Magnetoresistance from quenching of spin quantum correlation in organic semiconductors

We present a theory of organic magnetoresistance (OMR) based on the quenching of the quantum correlation between the carrier’s spin and its local environment when the incoherent hopping takes place. We prove that this process contributes a spin-dependent prefactor to the attempt-to-escape frequency in the hopping rate, with its value modulated by the magnetic field. The resulting OMR exhibits a positive Lorentzian saturation component and a negative small-field component, which are independent of model parameters. These behaviors, with their isotope effects, are in good agreement with experimental results.     

Temperature and field-dependence of hopping conduction in organic semiconductors

Electrical characteristics of the hopping transport in organic semiconductors are studied theoretically. Based on percolation theory of hopping between localized states, an analytical mobility model is obtained. This model is applied to the analysis of both the electric field dependence and the temperature dependence of the mobility. The results agree quantitatively with recent experimental data.     

Determination of diffusion lengths in organic semiconductors: Correlation with solar cell performances

Organic semiconductors are promising candidates for future applications in solar energy conversion. Recent investigations of bulk heterojunction (BHJ) semiconductors have suggested a density of states and transport mechanisms by multiple trapping close to those observed in disordered inorganic thin films. That is why we have applied to BHJ thin films experiments that are currently used for disordered semiconductors. In addition to the steady state photoconductivity we have tested the ability of the steady state photocarrier grating (SSPG) technique to provide information on the minority carrier diffusion length. We found that SSPG can be applied to P3HT:PCBM thin films leading, for the best sample, to a diffusion length of the order of 125 nm. From the comparison of the transport parameters obtained on thin films with the performances of the devices integrating the latter, we conclude that SSPG is a very powerful tool for optimizing the BHJ thin film properties before their incorporation in solar devices.     

Theoretical investigation of voltage effect on magnetoresistance in an organic small molecule device

We theoretically study the voltage effect on organic magnetoresistance (OMAR) in a weak disordered small molecule device on the basis of the quantum dynamics. It is found that with the increase of the voltage, the OMAR effect is reduced. The results show a good agreement with the experimental data. In addition, the carrier density effect on OMAR has also been discussed.     

Molecular orientation and thermal stability of thin-film organic semiconductors

Molecular orientation in organic semiconductors plays a critical role in maximizing external quantum efficiencies of organic light-emitting diodes. It was generally believed that the molecular packing of organic semiconductors is either amorphous or liquid-crystal-like with a preferred molecular orientation distributed uniformly throughout the film. In this paper, however, we report that the orientation of organic molecules in physical-vapor deposited films varies drastically depending on thickness. The thermal stability of the molecular network, measured by its characteristic glass transition temperature, also varies as a function of the film thickness. Based on a two-layered film-structure model, we propose a simple function to quantify the molecular dipole orientation S parameter as a function of film thickness. This function describes well experimental data. In addition to contributing to external quantum efficiency, the molecular orientation parameter S is found to have a strong impact on disruptive change in material density after thermal anneal and glass transition.     

Assessing the width of Gaussian density of states in organic semiconductors

The Density of States (DOS) is an ingredient of critical importance for the accurate physical understanding of the optoelectronic properties of organic semiconductors. The disordered nature of this class of materials, though, renders the task of determining the DOS far from trivial. Its extraction from experimental measurements is often performed by driving the semiconductor out of thermal equilibrium and therefore requires making assumptions on the charge transport properties of the material under examination. This entanglement of DOS and charge transport models is unfavorable since transport mechanisms in organic semiconductors are themselves still subject of debate. To avoid this, we propose an alternative approach which is based on populating and probing the DOS by means of capacitive coupling in Metal Insulator Semiconductors (MIS) structures while keeping the semiconductor in thermal equilibrium. Assuming a Gaussian shape, we extract the DOS width by numerical fitting of experimental Capacitance–Voltage curves, exploiting the fact that the DOS width affects the spatial distribution of accumulated charge carriers which in turn concurs to define the MIS capacitance. The proposed approach is successfully tested on two benchmark semiconducting polymers, one of n-type and one of p-type and it is validated by verifying the robustness of the extraction procedure with respect to varying the insulator electrical permittivity. Finally, as an example of the usefulness and effectiveness of our approach, we study the static characteristics of thin film transistors based on the aforementioned polymers in the framework of the Extended Gaussian Disorder transport model. Thanks to the extracted DOS widths, the functional dependence of current on the gate voltage is nicely predicted and physical insight on transistor operation is achieved.     

The relevance of correct injection model to simulate electrical properties of organic semiconductors

In this work we demonstrate how a full comprehensive model can be used to understand the electrical behavior of actual organic devices. We address all the aspects which need to be taken into account for realistic simulations of a wide range of device structures and configurations. In particular we stress the relevance of the correct modeling of contact/organic interfaces. The model is applied to perform predictive simulations of organic light-emitting diodes and to deduce how a full experimental characterization of an organic device should be performed in order to completely grasp its electrical behavior.     

