Morphology of polymer solar cells

Publication: ResearchPh.D. thesis – Annual report year: 2014

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Organic electronic devices are an intense area of research. While some devices, such as organic light emitting diodes (OLED) have matured and are found in a vast amount of consumer electronic devices, their energy producing counterpart, organic photovoltaics (OPV), are still in the process of making the transition from the laboratory into the commercial market. One of the biggest challenges in this process is upscaling the production. The object of this thesis is to investigate the morphology of OPV devices produced from pilot scale roll to roll (R2R) coaters. OPV devices still struggle with low performance, and the morphology is known to have a critical impact on the performance of a device. Several studies have tried to identify the optimal morphology of OPV devices and how to achieve it. Most work has been focused on OPVs produced by spin coating in a small laboratory scale. Devices produced by R2R coating, which works fundamentally different, have not been studied. Traditional production of OPV has required the use of toxic solvents. A new environmentally friendly approach using water based inks, made of nanoparticles, is now being tested. However, nothing is known about the morphology of the active layer of the solar cells when produced with water based inks using R2R coating. Using a broad range of scattering and imaging techniques, cells coated with water based inks were investigated, and compared to their spin coated counterpart. Two challenges to be addressed were small domain size to be studied, in the nanometer regime, and the poor contrast due to the similarity of the organic materials. The physical impact of the ink and the process of coating it, was investigated by electron microscopy, X-ray scattering, hard X-ray ptychography and soft X-ray transmission imaging. Utilizing the robustness and high resolution of transmission electron microscopy, different preparations of inks were studied. Electron microscopy offers good visualization, but lacks contrast to distinguish similar organic materials, such as P3HT and PCBM, two components of the active layer. Electron diffraction yields information about the crystal structure of the samples but have a coarse spot size. X-ray scattering is a well known technique for measuring shapes, sizes, crystal structures and orientation. Both small- and wide-angle scattering were used to measure the crystallinity of the layers as a function of polymer, type of ink, annealing etc. Ptychography is a new state of the art X-ray imaging technique based on coherent scattering. Together with Scanning X-ray Transmission Microscopy (STXM) it has been used in this study to inspect the morphology of the active layer taken from working solar cells. Ptychography offers desirable properties such as potentially high resolution, quantitative contrast and possibility for tomography. Both these X-ray imaging techniques were used to measure the samples with high spatial and chemical resolution. In addition, these experiments explored and reviewed the viability of ptychography as a characterization technique for OPVs evaluated. The ink studies showed that the nanoparticles in the active layer were disrupted. Dense parts of the nanoparticles could be observed surrounded by a bulk of less dense material. The same pattern was seen in preparations made by both coating methods. A difference, observed between the two methods was that the layer produced by R2R consisted of aggregates of particles. The particles in the spin coated samples were uniformly distributed. Furthermore, this thesis focuseds on developing, and testing, a new method for high throughput characterization of OPV devices. An advantage with R2R coating is the continuous production of layers and the possibility to change production parameters continuously during the process. It would therefore be an advantage if the characterization could also be done continuously. For this purpose a small film winder-underwinder was tested. The crystalline structure was measured using small angle X-ray scattering on three samples. The high spatial resolution obtained, made it possible to see the changes in crystalline structure as a function of coating paramters. These changes would not have been possible to see using a series of spin coated samples.
Original languageEnglish
PublisherDepartment of Energy Conversion and Storage, Technical University of Denmark
Number of pages208
StatePublished - 2013
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