N-type Polymer Semiconductors Research Articles

N-Type semiconducting polymers based on an bithiophene imide-bridged isoindigo for organic field-effect transistors

n-Type polymer semiconductors are essential for enabling high-performance organic electronic devices; however, their development still remains a challenge. Herein, we reported a new electron-deficient building block, i.e. bithiophene imide (BTI)-bridged isoindigo (IID), BTIID, through a Knoevenagel condensation reaction between the bisaldehyde BTI and the alkylated indolinones. Three donor-acceptor (D-A) copolymers were then successfully synthesized via standard Stille coupling reaction based on this novel building block, which exhibit highly planar backbones and low-lying lowest unoccupied molecular orbitals (LUMO) energy levels. When integrated into organic field-effect transistors (OFETs), these polymers demonstrated unipolar n-type transport characteristics with a highest electron mobility of 5.79 × 10−4 cm2 V−1 s−1. Our research results indicate that the design of strong electron-withdrawing units via integrating two different electron-acceptor units into one single structure represents a promising strategy for developing n-type polymer semiconductors with low-lying LUMO energy levels.

Functionalized Phenanthrene Imide-Based Polymers for n-Type Organic Thin-Film Transistors.

High-performing n-type polymers are crucial for the advance of organic electronics field, however strong electron-deficient building blocks with optimized physicochemical properties for constructing them are still limited. The imide-functionalized polycyclic aromatic hydrocarbons (PAHs) with extended π-conjugated framework, high electron deficiency and good solubility serve as promising candidates for developing high-performance n-type polymers. Among the PAHs, phenanthrene (PhA) features a well-delocalized aromatic π-system with multiple modifiable active sites . However, the PhA-based imides are seldom studied, mainly attributed to the synthetic challenge. Herein, we report two functionalized PhAs, CPOI and CPCNI, by simultaneously incorporating imide with carbonyl or dicyanomethylene onto PhA. Notably, the dicyanomethylene-modified CPCNI exhibits a well stabilized LUMO energy level (-3.84 eV), attributed to the synergetic inductive effect from imide and cyano groups. Subsequently, based on CPOI and CPCNI, two polymers PCPOI-Tz and PCPCNI-Tz were developed. Applied to organic thin-film transistors, owing to the strong electron-deficiency of CPCNI, polymer PCPCNI-Tz shows an improved electron mobility and largely decreased threshold voltage compared with PCPOI-Tz. This work affords two structurally novel electron-deficient building blocks and highlights the effectiveness of dual functionalization of PhAs with strong electron-withdrawing groups for devising n-type polymers.

Cyano-Functionalized Pyrazine: A Structurally Simple and Easily Accessible Electron-Deficient Building Block for n-Type Organic Thermoelectric Polymers.

Developing low-cost and high-performance n-type polymer semiconductors is essential to accelerate the application of organic thermoelectrics (OTEs). To achieve this objective, it is critical to design strong electron-deficient building blocks with simple structure and easy synthesis, which are essential for the development of n-type polymer semiconductors. Herein, we synthesized two cyano-functionalized highly electron-deficient building blocks, namely 3,6-dibromopyrazine-2-carbonitrile (CNPz) and 3,6-Dibromopyrazine-2,5-dicarbonitrile (DCNPz), which feature simple structures and facile synthesis. CNPz and DCNPz can be obtained via only one-step reaction and three-step reactions from cheap raw materials, respectively. Based on CNPz and DCNPz, two acceptor-acceptor (A-A) polymers, P(DPP-CNPz) and P(DPP-DCNPz) are successfully developed, featuring deep-positioned lowest unoccupied molecular orbital (LUMO) energy levels, which are beneficial to n-type organic thin-film transistors (OTFTs) and OTEs performance. An optimal unipolar electron mobility of 0.85 and 1.85 cm2 V-1 s-1 is obtained for P(DPP-CNPz) and P(DPP-DCNPz), respectively. When doped with N-DMBI, P(DPP-CNPz) and P(DPP-DCNPz) show high n-type electrical conductivities/power factors of 25.3 S cm-1 /41.4 μW m-1 K-2 , and 33.9 S cm-1 /30.4 μW m-1 K-2 , respectively. Hence, the cyano-functionalized pyrazine CNPz and DCNPz represent a new class of structurally simple, low-cost and readily accessible electron-deficient building block for constructing n-type polymer semiconductors.

