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Two conjugated PPE-PPV copolymers were studied as electron donor materials in bulk heterojunction organic solar cells in combination with a library of electron acceptor fullerene derivatives. It was shown that molecular structure and solubility of the fullerene counterpart significantly affect the photovoltaic performance of both polymers. Use of [60]PCBM as an electron acceptor material yielded quite moderate power conversion efficiencies. The best results were achieved when fullerene derivatives with suitable molecular structures and solubilities were applied. The obtained results suggest that every newly designed conjugated polymer should be evaluated in solar cells using a library of fullerene derivatives instead of just conventional PCBMs. We believe that only this combinatorial approach might bring the best performing donor/acceptor combinations for future generations of efficient organic solar cells.

New poly( p -phenylenevinylene) (PPV)-type conducting polymers containing different concentrations of thiophene and fluorene functional units were investigated in this work and the photophysical and electrochemical properties were evaluated. We observed a dependence of these properties on the concentration of thiophene units in the polymer backbone. The hole mobilities were estimated to be on the order of 10 −6 cm 2 V −1 s −1 . The polymers were combined with different concentrations of a soluble fullerene derivative (PCBM) and applied in bulk-heterojunction photovoltaic cells. The effects of PCBM concentration and of annealing (post-production treatment) on these devices were investigated. The best results were obtained for the materials containing higher concentrations of thiophene units.

Novel fullerene derivatives bearing thiophene and furan residues were synthesized and studied as electron acceptor materials in bulk heterojunction organic solar cells, together with poly(3-hexylthiophene) (P3HT) as the donor polymer. Some compounds showed large nanomorphological inhomogenities in blends with P3HT; in particular, clusters with dimensions in the range of 100-1000 nm were formed. However, some blends that showed such large clusters yielded at the same time high power conversion efficiencies in photovoltaic devices, approaching 3.7 %. This is in sharp contrast with previously studied systems, in which a substantial phase separation always resulted in a poor photovoltaic performance. We assume that the attachment of thienyl or furyl groups to the fullerene cage results in a certain ordering of the designed fullerene derivatives I-IX with P3HT in photoactive blends. Both the fullerene derivative and P3HT might assemble via pi-pi stacking of the thiophene units to form the nanostructures observed in the films by optical and atomic force microscopy. The presence of ordered donor and acceptor counterparts in these nanostructures results in superior photovoltaic device operation.

The preparation of 27 different derivatives of C60 and C70 fullerenes possessing various aryl (heteroaryl) and/or alkyl groups that are appended to the fullerene cage via a cyclopropane moiety and their use in bulk heterojunction polymer solar cells is reported. It is shown that even slight variations in the molecular structure of a compound can cause a significant change in its physical properties, in particular its solubility in organic solvents. Furthermore, the solubility of a fullerene derivative strongly affects the morphology of its composite with poly(3-hexylthiophene), which is commonly used as active material in bulk heterojunction organic solar cells. As a consequence, the solar cell parameters strongly depend on the structure and the properties of the fullerene-based material. The power conversion efficiencies for solar cells comprising these fullerene derivatives range from negligibly low (0.02%) to considerably high (4.1%) values. The analysis of extensive sets of experimental data reveals a general dependence of all solar cell parameters on the solubility of the fullerene derivative used as acceptor component in the photoactive layer of an organic solar cell. It is concluded that the best material combinations are those where donor and acceptor components are of similar and sufficiently high solubility in the solvent used for the deposition of the active layer.

We investigated donor−acceptor bilayer heterojunctions formed by deposition of solution-processed pyrrolidinofullerenes bearing chelating pyridyl groups (PyFs) on vacuum-evaporated films of zinc phthalocyanine (ZnPc). It is shown that coordination complexes are formed at the interface between these donor and acceptor components; such association facilitates photoinduced charge separation and results in improved performance of the photovoltaic devices. Thus, the bilayer photovoltaic cells fabricated from different pyrrolidinofullerenes and ZnPc exhibit short circuit current (Isc) densities in the range of 3−5 mA/cm2, open circuit voltages (Voc) of 400−600 mV, and fill factors (FF) of 40−50% that correspond to power conversion efficiencies (η) of up to 1.5% under 100 mW/cm2 simulated AM1.5 illumination. The reference cells based on the nonchelating fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor component yield lower power conversion efficiencies (0.4−0.6%); the performanc...

