Energy-Level Manipulation in Novel Indacenodithiophene-Based Donor-Acceptor Polymers for Near-Infrared Organic Photodetectors

Customizable Organic Charge-Transfer Cocrystals for the Dual-Mode Optoelectronics in the NIR (II) Window

Organic molecules have been regarded as ideal candidates for near-infrared (NIR) optoelectronic active materials due to their customizability and ease of large-scale production. However, constrained by the intricate molecular design and severe energy gap law, the realization of optoelectronic devices in the second near-infrared (NIR (II)) region with required narrow band gaps presents more challenges. Herein, we have originally proposed a cocrystal strategy that utilizes intermolecular charge-transfer interaction to drive the redshift of absorption and emission spectra of a series BFXTQ (X = 0, 1, 2, 4) cocrystals, resulting in the spectra located at NIR (II) window and reducing the optical bandgap to ∼0.98 eV. Significantly, these BFXTQ-based optoelectronic devices can exhibit dual-mode optoelectronic characteristics. An investigation of a series of BFXTQ-based photodetectors exhibits detectivity (D*) surpassing 1013 Jones at 375 to 1064 nm with a maximum of 1.76 × 1014 Jones at 1064 nm. Moreover, the radiative transition of CT excitons within the cocrystals triggers NIR emission over 1000 nm with a photoluminescence quantum yield (PLQY) of ∼4.6% as well as optical waveguide behavior with a low optical-loss coefficient of 0.0097 dB/μm at 950 nm. These results promote the advancement of an emerging cocrystal approach in micro/nanoscale NIR multifunctional optoelectronics.

A Three-in-One Hybrid Strategy for High-Performance Semiconducting Polymers Processed from Anisole

The development of semiconducting polymers with good processability in green solvents and competitive electrical performance is essential for realizing sustainable large-scale manufacturing and commercialization of organic electronics. A major obstacle is the processability-performance dichotomy that is dictated by the lack of ideal building blocks with balanced polarity, solubility, electronic structures, and molecular conformation. Herein, through the integration of donor, quinoid and acceptor units, an unprecedented building block, namely TQBT, is introduced for constructing a serial of conjugated polymers. The TQBT, distinct in non-symmetric structure and high dipole moment, imparts enhanced solubility in anisole-a green solvent-to the polymer TQBT-T. Furthermore, PTQBT-T possess a highly rigid and planar backbone owing to the nearly coplanar geometry and quinoidal nature of TQBT, resulting in strong aggregation in solution and localized aggregates in film. Remarkably, PTQBT-T films spuncast from anisole exhibit a hole mobility of 2.30 cm2 V-1 s-1, which is record high for green solvent-processable semiconducting polymers via spin-coating, together with commendable operational and storage stability. The hybrid building block emerges as a pioneering electroactive unit, shedding light on future design strategies in high-performance semiconducting polymers compatible with green processing and marking a significant stride towards ecofriendly organic electronics.

An n-Type All-Fused-Ring Molecule with Photoresponse to 1000 nm for Highly Sensitive Near-Infrared Photodetector

Most of all-fused-ring π-conjugated molecules have wide or medium bandgap and show photo response in the visible range. In this work, an all-fused-ring n-type molecule, which exhibits an ultrasmall optical bandgap of 1.22 eV and strong near-infrared (NIR) absorption with an onset absorption wavelength of 1013 nm is reported. The molecule consists of 14 aromatic rings and has electron donor-acceptor characteristics. It exhibits excellent n-type properties with low-lying HOMO/LUMO energy levels of -5.48 eV/-3.95 eV and high electron mobility of 7.0 × 10^-4  cm²  V^-1  s^-1 . Most importantly, its thin film exhibits a low trap density of 5.55 × 10¹⁶  cm^-3 because of the fixed molecular conformation and consequently low conformation disorder. As a result, organic photodetector (OPD) based on the compound exhibits a remarkably low dark current density (J_d ) of 2.01 × 10^-10  A cm^-2 at 0 V. The device shows a shot-noise-limited specific detectivity (D_sh *) of exceeding 10¹³  Jones at 400-1000 nm wavelength region with a peak specific detectivity of 4.65 × 10¹³  Jones at 880 nm. This performance is among the best reported for self-powered NIR OPDs.