Enabling close backbone stacking in near-amorphous n-type polymer semiconductors via side-chain engineering

This work demonstrates that the close π–π stacking distance is a more important factor to the electron mobility than crystallinity in near-amorphous polymer semiconductors.

A Rational Design of Isoindigo-Based Conjugated Microporous n-Type Semiconductors for High Electron Mobility and Conductivity.

The development of n-type organic semiconductors has evolved significantly slower in comparison to that of p-type organic semiconductors mainly due to the lack of electron-deficient building blocks with stability and processability. However, to realize a variety of organic optoelectronic devices, high-performance n-type polymer semiconductors are essential. Herein, conjugated microporous polymers (CMPs) comprising isoindigo acceptor units linked to benzene or pyrene donor units (BI and PI) showing n-type semiconducting behavior are reported. In addition, considering the challenges of deposition of a continuous and homogeneous thin film of CMPs for accurate Hall measurements, a plasma-assisted fabrication technique is developed to yield uniform thin films. The fully conjugated 2D networks in PI- and BI-CMP films display high electron mobility of 6.6 and 3.5cm2 V-1 s-1 , respectively. The higher carrier concentration in PI results in high conductivity (5.3mScm-1 ). Both experimental and computational studies are adequately combined to investigate structure-property relations for this intriguing class of materials in the context of organic electronics.

Synthesis and Optoelectronic Applications of dπ-pπ Conjugated Polymers with a Di-metallaaromatic Acceptor.

The development of conjugated polymers especially n-type polymer semiconductors is powered by the design and synthesis of electron-deficient building blocks. Herein, a strong acceptor building block with di-metallaaromatic structure was designed and synthesized by connecting two electron-deficient metallaaromatic units through a π-conjugated bridge. Then, a double-monomer polymerization methodology was developed for inserting it into conjugated polymer scaffolds to yield metallopolymers. The isolated well-defined model oligomers indicated polymer structures. Kinetic studies based on NMR and UV/Vis spectroscopies shed light on the polymerization process. Interestingly, the resulted metallopolymers with dπ-pπ conjugations are very promising electron transport layer materials which can boost photovoltaic performance of an organic solar cell, with power conversion efficiency up to 18.28% based on the PM6:EH-HD-4F non-fullerene system. This work not only provides a facile route to construct metallaaromatic conjugated polymers with various functional groups, but also discovers their potential applications for the first time.

Organic Thermoelectric Materials from n-Type Polymer Semiconductors.

Organic thermoelectric (OTE) materials have aroused increasing attention due to their potential application in wearable/flexible TE generator for energy harvesting. To meet the practical application of thermoelectric device, high-performance p- and n-type OTE materials are both necessary. In the past few years, n-type polymer semiconductors have witnessed the rapid development in OTE field, however, their thermoelectric performance isstill lagging behind p-type counterparts. Therefore, more efforts should be made to enhance n-type polymer TE performance, including development of novel molecule structure, synthesis of new dopants and optimization of device engineering. In this review, we highlight the effects of molecular structure and side chain length and polarity of n-type polymer semiconductors on the TE performance such as electron transport, electrical conductivity, and power factor. Finally, further perspective and challenge are also discussed to provide useful guideline for the design of high-performance n-type OTE materials.

A High-Performance n-Type Thermoelectric Polymer from C-H/C-H Oxidative Direct Arylation Polycondensation.

n-Type conjugated polymers (CPs) are crucial in the applications of organic electronics. Direct coupling of electron-deficient C-H monomer via selective C-H activation, namely C-H/C-H oxidative direct arylation polycondensation (Oxi-DArP), is an ideal approach toward such CPs. Herein, Oxi-DArP is firstly adopted to synthesize a high-performance n-type CP using a newly developed monomer, i.e., 3,6-di(thiazol-5-yl)-diketopyrrolopyrrole (Tz-5-DPP). Tz-5-DPP based homopolymer PTz-5-DPP with a molecular weight of 22 kDa has been synthesized via Oxi-DArP. After n-doping, PTz-5-DPP films exhibited electric conductivity values up to 8 S cm-1 and power factors (PFs) up to 106 μW m-1 K-2 . Notably, this PF value is the highest for n-type polymer thermoelectric materials to date. The Oxi-DArP synthesis and the excellent n-type performance of the polymer make this work an important step toward the straightforward and sustainable preparation of high-performance n-type polymer semiconductors.

Record-High Electron Mobility Exceeding 16 cm2 V- 1 s- 1 in Bisisoindigo-Based Polymer Semiconductor with a Fully Locked Conjugated Backbone.

