In bulk-heterojunction-based organic photovoltaics, strong light-absorbing ability is one of the most important advantages of nonfullerene acceptors prior to fullerene derivatives. Herein, a series of electron-donating units was asymmetrically connected at the imide positions of perylene diimide (PDI), which is a classic-electron acceptor building block. The photoinduced electron-transfer behaviors were compared in different substitutions by steady/time-resolved spectroscopy and theoretical simulation. The electron-donating ability as well as the donor–acceptor distance has a significant impact on the photoinduced electron transition from electron-donating units to the PDI core. Because of the process of charge transfer at excited states, a fast nonradiative deactivation is observed for the PDIs with relatively strong electron substituent. These results would help us to understand the electron-transfer processes from donor to acceptor in the case of the excited electron-acceptor molecule.

Evaporated naked silver nanoparticles (Ag-NPs) were embedded in the isolated layer of PTB7-based organic field-effect transistors (OFETs), where their electric bistability behavior was successfully activated by photo-irradiation. These devices showed no obvious memory behavior in the dark; while under weak photo-irradiation (0.015 mW cm−2), memory windows of 12.5–35 V have been achieved. Different operation modes have been designed to exhibit the memory behavior and to explore the origin of the formation of a stable charge state. The immobilized photo-generated electrons supplied an additional photo-generated electric field, which confined the trapped charge in the charge storage media after the gate voltage and photo-irradiation being removed. The photo-irradiation created more charges at the interface, and the presence of Ag-NPs allowed effective charge storage of the majority carriers, which increased the drain current and lowered the gate-operating voltage. This method is a novel approach to light-activated electric bistability based charge storage.

Generally in organic electronics, high-mobility organic materials require strong intermolecular interaction for delocalized electrons, while high luminescent efficiency needs localization of excitons. Thus, simultaneous enhancement of luminescent efficiency and charge mobility are theoretically difficult. In this work the simultaneous enhancement was realized by aggregation induced complanation in conformation pre-organization of a fluorene-based copolymer. Polymer chain conformation has been tesitifed as tending to be more planar in a poor solvent induced aggregate than in a good solvent, which is the same as that in an excited and charged state. This complanation effectively decreases conformation relaxation during excitation and charge transfer, which simultaneously enhances emission efficiency and charge mobility, and eventually improves electroluminescent efficiency in the PLED. This result will inspire new ideas for conjugated polymers, especially in processing film fabrication for high efficiency devices.

Gold nanoparticles are doped into a low refractive buffer layer (PMMA) to promote the amplified spontaneous emission (ASE) performance of a red-emissive polymer (MEH-PPV) in a slab waveguide device structure: Quartz/PMMA/MEH-PPV/Air. This facile device structure with the PMMA buffer layer results in a significant ∼12 fold enhancement of the ASE intensity and lowers the gain threshold by ∼40%. Remarkably, the enhanced ASE intensity beyond the gain threshold is much stronger than the control device without gold nanoparticles. Our results show that the surface plasmon resonance of gold nanoparticles in the slab waveguide structure can effectively increase the density of excited states (gain) and decrease energy losses (loss) and can be easily fabricated by solution processing methods. Our investigations indicate that buffer layer modification with metal nanoparticles exhibits huge potential for practical applications in the performance enhancement of organic lasers.

Efficient organic electroluminescent materials with both high solid-state fluorescence efficiency and high excitons usage efficiency for use in organic light-emitting diodes (OLEDs) are relatively rare. We report two isomeric-phenylethylene compounds with different positions and spatial orientation of the –CN substituent on the vinylene groups, namely α-CN-APV and β-CN-APV. The synthesis, characterization, crystal structure, optical, electrochemical, thermal, and electroluminescence (EL) properties of the two compounds are discussed in detail. The crystal structure of the β-CN-APV shows tight solid-state organization because of two, oppositely pointed, vertically aligned, hydrogen bonds between the two cyano groups and the two vinylene hydrogens (CN⋯H–CC), while the α-CN-APV showed more flexible molecular structure with absence of vinylene hydrogen intermolecular interactions in the crystal. Such tight intermolecular stacking of β-CN-APV ensures the high solid-state fluorescence quantum efficiency. Both the compounds exhibit hybrid local and charge transfer (HLCT) excited states, which facilitate the population of singlet excitons through the reverse intersystem crossing (RISC) process from the high lying triplet states. Experimental and theoretical investigation indicated that the β-CN-APV, when compared to the α-CN-APV, showed higher solid-state fluorescence quantum efficiency and also higher excitons usage efficiency, which eventually provided much higher external quantum efficiency and brightness in the corresponding EL devices.

