Organic Optoelectronic Devices Research Articles

Influence of molecular stacking pattern on excited state dynamics of copper phthalocyanine films

Photophysical processes occurring within organic semiconductors is important for designing and fabricating organic solar cells. Copper phthalocyanine (CuPc) is a typical electron acceptor. In this work, the triplet exciton lifetime is prolonged by altering the molecular stacking pattern of the CuPc film. For CuPc thin films, the excited state decays are mainly determined by the triplet-triplet annihilation process. The ultrafast transient absorption measurements indicate that the primary annihilation mechanism is one-dimensional exciton diffusion collision destruction. The decay kinetics show a clearly time-dependent annihilation rate constant with γ∝t−1/2. Annihilation rate constants are determined to be γ0 = (2.87±0.02)×10−20 cm3·s−1/2 and (1.42±0.02)×10−20 cm3·s−1/2 for upright and lying-down configurations, respectively. Compared to the CuPc thin film with an upright configuration, the thin film with a lying-down configuration shows longer exciton lifetime and higher absorbance, which are beneficial to organic solar cells. The results in this work have important implications on the design and mechanistic understanding of organic optoelectronic devices.

Antisolvent treatment of copper(i) thiocyanate (CuSCN) hole transport layer for efficiency improvements in organic solar cells and light-emitting diodes

A facile antisolvent treatment of CuSCN hole transport layers can be readily applied to improve carrier transport and efficiencies of 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

Synthesis, optical and electrochemical properties of 4,4'-bibenzo[c]thiophene derivatives.

We designed and synthesized unsubstituted 4,4′-bibenzo[c]thiophene 4,4′-BBT and its silyl-substituted derivatives 1,1′-Si-4,4′-BBT and 1,1′,3,3′-Si-4,4′-BBT with one or two tert-butyldimethylsilyl groups on each thiophene ring, as new π-building blocks in emitters, photosensitizers and semiconductors for organic optoelectronic devices. The characterization of 4,4′-BBT, 1,1′-Si-4,4′-BBT and 1,1′,3,3′-Si-4,4′-BBT was successfully determined by FTIR, 1H and 13C NMR measurements, high-resolution mass spectrometry (HRMS) analysis, photoabsorption and fluorescence spectroscopy, cyclic voltammetry (CV) and density functional theory (DFT) calculations. Moreover, a single-crystal X-ray structural analysis was successfully made for 1,1′-Si-4,4′-BBT and 1,1′,3,3′-Si-4,4′-BBT. The photoabsorption and fluorescence maxima (λabsmax and λflmax) of the three 4,4′-bibenzo[c]thiophene derivatives in toluene exhibit bathochromic shifts in the order of 4,4′-BBT (359 nm and 410 nm) < 1,1′-Si-4,4′-BBT (366 nm and 420 nm) < 1,1′,3,3′-Si-4,4′-BBT (371 nm and 451 nm). The HOMO and LUMO energy levels rise in the order of 4,4′-BBT (−5.55 eV and −2.39 eV) < 1,1′-Si-4,4′-BBT (−5.45 eV and −2.34 eV) < 1,1′,3,3′-Si-4,4′-BBT (−5.34 eV and −2.30 eV), but the rise of the HOMO energy level is larger than that of the LUMO energy level, resulting in the bathochromic shift of the photoabsorption band from 4,4′-BBT to 1,1′,3,3′-Si-4,4′-BBT. The fluorescence quantum yields (Φfl) of 4,4′-BBT, 1,1′-Si-4,4′-BBT and 1,1′,3,3′-Si-4,4′-BBT in toluene are 0.41, 0.41 and 0.36, respectively. It is worth mentioning that in the solid state 1,1′-Si-4,4′-BBT and 1,1′,3,3′-Si-4,4′-BBT show relatively high Φfl-solid values of 0.22 and 0.25, respectively, whereas 4,4′-BBT exhibits poor solid-state fluorescence properties (Φfl-solid < 0.02). This work provides an efficient synthetic method for the 4,4′-bibenzo[c]thiophene derivatives and their photophysical properties in the solution and solid state, electrochemical properties and X-ray crystal structures.

