High-performance Organic Optoelectronic Devices Research Articles

Individually Tunable Energy Levels of Oligomers Based on N-B←N Units.

Opto-electronic properties and device performance of organic semiconductors are mainly determined by energy levels of their frontier molecular orbitals, e.g. lowest unoccupied molecular orbital (ELUMO) and highest occupied molecular orbital (EHOMO) in the ground state, first singlet state (ES1) and first triplet state (ET1) in the excited state. These energy levels are always intricately intertwined. Herein, we report a series of monodisperse oligomers based on double B←N bridged bipyridine (BNBP) units. With the increasing number of repeating units, the oligomers exhibit gradually downshifted ELUMO and nearly unchanged EHOMO due to the different distribution of the frontier molecular orbitals of the oligomers. Moreover, the oligomers exhibit gradually decreasing ES1 and nearly unchanged ET1 because of the different contributions of the charge transfer component in the excited state. This work provides new insight into energy level tuning of organic semiconductors, which is important for high-performance organic opto-electronic devices.

Visualization of Nanocrystallites in Organic Semiconducting Blends Using Cryo-Electron Microscopy.

The efficiency of an organic solar cell is highly dependent on the complex, interpenetrating morphology, and molecular order within the composite phases of the bulk heterojunction (BHJ) blend. Both these microstructural aspects are strongly influenced by the processing conditions and chemical design of donor/acceptor materials. To establish improved structure-function relationships, it is vital to visualize the local microstructural order to provide specific local information about donor/acceptor interfaces and crystalline texture in BHJ blend films. The visualization of nanocrystallites, however, is difficult due to the complex semi-crystalline structure with few characterization techniques capable of visualizing the molecular ordering of soft materials at the nanoscale. Here, it is demonstrated how cryo-electron microscopy can be utilized to visualize local nanoscale order. This method is used to understand the distribution/orientation of crystallites in a BHJ blend. Long-range (>300nm) texturing of IEICO-4F crystallites oriented in an edge-on fashion is observed, which has not previously been observed for spin-coated materials. This approach provides a wealth of quantitative information about the texture and size of nanocrystallites, which can be utilized to understand charge generation and transport in organic film. This study guides tailoring the material design and processing conditions for high-performance organic optoelectronic devices.

Structure-Property Relationship of Macrocycles in Organic Photoelectric Devices: A Comprehensive Review.

Macrocycles have attracted significant attention from academia due to their various applications in organic field-effect transistors, organic light-emitting diodes, organic photovoltaics, and dye-sensitized solar cells. Despite the existence of reports on the application of macrocycles in organic optoelectronic devices, these reports are mainly limited to analyzing the structure-property relationship of a particular type of macrocyclic structure, and a systematic discussion on the structure-property is still lacking. Herein, we conducted a comprehensive analysis of a series of macrocycle structures to identify the key factors that affect the structure-property relationship between macrocycles and their optoelectronic device properties, including energy level structure, structural stability, film-forming property, skeleton rigidity, inherent pore structure, spatial hindrance, exclusion of perturbing end-effects, macrocycle size-dependent effects, and fullerene-like charge transport characteristics. These macrocycles exhibit thin-film and single-crystal hole mobility up to 10 and 26.8 cm2 V-1 s-1, respectively, as well as a unique macrocyclization-induced emission enhancement property. A clear understanding of the structure-property relationship between macrocycles and optoelectronic device performance, as well as the creation of novel macrocycle structures such as organic nanogridarenes, may pave the way for high-performance organic optoelectronic devices.

Modification of Electrode Interface with Fullerene-Based Self-Assembled Monolayer for High-Performance Organic Optoelectronic Devices

Efficient charge transfer between organic semiconductors and electrode materials at electrode interfaces is essential for achieving high-performance organic optoelectronic devices. For efficient charge injection and extraction at the electrode interface, an interlayer is usually introduced between the organic active layer and electrode. Here, a simple and effective approach for further improving charge transfer at the organic active layer–interlayer interface was presented. Treatment of the zinc oxide (ZnO) interlayer, a commonly used n-type interlayer, with a fullerene-based self-assembled monolayer (SAM) effectively improved electron transfer at the organic–ZnO interface, without affecting the morphology and crystalline structure of the organic active layer on the cathode interlayer. Furthermore, this treatment reduced charge recombination in the device, attributed to the improved charge extraction and reduction of undesirable ZnO-donor polymer contacts. The photocurrent density and power conversion efficiency of organic solar cells employing the fullerene-SAM-treated interlayer were ~10% higher than those of the device employing the nontreated interlayer. This improvement arises from the enhanced electron extraction and reduced charge recombination.

