Helmut Neugebauer

In situ spectroelectrochemical characterization of poly(3,4-ethylenedioxythiophene)

The electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT), in diAerent electrolyte‐solvent media was studied with cyclic voltammetry, in situ UV‐VIS-spectroelectrochemistry, electrochemical quartz crystal microbalance technique (EQCM) and with in situ Fourier transform infrared (FTIR) spectroscopy using external and internal reflection techniques. The eAect of polymerization current density and monomer concentration on the formation of the film structure was studied. The redox reactions and the stability of charged films of diAerent thickness were studied with cyclic voltammetry and open circuit potentiometric measurements. FTIR spectra were recorded in situ during step-wise and continuous potential cycling of the polymer films in diAerent electrolytes. A characterization of the doping induced IR-bands has been made. # 1999 Elsevier Science Ltd. All rights reserved.

Stability and photodegradation mechanisms of conjugated polymer/fullerene plastic solar cells

Degradation studies of poly(2-methoxy-5-(3@,7@-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV), fullerenes ((6,6)-phenyl C 61 - butyric acid methyl ester (PCBM) and C 60 ), and mixtures, which are the photoactive components in plastic solar cells, are shown. The degradation processes of the individual components and of a 1 : 3 mixture are characterized by attenuated total re#ection Fourier transform infrared (ATR-FTIR) spectroscopy and by current/voltage (I}<) measurements of devices under the in#uence of light and oxygen. A faster degradation rate was found for the polymer compared with C 60 . In composites with fullerenes, the stability of MDMO-PPV is enhanced due to the fast electron transfer to C 60 . ( 2000 Elsevier Science B.V. All rights reserved.

Stabilization of the nanomorphology of polymer–fullerene “bulk heterojunction” blends using a novel polymerizable fullerene derivative

The morphological stabilization of donor–acceptor blends for bulk heterojunction solar cells can be achieved by cross-linking of the small molecular phase in the polymer matrix using a polymerizable fullerene derivative. In a comparative study the morphology of polymer–fullerene blend films was investigated using poly(3-hexylthiophene) (P3HT) as the polymer and C61-butyric acid methyl ester (PCBM) or the newly synthesized polymerizable fullerene derivative, C61-butyric acid glycidol ester, PCBG, as the acceptor molecule, respectively. Changes in the nanomorphology due to heat treatment of the films were studied by means of atomic force microscopy (AFM), transmission electron microscopy (TEM) and photoluminescence (PL) studies. The polymerization process was monitored with infrared absorption studies. As demonstrated by these comparative studies this newly synthesized fullerene gives considerable stabilization of the solid state morphology in these blends. Such prevention of the long term, high temperature instability of bulk heterojunction morphology displays an important route to increase the operational stability of plastic solar cells in future applications.

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.

Structural and electronic transitions in polyaniline: A Fourier transform infrared spectroscopic study

Fourier transform infrared (FTIR) spectroscopic studies are carried out on different, potentiometric well defined oxidation states of polyaniline in aqueous acidic and organic electrolytes. During the oxidation process ring structures transform from benzenoid into quinoid states. The fully reduced state of polyaniline shows differences in the anion contents in acidic and organic electrolytes. The 400 mV vs saturated calomel electrode (SCE) oxidized state has the maximum number of the intercalated anions in aqueous acidic media in accordance with supporting potentiometric titration experiments. This conducting form of polyaniline shows similar FTIR spectra in organic as well as in acidic media. For the oxidized state at 800 mV vs SCE, a deintercalation of anions in aqueous acidic, or further intercalation in organic electrolyte is observed. Beyond 800 mV vs SCE, polyaniline shows degradation processes in aqueous acidic media which are found to proceed via formation of benzoquinone‐like structures and final...

Vibrational signatures of electrochemical p- and n-doping of poly(3,4-ethylenedioxythiophene) films: an in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) study

Abstract The vibrational spectroscopic properties of electrochemically synthesized poly(3,4-ethylenedioxythiophene) (PEDOT) films were studied with in situ Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy. PEDOT films were p-doped (oxidized) and n-doped (reduced) electrochemically. The doping induced infrared absorption spectra (IRAV spectra) during p- and n-doping of PEDOT films were studied comparatively. The results are discussed within the framework of a stronger localization of charges in the case of n-doping compared to p-doping.

Protonation and electrochemical redox doping processes ofpolyaniline in aqueous solutions: Investigations using insitu FTIR-ATR spectroscopy and a new dopingsystem

Polyaniline (PANI), one of the most interesting members of the group of conducting polymers, has been the subject of intense experimental and theoretical studies because of its outstanding behaviour during electrochemical redox processes and during changes of pH. In this work the electrochemical doping and protonation processes of PANI are described by using in situ FTIR-ATR and a new doping system NaReO 4 /HReO 4 . Cyclic voltammetry shows that above ca. pH 4 only one wave response can be obtained. With increasing acidity of the electrolyte, a two-wave response develops, the splitting of the waves occurring at pH ca. 3.5–4. By in situ measurements during the base–acid transition process, a distinct picture of protonation of different forms of PANI has been obtained. A scheme for the dopant concentration, as a function of the electrode potential and pH, is presented. The results show that the amount of anions involved in the doping and dedoping processes depends on the pH and the potential. Maximum doping occurs at the intermediate oxidized metallic state. During both processes the electronic structure of the polymer and also the number of protons bound to nitrogen atoms are altered. The mechanisms of the changes under different redox and acid–base conditions are described.

Negative capacitance in organic semiconductor devices: Bipolar injection and charge recombination mechanism

The authors report negative capacitance at low frequencies in organic semiconductor based diodes and show that it appears only under bipolar injection conditions. They account quantitatively for this phenomenon by the recombination current due to electron-hole annihilation. Simple addition of the recombination current to the well established model of space charge limited current in the presence of traps yields excellent fits to the experimentally measured admittance data. The dependence of the extracted characteristic recombination time on the bias voltage is indicative of a recombination process which is mediated by localized traps.

Charge Carrier Lifetime and Recombination in Bulk Heterojunction Solar Cells

In this paper, the main photocurrent density and power conversion efficiency limiting mechanisms in bulk heterojunction solar cells are discussed with the emphasis on recombination processes. Charge extraction by linearly increasing voltage, time of flight, and other methods that allow the carrier lifetime and recombination to be studied experimentally in operating solar cells are discussed. It is shown that non-Langevin recombination is required for high-performance organic photovoltaic devices, which typically have low charge carrier mobility. Long charge carrier lifetime, exceeding carrier transit time through the film, can be achieved when non-Langevin recombination is observed. Langevin-type recombination dominates in most low-efficiency solar cells, whereas non-Langevin recombination is present in high efficient, e.g., annealed poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester blend devices. The film nanomorphology plays a crucial role governing the charge transport and the carrier lifetime. Double injection current with non-Langevin carrier recombination is demonstrated in high-efficiency devices, which strongly exceeds the injection current with Langevin recombination, due to the high carrier concentration attainable under non-Langevin recombination. Several different models explaining the non-Langevin recombination in organic solar cells are reviewed. Requirements for charge carrier mobility and recombination to maximize power conversion efficiency in organic photovoltaic devices are outlined.

Stability Studies and Degradation Analysis of Plastic Solar Cell Materials by FTIR Spectroscopy

Results of controlled degradation experiments performed with the individual components and with the actual mixture used in plastic solar cells are shown. A testing procedure for the stability and for degradation effects under illumination in controlled atmosphere using FTIR-ATR spectroscopy is presented. The method yields rapid information on the degradation processes on molecular level within a few hours.