Efficient and bright polymer light emitting field effect transistors

Light emitting field effect transistors (LEFETs) are emerging as a multi-functional class of optoelectronic devices. LEFETs can simultaneously execute light emission and the standard logic functions of a transistor in a single architecture. However, current LEFET architectures deliver either high brightness or high efficiency but not both concurrently, thus limiting their use in technological applications. Here we show an LEFET device strategy that simultaneously improves brightness and efficiency. The key step change in LEFET performance arises from the bottom gate top-contact device architecture in which the source/drain electrodes are semitransparent and the active channel contains a bi-layer comprising of a high mobility charge-transporting polymer, and a yellow–green emissive polymer. A record external quantum efficiency (EQE) of 2.1% at 1000 cd/m² is demonstrated for polymer based bilayer LEFETs.     

Unlocking the full potential of light emitting field-effect transistors by engineering charge injection layers

Light emitting field-effect transistors (LEFETs) are a class of next generation devices which combine the switching properties of field-effect transistors (FETs) with light emitting capabilities of organic light-emitting diodes (OLEDs) in a single device architecture. Current LEFET architectures suffer from inefficient charge injection of electrons and holes from the source and drain electrodes, leading to unbalanced charge transport and hence poor device performance. Here we report a simple fabrication method for LEFETs that delivers asymmetric source and drain electrodes comprised of low and high work function materials. The interdigitated low and high work function source–drain electrodes consist of combinations of organic materials, salts, metal oxides and metals. Using this method we were able to obtain a maximum EQE of up to 1.2% in a single layer device with Super Yellow as the active material.     

Charge transport and density of trap states in balanced high mobility ambipolar organic thin-film transistors

We report on charge transport and density of trap states (trap DOS) in ambipolar diketopyrrolopyrrole-benzothiadiazole copolymer thin-film transistors. This semiconductor possesses high electron and hole field-effect mobilities of up to 0.6 cm²/V-s. Temperature and gate-bias dependent field-effect mobility measurements are employed to extract the activation energies and trap DOS to understand its unique high mobility balanced ambipolar charge transport properties. The symmetry between the electron and hole transport characteristics, parameters and activation energies is remarkable. We believe that our work is the first charge transport study of an ambipolar organic/polymer based field-effect transistor with room temperature mobility higher than 0.1 cm²/V-s in both electrons and holes.     

Interface engineering to enhance charge injection and transport in solution-deposited organic transistors

Soluble small molecule organic semiconductors combine the high-performance of small molecule organic semiconductors with the versatile processability of polymeric materials, but the control of device performance and uniformity is challenged by the complex film microstructure formed in these materials, and its strong dependence on processing conditions. These films crystallize via a nucleation and growth mechanism that can be difficult to control. In this study we used highly fluorinated self-assembled monolayers (SAMs) to modify the surface of the source and drain contacts and improve the performance of organic thin-film transistors (OTFTs) through controlling film microstructure and lowering the contact resistance. We reached charge carrier mobilities as high as 5.7 cm²/V in 2,8-Difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT), one order of magnitude greater than what we obtained in devices on untreated substrates, and on par with the value reported for single crystal devices. Kelvin probe measurements distinguished an increase in the work function between 0.28 eV and 0.5 eV, depending on the molecular structure of the SAM. Selected area electron diffraction (SAED) confirmed the preferential “edge-on” molecular orientation of the semiconductor. We discuss the device performance in relation to the film morphology and contact resistance.     

Low voltage operating organic light emitting transistors with efficient charge blocking layer

Charge injection/blocking layers play important roles in the performances of organic electronic devices. Their incorporation into organic light emitting transistors has been limitted, due to generally high operating voltages (above 60 V) of these devices. In this work, two hole blocking molecules are integrated into tris-(8-hydroxyquinoline) aluminum (Alq₃) based light emitting transistors under operating voltage as low as 5 V. The effects of hole blocking and electron injection are decoupled through the differences in the energy levels. Significantly improved optical performance is achieved with the molecule of suitable energy level for electron injection. Surprisingly, a decreased performance is observed in the case of another hole blocking molecule evidencing that charge injection overweighs charge blocking in this device architecture.     

Highly enhanced light extraction from organic light emitting diodes with little image blurring and good color stability

We report a highly enhanced light extraction from a top emission organic light emitting diode with little image blurring and color variation with viewing angle. Direct integration of a high refractive index micro lens array on the top of the transparent indium zinc oxide top electrode of a green phosphorescent OLED showed a significant enhancement of light extraction to get EQE of 44.7% from 27.6%, the power efficiency of 134.7 lm/w from 85.9 lm/W and the current efficiency of 217.2 cd/A from 120.7 cd/A without image blurring. In addition, the device showed excellent color stability on viewing angle with Commission Internationale de l’Eclairage (CIE) coordinate of Δx = 0.01, Δy = 0.01 as the viewing angle varied from 0° to 60°.     

