Thiazole Imide-Based All-Acceptor Homopolymer: Achieving High-Performance Unipolar Electron Transport in Organic Thin-Film Transistors

On-Demand Catalysed n-Doping of Organic Semiconductors

A new approach to control the n-doping reaction of organic semiconductors is reported using surface-functionalized gold nanoparticles (f-AuNPs) with alkylthiols acting as the catalyst only upon mild thermal activation. To demonstrate the versatility of this methodology, the reaction of the n-type dopant precursor N-DMBI-H with several molecular and polymeric semiconductors at different temperatures with/without f-AuNPs, vis-à-vis the unfunctionalized catalyst AuNPs, was investigated by spectroscopic, morphological, charge transport, and kinetic measurements as well as, computationally, the thermodynamic of catalyst activation. The combined experimental and theoretical data demonstrate that while f-AuNPs is inactive at room temperature both in solution and in the solid state, catalyst activation occurs rapidly at mild temperatures (~70 °C) and the doping reaction completes in few seconds affording large electrical conductivities (~10-140 S cm-1). The implementation of this methodology enables the use of semiconductor+dopant+catalyst solutions and will broaden the use of the corresponding n-doped films in opto-electronic devices such as thin-film transistors, electrochemical transistors, solar cells, and thermoelectrics well as guide the design of new catalysts.

Tailoring the Density of State of n-Type Conjugated Polymers through Solvent Engineering for Organic Electrochemical Transistors

Conjugated polymers with ethylene glycol-type side chains are commonly used as channel materials in organic electrochemical transistors (OECTs). To improve the performance of these materials, new chemical structures are often created through synthetic routines. Herein, we demonstrate that the OECT performance of these polymers can also be improved by changing their density-of-state (DOS) profile through solvent engineering. Depending on the solvent polarity, it solvates the backbone and side chains of the conjugated polymer differently, leading to differences in molecule orientation, π-stacking paracrystallinity, and film defects, such as grain boundaries and pinholes. This then results in a change in the DOS profile of the polymer. A more intense and narrow-width DOS distribution is usually observed in organic films with an "edge on" orientation and fewer film defects, while films with a "face on" orientation and apparent defects show a broadened DOS profile. The OECT devices that use the polymer film with a more intense and narrow-width DOS profile exhibit a better-normalized transconductance and figure-of-merit μC* than those with a broadened DOS profile (0.74 to 4.29 S cm-1 and 3.5 to 14.3 F cm-1 V-1 s-1). This study provides useful insights into how the DOS profile affects the mixed ionic-electronic conduction performance and presents a new avenue for improving n-type OECT materials.

Counterion docking: a general approach to reducing energetic disorder in doped polymeric semiconductors

Molecular doping plays an important role in controlling the carrier concentration of organic semiconductors. However, the introduction of dopant counterions often results in increased energetic disorder and traps due to the molecular packing disruption and Coulomb potential wells. To date, no general strategy has been proposed to reduce the counterion-induced structural and energetic disorder. Here, we demonstrate the critical role of non-covalent interactions (NCIs) between counterions and polymers. Employing a computer-aided approach, we identified the optimal counterions and discovered that NCIs determine their docking positions, which significantly affect the counterion-induced energetic disorder. With the optimal counterions, we successfully reduced the energetic disorder to levels even lower than that of the undoped polymer. As a result, we achieved a high n-doped electrical conductivity of over 200 S cm-1 and an eight-fold increase in the thermoelectric power factor. We found that the NCIs have substantial effects on doping efficiency, polymer backbone planarity, and Coulomb potential landscape. Our work not only provides a general strategy for identifying the most suitable counterions but also deepens our understanding of the counterion effects on doped polymeric semiconductors.

Achieving Ideal and Environmentally Stable n-Type Charge Transport in Polymer Field-Effect Transistors

Realizing ideal charge transport in field-effect transistors (FETs) of conjugated polymers is crucial for evaluating device performance, such as carrier mobility and practical applications of conjugated polymers. However, the current FETs using conjugated polymers as the active layers generally show certain non-ideal transport characteristics and poor stability. Here, ideal charge transport of n-type polymer FETs is achieved on flexible polyimide substrates by using an organic-inorganic hybrid double-layer dielectric. Deposited conjugated polymer films show highly ordered structures and low disorder, which are supported by grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine structure, and molecular dynamics simulations. Furthermore, the organic-inorganic hybrid double-layer dielectric provides low interfacial defects, leading to excellent charge transport in FETs with high electron mobility (1.49 ± 0.46 cm2 V-1 s-1) and ideal reliability factors (102 ± 7%). Fabricated polymer FETs show a self-encapsulation effect, resulting in high stability of the FET charge transport. The polymer FETs still work with high mobility above 1 cm2 V-1 s-1 after storage in air for more than 300 days. Compared with state-of-the-art conjugated polymer FETs, this work simultaneously achieves ideal charge transport and environmental stability in n-type polymer FETs, facilitating rapid device optimization of high-performance polymer electronics.

