for higher PCBM mixing ratios above 50 wt.%, a new additional peak was observed at g = 2.0022, which is characteristic of the pπ-orbital axis, indicating the occurrence of flat-on P3HT domains. Ambipolar charge transport was also observed in field-effect transistors using P3HT/PCBM on parylene. With increasing PCBM fraction, from 0 to 100 wt.%, the field-effect mobility of holes decreased from 10 −3 to 10 −6 cm 2/Vs, while that of electrons increased from 10 −5 to 10 −2 cm 2/Vs. These results demonstrate that the flat-on orientation strongly reduces the charge carrier transport for holes because two-dimensional charge transport assisted by π– π interaction is interrupted by the presence of the flat-on structures.

Microscopic mechanisms behind the high mobility in rubrene single-crystal transistors as revealed by field-induced electron spin resonance

Microscopic mechanisms behind the high mobility in rubrene single-crystal transistors as revealed by field-induced electron spin resonance

Electron Spin Resonance of Charge Carriers in Organic Field-Effect Devices

We have developed a field-induced electron spin resonance (ESR) technique to detect charge carriers with spins that are generated at the interface of organic field-effect devices. ESR signals have been successfully observed in the devices of high-mobility organic materials, such as regioregular poly(3-alkylthiophene) (RR-P3AT), in the case of polymer, and pentacene, in the case of small organic molecules. ESR signals of these devices provide valuable information concerning the spin–charge relationship, the spatial extent of charge carriers and molecular orientations at the device interfaces. Instead of Al2O3 gate insulators initially used in devices for ESR, more conventional SiO2 gate insulators on the relatively low-doped Si substrates have been introduced in obtaining ESR signals without lowering the Q-value of the ESR cavity. We present gate-bias dependence of ESR signals of RR-P3AT devices with different gate insulators, where the spins tend to saturate above the charge concentration of about 0.2%, regardless of gate insulating materials. This fact indicates that the charge carriers of the polymers change from polarons with spins to spinless bipolarons or polaron pairs at higher doping levels. Anisotropic ESR spectra of unpaired π-electrons in the devices with SiO2 gate insulators have revealed preferential orientation of the molecular plane, consistent with the formation of self-organized lamellar structures of RR-P3AT as in the case of Al2O3 gate insulators. Motional narrowing of ESR spectra has been observed, depending on the alkyl-chain lengths of RR-P3AT, which could be related to the reported alkyl-chain dependence of carrier mobility.

Direct Observation of Field-Induced Carrier Dynamics in Pentacene Thin-Film Transistors by Electron Spin Resonance Spectroscopy

We report our recent studies on interfacial charge transport in organic transistors on the basis of electron spin resonance (ESR) spectroscopy of field-induced carriers. We successfully observed motional narrowing effects in field-induced ESR spectra for relatively high-mobility (∼0.6 cm2 V-1 s-1) pentacene thin-film transistors (TFTs); Lorentz-type resonance spectra become narrower as the carrier motion becomes more active with increasing temperature or gate voltage. The variation of spectral linewidth is within the range of 30–200 µT, from which we conclude that the charge transport is dominated by the traps with trap residence time of 4 ns. Meanwhile, it was also reported for lower-mobility (∼0.01 cm2 V-1 s-1) pentacene TFTs that ESR spectra do not show motional narrowing and that the linewidth remains unchanged by gate voltage or temperature at about 200 µT. On the basis of these examinations, we discuss the microscopic nature of field-induced carrier dynamics in organic TFTs associated with the origin of linewidth in ESR spectra.

ESR studies of ambipolar charge carriers in metal–insulator–semiconductor diodes of regioregular poly(3-hexylthiophene)/PCBM composites

We have applied field-induced electron spin resonance (FI-ESR) method to organic metal–insulator–semiconductor (MIS) diodes using regioregular poly(3-hexylthiophene) (RR-P3HT) and [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) composites as the active layer, Al 2O 3 as the gate insulator and gold (Au) or aluminum (Al) as the top electrode. In the case of Al electrode, clear FI-ESR signals are observed with g ∼ 2.002 and 1.999 for negative and positive gate voltage ( V G), respectively, demonstrating that these signals with former and the latter g-values correspond to field-induced positive polarons of RR-P3HT and PCBM radical anions, respectively. Both RR-P3HT and PCBM signals monotonically increase as | V G| increases. The threshold voltages ( V th) for field-induced ambipolar charge carriers coincide well with those of capacitance measurements with hysteresis behaviors. On the other hand, in the case of Au electrode, only positive polarons signals are clearly observed and PCBM signals are nearly undetected. These different behaviors of FI-ESR signals between Al and Au top electrodes are discussed in relation to the difference of work functions.

Spatial Extent of Wave Functions of Gate-Induced Hole Carriers in Pentacene Field-Effect Devices as Investigated by Electron Spin Resonance

An electron spin resonance (ESR) method is applied to a pentacene field-effect device to investigate gate-induced hole carriers in such devices. Clear field-induced ESR signals are observed, demonstrating that all of the field-injected carriers have S = 1/2 spins. Anisotropic ESR signals due to unpaired pi electrons show the molecular orientation at the interface in the devices. The spatial extent of the spin density distribution (wave function) of the carriers is evaluated from the ESR linewidth, accounting for the hyperfine structure, to be of the order of 10 molecules.

ESR studies of field-induced polarons in MIS diode structures with self-organized regioregular poly(3-hexylthiophene)

We report electron spin resonance (ESR) studies on field-induced charge carriers in regioregular poly(3-hexylthiophene) (RR-P3HT) using metal-insulator-semiconductor (MIS) diode structures with self-organized RR-P3HT as the active semiconductor layer and Al 2O 3 as an insulating layer. The field-induced ESR signals ( g ∼ 2.002) were clearly observed; their intensities monotonically increased as the absolute value of the gate bias increased in the accumulation mode with a saturation behavior at higher voltages. The ESR signals were consistent with those of the photogenerated positive polarons in RR-P3HT, demonstrating that the carriers are polarons. The transient responses of the field-induced ESR intensity show a fast component with a time constant of an order of less than 20 ms. The self-organized lamellar molecular orientation was confirmed by the anisotropic ESR signals due to π-electrons.

Electron Spin Resonance of Field-Induced Polarons in Regioregular Poly(3-alkylthiophene) Using Metal–Insulator–Semiconductor Diode Structures

We report electron spin resonance (ESR) studies on field-induced charge carriers in regioregular poly(3-hexylthiophene) (RR-P3HT) and regioregular poly(3-octylthiophene) (RR-P3OT) for metal–insulator–semiconductor (MIS) diode structures with Al 2 O 3 as an insulating layer. The field-induced ESR signals ( g ∼2.002) were clearly observed; their intensities monotonically increased as the absolute value of the gate bias increased in the accumulation mode with a saturation behavior at higher voltages. The ESR signals were consistent with those of the photogenerated positive polarons in RR-P3HT and RR-P3OT, demonstrating that the carriers are polarons. The transient responses of the field-induced ESR intensity show fast and slow components. The self-organized lamellar molecular orientation was confirmed by the anisotropic ESR signals due to π-electrons, which is semiquantitatively reproduced by an ESR simulation.