B←N-Incorporated Dibenzo-azaacene with Selective Near-Infrared Absorption and Visible Transparency.

Organic compounds with selective near-infrared absorption and visible transparency are very desirable for fabrication of transparent/semitransparent optoelectronic devices. Herein, we develop a molecule with selective near-infrared absorption property, QBNA-O, in which four B←N units are incorporated to the core and two benzodioxin groups are introduced at the termini of the dibenzo-azaacene skeleton. QBNA-O exhibits a small optical gap of 1.39 eV due to the strong electron-donating benzodioxin groups and the strong electron-withdrawing B←N units. In toluene solution, QBNA-O shows a strong absorption peak at 856 nm with the full width at half maximum (FWHM) of only 41 nm as well as very weak absorption in the visible range from 380 nm to 760 nm. Thin films of QBNA-O exhibit the average visible transparency (AVT) of 78 % at the thickness of 205 nm and 90 % at the thickness of 45 nm. Solution-processed organic field-effect transistors (OFETs) of QBNA-O display ambipolar transporting behavior with the electron mobility of 0.52 cm2 V-1 s-1 and the hole mobility of 0.013 cm2 V-1 s-1 together with excellent air-stability. The selective NIR absorbing property and excellent charge transporting property imply that QBNA-O can be used to fabricate transparent organic optoelectronic devices.

Transient grating spectroscopy of photocarrier dynamics in semiconducting polymer thin films

While charge carrier dynamics and thermal management are both keys to the operational efficiency and stability for energy-related devices, experimental techniques that can simultaneously characterize both properties are still lacking. In this paper, we use laser-induced transient grating (TG) spectroscopy to characterize thin films of the archetypal organic semiconductor regioregular poly(3-hexylthiophene) and its blends with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester on glass substrates. While the thermal responses from the thin film and the substrate cannot be distinguished due to their similar thermal diffusivities, we show that the recombination dynamics of photocarriers in the organic semiconductor thin films occur on a similar timescale and can be separated from the thermal response. Our measurements indicate that the photocarrier dynamics are determined by multiple recombination processes and our extracted recombination rates are in good agreement with previous reports using other techniques. We further apply TG spectroscopy to characterize another conjugated polymer and a molecular fluorescent material to demonstrate its general applicability. Our study indicates the potential of transient grating spectroscopy to simultaneously characterize thermal transport and photocarrier dynamics in organic optoelectronic devices.

Synthesis and Photocyclization of Conjugated Diselenophene Pyrrole-2,5-dione Based Monomers for Optoelectronics

Imide-functionalized organic monomers and polymers are attractive in organic optoelectronic devices due to the strong electron-withdrawing ability of the carbonyl units. We report the synthesis of ...

Gold nanoparticles-decorated N,N'-dioctyl-3,4,9,10-perylene tetracarboxylic diimide active layer towards remarkably enhanced visible-light photoresponse of an n-type organic phototransistor

Metal nanostructures exhibit distinct physical properties from macroscopic objects, which have been widely applied to organic optoelectronic device. N, N'-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) is one of the few air-stable, high mobility n-type semiconductor, which is usually used to fabricate green or white light detectors due to its main absorption concentrated in the range 450 - 600 nm. In this paper, a high performance red light sensor was achieved by PTCDI-C8 based organic phototransistor (OPT) with the optimized gold nanoparticles (AuNPs) modified layer, the photosensitivity, photoresponsivity, specific detectivity and the external quantum efficiency (EQE) of which can reach 6.21 × 105, 24.12 A/W, 2.85 × 1011 Jones and 453.58% at 660 nm, respectively. Additionally, the AuNPs-modified OPT also gained a significant improvement in EQE under green light illumination at 532 nm compared with single PTCDI-C8 layer OPT. The photophysical mechanisms of introducing AuNPs to improve device performance were explored.

