title: Donor-acceptor polymers for organic solar cells
Creators: name: Saller, Christina
Publisher: name: University of Bayreuth
Year: 2018
Subjects: subject: Organic Solar Cells
subject: Organic Photovoltaics
subject: Donor-Acceptor Polymers
subject: Multilayer Solar Cells
subject: Crosslinking
subject: Diffusion
subject: 500
subject: 530
subject: 540

description: Photovoltaics present an interesting option for the energy production using renewable sources. As the sun provides inexhaustible amounts of energy, the global energy consumption could in principle be covered by electricity from solar cells. Organic solar cells present a promising alternative to inorganic devices, although the lower efficiencies and the lack of long-term stability hinder the commercialization so far. In bulk heterojunction solar cells, the efficiency decrease arises from the diffusion of small molecule acceptors. Furthermore, the deposition of a second material on top of a first layer leads to the damage of underlying layers in multilayer devices as well as the dissolution of nanostructured morphologies. For an efficient improvement of organic solar cells, the device degradation has to be extensively studied and strategies for the fabrication of long-term stable devices have to be deduced.Crosslinking is a versatile option to achieve increased stability. The diffusion of small acceptor molecules in bulk heterojunction solar cells can be limited by deploying a crosslinked donor polymer matrix. The insolubility of a crosslinked polymer layer or interface structure allows the deposition of a second layer on top without damage of the underlying device. In addition, the understanding and improvement of organic photovoltaic devices require extensive studies on both charge carrier generation and recombination processes at the donor-acceptor interface of the active layer. This thesis deals with the modification of the well-established low bandgap polymers PCDTBT and PCPDTBT that are applied as donor materials in organic planar heterojunction solar cells. By this means, contributions to both optimized device fabrication as well as basic investigations were achieved. The synthetic modifications are classified into two parts. On the one hand, PCDTBT and PCPDTBT derivatives bearing crosslinkable units at the side chains of the donor monomers were realized. Oxetane was selected as the crosslinkable group because of its compatibility with the Suzuki coupling reaction and the possibility to initiate the crosslinking reaction by acid vapour. On the other hand, the polymer backbone of PCDTBT was modified with triphenyldiamine moieties exhibiting good hole transport properties. Furthermore, the attachment of aliphatic spacers to the acceptor moiety received a series of copolymers exhibiting different characteristics. Besides the chemical, thermal, optical and electronic characterisation of the synthesized polymers, the modifications of the polymers were evaluated. This included studies about the polymer properties in dependence of the crosslinking process and conditions as well as investigations about the variation of the polymer characteristics due to the incorporation of the additional donor monomer and the aliphatic sidechains at the acceptor units. One part of this thesis deals with a first three-layer device realized by solution processing due to the application of a crosslinked and insoluble polymer interlayer that allows the spin coating of the donor polymer on top. An increased external quantum efficiency as well as a higher efficiency of 1.8% compared to the bilayer reference cells with 1.6% efficiency were achieved with the three-layer cell due to the exciton blocking effect of the interlayer. The low bandgap copolymer series realized by the incorporation of an additional comonomer and aliphatic spacers in the PCDTBT structure was used to conduct fundamental investigations. The PCDTBT modified with triphenyldiamine moieties was chosen for the evaluation of the intrinsic contribution of the acceptors C60 and PCBM to the overall photocurrent. EQE measurements of single layer cells in dependence of the excitation energy revealed that the delocalization of the excitons and thus the intrinsic dissociation of both acceptors was enhanced with increasing photon energy. In a further study, the monomolecular and bimolecular recombination mechanisms in organic solar cells were addressed by the use of the PCDTBT-based donor polymer with triphenyldiamine units and aliphatic spacers at the acceptor monomer. Current-voltage characteristics showed that the fill factor decreased with increasing donor layer thickness at low light intensities due to geminate recombination. Higher light intensities induced non-geminate recombination that became higher for increasing layer thickness and was responsible for the further decreasing fill factor. A further aspect of this work was the investigation of the diffusion behaviour of C60 through different polymers with and without additional triphenyldiamine unit and aliphatic sidechains using a novel bilayer setup. A crosslinked polyfluorene was used to verify the novel bilayer setup in comparison to previously used three-layer samples. Temperature dependent photoluminescence measurements showed that fullerene diffusion occurs already below the glass transition temperature because of the local motion of sidechains that allows the diffusion of the small molecule acceptor. In conclusion, this work contributes to both fundamental research topics covering photogeneration and charge carrier recombination and device fabrication studies towards the optimization of organic solar cells with respect to long-term stability. Different donor polymers were thus synthesized with modifications to the basic polymer structures of PCDTBT and PCPDTBT like the introduction of the crosslinking ability or the incorporation of an additional comonomer and aliphatic spacers.