This study investigates the effect of the molecular structure of three different donor units, naphthalene (Np), bithiophene (BT), and thiophene–vinylene–thiophene (TVT), in isoindigo (IIG)-based donor –acceptor conjugated polymers (PIIG-Np, PIIG-BT and PIIG-TVT) on the charge carrier mobility of organic field-effect transistors (OFETs). The charge transport properties of three different IIG-based polymers strongly depend on donor units. PIIG–BT OFETs showed 50 times higher hole mobility (0.63 cm² V⁻¹ s⁻¹) than PIIG–TVT and PIIG–Np ones of ≈ 0.01 cm² V⁻¹ s⁻¹ with CYTOP dielectric though the BT units have less planarity than the TVT and Np units. The reasons for the different mobility in IIG-based polymers are studied by analyzing the energy structure by absorption spectra, calculating transport levels by density functional theory, investigating the in- and out-of-plane crystallinity of thin film by grazing-incidence wide-angle X-ray scattering, and extracting key transport parameters via low-temperature measurements. By combining theoretical, optical, electrical, and structural analyses, this study finds that the large difference in OFET mobility mainly originates from the transport disorders determined by the different microcrystal structure, rather than the intrinsic transport properties in isolated chains for different polymers.

Interdependence of chemical structure, thin-film morphology, and transport properties is a key, yet often elusive aspect characterizing the design and development of high-mobility, solution-processed polymers for large-area and flexible electronics applications. There is a specific need to achieve >1 cm² V⁻¹ s⁻¹ field-effect mobilities (μ) at low processing temperatures in combination with environmental stability, especially in the case of electron-transporting polymers, which are still lagging behind hole transporting materials. Here, the synthesis of a naphthalene-diimide based donor–acceptor copolymer characterized by a selenophene vinylene selenophene donor moiety is reported. Optimized field-effect transistors show maximum μ of 2.4 cm² V⁻¹ s⁻¹ and promising ambient stability. A very marked film structural evolution is revealed with increasing annealing temperature, with evidence of a remarkable 3D crystallinity above 180 °C. Conversely, transport properties are found to be substantially optimized at 150 °C, with limited gain at higher temperature. This discrepancy is rationalized by the presence of a surface-segregated prevalently edge-on packed polymer phase, dominating the device accumulated channel. This study therefore serves the purpose of presenting a promising, high-electron-mobility copolymer that is processable at relatively low temperatures, and of clearly highlighting the necessity of specifically investigating channel morphology in assessing the structure–property nexus in semiconducting polymer thin films.

Thin film electronic and optoelectronic devices demand electrodes with a work function (Φ) that is sufficiently low to facilitate the transport of electrons in and out of the lowest unoccupied molecular orbital of a given semiconductor. Herein, phenothiazine-, carbazole-, and fluorene-based phenylquinoline derivatives as efficient interfacial layer (IL) materials for solution-processable organic and metal oxide electronic devices are reported. The IL is applied on top of a charge injection electrode in various solution-processed devices, including n-channel organic thin-film transistors (OTFTs) with [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) 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)] and amorphous indium gallium zinc oxide (IGZO) transistors, and also in organic photovoltaics (OPVs). Both PC₇₁BM- and P(NDI2OD-T2)-based n-channel OTFTs with IL show enhanced mobility by more than 200% compared to bare Au electrode. IGZO transistors showed much improved mobility of 15.3 cm² V⁻¹ s⁻¹ with an IL compared to bare Au (0.6 cm² V⁻¹ s⁻¹ ) device. A significantly improved power conversion efficiency (PCE) of 7.63% is obtained for IL utilizing the poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7):PC₇₁BM based OPVs compared to 4.75% of control device. Ultraviolet photoelectron spectroscopy study reveals that phenylquinoline derivatives significantly lower the Φ of Au, thus facilitating electron injection/extraction in the device.