Tuning the nanostructure and molecular orientation of high molecular weight diketopyrrolopyrrole-based polymers for high-performance field-effect transistors

As a versatile class of semiconductors, diketopyrrolopyrrole (DPP)-based conjugated polymers are well suited for applications of next-generation plastic electronics because of their excellent and tunable optoelectronic properties via a rational design of chemical structures. However, it remains a challenge to unravel and eventually influence the correlation between their solution-state aggregation and solid-state microstructure. In this contribution, the solution-state aggregation of high molecular weight PDPP3T is effectively enhanced by solvent selectivity, and a fibril-like nanostructure with short-range and long-range order is generated and tuned in thin films. The predominant role of solvent quality on polymer packing orientation is revealed, with an orientational transition from a face-on to an edge-on texture for the same PDPP3T. The resultant edge-on arranged films lead to a significant improvement in charge transport in transistors, and the field-effect hole mobility reaches 2.12 cm² V^-1 s^-1 with a drain current on/off ratio of up to 10⁸. Our findings offer a new strategy for enhancing the device performance of polymer electronic devices.

Dual-Band, Efficient Self-Powered Organic Photodetectors with Isotype Subphthalocyanine-Based Heterojunctions toward Secure Optical Communications

High-performance heterojunction organic photodetectors (OPDs) are of great significance in optical detecting technology due to their tailorable optoelectronic properties. Herein, we designed and synthesized three n-type subphthalocyanine (SubPc) derivatives PhO-BSubPcF₁₂, CHO-PhO-BSubPcF₁₂, and NO₂-PhO-BSubPcF₁₂ via axial nonhalogen substitution on fluorinated SubPc. These SubPc derivatives exhibit improved intramolecular charge transfer, high electron mobilities, optimized energy levels, and good thermal stability. The novel isotype p-n SubPc heterojunctions are evaluated as photosensitive layers in OPDs, which show a UV-visible dual-band response and self-powered effect. The optimal OPD with Br-BSubPc/NO₂-PhO-BSubPcF₁₂ presents stable and superior performances with a high responsivity (R) of 0.14 A W^-1, a peak external quantum efficiency (EQE) of 30.6%, and an extremely low dark current of 0.92 nA cm^-2 under a 570-595 nm illumination without a bias voltage. It has outperformed most of the reported SubPc-based OPDs. The better interfacial contact of p-n SubPc derivatives leads to a large depletion region with decreased trap densities as well as a low carrier recombination rate, which is conducive to the photoinduced carriers' separation and well-balanced transport, resulting in high device performances. Moreover, a secure communication strategy is successfully demonstrated by dual-band optimal OPD. This work is expected to provide some guidance for molecular engineering and device performance toward multifunctional electronics.

Electron-donating amine-interlayer induced n-type doping of polymer:nonfullerene blends for efficient narrowband near-infrared photo-detection

Inherently narrowband near-infrared organic photodetectors are highly desired for many applications, including biological imaging and surveillance. However, they suffer from a low photon-to-charge conversion efficiencies and utilize spectral narrowing techniques which strongly rely on the used material or on a nano-photonic device architecture. Here, we demonstrate a general and facile approach towards wavelength-selective near-infrared phtotodetection through intentionally n-doping 500-600 nm-thick nonfullerene blends. We show that an electron-donating amine-interlayer can induce n-doping, resulting in a localized electric field near the anode and selective collection of photo-generated carriers in this region. As only weakly absorbed photons reach this region, the devices have a narrowband response at wavelengths close to the absorption onset of the blends with a high spectral rejection ratio. These spectrally selective photodetectors exhibit zero-bias external quantum efficiencies of ~20-30% at wavelengths of 900-1100 nm, with a full-width-at-half-maximum of ≤50 nm, as well as detectivities of >10¹² Jones.

