@phdthesis{Sun2024,
  author    = {Sun, Bowen},
  title     = {Energy losses in low-offset organic solar cells},
  doi       = {10.25932/publishup-62143},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-621430},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xxi, 190},
  year      = {2024},
  abstract  = {Organic solar cells (OSCs) represent a new generation of solar cells with a range of captivating attributes including low-cost, light-weight, aesthetically pleasing appearance, and flexibility. Different from traditional silicon solar cells, the photon-electron conversion in OSCs is usually accomplished in an active layer formed by blending two kinds of organic molecules (donor and acceptor) with different energy levels together. The first part of this thesis focuses on a better understanding of the role of the energetic offset and each recombination channel on the performance of these low-offset OSCs. By combining advanced experimental techniques with optical and electrical simulation, the energetic offsets between CT and excitons, several important insights were achieved: 1. The short circuit current density and fill-factor of low-offset systems are largely determined by field-dependent charge generation in such low-offset OSCs. Interestingly, it is strongly evident that such field-dependent charge generation originates from a field-dependent exciton dissociation yield. 2. The reduced energetic offset was found to be accompanied by strongly enhanced bimolecular recombination coefficient, which cannot be explained solely by exciton repopulation from CT states. This implies the existence of another dark decay channel apart from CT. The second focus of the thesis was on the technical perspective. In this thesis, the influence of optical artifacts in differential absorption spectroscopy upon the change of sample configuration and active layer thickness was studied. It is exemplified and discussed thoroughly and systematically in terms of optical simulations and experiments, how optical artifacts originated from non-uniform carrier profile and interference can manipulate not only the measured spectra, but also the decay dynamics in various measurement conditions. In the end of this study, a generalized methodology based on an inverse optical transfer matrix formalism was provided to correct the spectra and decay dynamics manipulated by optical artifacts. Overall, this thesis paves the way for a deeper understanding of the keys toward higher PCEs in low-offset OSC devices, from the perspectives of both device physics and characterization techniques.},
  language  = {en}
}
@phdthesis{Albrecht2014,
  author    = {Albrecht, Steve},
  title     = {Generation, recombination and extraction of charges in polymer},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-72285},
  school      = {Universit{\"a}t Potsdam},
  pages     = {144},
  year      = {2014},
  abstract  = {A dramatic efficiency improvement of bulk heterojunction solar cells based on electron-donating conjugated polymers in combination with soluble fullerene derivatives has been achieved over the past years. Certified and reported power conversion efficiencies now reach over 9\% for single junctions and exceed the 10\% benchmark for tandem solar cells. This trend brightens the vision of organic photovoltaics becoming competitive with inorganic solar cells including the realization of low-cost and large-area organic photovoltaics. For the best performing organic materials systems, the yield of charge generation can be very efficient. However, a detailed understanding of the free charge carrier generation mechanisms at the donor acceptor interface and the energy loss associated with it needs to be established. Moreover, organic solar cells are limited by the competition between charge extraction and free charge recombination, accounting for further efficiency losses. A conclusive picture and the development of precise methodologies for investigating the fundamental processes in organic solar cells are crucial for future material design, efficiency optimization, and the implementation of organic solar cells into commercial products. In order to advance the development of organic photovoltaics, my thesis focuses on the comprehensive understanding of charge generation, recombination and extraction in organic bulk heterojunction solar cells summarized in 6 chapters on the cumulative basis of 7 individual publications. The general motivation guiding this work was the realization of an efficient hybrid inorganic/organic tandem solar cell with sub-cells made from amorphous hydrogenated silicon and organic bulk heterojunctions. To realize this project aim, the focus was directed to the low band-gap copolymer PCPDTBT and its derivatives, resulting in the examination of the charge carrier dynamics in PCPDTBT:PC70BM blends in relation to by the blend morphology. The phase separation in this blend can be controlled by the processing additive diiodooctane, enhancing domain purity and size. The quantitative investigation of the free charge formation was realized by utilizing and improving the time delayed collection field technique. Interestingly, a pronounced field dependence of the free carrier generation for all blends is found, with the field dependence being stronger without the additive. Also, the bimolecular recombination coefficient for both blends is rather high and increases with decreasing internal field which we suggest to be caused by a negative field dependence of mobility. The additive speeds up charge extraction which is rationalized by the threefold increase in mobility. By fluorine attachment within the electron deficient subunit of PCPDTBT, a new polymer F-PCPDTBT is designed. This new material is characterized by a stronger tendency to aggregate as compared to non-fluorinated PCPDTBT. Our measurements show that for F-PCPDTBT:PCBM blends the charge carrier generation becomes more efficient and the field-dependence of free charge carrier generation is weakened. The stronger tendency to aggregate induced by the fluorination also leads to increased polymer rich domains, accompanied in a threefold reduction in the non-geminate recombination coefficient at conditions of open circuit. The size of the polymer domains is nicely correlated to the field-dependence of charge generation and the Langevin reduction factor, which highlights the importance of the domain size and domain purity for efficient charge carrier generation. In total, fluorination of PCPDTBT causes the PCE to increase from 3.6 to 6.1\% due to enhanced fill factor, short circuit current and open circuit voltage. Further optimization of the blend ratio, active layer thickness, and polymer molecular weight resulted in 6.6\% efficiency for F-PCPDTBT:PC70BM solar cells. Interestingly, the double fluorinated version 2F-PCPDTBT exhibited poorer FF despite a further reduction of geminate and non-geminate recombination losses. To further analyze this finding, a new technique is developed that measures the effective extraction mobility under charge carrier densities and electrical fields comparable to solar cell operation conditions. This method involves the bias enhanced charge extraction technique. With the knowledge of the carrier density under different electrical field and illumination conditions, a conclusive picture of the changes in charge carrier dynamics leading to differences in the fill factor upon fluorination of PCPDTBT is attained. The more efficient charge generation and reduced recombination with fluorination is counterbalanced by a decreased extraction mobility. Thus, the highest fill factor of 60\% and efficiency of 6.6\% is reached for F-PCPDTBT blends, while 2F-PCPDTBT blends have only moderate fill factors of 54\% caused by the lower effective extraction mobility, limiting the efficiency to 6.5\%. To understand the details of the charge generation mechanism and the related losses, we evaluated the yield and field-dependence of free charge generation using time delayed collection field in combination with sensitive measurements of the external quantum efficiency and absorption coefficients for a variety of blends. Importantly, both the yield and field-dependence of free charge generation is found to be unaffected by excitation energy, including direct charge transfer excitation below the optical band gap. To access the non-detectable absorption at energies of the relaxed charge transfer emission, the absorption was reconstructed from the CT emission, induced via the recombination of thermalized charges in electroluminescence. For a variety of blends, the quantum yield at energies of charge transfer emission was identical to excitations with energies well above the optical band-gap. Thus, the generation proceeds via the split-up of the thermalized charge transfer states in working solar cells. Further measurements were conducted on blends with fine-tuned energy levels and similar blend morphologies by using different fullerene derivatives. A direct correlation between the efficiency of free carrier generation and the energy difference of the relaxed charge transfer state relative to the energy of the charge separated state is found. These findings open up new guidelines for future material design as new high efficiency materials require a minimum energetic offset between charge transfer and the charge separated state while keeping the HOMO level (and LUMO level) difference between donor and acceptor as small as possible.},
  language  = {en}
}