The p-type Project- Development of Alternative Sensitisers and p-type Materials for Integration into Tandem Devices

Abstract

Photovoltaics are a key player in the push to provide the world with energy from sustainable sources. Tandem dye-sensitised solar cells are one approach to the integration of PV generation into daily life, but efficiency of these devices is limited by the properties of the photocathode. By replacing the thin-film NiO semiconductor with a material with better transparency, charge transport capabilities, or lower valence band potential, some of the problems with this technology can be addressed. The aim of this work is to define an appropriate sensitising material for high- throughput materials screening during the development of alternative p-type semiconductors for tandem DSSCs. The work in this thesis encompasses a range of sensitisers for light harvesting within dye and quantum dot sensitised solar cells. The introductory Chapters, 1 and 2, highlight the importance of photovoltaics in the context of the world’s energy demand and provide background and insight into the state of the field. They also illustrate some of the most important techniques used throughout the course of these projects, and the experimental details for the fabrication of DSSCs. In Chapter 3, two classes of sensitisers are integrated into n-type DSSCs and optimised. Firstly, an iridium (III) complex with panchromatic absorption response up to 700 nm is investigated as an alternative to ruthenium-based dyes. After optimisation using blocking and scattering layers on the TiO2, an efficiency of 0.49 % was achieved for these devices. However, the application and poor sustainability of iridium-based dyes was a drawback for these devices, and a second approach was taken. The sensitisers derived from natural products, namely the seeds of the Peruvian-native prickly pear (Opuntia Soehrensii), were applied to the previously optimised TiO2 thin films and tested in devices. Promising efficiencies were then improved via the addition of citric acid to stabilise the photosensitive dye, and an efficiency of 1.41 % was achieved with current output remaining stable after 7 days. Upon testing in low-light conditions, the devices also produced a PCE of 4 %. Within Chapter 4, dyes for p-type DSSCs were investigated. Collaborators produced a series of dyes based on triarylamine cores, indolium acceptors and pyridine anchors. Each of these dyes were integrated into p-DSSCs, and the architecture combining two indolium acceptor groups with a phenyl pyridine anchor resulted in devices with the best ii performance of the series (η =0.097 %). While the performance was attributed to the absorption profile of the dye extending to almost 700 nm, the poor dye loading limited the absorption capability of the devices, and therefore the performance. Another study was undertaken using a further red-shifted BODIPY dye (RJ3) synthesised by colleagues. This dye was optimised for absorption onto NiO using two carboxylic acid anchoring groups and was highly emissive. Champion p-DSSCs containing this dye reached efficiencies over 0.1 %, allowing preliminary tandem devices to be constructed using a D35-sensitized TiO2 photoanode. While poor current matching limited the output of the tandem cells, RJ3 was shown to be a promising dye for application in p-type devices. The final results Chapter covers the development of quantum dot sensitisers for the screening of alternative p-type materials. Initial tests with PbS quantum dots synthesised ex-situ in a hot injection process gave poor results, due to low loading, so a new approach was taken in which CdS nanofilms were deposited directly onto films via SILAR. This process was then automated using robots constructed with low-cost and widely available LEGO pieces, allowing for in-laboratory upscale of film production. The CdS-sensitized NiO films were incorporated into devices containing a polysulphide gel-based electrolyte and a new counter electrode material, nickel foam. CdS films were also co-sensitised with the RJ3 dye investigated in Chapter 4 in order to extend the spectral response of the photocathode, which improved the device efficiency to over 1 %. Work with collaborators from IITH is also discussed in this chapter, in which new materials were introduced via SILAR onto NiO films, which were subsequently integrated into tandem devices. Chapter 6 summarises the key findings in this thesis and discusses the scope for continuation of the work