Organic photovoltaics has emerged as a promising technology for electricity generation. The essential component in an organic solar cell is the bulk heterojunction absorber layer, typically a blend of an electron donor and an electron acceptor. Efforts have been made to design new materials such as donor polymers and novel acceptors to improve the power conversion efficiencies. New fullerene free acceptors providing low cost synthesis routes and tenability of their optoelectronic and electrochemical properties have been designed. Despite the efforts, still not much is known about the photopysical processes in these blends that limit the performance. In this respect, time-resolved spectroscopy such as transient absorption and time-resolved photoluminescence, can provide in-depth insight into the various (photo) physical processes in bulk heterojunction solar cell. In this thesis, PCE10 was used as donor and paired with different non fullerene acceptors. In the first part of this thesis the impact of the core structure (cyclopenta-[2, 1-b:3, 4-b’]dithiophene (CDT) versus indacenodithiophene (IDTT)) of malononitrile (BM)-terminated acceptors, abbreviated as CDTBM and IDTTBM, on the photophysical characteristics of BHJ solar cells is reported. The IDTT-based acceptor achieves power conversion efficiencies of 8.4%, higher than the CDT-based acceptor (5.6%), due to concurrent increase in short-circuit current and open-circuit voltage. Using (ultra)fast transient spectroscopy we demonstrate that reduced geminate recombination in PCE10: IDTTBM blends is the reason for the difference in short-circuit currents. External quantum efficiency measurements indicate that the higher energy of interfacial charge-transfer states observed for the IDTT-based acceptor blends is the origin of the higher open-circuit voltage. In the second part of this thesis, I report the impact of acceptor side chains on the photo-physical processes of BHJ solar cells using three different IDT-based acceptors, namely O-IDTBR, EH-IDTBR and O-IDTBCN blended with PCE10. Power conversion efficiencies as high as 10 % were achieved. The transient absorption spectroscopy experiments provide insight into sub-picosecond exciton dissociation and charge generation which is followed by nanosecond triplet state formation in PCE10:O-DTBR and PCE10:EH-IDTBR blends, while in O-IDTBCN triplets are not observed. Time delayed collection field experiments (TDCF) were performed to address the charge carrier generation and examine its dependence on the electric field.

The state-of-the-art organic solar cells (OSC) use bulk heterojunction (BHJ) blend architecture in the photo-active layer. The BHJ is formed by finely mixing polymer donor and small molecule acceptor, which was predominantly fullerene derivatives until the last five years. However, the emergence of non-fullerene acceptor (NFA) materials has been the viable alternative to overcome high synthetic costs, limited optical absorption, and poor bandgap tunability of fullerene-based acceptors. These unique properties of NFA has resulted in a rapid improvement of OSC efficiency and opened doors for wide variety of applications including building integrated photovoltaics, green houses and agrivoltaics. Despite these advantages, the shorter device lifetime under light and heat is a major concern for their commercialization. This dissertation is focused on improving poor photo- and thermal stability of high efficiency OSC based on the widely used NFA, ITIC and Y-series derivatives. The light-induced changes in the acceptor molecular structure and the active layer nanostructure results in the photo-induced traps in photo-aged devices. The selective addition of third component to the active layer impedes the changes in the active layer nanostructure and suppress trap formation. Under constant thermal stress, the growth of acceptor crystals results increases the trap-assisted recombination in thermally aged devices. Similar to photo-stability the selective addition of third or more component/s arrests the crystal growth by minimizing the Gibbs free energy. The results suggest that the fabricated hexanary and ternary OSC display a superior thermal stability than the respective binary devices. In addition, the hexanary devices displayed thickness independent thermal stability, which is essential for the active layer thermal stability printed via high throughput techniques.

The growing concern about climate change is pushing the global community towards greener solutions to cut down the greenhouse gases emissions. As such, producing energy from sustainable sources becomes mandatory to achieve the net zero emissions goal by 2050, as set by the United Nations. Solar panels offer the possibility to generate power from light harvesting, but it’s the use of organic materials that offers great advantages in terms of functionality and life-cycle. In particular, organic semiconductors properties such as their tunable colours, lightweight, flexibility, and semi-transparency enable the use of Organic Photovoltaics (OPV) in building façades and contribute to the realisation of Net Zero Energy Buildings (NZEB). However, the OPV scalability to terawatts of installed capacity is still non competitive with respect to its cost when compared to the conventional inorganic silicon-based technologies. One of the reasons is the lower performance achieved by the state-of-the-art OPV devices, whose active layer (the film where the light is absorbed and converted into free charges, electrons and holes, i.e. electricity) is typically composed of a blend made of an electron donor material (conjugated polymer) and a smaller compound as electron acceptor (Non-Fullerene Acceptor, NFA). A crucial factor determining the low performance of OPVs made with NFAs is related to their poor charge transport properties (e.g. low electron mobility and high recombination), which are intimately related to how these molecules are arranged in the solid film, i.e. their molecular organisation. Great progress was made in the field of organic electronics to obtain higher mobility by understanding the crystalline behaviour of organic molecules from their single crystals, and using these knowledge in the design of new compounds with the desired properties. At the beginning of this thesis project, little was known about the solid-state organisation of NFAs as very few single crystal structures were disclosed. For these reasons, we were first dedicated to the study of the intrinsic propensity of NFAs to crystallise by growing single crystals. At this fundamental level, we found that the NFA packing geometry is strongly affecting the isotropy of the charge transport, and potentially the electron mobility. On a following step, we developed a methodology to track the NFA packing geometry as we move from ideal systems (single crystals) to the most complex scenario of the solar cell active layer films, which include a donor and an acceptor (NFA) component. We discovered that NFAs generally tracks their packing motif from single crystals to blend films, and we quantified the benefit of using crystalline compounds with specific packing geometry in terms of electron mobility. Interestingly, we also found that these motifs are not necessary to obtain high performance in organic solar cells as the efficiency is mostly driven by charge recombination and domain purity, rather than electron mobility.

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