Interfacial optimisation in colloidal quantum dot solar cells

Colloidal quantum dots (QDs) have attracted tremendous attention for the application in advance photoelectric devices. They possess many advantages, such as low-cost solution processability, bandgap tunability, ease of property modification via surface engineering and efficient multiple exciton generation effect (MEG). A significant amount of research effort has been conducted in last decade, particularly in the field of photovoltaics (PV). Among all types of QDs, lead selenide (PbSe) and lead sulfide (PbS) QDs are particularly interesting due to their relatively good stability and advantage in carrier transport. However, the development in these QD solar cells (QDSCs) is inhibited by many problems such as limited carrier extraction, lack of surface passivation and difficulty of doping QDs in QDSCs. This thesis aims to identify and address some of the main problems through different approaches of engineering the interfaces in QDSCs. The introduction of an antimony-doped SnO2 buffer layer at the PbSe/titanium dioxide interface was investigated. It was found that this buffer layer can have shunt-blocking effect at the interfaces and inhibit current leakage. In addition, the energy band alignment among the layers within the device can be optimised by the buffer layer. These overall improved the carrier extraction in a PbSe QDSC. The device structure design in this work provides guidance to fabricate devices with potential MEG effect. Non-radiative recombination at the QD hole transport layer (HTL) and metal contact interface in a PbS QD solar cell was found to be effectively supressed by introducing a zinc salt post-treatment on HTL. Zinc acetate was identified to be the best source for HTL passivation, as more comprehensive passivation was achieved by extensive zinc incorporation into the PbS HTL, while essential p-doping oxidation was preserved. These eventually led to suppression in non-radiative recombination and improved the device efficiency from 8.7% to 10.2%. This work emphasises the importance of passivating the surface of HTL in a QDSC. N-type PbSe QD layer was fabricated with bismuth iodide ligands in order to replace conventional metal oxide layers which usually require high-temperature processes. With the Bi3+ doped PbSe QD film as the n-type layer, a proof-of-concept PbSe QDSC has been fabricated with a PCE of 4.7%. This demonstrates the potential of producing QDSC purely based on QD junctions