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.

Multi-Selenophene Incorporated Thiazole Imide-Based n-Type Polymers for High-Performance Organic Thermoelectrics

Developing polymers with high electrical conductivity (σ) after n-doping is a great challenge for the advance of the field of organic thermoelectrics (OTEs). Herein, we report a series of thiazole imide-based n-type polymers by gradually increasing selenophene content in polymeric backbone. Thanks to the strong intramolecular noncovalent N⋅⋅⋅S interaction and enhanced intermolecular Se⋅⋅⋅Se interaction, with the increase of selenophene content, the polymers show gradually lowered LUMOs, more planar backbone, and improved film crystallinity versus the selenophene-free analogue. Consequently, polymer PDTzSI-Se with the highest selenophene content achieves a champion σ of 164.0 S cm-1 and a power factor of 49.0 μW m-1  K-2 in the series when applied in OTEs after n-doping. The σ value is the highest one for n-type donor-acceptor OTE materials reported to date. Our work indicates that selenophene substitution is a powerful strategy for developing high-performance n-type OTE materials and selenophene incorporated thiazole imides offer an excellent platform in enabling n-type polymers with high backbone coplanarity, deep-lying LUMO and enhanced mobility/conductivity.

Dipyridyl-Fused Quinoxalineimide (DPQI): A Strong Electron-Withdrawing Building Block for n-Type Polymer Semiconductors

Exploration of new electron-withdrawing building blocks plays a key role in the development of n-type organic semiconductors. Herein, a strong electron-withdrawing building block, dipyridyl-fused quinoxalineimide (DPQI), was successfully designed and synthesized. Single-crystal structure reveals that DPQI molecule possesses a completely planar backbone, which is beneficial for charge transport. For comparison, dibenzo-fused quinoxalineimide (DBQI) was also synthesized. The frontier molecular orbital (FMO) energy levels downshift with the incorporation of nitrogen atoms onto the π-conjugated backbone of quinoxalineimide. Two acceptor-acceptor (or all-acceptor) polymers P(BTI-DBQI) and P(BTI-DPQI) based on DBQI and DPQI were synthesized, respectively. Two polymers exhibit deep lowest-unoccupied molecular orbital (LUMO) levels (~-3.5 eV). Additionally, P(BTI-DPQI) exhibits unipolar n-type charge transport with μe of 1.4×10-4  cm2  V-1  s-1 in the organic field-effect transistors (OFET), which render them highly attractive for developing n-type semiconductors device. This work demonstrates that DPQI is a promising building block for constructing n-type polymer semiconductors.

Rational Design of Novel Conjugated Terpolymers Based on Diketopyrrolopyrrole and Their Applications to Organic Thin-Film Transistors

Organic polymer semiconductor materials, due to their good chemical modifiability, can be easily tuned by rational molecular structure design to modulate their material properties, which, in turn, affects the device performance. Here, we designed and synthesized a series of materials based on terpolymer structures and applied them to organic thin-film transistor (OTFT) device applications. The four polymers, obtained by polymerization of three monomers relying on the Stille coupling reaction, shared comparable molecular weights, with the main structural difference being the ratio of the thiazole component to the fluorinated thiophene (Tz/FS). The conjugated polymers exhibited similar energy levels and thermal stability; however, their photochemical and crystalline properties were distinctly different, leading to significantly varied mobility behavior. Materials with a Tz/FS ratio of 50:50 showed the highest electron mobility, up to 0.69 cm2 V-1 s-1. Our investigation reveals the fundamental relationship between the structure and properties of materials and provides a basis for the design of semiconductor materials with higher carrier mobility.