Electron-Deficient Conjugated Materials via p-π* Conjugation with Boron: Extending Monomers to Oligomers, Macrocycles, and Polymers.

The extension of conjugated organoboranes from monomeric species to oligomers, macrocycles, and polymers offers access to a plethora of fascinating new materials. The p-π* conjugation between empty orbitals on boron and the conjugated linkers not only affects the electronic structure and optical properties, but also enables mutual interactions between electron-deficient boron centers. The unique properties of these electron-deficient π-conjugated systems are exploited in highly luminescent materials, organic optoelectronic devices, and sensing applications.

Step-Economical Synthesis of Conjugated Polymer Materials Composed of Three Components: Donor, Acceptor, and π Units.

Conjugated polymers have immense potential for their use as semiconducting materials in organic optoelectronic devices. The improvement of synthetic methods for conjugated polymers is important for the practical application of conjugated polymers. For mass production, synthetic methods must be developed by considering the concerns regarding cost and environment. Reduction in the number of synthetic steps is an efficient approach to address these concerns. The utilization of direct CH functionalization is a reasonable strategy in monomer and polymer syntheses, because the prefunctionalization steps for CC bond formation can be eliminated. This review summarizes the recent developments in the efficient syntheses of conjugated polymers as well as their monomers via direct arylation (CH/CX coupling) and cross-dehydrogenative coupling (CH/CH coupling) reactions.

(Invited) Atomic Layer Deposition of Functional Films for Organic Optoelectronic Devices

Among the advanced electronic devices, transparent flexible organic electronic devices with rapid development are the most promising technologies to customers and industries. However, thin film encapsulation (TFE) and transparent oxide conductive (TOC) of organic devices still remain a big challenge, because of the difficulty in low temperature and low plasma power fabricating process. Atomic layer deposition (ALD) is increasingly used in the field of organic electronics. However, the deposition of ALD outside the temperature window still cannot be stably implemented. In this study, transient steric hindrance caused by gas-phase molecules at low-temperature (80℃) was investigated. In order to mitigate the effect of this transient hindrance, a process of consecutive short-pulses was adopted in fabricating TFE and TOC. Overall, the proposed idea would help low temperature ALD for organic electronics become mature and be widely promoted.

Transparent Hole-Transporting Materials Containing Partially Oxygen-Bridged Triphenylamine Skeletons for Efficient Perovskite Solar Cells

Transparent organic semiconductors are desirable for organic optoelectronic devices. While electrical performance is often improved by increasing the p-conjugation length, the resulting red-shifted absorption profile typically leads to loss of transparency. Therefore, it is a great challenge to design and develop organic semiconductor materials that possess both high electrical performance and transparency in visible light region. We have previously developed promising transparent hole-transporting materials (HTMs) based on the quasiplanar skeletons of partially oxygen-bridged triphenylamines.In the present study, we have designed new transparent HTMs by introducing diarylamines into the 7-position of a quasiplanar partially oxygen-bridged triphenylamine. The combination of an oxygen-bridged triphenylamine strong donor and a diarylamine weak donor makes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) localize on the oxygen-bridged triphenylamine and diarylamine, respectively, leading to minimal orbital overlap between the HOMO and the LUMO, therefore, weak absorption. Three HTMs, namely, HND-Cbz, HND-DTP, and HND-NOMe, were synthesized by connecting a partially oxygen-bridged triphenylamine structure to carbazole, dithieno[3,2-b:2′,3′-d]pyrrole, and bis(4-methoxyphenyl)amine through Buchwald-Hartwig amination with yields of 91%, 70%, and 88%, respectively. All of three materials are amorphous and thermally stable with decomposition temperatures higher than 350 °C. The vacuum-deposited thin films are transparent, with absorption edges from 405 nm to 419 nm. The space-charge limited current (SCLC) hole mobilities of HND-Cbz, HND-DTP, and HND-NOMe are 1.3 x 10- 7, 3.5 x 10- 5 and 1.4 x 10- 4 cm2 V- 1 s- 1, respectively while the HOMO energy levels are –5.34, –5.30 and –5.08 eV. The HOMO levels lie above the corresponding valence band of typical perovskites (e.g. –5.45 eV for MAPbI3 or –5.56 for Cs0.05FA0.15MA0.80PbI2.75Br0.25), making these molecules potentially good candidates for hole-transporting materials in perovskite solar cells. To verify this, perovskite solar cells having the structures ITO/SnO2/Cs0.05FA0.15MA0.80PbI2.75Br0.25/HTM/Au for the solution processed HTM and ITO/SnO2/Cs0.05FA0.15MA0.80PbI2.75Br0.25/HTM/MoO3/Au/MoO3 for the vacuum processed HTM layer, were fabricated. With HND-Cbz, HND-DTP and HND-NOMe as the HTM, the best power conversion efficiency reached 13.7%, 15.0% and 17.2%, respectively. Figure 1

