High-performance N-type Organic Semiconductors Research Articles

Ring Fusion Elevates the Electronic Mobility of Azabenzannulated Perylene Diimide.

The backwardness of n-type organic semiconductors still exists compared with the p-type counterparts. Thus, the development of high-performance n-type organic semiconductors is of great importance for organic electronic devices and their integrated circuits. In recent years, azabenzannulated perylene diimide (PDI), as one of immense bay-region-annulated PDI derivatives, has drawn considerable attentions. However, the electronic mobilities of azabenzannulated PDI derivatives are barely satisfactory. In this contribution, the peripheral benzene ring in azabenzannulated PDI 2 was fused to the ortho position by intramolecular C-H arylation cyclization. This endows the resultant azabenzannulated PDI 4 a planar configuration as well as electron deficient pentagonal ring. As a result, the electronic mobility of 4 is almost two orders of magnitude higher than that of the nonfused azabenzannulated PDI 2. This work shall pave a new avenue in elevating the performance of azabenzannulated PDI in organic electronics.

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.

Cyano-functionalized organic and polymeric semiconductors for high-performance n-type organic electronic devices

Cyano functionalization as an effective strategy to construct high-performance n-type semiconductors is widely investigated in organic electronics.

Benzothiadiazole versus Thiazolobenzotriazole: A Structural Study of Electron Acceptors in Solution-Processable Organic Semiconductors.

Despite the rapid progress of organic electronics, developing high-performance n-type organic semiconductors is still challenging. Donor-acceptor (D-A) type conjugated structures have been an effective molecular design strategy to achieve chemically-stable semiconductors and the appropriate choice of the acceptor units determines the electronic properties and device performances. We have now synthesized two types of A1 -D-A2 -D-A1 type conjugated molecules, namely, NDI-BTT-NDI and NDI-TBZT-NDI, with different central acceptor units. In order to investigate the effects of the central acceptor units on the charge-transporting properties, organic field-effect transistors (OFETs) were fabricated. NDI-TBZT-NDI had shallower HOMO and deeper LUMO levels than NDI-BTT-NDI. Hence, the facilitated charge injection resulted in ambipolar transistor performances with the optimized hole and electron mobilities of 0.00134 and 0.151 cm2 V-1 s-1 , respectively. In contrast, NDI-BTT-NDI displayed only an n-channel OFET performance with the electron mobility of 0.0288 cm2 V-1 s-1 . In addition, the device based on NDI-TBZT-NDI showed a superior air stability to that based on NDI-BTT-NDI. The difference in these OFET performances was reasonably explained by the contact resistance and film morphology. Overall, this study demonstrated that the TBZ acceptor is a promising building block to create n-type organic semiconductors.

Asymmetrical and symmetrical naphthalene monoimide fused perylene diimide acceptors for organic solar cells

Perylene diimide (PDI) and its derivatives have received intensive research attention due to their promising applications in organic electronics, such as organic solar cells (OSCs) and organic field-effect transistors (OFETs). Fusing PDI with other arene imide is a robust strategy for the construction of high performance n-type organic semiconductors. In this work, we report two PDI derivatives (asymmetrical FPN and symmetrical FNPN) by fusing naphthalene monoimide (NMI) moieties at the bay regions. The optoelectronic characteristics of FPN and FNPN have been investigated. Using PBDB-T as donor material, FPN and FNPN based OSCs provide optimal power conversion efficiency (PCE) of 3.13% and 1.79%, respectively. The higher PCE of FPN based OSCs may be ascribed to the asymmetrical configuration-induced large natural dipole moment and the reinforced intermolecular forces, suggesting that the introduction of asymmetric structure is an effective strategy to improve the photovoltaic performance of PDI based OSCs.

A co-crystallization strategy toward high-performance n-type organic semiconductors through charge transport switching from p-type planar azaacene derivatives

The co-crystallization strategy has offered an efficient route to achieve high-performance n-type semiconductors through charge-transport switching from pristine p-type azaacene derivatives.

Cocrystal engineering of molecular rearrangement: a “turn-on” approach for high-performance N-type organic semiconductors

From TCAF to binary α-phase cocrystals, an electron mobility of 0.06 cm2 V−1 s−1 was found. Then by molecular rearrangement, β-cocrystals from heterogeneous nucleation growth showed a remarkable mobility up to 0.88 cm2 V−1 s−1.