Semiconducting Polymers Based on Simple Electron-Deficient Cyanated trans-1,3-Butadienes for Organic Field-Effect Transistors

Developing high-performance but low-cost n-type polymers remains a significant challenge in the commercialization of organic field-effect transistors (OFETs). To achieve this objective, it is essential to design the key electron-deficient units with simple structures and facile preparation processes, which can facilitate the production of low-cost n-type polymers. Herein, by sequentially introducing fluorine and cyano functionalities onto trans-1,3-butadiene, we developed a series of structurally simple but highly electron-deficient building blocks, namely 1,4-dicyano-butadiene (CNDE), 3-fluoro-1,4-dicyano-butadiene (CNFDE), and 2,3-difluoro-1,4-dicyano-butadiene (CNDFDE), featuring a highly coplanar backbone and deep-positioned lowest unoccupied molecular orbital (LUMO) energy levels (-3.03-4.33 eV), which render them highly attractive for developing n-type semiconducting polymers. Notably, all these electron-deficient units can be easily accessed by a two-step high-yield synthetic procedure from low-cost raw materials, thus rendering them highly promising candidates for commercial applications. Upon polymerization with diketopyrrolopyrrole (DPP), three copolymers were developed that demonstrated unipolar n-type transport characteristics in OFETs with the highest electron mobility of >1 cm2  V-1  s-1 . Hence, CNDE, CNFDE, and CNDFDE represent a class of novel, simple, and efficient electron-deficient units for constructing low-cost n-type polymers, thereby providing valuable insight for OFET applications.

Designing Electron-Deficient Diketone Unit Based Non-Fused Ring Acceptors with Amplified Optoelectronic Features for Highly Efficient Organic Solar Cells: A DFT Study

Organic solar cells (OSCs) made of electron-acceptor and electron-donor materials have significantly developed in the last decade, demonstrating their enormous potential in cutting-edge optoelectronic applications. Consequently, we designed seven novel non-fused ring electron acceptors (NFREAs) (BTIC-U1 to BTIC-U7) using synthesized electron-deficient diketone units and reported end-capped acceptors, a viable route for augmented optoelectronic properties. The DFT and TDDFT approaches were used to measure the power conversion efficiency (PCE), open circuit voltage (Voc), reorganization energies (λ_h, λ_e), fill factor (FF), light harvesting efficiency (LHE) and to evaluate the potential usage of proposed compounds in solar cell applications. The findings confirmed that the photovoltaic, photophysical, and electronic properties of the designed molecules BTIC-U1 to BTIC-U7 are superior to those of reference BTIC-R. The TDM analysis demonstrates a smooth flow of charge from the core to the acceptor groups. Charge transfer analysis of the BTIC-U1:PTB7-Th blend revealed orbital superposition and successful charge transfer from HOMO (PTB7-Th) to LUMO (BTIC-U1). The BTIC-U5 and BTIC-U7 outperformed the reference BTIC-R and other developed molecules in terms of PCE (23.29% and 21.18%), FF (0.901 and 0.894), normalized Voc (48.674 and 44.597), and Voc (1.261 eV and 1.155 eV). The proposed compounds enclose high electron and hole transfer mobilities, making them the ideal candidate for use with PTB7-Th film. As a result, future SM-OSC design should prioritize using these constructed molecules, which exhibit excellent optoelectronic properties, as superior scaffolds.

Recent advances in n-type and ambipolar organic semiconductors and their multi-functional applications

Organic semiconductors have received broad attention and research interest due to their unique integration of semiconducting properties with structural tunability, intrinsic flexibiltiy and low cost. In order to meet the requirements of organic electronic devices and their integrated circuits, p-type, n-type and ambipolar organic semiconductors are all necessary. However, due to the limitation in both material synthesis and device fabrication, the development of n-type and ambipolar materials is quite behind that of p-type materials. Recent development in synthetic methods of organic semiconductors greatly enriches the range of n-type and ambipolar materials. Moreover, the newly developed materials with multiple functions also put forward multi-functional device applications, including some emerging research areas. In this review, we give a timely summary on these impressive advances in n-type and ambipolar organic semiconductors with a special focus on their synthesis methods and advanced materials with enhanced properties of charge carrier mobility, integration of high mobility and strong emission and thermoelectric properties. Finally, multi-functional device applications are further demonstrated as an example of these developed n-type and ambipolar materials.

Synthesis and Derivatization of an Isomerized Bithiophene Imide (iBTI) Acceptor with a Controllably Twisted Backbone

A heptagonal isomerized bithiophene imide (iBTI) acceptor has been effectively synthesized on a gram scale. Its series of β-, α',β-, α,α'-, α,α',β-, and α,α',β,β'-substituted derivatives can be obtained by controlling brominated sites. Single-crystal analyses indicate that the torsion angle of the imide backbone depends on the number and rigidity of β-substituted groups. Furthermore, the helical chirality of tetrasubstituted and [7]helicene-like derivatives based on iBTI shows great promise for the construction of chiral semiconductor materials.