Theoretical investigation on organic magnetoresistance based on Zeeman interaction

Based on the magnetic field-spin Zeeman interaction, a mechanism is suggested to explain the organic magnetoresistance (OMR). It is found that a considerable magnetoresistance (MR) could be obtained in organic semiconductors (OSCs). The value of MR is sensitive to the carrier concentration and the mobility (or the charge-transfer integral). We speculate that the MR in organic materials should be phonon dependent. This investigation is consistent with the present experimental observations on OMR.     

Transition metal oxides on organic semiconductors

Transition metal oxides (TMOs) on organic semiconductors (OSs) structure has been widely used in inverted organic optoelectronic devices, including inverted organic light-emitting diodes (OLEDs) and inverted organic solar cells (OSCs), which can improve the stability of such devices as a result of improved protection of air sensitive cathode. However, most of these reports are focused on the anode modification effect of TMO and the nature of TMO-on-OS is not fully understood. Here we show that the OS on TMO forms a two-layer structure, where the interface mixing is minimized, while for TMO-on-OS, due to the obvious diffusion of TMO into the OS, a doping-layer structure is formed. This is evidenced by a series of optical and electrical studies. By studying the TMO diffusion depth in different OS, we found that this process is governed by the thermal property of the OS. The TMO tends to diffuse deeper into the OS with a lower evaporation temperature. It is shown that the TMO can diffuse more than 20 nm into the OS, depending on the thermal property of the OS. We also show that the TMO-on-OS structure can replace the commonly used OS with TMO doping structure, which is a big step toward in simplifying the fabrication process of the organic optoelectronic devices.     

Time-resolved kelvin probe force microscopy to study population and depopulation of traps in electron or hole majority organic semiconductors

We present a time-resolved Kelvin Probe Force Microscopy (KPFM) technique that can record carrier motion on the scale of milliseconds, appropriate for polycrystalline materials like organic semiconductors. The organic semiconductors are studied in a transistor geometry to which we apply a step voltage to the back-gate. We record the change in potential at a specific location and observe the times associated with filling and emptying majority carrier traps, observed in hole majority carrier poly(3-hexylthiophene-2,5-diyl) (P3HT) and a perylene diimide electron majority carrier, PDI-CN2. We see signs of bias stress with repeated measurements in P3HT.     

Analysis of charge transfer complex at the interface between organic and inorganic semiconductors

Improving the electrical performance of organic semiconductors is critical to use them for optoelectronic applications. In this study, we analyze the mechanism of charge transfer complex (CTC) formation at the interface between organic and inorganic semiconductors through extensive optical and electrical measurements. N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) and molybdenum oxide (MoO₃) were sequentially deposited to form a donor/accepter heterojunction structure. The CTC formation and conductivity of the films were determined using UV–visible spectroscopy and transmission line method, respectively. Compared with the single layer devices, the donor/accepter heterojunction exhibits significantly enhanced conductivity. In addition, the conductivity and CTC generation efficiency of the heterojunction display strong dependence on NPB layer thickness, which originates from the variation of dipole interactions at the heterojunction interface. These results provide useful insights on interfacial doping properties, which is potentially beneficial for enhancing the understanding of organic/inorganic interfaces.     

Dialkylated dibenzotetrathienoacene derivative as semiconductor for organic field effect transistors

A novel semiconductor material based on dialkylated thienoacene is designed and synthesized. The dihexyl-substituted dibenzotetrathienoacene derivative C6-DBTTA exhibits high stability which is evidenced by thermogravimetric analysis (TGA), UV–vis spectroscopy and electrochemistry. X-ray diffraction measurements of the vacuum-evaporated thin films show strong diffraction and indicate that the molecules are stacked nearly perpendicular to the substrate. AFM images reveal that the morphology of thin films depended on the deposition temperature. Thin film FETs devices based on C6-DBTTA were constructed and showed high mobility up to 0.48 cm² V⁻¹ s⁻¹ and an on/off ratio over 10⁷. These results suggest that this new dihexylated thienoacene is an important organic semiconductor for field effect transistors.     

BODIPY derivatives as n-type organic semiconductors: Isomer effect on carrier mobility

In the present work, two dipyrro-boradiazaindacenes (BODIPY) derivatives functioning as novel high-performance organic semiconductors are investigated by theoretical method. These two isomeric complexes are demonstrated to have large electron-transfer mobility, which means they are favor to be n-type organic semiconductors. The highest electron-transfer mobility appears at the same packing style in two crystals. The intermolecular distances of the packing style are nearly same, 4.994 Å in crystal 1 and 5.283 Å in crystal 2. However, their electron-transfer mobility changes significantly. The mobility of crystal 2 with better planar molecular structure is 0.291 cm² V⁻¹ s⁻¹, which is 13 times larger than that of crystal 1 as 0.022 cm² V⁻¹ s⁻¹. The significant difference of carrier mobility is ascribed to the little structural difference of these two isomers. It has been demonstrated that both crystal 1 and 2 show remarkable anisotropic behavior. This study will undoubtedly provide a new understanding of isomerization on designing novel organic semiconductors.     