Selenium-containing core-expanded naphthalene diimides for high performance n-type organic semiconductors

The incorporation of heavy atoms into molecular backbone is an extremely straightforward strategy for fine-tuning the optoelectronic properties of organic semiconductors. However, it is rarely studied in n-type small molecules. Herein, by selenium substitution of NDI3HU-DTYM2, two Se-decorated core-expanded naphthalene diimides (NDI) derivatives DTYM-NDI3HU-DSYM ( 1 ) and NDI3HU-DSYM2 ( 2 ) were synthesized. In comparison with the reference S-containing compound NDI3HU-DTYM2, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of 1 and 2 were fine-tuned with ∆HOMO of about 0.2 eV, ∆LUMO of 0.1 eV and the narrowed HOMO-LUMO gaps. More surprisingly, the as-spun organic thin film transistors (OTFTs) based on 1 and 2 both showed µ e,sat values as high as 1.0 cm2 V−1 s−1, which are 2-fold higher than that of NDI3HU-DTYM2 with the same device structure and measurement conditions. In addition, the single crystal OFET devices based on Se-containing compound NDI2BO-DSYM2 showed a high µ e,sat value of 1.30 cm2 V−1 s−1. The molecular packing of NDI2BO-DSYM2 in single crystals (two-dimensional supramolecular structure formed by intermolecular Se···Se interactions) is quite different from that of a S-containing compound NDI-DTYM2 (one dimensional supramolecular structure formed by intermolecular π-π stacking). Therefore, the Se substitution can cause dramatic change about molecular stacking model, giving rise to high n-type OTFT performance. Our results demonstrated an effective strategy of the heavy atom effect for designing novel organic semiconductors.

Robust, high-performance n-type organic semiconductors

Organic semiconductors (OSCs) are important active materials for the fabrication of next-generation organic-based electronics. However, the development of n-type OSCs lags behind that of p-type OSCs in terms of charge-carrier mobility and environmental stability. This is due to the absence of molecular designs that satisfy the requirements. The present study describes the design and synthesis of n-type OSCs based on challenging molecular features involving a π-electron core containing electronegative N atoms and substituents. The unique π-electron system simultaneously reinforces both electronic and structural interactions. The current n-type OSCs exhibit high electron mobilities with high reliability, atmospheric stability, and robustness against environmental and heat stresses and are superior to other existing n-type OSCs. This molecular design represents a rational strategy for the development of high-end organic-based electronics.

Reduction Treatment of Molecular-Doped Polymer Semiconductors for High-Performance N-Channel Organic Field-Effect Transistors

Here, we study an efficient reduction treatment for improving the electron transport properties in n-type organic field-effect transistors (OFETs) based on solution-processed polymer semiconductors. If complementary-type printed electronic circuits are to be enabled, equivalently high-performance p-type and n-type organic semiconductors have to develop. Most organic semiconductors have p-type performance superior to the n-type performance. Therefore, high-performance n-type organic semiconductors and their doping process are essential. Reduced-state viologen is known to be a powerful molecular dopant via donating electrons to the conjugated polymer semiconductors. However, the reduced-state viologen readily converts to its initial ionic species under ambient conditions, which leads to insufficient n-doping performance, especially for low electron affinity organic semiconductors. N-doped polymer semiconductors using pristine viologen molecular dopants show inadequate n-channel OFET behavior. For enhancing the electron transfer doping process, solution-based reduction treatment is used to improve remarkably the performance of n-channel OFETs based on initially p-type dominant donor-acceptor-type ambipolar polymer semiconductors. This solution-processed organic active channel layer can be produced at low cost and can be applied to flexible electronic and optoelectronic devices by using various graphic art printing techniques.

Quadruply B←N-Fused Dibenzo-azaacene with High Electron Affinity and High Electron Mobility.

For many years, organoboron compounds have been expected to show excellent electron-injecting and -transporting properties. However, lowest unoccupied molecular orbital (LUMO) energy levels (ELUMO) of B-containing π-conjugated molecules are mostly higher than -4.0 eV and their electron mobilities are usually less than 10-2 cm2 V-1 s-1. In this work, we experimentally prove the remarkably high electron affinity and high electron mobility of organoboron compounds. Our strategy is to incorporate multiple boron-nitrogen coordination bonds (B←N) into azaacenes. We synthesized quadruply B←N-fused dibenzo-azaacene (QBNA) through one-pot multifold borylation cyclization reaction. The incorporation of four B←N units greatly changes the electronic structures and properties and significantly downshifts the electronic energy levels of QBNA. QBNA shows a ELUMO of as low as -4.58 eV, which is among the lowest for n-type organic semiconductors. Single-crystal organic field-effect transistors of QBNA display unipolar n-type characteristic with an electron mobility of up to 1.60 cm2 V-1 s-1 together with excellent ambient stability. This study thus provides a design strategy for high-performance n-type organic semiconductors and high electron-affinity π-systems based on organoboron chemistry.

