In recent years, metal-organic framework nanosheets (MONs) have emerged as novel two-dimensional materials with enormous potential for use in advanced electronic devices. MONs comprise of organic linkers that are linked with metal ions or clusters in two-dimensions. They are most notable for their high surface area, nanoscopic dimensions, physical flexibility, and diverse chemical functionalities. With their modular design, MONs can be systematically modified through substitution of different ligands and metal ions which allows for systematic tuning of their optoelectronic properties. In this thesis, MONs are explored as potential materials for enhancing the performance of a range of organic photovoltaic (OPV) devices.
Chapter 1 introduces photovoltaics, outlining the background theory of OPVs, their performance mechanism and provides an overview of the progress in ternary OPVs. Chapter 2 is an introduction to MONs, detailing their structure, synthesis, and applications in electronics. This chapter also covers the aims and objectives of this thesis. Chapter 3 is the experimental methods chapter.
In Chapter 4, Zn2(ZnTCPP) MONs (where TCPP = tetracarboxyphenyl porphyrin) were synthesised using liquid exfoliation. The MONs were found to approach monolayer thicknesses and their optoelectronic properties were found to be ideally suited for incorporation into the active layer a polythiophene-fullerene based OPVs. P3HT-MON-PCBM ternary blend bulk heterojunctions were therefore developed. Upon optimisation, the ternary OPV devices were found to outperform the reference devices with the champion MON devices at 5.2% PCE as compared to the references at 2.6%. Detailed mechanistic investigations were carried out to probe their performance enhancement. The incorporation of MONs was found to lead to creation of highly crystalline P3HT domains in the films, which resulted in improved light absorption, higher hole mobility and reduced grain sizes. This work provides the first example of incorporation of MONs into the active layer of an OPV device and demonstrates their potential as additives for enhancing the performance of OPVs.
In Chapter 5 the effect of different metal ions and ligands on the energy level alignment of MONs was explored. The aim was to identify the key structural and electronic features that should be considered while designing MONs for OPV applications. Upon comparing Cu2(CuTCPP) and Cu2(ZnTPyP), (where TCPP = tetracarboxyphenyl porphyrin and TPyP = tetraphenyl porphyrin) as additives in P3HT-PCBM system, the device performance doubled with Zn2(ZnTCPP), remained unaffected with Cu2(ZnTPyP) and halved with Cu2(CuTCPP). The energy level alignment in different systems was evaluated using photoemission techniques. The choice of metal ions was found to only have a small impact on the ionization energies of the MON and was not sufficient to explain the variations in device performances observed. The size of the nanosheets was found to play a significant role in influencing the device performance. Large sized nanosheets were detrimental to the power conversion efficiencies because they reduce the interface between the donor P3HT and acceptor PCBM. This work identified the key structural and electronic features to be considered while choosing MONs for OPV applications.
In Chapter 6 the general applicability of MONs to other donor-acceptor OPV systems is investigated. A range of commonly used polymer-fullerene devices were selected ranging from fully amorphous to fully crystalline systems. The addition of MONs to devices based on fully crystalline or amorphous donor polymers showed only small or non-significant improvements in their PCE upon incorporation of MONs. In contrast, the addition of MONs to semi-crystalline polymers showed remarkable improvements in performance. In particular, the PCE of PffBT4T-2OD-PCBM based devices increased from 10.6% to 12.23 % with the inclusion of MONs resulting in the best performing fullerene based OPV reported so far. Detailed mechanistic investigations of these devices showed that MONs promote a higher degree of crystallinity in the films by forcing a more face-on orientation of the polymer chains. This favours the charge transport direction resulting in higher charge mobilities, better light absorption and smaller well-defined grain sizes. This study not only demonstrates the general applicability of MONs as templates for semi-crystalline polymers but also offers important insights towards improving the nanoscale morphology in OPVs.
Overall, this thesis demonstrates the application of MONs in OPVs as additives for improved device performance. In addition to enhancing absorption and acting as electron donors within OPV devices, MONs can act as templates that improve the morphology of semi-crystalline polymers resulting in significant improvements in performance. Matching the energy levels of the MONs to the OPV devices and optimising exfoliation to ensure monolayer nanosheets are used are key to designing MONs for thin-film OPV applications. With their tuneable properties and nanoscopic dimensions, MONs therefore have significant potential to enhance the performance of a wide range of organic electronic devices.
