Investigation of the operational stability of metal-halide perovskite solar cells

Abstract

Man-made climate change is a threat to the world as we know it and renewable energies must be employed to a great extent to fight it. Photovoltaic solar cells, which convert the energy of photons into electric energy, provide an increasingly important source of clean energy. In the last decade, a new class of materials has attracted tremendous attention to the scientific community due to their outstanding photovoltaic properties: Metal-halide perovskites. The efficiency of solar cells utilising perovskite materials as light absorber has sky-rocketed to values competing with established technologies like silicon photovoltaics. Yet, several roadblocks remain to be overcome on the path towards the commercialisation of this young technology. One of the critical issues to be overcome is the operational stability of perovskite solar cells (PSCs). This dissertation presents an Investigation of the Operational Stability of Metal-Halide Perovskite Solar Cells. For this purpose, a high-throughput ageing system for perovskite solar cells was constructed. The system is designed to test hundreds of solar cells in parallel under the most realistic electric load condition maximum power point tracking (MPPT), which requires specialised electronics. The ageing system allows to investigate the operational stability in controlled atmosphere, temperature, and illumination with statistical relevance. The ageing system is employed to explore the relation between different laboratory testing scenarios and a realistic stability assessment in outdoor tests. A complete set of constant illumination indoor testing, cycled illumination indoor testing, and real-world outdoor testing is presented, performed on four different types of PSCs. A distinctly different ageing behaviour is observed between the different PSCs among the indoor experiments, despite the only difference being the introduction of light cycling. The shape of the MPPT as well as the maximum efficiency reached are found to change significantly with the number of cycles for devices utilising the organic self-assembled monolayer MeO-2PACz as hole transport layer (HTL) in solar cells with a Cs5(MA15FA85)95Pb(I85Br15)3 perovskite absorber. Strikingly, this behaviour is not reflected in the constant illumination test. This highlights that the effects causing the initial transient behaviour observed under maximum power tracking play a crucial role for PSC stability. The comparison to outdoor tests reveals a good correlation between the cycled illumination test and the outdoor behaviour for three of the four solar cell stacks tested. The commonly used constant illumination test is shown to possess little predictive power towards realistic outdoor behaviour. The most likely reason is that in a constant illumination test the initial transient only occurs once, while it is repeatedly occurring in cycled illumination testing as well as in real-world operation where solar cells are subjected to the diurnal rhythm of day and night. Furthermore, three studies are presented, performed to investigate the factors influencing the operational stability and to elucidate the mechanisms behind the observed degradation. In the first study, the influence of a variation of both electron transport layer and hole transport layer on the ageing behaviour of devices with a Cs5(MA15FA85)95Pb(I85Br15)3 perovskite absorber is found to be large. The combination of the organic self-assembled monolayer MeO-2PACz as hole transport layer and a double layer of C60 and SnO2 as electron transport stack is found to be most stable among the tested variations. In the second study, the hole transport layer is varied between the metal oxide NiO and MeO-2PACz in devices with Cs5(MA15FA85)95Pb(I85Br15)3 as light absorber. Devices are aged at an elevated temperature of 85 °C to explore irreversible degradation. As-prepared and aged devices are characterised and a fundamentally different degradation mechanism between the two hole-transport layers is identified. Time-resolved photoluminescence imaging under light soaking is shown to be an elucidative tool to explore the degradation of full devices. The third study reports on the influence of the grainsize of a CH3NH3PbI3 absorber on the ageing behaviour under continuous illumination at 25 °C. The long-term stability, evaluated via the linear slope of the ageing tracks, is found to be directly dependent on the grainsize. Solar cells with larger grains show higher performance as well as a reduced ageing slope, indicating more stable cells. Additionally, a change in initial transient behaviour in dependency of the grainsize is found; devices with smaller grains show a shorter initial transient. This is relevant with respect to the previous finding that the transient behaviour plays a crucial role for a realistic stability assessment of PSCs.