An Amorphous n-Type Conjugated Polymer with an Ultra-Rigid Planar Backbone

High charge carrier mobility polymer semiconductors are always semi-crystalline. Amorphous conjugated polymers represent another kind of polymer semiconductors with different charge transporting mechanism. Here we report the first near-amorphous n-type conjugated polymer with decent electron mobility, which features a remarkably rigid, straight and planar polymer backbone. The molecular design strategy is to copolymerize two fused-ring building blocks which are both electron-accepting, centrosymmetrical and planar. The polymer is the alternating copolymer of double B←N bridged bipyridine (BNBP) unit and benzobisthiazole (BBTz) unit. It shows a decent electron mobility of 0.34 cm²  V^-1  s^-1 in organic field-effect transistors. The excellent electron transporting property of the polymer is possibly due to the ultrahigh backbone stiffness, small π-π stacking distance, and high molecular weight.

Thiazole Imide-Based All-Acceptor Homopolymer with Branched Ethylene Glycol Side Chains for Organic Thermoelectrics

n-Type semiconducting polymers with high thermoelectric performance remain challenging due to the scarcity of molecular design strategy, limiting their applications in organic thermoelectric (OTE) devices. Herein, we provide a new approach to enhance the OTE performance of n-doped polymers by introducing acceptor-acceptor (A-A) type backbone bearing branched ethylene glycol (EG) side chains. When doped with 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI), the A-A homopolymer PDTzTI-TEG exhibits n-type electrical conductivity (σ) up to 34 S cm^-1 and power factor value of 15.7 μW m^-1  K^-2 . The OTE performance of PDTzTI-TEG is far greater than that of homopolymer PBTI-TEG (σ=0.27 S cm^-1 ), indicating that introducing electron-deficient thiazole units in the backbone further improves the n-doping efficiency. These results demonstrate that developing A-A type polymers with EG side chains is an effective strategy to enhance n-type OTE performance.

A New Dibenzoquinoxalineimide-Based Wide-Bandgap Polymer Donor for Polymer Solar Cells

The molecular design of a wide-bandgap polymer donor is critical to achieve high-performance organic photovoltaic devices. Herein, a new dibenzo-fused quinoxalineimide (BPQI) is successfully synthesized as an electron-deficient building block to construct donor-acceptor (D-A)-type polymers, namely P(BPQI-BDT) and P(BPQI-BDTT), using benzodithiophene and its derivative, which bears different side chains, as the copolymerization units. These two polymers are used as a donor, and the narrow bandgap (2,20-((2Z,20Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo [3,4-e]thieno[2,″30':4',50]thieno[20,30:4,5]pyrrolo[3,2g]thieno[20,30:4,5]thieno[3,2-b]indole-2,10 diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) Y6 is used as an acceptor to fabricate bulk heterojunction polymer solar cell devices. Y6, as a non-fullerene receptor (NFA), has excellent electrochemical and optical properties, as well as a high efficiency of over 18%. The device, based on P(BPQI-BDTT):Y6, showed power conversion efficiencies (PCEs) of 6.31% with a JSC of 17.09 mA cm-2, an open-circuit voltage (VOC) of 0.82 V, and an FF of 44.78%. This study demonstrates that dibenzo-fused quinoxalineimide is a promising building block for developing wide-bandgap polymer donors.

Recent Structural Engineering of Polymer Semiconductors Incorporating Hydrogen Bonds

Highly planar, extended π-electron organic conjugated polymers have been increasingly attractive for achieving high-mobility organic semiconductors. In addition to the conventional strategy to construct rigid backbone by covalent bonds, hydrogen bond has been employed extensively to increase the planarity and rigidity of polymer via intramolecular noncovalent interactions. This review provides a general summary of high-mobility semiconducting polymers incorporating hydrogen bonds in field-effect transistors over recent years. The structural engineering of the hydrogen bond-containing building blocks and the discussion of theoretical simulation, microstructural characterization, and device performance are covered. Additionally, the effects of the introduction of hydrogen bond on self-healing, stretchability, chemical sensitivity, and mechanical properties are also discussed. The review aims to help and inspire design of new high-mobility conjugated polymers with superiority of mechanical flexibility by incorporation of hydrogen bond for the application in flexible electronics.

