Pinning energies of organic semiconductors in high-efficiency organic solar cells

With the emergence of new materials for high-efficiency organic solar cells(OSCs),understanding and finetuning the interface energetics become increasingly important.Precise determination of the so-called pinning energies,one of the critical characteristics of the material to predict the energy level alignment(ELA)at either electrode/organic or organic/organic interfaces,are urgently needed for the new materials.Here,pinning energies of a wide variety of newly developed donors and nonfullerene acceptors(NFAs)are measured through ultraviolet photoelectron spectroscopy.The positive pinning energies of the studied donors and the negative pinning energies of NFAs are in the same energy range of 4.3−4.6 eV,which follows the design rules developed for fullerene-based OSCs.The ELA for metal/organic and inorganic/organic interfaces follows the predicted behavior for all of the materials studied.For organic-organic heterojunctions where both the donor and the NFA feature strong intramolecular charge transfer,the pinning energies often underestimate the experimentally obtained interface vacuum level shift,which has consequences for OSC device performance.

Wavelength-tunable organic semiconductor lasers based on elastic distributed feedback gratings

Wavelength-tunable organic semiconductor lasers based on mechanically stretchable polydimethylsiloxane (PDMS) gratings were developed. The intrinsic stretchability of PDMS was explored to modulate the period of the distributed feedback gratings for fine tuning the lasing wavelength. Notably, elastic lasers based on three typical light-emitting molecules show com-parable lasing threshold values analogous to rigid devices and a continuous wavelength tunability of about 10 nm by mechanic-al stretching. In addition, the stretchability provides a simple solution for dynamically tuning the lasing wavelength in a spec-tral range that is challenging to achieve for inorganic counterparts. Our work has provided a simple and efficient method of fab-ricating tunable organic lasers that depend on stretchable distributed feedback gratings, demonstrating a significant step in the advancement of flexible organic optoelectronic devices.

Hybrid Field-Effect Transistors and Photodetectors Based on Organic Semiconductor and CsPbI_3 Perovskite Nanorods Bilayer Structure

The outstanding performances of nanostructured allinorganic CsPbX_3(X = I, Br, Cl) perovskites in optoelectronic applications can be attributed to their unique combination of a suitable bandgap, high absorption coefficient, and long carrier lifetime, which are desirable for photodetectors. However, the photosensing performances of the CsPbI_3 nanomaterials are limited by their low charge-transport efficiency. In this study, a phototransistor with a bilayer structure of an organic semiconductor layer of 2,7-dioctyl [1] benzothieno[3,2-b] [1] benzothiophene and CsPbI_3 nanorod layer was fabricated. The high-quality CsPbI_3 nanorod layer obtained using a simple dip-coating method provided decent transistor performance of the hybrid transistor device.The perovskite layer efficiently absorbs light, while the organicsemiconductor layer acts as a transport channel for injected photogenerated carriers and provides gate modulation. The hybrid phototransistor exhibits high performance owing to the synergistic function of the photogating effect and field effect in the transistor,with a photoresponsivity as high as 4300 A W^(-1), ultra-high photosensitivity of 2.2 9 106, and excellent stability over 1 month.This study provides a strategy to combine the advantages of perovskite nanorods and organic semiconductors in fabrication of high-performance photodetectors.

2D organic semiconductors, the future of green nanotechnology

The discovery of 2D organic semiconductors of atomically thin structures has attracted great attention due to their emerging optical, electronic, optoelectronic and mechatronic properties. Recent progress in such organic nanostructures has opened new opportunities for engineering material properties in many ways, such as, 0D/1D/2D nanoparticles hybridization, strain engineering, atomic doping etc. Moreover, 2D organic nanostructures exhibit a unique feature of bio–functionality and are highly sensitive to bio-analytes. Such peculiar behavior in 2D organics can be utilized to design highly-efficient bio-sensors. Also, a bio-molecular integrated electronic/optoelectronic device with enhanced performance can be attained. Furthermore, the bio-degradable, biocompatible, biometabolizable, non-toxic behaviour and natural origin of organic nanomaterials can address the current ecological concerns of increasing inorganic material based electronic waste. This review highlights the benefits of 2D organic semiconductors. Considering the importance of strategic techniques for growing thin 2D organic layers,this review summarizes progress towards this direction. The possible challenges for long-time stability and future research directions in 2D organic nano electronics/optoelectronics are also discussed. We believe that this review article provides immense research interests in organic 2D nanotechnology for exploiting green technologies in the future.