The Stoichiometry of TCNQ-Based Organic Charge-Transfer Cocrystals

Organic charge-transfer cocrystals (CTCs) have attracted significant research attention due to their wide range of potential applications in organic optoelectronic devices, organic magnetic devices, organic energy devices, pharmaceutical industry, etc. The physical properties of organic charge transfer cocrystals can be tuned not only by changing the donor and acceptor molecules, but also by varying the stoichiometry between the donor and the acceptor. However, the importance of the stoichiometry on tuning the properties of CTCs has still been underestimated. In this review, single-crystal growth methods of organic CTCs with different stoichiometries are first introduced, and their physical properties, including the degree of charge transfer, electrical conductivity, and field-effect mobility, are then discussed. Finally, a perspective of this research direction is provided to give the readers a general understanding of the concept.

Effect of Bulky Functional Groups on Donor and Acceptor Interactions and their Photoluminescence Properties.

Material designs that use donor and acceptor units are often found in organic optoelectronic devices. Molecular level insight into the interactions between donors and acceptors are crucial for understanding how such interactions can modify the optical properties of the organic optoelectronic materials. In this paper, tris(4-(tert-butyl)phenyl)amine (pTPA) was synthesized as a donor in order to compare with unmodified triphenylamine (TPA) in a donor-acceptor system by having 2,4,6-triphenyl-1,3,5-triazine (TRZ) as an acceptor. Dimerization of donors and acceptors occurred in solvent when the concentration of solute is high. At 0 K, using a polarizable continuum model, the nitrogen atom of TPA is found to stack on top of the center of triazine of TRZ, whereas such alignment is offset in pTPA and TRZ. We attributed such alignment in TPA-TRZ as the result of attractive interactions between partial localization of 2pz electrons at the nitrogen atom of TPA and the π deficiency of triazine in TPA-TRZ. By taking into account random motions of the solvent effect at 300 K in quantum molecular dynamics and classical molecular dynamics simulations to interpret the marked difference in emission spectra between TPA-TRZ and pTPA-TRZ, it was revealed that the attractive interaction between pTPA and TRZ in toluene is weaker than TPA and TRZ. Because of the weaker attractive interaction between pTPA and TRZ in toluene, the dimers adopted numerous ground state conformations resulting in broad emission bands superimposed with multiple small Gaussian peaks. This is in contrast to TPA-TRZ which has only one dominant dimer conformation. This study demonstrates that the strength of intermolecular interactions between donors and acceptors should be taken into consideration in designing supramolecular structures.

High colloidal stability ZnO nanoparticles independent on solvent polarity and their application in polymer solar cells

Significant aggregation between ZnO nanoparticles (ZnO NPs) dispersed in polar and nonpolar solvents hinders the formation of high quality thin film for the device application and impedes their excellent electron transporting ability. Herein a bifunctional coordination complex, titanium diisopropoxide bis(acetylacetonate) (Ti(acac)2) is employed as efficient stabilizer to improve colloidal stability of ZnO NPs. Acetylacetonate functionalized ZnO exhibited long-term stability and maintained its superior optical and electrical properties for months aging under ambient atmospheric condition. The functionalized ZnO NPs were then incorporated into polymer solar cells with conventional structure as n-type buffer layer. The devices exhibited nearly identical power conversion efficiency regardless of the use of fresh and old (2 months aged) NPs. Our approach provides a simple and efficient route to boost colloidal stability of ZnO NPs in both polar and nonpolar solvents, which could enable structure-independent intense studies for efficient organic and hybrid optoelectronic devices.