Acceptor Modulation Strategies for Improving the Electron Transport in High-Performance Organic Field-Effect Transistors

High-performance ambipolar and electronic type semiconducting polymers are essential for fabricating various organic optoelectronic devices and complementary circuits. This review summarizes the strategies of improving the electron transport of semiconducting polymers via acceptor modulation strategies, which include the use of single, dual, triple, multiple, and all acceptors as well as the fusion of multiple identical acceptors to obtain new heterocyclic acceptors. To further improve the electron transport of semiconducting polymers, the introduction of strong electron-withdrawing groups can enhance the electron-withdrawing ability of donors and acceptors, thereby facilitating electron injection and suppressing hole accumulation. In addition, the relationships between the molecular structure, frontier molecular orbital energy levels, thin film morphology, microstructure, processing conditions, and device performances are also comprehensively discussed. Finally, the challenges encountered in this research area are proposed and the future outlook is presented.

Polymer Acceptors for High-Performance All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) have attracted considerable attention owing to their pronounced advantages of excellent mechanical flexibility/stretchability and greatly enhanced device stability as compared to other types of organic solar cells (OSCs). Thanks to the extensive research efforts dedicated to the development of polymer acceptors, all-PSCs have achieved remarkable improvement of photovoltaic performance, recently. This review summarizes the recent progress of polymer acceptors based on the key electron-deficient building blocks, which include bithiophene imide (BTI) derivatives, boron-nitrogen coordination bond (B←N)-incorporated (hetero)arenes, cyano-functionalized (hetero)arenes, and fused-ring electron acceptors (FREAs). In addition, single-component-based all-PSCs are also briefly discussed. The structure-property correlations of polymer acceptors are elaborated in detail. Finally, we offer our insights into the development of new electron-deficient building blocks with further optimized properties and the polymers built from them for efficient all-PSCs.

Influence of Fluorine Substitution on the Photovoltaic Performance of Wide Band Gap Polymer Donors for Polymer Solar Cells

The design and development of wide band gap (WBG) polymer donors are critical for achieving high power conversion efficiencies (PCEs) in polymer solar cells. In this work, four WBG polymer donors, Q4, Q5, Q6, and Q7, with different numbers and positions of fluorine substitution (n = 0, 2, 2, and 4, respectively) were prepared, and the effect of fluorination on their photovoltaic performance was systematically investigated. When blended with a small-molecule electron acceptor MeIC, the devices based on Q4, Q5, Q6, and Q7 showed PCEs of 10.34, 11.06, 5.26, and 0.48%, respectively. When coupled with a low band gap polymer acceptor PYIT to fabricate all-polymer solar cells (all-PSCs), while the other three polymers (Q5-Q7) exhibited much lower PCEs in the range of 0.12-6.71%, the Q4 polymer-based all-PSCs showed the highest PCE of 15.06%, comparable to that of the devices fabricated with the star polymer donor PM6 (PCE = 15.00%). Detailed physicochemical and morphological studies revealed that an over-substitution of F in Q7 results in undesired low-lying HOMO levels and phase separation with the acceptors, thus resulting in its inferior PCEs. Moreover, the less F-substitution and controlling of the positions of F-substitution position in Q4 and Q5 can improve the HOMO energy level matching as well as morphologies between these two polymers with the acceptors, which in turn gives rise to higher performances. Clearly, our results indicate that Q4 is a promising donor candidate for high-performance all-PSCs, and the fine-tuning of both the number and positions of F-substitution in the polymer backbone is essential in developing high-performance WBG polymer donors.

Transition metal-catalysed molecular n-doping of organic semiconductors

Chemical doping is a key process for investigating charge transport in organic semiconductors and improving certain (opto)electronic devices^1-9. N(electron)-doping is fundamentally more challenging than p(hole)-doping and typically achieves a very low doping efficiency (η) of less than 10%^1,10. An efficient molecular n-dopant should simultaneously exhibit a high reducing power and air stability for broad applicability^1,5,6,9,11, which is very challenging. Here we show a general concept of catalysed n-doping of organic semiconductors using air-stable precursor-type molecular dopants. Incorporation of a transition metal (for example, Pt, Au, Pd) as vapour-deposited nanoparticles or solution-processable organometallic complexes (for example, Pd₂(dba)₃) catalyses the reaction, as assessed by experimental and theoretical evidence, enabling greatly increased η in a much shorter doping time and high electrical conductivities (above 100 S cm^-1; ref. ¹²). This methodology has technological implications for realizing improved semiconductor devices and offers a broad exploration space of ternary systems comprising catalysts, molecular dopants and semiconductors, thus opening new opportunities in n-doping research and applications^{12, 13}.