In-situ/operando characterization techniques for organic semiconductors and devices

The increasing demands of multifunctional organic electronics require advanced organic semiconducting materials to be developed and significant improvements to be made to device performance. Thus, it is necessary to gain an in-depth understanding of the film growth process, electronic states, and dynamic structure-property relationship under realistic operation conditions, which can be obtained by in-situ/operando characterization techniques for organic devices. Here, the up-todate developments in the in-situ/operando optical, scanning probe microscopy, and spectroscopy techniques that are employed for studies of film morphological evolution, crystal structures, semiconductor-electrolyte interface properties, and charge carrier dynamics are described and summarized. These advanced technologies leverage the traditional static characterizations into an in-situ and interactive manipulation of organic semiconducting films and devices without sacrificing the resolution, which facilitates the exploration of the intrinsic structure-property relationship of organic materials and the optimization of organic devices for advanced applications.

High-Performance Perovskite Quantum Dot Solar Cells Enabled by Incorporation with Dimensionally Engineered Organic Semiconductor

Perovskite quantum dots(PQDs)have been considered promising and effective photovoltaic absorber due to their superior optoelectronic properties and inherent material merits combining perovskites and QDs.However,they exhibit low moisture stability at room humidity(20-30%)owing to many surface defect sites generated by inefficient ligand exchange process.These surface traps must be re-passivated to improve both charge transport ability and moisture stability.To address this issue,PQD-organic semiconductor hybrid solar cells with suitable electrical properties and functional groups might dramatically improve the charge extraction and defect passivation.Conventional organic semiconductors are typically low-dimensional(1D and 2D)and prone to excessive self-aggregation,which limits chemical interaction with PQDs.In this work,we designed a new 3D star-shaped semiconducting material(Star-TrCN)to enhance the compatibility with PQDs.The robust bonding with Star-TrCN and PQDs is demonstrated by theoretical modeling and experimental validation.The Star-TrCN-PQD hybrid films show improved cubic-phase stability of CsPbI_(3)-PQDs via reduced surface trap states and suppressed moisture penetration.As a result,the resultant devices not only achieve remarkable device stability over 1000 h at 20-30%relative humidity,but also boost power conversion efficiency up to 16.0%via forming a cascade energy band structure.

Investigation on the Trap Signature in Organic Semiconductor Turmeric Film Through Current-Voltage Analysis

The analytical description of the trap signature in the charge conduction process of turmeric dye-based organic semiconductor has been presented in this study.An analytical explanation of the built-in potential Ⅴx-Ⅴ graph that emphasizes the presence of trapping states has been provided.Differential analysis of current-voltage(Ⅰ-Ⅴ)characteristics has also been conducted to verify the trap signature of the carrier in the device.The non-monotonous decrement of the G(Ⅴ)-Ⅴ plot verifies the trap signature.The values of trap energy(Et)and trap factor(θ)have been derived from the logarithmic Ⅰ-Ⅴ relationship.From the analysis of the semilogarithmic Ⅰ-Ⅴ plot,the barrier height(ϕbi)of the device has also been determined.The overallⅠ-Ⅴcurve has been taken into account to examine the Richardson-Schottky and Poole-Frenkel effects on the trap-assisted charge conduction process.From the results of the experiment,the Schottky effect has been observed to be effective,which leads to a bulk-limited charge conduction process.

Unsymmetrical donor—acceptor—donor—acceptor type indoline based organic semiconductors with benzothiadiazole cores for solution-processed bulk heterojunction solar cells

Bulk heterojunction(BHJ) solar cells based on small molecules have attracted potential attention due to their promise of conveniently defined structures, high absorption coefficients, solution process-ability and easy fabrication. Three D—A—D—A type organic semiconductors(WS-31,WS-32 and WS-52) are synthesized, based on the indoline donor and benzotriazole auxiliary acceptor core, along with either bare thiophene or rigid cyclopentadithiophene as π bridge, rhodanine or carbonocyanidate as end-group. Their HOMO orbitals are delocalized throughout the whole molecules. Whereas the LUMOs are mainly localized on the acceptor part of structure, which reach up to benzothiadiazole, but no distribution on indoline donor. The first excitations for WS-31 and WS-32 are mainly originated by electron transition from HOMO to LUMO level, while for WS-52, partly related to transition between HOMO and LUMO+1 level. The small organic molecules are applied as donor components in bulk heterojunction(BHJ) organic solar cells, using PC_(61)BM as acceptor material to check their photovoltaic performances. The BHJ solar cells based on blended layer of WS-31:PC_(61)BM and WS-32:PC_(61)BM processed with chloroform show overall photoelectric conversion efficiency(PCE) of 0.56% and 1.02%, respectively. WS-32 based BHJ solar cells show a higher current density originated by its relatively larger driving force of photo-induced carrier in photo-active layer to LUMO of PC_(61)BM.