Simplified synthetic routes for low cost and high photovoltaic performance n-type organic semiconductor acceptors

The application of polymer solar cells (PSCs) with n-type organic semiconductor as acceptor requires further improving powder conversion efficiency, increasing stability and decreasing cost of the related materials and devices. Here we report a simplified synthetic route for 4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno [1,2-b:5,6-b’] dithiophene by using the catalyst of amberlyst15. Based on this synthetic route and methoxy substitution, two low cost acceptors with less synthetic steps, simple post-treatment and high yield were synthesized. In addition, the methoxy substitution improves both yield and efficiency. The high efficiency of 13.46% was obtained for the devices with MO-IDIC-2F (3,9-bis(2-methylene-5 or 6-fluoro-(3-(1,1-dicyanomethylene)-indanone)-4,4,9,9-tetrahexyl-5,10-dimethoxyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b’] dithiophene) as acceptor. Based on the cost analysis, the PSCs based on MO-IDIC-2F possess the great advantages of low cost and high photovoltaic performance in comparison with those PSCs reported in literatures. Therefore, MO-IDIC-2F will be a promising low cost acceptor for commercial application of PSCs.

(Semi)ladder-Type Bithiophene Imide-Based All-Acceptor Semiconductors: Synthesis, Structure-Property Correlations, and Unipolar n-Type Transistor Performance.

Development of high-performance unipolar n-type organic semiconductors still remains as a great challenge. In this work, all-acceptor bithiophene imide-based ladder-type small molecules BTI n and semiladder-type homopolymers PBTI n ( n = 1-5) were synthesized, and their structure-property correlations were studied in depth. It was found that Pd-catalyzed Stille coupling is superior to Ni-mediated Yamamoto coupling to produce polymers with higher molecular weight and improved polymer quality, thus leading to greatly increased electron mobility (μe). Due to their all-acceptor backbone, these polymers all exhibit unipolar n-type transport in organic thin-film transistors, accompanied by low off-currents (10-10-10-9 A), large on/off current ratios (106), and small threshold voltages (∼15-25 V). The highest μe, up to 3.71 cm2 V-1 s-1, is attained from PBTI1 with the shortest monomer unit. As the monomer size is extended, the μe drops by 2 orders to 0.014 cm2 V-1 s-1 for PBTI5. This monotonic decrease of μe was also observed in their homologous BTI n small molecules. This trend of mobility decrease is in good agreement with the evolvement of disordered phases within the film, as revealed by Raman spectroscopy and X-ray diffraction measurements. The extension of the ladder-type building blocks appears to have a large impact on the motion freedom of the building blocks and the polymer chains during film formation, thus negatively affecting film morphology and charge carrier mobility. The result indicates that synthesizing building blocks with more extended ladder-type backbone does not necessarily lead to improved mobilities. This study marks a significant advance in the performance of all-acceptor-type polymers as unipolar electron transporting materials and provides useful guidelines for further development of (semi)ladder-type molecular and polymeric semiconductors for applications in organic electronics.

6,6′-Diaryl-substituted biazulene diimides for solution-processable high-performance n-type organic semiconductors

High electron mobility derived from dense molecular packing induced by the dipolar moment of azulene units.

Charge-transfer mobility and electrical conductivity of PANI as conjugated organic semiconductors.

The intramolecular charge transfer properties of a phenyl-end-capped aniline tetramer (ANIH) and a chloro-substituted derivative (ANICl) as organic semiconductors were theoretically studied through the first-principles calculation based on the Marcus-Hush theory. The reorganization energies, intermolecular electronic couplings, angular resolution anisotropic mobilities, and density of states of the two crystals were evaluated. The calculated results demonstrate that both ANIH and ANICl crystals show the higher electron transfer mobilities than the hole-transfer mobilities, which means that the two crystals should prefer to function as n-type organic semiconductors. Furthermore, the angle dependence mobilities of the two crystals show remarkable anisotropic character. The maximum mobility μmax of ANIH and ANICl crystals is 1.3893 and 0.0272 cm2 V-1 s-1, which appear at the orientation angles near 176°/356° and 119°/299° of a conducting channel on the a-b reference plane. It is synthetically evaluated that the ANIH crystal possesses relatively lower reorganization energy, higher electronic coupling, and electron transfer mobility, which means that the ANIH crystal may be the more ideal candidate as a high performance n-type organic semiconductor material. The systematic theoretical studies on organic crystals should be conducive to evaluating the charge-transport properties and designing higher performance organic semiconductor materials.