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 0 ,7 0 -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.

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.

We report on photoinduced charge transfer and energy transfer reactions using a new cyano-substituted naphthalene-vinylene type conjugated polymer. Optical absorption and emission spectra of thin films of mixtures of this polymer with the fullerene derivative 1-(3-(methoxycarbonyl)propyl)-1-phenyl-(6,6)C 61 (PCBM) exhibit several interesting features such as luminescence quenching and polaronic photoinduced absorption (PIA) of charges indicating photoinduced charge transfer. On the other hand this new polymer exhibits energy transfer phenomena in blends with MDMO-PPV (poly((2-methoxy-5-((3,7-dimethyloctyl)oxy)-1,4-phenylene)vinylene)). Photovoltaic devices and light emitting diode (LED) devices have been fabricated and characterized using this novel conjugated polymer in composite with fullerenes and MDMO-PPV’s, respectively.

The photoinduced charge transfer between tetracyano-anthraquino-dimethane (TCAQ) derivatives and poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO−PPV) has been studied by photoinduced absorption (PIA) spectroscopy in the VIS and IR spectral region and by light induced electron spin resonance (LESR). Studies on three different TCAQ derivatives with different side chain substitutions are reported. Among them a supermolecule with TCAQ attached to a fullerene have been synthesized to serve as tandem electron acceptor. The photoinduced absorption in the Vis/near-IR range shows a broad plateau around 1.8 eV followed by two peaks at 1.35 and 1.24 eV with an additional broad feature below 0.5 eV for all three acceptors in composite with MDMO−PPV. All PIA features have a power law excitation intensity dependence with an exponent close to 0.5 as expected for bimolecular kinetics. LESR studies show one absorption at a g-factor of 2.0028 with a width (peak-to-peak) of 3.5 G, originating from TCAQ ...

Recent developments in conjugated polymer-based photovoltaic elements have been reviewed. The photophysics of such photoactive devices is based on the photoinduced electron transfer from donor-type semiconducting conjugated polymers onto acceptor-type conjugated polymers or acceptor molecules such as Buckminsterfullerene, C60. Photoinduced electron transfer in solid composite films of fullerenes embedded into conjugated polymers is reversible, ultrafast (within 300 fs) with a quantum efficiency approaching unity, and metastable. Similar to the first step in natural photosynthesis, this photoinduced electron transfer leads to a number of potentially interesting applications, which include sensitization of the photoconductivity and photovoltaic phenomena. Furthermore, using the conjugated polymer donors in polymer blends with another conjugated polymer acceptor, similar photovoltaic elements have been realized. Examples of photovoltaic architectures are discussed with their potential in terrestrial solar energy conversion.

We present near-steady-state photoinduced absorption (PIA), photoluminescence, and light-induced electron spin resonance (LESR) studies on photoinduced electron transfer reactions from poly(bis-2,5-epi-cholestanoxy-1,4-phenylene vinylene) (BeCHA-PPV) as a donor to Buckminsterfullerene (C60) and a number of functionalized fullerenes as acceptors. We show that functionalized fullerenes with a solubilizing side chain linked to C60 in a [5,6] (fulleroid) or [6,6] (methanofullerene) fashion, are efficient electron acceptors towards BeCHA-PPV, as evidenced from changes in the PIA, the efficient quenching of the photoluminescence, and the LESR spectra of BeCHA-PPV cation and fullerene anion radicals. The increased miscibility of the functionalized fullerenes with the conjugated polymer as compared to C60, can be important for the preparation of future electrooptic devices, based on the photoinduced charge separation.

