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.

Two small molecule donor materials (DTGe(FBTTh₂)₂ and DTGe(FBTBFu)₂) incorporating the dithienogermole (DTGe) moiety with fluorobenzothiadiazole (FBT) and bithiophene (Th₂) or benzofuran (BFu) end-capping groups are synthesized and their properties as donor materials in small molecule bulk heterojunction type (BHJ) solar cells are investigated. The DTGe(FBTTh₂)₂ with Th₂ end groups shows outstanding solar cell characteristics with efficiencies up to 6.4% using a standard BHJ architecture and 7.3% using a ZnO optical spacer, while the BFu end-capped DTGe(FBTBFu)₂ has slightly wider band gaps and yields slightly higher open circuit voltage (V_OC) at the expense of short circuit current (J_SC) and fill factor (FF). In this study, the DTGe-based molecules are systematically compared to the dithienosilole (DTSi)-based analogues, which are currently among the highest power conversion efficiency (PCE) small molecule solar cell donor materials known. The J_SC produced by the DTGe molecule is found to be similar to, or slightly higher than the Si analogue, despite similar absorption characteristics, however, the PCE is similar to the Si analogues due to small decreases in V_OC and FF. This report marks the first small molecule BHJ based on a Ge-containing heterocycle with PCE over 7%.

Considering that a high compatibility at hybrid organic/inorganic interfaces can be achieved using polar and hydrophilic functionalities, this approach is used to improve inverted polymer solar cell performance by introducing nonionic phosphonate side chains (at 0%, 5%, 15%, and 30% substitution levels) into a series of isoindigo-based polymers (PIIGDT-Pn). This approach led to ≈20% improvement in power conversion efficiency compared to a nonmodified control polymer, via an increased short-circuit current (J _SC). This enhancement is believed to stem from reduced nongerminate recombination and improved charge carried extraction when the level of phosphonate substitution is optimized. These results are substantiated by a combination of detailed electrical measurements including space-charged limited current modeling, light intensity–dependent photocurrent (J _ph) analysis, and morphological studies (grazing-incidence wide-angle X-ray scattering and atomic force microscopy). This is the first practical report demonstrating the use of nonionic polar side chains to control charge carrier dynamics in an existing photovoltaic polymer structure. It is envisioned that this simple strategy may be applied to other material systems and yield new materials with the potential for even higher performance.

Based on the integrated consideration and engineering of both conjugated backbones and flexible side chains, solution-processable polymeric semiconductors consisting of a diketopyrrolopyrrole (DPP) backbone and a finely modulated branching side chain (ε-branched chain) are reported. The subtle change in the branching point from the backbone alters the π−π stacking and the lamellar distances between polymer backbones, which has a significant influence on the charge-transport properties and in turn the performances of field-effect transistors (FETs). In addition to their excellent electron mobilities (up to 2.25 cm² V⁻¹ s⁻¹), ultra-high hole mobilities (up to 12.25 cm² V⁻¹ s⁻¹) with an on/off ratio (I_on/I_off) of at least 10⁶ are achieved in the FETs fabricated using the polymers. The developed polymers exhibit extraordinarily high electrical performance with both hole and electron mobilities superior to that of unipolar amorphous silicon.

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.

Together with ring-opening metathesis polymerization, a controlled protocol of a progressive addition of the ultrafast-initiating and more reactive third-generation Grubbs catalyst yields a dendronized polymer (denpol) containing multi-anthraquinone-terminated dendrons (AQ-ter-denpol) (M_n=14.0 kDa) with a unimodal molar mass distribution (polydispersity index=1.20), fully characterized by ¹H nuclear magnetic resonance spectroscopy. AQ-ter-denpol is investigated as an organic cathode material for rechargeable Li-ion batteries, with a view to its unique morphology. Our methodologies establish an opportunity for developing a new generation of organic electrodes based on denpols.

Systematic creation of polymeric semiconductors from novel building blocks is critical for improving charge transport properties in organic field-effect transistors (OFETs). A series of ultralow-bandgap polymers containing thienoisoindigo (TIIG) as a thiophene analogue of isoindigo (IIG) is synthesized. The UV-Vis absorptions of the TIIG-based polymers (PTIIG-T, PTIIG-Se, and PTIIG-DT) exhibit broad bands covering the visible to near-infrared range of up to 1600 nm. All the polymers exhibit unipolar p-channel operations with regard to gold contacts. PTIIG-DT with centrosymmetric donor exhibits a maximum mobility of 0.20 cm² V⁻¹ s⁻¹ under gold contacts, which is higher than those of the other polymers containing axisymmetric donors. Intriguingly, OFETs fabricated with aluminum electrodes show ambipolar charge transport with hole and electron mobilities of up to 0.28 (PTIIG-DT) and 0.03 (PTIIG-T) cm² V⁻¹ s⁻¹, respectively. This is a record value for the hitherto reported TIIG-based OFETs. The finding demonstrates that TIIG-based polymers can potentially function as either unipolar or ambipolar semiconductors without reliance on the degree of electron affinity of the co-monomers.

