Dynamical and statistical features of soliton interactions in the focusing Gardner equation.

In this paper, the dynamical properties of soliton interactions in the focusing Gardner equation are analyzed by the conventional two-soliton solution and its degenerate cases. Using the asymptotic expressions of interacting solitons, it is shown that the soliton polarities depend on the signs of phase parameters, and that the degenerate solitons in the mixed and rational forms have variable velocities with the time dependence of attenuation. By means of extreme value analysis, the interaction points in different interaction scenarios are presented with exact determination of positions and occurrence times of high transient waves generated in the bipolar soliton interactions. Next, with all types of two-soliton interaction scenarios considered, the interactions of two solitons with different polarities are quantitatively shown to have a greater contribution to the skewness and kurtosis than those with the same polarity. Specifically, the ratios of spectral parameters (or soliton amplitudes) are determined when the bipolar soliton interactions have the strongest effects on the skewness and kurtosis. In addition, numerical simulations are conducted to examine the properties of multi-soliton interactions and their influence on higher statistical moments, especially confirming the emergence of the soliton interactions described by the mixed and rational solutions in a denser soliton ensemble.

Efficient and air-stable n-type doping in organic semiconductors.

Chemical doping of organic semiconductors (OSCs) enables feasible tuning of carrier concentration, charge mobility, and energy levels, which is critical for the applications of OSCs in organic electronic devices. However, in comparison with p-type doping, n-type doping has lagged far behind. The achievement of efficient and air-stable n-type doping in OSCs would help to significantly improve electron transport and device performance, and endow new functionalities, which are, therefore, gaining increasing attention currently. In this review, the issue of doping efficiency and doping air stability in n-type doped OSCs was carefully addressed. We first clarified the main factors that influenced chemical doping efficiency in n-type OSCs and then explain the origin of instability in n-type doped films under ambient conditions. Doping microstructure, charge transfer, and dissociation efficiency were found to determine the overall doping efficiency, which could be precisely tuned by molecular design and post treatments. To further enhance the air stability of n-doped OSCs, design strategies such as tuning the lowest unoccupied molecular orbital (LUMO) energy level, charge delocalization, intermolecular stacking, in situ n-doping, and self-encapsulations are discussed. Moreover, the applications of n-type doping in advanced organic electronics, such as solar cells, light-emitting diodes, field-effect transistors, and thermoelectrics are being introduced. Finally, an outlook is provided on novel doping ways and material systems that are aimed at stable and efficient n-type doped OSCs.

Nonfullerene Acceptor Featuring Unique Self-Regulation Effect for Organic Solar Cells with 19 % Efficiency.

The blend nanomorphology of electron-donor (D) and -acceptor (A) materials is of vital importance to achieving highly efficient organic solar cells. Exogenous additives especially aromatic additives are always needed to further optimize the nanomorphology of blend films, which is hardly compatible with industrial manufacture. Herein, we proposed a unique approach to meticulously modulate the aggregation behavior of NFAs in both crystal and thin film nanomorphology via self-regulation effect. Nonfullerene acceptor Z9 was designed and synthesized by tethering phenyl groups on the inner side chains of the Y6 backbone. Compared with Y6, the tethered phenyl groups participated in the molecular aggregation via the π-π stacking of phenyl-phenyl and phenyl-2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2F) groups, which induced 3D charge transport with phenyl-mediated super-exchange electron coupling. Moreover, ordered molecular packing with suitable phase separation was observed in Z9-based blend films. High power conversion efficiencies (PCEs) of 19.0 % (certified PCE of 18.6 %) for Z9-based devices were achieved without additives, indicating the great potential of the self-regulation strategy in NFA design.

Design of Near-Infrared Nonfullerene Acceptor with Ultralow Nonradiative Voltage Loss for High-Performance Semitransparent Ternary Organic Solar Cells.

