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In the past decades organic thin film transistors (OTFTs) have been notably studied due to their interesting properties. Not only they can be processed by simple methods such as inkjet printing but also open the doors to new applications for cheap plastic electronics including electronic tags, biosensors, flexible screens,… However, the measured field-effect mobility in OTFTs is relatively low compared to inorganic devices. Generally, such low field-effect mobility values result from extrinsic effects such as grain boundaries or imperfect interfaces with source and drain electrodes. It has been shown that reducing the number of grain boundaries between the source and drain electrodes improves the field effect mobility.1-3 Therefore, it is important to understand the transport mechanisms by studying the structure of organic thin films and local electrical properties within the channel and at the interfaces with source and drain electrodes in order to improve the field-effect mobility in OTFTs. Kelvin probe force microscopy (KPFM) is an ideal tool for that purpose since it allows to simultaneously investigation of the local structure and the electrical potential distribution in electronic devices. In this work, the structure and the electrical properties of OTFTs based on dioctylterthiophene (DOTT) were studied. The transistors were fabricated by spin-coating of DOTT on the transistor structures with treated (silanized) and untreated channel oxide. The potential profiles across the channel and at the metal-electrode interfaces were measured by KPFM. The effect of surface treatment on hysteresis effects was also studied. Smaller crystals and a lower threshold voltage were observed for the silanized devices. Hysteresis effects appeared to be less important in modified devices compared to the untreated ones.
We report high-efficiency phosphorescent blue OLEDs with an organic three stacked structure. Using a high-triplet-energy-hole transporting material of TAPC and a high-triplet-energy-electron transporting material of TmPyPB, the organic three stacked structure has been realized with three new narrow band-gap blue host materials. These host materials have bipolar characteristics and high triplet energy of >2.8 eV. Very low onset voltages of 2.8~3.0 V and driving voltages of 4.2~4.6 V to obtain a brightness of 1000 cd/m2 are achieved in this three stacked device configuration. Maximum external quantum efficiency above 20% is reported.
In this paper we study the density of states in n-type N,N’-ditridecylperylene-3,4,9,10-tetracarboxylic diimide organic semiconductor using two different methods. The first one is based on the temperature dependence of the channel conductance in field-effect transistors. The second one is based on the subgap optical absorption coefficient measured using the Photothermal Deflection Spectroscopy technique. Both techniques allow estimating the distribution of localized states in the band gap of the semiconductor.
Donor-acceptor mixed-stack charge-transfer (CT) compounds can be regarded as a model system for charge carrier separation in molecular-scale donor-acceptor heterojunctions. Here we investigated fundamental photocarrier generation characteristics in single crystals of a donoracceptor mixed-stack system, phenothiazine-tetracyanoquinodimethane (PTZ-TCNQ). The laser beam-induced current (LBIC) measurement on the crystals allowed the discrimination between the exciton and the photocarrier diffusion on the basis of the observed spatial decay profiles. We found that the photocarriers are directly generated by higher-lying CT band excitation and exhibit extremely long diffusion length reaching more than 10 μm. We discuss the origin of the efficient photocarrier generation in terms of the geminate electron-hole pair formation.
A comparison of the photocurrent spectra of organic bulk heterojunction solar cells of various thicknesses is presented. Increasing the thickness of the active layer in both MDMO-PPV /PCBM and P3HT/PCBM solar cells reduces the magnitude of the photocurrent due to the low mobility of the photogenerated holes. Measurements show that the photocurrent reduction is predominately due to a loss in carriers generated at the polymer absorption maximum, while the low energy response is relatively unaffected. In a thick enough sample, the low energy response (1.5-2 eV) dominates, and a photocurrent peak is no longer observed at the main absorption maximum (2.6 eV). The results imply that hole transport is blocked for carriers generated in the polymer at higher energy. Because these holes are generated at the absorption maximum their low mobility could be a major factor limiting solar cell efficiency.
