Volume 55 Issue 4, April 2023

With recent advancements in the Internet of Things (IoT), indoor organic photovoltaic devices (iOPVs) have attracted increasing attention because of their potential utility as self-sustainable, eco-friendly power sources. This review highlights emerging iOPV technologies based on π-conjugated polymers and oligomeric materials and outlines their fundamental principles and characterization techniques.

Chemical design strategies developed in the last decade for conjugated polymer binders in lithium-ion batteries (LIBs) are reviewed here. The first part of this review discusses the mechanism of operation and role of binders in LIBs, and the importance of conductivity in advanced binder systems. The second part of this review gives an account of various conducting polymer binders developed for cathodes and anodes of LIBs. The review concludes with an emphasis on sustainable synthetic design strategies for next-generation conducting polymer binders for greener electrochemical energy storage systems.

As the performance of organic semiconductors has increased so has their molecular structure. In this focused review, we introduce the concept of synergistic catalysis, which involves the use of two or more catalysts with orthogonal reactivity to enable reactions that are not possible with the use of a single catalyst, to synthesize π-conjugated polymers. Synergistic catalysis allows for controlled polymerizations, room-temperature reactions, and/or polymerizations with greater regioselectivity, opening the door to more time-, labor-, cost-, and energy-saving synthetic methods.

We describe here the new concept of developing stimuli-responsive luminochromic materials based on frustrated element-blocks. We termed “frustrated” element-blocks as those that show a relatively larger degree of structural relaxation in the excited state but are structurally restricted. Enhanced sensitivity toward environmental changes and external stimuli can be observed.

Intrinsically stretchable semiconducting polymer materials have been sought for meeting requirements in the development of wearable intelligent electronic devices. In this focus review, our recent progress in main-chain engineering for development of intrinsically stretchable n-type semiconducting materials is described. The topics include four strategies, namely, (i) a conjugation-break spacer approach, (ii) a block copolymer approach, (iii) an all-conjugated statistical terpolymer approach, and (iv) a sequence random copolymer approach, in which specially designed stress-relaxation units or sequences are incorporated along the main chains of naphthalene-diimide-based n-type semiconducting polymers.

In comparison to the ubiquity of their chalcogenic thiophene analogs, selenophene-incorporated conjugated polymers are relatively scarce in the organic electronics literature. However, this scarcity belies the tremendous utility of selenophene in high-performance optoelectronic materials. In this focus review, we discuss the key developments in selenophene-based conjugated polymer research from the early polyselenophene investigations, through the detailed study of regioregular poly(3-hexylselenophene), to the state-of-the-art donor-acceptor polymers that have enabled advancements in the performance and functionality of various optoelectronic devices. Finally, we end with our perspective on the future of this field.

We demonstrate the synthesis of head-to-tail (HT) regioregular poly(3-hexylthiophene) (P3HT) with controlled molecular weight via direct arylation polymerization (DArP). The key to control is using 2-phenylthiazole (2), which has a C–H bond with high reactivity for direct arylation, as the end-capping reagent. The DArP of 2-bromo-3-hexylthiophene (1) in the presence of 2 ([1]₀/[2]₀ = 10–80), Herrmann-Beller catalyst, and P(o-Me₂NC₆H₄)₃ afforded HT-regioregular P3HT (HT = 99%) capped with a 2-phenylthiazole-5-yl group. The number-average molecular weight (M_n) of P3HT agreed well with the value calculated by assuming the formation of one polymer chain per molecule of 2 (polydispersity index: Ð = 1.8–2.1).

C–H/C–I direct arylation reactions for monomer synthesis and C–H/C–Br direct arylation polycondensation were investigated for the short-step synthesis of promising medium-bandgap polymers (PBDB-T) for OPV applications. Polycondensation and end-capping reactions using direct arylation have been established, enabling the synthesis of polymers without Br termini. The OPV performance of the synthesized polymer was lower than that of the benchmark polymer synthesized using the conventional method, despite having similar molecular weights. Possible factors for low OPV performance and future challenges include structural defects, impurities, and large molecular weight distribution.