Considering there is growing interest in the superior charge transport in the (E)-2-(2-(thiophen-2-yl)-vinyl)thiophene (TVT)-based polymer family, an essential step forward is to provide a deep and comprehensive understanding of the structure–property relationships with their polymer analogs. Herein, a carefully chosen set of DPP-TVT-n polymers are reported here, involving TVT and diketopyrrolopyrrole (DPP) units that are constructed in combination with varying thiophene content in the repeat units, where n is the number of thiophene spacer units. Their OFET characteristics demonstrate ambipolar behavior; in particular, with DPP-TVT-0 a nearly balanced hole and electron transport are observed. Interestingly, the majority of the charge-transport properties changed from ambipolar to p-type dominant, together with the enhanced hole mobilities, as the electron-donating thiophene spacers are introduced. Although both the lamellar d-spacings and π-stacking distances of DPP-TVT-n decreased with as the number of thiophene spacers increased, DPP-TVT-1 clearly shows the highest hole mobility (up to 2.96 cm² V⁻¹ s⁻¹) owing to the unique structural conformations derived from its smaller paracrystalline distortion parameter and narrower plane distribution relative to the others. These in-depth studies should uncover the underlying structure–property relationships in a relevant class of TVT-like semiconductors, shedding light on the future design of top-performing semiconducting polymers.

Despite extensive progress in organic field-effect transistors, there are still far fewer reliable, high-mobility n-type polymers than p-type polymers. It is demonstrated that by using dopants at a critical doping molar ratio (MR), performance of n-type polymer poly[[N,N9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)] (P(NDI2DO-T2)) field-effect transistors (FETs) can be significantly improved and simultaneously optimized in mobility, on–off ratio, crystallinity, injection, and reliability. In particular, when using the organic dopant bis(cyclopentadienyl)–cobalt(II) (cobaltocene, CoCp₂) at a low concentration (0.05 wt%), the FET mobility is increased from 0.34 to 0.72 cm² V^–1 s^–1, and the threshold voltage was decreased from 32.7 to 8.8 V. The relationship between the MR of dopants and electrical characteristics as well as the evolution in polymer crystallinity revealed by synchrotron X-ray diffractions are systematically investigated. Deviating from previous discoveries, it is found that mobility increases first and then decreases drastically beyond a critical value of MR. Meanwhile, the intensity and width of the main peak of in-plane X-ray diffraction start to decrease at the same critical MR. Thus, the mobility decrease is correlated with the disturbed in-plane crystallinity of the conjugated polymer, for both organic and inorganic dopants. The method provides a simple and efficient approach to employing dopants to optimize the electrical performance and microstructure of P(NDI2DO-T2).

The two small molecules, quinoidal bithiophene (QBT) and quinoidal biselenophene (QBS), are designed based on a quinoid structure, and synthesized via a facile synthetic route. These quinoidal molecules have a reduced band gap and an amphoteric redox behavior, which is caused by an extended delocalization. Due to such properties, organic field-effect transistors based on QBT and QBS have shown balanced ambipolar characteristics. After thermal annealing, the performances of the devices are enhanced by an increase in crystallinity. The field-effect hole and electron mobilities are measured to be 0.031 cm² V⁻¹ s⁻¹ and 0.005 cm² V⁻¹ s⁻¹ for QBT, and 0.055 cm² V⁻¹ s⁻¹ and 0.021 cm² V⁻¹ s⁻¹ for QBS, respectively. In addition, we investigate the effect of chalcogen atoms (S and Se) on the molecular properties. The optical, electrochemical properties and electronic structures are mainly dominated by the quinoidal structure, whereas molecular properties are scarcely affected by either type of chalcogen atom. The main effect of the chalcogen atoms is ascribed to the difference of crystallinity. Due to a strong intermolecular interaction of the selenophene, QBS exhibits a higher degree of crystallinity, which leads to an enhancement of both hole and electron mobilities. Consequently, these types of quinoidal molecules are found to be promising for use as ambipolar semiconductors.

