Vacuum-deposited Small Molecules Research Articles

Interfacial molecular screening of polyimide dielectric towards high-performance organic field-effect transistors

The compatibility of the gate dielectrics with semiconductors is vital for constructing efficient conducting channel for high charge transport. However, it is still a highly challenging mission to clearly clarify the relationship between the dielectric layers and the chemical structure of semiconductors, especially vacuum-deposited small molecules. Here, interfacial molecular screening of polyimide (Kapton) dielectric in organic field-effect transistors (OFETs) is comprehensively studied. It is found that the semiconducting small molecules with alkyl side chains prefer to form a high-quality charge transport layer on polyimide (PI) dielectrics compared with the molecules without alkyl side chains. On this basis, the fabricated transistors could reach the mobility of 1.2 cm2 V−1 s−1 the molecule with alkyl side chains on bare PI dielectric. What is more, the compatible semiconductor and dielectric would further produce a low activation energy (EA) of 3.01 meV towards efficient charge transport even at low temperature (e.g., 100 K, 0.9 cm2 V−1 s−1). Our research provides a guiding scheme for the construction of high-performance thin-film field-effect transistors based on PI dielectric layer at room and low temperatures.

Structure-Property Relation in Organic-Metal Oxide Hybrid Phototransistors.

We report an optoelectronic device consisting of a solution-processed indium gallium zinc oxide (IGZO) thin-film transistor and vacuum-deposited small organic molecules. Depending on the configurations of the organic materials, either bulk heterojunction or planar heterojunction (PHJ), the device assumes the functionality of either a photosensor or a photoinduced memory, respectively. Under λ = 625 nm light illumination, the photosensor shows response and recovery time of ∼50 ms, responsivity of ∼5 mA/W, sensitivity above 104, and a linear response. The mechanism of the photoinduced memory is studied experimentally and verified using a device simulation. We find that the memory is due to long charge retention time at the organic PHJ interface which is stable for over 9 days. It is correlated with the low leakage current found in ordered organic junctions having low subgap tail states. The presented integration of the PHJ with the transistor constitutes a new design of write-once-read-many-times memory device that is likely to be attractive for low-cost applications.

High brightness and efficiency of polymer-blend based light-emitting layers without the assistance of the charge-trapping effect.

Polymeric light-emitting materials have been developed recently as an attractive solution-processable alternative to conventional vacuum-deposited small molecules in organic/polymeric light-emitting diodes, but they are still limited in terms of their performance, especially with low luminance and efficiency. We report on some noteworthy characteristics of a new type of single emitting layer (EML), composed of a blend of a host blue-emitting polyspirobifluorene-based copolymer and a guest yellow-emitting poly(p-phenylene vinylene) derivative copolymer. These host and guest polymers have nearly identical highest occupied molecular orbital levels of about 5.2 eV, and lowest unoccupied molecular orbital levels of about 2.4 eV and 2.9 eV, respectively, minimizing the prevailing charge-trapping properties of their blend. Even in the absence of the charge-trapping effect, it is shown that very bright green electroluminescent (EL) emission with a maximum luminance of ~142,000 cd/m2 can be realized for the blended host:guest EML at a moderate concentration (~5 wt%) of the guest polymer. Current efficiency is also observed to be up to ~14 cd/A, which is much higher than those (3.6~5.1 cd/A) of reference devices with pure host or pure guest polymeric EMLs. Moreover, there is a small change in green color emission, with CIE coordinates of (0.35, 0.60) even at high luminance, showing good color stability of the EL emission from the blended EML. These significant improvements in device performance are mainly attributed to efficient Förster resonance energy transfer between the host and guest polymers in the blended EML. Together with its simple structure and easy processability, the high brightness and efficiency of our blended polymeric EML provides a new platform for the development of solution-processable light-emitting devices and/or advanced emissive display devices.

Increased Electromer Formation and Charge Trapping in Solution-Processed versus Vacuum-Deposited Small Molecule Host Materials of Organic Light-Emitting Devices

We investigate and compare between organic light-emitting devices (OLEDs) fabricated by solution-coating versus vacuum-deposition. Electroluminescence, photoluminescence, and chromatographic measurements on typical OLED host materials reveal significant electromer formation in layers fabricated by solution-processing, pointing to stronger intermolecular interactions in these systems. Delayed electroluminescence measurements reveal that solution-processed layers also have increased charge traps. The findings provide insights on the morphological differences between solution-processed and vacuum-deposited materials and shed light on the root causes behind the lower electroluminescence stability of solution-processed OLEDs.

Very high hole drift mobility in neat and doped molecular thin films for normal and inverted perovskite solar cells

Hole transporting layers (HTLs) have critical roles in the performance of perovskite solar cells. In this study, films of the vacuum-deposited small molecule of Tris[4-(5-phenylthiophene-2-yl)phenyl]amine (TPTPA) are demonstrated as efficient HTL materials for perovskite solar cells. The horizontal molecular stacking of the TPTPA neat film leads to the exceptionally high hole drift mobility of 7 × 10−3cm2V−1s−1, among the highest values observed in the organic molecular neat films. By doping with MoO3, a clear charge transfer state is formed and the hole mobility further increases to 3.6 × 10−2cm2V−1s−1. Both normal- and inverted-structured perovskite solar cells utilizing these HTLs are prepared and optimized; they show high power conversion efficiencies of 17–18%.

