D. Meissner

Modeling the optical absorption within conjugated polymer/fullerene-based bulk-heterojunction organic solar cells

Abstract In this paper, we report our results on the modeling of the optical properties of the bulk-heterojunction “plastic solar cells”, consisting of a solid-state blend of the conjugated polymer poly-[2-(3,7-dimethyloctyloxy)-5-methyloxy]-para-phenylene-vinylene and the fullerene C 60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C 61 . Upon illuminating these cells with the standard AM 1.5 solar spectrum, the short circuit current can be determined for any given internal quantum efficiency as a function of the active layer thickness. In addition, the depth profiles of photoinduced charge generation rates are calculated. Based on the agreement of this modeling with experimentally determined efficiencies of these solar cells, an internal quantum efficiency of about 80% has been estimated.

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.

Optical constants of conjugated polymer/fullerene based bulk-heterojunction organic solar cells

The performance of organic solar cells consisting of multiple layers, which are a few hundred nanometers thick, is determined by strong optical interference effects. In order to model their optical and photoelectrical behavior, we determined the optical constants of all components of this system. This was done by fitting model dielectric functions to reflection and transmission spectra of all layers in the solar cell device. We put a special emphasis on understanding the optical behavior of the photoactive bulk heterojunction film, which consists of a composite of semiconducting polymers with fullerenes.

The influence of materials work function on the open circuit voltage of plastic solar cells

c ¨¨ ¨ Abstract Conjugated polymeryfullerene plastic solar cells of the first generation were consisting of two distinct layers, made of the donor polymer and of the acceptor fullerene, respectively, sandwiched between two metal contacts. By mixing the polymer and the fullerene components, thus replacing a single flat junction with an interpenetrating network bulk-heterojunction, the device efficiency was dramatically improved. As a further step to proceed with the development of plastic solar cells, we developed the bulk diffusion bilayer approach, allowing the creation of donor-acceptor diffused interfaces with less restrictions to the phase compatibility of the two components. For a novel series of fullerenes, the bulk diffusion bilayer approach is shown to yield devices with comparable efficiencies as the blend bulk heterojunction approach. Bulk-heterojunction devices show unusually high open circuit voltage (V ) values. These values cannot be explained by the metal-insulator-metal (MIM) model that has been OC often used for organic light emitting diodes. In order to investigate the origin of the V in bulk-heterojunction plastic solar cells, OC we have prepared PPV based devices varying both the metal negative contact and the fullerene acceptor. Fullerene derivatives with varying acceptor strength, (i.e. the first reduction potential ) were used as electron acceptors in bulk-heterojunction plastic solar cells produced with the blend as well as the diffusion bilayer approach. The open circuit voltage of the devices was found to correlate directly with the acceptor strength of the fullerenes, while it was rather insensitive to variations in the workfunction of the negative electrode metal. These results suggest that the quasi-Fermi level of the fullerene pins the Fermi level of the evaporated negative metal contact. 2002 Elsevier Science B.V. All rights reserved.

Photoinduced charge transfer in composites of conjugated polymers and semiconductor nanocrystals

Photoinduced electron transfer in composites of CdSe and InP nanocrystals (NC) with a conjugated polymer [2-methoxy-5-(3-,7-dimethyl-octyloxy)-1, 4-phenylene vinylene] (OC1C10-PPV) is studied by means of light-induced electron spin resonance (LESR), photoluminescence (PL) as well as quasi steady-state photoinduced absorption (PIA) spectroscopy. The quenching of PL, the occurrence of two new optical absorption bands, and the formation of light-induced paramagnetic species on the polymer chain are altogether interpreted as an electron transfer between a donor polymer and acceptor NC in the excited state. In particular the LESR provides evidence of an electron transfer rather than energy transfer due to an overlap of absorption and emission bands of NC and the conjugated polymer. The g-factor of the signal is consistent with the cationic origin of OC1C10-PPV radicals. Additionally, the LESR signal is sensitive to NC surface coating being smaller in blends with the tri-n-octylphosphine oxide–tri-n-octylphosphine (TOPO–TOP) coated CdSe surface and completely absent in blends with TOPO–TOP–InP. LESR probes predominantly positive polarons on the conjugated polymer chain generated at room temperature in the course of an electron transfer to NC. This is very different from the situation in blends of OC1C10-PPV with fullerenes, another promising photovoltaic acceptor, where the photogenerated cation and anion were both observed in the LESR. In PIA we found contributions of both polymer polarons and electrons on the NC. The recombination kinetics shows a broad distribution of lifetimes, which is characteristic for dispersive (diffusive) recombination processes with fractional power frequency dependence.