Aminonaphthalimide-Based Molecular Cathode Interlayers for As-Cast Organic Solar Cells

A series of imide-based small molecules, namely NA, NAA, and NEA with simple structures, were designed and synthesized by introducing different amine side-chains into the benzene unit of imide, which were used as cathode interfacial materials in organic solar cells (OSCs). The amine side-chain substitution positions were systematically investigated with these small-molecule imides. Compared with NA without amide chains-NAA, and NEA, with 3-dimethylaminopropylamine and ethylenediamine chains, respectively-show bathochromic shifts in absorption, decreased band gaps, and higher highest occupied molecular orbital (HOMO) energy levels. A power conversion efficiency (PCE) of 15.04 % was obtained with the NEA-based as-cast OSCs with a high open-circuit voltage and fill factor for PM6 : Y6 blend and the maximum PCE of 15.80 % was reached for as-cast PM6 : Y6 : IT-M ternary OSCs. NEA exhibits better conductivity, higher electron mobility, and stronger the capability of lower work function of cathode among three molecules, affording OSCs with better photovoltaic performance. Additionally, these three molecules show excellent thermal stability both in solution and in films at 150 °C. The results indicate that imide-based small molecules are promising cathode interfacial materials for commercial OSCs.

n-Type Organic and Polymeric Semiconductors Based on Bithiophene Imide Derivatives

ConspectusIn the last three decades, p-type (hole-transporting) organic and polymeric semiconductors have achieved great success in terms of materials diversity and device performance, while the development of n-type (electron-transporting) analogues greatly lags behind, which is limited by the scarcity of highly electron-deficient building blocks with compact geometry and good solubility. However, such n-type semiconductors are essential due to the existence of the p-n junction and a complementary metal oxide semiconductor (CMOS)-like circuit in organic electronic devices. Among various electron-deficient building blocks, imide-functionalized arenes, such as naphthalene diimide (NDI) and perylene diimide (PDI), have been proven to be the most promising ones for developing n-type organic and polymeric semiconductors. Nevertheless, phenyl-based NDI and PDI lead to sizable steric hindrance with neighboring (hetero)arenes and a high degree of backbone distortion in the resultant semiconductors, which greatly limits their microstructural ordering and charge transport. To attenuate the steric hindrance associated with NDI and PDI, a novel imide-functionalized heteroarene, bithiophene imide (BTI), was designed; however, the BTI-based semiconductors suffer from high-lying frontier molecular orbital (FMO) energy levels as a result of their electron-rich thiophene framework and monoimide group, which is detrimental to n-type performance.In this Account, we review a series of BTI derivatives developed via various strategies, including ring fusion, thiazole substitution, fluorination, cyanation, and chalcogen substitution, and elaborate the synthesis routes designed to overcome the synthesis challenges due to their high electron deficiency. After structural optimization, these BTI derivatives can not only retain the advantages of good solubility, a planar backbone, and small steric hindrance inherited from BTI but also have greatly suppressed FMO levels. These novel building blocks enable the construction of a great number of n-type organic and polymeric semiconductors, particularly acceptor-acceptor (or all-acceptor)-type polymers, with remarkable performance in various devices, including electron mobility (μ_e) of 3.71 cm² V^-1 s^-1 in organic thin-film transistors (OTFTs), a power conversion efficiency (PCE) of 15.2% in all-polymer solar cells (all-PSCs), a PCE of 20.8% in inverted perovskite solar cells (PVSCs), electrical conductivity (σ) of 0.34 S cm^-1 and a power factor (PF) of 1.52 μW m^-1 K^-2 in self-doped diradicals, and σ of 23.3 S cm^-1 and a PF of ∼10 μW m^-1 K^-2 in molecularly n-doped polymers, all of which are among the best values in each type of device. The structure-property-device performance correlations of these n-type semiconductors are elucidated. The design strategy and synthesis of these novel BTI derivatives provide important information for developing highly electron-deficient building blocks with optimized physicochemical properties. Finally, we offer our insights into the further development of BTI derivatives and semiconductors built from them.