Semitransparent organic solar cells (ST-OSCs) are considered as one of the most valuable applications of OSCs and a strong contender in the market. However, the optical band gap of current high-performance ST-OSCs is still not low enough to achieve the optimal balance between power conversion efficiency (PCE) and average visible transmittance (AVT). An N-substituted asymmetric nonfullerene acceptor SN with over 40 nm bathochromically shifted absorption compared to Y6 was designed and synthesized, based on which the device with PM6 as donor obtained a PCE of 14.3 %, accompanied with a nonradiative voltage loss as low as 0.15 eV. Meanwhile, ternary devices with the addition of SN into PM6 : Y6 can achieve a PCE of 17.5 % with an unchanged open-circuit voltage and improved short-circuit current. Benefiting from extended NIR absorption and lowered voltage loss, ST-OSCs based on PM6 : SN : Y6 were fabricated and the optimized device demonstrated a PCE of 14.0 % at an AVT of 20.2 %, which is the highest PCE at an AVT over 20 %.

Carbon-Bridged 1,2-Bis(2-thienyl)ethylene: An Extremely Electron Rich Dithiophene Building Block Enabling Electron Acceptors with Absorption above 1000 nm for Highly Sensitive NIR Photodetectors.

The emerging donor-acceptor-donor (A-D-A)-type nonfullerene acceptors (NFAs) featuring near-infrared (NIR) photoresponsivity have greatly boosted the development of organic photovoltaics (OPVs) and display great potential for sensitive NIR organic photodetectors (OPDs). However, NIR NFAs with absorption above 1000 nm, which is of great importance for application in NIR OPDs for bioimaging, remote communication, night surveillance, etc., are still rare due to the scarcity of strong electron-rich cores. We report herein a new dithiophene building block, namely PDT, which exhibits the strongest electron-donating ability among the widely used dithiophene building blocks. By applying PDT and PDTT as the electron-donating cores and DFIC as the electron-accepting terminals, we developed two new NIR electron acceptors, PDTIC-4F and PDTTIC-4F, with optical absorptions up to 1030 nm, surpassing that of the well-known O6T-4F acceptor. In comparison with the carbon-oxygen-bridged core COi8 in O6T-4F, the synthetic complexity of PDT and PDTT is significantly reduced. Conventional OPV devices based on PM6:PDTTIC-4F display power conversion efficiencies (PCEs) of up to 10.70% with a broad external quantum efficiency (EQE) response from the ultraviolet-visible to the infrared, leading to a high short-circuit current density (Jsc) of 25.90 mA cm-2. Encouraged by these results, we investigated inverted PM6:PDTTIC-4F-based OPD devices by suppressing the dark current via modulation of the film thickness. The optimal OPD device exhibits compelling performance metrics that can compete with those of commercial silicon photodiodes: a record responsivity of 0.55 A W-1 (900 nm) among photodiode-type OPDs and excellent shot-noise-limited specific detectivity (Dsh*) of over 1013 jones.

n-Type Molecular Photovoltaic Materials: Design Strategies and Device Applications.

The use of photovoltaic technologies has been regarded as a promising approach for converting solar energy to electricity and mitigating the energy crisis, and among these, organic photovoltaics (OPVs) have attracted broad interest because of their solution processability, flexibility, light weight, and potential for large-area processing. The development of OPV materials, especially electron acceptors, has been one of the focuses in recent years. Compared with fullerene derivates, n-type non-fullerene molecules have some unique merits, such as synthetic simplicity, high tunability of the absorption and energy levels, and small energy loss. In the last 5 years, organic solar cells based on n-type non-fullerene molecules have achieved a significant breakthrough in the power conversion efficiency from approximately 4% to over 17%, which is superior to those of fullerene-based solar cells; meanwhile, n-type non-fullerene molecules have created brand new opportunities for the application of OPVs in some special situations. This Perspective analyzes the key design strategies of high-performance n-type molecular photovoltaic materials and highlights instructive examples of their various applications, including in ternary and tandem solar cells, high-efficiency semitransparent solar cells for power-generating building facades and windows, and indoor photovoltaics for driving low-power-consumption devices. Moreover, to accelerate the pace toward commercialization of OPVs, the existing challenges and future directions are also reviewed from the perspectives of efficiency, stability, and large-area fabrication.

Conjugation-Curtailing of Benzodithionopyran-Cored Molecular Acceptor Enables Efficient Air-Processed Small Molecule Solar Cells.