Investigation of magneto-photoluminescence and magneto-photocurrent in rubrene-based blending devices

In order to investigate the microscopic evolution process of different types of excitons in rubrene blending systems, this study fabricated a series of organic optoelectronic devices consisting of 5,6,11,12-tetraphenylnaphthacene (rubrene) blended with Tris(2,4,6-triMethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB). It then systematically measured the photoluminescence and photocurrent magnetic field responses of the devices with different blending proportions at different temperatures and applied voltages. The study found that the temperature-dependent magneto-photoluminescence (MPL) curves of the blending devices presented a W-like line-shape under zero bias voltage, while the magneto-photocurrent (MPC) curves showed an unreported M-like line-shape. The temperature dependences of high field effect and low field effect for the MPC curves demonstrated different evolution regulations. At the same time, the amplitudes of MPC curves changed from positive to negative signs with positive bias current. By analyzing these interesting variations in the curve line-shapes, we found that the W-like line-shape curves are caused by a magnetic-field-mediated singlet fission (STT) process. The rise in the MPC curve at low magnetic field can be attributed to the effect of the intersystem crossing (ISC) process, and the fall in the MPC curve at high magnetic field is caused by the triplet-charge annihilation (TQA) process. The positive to negative change in MPC amplitude is related to the intensity of electric injection carriers. This work not only contributes to the understanding of the microscopic mechanism of photoluminescence and photocurrent in organic optoelectronic devices, it can also serve as a reference for further optimizing the performance of rubrene-based optoelectronic diodes in the future.

Research progress of rubrene as an excellent multifunctional organic semiconductor

Rubrene, a superstar in organic semiconductors, has achieved unprecedented achievements in the application of electronic devices, and research based on its various photoelectric properties is still in progress. In this review, we introduced the preparation of rubrene crystal, summarized the applications in organic optoelectronic devices with the latest research achievements based on rubrene semiconductors. An outlook of future research directions and challenges of rubrene semiconductor for applications is also provided.