A Distannylated Monomer of a Strong Electron-Accepting Organoboron Building Block: Enabling Acceptor-Acceptor-Type Conjugated Polymers for n-Type Thermoelectric Applications

Acceptor-acceptor (A-A) copolymerization is an effective strategy to develop high-performance n-type conjugated polymers. However, the development of A-A type conjugated polymers is challenging due to the synthetic difficulty. Herein, a distannylated monomer of strong electron-deficient double B←N bridged bipyridine (BNBP) unit is readily synthesized and used to develop A-A type conjugated polymers by Stille polycondensation. The resulting polymers show ultralow LUMO energy levels of -4.4 eV, which is among the lowest value reported for organoboron polymers. After n-doping, the resulting polymers exhibit electric conductivity of 7.8 S cm^-1 and power factor of 24.8 μW m^-1  K^-2 . This performance is among the best for n-type polymer thermoelectric materials. These results demonstrate the great potential of A-A type organoboron polymers for high-performance n-type thermoelectrics.

n-Type organic semiconducting polymers: stability limitations, design considerations and applications

This review outlines the design strategies which aim to develop high performing n-type materials in the fields of organic thin film transistors (OTFT), organic electrochemical transistors (OECT) and organic thermoelectrics (OTE). Figures of merit for each application and the limitations in obtaining these are set out, and the challenges with achieving consistent and comparable measurements are addressed. We present a thorough discussion of the limitations of n-type materials, particularly their ambient operational instability, and suggest synthetic methods to overcome these. This instability originates from the oxidation of the negative polaron of the organic semiconductor (OSC) by water and oxygen, the potentials of which commonly fall within the electrochemical window of n-type OSCs, and consequently require a LUMO level deeper than ∼-4 eV for a material with ambient stability. Recent high performing n-type materials are detailed for each application and their design principles are discussed to explain how synthetic modifications can enhance performance. This can be achieved through a number of strategies, including utilising an electron deficient acceptor-acceptor backbone repeat unit motif, introducing electron-withdrawing groups or heteroatoms, rigidification and planarisation of the polymer backbone and through increasing the conjugation length. By studying the fundamental synthetic design principles which have been employed to date, this review highlights a path to the development of promising polymers for n-type OSC applications in the future.

Recent structural evolution of lactam- and imide-functionalized polymers applied in organic field-effect transistors and organic solar cells

Organic semiconductor materials, especially donor-acceptor (D-A) polymers, have been increasingly applied in organic optoelectronic devices, such as organic field-effect transistors (OFETs) and organic solar cells (OSCs). Plenty of high-performance OFETs and OSCs have been achieved based on varieties of structurally modified D-A polymers. As the basic building block of D-A polymers, acceptor moieties have drawn much attention. Among the numerous types, lactam- and imide-functionalized electron-deficient building blocks have been widely investigated. In this review, the structural evolution of lactam- or imide-containing acceptors (for instance, diketopyrrolopyrrole, isoindigo, naphthalene diimide, and perylene diimide) is covered and their representative polymers applied in OFETs and OSCs are also discussed, with a focus on the effect of varied structurally modified acceptor moieties on the physicochemical and photoelectrical properties of polymers. Additionally, this review discusses the current issues that need to be settled down and the further development of new types of acceptors. It is hoped that this review could help design new electron-deficient building blocks, find a more valid method to modify already reported acceptor units, and achieve high-performance semiconductor materials eventually.

Amphipathic Side Chain of a Conjugated Polymer Optimizes Dopant Location toward Efficient N-Type Organic Thermoelectrics

There is no molecular strategy for selectively increasing the Seebeck coefficient without reducing the electrical conductivity for organic thermoelectrics. Here, it is reported that the use of amphipathic side chains in an n-type donor-acceptor copolymer can selectively increase the Seebeck coefficient and thus increase the power factor by a factor of ≈5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host-dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host-dopant distance contributes to a higher Seebeck coefficient for a given electrical conductivity. Finally, an optimized power factor of 18 µW m^-1 K^-2 is achieved in the doped polymer film. This work provides a facile molecular strategy for selectively improving the Seebeck coefficient and opens up a new route for optimizing the dopant location toward realizing better n-type polymeric thermoelectrics.