Charge transport and recombination in PDPP5T:[70]PCBM organic solar cells: The influence of morphology

The performance of organic bulk heterojunction solar cells is strongly dependent on the donor/acceptor morphology. Morphological parameters, such as the extent and the composition of donor- and acceptor-rich domains, influence both the charge generation and the charge transport throughout the active layer. This work focuses on a polymer:fullerene system based on a small bandgap diketopyrrolopyrrole–quinquethiophene alternating copolymer (PDPP5T) mixed with [6,6]-phenyl-C₇₁-butyric acid methyl ester ([70]PCBM) that is capable of efficiencies higher than 6%. By changing the processing conditions, the morphology can be varied from a coarse separated morphology, with fullerene domains (blobs) embedded in a polymer-rich matrix, to a completely mixed layer.     

Charge conduction study of phosphorescent iridium compounds for organic light-emitting diodes application

The charge conduction properties of a series of iridium-based compounds for phosphorescent organic light-emitting diodes (OLEDs) have been investigated by thin-film transistor (TFT) technique. These compounds include four homoleptic compounds: Ir(ppy)₃, Ir(piq)₃, Ir(Tpa-py)₃, Ir(Cz-py)₃, and two heteroleptic compounds Ir(Cz-py)₂(acac) and FIrpic. Ir(ppy)₃, Ir(piq)₃ and FIrpic are commercially available compounds, while Ir(Tpa-py)₃, Ir(Cz-py)₃ and Ir(Cz-py)₂(acac) are specially designed to test their conductivities with respect to the commercial compounds. In neat films, with the exception of FIrpic, all Ir-compounds possess significant hole transporting capabilities, with hole mobilities in the range of about 5 × 10⁻⁶–2 × 10⁻⁵ cm² V⁻¹ s⁻¹. FIrpic, however, is non-conducting as revealed by TFT measurements. We further investigate how Ir-compounds modify carrier transport as dopants when they are doped into a phosphorescent host material CBP. The commercial compounds are chosen for the investigation. Small amounts of Ir(ppy)₃ and Ir(piq)₃ (<10%) behave as hole traps when they are doped into CBP. The hole conduction of the doped CBP films can be reduced by as much as 4 orders of magnitude. Percolating conduction of Ir-compounds occurs when the doping concentrations of the Ir-compounds exceed 10%, and the hole mobilities gradually increase as their values reach those of the neat Ir films. In contrast to Ir(ppy)₃ and Ir(piq)₃, FIrpic does not participate in hole conduction when it is doped into CBP. The hole mobility decreases monotonically as the concentration of FIrpic increases due to the increase of the average charge hopping distance in CBP.     

Effect of various microlens parameters on enhancement of light outcoupling efficiency of organic light emitting diode

Since organic light emitting diode (OLED) is a multilayer device where each layer has different refractive index, total internal reflection (TIR) plays an important role in limiting the efficiency of an OLED. Due to the presence of TIR, a major portion of light is trapped within the device in various wave guiding modes. Of the total light trapped in an OLED, we address only the part that is lost due to wave guiding mode arising from refractive index mismatch at the glass-air interface. Microlens array, to improve luminance, is a method that can be externally applied to the OLEDs without altering its electrical characteristics and is easy to use. Microlens arrays ranging from 10 to 40 μm have been fabricated using an organic elastomeric material polydimethylsiloxane (PDMS) by mold transfer technique. Maximum improvement of 25% in outcoupling efficiency for blue OLED is reported upon using the microlens array with diameter 10 μm. For a given diameter of microlens, out-coupling efficiency of OLED increases as height to diameter (H/D) ratio of microlens array approaches 0.5 (perfect hemisphere). It is also observed that outcoupling efficiency increases with the diameter of microlens for a given H/D ratio. The best luminescence improvement was observed for blue OLED, which can be explained by the higher refractive index of PDMS at lower wavelengths.     

The role of local potential minima on charge transport in thin organic semiconductor layers

We have performed a systematic study of dependence of time-resolved photocurrent on the point of charge excitation within the organic semiconductor channel formed by two coplanar metal electrodes. The results confirm that spatial variation of electric field between the electrodes crucially determines transport of photogenerated charge carriers through the organic layer. Time-of-flight measurements of photocurrent demonstrate that the transit time of photogenerated charge carrier packets drifting between the two electrodes decreases with increasing travelling distance. Such counterintuitive result cannot be reconciled with the spatial distribution of electric field between coplanar electrodes, alone. It is also in contrast to expected role of space-charge screening of external electric field. Supported by Monte Carlo simulations of hopping transport in disordered organic semiconductor layer, we submit that the space-charge screens the external electric field and captures slower charge carriers from the photogenerated charge carrier packet. The remaining faster carriers, exhibit velocity distribution with significantly higher mean value and shorter transit time.     