Electron and hole transport in the organic small molecule α-NPD

Electron and hole transport properties of the organic small molecule N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine are investigated by space-charge-limited current measurements. The hole transport shows trap-free behavior with a mobility of 2.3 × 10−8 m2/Vs at vanishing carrier density and electric field. The electron transport, on the other hand, shows heavily trap-limited behavior, which leads to highly unbalanced transport. A trap concentration of 1.3 × 1024 m−3 was found by modeling the electron currents, similar to the universal trap concentration found in conjugated polymers. This indicates that electron trapping is a generic property of organic semiconductors, ranging from vacuum-deposited small-molecules to solution-processed conjugated polymers.

Molecular Packing and Arrangement Govern the Photo-Oxidative Stability of Organic Photovoltaic Materials

For long-term performance, chemically robust materials are desired for organic solar cells (OSCs). Illuminating neat films of OSC materials in air and tracking the rate of absorption loss, or photobleaching, can quickly screen a material’s photochemical stability. In this report, we photobleach neat films of OSC materials including polymers, solution-processed oligomers, solution-processed small molecules, and vacuum-deposited small molecules. Across the materials we test, we observe photobleaching rates that span 7 orders of magnitude. Furthermore, we find that the film morphology of any particular material impacts the observed photobleaching rate and that amorphous films photobleach faster than crystalline ones. In an extreme case, films of amorphous rubrene photobleach at a rate 2500 times faster than polycrystalline films. When we compare density to photobleaching rate, we find that stability increases with density. We also investigate the relationship between backbone planarity and chemical reactivit...

Maskless RGB color patterning of vacuum-deposited small molecule OLED displays by diffusion of luminescent dopant molecules.

A maskless RGB color patterning technique based on diffusion of luminescent dopant molecules is proposed here for vacuum-deposited small molecule OLED displays. The proposed maskless color patterning technique enables us to overcome challenging issues in OLED display manufacturing arising from shadow mask limitations. This approach utilizes selective diffusion of luminescent dopant molecules from a donor substrate to an acceptor substrate. Results show that sufficiently high doping levels can be achieved through this technique and that devices with performance similar to those produced by standard co-deposition can be easily produced. Red, green and blue OLEDs are successfully fabricated side by side on one substrate using this technique.

Soft contact transplanted nanocrystal quantum dots for light-emitting diodes: effect of surface energy on device performance.

To realize the full-color displays using colloidal nanocrystal quantum dot (QD)-based light emitting diodes (QLEDs), the emissive QD layer should be patterned to red (R), green (G), and blue (B) subpixels on a micrometer scale by the solution process. Here, we introduced a soft contact QD-transplanting technique onto the vacuum-deposited small molecules without pressure to pattern the QD layer without any damage to the prior organic layers. We examined the patternability of QDs by studying the surface properties of various organic layers systematically. As a result, we found that the vacuum-deposited 4,4',4″-tri(N-carbazolyl)triphenylamine (TCTA) layer is suitable for QD-transplanting. A uniform and homogeneous QD patterns down to 2 μm could be formed for all the RGB QDs (CdSe/CdS/ZnS, CdSe@ZnS, and Cd1-xZnxS@ZnS, respectively) with this method. Finally, we demonstrated the R, G, and B QLEDs by transplanting each QD onto the soft TCTA layer, exhibiting higher brightness (2497, 14 102, and 265 cd m(-2), respectively) and efficiency (1.83, 8.07, and 0.19 cd A(-1), respectively) than those of the previous QLEDs fabricated by other patterning methods. Because this pressure-free technique is essential for patterning and stacking the QDs onto the soft organic layer, we believe that both fundamental study and the engineering approach presented here are meaningful for the realization of the colloidal QD-based full-color displays and other optoelectronic devices.

Low operating voltage and low bias stress in top-contact SnCl2Pc/CuPc heterostructure-based bilayer ambipolar organic field-effect transistors

A low operating voltage (∼10 V) top contact-bottom gate ambipolar organic field-effect transistor (OFET) is fabricated using vacuum-deposited small molecules, SnCl2Pc and CuPc. The ambipolar OFET exhibits balanced carrier mobility and low bias-stress (characteristics time constant ∼105 s) for both n-channel and p-channels.

