C.J. Brabec

Realization of Large Area Flexible Fullerene - Conjugated Polymer Photocells: A Route to Plastic Solar Cells

Bulk donor — acceptor heterojunctions between conjugated polymers and fullerenes have been utilized for photovoltaic devices with quantum efficiencies of around 1%. These devices are based on the photoinduced, ultrafast electron transfer between non degenerate ground state conjugated polymers and fullerenes. In this work we present efficiency data of solar cells based on a soluble alkoxy poly(para phenylenevinylene) (MDMO-PPV) and a highly soluble fullerene derivative (PCBM). Small area (mm2) photovoltaic devices show energy conversion efficiencies ηe > 1% and charge carrier collection efficiencies ηc20%. We present efficiency and stability studies on large area (6 cm x 6 cm) flexible solar cells and compare them with small area devices. The process of large scale coating using printing techniques demands excellent Theological properties of the solution and the compound. We studied the influence of conventional polymers as additives for the photoactive compounds in order to improve homogenous film formation. Addition of small amounts (11 wt%) of conventional polymers does not alter the device characteristics. At higher concentrations of additives conversion efficiency drop down quickly.

Infrared spectroelectrochemical investigations on the doping of soluble poly(isothianaphthene methine) (PIM)

Attenuated total reflection Fourier transform infrared (ATR-FTIR) investigations on the doping processes in substituted poly(isothianaphtene methine) (PIM), a new low band-gap conjugated polymer, are reported. The doping was studied in situ during chemical p-doping (oxidation) by iodine and during electrochemical p-doping (oxidation) and n-doping (reduction). During both signs of doping, infrared active vibrational (IRAV) bands due to strong coupling of the electrons to lattice vibrations are observed. The results are compared to FT-Raman spectra. Electrochemical p-doping shows two different doping regimes depending on the electrochemical potential. The narrow linewidth and low absorption intensities of the IRAV bands indicate a strong localization of the doping induced charge carriers, which is ascribed to a tilted geometry of the conjugated backbone due to steric repulsion effects.Attenuated total reflection Fourier transform infrared (ATR-FTIR) investigations on the doping processes in substituted poly(isothianaphtene methine) (PIM), a new low band-gap conjugated polymer, are reported. The doping was studied in situ during chemical p-doping (oxidation) by iodine and during electrochemical p-doping (oxidation) and n-doping (reduction). During both signs of doping, infrared active vibrational (IRAV) bands due to strong coupling of the electrons to lattice vibrations are observed. The results are compared to FT-Raman spectra. Electrochemical p-doping shows two different doping regimes depending on the electrochemical potential. The narrow linewidth and low absorption intensities of the IRAV bands indicate a strong localization of the doping induced charge carriers, which is ascribed to a tilted geometry of the conjugated backbone due to steric repulsion effects.

Investigation of photoexcitations of conjugated polymer/fullerene composites embedded in conventional polymers

A promising approach to improve the processability of semiconducting polymers is their incorporation into host matrices formed by conventional polymers such as polyethylene ~PE! or polystyrene ~PS!. We have characterized the linear optical properties of several guest‐host systems by absorption and luminescence measurements and probed the excited states by photoinduced absorption measurements ~PIA! and light induced electron spin resonance ~LESR!. The interesting photophysics of conjugated polymer/fullerene donor‐acceptor system, showing an ultrafast photoinduced electron transfer and a metastable charge separation in the pristine state, has been investigated in such composites where the photoactive components are further embedded into a conventional polymer matrix. The composition of the blend and the relative concentration of the components are found to strongly influence the photoinduced interaction between the conjugated polymer and the fullerenes ~C60 and functionalized fullerenes!. Photoinduced electron transfer between a soluble polyphenylenevinylene ~PPV! derivative and C60 is observed in a system with PS as host when the concentration of the both electroactive components is 33%. In diluted composites with PS as matrix we find strong luminescence quenching of the PPV derivative but no electron transfer upon adding C60. Dominant photoexcitations in these PS systems are triplet states as follows from intensity dependencies and lifetime measurements. In diluted systems with PE as matrix neither charge transfer nor luminescence quenching has been observed.

