Photovoltaic devices from fullerene--oligophenyleneethynylene conjugates

Acetylene-based materials in organic photovoltaics

Fossil fuel alternatives, such as solar energy, are moving to the forefront in a variety of research fields. Organic photovoltaic systems hold the promise of a lightweight, flexible, cost-effective solar energy conversion platform, which could benefit from simple solution-processing of the active layer. The discovery of semiconductive polyacetylene by Heeger et al. in the late 1970s was a milestone towards the use of organic materials in electronics; the development of efficient protocols for the palladium catalyzed alkynylation reactions and the new conception of steric and conformational advantages of acetylenes have been recently focused the attention on conjugated triple-bond containing systems as a promising class of semiconductors for OPVs applications. We review here the most important and representative (poly)arylacetylenes that have been used in the field. A general introduction to (poly)arylacetylenes, and the most common synthetic approaches directed toward making these materials will be firstly given. After a brief discussion on working principles and critical parameters of OPVs, we will focus on molecular arylacetylenes, (co)polymers containing triple bonds, and metallopolyyne polymers as p-type semiconductor materials. The last section will deal with hybrids in which oligomeric/polymeric structures incorporating acetylenic linkages such as phenylene ethynylenes have been attached onto C₆₀, and their use as the active materials in photovoltaic devices.

Photoinduced energy and electron transfer in fullerene–oligophenyleneethynylene systems: dependence on the substituents of the oligomer unit

The photophysical properties of fullerene hybrid systems in which disymmetrically substituted linear oligophenyleneethynylene (OPE) substituents have been attached to C(60) through a pyrrolidine ring are discussed. These hybrid systems differ in both the length of the conjugated OPE backbone and in the type of terminating groups employed, i.e. tri-isopropylsilane (-Si(iPr)(3)) and N,N-di-n-butylaniline (PhN(nBu)(2)). The terminating group is found to be crucial in determining the fate of light absorbed by the hybrid. In CH(2)Cl(2) and benzonitrile, the PhN(nBu)(2) terminated hybrids undergo electron transfer with charge separation lasting as long as 390 ns in the more polar medium, as detected via near-infrared transient absorption spectroscopy. Under the same conditions the Si(iPr)(3) terminated hybrids show ultrafast OPE --> C(60) singlet energy transfer (k = 10(9)-10(10) s(-1)) followed by regular deactivation of the C(60) moiety, as determined via UV-VIR-NIR steady state and time-resolved spectroscopy. Only in polar benzonitrile such systems can undergo electron transfer to some extent (40% yield). The results here presented can be readily explained in light of the electrochemical properties of the hybrids. The low oxidation potentials of the PhN(nBu)(2) terminated systems allow the formation of low lying charge separated states ( approximately 1.45 eV) which, in Si(iPr)(3) terminated analogues, are shifted substantially upward ( approximately 1.90 eV) and become hardly accessible via direct excitation or sensitization of the C(60) singlet level (1.72 eV). These results, when examined in light of the performance of photovoltaic devices using these hybrids as active materials, show a nice structure-activity relationship supporting the appeal of the so-called molecular approach to photovoltaic devices.

Topological Effects of a Rigid Chiral Spacer on the Electronic Interactions in Donor–Acceptor Ensembles

Two triads (donor-spacer-acceptor), exTTF-BN-C60 (6) and ZnP-BN-C60 (7), in which electron donors (i.e., exTTF or ZnP) are covalently linked to C60 through a chiral binaphthyl bridge (BN), have been prepared in a multistep synthetic procedure starting from a highly soluble enantiomerically pure binaphthyl building block (1). Unlike other oligomeric bridges, with binaphthyl bridges, the conjugation between the donor and the acceptor units is broken and geometric conformational changes are facilitated. Consequently, distances and electronic interactions between the donor and C60 are drastically changed. Both donor-spacer-acceptor (D-s-A) systems (i.e., 6 and 7) exhibit redox processes that correspond to all three constituent electroactive units, namely, donor, BN, and C60. Appreciable differences were, however, observed when comparing triad 6, in which no significant exTTF-C60 interactions were noted, with D-s-A 7, whose geometry favors donor-acceptor and pi-pi interactions that result in ZnP-C60 electronic communication. This through-space interaction is, for example, reflected in the redox potentials. Excited-state studies, carried out by fluorescence and transient absorption spectroscopy, also support through-space rather than through-bond interactions. Although both triads form the corresponding radical-ion pair, that is, exTTF*+-BN-C60*- and ZnP*+-BN-C60*-, dramatic differences were found in their lifetimes: 165 micros and 730 ns, respectively.