Vinylene-bridged 5-alkoxy-6-fluorobenzo[c][1,2,5]thiadiazole (FOBTzE) as a novel acceptor unit and its copolymer PFOE4T were designed and synthesized. The incorporation of an electron-donating alkoxy group instead of a fluorine atom prevented strong aggregation tendency and changed the molecular orientation of the polymers from edge-on to bimodal orientation, which was expected to provide uniform thin-film with PC₆₁BM and promoted carrier transport. However, since PFOE4T has still strong aggregation ability and low solubility, the PFOE4T/PC₆₁BM blended film formed a large phase separation, resulting in a power conversion efficiency of only 4.52%.

PBDTNTz1 and PBDTNTz2 combining naphtho[1,2-d:5,6-d’]bis([1,2,3]triazole (NTz) and benzo[1,2-b:4,5-b’]dithiophene (BDT) were successfully synthesized as wide-band gap polymers for nonfullerene acceptor polymer solar cells. Bulk heterojunction solar cells with inverted structures using PBDTNTz2:ITIC showed a 2.39% power conversion efficiency. The device utilizing PBDTNTz1:ITIC presented a 7.37% PCE with a high V_oc of 0.94 V, an FF of 55.8, and a J_sc of 14.04 mA/cm². OFETs using PBDTNTz1 and PBDTNTz2 showed hole mobilities of 2.2 × 10⁻² cm²V⁻¹s⁻¹ and 5.0 × 10⁻² cm²V⁻¹s⁻¹ with current on/off ratios of 1 × 10⁴ and 9 × 10², respectively.

Azide-substituted polystyrene was reacted at both terminals of a poly(thieno[3,2-b]thiophen-2,5-diylbutadiynelene) derivative by a Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) click postfunctionalization to form a coil-rod-coil type triblock copolymer. The resulting block copolymer exhibited higher solubilities than the precursor polymer, but the electronic properties of the conjugated polymer main chain were retained in the solid thin film state.

The feasibility of using strongly electron-deficient naphthobispyrazine bisimide (NPI) as a building unit for n-type semiconducting polymers used in all-polymer organic photovoltaics (OPV) cells was investigated. The NPI-based polymers had sufficiently deep LUMO energy levels of ~−3.7 eV and narrow bandgaps of less than 1.5 eV, which matched well energetically with p-type polymers exhibiting wide optical bandgaps. The NPI-based polymers exhibited a clear photoresponse in all-polymer OPV cells blended with PTB7-Th as a benchmark p-type polymer and showed power conversion efficiencies of up to 1.6%.

Fused π−conjugated molecules have been utilized as key components in organic semiconductors due to their high charge-transport characteristics. In this study, we designed and synthesized a novel fused π−conjugated molecule composed of electron-donating benzodithiophene and electron-accepting naphthobisthiadiazole units and investigated its photo/electrochemical properties and semiconducting characteristics. The fused donor-acceptor-donor (DAD) molecule showed typical p-type characteristics with a hole mobility of 1.1 × 10⁻³ cm² V⁻¹ s⁻¹. We also achieved the synthesis of a D-A type copolymer containing the fused DAD structure as an electron donor unit.

Performance limiting factors in three end-group-brominated ITIC (β, γ, δ):PM6 bulk-heterojunction (BHJ) organic solar cells (OSCs) were investigated. The charge carrier dynamics (mobility and its relaxation) in these OSCs were evaluated by simultaneous time-of-flight (TOF)-time-resolved microwave conductivity (TRMC) measurement and photoinduced charge extraction by linearly increasing voltage (photo-CELIV) technique. The charge separation and recombination were found to be stronger power conversion efficiency-governing factors than mobilities and their balance, as suggested by regression analyses.

Ternary phase diagrams of the solvent/donor/acceptor system were studied to discuss the relationship between the photovoltaic properties of polymer solar cells and the phase-separated structures. The downshifted intersection points between the solvent drying pathway and binodal lines suggest the smaller domain sizes, and hence the higher exciton diffusion efficiency. The appropriate acceptor volume fraction close to the percolation threshold in mixed-phase shows high charge collection efficiency due to an effective charge transport and a suppressed bimolecular recombination.

Tricoordinate boron embedded conjugated polymers, so-called p−π* conjugated polymers, have been studied as optoelectronic and sensor materials. p−π* Conjugated polymers usually possess bulky aryl groups that kinetically stabilize the boron centers, and the bulky aryl substituents prohibit intermolecular interactions in the solid state. In this work, we synthesized new p−π* conjugated polymers based on a thiophene-fused thiaborin unit. The thiaborin-based p−π* conjugated polymers exhibited great red shifts in the absorption spectra measured in film state relative to those measured in solution, suggesting strong intermolecular interactions.