Polymer semiconductors with mobilities exceeding 10 cm2 V- 1 s- 1 , especially ambipolar and n-type polymer semiconductors, are still rare, although they are of great importance for fabricating polymer field-effect transistors (PFETs) toward commercial high-grade electronics. Herein, two novel donor-acceptor copolymers, PNFFN-DTE and PNFFN-FDTE, are designed and synthesized based on the electron-deficient bisisoindigo (NFFN) and electron-rich dithienylethylenes (DTE or FDTE). The copolymer PNFFN-DTE, containing NFFN and DTE, possesses a partially locked polymeric conjugated backbone, whereas PNFFN-FDTE, containing NFFN and FDTE, has a fully locked one. Fluorine atoms in FDTE not only induce the formation of additional CH∙∙∙F hydrogen bonds, but also lower frontier molecular orbitals for PNFFN-FDTE. Both PNFFN-DTE and PNFFN-FDTE form more ordered molecular packing in thin films prepared from a polymer solution in bicomponent solvent containing 1,2-dichlorobenzene (DCB) and 1-chloronaphthalene (with volume ratio of 99.2/0.8) than pure DCB. The two copolymers-based flexible PFETs exhibit ambipolar charge-transport properties. Notably, the bicomponent solvent-processed PNFFN-FDTE-based PFETs afford a high electron mobility of 16.67 cm2 V-1 s-1 , which is the highest electron-transport mobility for PFETs reported so far. The high electron mobility of PNFFN-FDTE is attributed to its fully locked conjugated backbone, dense molecular packing, and much matched LUMO energy level.

Intrinsic Charge Transport for DTzTI-Based All-Acceptor Homopolymer n-Type Organic Semiconductors: Roles of Conjugation Length and Orbital Delocalization

2,2′-Bithiazolothienyl-4,4′,10,10′-tetracarboxydiimide (DTzTI), a novel imide-functionalized thiazole, is envisioned as a candidate for an excellent building block for constructing all-acceptor homopolymers, and the resulting PDTzTI, which is the polymer of DTzTI, demonstrated unipolar n-type transport with an exceptional electron mobility (μe) of 1.61 cm2 V–1 s–1. Density functional theory (DFT) and the incoherent charge-hopping model at the molecular level are used to design and investigate the model compounds DTzTI and two novel fluorine- or selenium-substituted analogues, DTzTI-2F and DTzTI-4Se, in order to better understand the roles of conjugation length and orbital delocalization for intrinsic charge transport as well as to increase the electron mobility and ambient stability of DTzTI-based polymers. According to the DFT results, increasing the conjugation length (n, number of haploids) of homopolymer molecules could significantly lower the recombination energy, decrease the ELUMO–HOMO, improve the delocalization of the frontier molecular orbitals, and raise the electron’s transfer integral (Ve) between adjacent neighboring homopolymer molecules. This would make it easier to delocalize and transport charge carriers between chains, increasing the electron-transfer efficiency. Additionally, lowering the lowest unoccupied molecular orbital (LUMO) level below −4 eV with the substitution of fluorine or selenium would be very advantageous to ambient stability. 8DTzTI, 8DTzTI-2F, and 8DTzTI-4Se are anticipated to have μe values of 23.87, 19.44, and 29.07 cm2 V–1 s–1, respectively. The performance of all the three analogues is unipolar n-type. The bigger orbital delocalization and larger transfer integral resulting from the face-to-face π–π stacking produce significant electron mobility for DTzTI-4Se, demonstrating that larger delocalization of molecular orbitals will improve intermolecular conjugation and boost charge transport characteristics. A straightforward mathematical model of mobility and conjugation length is discussed, enabling a rapid computation of the theoretical mobilities for specific homopolymers of all-acceptor n-type semiconductor materials. Another method for enhancing the electron mobility and environmental stability of DTzTI-based unipolar n-type polymer semiconductors is selenium substitution.