Solid Film versus Solution-Phase Charge-Recombination Dynamics of exTTF–Bridge–C60 Dyads

The charge-recombination dynamics of two exTTF-C60 dyads (exTTF = 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene), observed after photoinduced charge separation, are compared in solution and in the solid state. The dyads differ only in the degree of conjugation of the bridge between the donor (exTTF) and the acceptor (C60) moieties. In solution, photoexcitation of the nonconjugated dyad C60-BN-exTTF (1) (BN = 1,1'-binaphthyl) shows slower charge-recombination dynamics compared with the conjugated dyad C60-TVB-exTTF (2) (TVB = bisthienylvinylenebenzene) (lifetimes of 24 and 0.6 micros, respectively), consistent with the expected stronger electronic coupling in the conjugated dyad. However, in solid films, the dynamics are remarkably different, with dyad 2 showing slower recombination dynamics than 1. For dyad 1, recombination dynamics for the solid films are observed to be tenfold faster than in solution, with this acceleration attributed to enhanced electronic coupling between the geminate radical pair in the solid film. In contrast, for dyad 2, the recombination dynamics in the solid film exhibit a lifetime of 7 micros, tenfold slower than that observed for this dyad in solution. These slow recombination dynamics are assigned to the dissociation of the initially formed geminate radical pair to free carriers. Subsequent trapping of the free carriers at film defects results in the observed slow recombination dynamics. It is thus apparent that consideration of solution-phase recombination data is of only limited value in predicting the solid-film behaviour. These results are discussed with reference to the development of organic solar cells based upon molecular donor-acceptor structures.

Cooperative Recognition of C60-Ammonium Substrates by a Ditopic Oligophenylenevinylene/Crown Ether Host

A new fullerene derivative with an ammonium subunit has been prepared. Its ability to form supramolecular complexes with oligophenylenevinylene derivatives bearing one or two crown ether moieties has been evidenced by electrospray mass spectrometry, and UV-visible and luminescence spectroscopy experiments. Interestingly, the assembly of the C60-ammonium cation with the oligophenylenevinylene derivative bearing two crown ether moieties leads to the cooperative formation of the 2:1 complex owing to intramolecular fullerene-fullerene interactions.

Pyrazolino[60]fullerene-Oligophenylenevinylene Dumbbell-Shaped Arrays: Synthesis, Electrochemistry, Photophysics, and Self-Assembly on Surfaces

Symmetrically substituted oligophenylenevinylene (OPV) derivatives bearing terminal p-nitrophenylhydrazone groups have been prepared and used for the synthesis of dumbbell-shaped bis(pyrazolino[60]fullerene)-OPV systems. In these triad arrays, the OPV-type fluorescence is dramatically quenched as a consequence of ultrafast OPV-->C60 singlet energy transfer. In its turn the fullerene singlet state is quenched by pyrazoline-->C60 electron transfer, in line with the behavior of the corresponding reference fullerene molecule. The occurrence of electron transfer in the multicomponent arrays is evidenced by recovery of fullerene fluorescence at 77 K in CH2Cl2 and in toluene at 298 K. Under these conditions the OPV-->C60 energy transfer is unaffected. The rate of this process turns out to be higher for the OPV trimer than for the corresponding pentameric OPV arrays, in agreement with energy-transfer theory expectations. Scanning tunneling microscopy (STM) and scanning force microscopy (SFM) revealed that the bis(pyrazolino[60]fullerene)-OPV can self-assemble into ordered layered crystalline architectures on the basal plane of highly oriented pyrolitic graphite.

