[Show abstract][Hide abstract] ABSTRACT: Conjugated Polymers with a HOMO-LUMO transition <2 eV, i.e. a low bandgap, respectively, have interesting and desired properties for some thin film optoelectronic devices like light emitting diodes and solar cells. In this contribution we present the implementation of the novel copolymer PTPTB, consisting of alternating electron-rich N -dodecyl-2,5-bis(2′thienyl)pyrrole (TPT and electron-deficient 2,1,3-benzothiadiazole (B) units, in light emitting diodes (LEDs ) and photovoltaic devices. The LEDs show emission in the near infrared region, peaking at 800 nm. The electroluminescence yield can be significantly enhanced by blending with the wide bandgap polymer poly (para-phenylene-vinylene) derivative MDMO-PPV. Bulk heterojunction devices of PTPTB blended with the C 60 derivative PCBM shows AM 1.5 efficiencies around 1%. The low bandgap of PTPTB allows collecting photons up to 750 nm.
[Show abstract][Hide abstract] ABSTRACT: We report on ex situ Raman characterization of rubrene thin films grown by hot-wall epitaxy on cleaved mica substrates. Analysis of the vibrational bands revealed that at earliest growth stages the film is amorphous. In particular, a broad band at 1373 cm(-1) proves the amorphous nature of the film. The rubrene molecules in amorphous phase are geometrically distorted, since the appearance of the Raman band at 1606 cm(-1) is only infrared active for rubrene molecules with the C(2h) symmetry group. Further growth leads to seeding of spherulites in the amorphous matrix and further to their coalescence. Raman bands from isolated spherulites embedded in an amorphous matrix and from coalesced spherulites show polarization dependence (depolarization ratio < 0.6), thus demonstrating their crystalline nature. It is also found that the breathing mode (1003 cm(-1)) represents the rubrene fingerprint feature independent of layer crystallinity.
[Show abstract][Hide abstract] ABSTRACT: A new series of oligothiophenes bearing electron deficient fluorene units have been prepared and characterised. The materials are functionalised by C 8 /C 11 alkyl chains or triethylene glycol side groups, yet the higher oligomers remain poorly soluble. The absorption characteristics of a sexithiophene analogue (compound 3) have been studied by UV-vis and photoinduced absorption spectroscopy. Photovoltaic cells have been fabricated from blends of 3 and fullerene derivative [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM). The photocurrent spectrum of the device matches the absorption spectrum of the sexithiophene system which incorporates an intramolecular charge transfer band arising from the 1,3-dithiole-fluorene units. A modest power conversion efficiency of 0.1% was achieved.
[Show abstract][Hide abstract] ABSTRACT: [Structure: see text] A novel covalently linked bis(fullerene)-phthalocyanine triad is reported, exhibiting long-lived photoinduced charge separation both in solution and in the solid state. The first demonstration of a working solar cell using triad 1 as the active material is also presented.
[Show abstract][Hide abstract] ABSTRACT: We report the photophysical behavior and photovoltaic performance of a poly(3,4-propylenedioxythiophene) (PProDOT) derivative, namely poly-[3,3-dihexyl-3,4-dihydro-2H-thieno(3,4-b)(1,4)dioxepine] (PProDOT(Hx)2), as an electron donor in bulk heterojunction solar cells blended with the acceptor 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM). Devices composed of a 1:4 (w:w) ratio of PProDOT(Hx)2/PCBM and spin coated from chlorobenzene were characterized by measuring current–voltage characteristics under simulated Air Mass 1.5 (AM1.5) conditions as well as the spectrally resolved photocurrent (IPCE). The influence of different preparation parameters like various blend ratios, spin coating from different solvents, and changing the metal contacts was studied. It was found that the photoluminescence of the polymer is quenched by a factor greater than 100 using blends consisting of PProDOT(Hx)2 and PCBM (3:2, w:w). Additionally, the photoactive blends were characterized by photoinduced absorption spectroscopy and the results suggest that charge transfer is occurring from PProDOT(Hx)2 to PCBM. Results from atomic force microscopy reveal that a bicontinuous network, with domain sizes on the order of 100–200 nm, results when a 1:4 blend of PProDOT(Hx)2/PCBM is spin coated from chlorobenzene.
Solar Energy Materials and Solar Cells 12/2006; 90(20-90):3531-3546. DOI:10.1016/j.solmat.2006.06.051 · 5.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: New donor-acceptor materials based on a polynorbornene framework to which both phthalocyanine and C60 electroactive pendant units are randomly attached have been prepared in good yield by ring-opening-metathesis polymerization (ROMP) in the presence of a Grubbs catalyst. A structurally related phthalocyanine homopolymer was also synthesized for comparison. A remarkable fluorescence quenching was observed in the homopolymer and accounts for PcPc interactions along the polymeric framework. As expected, the fluorescence quenching increases in the case of the polynorbornenes containing both Pc and C60 units owing to photoinduced electron transfer, which was further confirmed by transient absorption spectroscopy. Finally, preliminary solar cell devices made of one of the copolymers were constructed.