A better understanding of the carrier formation process for semiconducting polymers, especially in thin films, is essential for designing and constructing highly-functionalized polymeric optoelectronic devices. We herein examined effects of the crystalline structure and thermal molecular motion on photo-carrier formation for melt-crystalized thin films of monodisperse poly(3-hexylthiophene) (P3HT) with various molecular weights (MWs), which were prepared from polydisperse sample by preparative chromatography.

By integrating the strongly electron-deficient building blocks cyano-substituted benzochalcogenadiazole and diketopyrrolopyrrole, two A-A polymers were constructed and exhibited decent solubilities, low-lying LUMOs, and ultra-narrow bandgaps, thus leading to a predominant n-type transporting characteristics. When n-type doped with N-DMBI-H, the A-A polymers delivered an n-type performance with a power factor of 9.22 μW m⁻¹ K⁻². The design strategy is highly informative for further developing n-type organic thermoelectric materials.

Transition metal-free aldol condensation was applied to synthesize n-type conjugated polyelectrolyte (CPE), PIIGBDV-N. The quaternary ammonium groups with Br^- ions on the side chain enabled in situ n-doping during polymerization, which resulted in strong polaron absorption in the near infrared region and increased conductivity of the obtained CPE. PIIGBDV-N exhibited good electron transport capability and can be utilized as thickness-insensitive electron transporting layer in high-performance organic solar cell devices.

Stretchable mechanoelectric generators (MEGs) using ‘π-electrets’ composed of π-conjugated polymers modified with bulky, yet flexible alkyl side chains were fabricated. Facile control of the elastic modulus by blending two miscible alkylated π-conjugated polymers enables the laminated films in a MEG to have high stretching of up to 300%. The fabricated MEGs showed good conformability when applied to a soft object that underwent deformation.

Optically active through-space conjugated polymers were prepared from planar chiral pseudo-meta-disubstituted [2.2]paracyclophane. The corresponding optically active pseudo-meta-linked dimers were also prepared, and their optical and chiroptical properties were compared with those of the pseudo-para- and pseudo-ortho-linked dimers. Optically active pseudo-meta-linked dimers and polymers emitted intense circularly polarized luminescence with good anisotropy factors on the order of 10^–3. The electrical photoconductivities of the racemic pseudo-meta- and pseudo-para-linked polymers were also evaluated. A number of properties were explained by using time-dependent density functional theory calculations.

We report a thermally cross-linkable fluorescent polymer and its assembly to insoluble microbeads by hydrothermal annealing. A novel fluorene-based poly(triarylamine) bearing crosslinkable styryl groups on a fluorene unit is newly designed and synthesized by a Pd-catalyzed amination reaction. The polymer self-assembles into spherical microparticles and becomes insoluble to organic solvents by hydrothermal crosslinking without losing the spherical morphology. Mixing of the polymer with polystyrene enhances the photoluminescence quantum yield by suppressing the aggregation induced quenching and induces a spectral shift of the emission band to green and red after annealing.

The first main-chain type triphenylarsine (AsPh₃) polymers were synthesized by Suzuki-Miyaura polycondensation. The electronic and photophysical properties of the AsPh₃ polymers were comparison with model compounds by experimental and computational methods. As a result, it was found that the conjugated systems were expanded through the arsenic atoms. The present AsPh₃ polymers are promising as luminescent materials.

A cholesterol-functionalized poly(3-alkylesterfuran) (P3CHOLEF) was synthesized in three synthetic steps from 2-bromo-3-furoic acid. The polymer is synthesized by Suzuki-Miyaura cross-coupling polymerization using an N-heterocyclic carbene palladium catalyst. P3(CHOL)EF adopts a helical conformation, and the rigid side chain ensures the polymer remains folded in various solvents. This behavior is unique when compared to a similar polymer where the cholesterol group is replaced with S-3-octanol. When P3(CHOL)EF was dissolved in n-octane at room temperature and circularly polarized luminescence behavior was studied, a dissymmetry factor of 0.05 was recorded for the polymer.