Performance enhancement in organic solar cells and photodetectors enabled by donor phase optimization at the surface of hole transport layer

The domain purity, material crystallinity and distribution at the interface between the active layer and the transport layer have an important impact on the performance of organic solar cells (OSCs) and organic photodetectors (OPDs), while this focal issue has received less attention in previous studies. From this perspective, a new method to simultaneously enhance the performance of OSC and OPD is proposed, namely, using a sequential deposition method to first construct a compact stacking structure of dual-donor (D18-Cl:PTO2) eutectic in the donor layer, and then induce the ordered deposition of the acceptor (Y6). Compared with the conventional bulk heterojunction (BHJ), the active layer realized by this method not only improves the crystallinity and stacking order of the constituent material on the surface of the transport layer, but also regulates a good vertical distribution, which is conducive to improving the charge transport and extraction efficiency, reducing the leakage current, and enhancing the stability of the device. As a result, the OSC device based on the D18-Cl:PTO2/Y6 structure achieves a power conversion efficiency of up to 17.65% and good light-degradation stability, which is much better than that of BHJ-based OSC (PCE of 16.37%). For the OPD, the dark current at reverse bias is reduced by more than an order of magnitude, and the maximum responsivity is improved to 0.52 A/W through the optimization of the donor phase at the interface. Moreover, the strategy does not require additional post-processing compared to the BHJ preparation, which reduces the device construction cost and process complexity, providing an effective way for developing high-performance organic optoelectronic devices.

Single Crystal X-ray Structure Analysis for Molecular Assemblies of Organic Semiconductors toward High Performance Organic Optoelectronic Devices

Single Crystal X-ray Structure Analysis for Molecular Assemblies of Organic Semiconductors toward High Performance Organic Optoelectronic Devices

Thermal degradation of p-doped organic homojunction

The stability of doped organic semiconductors is one of the essential features to achieve stable and high-performance organic optoelectronic devices with low power consumption. In this study, the thermal degradation of an organic homojunction, consisting of an intrinsic organic layer and a molybdenum oxide (MoO3)-doped organic layer, is investigated by impedance spectroscopy (IS) under thermal heating. The IS analysis indicates that the thermal deformation of the intrinsic organic layer is more significant than that of the underlying doped organic layer. A charge-transfer complex absorption peak analysis by ultraviolet-visible spectroscopy confirms that the thermal degradation is related to the deformation of organic host molecules rather than to diffusion of dopants. These results show that the organic homojunction is degraded owing to the crystallization of intrinsic organic molecules at high temperatures, above the glass transition temperature (Tg), rather than because of disruption of the interface at the homojunction by dopant diffusion. This study shows that hole-transport molecules having high Tg should be selected to provide stable electronic devices with organic homojunctions, thus paving the way for the development of novel devices with higher performance.

Influence of partially-oxidized silver back electrodes on the electrical properties and stability of organic semiconductor diodes

Abstract Silver (Ag) is one of the most suitable metals for electrodes in high-performance organic optoelectronic devices due to its high conductivity and reflectivity, and low parasitic absorption loss at visible wavelengths. However, the electronic work function of Ag is not ideal for use as either the cathode or anode in many organic optoelectronic devices. In this report, we investigate the formation of an ultrathin surface oxide layer on a Ag electrode and its impact on hole injection into an organic conjugated polymer semiconductor. The surface oxide is formed by exposing the Ag electrode to a low-power O2/Ar plasma and which changes the electrical properties of the pure Ag electrode. We study the morphology and the chemical composition of the Ag surfaces after different plasma treatment times through X-ray photoelectron spectroscopy, scanning electron microscopy and dark-field optical microscopy. After plasma exposure the surface oxide is composed of both AgOx and Ag2CO3. As plasma exposure time increases from 1 s to 7 s, the fraction of AgOx increases while Ag2CO3 decreases gradually. Both the turn-on voltages and barrier heights of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) hole-only devices decrease with plasma treated Ag surfaces indicating that the work function of the Ag surface is increased by the surface oxide. F8BT hole-only devices with a thin MoO3 layer and a thicker AgOx layer on the electrode are found to be stable compared to thinner surface oxides and un-treated Ag surfaces.

Tuning dielectric-semiconductor interfacial properties in organic phototransistors for ultralow light detection

