Christoph Winder

Effect of LiF/metal electrodes on the performance of plastic solar cells

The insertion of thin interlayers of LiF under the negative metal electrode (Al and Au) of bulk heterojunction solar cells significantly enhances the fill factor and stabilizes high open circuit voltages. Compared to devices without the LiF interfacial layer, the white light efficiencies increase by over 20% up to ηeff∼3.3%. Substitution of the LiF by another insulating interlayer SiOx results in lower overall efficiencies. In the case of a LiF/Au electrode, substantial efficiency enhancement is observed compared to a pristine Au electrode and white light efficiencies up to ηeff∼2.3% are reported.

Low bandgap polymers for photon harvesting in bulk heterojunction solar cells

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.

Stabilization of the nanomorphology of polymer–fullerene “bulk heterojunction” blends using a novel polymerizable fullerene derivative

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 C61-butyric acid methyl ester (PCBM) or the newly synthesized polymerizable fullerene derivative, C61-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.

Transient optical studies of charge recombination dynamics in a polymer/fullerene composite at room temperature

The recombination kinetics of photogenerated charge carriers in a composite of poly[2-methoxy-5- (3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene], (MDMO–PPV) and the functionalised fullerene 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 are investigated at room temperature by transient absorption spectroscopy. The decay dynamics of positively charged MDMO–PPV polarons were found to be either monophasic or biphasic, depending upon the laser excitation density employed. The slower, power law, decay phase (100 ns–10 ms) is attributed to recombination dynamics of localized polarons, while the fast decay component (<20 ns) is attributed to recombination of relatively mobile polarons observed when the density of localized states is exceeded by the density of photogenerated polarons (∼1017 cm−3). The implications of these observations are discussed in relation to polymer/C60 photovoltaic cells.

Long-lived photoinduced charge separation for solar cell applications in phthalocyanine-fulleropyrrolidine dyad thin films{

The photophysical properties of a new dyad molecule composed of a covalently linked Zn-phthalocyanine (antenna/donor) and a C60 derivative (acceptor) have been investigated. We report experimental evidence of long-lived charge separation in the solid state with a lifetime several orders of magnitude higher than in solution. Such a long lifetime, unusual for phthalocyanine–fullerene dyads, is the basis for possible photovoltaic applications. A first demonstration of a working solar cell using phthalocyanine–fullerene dyads as the active material is presented. Though the power conversion efficiency under simulated solar illumination of 80 mW cm−2 is found to be moderate (0.02%), it is an encouraging result for application of C60 dyad molecules to photovoltaics.

Sensitization of low bandgap polymer bulk heterojunction solar cells

For efficiently harvesting the terrestrial solar spectrum in conjugated polymer based solar cells, low bandgap polymers with a bandgap -1.8 eV are needed. The photophysics of such low band gap conjugated polymers as well as their excited state interactions with electron acceptors such as fullerenes are of importance when using them in photovoltaic devices.In this work we present a device structural study on the soluble low bandgap polymer PTPTB, consisting of alternating electron-rich Ndodecyl-2,5-bis (29-thienyl)pyrrole (TPT) and electron-deficient 2,1,3-benzothiadiazole (B) units.The bandgap of this polymer, determined by electrochemistry and by optical absorption, is 1.6 eV. The performance of the photovoltaic devices is discussed in terms of spectrally resolved photocurrent measurements, AM1.5 measurements and temperature dependent I–V spectroscopy. Strategies to utilize this polymer for bulk heterojunction tandem solar cells with either a wide band gap polymer or in conjunction with strongly absorbing small molecular dyes are discussed. 2002 Elsevier Science B.V. All rights reserved.

Influence of disorder on the photoinduced excitations in phenyl substituted polythiophenes

Regioregular poly(3-(4′-(1″,4″,7″-trioxaoctyl)phenyl)thiophenes) (PEOPTs) exhibit interesting properties for the use in polymer electronics. Exposing thin films of the amorphous, disordered phase (orange phase) of the “as prepared” polymer to chloroform vapor or annealing them by heat treatment results in a redshift of the absorption maximum due to the formation of nanocrystals in an ordered phase (blue phase). As such, PEOPT thus is a very interesting conjugated polymeric material, which exhibits two different phases with well-defined order/disorder characters on one-and-the-same material. This property opens up the unique possibility to investigate the role of order/disorder on the photoexcited pattern without being obscured by the differences in chemical structure by using different materials with different crystallinity. The fact, that blue phase PEOPT exhibits absorption edges at relatively low energies around 1.8 eV, thereby demonstrating an enhanced spectral absorption range as compared to the oran...

Spectroscopic properties of PEDOTEHIITN, a novel soluble low band-gap conjugated polymer

Polymers with narrow band gap are expected to posses appreciably high RT conductivities, luminescence in the NIR and improved solar energy harvesting properties. Here we report the spectroscopic properties of a soluble and environmentally stable copolymer (PEDOTEHIITN) with a band-gap of ca. 1.1 eV. The combination of this outstandingly narrow band gap, processability and stability are promising for future electronic and optoelectronic applications.

Fluorene functionalised sexithiophenes—utilising intramolecular charge transfer to extend the photocurrent spectrum in organic solar cells

A new series of oligothiophenes bearing electron deficient fluorene units have been prepared and characterised. The materials are functionalised by C8/C11 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-C61 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.

A Fulleropyrrolidine-phthalocyanine dyad for photovoltaic applications

We report on photophysical properties of a novel dyad molecule having as antenna/donor a Zn-phthalocyanine derivative and as acceptor a C 60 derivative covalently attached. We found evidences for long living photoinduced electron transfer in solid state Photovoltate action of thin film devices of the dyad is demonstrated.