In this paper, a technique using mixed transition-metal oxides as contact interlayers to modulate both the electron- and hole-injections in ambipolar organic field-effect transistors (OFETs) is presented. The cesium carbonate (Cs₂CO₃) and vanadium pentoixide (V₂O₅) are found to greatly and independently improve the charge injection properties for electrons and holes in the ambipolar OFETs using organic semiconductor of diketopyrrolopyrrolethieno[3,2-b]thiophene copolymer (DPPT-TT) and contact electrodes of molybdenum (Mo). When Cs₂CO₃ and V₂O₅ are blended at various mixing ratios, they are observed to very finely and constantly regulate the Mo's work function from −4.2 eV to −4.8 eV, leading to high electron- and hole-mobilities as high as 2.6 and 2.98 cm² V⁻¹ s⁻¹, respectively. The most remarkable finding is that the device characteristics and device performance can be gradually controlled by adjusting the composition of mixed-oxide interlayers, which is highly desired for such applications as complementary circuitry that requires well matched n-channel and p-channel device operations. Therefore, such simple interface engineering in conjunction with utilization of ambipolar semiconductors can truly enable the promising low-cost and soft organic electronics for extensive applications.

Naphthalenediimide (NDI)-based polymers co-polymerized with thienyl units are an interesting class of polymer semiconductors because of their good electron mobilities and unique film microstructure. Despite these properties, understanding how the extension of the thienyl co-monomer affects charge transport properties remains unclear. With this goal in mind, we have synthesized a series of NDI derivatives of the parent 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 exhibited excellent electron mobility. The strategy comprises both the extension of the donor o-conjugation length and the heteroatomic fusion of the thiophene rings. These newly synthesized compounds are characterized experimentally and theoretically vis-à-vis with P(NDI2OD-T2) as the reference. UV-vis data and cyclic-voltammetry are adopted to assess the effect of the donor modification on the frontier energy levels and on the bandgap. Intra-molecular polaronic effects are accounted for by computing the internal reorganization energy with density functional theory (DFT) calculations. Finally electrons and holes transport is experimentally investigated in field-effect transistors (FETs), by measuring current-voltage characteristics at variable temperatures. Overall we have identified a regime where inter-molecular effects, such as the wavefunction overlap and the degree of energetic disorder, induced by the different donor group prevail over polaronic effects and are the leading factors in determining electrons mobility.

The selective tuning of the operational mode from ambipolar to unipolar transport in organic field-effect transistors (OFETs) by printing molecular dopants is reported. The field-effect mobility (μ_FET) and onset voltage (V_on) of both for electrons and holes in initially ambipolar methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) OFETs are precisely modulated by incorporating a small amount of cesium fluoride (CsF) n-type dopant or tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) p-type dopant for n-channel or p-channel OFETs either by blending or inkjet printing of the dopant on the pre-deposited semiconductor. Excess carriers introduced by the chemical doping compensate traps by shifting the Fermi level (E_F) toward respective transport energy levels and therefore increase the number of mobile charges electrostatically accumulated in channel at the same gate bias voltage. In particular, n-doped OFETs with CsF show gate-voltage independent Ohmic injection. Interestingly, n- or p-doped OFETs show a lower sensitivity to gate-bias stress and an improved ambient stability with respect to pristine devices. Finally, complementary inverters composed of n- and p-type PCBM OFETs are demonstrated by selective doping of the pre-deposited semiconductor via inkjet printing of the dopants.

For at least the past ten years printed electronics has promised to revolutionize our daily life by making cost-effective electronic circuits and sensors available through mass production techniques, for their ubiquitous applications in wearable components, rollable and conformable devices, and point-of-care applications. While passive components, such as conductors, resistors and capacitors, had already been fabricated by printing techniques at industrial scale, printing processes have been struggling to meet the requirements for mass-produced electronics and optoelectronics applications despite their great potential. In the case of logic integrated circuits (ICs), which constitute the focus of this Progress Report, the main limitations have been represented by the need of suitable functional inks, mainly high-mobility printable semiconductors and low sintering temperature conducting inks, and evoluted printing tools capable of higher resolution, registration and uniformity than needed in the conventional graphic arts printing sector.

