@phdthesis{Buerckstuemmer2011,
  author    = {B{\"u}rckst{\"u}mmer, Hannah},
  title     = {Merocyanine dyes for solution-processed organic bulk heterojunction solar cells},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-66879},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {The technology of organic photovoltaics offers the possibility of low-cost devices due to easy fabrication procedures and low material consumption and at the same time high flexibility concerning the applied substrates or design features such as the color palette. Owing to these benefits, this research field is highly active, being reflected by the continuously rising number of publications. Chapter 1 gives an extensive overview of a part of these reports, namely the field of solution-processed BHJ organic solar cells using small molecules as electron-donating materials. In the early years of this research area (2006-2008), well known hole transporting materials such as triphenylamine based chromophores, oligothiophenes and polyaromatic hydrocarbons were applied. However, many of these dyes lacked absorption at longer wavelengths and were therefore limited in their light harvesting qualities. Later, chromophores based on low band gap systems consisting of electron-donating and electron-accepting units showing internal charge transfer overcame this handicap. Today, donor-substituted diketopyrrolopyrroles (D-A-D chromophores), squaraines (D-A-D chromophores) and acceptor substituted oligothiophenes (A-D-A chromophores) are among the most promising dyes for small molecule based organic solar cells with PCEs of 4-5\%. This work is based on the findings of the groups of W{\"u}rthner and Meerholz, which tested merocyanine dyes for the first time in organic BHJ solar cells.4 According to the B{\"a}ssler theory85, the high dipolarity of these dyes should hamper the charge transport, but the obtained first results with PCE of 1.7\% proved the potenital of this class of dyes for this application. Merocyanine dyes offer the advantages of facile synthesis and purification, high tinctorial strength and monodispersity. Additionally, the electronic structure of the dyes, namely the absorption as well as the electrochemical properties, can be adjusted by using the right combination of donor and acceptor units. For these reasons, this class of dye is highly interesting for the application in organic solar cells. It was the aim of the thesis to build more knowledge about the potential and limitations of merocyanines in BHJ photovoltaic devices. By screening a variety of donor and acceptor groups a comprehensive data set both for the molecular materials as well as for the respective solar devices was generated and analyzed. As one focus, the arrangement of the chromophores in the solid state was investigated to gain insight about the packing in the solar cells and its relevance for the performance of the latter. To do so, X-ray single crystal analyses were performed for selected molecules. By means of correlations between molecular properties and the characteristics of the corresponding solar cells, several design rules to generate efficient chromophores for organic photovoltaics were developed. The different donor and acceptor moieties applied in this work are depicted in the following ...},
  subject      = {organische Solarzelle},
  language  = {en}
}
@phdthesis{Schafferhans2011,
  author    = {Schafferhans, Julia},
  title     = {Investigation of defect states in organic semiconductors: Towards long term stable materials for organic photovoltaics},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-57669},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {In this work, the trap states in the conjugated polymer P3HT, often used as electron donor in organic bulk heterojunction solar cells, three commonly used fullerene based electron acceptors and P3HT:PC61BM blends were investigated. Furthermore, the trap states in the blend were compared with these of the pure materials. Concerning the lifetime of organic solar cells the influence of oxygen on P3HT and P3HT:PC61BM blends was studied. The experimental techniques used to investigate the trap states in the organic semiconductors were (fractional) thermally stimulated current (TSC) and current based deep level transient spectroscopy (Q-DLTS). Fractional TSC measurements on P3HT diodes revealed a quasi-continuous trap distribution. The distribution suggested two different traps in P3HT with approximately Gaussian energy distributions and maxima at about 50 meV and 105 meV. Thereby, the former was attributed to the tail states within the regular Gaussian density of states due to the low activation energy. The latter, deeper traps, however, exhibited a strong dependence on oxygen. Exposure of the P3HT diodes to oxygen, ambient air and synthetic (dry) air all revealed an increase of the deeper traps density with exposure time in the same manner. While the lower limit of the trap density in non aged P3HT samples was in the range of (1.0 - 1.2)×10^22 m^-3, it was more than doubled after an exposure of 50 h to air. An increase of the trap density with oxygen exposure time was also seen in the Q-DLTS measurements accompanied with an increase of the temperature dependence of the emission rates, indicating an enhanced formation of deeper traps. Due to the raise in density of the deeper traps, the charge carrier mobility in P3HT significantly decreased, as revealed by photo-CELIV measurements, resulting in a loss in mobility of about two orders of magnitude after 100 h exposure to synthetic air. The increased trap density