Carbon Nanomaterials for Improved Performance of Perovskite Solar Cells Processed under High Humidity

Abstract

The work documented in this thesis concerns optimization of stacks of semiconducting materials for photovoltaic purposes. Hybrid organic-inorganic metal halide perovskite (MHP) is a promising semiconducting material for thin film solar cells, offering both high solar to electrical energy conversion efficiencies and cheap fabrication costs. However, several issues have so far acted as constraints to development of the technology such as sensitivity of commonly pursued fabrication techniques to atmospheric moisture and use of high temperature processed porous metal oxides which add to manufacturing costs and are incompatible with high-throughput roll-to-roll processing on flexible substrates. In addition, many of the prototypical charge selective materials used to achieve high performance are known to be inherently reactive with perovskites, especially in the presence of oxygen, moisture and even ultraviolet light in some cases. In order to fully realise the potential of perovskite solar cells it would be extremely advantageous to improve device performance, reproducibility and stability in uncontrolled humid environments; both from the perspective of commercialisation and of rendering the technology accessible to less specialist research labs. Initially, work focuses on finding optimal electron selective materials for planar solution processed perovskite devices under humid conditions. It is found that many of the most commonly used metal oxides are not well suited to the task, presenting large barriers to charge extraction resulting in low photocurrents. In addition, zinc oxide is found to accelerate perovskite decomposition to varying degrees depending on the method used for thin film fabrication. On the other hand, introduction of solution-processed C60 as a surface modifier is found to reduce the barrier to charge extraction for all of the metal oxides studied. Consequently, efforts are made to avoid metal oxide electron transport layers (ETLs) by optimizing C60-based ETLs. Thin layers of the polymer polyethyleneimine (PEI) are found to be effective in achieving consistently high photovoltaic conversion efficiencies (PCE) with champion cells delivering over 15.2mW/cm2 under AM1.5 illumination. Furthermore, these polymer-fullerene bilayers are demonstrated to be versatile ETLs by fabricating flexible photovoltaic devices on ITO-coated PET and PEDOT:PSS-coated PET as well as aluminium-doped zinc oxide (AZO) as a low cost solution-processed alternative to ITO. PEI is also found suitable for stabilizing graphene at concentrations of approximately 0.66mg/mL in butanol, thereby permitting the deposition of PEI-graphene composite films via spin coating. Optimized composites act as an effective barrier to perovskite decomposition whilst maintaining high PCE. Introduction of PbCl2 into the perovskite precursor mix improved both photocurrent output and open circuit voltage for devices made using single-step deposited perovskite via solvent-solvent extraction, although two-step deposition ultimately led to less cell-to-cell variability. Addition of 30 weight% lead chloride (PbCl2) is found to be crucial in guaranteeing full perovskite crystallization in highly humid environments, leading to much improved batch-to-batch reproducibility. Higher PbCl2 content is also demonstrated to improve performance of devices based on transparent AZO electrodes deposited by spray pyrolysis. Cells showed good stability, retaining 75% of initial PCE after over 600 hours of storage in ambient without encapsulation. Performance reproducibility issues are also raised regarding the hole transport layer (HTL) P3HT, with carbon nanotube (CNT)-PMMA composite layers found to give consistently high photocurrents. Champion devices with HTL stacks of P3HT / CNT / PMMA delivered a PCE of 16.2% and outperformed P3HT-only HTLs by maintaining 80% of initial PCE after 900 hours. A thicker PEI-C60 ETL is also demonstrated to improve device fill factor leading to a stabilized PCE of 15.5% for P3HT-free devices containing only thin CNT/ PMMA HTLs. Finally, efforts are made to incorporate graphene as a p-doped material for hole transport.