Synthetic Route to Conjugated Donor–Acceptor Polymer Brushes via Alternating Copolymerization of Bifunctional Monomers

Alternating donor–acceptor conjugated polymers, widely investigated due to their applications in organic photovoltaics, are obtained mainly by cross-coupling reactions. Such a synthetic route exhibits limited efficiency and requires using, for example, toxic palladium catalysts. Furthermore, the coating process demands solubility of the macromolecules, provided by the introduction of alkyl side chains, which have an impact on the properties of the final material. Here, we present the synthetic route to ladder-like donor–acceptor polymer brushes using alternating copolymerization of modified styrene and maleic anhydride monomers, ensuring proper arrangement of the pendant donor and acceptor groups along the polymer chains grafted from a surface. As a proof of concept, macromolecules with pendant thiophene and benzothiadiazole groups were grafted by means of RAFT and metal-free ATRP polymerizations. Densely packed brushes with a thickness up to 200 nm were obtained in a single polymerization process, without the necessity of using metal-based catalysts or bulky substituents of the monomers. Oxidative polymerization using FeCl₃ was then applied to form the conjugated chains in a double-stranded (ladder-like) architecture.

Copolymers Containing 1-Methyl-2-phenyl-imidazole Moieties as Permanent Dipole Generating Units: Synthesis, Spectroscopic, Electrochemical, and Photovoltaic Properties

New donor–acceptor conjugated alternating or random copolymers containing 1-methyl-2-phenylbenzimidazole and benzothiadiazole (P1), diketopyrrolopyrrole (P4), or both acceptors (P2) are reported. The specific feature of these copolymers is the presence of a permanent dipole-bearing moiety (1-methyl-2-phenyl imidazole (MPI)) fused with the 1,4-phenylene ring of the polymer main chain. For comparative reasons, polymers of the same main chain but deprived of the MPI group were prepared, namely, P5 with diketopyrrolopyrrole and P3 with both acceptors. The presence of the permanent dipole results in an increase of the optical band gap from 1.51 eV in P3 to 1.57 eV in P2 and from 1.49 eV in P5 to 1.55 eV in P4. It also has a measurable effect on the ionization potential (IP) and electrochemical band gap (E_g^CV), leading to their decrease from 5.00 and 1.83 eV in P3 to 4.92 and 1.79 eV in P2 as well as from 5.09 and 1.87 eV in P5 to 4.94 and 1.81 eV in P4. Moreover, the presence of permanent dipole lowers the exciton binding energy (E_b) from 0.32 eV in P3 to 0.22 eV in P2 and from 0.38 eV in P5 to 0.26 eV in P4. These dipole-induced changes in the polymer properties should be beneficial for photovoltaic applications. Bulk heterojunction solar cells fabricated from these polymers (with PC₇₁BM acceptor) show low series resistance (r_s), indicating good electrical transport properties. The measured power conversion efficiency (PCE) of 0.54% is limited by the unfavorable morphology of the active layer.

Influence of π-Endcaps on the Performance of Functionalized Quinolines for p-Channel OFETs

This study investigates the influence of aryl and ethynyl linkers as well the effect of various pi-end-groups on the performance of the quinoline-based organic field-effect transistors. A series of new functionalized quinolines with D-π-A-π-D and A-π-A-π-A architectures are designed and synthesized via the Sonagashira cross-coupling reaction. All the new compounds are well characterized and their photophysical properties are studied. The bottom gate-top contact-organic field-effect transistors devices are fabricated using the spin-coating technique. By employing the pre and post-annealing technique, films with uniform surface coverage are obtained. The variation in the end-groups results in versatile packing arrangements which determine their good charge transport properties. The p-channel transistor behavior is observed for all the new compounds. Among the molecules studied, methoxyphenyl and thiophen-2-yl terminal functionalized with D-π-A-π-D architecture exhibit the higher p-channel transistor characteristics with hole mobilities of 1.39 and 1.33 cm² V^-1 s^-1 , respectively. The good charge carrier mobilities are supported by an electron-donating methoxy group and thiophene as the end-groups with high highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) levels, extensive π-conjugation, and better self-assembly.