N. Serdar Sariciftci

2.5% efficient organic plastic solar cells

We show that the power conversion efficiency of organic photovoltaic devices based on a conjugated polymer/methanofullerene blend is dramatically affected by molecular morphology. By structuring the blend to be a more intimate mixture that contains less phase segregation of methanofullerenes, and simultaneously increasing the degree of interactions between conjugated polymer chains, we have fabricated a device with a power conversion efficiency of 2.5% under AM1.5 illumination. This is a nearly threefold enhancement over previously reported values for such a device, and it approaches what is needed for the practical use of these devices for harvesting energy from sunlight.

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.

Influence of the solvent on the crystal structure of PCBM and the efficiency of MDMO-PPV:PCBM ‘plastic’ solar cells

Two crystal structures of PCBM, obtained from different crystallisation solvents, are presented; a proposed link with solvent dependence of the efficiency of MDMO-PPV:PCBM solar cells is described.

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.

The effects of CdSe incorporation into bulk heterojunction solar cells

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.

Polymeric photovoltaic materials

Abstract 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.

Complexation of pyrrolidinofullerenes and zinc-phthalocyanine in a bilayer organic solar cell structure

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.

Core/shell nanomaterials in photovoltaics

Hybrid materials consist of inorganic nanoparticles embedded in polymer matrices. An advantage of these materials is to combine the unique properties of one or more kinds of inorganic nanoparticles with the film forming properties of polymers. Most of the polymers can be processed from solution at room temperature enabling the manufacturing of large area, flexible and light weight devices. To exploit the full potential for the technological applications of the nanocrystalline materials, it is very important to endow them with good processing attributes. The surface of the inorganic cluster can be modified during the synthesis by organic surfactants. The surfactant can alter the dispersion characteristic of the particles by initiating attractive forces with the polymer chains, in which the particles should be homogenously arranged. In this review, we present wet chemical methods for the synthesis of nanoparticles, which have been used as photovoltaic materials in polymer blends. The photovoltaic performance of various inorganic/organic hybrid solar cells, prepared via spin-coating will be the focus of this contribution.

Material solubility and molecular compatibility effects in the design of fullerene/polymer composites for organic bulk heterojunction solar cells

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.

PHOTOVOLTAIC PERFORMANCE OF PPE-PPV COPOLYMERS: EFFECT OF THE FULLERENE COMPONENT

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.