The complex admittance of the Si+/SiO2/Pentacene/Au (metal/oxide/pentacene) thin film junctions is investigated under ambient conditions. The results are compared with the ones obtained for the corresponding Si+/SiO2/Au junctions (i.e. a small part of the surface left free from pentacene) which constitutes the “reference” of our samples. This allows us to extract the “organic” part of the dielectric response from the whole spectrum. Our data clearly show that the admittance is decomposed in three main contributions. At low frequencies, a contribution attributed to proton diffusion through the oxide is seen. This diffusion is shown to be anomalous and is believed to be also at the origin of the bias stress effect observed in organic field effect transistors. At higher frequencies, two dipolar contributions are evidenced, attributed to defects located one at the organic/oxide interface or within the organic, and the other in the bulk of the oxide. These two dipolar responses show different dynamic properties that manifest themselves in the admittance in the form of a Debye contribution for the defects located in the oxide, and of a Cole-Cole contribution for the defects related to the organic.
Transparent conductive oxide less flexible dye-sensitized solar cells (TCO-less DSC) with flat and cylinder shapes are reported. The cell consists of a plastic cover, a flexible titania/dye sheet back contacted with a metal mesh sheet, a gel electrolyte sheet, and Pt layer on a Ti sheet. How to increase the efficiency were discussed. We concluded that making a titania/dye layer on a metal mesh sheet thinner and using a thinner electrolyte layer were effective for increasing the efficiency. A flat TCO-less DSC with 6.1 % efficiency and a cylindrical TCO-less DSC with 5.1 % efficiency are reported.
To improve the conversion efficiency of polymer photodetectors (PDs) fabricated by solution process, the properties of fluorene-type polymer photodetectors doped with iridium (Ir) and platinum (Pt) complexes were investigated. The devices based on poly(dioctylfluorene) and poly(dioctylfluorene-co-benzothiadiazole) (F8BT) had violet and blue sensitivity, respectively. Triplet materials can enhance the incident-photon-to-current conversion efficiency of the devices utilizing the fluorene-type polymers when their triplet levels are lower than the lowest excited singlet states of the host and higher than the lowest excited triplet states of the host. The transmission of a moving picture was successfully demonstrated using the bilayer F8BT device with green Ir complex as an opto-electrical conversion device. We demonstrate that the polymer PDs fabricated by solution process can be applied to short-range optical communication fields, such as opto-electrical conversion devices for optical links.
We have studied the effect of pentacene purity and evaporation rate on low-voltage organic thin-film transistors (OTFTs) prepared solely by dry fabrication techniques. The maximum field-effect mobility of 0.07 cm2/Vs was achieved for the highest pentacene evaporation rate of 0.32 Å/s and four-time purified pentacene. Four-time purified pentacene also led to the lowest threshold voltage of -1.1 V and inverse subthreshold slope of ∼100 mV/decade. In addition, pentacene surface was imaged using atomic force microscopy, and the transistor channel and contact resistances for various pentacene evaporation rates were extracted and compared to field-effect mobilities.
Short channel organic thin film transistors in bottom-gate, bottom contact configuration use typically gold metallization for the source and drain contacts because this metal can easily be cleaned from photoresist residuals by oxygen plasma or ultraviolet-ozone and allows also surface modification by self-assembled monolayers (e.g. thiols). Alternative low-cost bottom contact metallization for high performance short-channel organic thin film transistors are scarce because of the incompatibility of the bottom contact material with the cleaning step. In this work a new process flow, involving a temporary thin aluminum protection layer, is presented. Short channel (3.4 μm) pentacene transistors with lithographical defined and thiol modified silver source/drain bottom contacts (25 nm thick, on a 2 nm titanium adhesion layer) prepared according to this process achieved a saturation mobility of 0.316 cm2/(V.s), and this at a metal cost below 1% of the standard 30 nm thick gold metallization.