Highly fluorescent purine-containing conjugated copolymers with tailored optoelectronic properties

Conjugated copolymers containing electron donor and acceptor units in their main chain have emerged as promising materials for organic electronic devices due to their tunable optoelectronic properties.

Panchromatic Organoboron Molecules with Tunable Absorption Spectra

Panchromatic molecules, e. g. organic small molecules with wide absorption spectra, are very desirable for solar energy-related applications. Here, we report the development of a series of organoboron compounds composed of an organoboron core unit, two π-bridging units and two electron-withdrawing end-capping units. All seven molecules have the HOMO localized on the core unit and the LUMO delocalized on the whole conjugated backbone. They exhibit wide absorption spectra consisting of two strong absorption bands with the full width at half maximum of ca. 280 nm. These panchromatic compounds can be used as electron acceptors in organic solar cells. We elucidate the relationship between the chemical structures and opto-electronic properties of these organoboron panchromatic compounds. Increasing the electron-withdrawing capability of the core units results in a downshifted HOMO level as well as blueshifted long-wavelength absorption band with increased extinction coefficient. Extending the π-bridging units causes an increased HOMO level and blueshifted long-wavelength absorption band with increased extinction coefficients. Weakening the electron-withdrawing capability of the end-capping units leads to an upshifted LUMO level and blueshifted long-wavelength absorption peak with decreased extinction coefficient. This work provides insight into the absorption spectrum manipulation of panchromatic molecules and would pave the way for the development of solar energy-related applications.

Dithieno[3,2-b:2',3'-d]pyrrole-benzo[c][1,2,5]thiadiazole conjugate small molecule donors: effect of fluorine content on their photovoltaic properties

Two new small molecule donors, namely ICT4 and ICT6 with D₁-A-D₂-A-D₁ architecture having 2,4-bis(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (EHDTP, D₁) and 4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene (OBDT, D₂) as the terminal and central donor, and benzo[c][1,2,5]thiadiazole (BT for ICT4) and 5,6-difluorobenzo[c][1,2,5]thiadiazole (F2BT for ICT6) as the acceptor (A) moieties, are synthesized and their optical, electronic and photovoltaic properties are investigated. Both ICT4 and ICT6 have considerable solubility in various solvents and possess efficient light absorption ability [ε (×10⁵ mol^-1 cm^-1) is 0.99 and 1.06, respectively for ICT4 and ICT6] and appropriate frontier molecular orbital energy offsets with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). Bulk heterojunction solar cells (BHJSCs) are fabricated using ICT4/ICT6 and PC₇₁BM as donors and acceptors, respectively and BHJSCs with two-step annealed (thermal followed by solvent vapor annealing) active layers of ICT4 and ICT6 show overall power conversion efficiencies (PCEs) of 5.46% and 7.91%, respectively. The superior photovoltaic performance of the ICT6 based BHJSCs is due to the favourable morphology with a nanoscale interpenetrating network in the ICT6:PC₇₁BM active layer induced by the fluorine atoms on the BT acceptor, which significantly enhances the dissociation of excitons, charge transport and the charge collection efficiency, and suppresses bimolecular recombination in the BHJ. The observed higher PCE of 7.91% indicates that ICT6 is one of the best BT based donor material for small molecular BHJSCs.

Synergistic effect of processing additives and thermal annealing in organic solar cells: the “Morphology of Magic”

The synergistic effect of processing additives and thermal annealing in organic solar cells has been systematically investigated through morphology studies.

Novel Copolymers Based Tetrafluorobenzene and Difluorobenzothiadiazole for Organic Solar Cells with Prominent Open Circuit Voltage and Stability

Two novel copolymers based on benzothiadiazole (BT) or difluorobenzothiadizole (ffBT) with 2,2'-(perfluoro-1,4-phenylene)dithiophene (2TPF4), namely PBT-2TPF4 and PffBT-2TPF4, are synthesized for applications in polymer solar cells (PSCs). A noticeably high open-circuit voltage (V_oc ) of 1.017 and 0.87 V are achieved for PffBT-2TPF4 and PBT-2TPF4-based devices, respectively. Although only a moderate efficiency (5.7%) of PBT-2TPF4-based devices is obtained, it is first demonstrated that 2TPF4 is a promising acceptor block for construction of the donor copolymers which possess high V_oc , prominent crystallinity, and long-term stability, simultaneously. Besides, two thienyl flanking the tetrafluorophenylene can decrease torsion angle between conjugated units, resulting in a high coplanar structure of copolymers to enhance their charge carrier mobility. The findings may open a promising and practical way to accelerate the commercialization of PSCs by developing a series of new donor copolymers for efficient and long-term stable thickness bulk heterojunction PSCs.

Synthesis of Main Chain Purine-Based Copolymers and Effects of Monomer Design on Thermal and Optical Properties

The ability to incorporate diverse monomeric building blocks enables the development of advanced polymeric materials possessing a wide range of properties that suits them for myriad applications. Herein, that synthetic toolbox is expanded through the first report of purine-based copolymers in which purines are incorporated directly into the polymer main chain. Stille cross-coupling of dibromopurine monomers with benzodithiophene (BDT) comonomers is used to generate these "poly(purine)s", and variations in the substitution pattern of the purine monomer and BDT side-chains provides insight into the role of monomer design on their resultant thermal and photophysical properties. Specifically, thermal analyses show that poly(purine)s exhibit high thermal stability and high glass transition temperatures depending on the BDT side-chain substituents and substitution pattern of the purine-derived comonomer. Furthermore, optical properties measured via UV-vis and fluorescence spectroscopies show dependence on monomer substitution pattern. These findings demonstrate the viability of synthesizing poly(purine)s via metal-catalyzed cross-coupling reactions and highlight the potential to tailor poly(purine) properties via simple alterations of comonomers.

Synthesis and characterization of acceptor–donor–acceptor-based low band gap small molecules containing benzoselenadiazole

Acceptor–donor–acceptor-type compounds 5′,5″-(benzo[c][1,2,5]selenadiazole-4,7-diyl)bis(3′-dodecyl-2,2′-bithiophene-5-carbonitrile) (9) and 4,4′-(5,5′-(benzo[c][1,2,5]selenadiazole-4,7-diyl)bis(3-dodecylthiophene-5,2-diyl))dibenzonitrile (10) were designed and synthesized. These compounds differ in terminal positions, compound 9 with a thiophene-containing nitrile group and compound 10 with a phenyl-containing nitrile group. Both compounds have shown good thermal stability and low band gap. The band gaps of compounds 9 and 10 were 1.74 and 1.83 eV, respectively. These results indicate that they are promising materials for use in optoelectronics.