The electrical effects of metal nanoparticles are determined by the nature of a single nanoparticle (shape, size, surface) and their correlation with the nanoscale electronic structure. In this work, we report that electrical properties of evaporated silver nanoparticles can be controlled by different thicknesses and thermal annealing times. The particle size and size distribution were first fully characterized by the AFM and optical extinction spectra, and then their electrical properties such as current trapping and threshold voltage were studied by the organic field-effect transistor with a device structure of Si/SiO2/Ag-NPs/PMMA/PTB7/Ag. The results show that the thickness decrease and thermal annealing are effective ways for a lower charge trapping, which corresponds to smaller particle size and homogeneous particle distribution without particle aggregates. These results would be helpful for the optoelectronic applications of metal nanoparticles.

In π-conjugated optoelectronic functional materials, the excimer in the aggregate state drew much attention because of undesired low energy emission and fluorescence quenching. We report enhanced pure violet emission in meta-polyfluorene by restrained excimer formation in double-helix-like interchain entangled aggregates. It will be beneficial for optoelectric application of conjugated polymers, especially in the processing of film fabrication for high efficiency devices.

The enhanced performance of polymer light emitting diodes has been achieved by embedding an ultra-thin gold nanoparticle layer on the cathode. The performance of devices could be optimized simply by adjusting the thickness of gold, which exhibited the improved brightness from 17k cd m−2 to 20k cd m−2 and the enhanced luminous efficiency from 15.4 cd A−1 to 18.3 cd A−1, when a greenish emissive polymer P-PPV was applied as an emissive layer. The experimental results show that it is mainly attributed to the decreased holes and the increased electrons, resulting in the balanced electron–hole recombination and the shifted light emitting profile to the anode. The results demonstrate that gold nanoparticles constitute a feasible and effective route for achieving high-performance polymer optoelectronic devices through their electric properties.

The diffusion of constituent materials at interfaces is one of the key factors for device performance and stability. In this work, the effect of interfacial diffusion of a classic interfacial material PFN on device performance of polymer solar cells was studied quantitatively by doping PFN into active layer based on P3HT:PC61BM blend. The PCEs of devices with 550 ppm PFN decrease to half compared to those of the control devices without PFN, which are mainly attributed to the decrease of short-circuit current (Jsc) and fill factor (FF). Advanced analyses of equivalent circuit, absorption spectra, and atomic force microscopy indicates that the presence of PFN in the active layer increases the leakage current, decreases the aggregation of P3HT, and reduces the phase separation. This research reveals the physical mechanism of interfacial diffusion in device performance and provides a basis for further improving the performance and stability of PSCs.

An insoluble electrochemically cross-linked thin film is successfully applied as an interface layer between PEDOT:PSS and the light emitting layer in solution-processed polymer light-emitting diodes for enhanced device performance.

Inverted-type polymer light-emitting diodes with Au nanoparticles modified ITO cathode has exhibited improved brightness from 5900 to 15 000 cd m–2 (1.5-fold enhancement) and enhanced luminous efficiency from 4.4 to 10.5 cd A–1 (1.4-fold enhancement), when greenish emissive polymer-P-PPV was applied as active layer. Both the experimental and theoretical results show that it is mainly attributed to effective overlapping between local surface plasmon resonance induced by Au nanopartices and excitons quenching region at ZnO/P-PPV interface, which makes originally electrode-quenched excitons emissive and increases excitons efficiency.Keywords: Au NPs; electrode quenching; excitons; p-type semiconducting polymer; PLEDs

We reported a facile and controllable electrostatic adsorption of gold nanoparticles (Au NPs) on an indium tin oxide (ITO) anode, for applications in polymer light-emitting diodes (PLEDs). Au NPs (size 20 nm) were found to be well dispersed on a pre-treated ITO anode, and the density of Au NPs was well controlled from 0 to 60 μm−2 depending on the adsorption time. By using the Au NPs modified ITO as the anode, the optimized PLEDs showed a 60% enhancement in brightness and 40% enhancement in luminous efficiency. Our results showed that the enhancement of devices performance mainly originated from enhanced photoluminescence, the maximum of luminous efficiency appeared when the distance of Au NPs and light emission profile was about 40–80 nm, which indicated that the origin of enhanced luminescence is due to the “far-field” effect of the Au NPs layer rather than a classical localized surface plasma resonance.