In this review we discuss the photophysical properties of the supramolecular composites of two π-electron semiconductors; conjugated polymers as electron donors and Buckminsterfullerene as electron acceptor. Conjugated, polymeric semiconductors have been found to be effective donors upon photoexcitation of the valence band electrons across the bandgap into the conduction bands. The Buckminsterfullerene, C60, acts as a strong acceptor upon photoexcitation. Thus, the supramolecular composite of these two conjugated materials exhibit ultrafast, reversible, metastable photoinduced electron transfer and charge separation. This process, similar to the primary steps of photosynthesis, has been utilized in conjugated polymer/C60 based heterojunction as well as in metal-semiconductor-metal tunnel devices for effective conversion of the solar photon energy into electricity. Other related applications of the photophysics, including photolithographic and xerographic processes, are reviewed. Furthermore, recent developments on light emitting diodes and solar cells with anomalous behavior based on Buckminsterfullerene are critically reviewed. Quantum well-like heterostructures based on organic donor-acceptor layers are proposed to exhibit interesting photoinduced phenomena.

The synthesis, characterization and photovoltaic study of two novel derivatives of [70]fullerene, phenyl-C₇₁-propionic acid propyl ester ([70]PCPP) and phenyl-C₇₁-propionic acid butyl ester ([70]PCPB), are reported. [70]PCPP and [70]PCPB outperform the conventional material (6,6)-phenyl-C₇₁-butyric acid methyl ester ([70]PCBM) in solar cells based on poly(2-methoxy-5-{3',7'-dimethyloctyloxy}-p-phenylene vinylene) (MDMO-PPV) as a donor polymer using chlorobenzene (CB) or dichlorobenzene (DCB) as solvents. AFM data suggest that improvement of the device efficiency should be attributed to the increased phase compatibility between the novel C₇₀ derivatives and the polymer matrix. [70]PCPP and [70]PCBM showed more or less equally high performances in solar cells comprising poly(3-hexylthiophene) (P3HT) as a donor polymer. Optical modeling revealed that the application of [70]fullerene derivatives as acceptor materials in P3HT-based bulk heterojunction solar cells might give approximately 10 % higher short circuit current densities than using C₆₀-based materials such as [60]PCBM. The high solubility of [70]PCPP and [70]PCPB and their good compatibility with the donor polymers suggest these fullerene derivatives as promising electron acceptor materials for use in efficient bulk heterojunction organic solar cells.

Here, we report on a novel light sensing scheme based on a silicon/fullerene-derivative (methano-fullerene [6,6] phenylC61 butyric acid methyl ester – PCBM) heterojunction that allows the realization of optoelectronic devices for the detection of nearto mid-IR light. Despite the absent absorption of silicon and the fullerene-derivative in the IR a heterojunction of these materials absorbes and generates a photocurrentin the spectral range from 1.1 to 0.55 eV. Besides its scientific relevance, the simple fabrication process of the heterojunction (e.g., the fullerene-derivative is deposited by spin coating on Si) as well as its compatibility with the established complementary metaloxide semiconductor (CMOS) technology makes the presented hybrid approach a promising candidate for widespread applications.

[Structure: see text] A novel covalently linked bis([60]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.

A soluble low band gap polymer was synthesized and blended with a soluble fullerene derivative for the fabrication of plastic solar cells. The devices showed a remarkable harvest of photons, within the wide range of 300−900 nm, that were converted into current.

The possibility to use new organic semiconductor materials, in place of silicon wafers, in the fabrication of photovoltaic devices on substrates offer the prospect of lower manufacturing costs, particularly for large area applications. Thus, one of the most promising areas in fullerene research involves its potential application, mixed with conjugated polymers, in mimicking photosynthesis and in the related solar energy conversion. The tendency to phase segregation in blends of C60 derivatives and conjugated polymers has to be optimized to improve both charge photogeneration and transport in photovoltaic devices. In order to optimize device performances, a great deal of work has been devoted to the development of new device architectures and to elucidating the photophysical processes underlying the photovoltaic response. However, optimization of cells has been carried out with just a few materials and recent findings show that new developments can be very successful if a broader scope of materials in the composite layers becomes accessible. Thus, in this communication we present new approaches toward the synthesis of [60]fullerene derivatives specifically designed to be used in the fabrication of photovoltaic devices. (i) the first approach involves the synthesis of functionalized C60 and C70 derivatives with enhanced solubility; (ii) the second approach is directed to the synthesis of systems containing a conjugated moiety antenna covalently linked to the C60 unit. Together with the synthesis of these new materials, electrochemical and photophysical investigations have been also carried out. Photovoltaic devices have been fabricated with selected materials.

