Developing molecular sensitisers for NiO-based p-type dye-sensitised solar cells

Abstract

The investigation of NiO-based p-type dye-sensitised solar cells (p-DSCs) has attracted significant research interest as they remain the limiting factor for highly efficient tandem-DSCs. As the dye is the part of a DSC that absorbs light and drives the production of a photocurrent, the development of new sensitisers is crucial to improve device efficiencies. Due to the typically low NiO film thickness used (ca. 1.5 μm), organic dyes with high extinction coefficients are needed to maximise light harvesting. When incorporated into a tandem-DSC, the NiO p-DSC will be in combination with a TiO2 n-type DSC and should absorb a complementary wavelength range. As n-type devices have been optimised to harvest higher energy photons, for NiO we require dyes that can absorb across the longer-wavelength visible and near-IR regions. An anchoring unit, typically a group containing an acidic proton, is also essential for the dye to adsorb onto the semiconductor surface and to facilitate efficient charge transfer from NiO to the dye. In this thesis, novel BODIPY, indolium, porphyrin and indigo-based dyes have been synthesised and studied as sensitisers for p-DSCs. Each chapter focuses on a different aspect of dye design to provide an in-depth investigation into the requirements for an efficient device. ‘Push-pull’ dyes have been routinely used to construct efficient p-DSCs. These consist of electronic donor and acceptor groups, linked by a conjugated bridge, where the acceptor unit is spatially separated from the semiconductor surface. This is intended to promote a long-lived charge-separated state following electron transfer from NiO. A series of donor-acceptor BODIPY-triphenylamine dyes has been synthesised (1-5, Chapter 3) and characterised using spectroscopic and electrochemical techniques to investigate the effect of tuning donor-acceptor coupling on device efficiencies. When this coupling was limited, the lowest energy excitation was a π-π* transition localised on BODIPY, but as the coupling increased there was an increase in charge-transfer character. The dyes were integrated into p-DSCs and those with a π-π* transition localised on BODIPY performed modestly, with the performance largely unaffected by the nature of the donor/anchor group. The best performing dye had the highest degree of charge-transfer character and produced devices with efficiencies higher than the benchmark dye P1 and comparable to other highly-performing dyes reported. Many dyes used for p-DSCs contain a single carboxylic acid as an anchoring group. Multiple anchors are expected to increase dye stability and should induce a more rigid, structured dye monolayer. Dyes containing BODIPY or indolium acceptors and a novel vi pyrrole-based multidentate anchoring system were synthesised (6-8, Chapter 4) and incorporated into p-DSCs. Modest performances were observed for the BODIPY dyes that has been attributed to poor charge transfer from NiO, while higher efficiencies were obtained for the indolium dye that was limited by a low extinction coefficient. Despite this, high dye loading was seen for the indolium dye which could have potential applications for co-sensitisation. The synthesis of these dyes was hindered by the difficult hydrolysis of the two methyl carboxylates situated on the pyrrole to form the anchoring unit. This prompted an investigation into developing alternative routes to introduce anchoring units as a final synthetic step. While the BODIPY acceptors designed in Chapter 3 were successfully used to produce efficient devices, the most efficient dyes were those with higher charge-transfer character. These lacked most of the properties that attract interest toward BODIPYs, such as well-defined absorption/emission bands and high emission quantum yields. Therefore, the development of new acceptor groups was needed. Indigo derivatives have been routinely studied for their electron accepting properties and have found widespread use in organic photovoltaics. In Chapter 5, the development of dyes based on bay-annulated indigo is discussed. A major limitation to the synthesis was the poor solubility of the bay-annulated indigo core after halogenation. This was overcome by introducing solubilising branched chains to indigo prior to bay-annulation. A first-generation dye (9) containing a simple benzoic acid anchoring group was synthesised, characterised and investigated in p-DSCs. Somewhat low performances were obtained from these devices that has been attributed to low dye loading, likely due to the short anchor length compared to the bulky core. However, this study has proved the feasibility of bay-annulated indigo dyes for p-DSCs and has formed the initial groundwork to synthesise a more efficient dye. The synthetic difficulties encountered in Chapters 4-5 prompted an investigation into alternative routes of introducing anchoring groups as a final synthetic step. This culminated in the synthesis of triazole-bridged porphyrins (10-12, Chapter 6) using ‘click’ chemistry, where a range of different anchors could be introduced in a facile manner. In this series, the anchoring groups consisted of a carboxylic acid, a phosphonic acid and coumarin 3-carboxylic acid. Dye adsorption onto NiO was investigated using adsorption isotherms and kinetic studies and the phosphonate was determined to have a higher affinity for NiO and a higher rate constant for adsorption. The dyes containing the phosphonic acid and coumarin anchoring groups exhibited vii higher dye loading than the carboxylic acid. Therefore, devices made using these dyes had a higher light harvesting efficiency and produced higher photocurrents. In this chapter, a new method of introducing various anchoring groups as a final synthetic step has been developed, which can open new strategies for dye design. In conclusion, this thesis aimed to study the dye requirements for efficient p-type devices and provide guidelines for future dye design. Chapter 3 showed that higher device efficiencies were obtained with improved donor-acceptor coupling and Chapters 4 and 6 developed new routes for synthesising anchoring groups. The measurements carried out in Chapter 6 also showed that the phosphonate anchoring unit is more suitable for NiO that the widely used carboxylic acid group. A new acceptor based on indigo was developed for use in p-DSCs in Chapter 5 that could likely be improved using the anchoring groups designed in Chapter 6.