Small molecule solar cells (SMSCs) lag a long way behind polymer solar cells. A key limit is the less controllable morphology of small molecule materials, which can be aggravated when incorporating anisotropic nonfullerene acceptors. To fine-tune the blending morphology within SMSCs, a π-conjunction curtailing design is applied, which produces a efficient benzodithionopyran-cored molecular acceptor for nonfullerene SMSCs (NF-SMSCs). When blended with a molecular donor BDT3TR-SF to fabricate NF-SMSCs, the π-conjunction curtailed molecular acceptor NBDTP-M obtains an optimal power conversion efficiency (PCE) of up to 10.23%, which is much higher than that of NBDTTP-M of longer π-conjunction. It retains 93% of the PCE of devices fabricated in a glove box when all spin-coating and post-treating procedures are conducted in ambient air with relative humidity of 25%, which suggests the good air-processing capability of π-conjunction curtailed molecules. Detailed X-ray scattering investigations indicate that the BDT3TR-SF:NBDTP-M blend exhibits a blend morphology featuring fine interpenetrating networks with smaller domains and higher phase purity, which results in more efficient charge generation, more balanced charge transport, and less recombination compared to the low-performance BDT3TR-SF:NBDTTP-M blend. This work provides a guideline for molecular acceptors' design toward efficient, low-cost, air-processed NF-SMSCs.

Isomery-Dependent Miscibility Enables High-Performance All-Small-Molecule Solar Cells.

Nonfullerene polymer solar cells develop quickly. However, nonfullerene small-molecule solar cells (NF-SMSCs) still show relatively inferior performance, attributing to the lack of comprehensive understanding of the structure-performance relationship. To address this issue, two isomeric small-molecule acceptors, NBDTP-Fout and NBDTP-Fin , with varied oxygen position in the benzodi(thienopyran) (BDTP) core are designed and synthesized. When blended with molecular donor BDT3TR-SF, devices based on the two isomeric acceptors show disparate photovoltaic performance. Fabricated with an eco-friendly processing solvent (tetrahydrofuran), the BDT3TR-SF:NBDTP-Fout blend delivers a high power conversion efficiency of 11.2%, ranked to the top values reported to date, while the BDT3TR-SF:NBDTP-Fin blend almost shows no photovoltaic response (0.02%). With detailed investigations on inherent optoelectronic processes as well as morphological evolution, this performance disparity is correlated to the interfacial tension of the two combinations and concludes that proper interfacial tension is a key factor for effective phase separation, optimal blend morphology, and superior performance, which can be achieved by the "isomerization" design on molecular acceptors. This work reveals the importance of modulating the materials miscibility by interfacial-tension-oriented molecular design, which provides a general guideline toward efficient NF-SMSCs.

High-Performance Polymer Solar Cells Achieved by Introducing Side-Chain Heteroatom on Small-Molecule Electron Acceptor.

Side chains of photovoltaic materials play an important role in determining charge transport property, film morphology, and the corresponding device performance. In this work, two new acceptor materials, ATT-6 and ATT-7 with different side chains, m-hexylphenyl and m-hexyloxyphenyl on the indacenodithiophene, are designed and synthesized for applications in non-fullerene polymer solar cells. ATT-7 shows a higher absorption coefficient, increased crystallinity, and improved electron mobility in comparison with ATT-6. Using wide-bandgap polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PBDB-T) as donor, optimized devices based on PBDB-T:ATT-7 and PBDB-T:ATT-6 delivers power conversion efficiencies of 10.30% and 8.39%, respectively. The higher performance of ATT-7-based device can be attributed to efficient exciton dissociation, reduced bimolecular recombination, and enhanced and balanced charge carrier mobilities. These results indicate that side-chain modification is an easy but efficient way in the design of high-performance non-fullerene acceptors.

Stable Cross-Conjugated Tetrathiophene Diradical.

