A spatially resolved study on the Sn diffusion during the sintering process in the active layer of dye sensitised solar cells

Dye sensitised solar cells (DSSCs) use a mesoporous TiO2 scaffold, typically assisted by an adsorbed dye, as the main active element, responsible for the photon absorption, exciton generation and charge separation functionality. The sintering process employed in the TiO2 active layer fabrication plays a crucial role in the formation of the nanoparticle scaffold and hence the performance of a dye sensitised solar cell, as it allows the particles to form efficient inter-crystalline electric contacts to provide high electron conductivity. The sintering temperature, with typical values in the range of 450–600 °C, is of particular importance for the formation as it reduces the amount of unwanted organics between the individual crystallites and determines the formation of interfaces between the nanoparticles. Furthermore, the cell design requires a conductive transparent top electrode which is typically made of fluorinated tin oxide or indium tin oxide. Here we report on a highly spatially resolved scanning electron microscopy study including focussed ion beam (FIB) milling and energy dispersive X-ray (EDX) mapping of the distribution of all relevant elements within a DSSC subsequent to a classical sintering process. We find that the above quoted temperatures cause the Sn of the transparent conductive oxide (TCO) to migrate into the TiO2 scaffold, resulting in unwanted alterations in the composition of the complex scaffold which has a direct effect on the DSSC performance. One potential solution to this problem is the invention of novel concepts in the manufacturing of DSSCs using lower sintering temperatures.