Chemistry - An Asian Journal 08/2006; 1(1-2):148-54. DOI:10.1002/asia.200600090 · 3.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A novel polythienylenevinylene (PTV) and two new polythiophenes (PTs), featuring fused tetrathiafulvalene (TTF) units, have been prepared and characterized by ultraviolet-visible (UV-vis) and electron paramagnetic resonance (EPR) spectroelectrochemistry. All polymers undergo two sequential, reversible oxidation processes in solution. Structures in which the TTF species is directly linked to the polymer backbone (2 and 4) display redox behavior which is dictated by the fulvalene system. Once the TTF is spatially removed from the polymer chain by a nonconjugated link (polymer 3), the electroactivity of both TTF and polythiophene moieties can be detected. Computational studies confirm the delocalization of charge over both electroactive centers (TTF and PT) and the existence of a triplet dication intermediate. PTV 4 has a low band gap (1.44 eV), is soluble in common organic solvents, and is stable under ambient conditions. Organic solar cells of polymer 4:[6,6]-phenyl-C(61) butyric acid methyl ester (PCBM) have been fabricated. Under illumination, a photovoltaic effect is observed with a power conversion efficiency of 0.13% under AM1.5 solar simulated light. The onset of photocurrent at 850 nm is consistent with the onset of the pi-pi absorption band of the polymer. Remarkably, UV-vis spectroelectrochemistry of polymer 4 reveals that the conjugated polymer chain remains unchanged during the oxidation of the polymer.
The Journal of Physical Chemistry B 03/2006; 110(7):3140-52. DOI:10.1021/jp057256h · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The morphological stabilization of donor–acceptor blends for bulk heterojunction solar cells can be achieved by cross-linking of the small molecular phase in the polymer matrix using a polymerizable fullerene derivative. In a comparative study the morphology of polymer–fullerene blend films was investigated using poly(3-hexylthiophene) (P3HT) as the polymer and C 61 -butyric acid methyl ester (PCBM) or the newly synthesized polymerizable fullerene derivative, C 61 -butyric acid glycidol ester, PCBG, as the acceptor molecule, respectively. Changes in the nanomorphology due to heat treatment of the films were studied by means of atomic force microscopy (AFM), transmission electron microscopy (TEM) and photoluminescence (PL) studies. The polymerization process was monitored with infrared absorption studies. As demonstrated by these comparative studies this newly synthesized fullerene gives considerable stabilization of the solid state morphology in these blends. Such prevention of the long term, high temperature instability of bulk heterojunction morphology displays an important route to increase the operational stability of plastic solar cells in future applications.
[Show abstract][Hide abstract] ABSTRACT: In situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroelectrochemistry during oxidation (p-doping) and reduction (n-doping) as well as photoinduced infrared spectroscopy of a random copolymer of 2,3,5,6-tetrafluoro-1,4-phenylenevinylene and 2,5-dioctyloxy-1,4-phenylenevinylene [co(TFPV-DOPV)] is presented. The infrared active vibration (IRAV) patterns obtained during electrochemical oxidation (p-doping) and reduction (n-doping) are different. The copolymer shows photoinduced IRAV bands that are similar to those obtained during electrochemical reduction (n-doping) in contrast to most of the other conjugated polymers which show similarities with oxidation (p-doping) IRAV bands. To our knowledge this is the only p- and n-dopable polymer with different IRAV patterns of charge carriers of both signs, and a similarity between n-doping and photodoping IRAV bands.
[Show abstract][Hide abstract] ABSTRACT: The relation between the nanoscale morphology and associated device properties in conjugated polymer/fullerene bulk-heterojunction "plastic solar cells" is investigated. We perform complementary measurements on solid-state blends of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV) and the soluble fullerene C-60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C-61 (PCBM), spin-cast from either toluene or chlorobenzene solutions. The characterization of the nanomorphology is carried out via scanning electron microscopy (SEM) and atomic force microscopy (AFM), while solar-cell devices were characterized by means of current-voltage (I-V) and spectral photocurrent measurements. In addition, the morphology is manipulated via annealing, to increase the extent of phase separation in the thin-film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials under thermal treatment. Furthermore the photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase-separated blends cast from toluene. For toluene-cast films the scale of phase separation depends strongly on the ratio of MDMO-PPV to PCBM, as well as on the total concentration of the casting solution. Finally we observe small beads of 20-30 nm diameter, attributed to MDMO-PPV, in blend films cast from both toluene and chlorobenzene.