Polyethylene interfacial dielectric layer for organic semiconductor single crystal based field-effect transistors

Organic semiconductor single crystals(OSSCs) have shown their promising potential in high-performance organic field-effect transistors(OFETs). The interfacial dielectric layers are critical in these OFETs as they not only govern the key semiconductor/dielectric interface quality but also determine the growth of OSSCs by their wetting properties. However, reported interfacial dielectric layers either need rigorous preparation processes, rely on certain surface chemistry reactions, or exhibit poor solvent resistance, which limits their applications in low-cost, large-area, monolithic fabrication of OSSC-based OFETs. In this work, polyethylene(PE) thin films and lamellar single crystals are utilized as the interfacial dielectric layers, providing solvent resistive but wettable surfaces that facilitate the crystallization of 6,13-bis(tri-isopropylsilylethynyl)pentacene(TIPS-PEN) and 6,13-bis(triisopropylsilylethynyl)-5,7,12,14-tetraazapentacene(TIPS-TAP). As evidenced by the presence of ambipolar behavior in TIPS-PEN single crystals and the high electron mobility(2.3 ± 0.34 cm^(2)V^(-1)s^(-1)) in TIPS-TAP single crystals, a general improvement on electron transport with PE interfacial dielectric layers is revealed, which likely associates with the chemically inertness of the saturated C-H bonds. With the advantages in both processing and device operation, the PE interfacial dielectric layer potentially offers a monolithic way for the enhancement of electron transport in solution-processed OSSC-based OFETs.

Highly efficient solution-processed organic photovoltaics enabled by improving packing behavior of organic semiconductors

Solution processability is a unique property of organic semiconductors. The compact and regular π-π stacking between molecules is paramount in the performance of organic optoelectronic devices. However, it is still a challenge to improve their stacking quality without sacrificing the solution-processability from the aspect of materials design. Here, delicately engineered additives are presented to promote the formation of ordered aggregation of conjugated molecules by regulating their nucleation and growth dynamics. Intriguingly, the long-chain BTP-eC9-4F molecules can realize ordered aggregation comparable to short-chain ones without sacrificing processability. The domain size of BTP-eC9-4F aggregation is enlarged from 24.2 to 32.2 nm in blend films.Thereby exciton diffusion and charge transport become faster, contributing to the suppression of recombination losses. As a result, a power conversion efficiency of 19.2% is achieved in D18:BTP-eC9-4F based organic photovoltaics. Our findings demonstrate a facile strategy to improve the packing quality of solution-processed organic semiconductors for high-efficiency photovoltaics and beyond photovoltaics.

Multilayered organic semiconductors for high performance optoelectronic stimulation of cells

The efficiency of devices for bioelectronic applications,including cell and tissue stimulation,is heavily dependent on the scale and the performance level.With miniaturization of stimulation electrodes,achieving a sufficiently high current pulse to elicit action potentials becomes an issue.Herein we report on our approach of vertically stacking organic p-n junctions to create highlyefficient multilayered organic semiconductor(MOS)photostimulation device.A tandem arrangement substantially increases the photovoltage and charge density without sacrificing lateral area,while not exceeding 200-500 nm of thickness.These devices generate 4 times higher voltages and at least double the charge densities over single p-n junction devices,which allow using lower light intensities for stimulation.MOS devices show an outstanding stability in the electrolyte that is extremely important for forthcoming in vivo experiments.Finally,we have validated MOS devices performance by photostimulating fibroblasts and neuroblasts,and found that using tandem devices leads to more effective action potential generation.As a result,we obtained up to 4 times enhanced effect in cell growth density using 3 p-n layered devices.These results corroborate the conclusion that MOS technology not only can achieve parity with state-of-the-art silicon devices,but also can exceed them in miniaturization and performance for biomedical applications.