Core/shell nanomaterials in photovoltaics

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

Photovoltaics of interconnected networks

All three types of organic solar cells i.e. sensitised photoelectrochemical cells, molecular organic and polymeric plastic cells, work best when utilising interpenetrating networks. Although the basic construction is equal, the current understanding and the modelling of devices by different groups are contradictory. Whereas photoelectrochcmists use classical models based on the solidlelectrolyte junctions, the polymeric, organic device community seems to prefer to treat the network system as an intrinsic material sandwiched between metals of different work function (metal-insulator-metal MIM devices). Both approaches will be discussed with recent experiments performed with molecular organic solar cells.

Semiconductor photocatalysis type B: synthesis of unsaturated α-amino esters from imines and olefins photocatalyzed by silica-supported cadmium sulfide

Novel unsaturated N-phenyl-alpha-amino esters were synthesized in isolated yields of 50 to 10% by visible light irradiation of methanolic suspensions of silica-supported cadmium sulfide in the presence of methyl (2Z)-phenyl(phenylimino)acetate and various cyclic olefins. A semiconductor photocatalysis mechanism is proposed for this linear addition reaction. The light-generated electron-hole pair in the oxidative step induces a dissociative electron transfer from the olefin to CdS affording a proton and an allylic radical, whereas in the reductive step an alpha-aminobenzyl radical is formed in a proton coupled reaction. Heterocoupling of these intermediate radicals leads to the corresponding addition products. As the only by-product the hydrogenated imine is obtained in comparable amounts through the subsequent photoreduction of the alpha-aminobenzyl radical. Supporting the catalyst on silica made the reaction three times faster as compared to neat CdS. When the surface OH groups of CdS were removed through alkylation, the reaction between ArCH=NAr (Ar = p-C1C6H4) and cyclopentene was completely inhibited, but occurred again upon replacing the aldimine by its hydrochloride salt. This indicates that the protonated aldimine is involved in the reductive reaction step. Protonation makes this reduction easier by 0.1 V as indicated by the shift of the reduction potential of methyl (2Z)-phenyl(phenylimino)acetate upon addition of glacial acetic acid to the acetonitrile solution.

Electrodeposition of nanostructured diamond-like films by oxidation of lithium acetylide

Diamond-like carbon (DLC) films have been deposited by anodic oxidation of 4 M solution of lithium acetylide in dimethylsulfoxide on the surface of stainless steel or nickel electrode at room temperature and moderate anodic current densities (0.2–2.0 mA/cm2) in the range of electrode potentials 0.3–2.5 V (vs. sat. Ag|AgCl reference electrode). Electrodeposited DLC coatings represented complete and optically transparent films of a thickness 50–100 nm having dark island inclusions with a diameter 0.8–5.0 μm. The concentration and average size of these particles increased with the prolongation of deposition time. Micro-Raman spectra obtained by the focusing of laser beam onto these dark inclusions are characterized by a broad peak centered at 1500 cm−1 and weak peak at 1200 cm−1. With a defocused laser beam, there appear two well-distinguished peaks on the integrated Raman spectra – at 1530 and 1130 cm−1. Analysis of Raman spectra with the use of a Breit–Wigner–Fano lineshape and spectrum deconvolution indicates that the electrodeposited films consist of diamond-like nanostructured carbon with a high content (70–80%) of sp3 phase.

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.