[Show abstract][Hide abstract] ABSTRACT: The phthalocyanine–fullerene dyad Pc-C60 is an intramolecular donor–acceptor system that can be used as photoactive material in photovoltaic devices. The photocurrent shows maxima below 400 and 700 nm, but only a small response in the range around 500 nm. To improve the spectral match to the solar emission spectrum, an energy transfer process from the conjugated polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene] MDMO-PPV (antenna system) to the dyad compound can be used. The photophysics of the energy transfer process was studied with luminescence measurements and sub-picosecond time-resolved transmission changes. The influence of the polymeric antenna system on the photovoltaic device properties was measured with spectrally resolved photocurrent (IPCE) measurements. Photocurrent in nearly the full visible spectral range was obtained. In addition, Pc-C60 was also added as third component to MDMO-PPV:PCBM mixtures. The photocurrent around 700 nm was increased, however with reduction in the short-circuit current under white light illumination.
Solar Energy Materials and Solar Cells 06/2004; 83(2-3-83):201-209. DOI:10.1016/j.solmat.2004.02.025 · 5.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Encouraging progress has been made over the last few years in the field of photovoltaic solar cells using organic materials. Among the different concepts which have been proposed, the bulk heterojunction approach (formed by blending donor type conjugated polymers and acceptors like fullerenes) is attractive and significant progress in improving the power conversion efficiency of these devices was obtained recently. One of the possible improvements of this type of solar cells is the implementation of new materials absorbing the red and near infrared part of the solar spectrum, where the maximal photon flux of the sun is located. In this article, we will describe this strategy of photon harvesting in organic solar cells and review recent advances in the search for new materials as candidates for polymeric absorbers.
[Show abstract][Hide abstract] ABSTRACT: The discovery of a photoinduced charge transfer interaction between conjugated polymers and fullerenes has led to intensive spectroscopic investigations of solid state composites of these materials. Based on this charge transfer reaction, high efficient solar cell devices have been developed using the concept of bulk heterojunctions, i.e. blending of the conjugated polymer and fullerene. In this work, we present a spectroscopic study on such devices under working conditions. These devices were investigated by means of photoinduced absorption spectroscopy in reflection geometry at different temperatures and different applied external electric fields. Significant changes in the photoinduced signal were observed upon changing the applied voltage. These changes can be attributed to persistent charge carriers, charge carrier transport and charge carrier injection. From this the mean polaron lifetime, the built-in potential and the fraction of persistent charge carriers was estimated.
Thin Solid Films 03/2004; s 451–452:60–63. DOI:10.1016/j.tsf.2003.11.037 · 2.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The absorption pattern of infrared active vibrations (IRAVs) of p- and n-doped thiophene based conjugated polymers with low electronic band gaps are compared. The polymers were doped electrochemically and studied in situ with attenuated total reflection Fourier transform infrared spectroelectrochemistry. Substantial spectral differences were found for polyethylenedioxythiophene (PEDOT), a polyisothianaphthenemethine derivative (PIM) and two polydithienothiophenes (PDTT1 and PDTT2), whereas PDTT3 has no significant differences in the IRAV spectra. The results are related both to structural effects with selective participation of different chain units (for PIM) and to different localisation of the doping induced charge carriers of both signs (for PEDOT, PDTT1 and PDTT2).
[Show abstract][Hide abstract] ABSTRACT: The use of conjugated polymers as antenna systems in photovoltaic devices with phthalocyanine-fullerene dyad compounds is described. With luminescence measurements and time resolved transmission changes, energy transfer processes are studied. The influence on photovoltaic device properties is presented.
[Show abstract][Hide abstract] ABSTRACT: In this work, we demonstrate the fabrication of hot wall epitaxially grown para-sexiphenyl films for light emitting diodes and PSP/C60 bi-layer structures for photovoltaic cells. Para-sexiphenyl films display blue electroluminescence, which shows a spectrum coinciding with its photoluminescence. Photovoltaic devices are also fabricated and their external quantum and power conversion efficiencies are presented and discussed.
[Show abstract][Hide abstract] ABSTRACT: The discovery of a photoinduced charge transfer interaction between conjugated polymers and fullerenes led to intensive spectroscopic investigations as well as to high efficiency photovoltaic devices. Up to now, most spectroscopic investigations were performed on thin films, whereas little is known about the behavior of the charge carriers under working conditions of the device with an applied electric field.In this work, we investigated thin film photovoltaic devices by means of photoinduced absorption spectroscopy. Significant changes were observed for the polaron signal as well as the lifetime of the positive polaron on the conjugated polymer backbone upon changing the applied voltage on the device. The results are discussed in comparison with current–voltage characteristics of the devices.