1,2,5,6-Naphthalene Diimides:A Class of Promising Building Blocks for Organic Semiconductors

Organic semiconductors have drawn extensive atten-tion due to their optoelectronic properties and wide applications in organic optoelectronics.In comparison with the popular 1,4,5,8-naphthalene diimides(1,4,5,8-NDIs),the angular-shaped 1,2,5,6-NDIs have exhibited tunable photophysical properties,self-as-sembly behaviors and charge transporting properties.Due to these unique features,1,2,5,6-NDIs show great potential for construction of high performance n-func-tional materials.In this review,we highlight the recent advances and future prospects of 1,2,5,6-NDI-basedπ-systems in the field of organic optoelectronics,in-cluding molecular design,synthesis,structure-prop-erty relationships as well as the applications in high performance organic field-effect transistors,organic photovoltaics,perovskite solar cells,and so on.

Infrared Photodetectors and Image Arrays Made with Organic Semiconductors

Combining the strategies of introducing larger heteroatom,regio-regular backbone and extended branching position of side-chain,we developed polymer semiconductors(PPCPD)with narrow band-gap to construct the photosensing layer of thin-film photodiodes and image arrays.The spectral response of the resulting organic photodiodes spans from the near ultra-violet to short-wavelength infrared region.The performance of these short-wavelength infrared photodiodes in 900–1200 nm range achieved a level competitive with that of indium gallium arsenide-based inorganic crystalline detectors,exhibiting a specific detectivity of 5.55×1012 Jones at 1.15µm.High photodetectivity and quantum efficiency in photodiode with amorphous/nanocrystalline thin-films of 100–200 nm thickness enabled high pixel-density image arrays without pixel-level-patterning in the sensing layer.1×256 linear diode arrays with 25µm×25µm pixel pitch were achieved,enabling high pixel-density short-wavelength infrared imaging at room temperature.

Grain boundary engineering of organic semiconductor films in organic transistors

Organicfield-effect transistors(OFETs)show great application potential in organic electronic and optoelectronicfields due to their excellent mechanicalflexibility,low cost,and solution processing.However,grain boundaries(GBs)disrupt the aggrega-tion state of organic semiconductor(OSC)films and hinder electrical performance and stability,which limits the application of OFETs.Besides,the sensitive nature of GBs is widely used in sensing,but detailed descriptions of the GBs are scarce.This review aims tofill this knowledge gap.The role of GBs and their effect on the per-formance and stability of OFETs are analyzed,followed by a detailed summary of the characterization of GBs.Then,strategies for suppressing the negative effects of GBs and utilizing the sensitive nature of GBs for application are proposed.Finally,potential research directions for GBs in OFETs are discussed.

Unraveling the Effect of Stacking Configurations on Charge Transfer in WS_(2) and Organic Semiconductor Heterojunctions

Photoinduced interfacial charge transfer plays a critical role in energy conversion involving van der Waals(vdW)heterostructures constructed of inorganic nanostructures and organic materials.However,the effect of molecular stacking configurations on charge transfer dynamics is less understood.In this study,we demonstrated the tunability of interfacial charge separation in a type-Ⅱ heterojunction between monolayer(ML)WS_(2) and an organic semiconducting molecule[2-(3″′,4′-dimethyl-[2,2′:5′,2′:5″,2″′-quaterthiophen]-5-yl)ethan-1-ammonium halide(4Tm)]by rational design of relative stacking configurations.The assembly between ML-WS_(2) and the 4Tm molecule forms a face-to-face stacking when 4Tm molecules are in a selfaggregation state.In contrast,a face-to-edge stacking is observed when 4Tm molecule is incorporated into a 2D organic-inorganic hybrid perovskite lattice.The face-to-face stacking was proved to be more favorable for hole transfer from WS_(2) to 4Tm and led to interlayer excitons(IEs)emission.Transient absorption measurements show that the hole transfer occurs on a time scale of 150 fs.On the other hand,the face-to-edge stacking resulted in much slower hole transfer without formation of IEs.This inefficient hole transfer occurs on a similar time scale as A exciton recombination in WS_(2),leading to the formation of negative trions.These investigations offer important fundamental insights into the charge transfer processes at organic−inorganic interfaces.

