Enhancing the Light Absorption of Solar Cells with Carbon Nanotubes

The energy demand of the world is growing fast, it is therefore one of the greatest scientific challenges to develop energy sources or techniques to satisfy those future demands. Solar cells have aroused attention as clean energy source due to their light harvesting ability while being easy to install everywhere. The inorganic silicon solar cells still dominate the market due to the silicon’s abundance in semiconductor industry, their high efficiency and high-stability which results in a great performance-to-cost ratio. Rapid advances in emerging fields have led to high efficiencies of third generation solar cells like perovskite devices that are already comparable to silicon solar cells. Organic photovoltaics are also catching up with regard to efficiency, in the last 5 years they have developed from 12 % to over 18 % based on new organic semiconductor materials. This performance increase of the last years can be attributed to the development of non-fullerene acceptors (NFA), which typically consists of a fused ring system. Their broad spectral absorption is advantageous compared to standard fullerene acceptors, as well as the higher extinction coefficient and tunability of energy levels for tailored donor/acceptor pairs. Simpler and environmentally friendly synthesis routes and thus lower costs are required for the breakthrough of organic solar cells. Carbon nanotubes (CNTs) are a versatile material with multiple potential functions for organic photovoltaics. In principle, all elements of a solar cell, from the light sensitive component to carrier selective contacts, layers for passivation and transparent conducting films can be replaced by carbon nanotubes and their composites. The barriers to their application in industry are diminishing rapidly like the advanced dispersion and separation techniques with dramatically increase in yield and purity of single chiral species. Currently, polymer-sorting in toluene is a simple procedure which achieves high purity, but yields are low. On the other hand, aqueous sorting provides access to many more chiral species and higher yields. The narrow absorption bands and the tailorable electronic property of CNTs are useful for transparent electrodes and hole transport layers. Especially, CNTs as a passivation layer and hole selective contact in silicon photovoltaics have already been demonstrated to be competitive with current industrial cells. However, CNTs in the active layer of organic solar cells still face many challenges that need to be addressed. Primarily these are associated with improvements in the light absorption of the solar cells and the correspondingly low efficiency. In the thesis, different species of SWCNTs were tested to show that is possible to use the entire range of semiconducting CNTs (small and large diameter) and strategies to reduce excitonic trapping in mixtures of several species are required. The interesting questions for CNT based solar cells are what kind of nanotubes are favourable in PV and how can those be integrated into a device. Improving the light absorption of CNT/C60 cells is highly desired and can be accomplished by exchanging established materials like fullerene acceptors or adding materials such as dyes which absorb photons in other spectral regions. In this regard, CNTs can be used as cavity for dyes which have to transfer their absorbed energy to the nanotube to gain higher efficiencies. The replacement of fullerene acceptors with non-fullerene acceptors serves the purpose of broader light absorption and can simultaneously provide a solution to the accessibility issue of larger diameter CNTs. Nevertheless, the CNT film thickness will still be limited by the short exciton diffusion length. Considering this limitation, it is important to continue research on blended systems to optimize morphology and suppress recombination. The light absorption of such bulk heterojunctions can be completed by changing to broadband components or the addition of a third material that ideally facilitates a cascade for electron transfer. This strategy has a much higher probability of reaching competitive efficiencies because these ternary systems are built on established materials combinations. Alternatively, CNT based solar cells can be combined with established photovoltaic devices to a tandem stack for absorption extension into the infrared