Chakkooth Vijayakumar

Thienoisoindigo-based low-band gap polymers for organic electronic devices

We synthesized a series of new low-band gap donor–acceptor copolymers containing 4,4′-bis(alkyl)-[6,6′-bithieno[3,2-b]pyrrolylidene]-5,5′(4H,4′H)-dione. This acceptor unit, so-called dithienoketopyrrole (DTKP), is an analogue of isoindigo, the phenyl rings of which are replaced by thiophenes. Donor moieties such as benzodithiophene, cyclopentadithiophene, fluorene, and dithienothiophene are polymerized with DTKP in an alternating fashion by Stille or Suzuki–Miyaura coupling methods. Exceedingly low-band gaps (Eg = 1.0–1.6 eV) were achieved in these copolymers through internal charge transfer interactions between the donor and acceptor moieties. The structural, photophysical, and electrochemical properties of the resultant copolymers were characterized, and field-effect transistor (FET) mobilities were measured. The copolymers showed electronic absorption spectra extending to the near infrared region (600–1400 nm) with absorption maxima at 745–971 nm, along with a low-lying LUMO of −3.8 eV. Density functional theory (DFT) calculation indicated high planarity for the copolymer backbone when compared to that of its phenyl-isoindigo counterparts. FET hole mobilities on the order of 10−4 to 10−3 cm2 V−1 s−1 were obtained, demonstrating a feasibility to use them in organic photovoltaic cells.

Oligofluorene-based electrophoretic nanoparticles in aqueous medium as a donor scaffold for fluorescence resonance energy transfer and white-light emission

Self-assembled, surfactant-free organic nanoparticles of an oligofluorene derivative with high colloidal stability were prepared in aqueous medium. The blue emission of the nanoparticles was tuned to white through fluorescence resonance energy transfer (FRET) by encapsulating an orange–red emitting dye (1 mol%) within the nanoparticle scaffold.

A Versatile Approach to Organic Photovoltaics Evaluation Using White Light Pulse and Microwave Conductivity

State-of-the-art low band gap conjugated polymers have been investigated for application in organic photovoltaic cells (OPVs) to achieve efficient conversion of the wide spectrum of sunlight into electricity. A remarkable improvement in power conversion efficiency (PCE) has been achieved through the use of innovative materials and device structures. However, a reliable technique for the rapid screening of the materials and processes is a prerequisite toward faster development in this area. Here we report the realization of such a versatile evaluation technique for bulk heterojunction OPVs by the combination of time-resolved microwave conductivity (TRMC) and submicrosecond white light pulse from a Xe-flash lamp. Xe-flash TRMC allows examination of the OPV active layer without requiring fabrication of the actual device. The transient photoconductivity maxima, involving information on generation efficiency, mobility, and lifetime of charge carriers in four well-known low band gap polymers blended with phenyl-C(61)-butyric acid methyl ester (PCBM), were confirmed to universally correlate with the PCE divided by the open circuit voltage (PCE/V(oc)), offering a facile way to predict photovoltaic performance without device fabrication.

Oligofluorene-based nanoparticles in aqueous medium: hydrogen bond assisted modulation of functional properties and color tunable FRET emission

Fluorene-based linear π-conjugated oligomers with different end functional groups having zero- (OF1), one- (OF2), two- (OF3) and three-point (OF4) hydrogen bonding sites were synthesized and characterized. By using a reprecipitation method, self-assembled nanoparticles were prepared in aqueous medium. The spherical shape and amorphous nature of nanoparticles were established by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Zeta potential measurements showed that nanoparticles of OF2–4 have good colloidal stability, whereas those of OF1 have only moderate stability indicating that the hydrogen bonding groups in OF2–4 interact with the polar water molecules providing stability to the assembly. However, the interior of the nanoparticles remained non-polar, thus providing a conducive medium for hydrogen bonding between the oligofluorene molecules. This leads to varying interchromophore interactions in OF1–4 in the nanoparticle state depending on the H-bonding strength of the end groups. Dynamic light scattering (DLS) studies revealed that under identical conditions, the size of the nanoparticles decreased with increasing number of hydrogen bonding sites in the molecule. The interchromophore interactions were evident from the UV-Vis absorption and fluorescence studies. Bright blue fluorescence of the molecules in solution undergoes quenching in the nanoparticle state. The fluorescence quenching significantly increases from OF1 to OF4 indicating enhanced interaction between chromophores with increasing number of hydrogen bonding sites in the molecules. The nanoparticles were used as a donor scaffold for fluorescence resonance energy transfer (FRET) by encapsulating varying amounts of an orange red emitting neutral dye (D1) thereby achieving colour tunable emission including white. FRET studies were also conducted with a cationic dye (D2) adsorbed on the negatively charged nanoparticle surface. The FRET efficiency with both dyes showed direct correlation with the number of hydrogen bonding sites in the molecules.

Optoelectronic properties of dicyanofluorene-based n-type polymers.

Three new donor-acceptor-type copolymers (P1-P3) consisting of dicyanofluorene as acceptor and various donor moieties were designed and synthesized. Optoelectronic properties were studied in detail by means of UV-visible absorption and fluorescence spectroscopy, cyclic voltammetry, space-charge-limited current (SCLC), flash-photolysis time-resolved microwave conductivity (FP-TRMC), and density functional theory (DFT). All polymers showed strong absorption in the UV-visible region and the absorption maximum undergoes redshift with an increasing number of thiophene units in the polymer backbone. SCLC analysis showed that the electron mobilities of the polymers in the bulk state were 1 to 2 orders higher than that of the corresponding hole mobilities, which indicated the n-type nature of the materials. By using FP-TRMC, the intrapolymer charge-carrier mobility was assessed and compared with the interpolymer mobility obtained by SCLC. The polymers exhibited good electron-accepting properties sufficiently high enough to oxidize the excited states of regioregular poly(3-hexylthiophene) (P3HT (donor)), as evident from the FP-TRMC analysis. The P3 polymer exhibited the highest FP-TRMC transients in the pristine form as well as when blended with P3HT. Use of these polymers as n-type materials in all-polymer organic solar cells was also explored in combination with P3HT. In accordance with the TRMC results, P3 exhibited superior electron-transport and photovoltaic properties to the other two polymers, which is explained by the distribution of the energy levels of the polymers by using DFT calculations.

Optical and electrical properties of dithienothiophene based conjugated polymers: medium donor vs. weak, medium, and strong acceptors

Herein we report the systematic study of the structure–property relationship of a few dithienothiophene (DTT) based donor–acceptor conjugated polymers using various techniques such as UV-vis absorption and fluorescence spectroscopy, cyclic voltammetry (CV), flash-photolysis time-resolved microwave conductivity (FP-TRMC), density functional theory (DFT), X-ray diffraction (XRD), and field-effect transistor (FET). A medium donor, DTT, was coupled in an alternating fashion with thiazole-based weak, medium, and strong acceptors. Though the optical properties showed good correlation with the donor–acceptor strength, the FET properties indicated significant deviation. The XRD analysis and DFT calculations revealed that the deviation is caused by the difference in structural ordering of the polymers in the film state. Since the FP-TRMC analysis reflects the properties of semiconducting organic materials at the molecular level such as the donor–acceptor strength and structural ordering in the film state, it showed good correlation with FET properties. Thus the present work illustrates that the study of charge carrier generation and mobility dynamics by FP-TRMC is a valuable addition to the conventional structure–property analysis methods, and is reliable to find the suitability of conjugated polymers for electronic device applications.