A. Dhanabalan

Synthesis and Characterization of a Low Bandgap Conjugated Polymer for Bulk Heterojunction Photovoltaic Cells

Low optical bandgap conjugated polymers may improve the efficiency of organic photovoltaic devices by increasing the absorption in the visible and near infrared region of the solar spectrum. Here we demonstrate that condensation polymerization of 2,5-bis(5-trimethylstannyl-2-thienyl)-N-dodecylpyrrole and 4,7-dibromo-2,1,3-benzothiadiazole in the presence of Pd(PPh3)2Cl2 as a catalyst affords a novel conjugated oligomeric material (PTPTB), which exhibits a low optical bandgap as a result of the alternation of electron-rich and electron-deficient units along the chain. By varying the molar ratio of the monomers in the reaction and fractionation of the reaction product, two different molecular weight fractions (PTPTB-I and PTPTB-II, see Experimental section) were isolated, containing 5–17 and 13–33 aromatic units respectively, as inferred from matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Thin films of PTPTB-I and PTPTB-II exhibit an optical bandgap of 1.60 and 1.46 eV, respectively. Photoinduced absorption (PIA) and photoluminescence spectroscopy of blends of PTPTB-I and a methanofullerene (1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C61, PCBM) gave direct spectral evidence of the photoinduced electron-transfer reaction from PTPTB-I as a donor to the fullerene derivative as an acceptor. Thin PTPTB-I:PCBM composite films were sandwiched between indium tin oxide/poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (ITO/PEDOT:PSS) and Al electrodes to prepare working photovoltaic devices, which show an open circuit voltage of 0.67 V under white-light illumination. The spectral dependence of the device shows an onset of the photocurrent at 1.65 eV (750 nm).

Modern physicochemical research on Langmuir monolayers

Recent developments in characterising Langmuir monolayers of a variety of film-forming materials and employing several physicochemical techniques are reviewed. The extension of the LB method to non-amphiphilic substances, especially macromolecular systems, has increased the need of a thorough understanding of Langmuir film properties, which requires characterising techniques that provide complementary information. Since there is vast literature in the subject, only selected examples are given of results that illustrate the potential of the techniques discussed.

Preparation and characterization of composite LB films of lignin and cadmium stearate

Abstract We report on studies of composite Langmuir films containing lignin (extracted from sugar cane bagasse) and cadmium stearate and their subsequent transfer as Langmuir–Blodgett (LB) films. Pure lignin was found to form non-monomolecular films at the air-water interface. Composite monolayers of lignin and cadmium stearate with varying compositions have been obtained and characterized by surface pressure and surface potential isotherm studies which essentially indicated the formation of a meta-stable composite monolayer at the air-water interface. Nevertheless, these composite monolayers could be uniformly transferred onto solid substrates under optimized deposition conditions. Transferred multilayer LB films were characterized by UV-vis and Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and surface potential measurements. FTIR studies confirmed the transfer of lignin along with cadmium stearate. UV-vis results revealed a layer-by-layer transfer of the composite monolayer. In the composite film, both cadmium stearate and lignin were present as separate domains and the stacking order in the cadmium stearate domains was found to be influenced by the incorporation of lignin, as evidenced from XRD. Surface potential measurements indicated a good film uniformity.

Langmuir and Langmuir–Blodgett films of a homopolymer of Disperse Red-13

We report on the formation of stable and uniformly transferable Langmuir monolayers of a methacrylic homopolymer of Disperse Red-13 (HPDR13). There was no need of covalently attaching long alkyl side chains or mixing the homopolymer with film forming materials. HPDR13 was derived from DR13-methacrylate by a homo polymerization reaction in the presence of 2,2′-azobisisobutyronitrile as initiator. Surface pressure isotherms were obtained under various subphase temperatures, pH conditions and at several compression speeds. The monolayer has also been characterized by surface potential isotherms. The results indicated that, in contrast to the poor stability of a methacrylate-DR13 monomer monolayer, stable monolayers of HPDR13 could be obtained at the air–water interface. A uniform layer by layer transfer has been demonstrated by the linear increase of maximum absorption with increasing film thickness. A red shift observed in the UV–Vis spectra of the Langmuir–Blodgett (LB) films revealed a possible J-type aggregation of molecules. LB films have also been characterized by Fourier Transform Infra Red (FTIR) spectroscopy.