A tetracyano quinoidal tetrathiophene, having a central bi(thieno[3,4-c]pyrrole-4,6-dione) acceptor, has been studied. The recovered aromaticity of the thiophenes produces a diradical species with cross-conjugation between the inter-dicyano and inter-dione acceptor paths. A diradical character of y0 =0.61 and a singlet-triplet gap of -2.76 kcal mol-1 were determined. Competition between the two cross-conjugated paths enhances the disjointed character of the SOMOs and results in the confinement of the diradical to the molecular center, enabling a thermodynamic diradical stabilization featuring a half-life of 262 hours. Cross-conjugation effects have been also addressed in the anionic species (up to a radical trianion).

Air-Stable n-Type Thermoelectric Materials Enabled by Organic Diradicaloids.

Air-stable n-type thermoelectric materials are recognized as an important and challenging topic in organic thermoelectrics (OTEs) because conventional n-type OTE materials prepared by chemical doping are highly volatile upon exposure to air. Besides, doping efficiency and microstructure are hard to control with the incorporation of external dopants. We report herein the design and synthesis of unconventional n-type OTE materials based on the diradicaloids 2DQQT-S and 2DQQT-Se, which are proved to be neutral single-component organic conductors that exhibit an unprecedented air stability. Without external n-doping, a pristine film of 2DQQT-Se shows an electrical conductivity as high as 0.29 S cm-1 delivering a power factor of 1.4 µW m-1 K-2 . Under ambient conditions, no decay in electrical conductivity is observed for over 260 hours. This work demonstrates that diradicaloids are promising candidates for air-stable and high-performance OTE materials.

2,2'-Diamino-6,6'-diboryl-1,1'-binaphthyl: A Versatile Building Block for Temperature-Dependent Dual Fluorescence and Switchable Circularly Polarized Luminescence.

Temperature-dependent dual fluorescence and switchable circularly polarized luminescence (CPL) are two highly pursued but challenging properties for small organic molecules (SOMs). We herein disclose a triarylborane π-system based on a 2,2'-diamino-6,6'-diboryl-1,1'-binaphthyl scaffold that can serve as a versatile building block for achieving these two properties by simply choosing different amino groups. BNMe2 -BNaph with less bulky dimethylamino groups displays temperature-dependent dual fluorescence, and can thus be used as a highly sensitive ratiometric fluorescence thermometer. On the other hand, BNPh2 -BNaph with bulky diphenylamino groups exhibits intense fluorescence in both solution and in the solid state. A change of solvent from nonpolar cyclohexane to highly polar MeCN not only shifts the CPL position to much longer wavelength but also inverts the CPL sign. In addition, the complexation of BNPh2 -BNaph with fluoride greatly enhances the CPL intensity.

Thieno[3,4-b]thiophene-Based Novel Small-Molecule Optoelectronic Materials.

Because of the tailorable photoelectric properties derived from judicious molecular design and large-area and low-temperature processability especially on flexible substrates, design and synthesis of new organic π-functional materials is always a central topic in the field of organic optoelectronics, which siginificantly contributed to the development of high-performance optoelectronic devices such as organic photovoltaics (OPVs), organic field-effect transistors (OFETs), and organic light-emitting diodes (OLEDs). Compared with polymers, small molecules with well-defined molecular structures benefit the establishment of structure-property relationships, which may provide valuable guidelines for the design of new optoelectronic materials to further promote the device performance. New building blocks are essential for the construction of optoelectronic materials. As is well recognized, thiophene-based functional materials have played an indispensable role in the development of organic optoelectronics. Compared with six-membered benzene, five-membered thiophene shows weaker aromaticity and lower steric hindrance and may provide extra sulfur-sulfur interactions in solid state. Among various thiophene building blocks, thieno[3,4-b]thiophene (TbT) is an asymmetric fused bithiophene containing four functionalization positions, in which the proaromatic thiophene can effectively stabilize the quinoidal resonance of the aromatic thiophene. Thus, TbT exhibits a unique characteristic of quinoid-resonance effect that is powerful to modulate electronic structures. Although the application of TbT in polymer donor materials represented by PTB-7 has achieved a great success, its application in small-molecule optoelectronic materials is almost an untouched field. In this Account, we summerize the rational design of a series of TbT-based small-molecule optoelectronic materials designed and optimized by quinoid-resonance effect, regiochemistry, and side-chain engineering and demonstrate the crucial effect of TbT building blocks on the electronic structures, photophysical and charge transport properties, and photovoltaic performance. With well-defined regioregular oligothieno[3,4-b]thiopenes, we revealed the quinoid-resonance effect of the TbT moiety and its geometric origin. TbT-based small molecules exhibit full-color tunable emissions in the visible to near-infrared regions and excellent performance in OFETs and OPVs. For instance, TbT-based quinoidal molecules with near-infrared fluorescence quantum yields up to 53.1% and TbT-based aromatic molecules with full-color-tunable emissions and high fluorescence quantum yields approaching 100% in polar solvent were designed and synthesized. Solution-processable ambient-stable n-channel organic thin-film transistors based on two-dimensional π-expanded quinoidal terthiophenes with distal or proximal sulfur orientations (2DQTTs) realized a record electron mobility of 5.2 cm2 V-1 s-1. Furthermore, TbT-based electron donor and electron acceptor materials were successfully designed for OPV applications delivering high power conversion efficiencies up to 9.26% and 10.07%, respectively. We believe that new TbT-based small-molecule materials designed by a synergy of molecular engineering strategy may not only further promote OFET and OPV performance but also realize more unique applications.

