Simple π-Conjugated Polymers Based on Bithiazole for Nonfullerene Organic Photovoltaics

Thiazole, as a family of five-membered heteroaromatic rings, is an interesting building unit that can play a role in coplanarizing the backbone as well as deepening the HOMO energy level, which is beneficial for the design of π-conjugated polymers for the photoactive materials in organic photovoltaics (OPVs). Here, we designed and synthesized π-conjugated polymers with simple chemical structures, which consist of 2,2'-bithiazole or 5,5'-bithiazole and alkylthiophenes as the polymer backbone. In fact, the polymers can be easily synthesized in much fewer steps compared to the typical high-performance polymers based on fused heteroaromatic rings. Interestingly, PTN5 exhibited a markedly higher ordered structure than PTN2. This was likely ascribed to the more coplanar and rigid backbone of PTN5 than that of PTN2 originating in the effectively arranged S···N interaction. As a result, the nonfullerene photovoltaic cell based on PTN5 showed a PCE of 12.2%, which was much higher than the cell based on PTN2 (4.3%) and was high for the polymers consisting of only nonfused rings. These results demonstrate that thiazole-based polymers are promising photoactive materials for OPVs and emphasize the importance of careful molecular design utilizing noncovalent interactions.

The Characteristics of the Metal-Free and Non-Conjugated Polymer Film with Self-Assembled Nanoparticles

It is shown in this paper that a polymer, MA-PEG 1000-DGEBA (MP1D), exhibits antireflection, substrate-dependent photoluminescence (SDP), wide band-gap, and photoconduction characterization. MP1D was synthesized from maleic anhydride, polyethylene glycol 1000, and bisphenol-A diglycidyl ether. Self-assembled nanoparticles embedded in MP1D film and ranging from 2.5 to 31.6 nm are observed, which could be expected as scatterers to enhance light trapping and extraction. The size of the nanoparticle increases with the concentration of the MP1D solution. Besides solution concentration, the nanoparticle dimension could be modified by the chain length of polyethylene glycol in the polymer synthesis. The effects of solution concentration, annealing temperature, annealing period, and substrate on the photoluminescence (PL) of MP1D films are examined. Increasing solution concentration increases PL intensity. However, aggregation-caused quenching is explicit as the solution concentration exceeds 100 mM. PL intensity increases with annealing temperature, which could be attributed to crystallinity improvement. PL intensity increases with increasing the annealing period from 0.5 to 2 h. Nonetheless, as the annealing period exceeds 2 h, PL quenching is emerging, which could be due to aggregation. It is expected that MP1D could be a promising candidate for host materials and MP1D film could play a multifunctional role (antireflective and light-trapping functions) in optoelectronics.

Acceptor engineering for NIR-II dyes with high photochemical and biomedical performance

It is highly important and challenging to develop donor-acceptor-donor structured small-molecule second near-infrared window (NIR-II) dyes with excellent properties such as water-solubility and chem/photostability. Here, we discovery an electron acceptor, 6,7-di(thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (TQT) with highest stability in alkaline conditions, compared with conventional NIR-II building block benzobisthiadiazole (BBT) and 6,7-diphenyl-[1,2,5] thiadiazolo[3,4-g]quinoxaline (PTQ). The sulfonated hydrophilic dye, FT-TQT, is further synthesized with 2.13-fold increased quantum yield than its counterpart FT-BBT with BBT as acceptor. FT-TQT complexed with FBS is also prepared and displays a 16-fold increase in fluorescence intensity compared to FT-TQT alone. It demonstrates real-time cerebral and tumor vessel imaging capability with µm-scale resolution. Dynamic monitoring of tumor vascular disruption after drug treatment is achieved by NIR-II fluorescent imaging. Overall, TQT is an efficient electron acceptor for designing innovative NIR-II dyes. The acceptor engineering strategy provides a promising approach to design next generation of NIR-II fluorophores which open new biomedical applications.

Small molecule NIR-II fluorophores are of interest for a range of applications but can suffer from chemical and photostability issues. Here, the authors report on the development of an acceptor molecule with improved stability in alkaline conditions expanding the range of possible applications.

