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The understanding of the optical characteristics and the influence of thermally induced crystallization is vital to evaluate newly synthesized organic semiconductors for their potential applications. Variable‐angle spectroscopic ellipsometry (VASE) is employed to determine the complex dielectric function of two recently developed discrete oligomers based on naphthalene diimide (NDI) and bithiophene (T2), applying an anisotropic optical model. It is shown that the as‐cast films are optically near‐isotropic, while the annealed films exhibit pronounced anisotropies with respect to the orientation of the π−π* transition. Differences between the two distinct molecules are observed and discussed. The degree of optical anisotropy is quantified using the average molecular orientation angle φ, associated with the orientation of the π−π* transition dipole, and the related orientation order parameter S. The molecules are shown to form crystallites of several microns in size upon annealing. Imaging ellipsometry is thus additionally employed to study their optical anisotropy on a microscopic scale, and the agreement with results from macroscopic measurements is evaluated. [ABSTRACT FROM AUTHOR]

Doped organic semiconductors play a central role in the development of several innovative optoelectronic and energy harvesting applications. Currently, the realization of thermoelectric generators, which require both hole‐ and electron‐transporting materials with high electrical conductivity, is strongly hindered by the scarce availability of stable solution‐processable n‐dopants and their limited efficiency. Herein, the synthesis of 4‐(1,3‐dimethyl‐2,3‐dihydro‐1H‐benzimidazol‐2‐yl)‐dibenzazepine (IStBI), a novel derivative belonging to the well‐known family of the benzimidazoline compounds, is presented. The functionalization with the planarized and rigid iminostilbene substituent allows, without significantly affecting the compound electronic structure, an efficient intercalation of the dopant molecules inside the ordered regions of thin films of the benchmark n‐type polymer poly(N,N′‐bis‐2‐octyldodecylnaphthalene‐1,4,5,8‐bis‐dicarboximide‐2,6‐diyl‐alt‐5,5′‐2,2′‐bithiophene) P(NDI2OD‐T2). Consequently, a maximum electrical conductivity of (1.14 ± 0.13) × 10−2 S cm−1 is recorded, exceeding by one order of magnitude what previously achieved upon solution doping of the reference P(NDI2OD‐T2) with benzimidazoline derivatives. The thermoelectric power factor is also simultaneously increased. The findings confirm that tailoring of the dopant chemical structure to improve structural interactions with the host semiconductors can be employed as a successful strategy to achieve more effective n‐doping, helping to close the performance gap with p‐type materials. [ABSTRACT FROM AUTHOR]

Abstract Doped organic semiconductors play a central role in the development of several innovative optoelectronic and energy harvesting applications. Currently, the realization of thermoelectric generators, which require both hole‐ and electron‐transporting materials with high electrical conductivity, is strongly hindered by the scarce availability of stable solution‐processable n‐dopants and their limited efficiency. Herein, the synthesis of 4‐(1,3‐dimethyl‐2,3‐dihydro‐1H‐benzimidazol‐2‐yl)‐dibenzazepine (IStBI), a novel derivative belonging to the well‐known family of the benzimidazoline compounds, is presented. The functionalization with the planarized and rigid iminostilbene substituent allows, without significantly affecting the compound electronic structure, an efficient intercalation of the dopant molecules inside the ordered regions of thin films of the benchmark n‐type polymer poly(N,N′‐bis‐2‐octyldodecylnaphthalene‐1,4,5,8‐bis‐dicarboximide‐2,6‐diyl‐alt‐5,5′‐2,2′‐bithiophene) P(NDI2OD‐T2). Consequently, a maximum electrical conductivity of (1.14 ± 0.13) × 10−2 S cm−1 is recorded, exceeding by one order of magnitude what previously achieved upon solution doping of the reference P(NDI2OD‐T2) with benzimidazoline derivatives. The thermoelectric power factor is also simultaneously increased. The findings confirm that tailoring of the dopant chemical structure to improve structural interactions with the host semiconductors can be employed as a successful strategy to achieve more effective n‐doping, helping to close the performance gap with p‐type materials.