High mobility and low operation voltage organic field effect transistors by using polymer-gel dielectric and molecular doping

In this work, we present a method to increase the performance in solution processed organic field effect transistors (OFET) by using gel as dielectric and molecular doping to the active organic semiconductor. In order to compare the performance improvement, Poly (methylmethacrylate) (PMMA) and Poly (3-hexylthiophene-2,5-diyl) P3HT material system were used as a reference. Propylene carbonate (PC) is introduced into PMMA to form the gel for using as gate dielectric. The mobility increases from 5.72×10⁻³ to 0.26 cm² V s^–1 and operation voltage decreases from −60 to −0.8 with gel dielectric. Then, the molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is introduced into P3HT via co-solution. The mobility increases up to 1.1 cm² V s^–1 and the threshold voltage downs to −0.09 V with doping. The increase in performance is discussed in terms of better charge inducing by high dielectric properties of gel and trap filling due to the increased carrier density in active semiconductor by molecular doping.     

Theoretical investigation on magnetic field effect in organic devices with asymmetrical molecules

A mechanism of organic magnetic field effect (OMFE) based on Lorentz effect in organic light-emitting devices with asymmetrical molecules is suggested and the magnetoconductance (MC) value is calculated. By considering the collision of a positive and a negative charged polaron and exciton formation in the organic layer through a non-adiabatic dynamic process, it is found that the exciton yield can be changed by applying a magnetic field due to the Lorentz effect on the moving polarons’ phase factors. By calculating the current through the device and the MC, we obtain that the calculated MC is well consistent to some experimental observations. It is also found that the MC value is sensitive to the asymmetrical structure and the electron–phonon (e–ph) coupling of the organic material, which explains why magnetic effect in an organic semiconductor is much more apparent than its inorganic counterpart.     

Directed self-assembly of organic semiconductors via confined evaporative capillary flows for use in organic field-effect transistors

We fabricated well-defined 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) crystal arrays for use in electronic applications via a simple but effective method, the confined evaporative capillary flow (CEC) method. This has been accomplished by systematically controlling the contact line pinning at the edge of glass stylus and the outward hydrodynamic flow within the drying droplet with various processing solvents and surface properties of the substrate during solidification. We found that after CEC coating of TIPS-PEN solution dissolved into toluene onto SiO₂ surface, ribbon-shaped TIPS-PEN crystals were well developed with a width of 20–100 μm and length of 300 μm – 2 mm, which is presumably owing to optimized capillary evaporation. Specifically, TIPS-PEN crystals present highly preferred crystal orientation along the (l 0 0) axis, which can lead to efficient charge transport in a lateral direction. Thus, TIPS-PEN field-effect transistors (FETs) exhibited a good hole mobility of 0.72 cm²/Vs.     

Flexible organic phototransistors based on a combination of printing methods

Highly photosensitive organic phototransistors (OPTs) are successfully demonstrated on a flexible substrate using all-solution process as well as a combination of printing methods which consist of roll-to-plate reverse offset printing (ROP), inkjet printing and bar coating. Excellent electrical switching characteristics are obtained from heterogeneous interfacial properties of the reverse-offset-printed silver nanoparticle electrode and the inkjet-printed p-channel polymeric semiconductor. In particular, the OPTs exhibit remarkably photosensitivity with a photo-to-dark current ratio exceeding 5 orders. This optoelectronic properties of the combinational printed OPTs are theoretically and experimentally studied, and found the comparable tendency. In addition, excellent mechanical stability is observed with up to 0.5% of strain applied to the OPTs. Hence, by manufactured with a combination of various graphic art printing methods such as roll-to-plate ROP, inkjet printing, and bar coating, these devices are very promising candidates for large-area and low-cost printed and flexible optoelectronics applications.     

Pivotal factors in solution-processed,non-fullerene,all small-molecule organic solar cell device optimization

The importance of device structure and active-layer processing when screening non-fullerene acceptors was demonstrated through the organic solar cell device performance optimization of a solution processable non-fullerene, all small-molecule bulk heterojunction (BHJ) blend. Key tuning parameters were identified; notably, the largest improvement in performance was achieved by switching from the conventional device architecture (ITO/PEDOT:PSS/D-A BHJ/Ca/Al) to an inverted structure (ITO/ZnO/D-A BHJ/MoOx/Ag), approximately doubling the power conversion efficiency from best cells of 0.5%–1.0%, demonstrating the importance of investigating more than a single architecture when screening novel non-fullerene acceptors.