An easily made thienoacene comprising seven fused rings for ambient-stable organic thin film transistors

A novel easily made thienoacene-based organic semiconductor, i.e., dinaphtho[3,4-d:3′,4′-d′]benzo[1,2-b:4,5-b′]dithiophene (Ph5T2), was synthesized in high yield, and its thermal stability, electrochemical properties, thin-film morphology and field-effect mobility were investigated. Ph5T2 exhibit excellent thermal stability with a decomposition temperature (T_d) of 427 °C. Thin-film X-ray diffraction (XRD) and atomic force microscopy (AFM) characterizations indicate that Ph5T2 can form highly ordered films with large domain size on the para-sexiphenyl (6P)-modified substrates. Organic thin-film transistors (OTFTs) with top-contact geometry based on Ph5T2 exhibit mobilities up to 1.2 cm² V⁻¹ s⁻¹ in ambient. The devices are highly stable and exhibit almost no performance degradation during 3 months storage under ambient conditions with relative humidity up to 80%.     

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.     

Charge transport properties of bulk-heterojunction organic solar cells investigated by displacement current measurement technique

We have investigated charge transport properties of bulk-heterojunction (BH) solar cells in which P3HT (Poly(3-hexylthiophene)) and PCBM ([6,6]-phenyl-C₆₁-butyric acid methyl ester) are used as the active layer, by using the displacement current measurement (DCM) method. In order to investigate the charge transport properties of the BH solar cells, we fabricated a dedicated device that consists of P3HT and PCBM, and used the DCM method to measure the charge distribution of the devices with different composition ratios of P3HT and PCBM. DCM data suggested that a BH film with 50 wt% of PCBM exhibits a preferable charge transport property suited for BH solar cells. We confirmed that the DCM results are consistent with the measured performance of the BH solar cells, indicating that the DCM method is a simple and effective method for optimizing the structure of BH solar cells as well as other electronic devices composed of binary materials.     

面向彩色有机微显示的有机白光顶发射器件

以比铝、银等金属材料透光性更好的铜作为白光有机顶发射器件的顶电极,将其应用到基于Al底电极的蓝、黄互补色顶发射白光有机电致发光器件(TEWOLED),通过合理设计器件结构,制备出的器件具有较低的驱动电压和较高的效率,4V下亮度超过1 000cd/m2、功率效率达到28.5lm/W,效率滚降较小。我们利用p型电学掺杂结构和电子注入缓冲层结构分别解决了铝和铜电极功函数同空穴传输层的HOMO能级和电子传输层的LUMO能级不匹配问题,并通过TcTa光学覆盖层的调节作用使器件具有较好的光谱稳定性。基于Cu顶电极的TEWOLED与采用Al作为互连金属的CMOS工艺兼容,我们将该器件与硅基CMOS驱动电路结合,获得了SVGA白光有机微显示器件,为彩色有机发光微显示的实现奠定了基础。     

Controlling organization of conjugated polymer films from binary solvent mixtures for high performance organic field-effect transistors

We investigate the effect of a binary solvent blend as a solvent for poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} P(NDI2OD-T2) on the characteristics of n-channel organic field-effect transistors (OFETs). To make the binary solvent blend, the low-boiling-point non-solvent propylene glycol methyl ether acetate (PGMEA, b.p ∼146 °C) is added to the high-boiling-point good solvent 1,2-dichlorobenzene (O-DCB, b.p ∼180 °C) at various mixing ratio from 0 to 40 v%. UV–vis spectra of P(NDI2OD-T2) solution dissolved in the binary solvent clearly show the formation of polymer aggregates through a gradual red shift of the intramolecular charge transfer band with the addition of high concentrations of non-solvent PGMEA. Higher edge-on oriented crystallinity is observed for P(NDI2OD-T2) films spin-coated from the binary solvent with 5–10 v% PGMEA by out-of-order x-ray diffraction. P(NDI2OD-T2) films are applied as the active layer in top-gate/bottom-contact OFETs. Improved n-type field-effect mobility of the P(NDI2OD-T2) semiconducting layer up to 0.59 cm²/Vs was achieved for on-center spin coated films compared to 1.03 cm²/Vs for off-center (parallel alignment) spin-coated films respectively employing the binary solvent with 10 v% PGMEA.     