The Optical Properties of Metal-Free Polymer Films with Self-Assembled Nanoparticles

In this paper, it is reported that a metal-free and non-conjugated polymer, MA-PEG 8000-BADGE (MP8B), exhibits an antireflective property and substrate-dependent photoluminescence (SDP). MP8B was constructed from maleic anhydride, poly(ethylene glycol) and bisphenol-A diglycidyl ether. Self-assembled nanoparticles are found in MP8B and can prospectively act as scattering centers to improve light trapping and extraction. MP8B films prepared from MP8B solutions have been characterized by photoluminescence (PL), atomic force microscopy (AFM), tunnelling electron microscope (TEM), reflectance, transmittance, and UV-Vis absorption spectrum. MP8B films can suppress light reflection and enhance light transmission. The PL spectrum of MP8B film on ITO peaks at approximately 538 nm, spanning from 450 to 660 nm at a concentration of 25 mM. Meanwhile, the effects of concentration and substrate on the PL of MP8B films are also investigated in this study. Surface roughness becomes larger with concentration. A red shift of the PL spectrum is observed as solution concentration increases. Meanwhile, aggregation-caused quenching (ACQ) is insignificant. Moreover, the PL spectra of MP8B films show a substrate-dependent phenomenon due to dielectric screening. The optical band-gap energy of MP8B is approximately 4.05 eV. It is concluded that MP8B is a promising candidate for a host material, and its film can be utilized as a multifunctional layer (i.e., antireflective and light-scattering functions) for optoelectronic applications.

Synthesis and characterization of an electron-deficient conjugated polymer based on pyridine-flanked diketopyrrolopyrrole

We report a pyridine-flanked diketopyrrolopyrrole monomer and all-acceptor conjugated polymers synthesized therefrom. Photophysical, thermal and electrochemical properties of the polymers have been determined.

Synthesis of novel conjugated polymers based on benzo[1,2-d:4,5-d′]-bis([1,2,3]triazole) for applications in organic field-effect transistors

New conjugated polymers based on 4,8-bis(5-bromothiophen-2-yl)-2,6-bis(2-octyldodecyl)-2H-benzo[1,2-d:4,5-d′]- bis([1,2,3]triazole)-6-ium-5-ide (BTBBTa) displayed the highest hole mobility of 0.21 cm² V⁻¹ s⁻¹.

Modulating Surface Morphology and Thin-Film Transistor Performance of Bi-thieno[3,4-c]pyrrole-4,6-dione-Based Polymer Semiconductor by Altering Preaggregation in Solution

Due to their strong intermolecular interactions, polymer semiconductors aggregate in solution even at elevated temperature. With the aim to study the effect of this kind preaggregation on the order of thin films and further transistor performance, bi-thieno[3,4-c]pyrrole-4,6-dione and fluorinated oligothiophene copolymerized polymer semiconductor P1, which shows strong temperature-dependent aggregation behavior in solution, is synthesized. Its films are deposited through a temperature-controlled dip-coating technique. X-ray diffraction and atomic force microscopy results reveal that the aggregation behavior of P1 in solution affects the microstructures and order of P1 films. The charge transport properties of P1 films are investigated with bottom-gate top-contacted thin-film transistors. The variation of device performance (from 0.014  to 1.03 cm² V^-1 s^-1) demonstrates the importance of optimizing preaggregation degree. The correlation between preaggregation degree and transistor performance of P1 films is explored.

Effects of Dielectric Material, HMDS Layer, and Channel Length on the Performance of the Perylenediimide-Based Organic Field-Effect Transistors

It is well-known that the improvement in the performance of organic field-effect transistors (OFETs) relies primarily on growth properties of organic molecules on gate dielectrics, their interface behavior, and on understanding the physical processes occurring during device operation. In this work, the relation of varying the dielectric materials in an n-type OFET device based on 1,7-dibromo-N,N'-dioctadecyl-3,4,9,10-perylenetetracarboxylic diimide (Br₂PTCDI-C18) molecule on a low-cost glass substrate at different channel lengths is reported, which is conceptually very important and fundamental in the context of device performance. Anodized alumina (Al₂O₃) along with dielectric films of polyvinyl alcohol (PVA) or polymethylmethacrylate (PMMA) was used to fabricate the devices and study their influence on various transistor properties. In addition, the effects of a thin hexamethyldisilazane (HMDS) layer on the performance of OFETs including their contact resistances were studied with the channel length variations. The devices with PVA dielectric material exhibited the maximum mobility values of 0.012-0.025 cm² V^-1 s^-1 irrespective of varying channel lengths from 25 to 190 μm. The bias-stress measurements were recorded to realize the effects of the channel length and HMDS layer on the stability of the devices. The on/off ratios and electrical stabilities of these devices were enhanced significantly by modifying the surface of the PVA dielectric layer using a thin layer of HMDS. Similarly, in the case of PMMA dielectric layer, a drastic enhancement in the on/off ratio and bias-stress stability was observed. Characterization of all devices at different channel lengths using different dielectric materials permitted us to identify the effects of contact resistance on OFET devices. The stability of the devices in relation to the bias-stress measurements of devices by varying channel lengths and surface modification was systematically investigated. A careful analysis of oxide gate dielectrics modified with polymer-based dielectric materials, contact resistance, influence of thin HMDS layer on the electrical properties, and other parameters on top-contact bottom-gated configured n-type OFET devices is presented herein.