Solution-processable polymeric semiconductors are the best candidates to fulfill the requirements for printed logic ICs on flexible substrates, due to their superior processability, ease of tuning of their rheology parameters, and mechanical properties. One of the strongest limitations has been mainly represented by the low charge carrier mobility (μ) achievable with polymeric, organic field-effect transistors (OFETs). However, recently unprecedented values of μ ∼ 10 cm²/Vs have been achieved with solution-processed polymer based OFETs, a value competing with mobilities reported in organic single-crystals and exceeding the performances enabled by amorphous silicon (a-Si). Interestingly these values were achieved thanks to the design and synthesis of donor-acceptor copolymers, showing limited degree of order when processed in thin films and therefore fostering further studies on the reason leading to such improved charge transport properties. Among this class of materials, various polymers can show well balanced electrons and holes mobility, therefore being indicated as ambipolar semiconductors, good environmental stability, and a small band-gap, which simplifies the tuning of charge injection. This opened up the possibility of taking advantage of the superior performances offered by complementary “CMOS-like” logic for the design of digital ICs, easing the scaling down of critical geometrical features, and achieving higher complexity from robust single gates (e.g., inverters) and test circuits (e.g., ring oscillators) to more complete circuits.

Here, we review the recent progress in the development of printed ICs based on polymeric semiconductors suitable for large-volume micro- and nano-electronics applications. Particular attention is paid to the strategies proposed in the literature to design and synthesize high mobility polymers and to develop suitable printing tools and techniques to allow for improved patterning capability required for the down-scaling of devices in order to achieve the operation frequencies needed for applications, such as flexible radio-frequency identification (RFID) tags, near-field communication (NFC) devices, ambient electronics, and portable flexible displays.

The effects of using a blocking dielectric layer and metal nanoparticles (NPs) as charge-trapping sites on the characteristics of organic nano-floating-gate memory (NFGM) devices are investigated. High-performance NFGM devices are fabricated using the n-type polymer semiconductor, 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)), and various metal NPs. These NPs are embedded within bilayers of various polymer dielectrics (polystyrene (PS)/poly(4-vinyl phenol) (PVP) and PS/poly(methyl methacrylate) (PMMA)). The P(NDI2OD-T2) organic field-effect transistor (OFET)-based NFGM devices exhibit high electron mobilities (0.4–0.5 cm² V⁻¹ s⁻¹) and reliable non-volatile memory characteristics, which include a wide memory window (≈52 V), a high on/off-current ratio (I_on/I_off ≈ 10⁵), and a long extrapolated retention time (>10⁷ s), depending on the choice of the blocking dielectric (PVP or PMMA) and the metal (Au, Ag, Cu, or Al) NPs. The best memory characteristics are achieved in the ones fabricated using PMMA and Au or Ag NPs. The NFGM devices with PMMA and spatially well-distributed Cu NPs show quasi-permanent retention characteristics. An inkjet-printed flexible P(NDI2OD-T2) 256-bit transistor memory array (16 × 16 transistors) with Au-NPs on a polyethylene naphthalate substrate is also fabricated. These memory devices in array exhibit a high I_on/I_off (≈10^{4 ± 0.85}), wide memory window (≈43.5 V ± 8.3 V), and a high degree of reliability.

A high-performance naphthalene diimide (NDI)-based conjugated polymer for use as the active layer of n-channel organic field-effect transistors (OFETs) is reported. The solution-processable n-channel polymer is systematically designed and synthesized with an alternating structure of long alkyl substituted-NDI and thienylene–vinylene–thienylene units (PNDI-TVT). The material has a well-controlled molecular structure with an extended π-conjugated backbone, with no increase in the LUMO level, achieving a high mobility and highly ambient stable n-type OFET. The top-gate, bottom-contact device shows remarkably high electron charge-carrier mobility of up to 1.8 cm² V⁻¹ s⁻¹ (I_on/I_off = 10⁶) with the commonly used polymer dielectric, poly(methyl methacrylate) (PMMA). Moreover, PNDI-TVT OFETs exhibit excellent air and operation stability. Such high device performance is attributed to improved π–π intermolecular interactions owing to the extended π-conjugation, apart from the improved crystallinity and highly interdigitated lamellar structure caused by the extended π–π backbone and long alkyl groups.