The behavior of intrinsic trace aggregate species based on π–π stacking in conjugated polymer and its influencing factors are significantly important for the color purity and stability of polymer light-emitting diodes. In this work, low-energy emission has been observed in the aggregate state of meta-linked polymer poly(9,9′-spirobifluorene-3,6-diyl). The origin of low-energy emission has been testified as an excimer emission because the absorption spectra for a well-dissolved dilute solution and that of the aggregated state are quite similar in shape, and the concentration/solvent dependence of low-energy emission in emission spectra is well consistent with character of an excimer. The excimer emission is only observed in meta-linked polymers but not para-linked ones, which indicates the importance of meta-linkage and the corresponding conformation in the formation of an excimer. The meta-linkage effectively broke the conjugation, which allows a fluorene segment as the excimer building block in the polymer, and induces chain distortion, where interchain interaction is not restrained by an adjacent fluorene segment. Especially when the adjacent 9-positions of fluorenes are in the same side of the backbone (defined as cis-conformation), local big chain distortion tends to induce a more effective segment interaction.

Corrosion and exciton quenching of commonly used interlayer PEDOT:PSS for OLEDs have been paid great attention, and an effective method is to insert a thin interlayer between PEDOT:PSS and an active layer. Herein, we report an electrochemical method (ECP) to deposit such an interlayer on PEDOT:PSS. Meanwhile, such an interlayer is designed as insoluble and conductive with electronic structures of a wide-bandgap and high triplet state level as well, which are particularly suitable for solution-processed multilayer phosphorescent OLEDs. Accordingly, the molecule SimCP2 was selected as the precursor for electrochemical deposition. The oxidation coupling reactions between the cabazoles in SimCP2 result in a deposition of cross-linked thin film on PEDOT:PSS layer. A solution-processed hyperbranched polymer (HBIr) with fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) as the core and 3,6-carbazole-co-2,8-dioctyldibenzothiophene-S,S-dioxide-3,7-diyl as the branch was used as a phosphorescent active layer to verify the effect of the new interlayer. When the ECP interlayer with an optimal thickness was inserted, the luminance of PhOLEDs device increased from 5000 cd m−2 of the control device to 7368 cd m−2, leading to an enhancement of luminous efficiency (LE) and external quantum efficiency (EQE) from 24.4 cd A−1 and 9.0% of the control device to 32.5 cd A−1 and 12%, respectively. The cross-linked ECP interlayer not only effectively protects the triplet excitons from quenching at the interface between the emitting layer (EML) and PEDOT:PSS, but also blocks the superfluous holes injected and the electrons passed through the EML, which greatly improves device performances. This study paves the way for future applications of ECP interlayers towards highly efficient flexible organic optoelectronic devices.

Gold nanoparticles are doped into a low refractive buffer layer (PMMA) to promote the amplified spontaneous emission (ASE) performance of a red-emissive polymer (MEH-PPV) in a slab waveguide device structure: Quartz/PMMA/MEH-PPV/Air. This facile device structure with the PMMA buffer layer results in a significant ∼12 fold enhancement of the ASE intensity and lowers the gain threshold by ∼40%. Remarkably, the enhanced ASE intensity beyond the gain threshold is much stronger than the control device without gold nanoparticles. Our results show that the surface plasmon resonance of gold nanoparticles in the slab waveguide structure can effectively increase the density of excited states (gain) and decrease energy losses (loss) and can be easily fabricated by solution processing methods. Our investigations indicate that buffer layer modification with metal nanoparticles exhibits huge potential for practical applications in the performance enhancement of organic lasers.

An insoluble electrochemically cross-linked thin film is successfully applied as an interface layer between PEDOT:PSS and the light emitting layer in solution-processed polymer light-emitting diodes for enhanced device performance.