Aldol condensation-enabled acceptor-acceptor type bithiophene imide polymer semiconductors for n‐type field-effect transistors

While the development of high-performing n-type polymer semiconductors remains a challenge, the involving of costly transition-metal catalysts and even toxic reagents during the preparation of these materials might also cause serious environmental issues. Herein, by devising two new aldehyde-functionalized bithiophene imide (BTI)-type building blocks, we designed and synthesized two acceptor-acceptor type n-type polymers, BTI-BD and BTI2-BD, on the basis of an environmentally benign, transition metal-free and thus cost-effective aldol condensation. The resulting polymers exhibited planar and rigid backbone conformations and low-lying LUMO energy levels close to −4.0 eV, thus leading to unipolar n-type tranport character with a highest electron mobility of 1.35 × 10−3 cm2 V−1 s−1 in organic field-effect transistors for the BTI2-BD polymer which features a higher molecular ordering in solid state. Our results demonstrate that the aldol polycondensation strategy could serve as a cheaper and more ecofriendly alternative to the convential transition metal-catalyzed cross-coupling reactions for constructing n-type acceptor-acceptor polymers with unipolar n-type charge transport characteristics.

Dithiene-fused quinoxalineimide-based all-acceptor polymers for n-type organic semiconductors

Integration of bithiophene imide (BTI) and quinoxaline (Qx) units into a new electron-deficient building block, dithiene-fused quinoxalineimide (DTQI), is an effective strategy for constructing n-type polymer semiconductors.

Spatially nanoconfined N-type polymer semiconductors for stretchable ultrasensitive X-ray detection

Polymer semiconductors are promising candidates for wearable and skin-like X-ray detectors due to their scalable manufacturing, adjustable molecular structures and intrinsic flexibility. Herein, we fabricated an intrinsically stretchable n-type polymer semiconductor through spatial nanoconfinement effect for ultrasensitive X-ray detectors. The design of high-orientation nanofiber structures and dense interpenetrating polymer networks enhanced the electron-transporting efficiency and stability of the polymer semiconductors. The resultant polymer semiconductors exhibited an ultrahigh sensitivity of 1.52 × 104 μC Gyair−1 cm−2, an ultralow detection limit of 37.7 nGyair s−1 (comparable to the record-low value of perovskite single crystals), and polymer film X-ray imaging was achieved at a low dose rate of 3.65 μGyair s−1 (about 1/12 dose rate of the commercial medical chest X-ray diagnosis). Meanwhile, the hybrid semiconductor films could sustain 100% biaxial stretching strain with minimal degeneracy in photoelectrical performances. These results provide insights into future high-performance, low-cost e-skin photoelectronic detectors and imaging.

Imide-Functionalized Fluorenone and Its Cyanated Derivative Based n-Type Polymers: Synthesis, Structure-Property Correlations, and Thin-Film Transistor Performance.

The development of high-performance n-type polymer semiconductors is powered by the design and synthesis of electron-deficient building blocks with optimized physicochemical properties. By meticulously installing an imide group onto fluorene and its cyanated derivative, we report here two very electron-deficient building blocks, imide-functionalized fluorenone (FOI) and its cyanated derivative (FCNI), both featuring a deep-lying lowest unoccupied molecular orbital energy level down to -4.05 eV and highly coplanar framework, endowing them ideal units for constructing n-type polymers. Thus, a series of polymers are built from them, exhibiting unipolar n-type transport character with a highest electron mobility of 0.11 cm2 V-1 s-1 . Hence, FOI and FCNI offer a remarkable platform for accessing high-performance n-type polymers and the imide functionalization of appropriate (hetero)arenes is a powerful strategy for developing polymers with deep-lying LUMOs for n-type organic electronics.

N-type polymer semiconductors incorporating para, meta, and ortho-carborane in the conjugated backbone

We report on three novel n-type conjugated polymer semiconductors incorporating carborane in the polymer backbone and demonstrate their applicability in optoelectronic devices. Comparing the optoelectronic properties of para-, meta-, and ortho-carborane isomers revealed similar energetic characteristics between the different polymers, with the carborane unit acting as a “conjugation breaker”, confining electron delocalisation to the conjugated moieties. The fabrication of all-polymer organic photovoltaic (OPV) devices and thin-film transistors (TFTs) revealed some differences in device performance between the polymers, with the meta-carborane based polymer exhibiting superior performance in both OPV and TFT devices.