Light gain amplification in microcavity organic semiconductor laser diodes under electrical pumping

Organic semiconductor is one of the most promising luminescent and lasing materials that can be chemically synthesized with a controllable performance and possess high cross-section of stimulated emission[1].Organic semiconductor laser diodes(OSLDs)can be prepared by simple processing technologies and integrated easily with other optoelectronic devices.As a result,OSLDs would

Intercalation and hybrid heterostructure integration of two-dimensional atomic crystals with functional organic semiconductor molecules

Van der Waals(vdW)integration affords semiconductor heterostructures without constrains of lattice matching and opens up a new realm of functional devices by design.A particularly interesting approach is the electrochemical intercalation of two-dimensional(2D)atomic crystal and formation of superlattices,which can provide scalable production of novel vdW heterostructures.However,this approach has been limited to the use of organic cations with non-functional aliphatic chains,therefore failed to take the advantage of the vast potentials in molecular functionalities(electronic,photonic,magnetic,etc.).Here we report the integration of 2D crystal(MoS_(2),WS_(2),highly oriented pyrolytic graphite(HOPG),WSe_(2) as model systems)with electrochemically inert organic molecules that possess semiconducting characteristics(including perylene-3,4,9,10-tetracarboxylic dianhydride(PTCDA),pentacene and fullerene),through on-chip electrochemical intercalation.An unprecedented long-range spatial feature of intercalation has been achieved,which allowed facile assembly of a vertical MoS_(2)-PTCDA-Si junction.The intercalated heterostructure shows significant modulation of the lateral transport,and leads to a molecular tunneling characteristic at the vertical direction.The general intercalation of charge neutral and functional molecules defines a versatile platform of inorganic/organic hybrid vdW heterostructures with significantly extended molecular functional building blocks,holding great promise in future design of nano/quantum devices.

Controlled 2D growth of organic semiconductor crystals by suppressing “coffee-ring” effect

Owing to enhanced charge transport efficiency arising from the ultrathin nature,two-dimensional(2D)organic semiconductor single crystals(OSSCs)are emerging as a fascinating platform for high-performance organic field-effect transistors(OFETs).However,ucoffee-ring"effect induced by an evaporation-induced convective flow near the contact line hinders the large-area growth of 2D OSSCs through a solution process.Here,we develop a new strategy of suppressing the"coffee-ring"effect by using an organic semiconductor:polymer blend solution.With the high-viscosity polymer in the organic solution,the evaporation-induced flow is remarkably weakened,ensuring the uniform molecule spreading for the 2D growth of the OSSCs.As an example,wafer-scale growth of crystalline film consisting of few-layered 2,7-didecylbenzothienobenzothiophene(C10-BTBT)crystals was successfully accomplished via blade coating.OFETs based on the crystalline film exhibited a maximum hole mobility up to 12.6 cm^2·V^-1·s^-1,along with an average hole mobility as high as 8.2 cm^2·V^-1·s^-1.Our work provides a promising strategy for the large-area growth of 2D OSSCs toward high-performance organic electronics.

Dual-function surfactant strategy for two-dimensional organic semiconductor crystals towards high-performance organic field-effect transistors

Two-dimensional(2D)organic semiconductor crystals(OSCs)are ideal platforms for investigating fundament materials as well as achieving high-performance organic field-effect transistors(OFETs).The surfactants played an important role in the 2DOSCs growth in previous studies.However,residual surfactants may cause performance degradation of devices.Herein,a simple and effective dual-function surfactant strategy is used to control the growth of large-area few-molecular-layer 2DOSCs.The introduction of phosphatidylcholine decreases the interfacial tension and improves the crystal growth dynamics,resulting in high-quality and large-area few-molecular-layer 2,6-bis(4-hexylphenyl)anthracene(C_6–DPA)2DOSC.The additive also passivates charge traps,boosting the mobility of 2DOSC-based OFETs by almost threefold.This method is also suitable for the growth of various high-quality 2DOSCs,opening up a new avenue for high-quality 2DOSCs towards high-performance OFETs.

Greater than 10 cm^(2) V^(−1) s ^(−1) : A breakthrough of organic semiconductors for field-effect transistors

Organic semiconductors have been receiving intensive attention due to the specific advantages of low-temperature processing ability,low-fabrication cost,flexibility,and so forth.The charge carrier mobility of higher than 10 cm^(2) V^(−1) s ^(−1)for organic semiconductors is of great importance to be studied since it presents a future promising research direction toward commercial microelectronic applications.With the significant progress of the discovery of novel organic molecules and the further improvements of device fabrication technology,some organic molecules can break the limit of our knowledge and show very high mobilities.In this review,organic polymers and small molecules with mobilities above 10 cm^(2) V^(−1) s ^(−1) are first introduced to provide the readers with a general understanding of the features and characteristics of high-performance organic semiconductors.Then,some important parameters,including the molecular structures,the device configurations,and the performance,are discussed in detail.Finally,the clues to obtain high mobility are summarized,and the perspective toward the future possible research directions are also provided.