Evaporated naked silver nanoparticles (Ag-NPs) were embedded in the isolated layer of PTB7-based organic field-effect transistors (OFETs), where their electric bistability behavior was successfully activated by photo-irradiation. These devices showed no obvious memory behavior in the dark; while under weak photo-irradiation (0.015 mW cm−2), memory windows of 12.5–35 V have been achieved. Different operation modes have been designed to exhibit the memory behavior and to explore the origin of the formation of a stable charge state. The immobilized photo-generated electrons supplied an additional photo-generated electric field, which confined the trapped charge in the charge storage media after the gate voltage and photo-irradiation being removed. The photo-irradiation created more charges at the interface, and the presence of Ag-NPs allowed effective charge storage of the majority carriers, which increased the drain current and lowered the gate-operating voltage. This method is a novel approach to light-activated electric bistability based charge storage.

In π-conjugated optoelectronic functional materials, the excimer in the aggregate state drew much attention because of undesired low energy emission and fluorescence quenching. We report enhanced pure violet emission in meta-polyfluorene by restrained excimer formation in double-helix-like interchain entangled aggregates. It will be beneficial for optoelectric application of conjugated polymers, especially in the processing of film fabrication for high efficiency devices.

We reported a facile and controllable electrostatic adsorption of gold nanoparticles (Au NPs) on an indium tin oxide (ITO) anode, for applications in polymer light-emitting diodes (PLEDs). Au NPs (size 20 nm) were found to be well dispersed on a pre-treated ITO anode, and the density of Au NPs was well controlled from 0 to 60 μm−2 depending on the adsorption time. By using the Au NPs modified ITO as the anode, the optimized PLEDs showed a 60% enhancement in brightness and 40% enhancement in luminous efficiency. Our results showed that the enhancement of devices performance mainly originated from enhanced photoluminescence, the maximum of luminous efficiency appeared when the distance of Au NPs and light emission profile was about 40–80 nm, which indicated that the origin of enhanced luminescence is due to the “far-field” effect of the Au NPs layer rather than a classical localized surface plasma resonance.

The enhanced performance of polymer light emitting diodes has been achieved by embedding an ultra-thin gold nanoparticle layer on the cathode. The performance of devices could be optimized simply by adjusting the thickness of gold, which exhibited the improved brightness from 17k cd m−2 to 20k cd m−2 and the enhanced luminous efficiency from 15.4 cd A−1 to 18.3 cd A−1, when a greenish emissive polymer P-PPV was applied as an emissive layer. The experimental results show that it is mainly attributed to the decreased holes and the increased electrons, resulting in the balanced electron–hole recombination and the shifted light emitting profile to the anode. The results demonstrate that gold nanoparticles constitute a feasible and effective route for achieving high-performance polymer optoelectronic devices through their electric properties.

Efficient organic electroluminescent materials with both high solid-state fluorescence efficiency and high excitons usage efficiency for use in organic light-emitting diodes (OLEDs) are relatively rare. We report two isomeric-phenylethylene compounds with different positions and spatial orientation of the –CN substituent on the vinylene groups, namely α-CN-APV and β-CN-APV. The synthesis, characterization, crystal structure, optical, electrochemical, thermal, and electroluminescence (EL) properties of the two compounds are discussed in detail. The crystal structure of the β-CN-APV shows tight solid-state organization because of two, oppositely pointed, vertically aligned, hydrogen bonds between the two cyano groups and the two vinylene hydrogens (CN⋯H–CC), while the α-CN-APV showed more flexible molecular structure with absence of vinylene hydrogen intermolecular interactions in the crystal. Such tight intermolecular stacking of β-CN-APV ensures the high solid-state fluorescence quantum efficiency. Both the compounds exhibit hybrid local and charge transfer (HLCT) excited states, which facilitate the population of singlet excitons through the reverse intersystem crossing (RISC) process from the high lying triplet states. Experimental and theoretical investigation indicated that the β-CN-APV, when compared to the α-CN-APV, showed higher solid-state fluorescence quantum efficiency and also higher excitons usage efficiency, which eventually provided much higher external quantum efficiency and brightness in the corresponding EL devices.