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.

Polymer-fullerene based solar cells with power conversion efficiencies up to 3 % have been reported in the recent years. In this contribution we will present the recent developments in the field of these devices and discuss the basic physical processes, on which these devices are based.

We report the synthesis and the spectroscopic properties of a novel soluble thiophene-based copolymer carrying pendant fullerene moieties (donor–acceptor double-cable polymer). Photoinduced absorption (PIA) experiments on spin cast films reveal a photoinduced electron transfer from the polythiophene chain onto the fullerene moieties. The performance of photovoltaic devices produced with this copolymer is also discussed.

Homogenous blends of a processable methanofullerene, [6,6]-Phenyl C61 - butyric acid methyl ester (PCBM), with poly (p-phenylene vinylene) (PPV) made from a novel nonionic precursor route were investigated by optical spectroscopy and by photocurrent measurements. The conversion process of the precursor polymer to the PPV in the blends with PCBM was monitored by IR absorption. In composites of PPV/PCBM strong quenching of the PPV luminescence was observed. LESR (light induced electron spin resonance) and PIA (photoinduced absorption) studies confirmed the occurrence of photoinduced electron transfer from the PPV to PCBM. Photovoltaic devices made from PPV/PCBM blends showed monochromatic power conversion efficiencies of app. 0.3%. The spectral photocurrent was observed to follow the absorption profile of the PPV.

The synthesis of a homologous series of oligo(p-phenylene vinylene)-fulleropyrrolidines (OPVn-C60, n = 1-4, where n is the number of phenyl rings) is described. The photophysical properties of these donor-acceptor dyads and the corresponding model compounds, α,ω-dimethyl-2,5-bis(2-(S)-methylbutoxy)-1,4-phenylene vinylene oligomers (OPVn, n = 2-4) and N-methylfulleropyrrolidine (MP-C60), are studied as a function of the conjugation length in solvents of different polarity and as thin films. Fast singlet energy transfer occurs after photoexcitation of the OPVn moiety of the dyads toward the fullerene moiety in an apolar solvent. Photoexcitation of the dyads in a polar solvent results in electron transfer for OPV3-C60 and OPV4-C60, and to some extent for OPV2-C60, but not for OPV1-C60. These results are compared to the results obtained for mixtures of OPVn and MP-C60 in the same solvents. The solvent-dependent change in free energy for charge separation of the donor-acceptor systems is calculated from the Weller equation, and the rate constants for energy and electron transfer are derived from the fluorescence lifetime and quenching. The results show that in a polar solvent electron transfer in these dyads is likely to occur via a two-step process, that is, a very fast singlet energy transfer prior to charge separation. In thin solid films of OPV3-C60 and OPV4-C60, a long-lived charge-separated state is formed after photoexcitation. The long lifetime in the film is attributed to the migration of charges to different molecules. A flexible photovoltaic device is prepared from OPV4-C60.

Regioregular polythiophenes baring 3-(p-methoxyethoxyethoxy)-phenyl substituents (PEOPT) show high photoluminescence efficiencies. Exposing thin films of this polymer to vapors of chloroform or annealing them by heat treatment results in a red shift of the absorption maximum due to solvent or heat induced ordering which gives rise to more planar conformations. The fact, that annealed thin films of PEOPT exhibit absorption edges at relatively low energies and thus have an enhanced spectral range makes them suitable for use in photodiodes / solar cells. The photoinduced charge generation efficiency in PEOPT is significantly enhanced by the addition of a strong electron acceptor like fullerene, as observed by quenching of the luminescence and by photoinduced absorption measurements in the infrared and UV-Vis regime. The efficiency of the photoinduced charge transfer from PEOPT to a methanofullerene is found to depend on the ordering of PEOPT in thin films.