The quality of the dielectric-semiconductor interface plays a very crucial role in determining the photosensory performances of the organic field effect transistors. Herein, we have investigated the influence of polymer molecular weight on the photosensing behaviour of the organic transistors. In this study, we find high molecular weight PMMA to be more effective as gate dielectric in phototransistor applications compared to the low molecular weight PMMA due to the better morphological arrangements of the active semiconducting layer on top of its surface. The maximum values of photosensitivity and responsivity improved substantially from 3.5 × 10 4 and 8.3 × 10 3 A/W in the low molecular weight PMMA devices to 10 5 and 6.9 × 10 4 A/W in the high molecular weight PMMA devices, respectively, under same intensity light irradiation. Moreover, we have also proposed a simple, yet highly efficient interface modification technique to enhance the photosensitivity of the transistors under low light illumination through the plasma treatment of the dielectric layer. After the interface modification of the high molecular weight PMMA devices, the photosensitivity value enhanced significantly from 3.67 × 10 2 to 7.08 × 10 3 under an ultralow optical power of 5 μW/cm 2 . Post interface modification improvements in the device photosensitivity values were attributed to the plasma induced generation of polar surface functionalities which effectively enhance the number of free holes in the channel region by trapping the photo-generated electrons at the interface. This report, thus, represents a step forward towards improving the performance of the phototransistors for their applications in high performance organic optoelectronic devices. • Polymer molecular weight influenced transistor photoresponse has been explored. • Semiconductor grain structure strongly correlates with the device photoresponse. • Strategy of dielectric air plasma treatment has been demonstrated. • Optimum plasma treatment time has been reached to achieve ultralow light sensitivity.

Scalable Fabrication of Highly Crystalline Organic Semiconductor Thin Film by Channel-Restricted Screen Printing toward the Low-Cost Fabrication of High-Performance Transistor Arrays.

Control over the morphology and crystallinity of small-molecule organic semiconductor (OSC) films is of key importance to enable high-performance organic optoelectronic devices. However, such control remains particularly challenging for solution-processed OSC devices because of the complex crystallization kinetics of small-molecule OSC materials in the dynamic flow of inks. Here, a simple yet effective channel-restricted screen-printing method is reported, which uses small-molecule OSCs/insulating polymer to yield large-grained small-molecule OSC thin-film arrays with good crystallization and preferred orientation. The use of cross-linked organic polymer banks produces a confinement effect to trigger the outward convective flow at two sides of the channel by the fast solvent evaporation, which imparts the transport of small-molecule OSC solutes and promotes the growth of small-molecule OSC crystals parallel to the channel. The small-molecule OSC thin-film array produced by screen printing exhibits excellent performance characteristics with an average mobility of 7.94 cm2 V-1 s-1 and a maximum mobility of 12.10 cm2 V-1 s-1 , which are on par with its single crystal. Finally, screen printing can be carried out using a flexible substrate, with good performance. These demonstrations bring this robust screen-printing method closer to industrial application and expand its applicability to various flexible electronics.

Direct Photolithography on Molecular Crystals for High Performance Organic Optoelectronic Devices.

Organic crystals are generated via the bottom-up self-assembly of molecular building blocks which are held together through weak noncovalent interactions. Although they revealed extraordinary charge transport characteristics, their labile nature represents a major drawback toward their integration in optoelectronic devices when the use of sophisticated patterning techniques is required. Here we have devised a radically new method to enable the use of photolithography directly on molecular crystals, with a spatial resolution below 300 nm, thereby allowing the precise wiring up of multiple crystals on demand. Two archetypal organic crystals, i.e., p-type 2,7-diphenyl[1]benzothieno[3,2- b][1]benzothiophene (Dph-BTBT) nanoflakes and n-type N, N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) nanowires, have been exploited as active materials to realize high-performance top-contact organic field-effect transistors (OFETs), inverter and p-n heterojunction photovoltaic devices supported on plastic substrate. The compatibility of our direct photolithography technique with organic molecular crystals is key for exploiting the full potential of organic electronics for sophisticated large-area devices and logic circuitries, thus paving the way toward novel applications in plastic (opto)electronics.

Universal selection rule for surfactants used in miniemulsion processes for eco-friendly and high performance polymer semiconductors

We demonstrate a universal polymer semiconductor colloid synthesis method to realize eco-friendly and high performance organic opto-electronic devices.

Precisely Patterned Growth of Ultra-Long Single-Crystalline Organic Microwire Arrays for Near-Infrared Photodetectors.

Owing to extraordinary properties, small-molecule organic micro/nanocrystals are identified to be prospective system to construct new-generation organic electronic and optoelectronic devices. Alignment and patterning of organic micro/nanocrystals at desired locations are prerequisite for their device applications in practice. Though various methods have been developed to control their directional growth and alignment, high-throughput precise positioning and patterning of the organic micro/nanocrystals at desired locations remains a challenge. Here, we report a photoresist-assisted evaporation method for large-area growth of precisely positioned ultralong methyl-squarylium (MeSq) microwire (MW) arrays. Positions as well as alignment densities of the MWs can be precisely controlled with the aid of the photoresist-template that fabricated by photolithography process. This strategy enables large-scale fabrication of organic MW arrays with nearly the same accuracy, uniformity, and reliability as photolithography. Near-infrared (NIR) photodetectors based on the MeSq MW arrays show excellent photoresponse behavior and are capable of detecting 808 nm light with high stability and reproducibility. The high on/off ratio of 1600 is significantly better than other organic nanostructure-based optical switchers. More importantly, this strategy can be readily extended to other organic molecules, revealing the great potential of photoresist-assisted evaporation method for future high-performance organic optoelectronic devices.