A narrow bandgap polymeric semiconductor, BOC-PTDPP, comprising alkyl substituted diketopyrrolopyrrole (DPP) and tert-butoxycarbonyl (t-BOC)-protected DPP, is synthesized and used in organic field-effect transistors (OFETs). The polymer films are prepared by solution deposition and thermal annealing of precursors featuring thermally labile t-BOC groups. The effects of the thermal cleavage on the molecular packing structure in the polymer thin films are investigated using thermogravimetric analysis (TGA), UV-vis spectroscopy, atomic force microscopy (AFM), Fourier transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD) analysis. Upon utilization of solution-shearing process, integrating the ambipolar BOC-PTDPP into transistors shows p-channel dominant characteristics, resulting in hole and electron mobilities as high as 1.32 × 10⁻² cm² V⁻¹ s⁻¹ and 2.63 × 10⁻³ cm² V⁻¹ s⁻¹, which are about one order of magnitude higher than those of the drop-cast films. Very intriguingly, the dominant polarity of charge carriers changes from positive to negative after the thermal cleavage of t-BOC groups at 200 °C. The solution-sheared films upon subsequent thermal treatment show superior electron mobility (μ_e = 4.60 × 10⁻² cm² V⁻¹ s⁻¹), while the hole mobility decreases by one order of magnitude (μ_h = 4.30 × 10⁻³ cm² V⁻¹ s⁻¹). The inverter constructed with the combination of two identical ambipolar OFETs exhibits a gain of ∼10. Reported here for the first time is a viable approach to selectively tune dominant polarity of charge carriers in solution-processed ambipolar OFETs, which highlights the electronically tunable ambipolarity of thermocleavable polymer by simple thermal treatment.

An easily accessible DPP-based small molecule (DMPA-DTDPP) has been synthesized by a simple and efficient route. The resulting molecule, when incorporated into a P3HT:PCBM-based BHJ solar cell, is found to significantly improve the efficiency. The utility of DMPA-DTDPP as an additive yields an increase in the short circuit current density (J_sc) because DMPA-DTDPP serves as an energy funnel for P3HT excitons at the P3HT:PCBM interfaces, resulting in an improved overall power conversion efficiency, compared to the P3HT:PCBM control device. Considering the trouble-free and cost effective synthesis of DMPA-DTDPP, it may prove very useful in high-performance solar cells.

As a characteristic feature of conventional conjugated polymers, it has been generally agreed that conjugated polymers exhibit either high hole transport property (p-type) or high electron transport property (n-type). Although ambipolar properties have been demonstrated from specially designed conjugated polymers, only a few examples have exhibited ambipolar transport properties under limited conditions. Furthermore, there is, as yet, no example with ‘equivalent’ hole and electron transport properties. We describe the realization of an equivalent ambipolar organic field-effect transistor (FET) by using a single-component visible–near infrared absorbing diketopyrrolopyrrole (DPP)-benzothiadiazole (BTZ) copolymer, namely poly[3,6-dithiene-2-yl-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-5’,5’’-diyl-alt-benzo-2,1, 3-thiadiazol-4,7-diyl] (PDTDPP-alt-BTZ). PDTDPP-alt-BTZ shows not only ideally balanced charge carrier mobilities for both electrons (▴_e = 0.09 cm²V⁻¹s⁻¹) and holes (▴_h = 0.1 cm²V⁻¹s⁻¹) but also its inverter constructed with the combination of two identical ambipolar FETs exhibits a gain of ∼35 that is much higher than usually obtained values for unipolar logic.

On the basis of theoretical considerations of the intramolecular charge transfer (ICT) effect, we have designed a series of donor (D)–acceptor (A) conjugated polymers based on bis-benzothiadiazole (BBT). A PPP-type copolymer of electron-rich 2,7-carbazole (CZ) and electron-deficient BBT units poly[N-(2-decyltetradecyl)-2,7-carbazole-co-7,7′-{4,4′-bis-(2,1,3-benzothiadiazole)}] (PCZ-BBT), a PPV-type copolymer poly[N-(2-decyltetradecyl)-2,7-carbazolevinylene-co-7,7′-{4,4′-bis-(2,1,3-benzothiadiazolevinylene)}] (PCZV-BBTV), and a tercopolymer based on carbazole, thiophene, and BBT poly[N-(2-decyltetradecyl)-2,7-(di-2-thienyl)carbazole-co-7,7′-{4,4′-bis-(2,1,3-benzothiadiazole)}] (PDTCZ-BBT) have been synthesized to understand the influence of BBT acceptor structure and linkage on the photovoltaic characteristics of the resulting materials. Both the HOMO and LUMO of the resulting polymers are found to be deeper-lying than those of benzothiadiazole-based polymers. The measured electrochemical band gaps (eV) are in the following order: PDTCZ-BBT (1.65 eV) < PCZV-BBTV (1.69 eV) < PCZ-BBT (1.75 eV). All the polymers provide a photovoltaic response when blended with a fullerene derivative as an electron acceptor. The best cell reaches a power conversion efficiency of 2.07 % estimated under standard solar light conditions (AM1.5G, 100 mW cm⁻²). We demonstrate for the first time that BBT-based polymers are promising materials for use in bulk-heterojunction solar cells.