Intermolecular Charge Transport: A Novel Design Paradigm

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Renewable energy is expected to play a major role in reaching our target to end the UK's contribution to global warming by the year 2050.1 Emerging thin-film technologies, such as perovskite solar cells (PSC), are widening the scope of photovoltaics (PVs) to applications beyond the effective capabilities of conventional silicon based PVs. These devices are able to operate under low light conditions and can be printed easily as cheap lightweight-flexible devices, making them suitable for integration within indoor and portable systems.2PSCs with high efficiencies of 25.2%, and 28% within silicon-based tandem cells, have been already demonstrated.3 They make use of abundant, low-cost starting materials and are less energy intensive to produce than conventional silicon solar cells.4 However, a key challenge remains the 'hole' transport material (HTM) which plays a major role in controlling the overall performance and cost of these devices.4 Light hitting the perovskite absorber causes excites its electrons to a higher energy level, leaving behind a positively charged 'hole'. The HTM then shuttles these holes away from the absorber and toward the electrode allowing a current to flow through the device. One problem is charge recombination, which limits efficiency. In addition, state-of-the-art HTMs, such as spiro-OMeTAD, are expensive and difficult to synthesise.4Novel HTMs, have been developed at a fraction of the cost of conventional materials, employing simple chemistry.4,5,6 Their synthesis can be carried out under ambient conditions,without the need for metal catalysis, and trivial isolation techniques furnish products in high yields and purities. By combining different core and side groups, HTM libraries can be created, tuning structures to optimise their performance. However, these novel materials still do not outperform state-of-the-art HTMs.This project aims to investigate intermolecular charge transport affected by the HTM. By combining theoretical and experimental approaches, we are looking to understand the improved charge transport properties of novel materials, synthesised using condensation chemistry, with disrupted conjugation in the backbone.4 Theoretical studies will be carried out to investigate the properties of known HTMs, including conductivity and charge carrier mobility. Based on our findings we aim to design and synthesise improved materials which will be tested, both as 'hole'-only devices and within PSCs.Computational studies of known HTMs will be conducted, studying the neutral and charged species as well as transitions from the ground state to the excited state. These results will be used to gain an insight into properties, such as charge carrier mobility, packing and solubility, that effect performance. Our findings will guide the design of novel HTMs with improved charge transport properties. Molecules will be synthesised that expand on our range of molecules, synthesised using condensation chemistry, initially with disrupted conjugation in the backbone.Performance-related optoelectronic and physical properties of the molecules, will be tested both in 'hole'-only devises and within PSCs. Molecular properties will be calculated from UV-visible absorption spectra and cyclic voltammetry experiments. The conductivity and charge-carrier mobilities of HTMs will be measured. Finally, thermal transitions, stability and profile of HTM films will be analysed. HTMs with the appropriate properties will be used to fabricate PSCs and the structure and PV characteristics of these devices will be studied. Using an iterative approach, our results will be used to optimise the design process and arrive at better performing HTMs.