Thieno[3,4-c]pyrrole-4,6-dione Oligothiophenes Have Two Crossed Paths for Electron Delocalization.

A new series of electron-deficient oligothiophenes, thieno[3,4-c]pyrrole-4,6-dione oligothiophenes (OTPDn ), from the monomer to hexamer, is reported. The optical and structural properties in the neutral states have been analyzed by absorption and emission spectroscopy together with vibrational Raman spectroscopy. In their reduced forms, these molecules could stabilize both anions and dianions in similar ways. For the dianions, two independent modes of electron conjugation of the charge excess were observed: the interdione path and the interthiophene path. The interference of these two paths highlighted the existence of a singlet diradical ground electronic state and the appearance of low-energy, thermally accessible triplet states. These results provide valuable insights into the device performance of TPD-based materials and for the rational design of new high-performance organic semiconductors.

Ullmann-Type Intramolecular C-O Reaction Toward Thieno[3,2-b]furan Derivatives with up to Six Fused Rings.

A new strategy for the efficient synthesis of thieno[3,2-b]benzofuran derivatives (15 examples) was achieved on the basis of successive regioselective intermolecular Suzuki and newly developed intramolecular Ullmann C-O reactions in up to a 70% overall yield. The fast intramolecular C-O reaction can be realized by an efficient catalytic combination of CuI/1,10-phenanthroline in up to a 97% yield. This method is suitable for the construction of highly fused thieno[3,2-b]furan-containing heterocycles including DTBDF and TTDBF. The π-π and hydrogen-bonding interactions observed for the C8-DTBDF single crystal suggest its great potential for OFET applications in the near future.

A Thieno[3,4-b]thiophene-Based Non-fullerene Electron Acceptor for High-Performance Bulk-Heterojunction Organic Solar Cells.

A thieno[3,4-b]thiophene-based electron acceptor, ATT-1, is designed and synthesized. ATT-1 exhibits a planar conjugated framework, broad absorption with a large absorption coefficient, and a slightly high LUMO energy level. Bulk-heterojunction (BHJ) solar cells based on PTB7-Th electron donor and ATT-1 electron acceptor delivered power conversion efficiencies of up to 10.07%, which is among the best performances reported for non-fullerene BHJ solar cells using PTB7-Th as the electron donor.

Dithienoindophenines: p-Type Semiconductors Designed by Quinoid Stabilization for Solar-Cell Applications.

Compared with the dominant aromatic conjugated materials, photovoltaic applications of their quinoidal counterparts featuring rigid and planar molecular structures have long been unexplored despite their narrow optical bandgaps, large absorption coefficients, and excellent charge-transport properties. The design and synthesis of dithienoindophenine derivatives (DTIPs) by stabilizing the quinoidal resonance of the parent indophenine framework is reported here. Compared with the ambipolar indophenine derivatives, DTIPs with the fixed molecular configuration are found to be p-type semiconductors exhibiting excellent unipolar hole mobilities up to 0.22 cm2 V-1 s-1 , which is one order of magnitude higher than that of the parent IP-O and is even comparable to that of QQT(CN)4-based single-crystal field-effect transistors (FET). DTIPs exhibit better photovoltaic performance than their aromatic bithieno[3,4-b]thiophene (BTT) counterparts with an optimal power-conversion efficiency (PCE) of 4.07 %.

