Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Malgras, Victor, Lead sulfide colloidal quantum dots passivation and optoelectronic characterization for photovoltaic device application, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4203
Photovoltaic energy conversion is one of the best alternatives to fossil fuel consumption. Petroleum resources are now close from depletion and their combustion is known to be responsible for releasing a considerable amount of greenhouse gas and carcinogenic airborne particles. Novel third-generation solar cells include a vast range of device architectures and materials aiming to overcome the factors limiting the current technologies. Among them, quantum dot based devices showed promising potential both as sensitizers and as colloidal nanoparticle film. p-doped PbS colloidal quantum dot (CQD) forming a heterojunction with a n-doped wide-band-gap semiconductor such as TiO2 or ZnO. Ultimately, this technology would lead to the assembly of a tandem-type cell with CQD films absorbing in different region of the solar. The confinement in these nanostructures is also expected to result in marginal mechanisms such as hot carrier collection and multiple exciton generation which would increase the theoretical conversion efficiency limit.
In this work, certain mechanisms linked with CQD film passivation are addressed using various measurement methods. X-ray photoelectron spectroscopy is investigated in depth in order to pinpointed species specific to the role of methanol during the ligand exchange and notable differences are observed in the surface states of films treated with 3-mercaptopropionic acid, thioglycolic acid and thiolactic acid. The removal of the initial oleic acid ligand following methanol rinsing clearly leaves the CQD unprotected against adventitious oxidation altering a single atomic monolayer of the nanoparticles, as confirmed by a broadening and blue shift of the first exciton energy observed on UV-Vis absorption spectroscopy. Through fluorescence spectroscopy, two in-gap states are identified in each sample and the non-uniform quenching after ligand exchange suggests that 1% of the charges injected in TiO2 are recapture by the deeper trap state. Thiolactic ligand treatment shows a notable enhanced protection against surface contamination and displayed negligible electronic configuration change compared to the untreated sample while providing similar quenching properties. It can be related to the fact that this molecule is more bulky due to its -CH3 group and undergoes more steric interactions with neighbouring ligands.
A whole assembly procedure is optimized, from the PbS CQDs synthesis to characterization of selective contacts based devices. Current-voltage analysis in dark conditions indicates that transport in such device appears to be strongly affected by space-charge limiting effects due to in-gap trap states distribution. The impact of TiO2 and MoOX selective contacts is also addressed. MoOX not only improves performance due to electron screening barrier, it also enhances stability significantly. No sign of free extracted charge is observed, indicating that the doping in the CQD film is negligible. Through time resolved charge extraction measurements, one can observe that recombination appears to be first dominated by relatively slow mechanisms and undergoes a fast acceleration. The time at which this acceleration occurs looks to be partly related to the MoOX thickness, suggesting that recombination with the external circuit might play a dominant role. Recombination regimes are addressed and appears to involve multiple mechanisms which cannot be simply fit with common first or second order reaction rates.