The orientational control of semiconducting polymers (SCPs) in floating films offers several advantages over conventional solution‐processing methods for device fabrication, and the unidirectional floating film transfer method (UFTM) has been applied to fabricate large‐area oriented p‐type SCP films. Here, UFTM can be used to prepare high‐performance n‐type SCP films of P(NDI2OD‐T2) is reported. A strong correlation between the degree of polymer orientation and the solvent used for the preparation of P(NDI2OD‐T2) floating films is observed. In particular, the size of the nanofibers aligned along the orientation direction prepared using chloroform is dramatically increased by adding a small amount of chlorobenzene. Microstructural characterization reveals that the P(NDI2OD‐T2) floating films are uniaxially aligned with edge‐on orientation, whereas the spin‐coated films are isotropic with face‐on orientation. Notably, the floating films of P(NDI2OD‐T2) prepared with solvent blending have higher electron mobility with ambipolar‐like device characteristics and high threshold voltage (VTH) of ≈25 V. Finally, using the one‐step immersion process, the introduction of an interlayer of hexa(ethylene glycol)‐dithiol between the source/drain contacts and P(NDI2OD‐T2) film results in a drastic improvement in device, giving unipolar n‐channel transistor characteristics with a VTH of ≈0 V, on/off ratio of >105, and mobility reliability factor of almost 100%. [ABSTRACT FROM AUTHOR]

Abstract The orientational control of semiconducting polymers (SCPs) in floating films offers several advantages over conventional solution‐processing methods for device fabrication, and the unidirectional floating film transfer method (UFTM) has been applied to fabricate large‐area oriented p‐type SCP films. Here, UFTM can be used to prepare high‐performance n‐type SCP films of P(NDI2OD‐T2) is reported. A strong correlation between the degree of polymer orientation and the solvent used for the preparation of P(NDI2OD‐T2) floating films is observed. In particular, the size of the nanofibers aligned along the orientation direction prepared using chloroform is dramatically increased by adding a small amount of chlorobenzene. Microstructural characterization reveals that the P(NDI2OD‐T2) floating films are uniaxially aligned with edge‐on orientation, whereas the spin‐coated films are isotropic with face‐on orientation. Notably, the floating films of P(NDI2OD‐T2) prepared with solvent blending have higher electron mobility with ambipolar‐like device characteristics and high threshold voltage (VTH) of ≈25 V. Finally, using the one‐step immersion process, the introduction of an interlayer of hexa(ethylene glycol)‐dithiol between the source/drain contacts and P(NDI2OD‐T2) film results in a drastic improvement in device, giving unipolar n‐channel transistor characteristics with a VTH of ≈0 V, on/off ratio of >105, and mobility reliability factor of almost 100%.

In this study, we synthesized and evaluated graft copolymers with polystyrene (PSt) as side chains on poly {[ N,N´ -bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt -5,5′-(2,2′ -bithiophene)} [P(NDI2OD-T2)], which is a typical n-type semiconducting polymer applied to in organic photovoltaics due to its intense optical absorption for the visible light, and high electron affinity. A well-controlled structure was confirmed by 1H-NMR analysis. It was found that enhanced electron mobilities in graft copolymer films were observed compared with the homopolymer. In order to clarify the role of PSt chains and the origins of the enhanced mobility, UV–vis absorption, electrochemical, morphological and thermal characteristics, and crystalline nature were investigated. As the results, it is suggested that the improved mobilities were attributed to the enhanced growth of π – π stacks, and increased crystallinity induced by PSt graft chains. [ABSTRACT FROM AUTHOR]

