RESUMO
Scattering techniques are a powerful tool for probing thin-film nanomorphologies but often require additional characterization by other methods. We applied the well-established grazing-incidence small-angle X-ray scattering (GISAXS) technique for a selection of energies around the absorption edge of sulfur to exploit the resonance effect (grazing incidence resonant tender X-ray scattering, GIR-TeXS) of the sulfur atoms within a poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) sample to gain information about the composition of the film morphology. With this approach, it is possible not only to identify structures within the investigated thin film but also to link them to a particular material combination.
RESUMO
The performance of semiconducting polymers strongly depends on their intra- and intermolecular electronic interactions. Therefore, the morphology and particularly crystallinity and crystal structure play a crucial role in enabling a sufficient overlap between the orbitals of neighboring polymers. A new solution-based in situ polymerization for the fabrication of native polythiophene thin films is presented, which exploits the film formation process to influence the polymer crystal structure in the resulting thin films. The synthesis of the insoluble polythiophene is based on an oxidative reaction in which the oxidizing agent, iron(III) p-toluenesulfonate (FeTos), initially oxidizes the monomers to enable the polymer chain growth and secondly the final polymers, thereby chemically doping the polythiophene. To exploit the fact that the doped polythiophene has a different crystal packing structure compared to the undoped polythiophene, we investigate the structural effect of this inherent doping process by varying the amounts of FeTos in the reaction mixture, creating polythiophene thin films with different degrees of doping. The structural investigation performed by means of grazing incidence wide-angle X-ray scattering (GIWAXS) suggests that the strongly doped polymer chains aggregate in a π-stacked manner in the film formation process. Moreover, this π-stacking can be maintained after the removal of the dopant molecules. GIWAXS measurements, molecular dynamics simulations, and spectroscopic analysis suggest the presence of polythiophene in a novel and stable crystal structure with an enhanced intermolecular interaction.
RESUMO
The macroscopic device performance of organic solar cells is governed by interface physics on a nanometer scale. A comb-like bilayer all-polymer morphology featuring a controlled enhancement in donor-acceptor interfacial area is employed as a model system to investigate the fundamental processes of exciton separation and polaron recombination in these devices. The different nanostructures are characterized locally by SEM/AFM, and the buried interdigitating interface of the final device architecture is statistically verified on a large area via advanced grazing incidence X-ray scattering techniques. The results show equally enhanced harvesting of photoexcitons in both donor and acceptor materials directly correlated to the total enhancement of interfacial area. Apart from this beneficial effect, the enhanced interface leads to significantly increased polaron recombination losses both around the open-circuit voltage and maximum power point, which is determined in complement with diode dark current characteristics, impedance spectroscopy, and transient photovoltage measurements. From these findings, it is inferred that a spatially optimized comb-like donor-acceptor nanonetwork alone is not the ideal morphology even though often postulated. Instead, the energetic landscape has to be considered. A perfect morphology for an excitonic solar cell must be spatially and energetically optimized with respect to the donor-acceptor interface.