Progress in the synthesis of imide-based N-type polymer semiconductor materials

Based on the development situation and challenge of organic photovoltaics (OPVs) and organic field-effect transistors (OFETs), it is necessary to develop N-type polymer building blocks with specific structures and performance. After decades of development, some excellent polymer receptor building blocks have been developed to construct N-type organic semiconductors, which have been applied in OFETs and OPVs. In this paper, four kinds of imide (bisthiophene imide BTI, bisthiazolimide BTz, naphthalimide NDI, and perylene imide PDI)-based N-type polymer semiconductor materials are introduced, and their applications in OFETs and OPVs are analyzed, too. The molecular structure design and the performance of corresponding materials are summarized to provide further guidance and reference for the design and development of high performance N-type polymer semiconductors.

A novel dry-blending method to reduce the coefficient of thermal expansion of polymer templates for OTFT electrodes

Among the patterning technologies for organic thin-film transistors (OTFTs), the fabrication of OTFT electrodes using polymer templates has attracted much attention. However, deviations in the electrode alignment occur because the coefficient of thermal expansion (CTE) of the polymer template is much higher than the CTE of the dielectric layer. Here, a novel dry-blending method is described in which SiO₂ nanoparticles are filled into a grooved silicon template, followed by permeation of polydimethylsiloxane (PDMS) into the SiO₂ nanoparticle gaps. The SiO₂ nanoparticles in the groove are extracted by curing and peeling off PDMS to prepare a PDMS/SiO₂ composite template with a nanoparticle content of 83.8 wt %. The composite template has a CTE of 96 ppm/°C, which is a reduction by 69.23% compared with the original PDMS template. Finally, we achieved the alignment of OTFT electrodes using the composite template.

Air-Stable and High-Performance Unipolar n-Type Conjugated Semiconducting Polymers Prepared by a "Strong Acceptor-Weak Donor" Strategy

Unipolar n-type conjugated polymer materials with long-term stable electron transport upon direct exposure to the air atmosphere are very challenging to prepare. In this study, three unipolar n-type donor-acceptor (D-A) conjugated polymer semiconductors (abbreviated as PNVB, PBABDFV, and PBAIDV) were successfully developed through a "strong acceptor-weak donor" strategy. The weak electron donation of the donor units in all three polymers successfully lowered the molecular energy levels by the acceptor units that strongly attracted electrons. Cyclic voltammetry demonstrated that all three polymers had low highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels near -6.0 and -4.0 eV, respectively. These results were consistent with the density functional theory calculations. The as-prepared polymers were then used to manufacture organic field-effect transistor (OFET) devices in bottom-gate/top-contact (BG/TC) configuration without any packaging protection. As expected, all devices exhibited unipolar electron transport properties. PBABDFV-based devices showed excellent field-effect performance and air stability, beneficial for straight-line molecular chain and closest π-π stacking distance to prevent water vapor and oxygen from diffusion into the active layer. This led to a maximum electron mobility (μ_e,max) of 0.79 cm² V^-1 s^-1 under air conditions. In addition, 0.50 cm² V^-1 s^-1 was still maintained after 27 days of storage in ambient environment. The near-ideal transfer curve of the PBABDFV-based OFET device in BG/TC configuration under vacuum was obtained with average mobility reliability factor (r_ave) reaching 88%.

Relieving the Photosensitivity of Organic Field-Effect Transistors

It is generally believed that the photoresponse behavior of organic field-effect transistors (OFETs) reflects the intrinsic property of organic semiconductors. However, this photoresponse hinders the application of OFETs in transparent displays as driven circuits due to the current instability resulting from the threshold voltage shift under light illumination. It is necessary to relieve the photosensitivity of OFETs to keep the devices stable. 2,6-diphenyl anthracene thin-film and single-crystal OFETs are fabricated on different substrates, and it is found that the degree of molecular order in the conducting channels and the defects at the dielectric/semiconductor interface play important roles in determining the phototransistor performance. When highly ordered single-crystal OFETs are fabricated on polymeric substrates with low defects, the photosensitivity (P) decreases by more than 10⁵ times and the threshold voltage shift (ΔV_T ) is almost eliminated compared with the corresponding thin-film OFETs. This phenomenon is further verified by using another three organic semiconductors for similar characterizations. The decreased P and ΔV_T of OFETs ensure a good current stability for OFETs to drive organic light-emitting diodes efficiently, which is essential to the application of OFETs in flexible and transparent displays.

Achieving efficient polymer solar cells based on benzodithiophene–thiazole-containing wide band gap polymer donors by changing the linkage patterns of two thiazoles

Two conjugated polymers with different combinations of two thiazoles were synthesized to study their photovoltaic performances.