We report a systematic study of more than 100 bicomponent systems composed of 19 different fullerene derivatives blended with 9 different conjugated polymers (including previously investigated poly(3-hexylthiophene)). It was shown that short circuit current density (JSC) and light power conversion efficiency (η) of the fullerene/polymer photovoltaic devices depend on the solubility of the fullerene components in the solvent used for the blend film deposition (chlorobenzene). The revealed dependences have unusual “double branch” character because many fullerene derivatives possessing similar solubilities showed different photovoltaic performances. This behavior was related to the peculiarities of the molecular structures of the fullerene derivatives. Substituents attached to the cyclopropane ring fused with the fullerene cage in methanofullerenes affected both the morphology of their composites with conjugated polymers and their photovoltaic performance. It was demonstrated that variation of the fullerene component blended with a conjugated polymer might easily change its photovoltaic performance by a factor of 3–4. The obtained results proved that design of appropriate fullerene derivatives and novel conjugated polymers are equally important tasks on the way towards highly efficient organic photovoltaics.

Hybrid solar cells based on CdSe nanoparticles and a PPV-type polymer containing fluorene and thiophene units (PFT) were investigated. The CdSe/PFT devices showed very low photocurrent and fill factor values, which was attributed to the poor charge transport in the TOPO-capped CdSe nanoparticle phase. Thus, ternary systems based on mixtures of PFT/CdSe and the fullerene derivative PCBM were investigated. The CdSe:PCBM ratio was varied, and nanoparticles with different sizes were also used. It was observed that for the optimized composition of 20 wt% PFT + 40 wt% CdSe + 40 wt% PCBM the devices presented higher photocurrents and efficiencies. The photophysical and electrochemical properties and microscopy images (AFM and HRTEM) of the ternary systems were systematically investigated to elucidate the mechanism of action of the inorganic nanoparticles in these ternary hybrid devices.

The synthesis of a novel bithiophene with a pendant fullerene substituent and its electrochemical polymerisation are reported; light induced ESR measurements on the electrodeposited films reveal a photoinduced electron transfer from the donor cable (polythiophene) to the pendant acceptor cable (fullerene moieties).

Bilayer organic solar cells were prepared using zinc-phthalocyanine (ZnPc) and a novel, highly soluble pyrollidinofullerene bearing three chelating pyridyl groups (PyF). The formation of supramolecular complexes between the two compounds is indicated by a significantly increased solubility of ZnPc in dichloromethane upon addition of the PyF. Spin-coating a film of PyF on a vacuum-evaporated film of ZnPc results in a solar cell yielding short circuit current (Isc) densities of 3mA∕cm2 and open-circuit voltages (Voc) of about 0.4V under 100mW∕cm2 simulated AM1.5 illumination. Solar cells prepared by substituting the PyF with a fullerene derivative forming no complexes with ZnPc show significantly lower photovoltaic conversion efficiencies.

Using the ultrafast photoinduced electron transfer with long-living charge separation in the conjugated polymer/fullerene thin films, photovoltaic devices have been fabricated. The photoinduced charge separation happens with internal quantum efficiency near unity. The performance of such “bulk heterojunction” photovoltaic devices is critically dependent on the transport properties of the interpenetrating network. The influence of the variation of different donor / acceptor materials on the sample morphology is monitored by atomic force microscopy (AFM), while I/V characteristics have been studied to evaluate the conversion efficiency.

The properties of a novel poly(bithiophene-fulleropyrrolidine), which belongs to the class of so called “double cable” polymers, are studied. The polymer consists of covalently linked electron acceptor moieties to an electron donor polymeric backbone. Photoinduced charge transfer is studied by photoinduced absorption spectroscopy and by light induced ESR spectroscopy.

Organic field-effect transistors (FETs) have been fabricated using an active region consisting of a conjugated polymer poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) blended with a soluble derivative of C60 (6,6)-phenyl C61-butyric acid methyl ester (PCBM). Such blends are promising for use in organic photovoltaic devices. Fabrication of the FET provides a technique for estimating both the hole and electron field-effect mobilities of the material in the active region by measuring the saturation current as a function of the gate voltage. The hole field-effect mobility for pristine MDMO-PPV was found to be 9×10−4 cm2/(Vs). Saturation of electron current could not be attained for pristine MDMO-PPV. Evidence for the saturation of electron current could be seen only at very high PCBM doping levels.