Linear π-conjugated systems derivatized with C60-fullerene as molecular heterojunctions for organic photovoltaics

This tutorial review covers recent contributions in the area of linear pi-conjugated systems bound to fullerenes in view of their application as active materials in photovoltaic devices. The first part discusses the concepts of double-cable polymer and molecular hetero-junction and presents several examples of chemically or electrochemically synthesized C60-derivatized conjugated polymers. The second and main part of the article concerns the various classes of C60-derivatized pi-conjugated oligomers designed in view of their utilization in single-component photovoltaic devices. Thus, C60-containing pi-conjugated systems such as oligoarylenevinylenes, oligoaryleneethynylenes and oligothiophenes are discussed on the basis of the relationships between molecular structure, photophysical properties and performances of the derived photovoltaic devices. A brief last section presents some recent examples of surface-attached molecular hetero-junctions based on self-assembled monolayers and discusses possible routes for future research.

Materials for organic solar cells: the C60/π-conjugated oligomer approach

This tutorial review surveys recent advances in the field of C60/pi-conjugated oligomer donor-acceptor ensembles. In particular, different synthetic strategies are discussed that were developed to link pi-conjugated oligomers, as versatile photoexcited state electron donors, to C60. We highlight relationships between the nature/structural aspects of pi-conjugated donor systems and a variety of physico-chemical features. Modifications of the oligomeric components are discussed under aspects of tailoring (i) the absorption cross-section of the chromophore in the visible region, (ii) the oxidation potential of the oligomeric donor moiety, (iii) the size, shape, or chemical makeup of the oligomer, and (iv) the stabilization of the charge-separated radical ion pairs. In the final section, the applicability of selected materials for the fabrication of photovoltaic devices is analyzed.

Photovoltaic Properties of Self-Assembled Monolayers of Porphyrins and Porphyrin−Fullerene Dyads on ITO and Gold Surfaces

A systematic series of ITO electrodes modified chemically with self-assembled monolayers (SAMs) of porphyrins and porphyrin-fullerene dyads have been designed to provide valuable insight into the development of artificial photosynthetic devices. First the ITO and gold electrodes modified chemically with SAMs of porphyrins with a spacer of the same number of atoms were prepared to compare the effects of energy transfer (EN) quenching of the porphyrin excited singlet states by the two electrodes. Less EN quenching was observed on the ITO electrode as compared to the EN quenching on the corresponding gold electrode, leading to remarkable enhancement of the photocurrent generation (ca. 280 times) in the porphyrin SAMs on the ITO electrode in the presence of the triethanolamine (TEA) used as a sacrificial electron donor. The porphyrin (H(2)P) was then linked with C(60) which can act as an electron acceptor to construct H(2)P-C(60) SAMs on the ITO surface in the presence of hexyl viologen (HV(2+)) used as an electron carrier in a three electrode system, denoted as ITO/H(2)P-C(60)/HV(2+)/Pt. The quantum yield of the photocurrent generation of the ITO/H(2)P-C(60)/HV(2+)/Pt system (6.4%) is 30 times larger than that of the corresponding system without C(60): ITO/H(2)P-ref/HV(2+)/Pt (0.21%). Such enhancement of photocurrent generation in the porphyrin-fullerene dyad system is ascribed to an efficient photoinduced ET from the porphyrin singlet excited state to the C(60) moiety as indicated by the fluorescence lifetime measurements and also by time-resolved transient absorption studies on the ITO systems. The surface structures of H(2)P and H(2)P-C(60) SAMs on ITO (H(2)P/ITO and H(2)P-C(60)/ITO) have been observed successfully in molecular resolution with atomic force microscopy for the first time.

From metal complexes to fullerene arrays: exploring the exciting world of supramolecular photochemistry fifteen years after its birth

After over 15 years of extensive research work in many laboratories worldwide, supramolecular photochemistry is a well-established and highly recognized branch of science. A brief retrospective view on the birth and infancy of this research area is given and some of the latest developments are discussed. In supramolecular photochemistry Ru(II) and Cu(I) diimmine complexes and C60 fullerenes are some of the most widely investigated chromophores and over the years big efforts have been made to implement and tune their photophysical and excited state properties, which are briefly reviewed. Thanks to a huge amount of synthetic and analytical research work, it has been possible to insert or combine these organic and inorganic subunits in a variety of fascinating supramolecular architectures. Some results concerned with photoinduced processes occurring in dyads, triads, catenanes, rotaxanes, dendrimers, and protonated self-assembled architectures are briefly illustrated. The overall picture stemming form the current state of the art in supramolecular photochemistry is that of a discipline gaining an increasing degree of multidisciplinarity. Interconnections with biology, physics and information technology are being established at a very fast pace, suggesting a bright future for this still young research field.