was attributed to p-doping of P3HT by the transfer of an electron to adsorbed oxygen. This effect was partially reversible by applying vacuum to the sample for several hours or, more significantly, by a thermal treatment of the devices in nitrogen atmosphere. The trap states in the methanofullerenes PC61BM, bisPC61BM and PC71BM were investigated by TSC measurements. PC61BM yielded a broad quasi-continuous trap distribution with the maximum of the distribution at about 75 meV. The comparison of the TSC spectra of the three methanofullerenes exhibited significant differences in the trap states with higher activation energies of the most prominent traps in bisPC61BM and PC71BM compared to PC61BM. This probably originates from the different isomers bisPC61BM and PC71BM consist of. Each of the isomers yields different LUMO energies, where the lower ones can act as traps. The lower limit of the trap density of all of the three investigated fullerene derivatives exhibited values in the order of 10^22 m^-3, with the highest for bisPC61BM and the lowest for PC61BM. By applying fractional TSC measurements on P3HT:PC61BM solar cells, it was shown that the trap distribution in the blend is a superposition of the traps in pure P3HT and PC61BM and additional deeper traps in the range of about 250 meV to 400 meV. The origin of these additional traps, which can not be related to the pure materials, was attributed to a higher disorder in the blend and P3HT/PC61BM interfaces. This conclusion was supported by standard TSC and Q-DLTS measurements performed on pristine and annealed P3HT:PC61BM blends, exhibiting a higher ratio of the deep traps in the pristine samples. The lower limit of the trap density of the investigated annealed solar cells was in the range of (6-8)×10^22 m^-3, which was considerably higher than in the pure materials. The influence of oxygen on P3HT:PC61BM solar cells was investigated by exposure of the devices to synthetic air under specific conditions. Exposure of the solar cells to oxygen in the dark resulted in a strong decrease in the power conversion efficiency of 60 \% within 120 h, which was only caused by a loss in short-circuit current. Simultaneous illumination of the solar cells during oxygen exposure strongly accelerated the degradation, resulting in an efficiency loss of 30 \% within only 3 h. Thereby, short-circuit current, open-circuit voltage and fill factor all decreased in the same manner. TSC measurements revealed an increase of the density of deeper traps for both degradation conditions, which resulted in a decrease of the mobility, as investigated by CELIV measurements. However, these effects were less pronounced than in pure P3HT. Furthermore, an increase of the equilibrium charge carrier density with degradation time was observed, which was attributed to oxygen doping of P3HT. With the aid of macroscopic simulations, it was shown that the doping of the solar cells is the origin of the loss in short-circuit current for both degradation conditions.},
  subject      = {Organischer Halbleiter},
  language  = {en}
}
@phdthesis{Baumann2011,
  author    = {Baumann, Andreas},
  title     = {Charge Transport and Recombination Dynamics in Organic Bulk Heterojunction Solar Cells},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-64915},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {The charge transport in disordered organic bulk heterojunction (BHJ) solar cells is a crucial process affecting the power conversion efficiency (PCE) of the solar cell. With the need of synthesizing new materials for improving the power conversion efficiency of those cells it is important to study not only the photophysical but also the electrical properties of the new material classes. Thereby, the experimental techniques need to be applicable to operating solar cells. In this work, the conventional methods of transient photoconductivity (also known as "Time-of-Flight" (TOF)), as well as the transient charge extraction technique of "Charge Carrier Extraction by Linearly Increasing Voltage" (CELIV) are performed on different organic blend compositions. Especially with the latter it is feasible to study the dynamics, i.e. charge transport and charge carrier recombination, in bulk heterojunction (BHJ) solar cells with active layer thicknesses of 100-200 nm. For a well performing organic BHJ solar cells the morphology is the most crucial parameter finding a trade-off between an efficient photogeneration of charge carriers and the transport of the latter to the electrodes. Besides the morphology, the nature of energetic disorder of the active material blend and its influence on the dynamics are discussed extensively in this work. Thereby, the material system of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) serves mainly as a reference material system. New promising donor or acceptor materials and their potential for application in organic photovoltaics are studied in view of charge dynamics and compared with the reference system. With the need for commercialization of organic solar cells the question of the impact of environmental conditions on the PCE of the solar cells raises. In this work, organic BHJ solar cells exposed to synthetic air for finite duration are studied in view of the charge carrier transport and recombination dynamics. Finally, within the framework of this work the technique of photo-CELIV is improved. With the modified technique it is now feasible to study the mobility and lifetime of charge carriers in organic solar cells under operating conditions.},
  subject      = {Photovoltaik},
  language  = {en}
}