Photo-induced electron transfer from a dithieno thiophene-based polymer to TiO2

Abstract A spectroscopical study of composites made with a new conjugated polymer, polydithienothiophene, and nanocrystalline TiO2 is carried out by optical absorption, photoluminescence and steady state photo-induced absorption spectroscopy. Sintered nanoporous TiO2 films have been stained by dipping the inorganic semiconductor films into dithienothiophene polymer solutions. The optical absorption spectra indicate that this polymer is well absorbed into nanoporous TiO2 films, showing a remarkable interaction with the inorganic semiconductor as compared to other alkyl substituted thiophene based conjugated polymers. The photoluminescence quenching and the steady state photo-induced absorption spectra evidence that TiO2 acts as an efficient electron acceptor toward the polymer in the excited state. These features indicate that this conjugated polymer is a promising material to develop new hybrid organic/inorganic photovoltaic cells.

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.

Highly ordered anisotropic nano-needles in para-sexiphenyl films

Atomic force microscopy (AFM) was used to investigate the early growth stage of highly ordered para-sexiphenyl thin films deposited by hot wall epitaxy on mica, in order to find the process controlling parameters. It was shown that the growth time and surface type are important parameters for controlling of the film morphology, in terms of the degree of anisotropy and long range order. X-Ray diffraction pole figure technique and transmission electron diffraction was also used to characterize the crystallographic structure of the thicker films.

CW-Photocurrent measurements of conjugated polymers and fullerenes blended into a conventional polymer matrix

In this work we present efficiency data of solar cells based on mixtures from soluble alkoxy PPV, a highly soluble fullerene derivative (PCBM) and three different conventional, non conjugated polymers, polystyrene (PS), polyvinylcarbazole (PVK) and polyvinylbenzenechlorid (PVBC). We studied the I/V characteristics of devices with various amounts of conventional polymer. Energy conversion efficiency ηe and carrier collection efficiencies ηc are calculated for these three composite devices and compared with the data for a pure conjugated polymer / fullerene device. We find that addition of up to 10% of conventional polymer to the electroactive composite is possible without lowering the efficiency of the device.

Photoinduced FT-IR spectroscopy of conjugated polymer/fullerene composites embedded into conventional host polymer matrices

In this work, we report on the investigation of photoexcited states in conjugated polymer (donor) - fullereae (acceptor) interpenetrating networks (polythiophene derivatives — PC61BM) embedded into conventional polymer hosts like polystyrene, polyvinylcarbazole, polycarbonate or polyvinylbenzenechloride, using photoinduced FT-IR spectroscopy. We discuss the influence of specific host polymer matrices on photoexcited states of the photoactive guests via photoinduced infrared activated (IRAV) modes.

Realization of large area flexible fullerene-conjugated polymer photocells: A route to plastic solar cells

Various interesting photophysical phenomena in composites of fullerenes and non degenerate ground state conjugated polymers with highly extended π-electrons in their main chain can be explained by the ultrafast electron transfer from the conjugated polymer (donor) to the fullerene (acceptor) upon illumination. This photoeffect was utilized for the production of small area (mm2) photovoltaic devices which show energy conversion efficiencies ηe>1% and carrier collection efficiencies ηc>20%. In this work we present efficiency and stability studies on large area (6 cm by 6 cm) flexible solar cells based on a soluble alkoxy PPV (3,7-dimethyloctyloxy methyloxy poly(phenylenevinylene)) and a highly soluble fullerene derivative, 1-(3-methoxycarbonyl)propyl-1 phenyl [5,6]C61 (PCBM). The enhanced solubility of PCBM compared to C60 allows a high fullerene-conjugated polymer ratio and strongly supports the formation of donor-acceptor bulk heterojunctions.

Electronic properties of polymer-fullerene solar cells studied with light-induced electron spin resonance and admittance spectroscopy

Within recent years, the development of polymer-fullerene plastic solar cells has made significant progress. In such devices, an efficient charge generation takes place via photoinduced charge transfer between the photoexcited conjugated polymer and acceptor-type fullerene molecules. Due to the paramagnetic nature of the radical species, the photoinduced charge transfer can be studied by means of light induced electron spin resonance (LESR) techniques. We carried out W-band (95 GHz) LESR at high magnetic field strengths. Two well separated line groups with a strong anisotropic structure were detected for the composite MDMOPPV: PCBM. From the line shape analysis, we obtained an environmental axial symmetry for the positive polaron P+ and a lower, rhomboedric symmetry for the fullerene anion. The signals were found to be independent of each other with different spin-lattice relaxation times; hence, the radical species can be investigated separately. In order to study the bulk transport properties, we carried out admittance spectroscopy on the ITO/PEDOT:PSS/MDMO-PPV:PCBM/Al device. Two frequency-dependent contributions to the device capacitance with the activation energies 9 meV and 177 meV were found. For the very shallow trap state, we assume a bulk impurity, whereas the latter one is assigned to an interfacial defect state, located at the composite- aluminium interface.