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.

Unraveling two distinct polymorph transition mechanisms in one n-type single crystal for dynamic electronics.

Cooperativity is used by living systems to circumvent energetic and entropic barriers to yield highly efficient molecular processes. Cooperative structural transitions involve the concerted displacement of molecules in a crystalline material, as opposed to typical molecule-by-molecule nucleation and growth mechanisms which often break single crystallinity. Cooperative transitions have acquired much attention for low transition barriers, ultrafast kinetics, and structural reversibility. However, cooperative transitions are rare in molecular crystals and their origin is poorly understood. Crystals of 2-dimensional quinoidal terthiophene (2DQTT-o-B), a high-performance n-type organic semiconductor, demonstrate two distinct thermally activated phase transitions following these mechanisms. Here we show reorientation of the alkyl side chains triggers cooperative behavior, tilting the molecules like dominos. Whereas, nucleation and growth transition is coincident with increasing alkyl chain disorder and driven by forming a biradical state. We establish alkyl chain engineering as integral to rationally controlling these polymorphic behaviors for novel electronic applications.

New semi-ladder polymers for ambipolar organic light-emitting transistors.

Organic light-emitting transistors (OLETs) combine the light-emitting and gate-modulated electrical switching functions in a single device. Over the past two decades, progress has been made in developing new fluorescent semiconductors and device engineering to improve the properties of OLETs. In this paper, we give a brief review of the achievement and disadvantages of the present polymer-based OLETs, while highlighting the recent developments in semi-ladder polymers from our lab for new electroluminescent materials. The special folded molecular structures and unique aggregation states make these polymers suitable for exploration as OLET materials. A short conclusion is provided with a discussion on the challenges and future perspectives in this field.

Synergy between Photoluminescence and Charge Transport Achieved by Finely Tuning Polymeric Backbones for Efficient Light-Emitting Transistor.

The lack of design principle for developing high-performance polymer materials displaying strong fluorescence and high ambipolar charge mobilities limited their performance in organic light-emitting transistors (OLETs), electrically pumped organic laser, and other advanced electronic devices. A series of semiladder polymers by copolymerization of weak acceptors (TPTQ or TPTI) and weak donors (fluorene (F) or carbazole (C)) have been developed for luminescent and charge transporting properties. It was found that enhanced planarity, high crystallinity, and a delicate balance in interchain aggregation obtained in the new copolymer, TPTQ-F, contributed to high ambipolar charge mobilities and photoluminescent quantum yield. TPTQ-F showed excellent performance in solution-processed multilayered OLET devices with an external quantum efficiency (EQE) of 5.3%.

Finely Designed P3HT-Based Fully Conjugated Graft Polymer: Optical Measurements, Morphology, and the Faraday Effect.

In this work, our group synthesized and characterized a fully conjugated graft polymer comprising of a donor-acceptor molecular backbone and regioregular poly(3-hexylthiophene) (RRP3HT) side chains. Here, our macromonomer (MM) was synthesized via Kumada catalyst transfer polycondensation reaction based on ditin-benzodithiophene (BDT) initiator. The tin content of MM was then investigated by inductively coupled plasma-mass spectrometry (ICP-MS), which allowed for accurate control of donor/acceptor monomer ratio of 1:1 for the following Stille coupling polymerization toward our graft polymer (BP). The structures of the polymers were then characterized by gel permeation chromatography (GPC), NMR, and elemental analysis. This was followed by the characterization of optical, electrochemical, and physical properties. The magneto-optical activity of graft polymer BP was then measured. It was found that, despite the presence of the acceptor backbone, the characteristic large Faraday rotation of RRP3HT was maintained in polymer BP, which exhibited a Verdet constant of 2.39 ± 0.57 (104) °/T·m.

Design of High-Performance Organic Light-Emitting Transistors.

Organic light-emitting transistors (OLETs) integrate the light-emitting and gate-modulated electrical switching functions in a single device. Over the past decades, progress has been made in developing new fluorescent semiconductors and device engineering that pushed efficiencies of OLET devices to 8%. However, this efficiency of transistors is still too low to be competitive with organic light-emitting diodes (OLEDs). Currently, there are relatively few suitable organic fluorescent semiconductors suitable for OLETs, and the mechanism of electroluminescence in the devices is still not fully understood. In this mini-review, we discuss the state of highly efficient OLETs and plausible approaches to those unsettled problems. Since this is a mini-review, we will not be able to cover all the excellent work in the literature. Readers are encouraged to read other excellent reviews published earlier.