New naphthalene diimide-based n-type copolymers: The effects of conjugated side chains

Two poly(naphthalene diimide)s (PNDIs) have been developed using p-phenylene ethynylene-based comonomers with either thiophene side chains (P(NDI-BT)) or bithiophene side chains (P(NDI-BBT)), and the effects of the length of conjugated side chains on electrochemical, optical, and morphological properties were investigated. Both polymers showed additional vibronic peaks due to the interchain π-π* transition, and a high absorption coefficient (α) of the order of 105 cm−1 was observed. P(NDI-BBT) gave higher α with narrower optical energy band gaps and lower lying the lowest unoccupied molecular orbital (LUMO) energy levels compared to those in P(NDI-BT). Interestingly, lower crystallinity was observed in P(NDI-BBT) than P(NDI-BT), which is the opposite of trends shown for reported polymers with conjugated side chains. Their charge carrier mobilities and photovoltaic properties were also investigated, and provide insight of designing NDI based n-type polymer semiconductors with conjugated side chians.

Intrinsically Stretchable n-Type Polymer Semiconductors through Side Chain Engineering

Intrinsically Stretchable <i>n</i>-Type Polymer Semiconductors through Side Chain Engineering

Cyano-Functionalized Bithiophene Imide-Based n-Type Polymer Semiconductors: Synthesis, Structure–Property Correlations, and Thermoelectric Performance

n-Type polymers with deep-positioned lowest unoccupied molecular orbital (LUMO) energy levels are essential for enabling n-type organic thin-film transistors (OTFTs) with high stability and n-type organic thermoelectrics (OTEs) with high doping efficiency and promising thermoelectric performance. Bithiophene imide (BTI) and its derivatives have been demonstrated as promising acceptor units for constructing high-performance n-type polymers. However, the electron-rich thiophene moiety in BTI leads to elevated LUMOs for the resultant polymers and hence limits their n-type performance and intrinsic stability. Herein, we addressed this issue by introducing strong electron-withdrawing cyano functionality on BTI and its derivatives. We have successfully overcome the synthetic challenges and developed a series of novel acceptor building blocks, CNI, CNTI, and CNDTI, which show substantially higher electron deficiencies than does BTI. On the basis of these novel building blocks, acceptor-acceptor type homopolymers and copolymers were successfully synthesized and featured greatly suppressed LUMOs (-3.64 to -4.11 eV) versus that (-3.48 eV) of the control polymer PBTI. Their deep-positioned LUMOs resulted in improved stability in OTFTs and more efficient n-doping in OTEs for the corresponding polymers with a highest electrical conductivity of 23.3 S cm-1 and a power factor of ∼10 μW m-1 K-2. The conductivity and power factor are among the highest values reported for solution-processed molecularly n-doped polymers. The new CNI, CNTI, and CNDTI offer a remarkable platform for constructing n-type polymers, and this study demonstrates that cyano-functionalization of BTI is a very effective strategy for developing polymers with deep-lying LUMOs for high-performance n-type organic electronic devices.

Acceptor–acceptor-type conjugated polymer semiconductors

Series of acceptor–acceptor-type conjugated polymer semiconductors have been developed and applied to the electron transport of OFETs and PSCs. This study summarized their advances in polymerization methods, molecular structures and device applications. The rapid development of electronic devices such as organic field-effect transistors (OFETs) and solar cells makes the research and development of electronic transport materials imminent. The acceptor–acceptor-type (A−A-type) conjugated n-type polymer semiconductors have caught much attention due to the outstanding advantages on excellent electron-accepting capabilities, the precise adjustment of energy levels and the mass production at low fabrication cost. This article systematically reviews the polymerization methods of A−A-type polymers and the recent advancements applied in OFETs and polymer solar cells (PSCs). The analyses of the synthesis and the relationship between device performances and polymer molecular structures may provide a constructive guidance for the further development of high-performance n-type polymer materials.

Side-chain optimization of perylene diimide-thiophene random terpolymer acceptors for enhancing the photovoltaic efficiency of all-polymer solar cells

Abstract Development of n-type polymer semiconductor materials for all-polymer solar cells (all-PSCs) receives increasingly attention. A series of new n-type random terpolymers with identical perylene diimide-thiophene alternating copolymer backbone and different composition of the side chains 2-octyldodecanyl and 2-ethylhexanyl have been designed and applied as the acceptor materials in all-PSCs. The all-PSCs based on bulk heterojunction blends with side-chain engineered random terpolymer P(1/1) as acceptor and PTB7-Th as donor give a high power conversion efficiency value of 6.18% with an open-circuit voltage of 0.74 V, a short-circuit current density of 14.71 mA cm−2, and a fill factor of 0.57. The results demonstrate that design of the poly(perylenediimide-alt-thiophene) random terpolymer acceptors via side-chain engineering is an alternative strategy for obtaining high-performance photovoltaic acceptor materials.