Investigation of charge carrier mobility and recombination in PBDTTPD thin layer structures

In this paper we present investigation of hole transport properties in sandwich and OFET structures with single active layer of PBDTTPD (Poly[(5,6-dihydro-5-octyl-4,6-dioxo-4H-thieno[3,4-c]pyrrole-1,3-diyl)[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]]). Sandwich structures were investigated by photo-CELIV and TOF techniques, obtained results showed strongly time dependent hole mobility and non-Gaussian DOS tail. Photogenerated charge carrier decay experiment demonstrated that bimolecular recombination coefficient is smaller than calculated Langevin recombination coefficient and this was explained by faster holes escaping recombination area and not participating in Langevin recombination process. Organic field-effect transistor structure was investigated by current transients technique to find hole mobility near the dielectric layer and to study OTS treatment influence on hole transport. The study of hole mobility dependence on temperature was performed in order to evaluate energetic disorder of interface DOS in the channel of OFET structures.     

Magnetic field effects on electroluminescence in phosphorescence organic light emitting diodes

In this letter, we presented a method to study the MFEs on the triplets in phosphorescent OLEDs. The magnetic electroluminescence (MEL) was obtained by doping a red phosphorescent guest with low concentration into a fluorescent host, where the guest and host can simultaneously emit. Experimentally two different MEL shapes of Lorentz and linear were observed, depending on the used host materials. We presented two different mechanisms to explain their difference. The diffusion process of triplets from host to guest and prolonged lifetime of triplet by magnetic field were attributed to the formation of the Lorentz shape, and it is considered that the linear shape was caused by magnetic field increased Dexter energy transfer rate and determined by the triplet energy difference between guest and host. It can be seen that the competition of two processes lead to the formation of the two different MEL shapes in the phosphorescent OLEDs.     

Charge-transfer complex modified bottom electrodes for high performance low voltage organic field-effect transistors and circuits

By using charge transfer complex silver and 2,3-dichloro-5,6-dicyano-p-benzoquinone (AgDDQ) modified silver as the bottom contact source/drain electrodes, high performance organic transistors and complementary inverter circuits using dinaphtho[2,3-b:2′,3’-f]thieno[3,2-b]thiophene (DNTT) as P-type organic semiconductors and N,N’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2) as N-type organic semiconductors were demonstrated. Devices with Ag-DDQ bottom contact electrodes exhibit good compatibility for both P and N-type organic semiconductors, the transistors and inverters exhibit excellent stability after storing in air ambient for more than 40 days. The fabrication process is compatible with photolithography technology, which is applicable for large area integrated circuits. All these results indicate the potential application of Ag-DDQ modified electrodes in all-organic, flexible, and low-power electronics.     

Characterization of ohmic contacts in polymer organic field-effect transistors

It is well known that contact resistance R_c limits the performance of organic field-effect transistors (OFETs) that have high field-effect mobilities (μ_FET ≳ 0.3 cm² V⁻¹ s⁻¹) and short channel lengths (L_ch ≲ 30 μm). The usual transfer-line method (TLM) to analyze R_c calls for extrapolation of total resistance to zero L_ch at constant drain and gate voltages. This requires an unrealistic assumption that R_c does not vary with source−drain current I_sd (nor with channel carrier density σ). Here we describe a self-consistent TLM analysis that instead imposes the condition of constant I_sd and σ. The results explicitly reveal the dependence of R_c on I_sd and σ. We further describe how this R_c(I_sd, σ) surface can be modelled to yield the specific contact resistivity ρ_c of the metal/organic semiconductor (OSC) interface, a key parameter that has so far been neglected in OFETs. We illustrate the application of these analyses to high-performance staggered top-gate bottom-contact poly(2,5-bis(alkyl)-1,4-dioxopyrrolo [3,4-c]pyrrole-3,6-diyl-terthiophene-2,5″-diyl) (DPPT2-T) OFETs fabricated on bottom Au source–drain electrode arrays, with high contact-corrected μ_FET of 0.5 cm² V⁻¹ s⁻¹. We show that when these electrodes are modified to impose weak, and then strong hole-doping of the DPPT2-T interface, R_c diminishes and its dispersion, i.e. dependence on I_sd and σ, weakens. The ultimate ρ_c attained for the strongly hole-doped contact is ca. 1 Ω cm², broadly independent of I_sd and σ, which we propose is a hallmark of a true metal/OSC ohmic contact. For comparison, the bare Au/DPPT2-T contact gives ρ_c of the order of 10 Ω cm² with a marked σ dependence. The lowest ρ_c reached here shortens the current transfer length down to ca. 5 μm, enabling short electrode lengths to be advantageously employed in technology.