Foldable semi-ladder polymers: novel aggregation behavior and high-performance solution-processed organic light-emitting transistors.

A critical issue in developing high-performance organic light-emitting transistors (OLETs) is to balance the trade-off between charge transport and light emission in a semiconducting material. Although traditional materials for organic light-emitting diodes (OLEDs) or organic field-effect transistors (OFETs) have shown modest performance in OLET devices, design strategies towards high-performance OLET materials and the crucial structure-performance relationship remain unclear. Our research effort in developing cross-conjugated weak acceptor-weak donor copolymers for luminescent properties lead us to an unintentional discovery that these copolymers form coiled foldamers with intramolecular H-aggregation, leading to their exceptional OLET properties. An impressive external quantum efficiency (EQE) of 6.9% in solution-processed multi-layer OLET devices was achieved.

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.

Design of a Quinoidal Thieno[3,4-b]thiophene-Diketopyrrolopyrrole-Based Small Molecule as n-Type Semiconductor.

A quinoidal small-molecule semiconductor QDPPBTT was synthesized. Organic thin-film transistor (OTFT) devices based on QDPPBTT showed an electron mobility as high as 0.13 cm2 V-1 s-1 and Ion /Ioff ratio of 106 under ambient conditions. We suggested that 2D extended π-conjugation and quinoid-enhancing effect had an important role in electron mobility and stability of n-type FET devices, which might be a good strategy in designing new material systems.

Quinoid-Resonant Conducting Polymers Achieve High Electrical Conductivity over 4000 S cm-1 for Thermoelectrics.

New conducting polymers polythieno[3,4-b]thiophene-Tosylate (PTbT-Tos) are prepared by solution casting polymerization. Through tuning the alkyl group of TbT, the electrical conductivity can be effectively enhanced from 0.0001 to 450 S cm-1. Interestingly, the electrical conductivity of PTbT-C1-Tos increases significantly from 450 S cm-1 at room temperature to 4444 S cm-1 at 370 K, which is disparate from polyethylenedioxythiophene-Tos exhibiting metallic conducting behavior. Quasi-reversible phase transformation with temperature from 3D crystallites to lamellar-stacking coincides with the increasing electrical conductivity of PTbT-C1-Tos with heating. Methyl-substituted PTbT-Tos with the best electrical property is further utilized for thermoelectrics and a power factor as high as 263 µW m-1 K-1 is obtained. It is believed that PTbT-Tos will be a promising family of conducting polymers for solution-processed organic electronics.

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.

Efficient Solution-Processed n-Type Small-Molecule Thermoelectric Materials Achieved by Precisely Regulating Energy Level of Organic Dopants.

To achieve efficient n-type doping, three dopants, 2-Cyc-DMBI-H, (2-Cyc-DMBI)2, and (2-Cyc-DMBI-Me)2, with precisely regulated electron-donating ability were designed and synthesized. By doping with a small-molecule 2DQTT-o-OD with high electron mobility, an unexpectedly high power factor of 33.3 µW m-1 K-2 was obtained with the new dopant (2-Cyc-DMBI-Me)2. Notably, with the intrinsically low lateral thermal conductivity of 0.28 W m-1 K-1, the figure of merit was determined to be 0.02 at room temperature. Thus, we have demonstrated that small molecules with high electron mobility and low-lying LUMO energy levels can achieve high doping efficiency and excellent thermoelectric properties by doping with n-type dopants featuring highly matched energy levels and excellent miscibility.

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 %.

Diaceno[a,e]pentalenes from homoannulations of o-alkynylaryliodides utilizing a unique Pd(OAc)2/n-Bu4NOAc catalytic combination.

A heterogeneous catalytic system, Pd(OAc)2/n-Bu4NOAc, for the efficient synthesis of diaceno[a,e]pentalenes via a tandem Pd catalytic cycle is reported. The catalytic partner n-Bu4NOAc played indispensable and versatile roles, acting as both the media for recovering active Pd(0) species and their stabilizer. A series of new diaceno[a,e]pentalenes were obtained in moderate to high yields, among which the octacyclic dianthracenopentalene was found to be highly emissive.