Thienoisoindigo-based small molecules and narrow bandgap polymers synthesized via C-H direct arylation coupling

Thienoisoindigo (TIIG) has emerged as an attractive building block for high-performance organic optoelectronic devices. Here we report the first synthesis of a series of π-conjugated TIIG-based small molecules and alternating copolymers via direct C–H arylation, which enables the efficient synthesis without use of flammable and toxic orgametallic reagents in fewer steps compared Suzuki and Stille coupling. The direct arylation coupling between TIIG and two respective mono-bromo aryl reactants clearly shows that the α-H is more reactive than the β-H in the thiophene unit of TIIG. The high regioselectivity of TIIG monomer warrants the successful synthesis of high-quality alternating copolymers with minimal structural defects. PTIIG-BT polymer synthesized via direct arylation polymerization (DAP) showed comparable molecular weight and hole mobility than the same polymer previously synthesized via Suzuki coupling. Moreover, the two new polymers (PTIIG-TF and PTIIG-2FBT) synthesized via DAP showed hole mobility up to 10−3 cm2 V−1 s−1 in FET devices fabricated and tested under ambient conditions. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2015–2031

Over 1.1 eV Workfunction Tuning of Cesium Intercalated Metal Oxides for Functioning as Both Electron and Hole Transport Layers in Organic Optoelectronic Devices

In this paper, over 1.1 eV continuous tuning of metal oxides workfunction is realized by cesium intercalation, making the metal oxide function as both electron transport layer and hole transport layer in organic optoelectronic devices. The demonstrated metal oxides are commonly used molybdenum oxide and vanadium oxide. The proposed approach of synthesizing cesium intercalated metal oxides has interesting properties of room‐temperature, ambient atmosphere, water free and solution process, favoring the formation of metal oxides as carrier transport layers at different regions in multilayered devices and large scale fabrication of organic optoelectronics at low cost. Besides the wide range of controllable workfunction adjustment, band structures, and electrical properties are investigated in detail, to understand the effects of cesium intercalation on metal oxides. The device results show that, using the proposed cesium intercalation approach, each of the two investigated metal oxides can function as both ETL and HTL in organic solar cells and organic light emitting diodes with very good device performances. Consequently, with the interesting properties in film synthesis, the proposed cesium intercalated metal oxides can achieve continuously workfunction tuning over a large range and contribute to evolution of the simple route for fabricating high performance organic optoelectronic devices.

Steric self-assembly of laterally confined organic semiconductor molecule analogues.

Self-assembly of planar molecules can be a critical route to control morphology in organic optoelectronic systems. In this study, Monte Carlo simulations were performed with polygonal disc analogues to planar semiconducting molecules under confinement. By examining statistically the molecular density and configurations of such analogues, we have observed that the symmetry of the confining medium can have a greater impact on the final densified particle configurations than the intramolecular interactions. Using the steric frustration imparted by confinement, novel self-assembled (partially) ordered phases are available. Our Monte Carlo simulations suggest new avenues to control ordering and morphology of planar molecules, which are critical for high-performance organic optoelectronic devices.

Electric double layers allow for opaque electrodes in high performance organic optoelectronic devices

We report that opaque electrodes can be used for high-performance organic optoelectronic devices, facilitated by the electric double layers (EDLs) formed in ionic liquids. For the photocell, gold/poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM)/ionic liquid/silver, the EDLs enable a large photocurrent response, without the electrodes being superimposed. The external quantum efficiency and responsivity can reach 61.2% and 272 mA/W, respectively. The specific detectivity can reach 1.9 × 1013 Jones, which is larger than silicon-based detectors. This type of architecture will renew the operation principle and material choice for organic photocells, because transparency is no longer an indispensable condition for the electrodes.

ChemInform Abstract: Organic Photoresponse Materials and Devices

AbstractReview: 452 refs.

Organic photoresponse materials and devices

Organic photoresponse materials and devices are critically important to organic optoelectronics and energy crises. The activities of photoresponse in organic materials can be summarized in three effects, photoconductive, photovoltaic and optical memory effects. Correspondingly, devices based on the three effects can be divided into (i) photoconductive devices such as photodetectors, photoreceptors, photoswitches and phototransistors, (ii) photovoltaic devices such as organic solar cells, and (iii) optical data storage devices. It is expected that this systematic analysis of photoresponse materials and devices could be a guide for the better understanding of structure-property relationships of organic materials and provide key clues for the fabrication of high performance organic optoelectronic devices, the integration of them in circuits and the application of them in renewable green energy strategies (critical review, 452 references).