Wet Imprinting of Channel-Type Superstructures in Nanostructured Titania Thin Films at Low Temperatures for Hybrid Solar Cells.

Hierarchically structured titania films, exhibiting interconnected foam-like nanostructures and large-scale channel-type superstructures, were achieved in an energy-saving way at low temperatures by a polymer template-assisted sol-gel synthesis in combination with a wet-imprinting process. The surface morphology was probed with scanning electron microscopy and atomic force microscopy, whereas the inner morphology was characterized with grazing incidence small-angle X-ray scattering measurements. Compared to the initial hybrid films, the titania films showed reduced structure sizes caused by removal of the polymer template. UV/Vis measurements showed an additional light-scattering effect at various angles of light incidence in the hierarchically structured titania films, which resulted in higher light absorption in the wet-imprinted active layer. To give proof of viability, the titania films were evaluated as photoanodes for dye-free hybrid solar cells. The dye-free layout allowed for low-cost fabrication, avoided problems related to dye bleaching, and was a more environmentally friendly alternative to using dyes. Under different angles of light incidence, the enhancement in the short-circuit current density was in good agreement with the improvement in light absorption in the superstructured active layer, demonstrating a positive impact of the superstructures on the photovoltaic performance of hybrid solar cells.

Codependence between Crystalline and Photovoltage Evolutions in P3HT:PCBM Solar Cells Probed with in-Operando GIWAXS.

We address the correlation between the crystalline state of photoactive materials in a model organic solar cell based on poly(3-hexylthiophene-2,5-diyl):phenyl-C60-butyric acid methyl ester (P3HT:PCBM) and the photovoltage in an in-operando investigation. I-V curves are simultaneously measured together with grazing incidence wide-angle X-ray scattering probing the crystalline state of the device active layer as a function of the operation time. The results show a high degree of correlation between open-circuit voltage VOC and the crystalline state of P3HT.

Effect of Blend Composition and Additives on the Morphology of PCPDTBT:PC71BM Thin Films for Organic Photovoltaics.

The use of solvent additives in the fabrication of bulk heterojunction polymer:fullerene solar cells allows to boost efficiencies in several low bandgap polymeric systems. It is known that solvent additives tune the nanometer scale morphology of the bulk heterojunction. The full mechanism of efficiency improvement is, however, not completely understood. In this work, we investigate the influences of blend composition and the addition of 3 vol % 1,8-octanedithiol (ODT) as solvent additive on polymer crystallization and both, vertical and lateral morphologies of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] and [6,6]-phenyl C71-butyric acid methyl ester (PCPDTBT:PC71BM) blend thin films processed from chlorobenzene-based solutions. The nanoscale morphology is probed with grazing incidence small- and wide-angle X-ray scattering as well as X-ray reflectivity and complemented with UV/vis spectroscopy. In PCPDTBT:PC71BM films the use of ODT is found to lower the solubility of fullerene in the polymer matrix and to promote polymer crystallization, both vertical and lateral microphase separation with morphological coarsening, and formation of a fullerene-rich topping layer. The enhanced photovoltaic performance is explained by these findings.

A Closer Look into Two-Step Perovskite Conversion with X-ray Scattering.

Recently, hybrid perovskites have gathered much interest as alternative materials for the fabrication of highly efficient and cost-competitive solar cells; however, many questions regarding perovskite crystal formation and deposition methods remain. Here we have applied a two-step protocol where a crystalline PbI2 precursor film is converted to MAPbI3-xClx perovskite upon immersion in a mixed solution of methylammonium iodide and methylammonium chloride. We have investigated both films with grazing incidence small-angle X-ray scattering to probe the inner film morphology. Our results demonstrate a strong link between lateral crystal sizes in the films before and after conversion, which we attribute to laterally confined crystal growth. Additionally, we observe an accumulation of smaller grains within the bulk in contrast with the surface. Thus, our results help to elucidate the crystallization process of perovskite films deposited via a two-step technique that is crucial for controlled film formation, improved reproducibility, and high photovoltaic performance.

A direct evidence of morphological degradation on a nanometer scale in polymer solar cells.

In situ measurement of a polymer solar cell using micro grazing incidence small angle X-ray scattering (µGISAXS) and current-voltage tracking is demonstrated. While measuring electric characteristics under illumination, morphological changes are probed by µGISAXS. The X-ray beam (green) impinges on the photo active layer with a shallow angle and scatters on a 2d detector. Degradation is explained by the ongoing nanomorphological changes observed.