Developing Quinoidal Fluorophores with Unusually Strong Red/Near-Infrared Emission.

Despite the dominant position of aromatic fluorophores, we report herein the design and synthesis of quinoidal fluorophores based on rarely emissive quinoidal bithiophene. Quinoidal bitheno[3,4-b]thiophene, QBTT-C6, consisting of cruciform-fused (E)-1,2-bis(5-hexylthiophen-2-yl)ethene and quinoidal bithiophene, shows a fluorescence quantum yield of 8.5%, 25-fold higher than that of the parent quinoidal QBT chromophore, but its maximum emission is at similar wavelengths. QBTT-Ar's featuring intramolecular charge transfer can further shift the maximum emission into the near-infrared region. The intramolecular charge transfer is programmably enhanced by tuning the substituents on the aryl groups from the electron-withdrawing trifluoromethyl to the electron-donating methoxy groups. Unexpectedly, a positive relationship between intramolecular charge transfer and fluorescence quantum yield is observed; as a result, QBTT-FL gives an unprecedentedly high fluorescence quantum yield of up to 53.1% for quinoidal oligothiophenes. With detailed photophysical and theoretical investigations, we demonstrate that the nonradiative intersystem crossing (S1 → T2) is significantly restrained in QBTT-Ar's, which can be attributed to the faster reverse intersystem crossing (T2 → S1) characteristic of a small activation energy. This work reveals the possibility for developing red/near-infrared fluorophores from the less explored quinoidal molecules because of their intrinsically narrow bandgaps.

Planarization, fusion, and strain of carbon-bridged phenylenevinylene oligomers enhance π-electron and charge conjugation: a dissectional vibrational Raman study.

We have used Raman spectroscopy to study the molecular and electronic structures of the radical cations and dications of carbon-bridged oligo(para-phenylenevinylene)s (COPVn, n = 1-6) possessing consecutive fused pentagons and hexagons, up to 19, along with COPV derivatives having electron-donating and -withdrawing groups. This study was made possible by the outstanding stability of the charged states of COPVs. We could untangle the effects of π-conjugation in the planar structure on the Raman frequency by distinguishing it from other structural effects, such as strain in the vinylene groups shared by the two pentagons. The analyses showed that the radical cations have benzo-quinoidal structures confined in the center of the molecule, as well as benzo-aromatic rings at the terminal sites. In contrast, dications of COPVn longer than n = 3 exhibit a biradicaloid character because of the recovery of aromaticity in the central rings and quinoidal rings at the terminal positions. These biradicaloids favor a singlet nature in their ground electronic states because of the double spin polarization. The introduction of electron-donating and -withdrawing groups on the termini of a COPV core affords, upon oxidation or reduction, a fully delocalized class III mixed valence system because of the high degree of conjugation of the COPV platform, which favors extensive charge delocalization.

Multifaceted Regioregular Oligo(thieno[3,4-b]thiophene)s Enabled by Tunable Quinoidization and Reduced Energy Band Gap.

Thiophene-based materials have occupied a crucial position in the development of organic electronics. However, the energy band gaps of oligo- and polythiophenes are difficult to modulate without resorting to push-pull electronic effects. We describe herein a new series of monodisperse oligo(thieno[3,4-b]thiophene) derivatives with well-defined regioregular structures synthesized efficiently by direct C-H arylation. These compounds show a unique palette of colors and amphoteric redox properties with widely tunable energy band gaps. The capacity to stabilize both cations and anions results in both anodic and cathodic electrochromism. Under excitation, these compounds can produce photoionized states able to interconvert into neutral triplet or form these through singlet exciton fission or intersystem crossing. These features arise from a progressive increase in quinoidization on a fully planar platform making the largest effective conjugation length among hetero-oligomers. Oligo(thieno[3,4-b]thiophene)s might represent the more distinctive family of oligothiophenes of this decade.