Excitons and Polarons in Organic Materials

ConspectusExcitons and polarons play a central role in the electronic and optical properties of organic semiconducting polymers and molecular aggregates and are of fundamental importance in understanding the operation of organic optoelectronic devices such as solar cells and light-emitting diodes. For many conjugated organic molecules and polymers, the creation of neutral electronic excitations or ionic radicals is associated with significant nuclear relaxation, the bulk of which occurs along the vinyl-stretching mode or the aromatic-quinoidal stretching mode when conjugated rings are present. Within a polymer chain or molecular aggregate, nuclear relaxation competes with energy- and charge-transfer, mediated by electronic interactions between the constituent units (repeat units for polymers and individual chromophores for a molecular aggregate); for neutral electronic excitations, such inter-unit interactions lead to extended excited states or excitons, while for positive (or negative) charges, interactions lead to delocalized hole (or electron) polarons. The electronic coupling as well as the local coupling between electronic and nuclear degrees of freedom in both excitons and polarons can be described with a Holstein Hamiltonian. However, although excitons and polarons derive from similarly structured Hamiltonians, their optical signatures are quite distinct, largely due to differing ground states and optical selection rules.In this Account, we explore the similarities and differences in the spectral response of excitons and polarons in organic polymers and molecular aggregates. We limit our analysis to the subspace of excitons and hole polarons containing at most one excitation; hence we omit the influence of bipolarons, biexcitons, and higher multiparticle excitations. Using a generic linear array of coupled units as a model host for both excitons and polarons, we compare and contrast the optical responses of both quasiparticles, with a particular emphasis on the spatial coherence length, the length over which an exciton or polaron possesses wave-like properties important for more efficient transport. For excitons, the UV-vis absorption spectrum is generally represented by a distorted vibronic progression with H-like or J-like signatures depending on the sign of the electronic coupling, Jex. The spectrum broadens with increasing site disorder, with the spectral area preserved due to an oscillator strength sum rule. For (hole) polarons, the generally stronger electronic coupling results in a mid-IR spectrum consisting of a narrow, low-energy peak (A) with energy near a vibrational quantum of the vinyl stretching mode, and a broader, higher-energy feature (B). In contrast to the UV-vis spectrum, the mid-IR spectrum is invariant to the sign of the electronic coupling, th, and completely resistant to long-range disorder, where it remains entirely homogeneously broadened. Even in the presence of short-range disorder, the width of peak A remains surprisingly narrow as long as |th| remains sufficiently large, a property that can be understood in terms of Herzberg-Teller coupling. Unlike for excitons, for polarons, the absorption spectral area decreases with increasing short-range disorder σ (i.e., there is no oscillator sum rule) reflective of a decreasing polaron coherence length. The intensity of the low-energy peak A in relation to B is an important signature of polaron coherence. By contrast, for excitons, the absorption spectrum contains no unambiguous signs of exciton coherence. One must instead resort to the shape of the steady-state photoluminescence spectrum. The Holstein-based model has been highly successful in accounting for the spectral properties of molecular aggregates as well as conjugated polymers like poly(3-hexylthiophene) (P3HT) in the mid-IR and UV-vis spectral regions.

Synthesis and Photoluminescence study of Strontium salts with 2-Methyl-8-Hydroxyquinoline and 8-Hydroxyquinoline

The pure green and turquoise green-emitting organometallic complex of strontium(II) with 8-Hydroxyquinoline and 2-methyl-8-hydroxyquinoline were synthesized by using Co-precipitation method. The structure of Sr(8-Hq)2 and Sr(2-Me-8Hq)2 was studied with Fourier transform infrared spectroscopy, and the vibrational peak of the Sr-O bond was found at 503 cm−1 and 499 cm−1 respectively. The X-Ray diffraction spectra are taken and compared with XRD data of raw materials to confirm the formation of the complex. Further UV-Absorption was studied, and the optical band gap is 2.813eV and 2.852eV, respectively, by using Taucs plot method. The photoluminescent study carried out in solution medium, and the emission wavelength found at 526nm and 509nm, and the color is confirmed using the CIE plot method, and the emission color is pure green and turquoise green. The complex may have significant applications as emitters in organic optoelectronic devices.

Strong Plasmon-Exciton Coupling in Ag Nanoparticle-Conjugated Polymer Core-Shell Hybrid Nanostructures.

Strong plasmon–exciton coupling between tightly-bound excitons in organic molecular semiconductors and surface plasmons in metal nanostructures has been studied extensively for a number of technical applications, including low-threshold lasing and room-temperature Bose-Einstein condensates. Typically, excitons with narrow resonances, such as J-aggregates, are employed to achieve strong plasmon–exciton coupling. However, J-aggregates have limited applications for optoelectronic devices compared with organic conjugated polymers. Here, using numerical and analytical calculations, we demonstrate that strong plasmon–exciton coupling can be achieved for Ag-conjugated polymer core-shell nanostructures, despite the broad spectral linewidth of conjugated polymers. We show that strong plasmon–exciton coupling can be achieved through the use of thick shells, large oscillator strengths, and multiple vibronic resonances characteristic of typical conjugated polymers, and that Rabi splitting energies of over 1000 meV can be obtained using realistic material dispersive relative permittivity parameters. The results presented herein give insight into the mechanisms of plasmon–exciton coupling when broadband excitonic materials featuring strong vibrational–electronic coupling are employed and are relevant to organic optoelectronic devices and hybrid metal–organic photonic nanostructures.