Multisource Evaporation of Perovskite Solar Cells
This thesis focuses on the emerging semiconductor family of metal-halide perovskites for their application in solar cells. Halide perovskite materials have shown several promising properties, such as high absorption coefficient, low exciton binding energy, high charge mobility, long diffusion length and low annealing temperature. Moreover, their tunable bandgaps, with emission from the blue to near-infrared spectrum region possible, demonstrates their potential for different optoelectronic applications, such as solar cells, light-emitting diodes and transistors. The device performance of perovskite solar cells has increased rapidly to 25.7% in 12 years. This incredible improvement is based on a myriad of research works, mostly on solution- processed perovskites, as solution processing allows rapid screening of experiment protocols in the lab. However, scaling up perovskite solar cells for real-world impact is critical to solving energy demand crisis and climate change by using renewable energy with low-carbon emissions. Vacuum deposition, an industrially compatible deposition method, can provide smooth, uniform and solvent-free thin films for solar cell fabrication. In this thesis, the deposition and characterisation of multisource evaporated perovskite films with MA-free composition for solar cells has been studied. Chapter 4 shows that the underlying substrate can affect the perovskite quality, form different grain sizes and preferred structure orientations. Excess PbI2 during the evaporation can improve the film moisture stability and photoluminescence quantum efficiency. The importance of deposition rate is also highlighted here, showing the issue with halide uniformity. Furthermore, non-radiative losses in p-i-n architecture has been identified. After optimising the amount of excess PbI2, solar cell device performance of 18.1% can be achieved, which was the highest MA-free perovskite solar cell from multisource evaporation system at the time of the work. In chapter 5, the loss between perovskite and hole-transporting layer is minimised, leading to an even further improved device performance of 20.7% from a bandgap of 1.62 eV perovskite. A tandem solar cell is a promising route to exceed the single-junction Shockley–Queisser limit. A wide bandgap perovskite with bandgap from 1.62 eV to 1.8 eV, fabricated by multisource evaporation, is explored by controlling the PbBr2 evaporation rate. The photoluminescence, time-resolved photoluminescence and phase segregation have been studied and these results indicate the most optimised material under these conditions for tandem solar cells has a 1.77 eV bandgap. Perovskite solar cells with these bandgap absorbers have been fabricated and tested. Their device open-circuit voltage (VOC) increases monotonically to 1.24 V for a bandgap of 1.77 eV perovskite, compared to a VOC of 1.1 V from a bandgap of 1.62 eV. To understand the non-radiative loss, PLQE measurements have been performed, and the surface passivation treatment by Ethane-1,2-diammonium iodide (EDAI) leads to an improvement in VOC and fill factor. The narrow bandgap perovskite with Pb/Sn composition is prepared and the film morphology, crystal structure, and photoluminescence has been characterised and optimised to make all-perovskite tandem solar cell. The same EDAI passivation is also useful for the Pb/Sn perovskite as surface passivation by further improving the device VOC and FF. The interconnect layer by atomic layer deposition for SnOX layer is developed to connect the wide and narrow bandgap perovskite. A 2-terminal all-perovskite tandem solar cell with a PCE of 24.1% is shown, where at least one subcell is prepared by multisource evaporation. Chapter 6 presents the versatility of evaporated perovskite in different applications. We have demonstrated a proof of concept of evaporated perovskite for perovskite/Si tandem solar cells, solar fuel device and large-scale deposition. Finally, chapter 7 provides a conclusion and outlook. To sum up, this thesis provides a deep understanding of the multisource evaporation process, non-radiative loss in the device, solar cells optimisation and all-perovskite tandem solar cells.
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