• The n-type polymer P(NDI2OD-T 2) shows strong aggregation in solution which is beneficial for alignment in films. • Film formation is related to liquid crystalline phases in highly concentrated solutions. • A structural model including texture and polymorphism and semicrystallinity is highlighted. • Decreased regioregularity leads to reduced aggregation and order in solution and films. • While transistor performance is robust towards regioregularity, decreasing regioregularity helps blend formation in OPV. Research developments on the n -type semiconducting donor-acceptor copolymer poly{[ N , N ′-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]- alt -5,5′-(2,2′-bithiophene) P(NDI2OD-T 2) are reviewed in terms of synthesis, structure-property relationships and device applications. The classical purely 2,6-linked regioregular (RR) polymers are compared to their regioirregular counterparts (RI) with 2,6- and 2,7-linked regioisomers. The inherent strong aggregation tendency of RR is highlighted and methods to fine-tune aggregation including regioregularity, solvent quality and deposition technique are compared. Alignment techniques and temperature treatment are used to control polymorphism, texture and semicrystallinity in films. A general finding is that aggregation within fibers dominates structure formation, and that the fiber direction is along the chain direction which is in contrast to classical poly(3-hexylthiophene). Evidence for liquid crystalline behavior in highly concentrated solutions and freezing upon film deposition is shown to be beneficial for the creation of highly anisotropic structures. While down-tuning of aggregation is beneficial for miscibility issues in blends, e.g. for organic photovoltaics, transistor performance is surprisingly independent of regioregularity and processing. This highlights the nature of P(NDI2OD-T 2) as conjugated redox polymer with rather localized redox units. [ABSTRACT FROM AUTHOR]

It is widely recognized that fullerene derivatives show several advantages as n-type materials in photovoltaic applications. However, conventional [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) exhibits weak absorption in the visible region, and poor morphological stability, due to the facile aggregation. For further improvement of the device performance and durability, utilization of n-type polymeric materials instead of PCBM is considered to be a good way to solve the problems. In this study, we fabricated completely polymer-based solar cells utilizing p- and n-type block copolymers consisting of poly(3-hexylthiophene) (P3HT) and poly{[N,N'-bis(2-octyldodecyl) naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} [P(NDI2OD-T2)], respectively, containing common polystyrene (PSt) inert blocks, which decreased the size of phase separated structures. Electron mobility in synthesized P(NDI2OD-T2)-b-PSt film enhanced by a factor of 8 compared with homopolymer. The root mean square roughness of the blend film of two block copolymers (12.2 nm) was decreased, compared with that of the simple homopolymers blend (18.8 nm). From the current density-voltage characteristics, it was confirmed that the introduction of PSt into both P3HT and P(NDI2OD-T2) improves short-circuit current density (1.16 to 1.73 mA cm-2 and power-conversion efficiency (0.24% to 0.32%). Better performance is probably due to the uniformity of the phase separation, and the enhancement of charge mobility. [ABSTRACT FROM AUTHOR]

Tuning the side chains of conjugated polymers is a simple, yet effective strategy for modulating their structural and electrical properties, but their impact on n-type conjugated polymers has not been studied extensively, particularly in the area of all-polymer solar cells (all-PSCs). Herein, the effects of side chain engineering of P(NDI2OD-T2) polymer (also known as Polyera Activink N2200) are investigated, which is the most widely used n-type polymer in all-PSCs and organic field-effect transistors (OFETs), on their structural and electronic properties. A series of naphthalenediimide-bithiophene-based copolymers (P(NDIR-T2)) is synthesized, with different side chains (R) of 2-hexyldecyl (2-HD), 2-octyldodecyl (2-OD), and 2-decyltetradecyl (2-DT). The P(NDI2HD-T2) exhibits more noticeable crystalline behaviors than P(NDI2OD-T2) and P(NDI2DT-T2), thereby facilitating superior 3D charge transport. For example, the P(NDI2HD-T2) shows the highest OFET electron mobility (1.90 cm2 V−1 s−1). Also, a series of all-PSCs is produced using different electron donors of PTB7-Th, PTB7, and PPDT2FBT. The P(NDI2HD-T2) based all-PSCs produce much higher power conversion efficiency (PCE) irrespective of the electron donors. In particular, the PTB7-Th:P(NDI2HD-T2) forms highly ordered, strong face-on interchain stackings, and has better intermixed bulk-heterojunction morphology, producing the highest PCE of 6.11% that has been obtained by P(NDIR-T2) based all-PSCs to date. [ABSTRACT FROM AUTHOR]