4,5,9,10-Pyrene Diimides: A Family of Aromatic Diimides Exhibiting High Electron Mobility and Two-Photon Excited Emission.

The design and synthesis of high-performance n-type organic semiconductors are important for the development of future organic optoelectronics. Facile synthetic routes to reach the K-region of pyrene and produce 4,5,9,10-pyrene diimide (PyDI) derivatives are reported. The PyDI derivatives exhibited efficient electron transport properties, with the highest electron mobility of up to 3.08 cm2 V-1 s-1 . The tert-butyl-substituted compounds (t-PyDI) also showed good one- and two-photon excited fluorescence properties. The PyDI derivatives are a new family of aromatic diimides that may exhibit both high electron mobility and good light-emitting properties, thus making them excellent candidates for future optoelectronics.

A Furan-Thiophene-Based Quinoidal Compound: A New Class of Solution-Processable High-Performance n-Type Organic Semiconductor.

The furan-thiophene-based quinoidal organic semiconductor, TFT-CN, is designed and synthesized. TFT-CN displays a high electron mobility of 7.7 cm(2) V(-1) s(-1) , two orders of magnitude higher than the corresponding thiophene-based derivative.

Thiophene-S,S-dioxidized indophenines as high performance n-type organic semiconductors for thin film transistors

Three thiophene-S,S-dioxidized indophenines with deep frontier energy levels are synthesized from isatins and thiophene, which exhibit n-type semiconductor performance with high electron mobility of up to 0.11 cm2 V−1 s−1 in thin film transistors.

Theoretical design of benzo[1,2-b:3,4-b′:5,6-b′′]tristhianaphthene and its derivatives as high performance organic semiconductors

In order to probe the effects of substituents (F and CN) attached to benzo[1,2-b:3,4-[Formula: see text]:5,6-[Formula: see text]]tristhianaphthene (BTTP) on their charge carrier transport properties, we investigated the characteristics of molecular structures and charge transport properties of BTTP and its derivatives (BTTP1, BTTP2, BTTP3, BTTP4, and BTTP5). Six crystal structures were predicted by the Monte Carlo-simulated annealing method with the embedded electrostatic potential charges method. Even a subtle change of geometrical structures may result in a great change of the reorganization energy. With increasing numbers of substituted fluorine atoms, the reorganization energy of the BTTP derivative increases, which is disadvantageous to the electron transport. In contrast, the attachment of the electron-withdrawing cyano groups to BTTP decreases the reorganization energy and raises the electron affinity, which is beneficial to electron injection and charge carrier stabilization. The introduction of cyano groups also results in an enhancement of [Formula: see text]–[Formula: see text] interaction and leads to an increase in the transfer integrals. Among the six compounds, the novel compound BTTP4 has the largest electron mobility (1.154[Formula: see text]cm[Formula: see text]) on account of its larger transfer integral and smaller reorganization energy, indicating that BTTP4 is a promising high-performance n-type organic semiconductor and worth to synthesize. The analysis of angular-resolution anisotropic mobilities for the BTTP and BTTP4 shows that it is helpful to control the orientations of the conducting channels for a better charge transport efficiency. This work provides a rational strategy for the design of high-performance n-type organic semiconductors from molecule to crystal structure.

Anisotropic electron-transfer mobilities in diethynyl-indenofluorene-dione crystals as high-performance n-type organic semiconductor materials: remarkable enhancement by varying substituents.

In this study, the electron-transfer properties of alkynylated indenofluorene-diones with various substituents (SiMe3, SiPr3, and SiPh3) that function as n-type organic semiconductors were comparatively investigated at the first-principles DFT level based on the Marcus-Hush theory. The reorganization energies are calculated by the adiabatic potential-energy surface method, and the coupling terms are evaluated through a direct adiabatic model. The maximum value of the electron-transfer mobility of SiPr3 is 0.485 cm(2) V(-1) s(-1), which appears at the orientation angle of the conducting channel on the reference plane a-b near to 172°/352°. The predicted maximum electron mobility value of SiPr3 is nearly 26 times larger than that of SiPh3. This may be attributed to the largest number of intermolecular π-π interactions. In addition, the mobilities in all three crystals show remarkable anisotropic behavior. The calculated results indicate that SiPr3 could be an ideal candidate as a high-performance n-type organic semiconductor material. Our investigations not only give us an opportunity to completely understand the charge transport mechanisms, but also provide guidelines for designing materials for electronic applications.