Design and synthesis of new processible donor-acceptor dyad and triads

We report the design, synthesis and characterizarion of new processible well-defined donor (D)-acceptor (A) dyads and A-D-A triads for use in photovoltaic devices. The donor oligomers, which consist either of oligoaniline, thiophene-pyrrole-thiophene, phenyl-pyrrole-phenyl, or phenyl-thiophene-pyrrole-thiophene-phenyl segments, were covalently attached to fullerene C 60 which acts an acceptor. All compounds were fully characterized using different analytical techniques.

Langmuir monolayers of lignins obtained with different isolation methods

Abstract The characteristics of Langmuir monolayers of lignins isolated via distinct methods, viz. acetone–oxygen–organosolv (termed Lignin-1), Kraft (Lignin-2), acetic acid–organosolv (Lignin-3) and acetone–organosolv (Lignin-4), are investigated using surface pressure and surface potential measurements. The overall behavior of these lignins is essentially the same, but some differences were observed which are attributed to the relative amount of carbonyl groups in the lignin skeleton. Lignin-1 and 3 displayed pressure–area isotherms considerably more expanded because they possess the largest amounts of carbonyls that are forced out of the water upon monolayer compression in the condensed phase. These carbonyls contribute negatively to the surface potential. For Lignin-1, where such groups appear in COOH moieties, the surface potential is much smaller than for the other lignins. Hysteresis and stability experiments showed that the lignin monolayers are metastable possibly owing to molecular reorganization at the air–water interface.

Optical storage in mixed Langmuir-Blodgett (LB) films of disperse Red 19

We report on the optical storage characteristics of mixed Langmuir-Blodgett (LB) films from an azobenzene polymer (disperse red 19 1,4 diphenylmethane polyurethane, DR19-MDI) and cadmium stearate. This mixed LB film showed a great remaining birefringence (80%) due to its high rigidity.

Design and synthesis of processible functional copolymers

Novel tailor-made AB, ABAC, and ABAA type heteroaromatic copolymers, in which B represents an N-alkylpyrrole unit, were synthesized via Pd-catalyzed Stille polymerization and characterized by a variety of analytical techniques. MALDI-TOF mass spectrometry allowed the detailed characterization of polymer weight and determination of end groups. By varying the nature of the repeating monomer units and their sequence, copolymers with promising properties for photovoltaic devices could be obtained.

Langmuir and Langmuir-Blodgett films from the N-hexyl-pyrrole-thiophene (AB) semi-amphiphilic copolymer

A report is made of Langmuir film of a new semi-amphiphilic alternating N-hexylpyrrole-thiophene AB copolymer (PHPT) that is amenable to transfer as Langmuir–Blodgett (LB) films. PHPT was synthesized by condensation copolymerization via Stille coupling between N-hexyl-2,5-bis(trimethylstannyl)pyrrole and 2,5-dibromothiophene in the presence of Pd(PPh3)2Cl2 as a catalyst in dry DMF and characterized with various analytical techniques. In contrast to homopolymers (e.g. poly(N-alkylpyrrole)s and poly(3-alkylthiophene)s), PHPT formed a reasonably stable monolayer, as inferred through surface pressure and surface potential isotherms, and Brewster angle microscopy (BAM). The dependence of monolayer characteristics on the spreading volume, subphase temperature and compression speeds was also investigated. Electrical characterization revealed the possibility of using PHPT LB films as gas sensors.

Implications of using m-cresol in the Langmuir-Blodgett processing of polyaniline

The use of m-cresol for the Langmuir–Blodgett (LB) processing of polyaniline has been investigated, in particular for probing possible secondary doping processes in LB films. Langmuir monolayers and LB films of polyaniline and polyaniline–cadmium stearate composites have been obtained with the use of camphor sulfonic acid (CSA) both in the presence and absence of m-cresol in the spreading solution. Expanded monolayers with higher molecular areas have been observed upon employing m-cresol. The UV-vis spectral changes for the polyaniline LB films caused by m-cresol were shown to depend both on the preparation conditions and on the doping state of the monolayer. Such effects are likely to originate from changes in molecular conformation since m-cresol is not retained in the transferred film. In addition, processing with m-cresol brought no change to the electrical conductivity nor to the stacking order of cadmium stearate domains in the transferred LB films.