The effect of charge extraction layers on the photo-stability of vacuum-deposited versus solution-coated organic solar cells

Organic solar cells (OSCs) are studied for their photo-stability in inert atmosphere. Polymer solar cells with a bulk heterojunction (BHJ) of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are contrasted with small molecule solar cells with a BHJ of chloroindium phthalocyanine (ClInPc) and C60-fullerene. A series of charge extraction layers at the hole and electron collecting contacts are examined for their role in OSC performance and stability. The inter-compatibilities of these extraction layers in vacuum-deposited small molecule OSCs (SM-OSCs) versus solution-coated polymer OSCs (P-OSCs) are explored. Through photo-stability studies, we show that interfacial extraction layers are necessary to avoid contact photo-degradation, which otherwise leads to strong reductions in OSC efficiencies. We also highlight certain extraction layer combinations that result in strong inter-electrode degradation, and we discuss incompatibilities in extraction layers among SM-OSCs versus P-OSCs. Our results suggest that the presence of excitons at the organic-electrode interface likely plays a critical role in contact photo-degradation. By minimizing contact photo-degradation, which dominates the majority of short-term OSC degradation, a new avenue for studying OSC stability behavior and opportunities to focus on other losses in OSCs become possible.

Self-organizing properties of triethylsilylethynyl-anthradithiophene on monolayer graphene electrodes in solution-processed transistors

Graphene has shown great potential as an electrode material for organic electronic devices such as organic field-effect transistors (FETs) because of its high conductivity, thinness, and good compatibility with organic semiconductor materials. To achieve high performance in graphene-based organic FETs, favorable molecular orientation and good crystallinity of organic semiconductors on graphene are desired. This strongly depends on the surface properties of graphene. Here, we investigate the effects of polymer residues that remain on graphene source/drain electrodes after the transfer/patterning processes on the self-organizing properties and field-effect characteristics of the overlying solution-processed triethylsilylethynyl-anthradithiophene (TES-ADT). A solvent-assisted polymer residue removal process was introduced to effectively remove residues or impurities on the graphene surface. Unlike vacuum-deposited small molecules, TES-ADT displayed a standing-up molecular assembly, which facilitates lateral charge transport, on both the residue-removed clean graphene and as-transferred graphene with polymer residues. However, TES-ADT films grown on the cleaned graphene showed a higher crystallinity and larger grain size than those on the as-transferred graphene. The resulting TES-ADT FETs using cleaned graphene source/drain electrodes therefore exhibited a superior device performance compared to devices using as-transferred graphene electrodes, with mobilities as high as 1.38 cm(2) V(-1) s(-1).

Vacuum-Deposited Small-Molecule Organic Solar Cells with High Power Conversion Efficiencies by Judicious Molecular Design and Device Optimization

Three new tailor-made molecules (DPDCTB, DPDCPB, and DTDCPB) were strategically designed and convergently synthesized as donor materials for small-molecule organic solar cells. These compounds possess a donor-acceptor-acceptor molecular architecture, in which various electron-donating moieties are connected to an electron-withdrawing dicyanovinylene moiety through another electron-accepting 2,1,3-benzothiadiazole block. The molecular structures and crystal packings of DTDCPB and the previously reported DTDCTB were characterized by single-crystal X-ray crystallography. Photophysical and electrochemical properties as well as energy levels of this series of donor molecules were thoroughly investigated, affording clear structure-property relationships. By delicate manipulation of the trade-off between the photovoltage and the photocurrent via molecular structure engineering together with device optimizations, which included fine-tuning the layer thicknesses and the donor:acceptor blended ratio in the bulk heterojunction layer, vacuum-deposited hybrid planar-mixed heterojunction devices utilizing DTDCPB as the donor and C(70) as the acceptor showed the best performance with a power conversion efficiency (PCE) of 6.6 ± 0.2% (the highest PCE of 6.8%), along with an open-circuit voltage (V(oc)) of 0.93 ± 0.02 V, a short-circuit current density (J(sc)) of 13.48 ± 0.27 mA/cm(2), and a fill factor (FF) of 0.53 ± 0.02, under 1 sun (100 mW/cm(2)) AM 1.5G simulated solar illumination.

Tandem organic photovoltaics using both solution and vacuum deposited small molecules

We demonstrate a tandem organic photovoltaic cell incorporating solution- and vacuum-deposited small molecules as the active layers. A blue and green-absorbing boron subphthalocyanine chloride:C70 graded heterojunction (HJ) sub-cell is combined with a green and red-absorbing functionalized squaraine/C70 bilayer HJ sub-cell, resulting in a tandem cell with a wavelength response from 350 nm to 800 nm. The efficiency of the cells depends on process conditions such as solvent annealing, resulting in nanocrystalline morphology that leads to improved charge and exciton transport compared with un-annealed cells. The incorporation of C70 in both sub-cells leads to an increase of short-circuit current by at least 30% compared to analogous cells using C60. The optimized power conversion efficiency of the tandem cell is 6.6% ± 0.1%, with an open-circuit voltage of 1.97 ± 0.1 V under simulated 1 sun, AM 1.5G illumination. The tandem cell voltage is equal to the sum of the constituent sub-cells, indicating that the transparent, Ag nanoparticle/MoO3 compound charge recombination layer interposed between the cells is nearly lossless.